Galaxies

28 pages, 1526 KiB

Open AccessFeature PaperReview

The UV Perspective of Low-Mass Star Formation

by P. Christian Schneider, H. Moritz Günther and Kevin France

Galaxies 2020, 8(1), 27; https://doi.org/10.3390/galaxies8010027 - 21 Mar 2020

Cited by 14 | Viewed by 3768

The formation of low-mass (

M_{★} ≲ 2 M_{⊙}

) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately [...] Read more.

The formation of low-mass (

M_{★} ≲ 2 M_{⊙}

) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited for studying star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV range. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows. Full article

(This article belongs to the Special Issue Star Formation in the Ultraviolet)

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11 pages, 1384 KiB

Open AccessArticle

Shaping Planetary Nebulae with Jets and the Grazing Envelope Evolution

by Noam Soker

Galaxies 2020, 8(1), 26; https://doi.org/10.3390/galaxies8010026 - 18 Mar 2020

Cited by 14 | Viewed by 2452

Abstract

I argue that the high percentage of planetary nebulae (PNe) that are shaped by jets show that main sequence stars in binary systems can accrete mass at a high rate from an accretion disk and launch jets. Not only does this allow jets [...] Read more.

I argue that the high percentage of planetary nebulae (PNe) that are shaped by jets show that main sequence stars in binary systems can accrete mass at a high rate from an accretion disk and launch jets. Not only does this allow jets to shape PNe, but this also points to the importance of jets in other types of binary systems and in other processes. These processes include the grazing envelope evolution (GEE), the common envelope evolution (CEE), and the efficient conversion of kinetic energy to radiation in outflows. Additionally, the jets point to the possibility that many systems launch jets as they enter the CEE, possibly through a GEE phase. The other binary systems in which jets might play significant roles include intermediate-luminosity optical transients (ILOTs), supernova impostors (including pre-explosion outbursts), post-CEE binary systems, post-GEE binary systems, and progenitors of neutron star binary systems and black hole binary systems. One of the immediate consequences is that the outflow of these systems is highly-non-spherical, including bipolar lobes, jets, and rings. Full article

(This article belongs to the Special Issue Workplans II: Workshop for Planetary Nebula Observations)

27 pages, 3748 KiB

Open AccessReview

Gamma-Ray Dark Matter Searches in Milky Way Satellites—A Comparative Review of Data Analysis Methods and Current Results

by Javier Rico

Galaxies 2020, 8(1), 25; https://doi.org/10.3390/galaxies8010025 - 17 Mar 2020

Cited by 21 | Viewed by 3363

Abstract

If dark matter is composed of weakly interacting particles with mass in the GeV-TeV range, their annihilation or decay may produce gamma rays that could be detected by gamma-ray telescopes. Observations of dwarf spheroidal satellite galaxies of the Milky Way (dSphs) benefit from [...] Read more.

If dark matter is composed of weakly interacting particles with mass in the GeV-TeV range, their annihilation or decay may produce gamma rays that could be detected by gamma-ray telescopes. Observations of dwarf spheroidal satellite galaxies of the Milky Way (dSphs) benefit from the relatively accurate predictions of dSph dark matter content to produce robust constraints to the dark matter properties. The sensitivity of these observations for the search for dark matter signals can be optimized thanks to the use of advanced statistical techniques able to exploit the spectral and morphological peculiarities of the expected signal. In this paper, I review the status of the dark matter searches from observations of dSphs with the current generation of gamma-ray telescopes: Fermi-LAT, H.E.S.S, MAGIC, VERITAS and HAWC. I will describe in detail the general statistical analysis framework used by these instruments, putting in context the most recent experimental results and pointing out the most relevant differences among the different particular implementations. This will facilitate the comparison of the current and future results, as well as their eventual integration in a multi-instrument and multi-target dark matter search. Full article

(This article belongs to the Special Issue The Role of Halo Substructure in Gamma-Ray Dark Matter Searches)

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6 pages, 258 KiB

Open AccessArticle

X-ray Observations of Planetary Nebulae since WORKPLANS I and Beyond

by Martín A. Guerrero

Galaxies 2020, 8(1), 24; https://doi.org/10.3390/galaxies8010024 - 17 Mar 2020

Cited by 3 | Viewed by 2071

Abstract

Planetary nebulae (PNe) were expected to be filled with hot pressurized gas driving their expansion. ROSAT hinted at the presence of diffuse X-ray emission from these hot bubbles and detected the first sources of hard X-ray emission from their central stars, but it [...] Read more.

Planetary nebulae (PNe) were expected to be filled with hot pressurized gas driving their expansion. ROSAT hinted at the presence of diffuse X-ray emission from these hot bubbles and detected the first sources of hard X-ray emission from their central stars, but it was not until the advent of Chandra and XMM-Newton that we became able to study in detail their occurrence and physical properties. Here I review the progress in the X-ray observations of PNe since the first WORKshop for PLAnetary Nebulae observationS (WORKPLANS) and present the perspective for future X-ray missions with particular emphasis on eROSITA. Full article

(This article belongs to the Special Issue Workplans II: Workshop for Planetary Nebula Observations)

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(bottom) Time evolution of the number of PNe detected by the Chandra (red) and XMM-Newton (blue) X-ray observatories. (top) Time evolution of the number of papers (black) and citations to these papers (blue) based on those X-ray observations (source Astronomycal Database System, ADS, at <a href="https://ui.adsabs.harvard.edu" target="_blank">https://ui.adsabs.harvard.edu</a>). Note that the left and right axes in this panel refer to numbers of papers and citations, respectively, as denoted by their labels and colors. The red-dashed line is a linear fit to the number of papers, implying a publication rate ≃3 papers per year on the X-ray emission from PNe. Full article ">Figure 2
Time evolution of the exposure time required for a Chandra ACIS-S observation to obtain a given number of counts for a plasma model typical of the diffuse X-ray emission in PNe. Simulations were obtained using the Chandra PIMMS v4.9 simulator, which includes the most up-to-date calibrations. A plasma with a typical 1.6 × 10<math display="inline"><semantics> <msup> <mrow/> <mn>6</mn> </msup> </semantics></math> K temperature, low hydrogen column density, and slightly subsolar chemical composition has been assumed. Full article ">Figure 3
Distribution of exposure time (left) and sensitivity (rigth) for PNe expected in the final eRASS map of the whole sky. Most PNe will be located in areas of the eRASS with total exposure times shorter than 20 ks, but a significant number of them, ≃200, will have longer exposure times. Accordingly, a significant number of PNe, those with long exposure times in the final eRASS, will have observations with sensitivities adequate to detect X-ray fluxes smaller than ≃2×10<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>−</mo> <mn>15</mn> </mrow> </msup> </semantics></math> erg cm<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>−</mo> <mn>2</mn> </mrow> </msup> </semantics></math> s<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> </semantics></math>. Full article ">

8 pages, 1771 KiB

Open AccessEditorial

WORKPLANS: Workshop on Planetary Nebula Observations

by Isabel Aleman, Jeronimo Bernard-Salas, Joel H. Kastner, Toshiya Ueta and Eva Villaver

Galaxies 2020, 8(1), 23; https://doi.org/10.3390/galaxies8010023 - 16 Mar 2020

Cited by 1 | Viewed by 2890

Abstract

This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The [...] Read more.

This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The specific aims of the WORKPLANS workshop series are (i) to discuss and prioritize the most important topics to be investigated by the PN community in the following years; (ii) to establish a network of excellent researchers with complementary expertise; (iii) to formulate ambitious observing proposals for the most advanced telescopes and instrumentation presently available (ALMA, SOFIA, VLT, GTC, HST, etc.), addressing those topics; and (iv) to develop strategies for major proposals to future observatories (JWST, ELT, SPICA, Athena, etc.). To achieve these goals, WORKPLANS II brought together experts in all key sub-areas of the PNe research field, namely: analysis and interpretation of PNe observational data; theoretical modeling of gas and dust emission; evolution from Asymptotic Giant Branch stars (PNe progenitors) to PNe; and the instrumentation and technical characteristics of the relevant observatories. Full article

(This article belongs to the Special Issue Workplans II: Workshop for Planetary Nebula Observations)

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19 pages, 1131 KiB

Open AccessArticle

Diagnosing Magnetic Field Geometry in Blazar Jets Using Multi-Frequency, Centimeter-Band Polarimetry and Radiative Transfer Modeling

by Margo Aller, Philip Hughes, Hugh Aller and Talvikki Hovatta

Galaxies 2020, 8(1), 22; https://doi.org/10.3390/galaxies8010022 - 10 Mar 2020

Cited by 1 | Viewed by 2393

Abstract

We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We [...] Read more.

We use multi-frequency linear polarization observations from the University of Michigan blazar program (UMRAO), in combination with radiative transfer simulations of emission from a relativistic jet, to investigate the time-dependent flow conditions, including magnetic field geometry, in an example blazar OT 081. We adopt a scenario incorporating relativistic shocks during flaring, and both ordered axial and helical magnetic field components and magnetic turbulence in the underlying flow; these constituents are consistent with the observed periods of ordered behavior in the polarization intermixed with stochastic variations. The simulations are able to reproduce the global features of the observed light curves, including amplitude and spectral evolution of the linear polarization, during four time periods spanning 25 years. From the simulations, we identify the signature of a weak-to-strong helical magnetic field on the polarization, but conclude that a dominant helical magnetic field is not consistent with the UMRAO polarization data. The modeling identifies time-dependent changes in the ratio of the ordered-to-turbulent magnetic field, and changes in the flow direction and Lorentz factor. These suggest the presence of jet-like structures within a broad envelope seen at different orientations. Full article

(This article belongs to the Special Issue Polarimetry as a Probe of Magnetic Fields in AGN Jets)

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Longterm UMRAO light curves illustrate the range of variability in consecutive outbursts in the emission from the example program source OT 081 (1749 + 096). From bottom to top, 30-day averages of the total flux density (S), polarized flux (P), and electric vector position angle (EVPA or X) are shown. Green crosses, blue circles, and red triangles denote the data at 14.5, 8.0, and 4.8 GHz respectively. Modeled epochs are indicated with time labels in the lower panel. Full article ">Figure 2
Comparison of the observed light curves in the form of daily-averaged data (bottom plots) and simulations (top plots) for epochs T1985 (left) and T1996 (right) showing time in years. The data at the three frequencies are symbol and color coded to match in the data and simulations. The simulated light curves were computed for 20 time steps using the adopted flow parameters listed in <a href="#galaxies-08-00022-t001" class="html-table">Table 1</a> and the values of the shock attributes listed in <a href="#app1-galaxies-08-00022" class="html-app">Appendix A</a> <a href="#galaxies-08-00022-t0A1" class="html-table">Table A1</a> and <a href="#galaxies-08-00022-t0A2" class="html-table">Table A2</a>. Upward black arrows mark the adopted shock onset times. Full article ">Figure 3
Comparison of the the data (bottom plots) and simulations (top plots) for T2008 (left) and T2010 (right). Symbols, number of time steps in the simulation, and arrows are as in <a href="#galaxies-08-00022-f002" class="html-fig">Figure 2</a>. The shock parameters are given in <a href="#app1-galaxies-08-00022" class="html-app">Appendix A</a> <a href="#galaxies-08-00022-t0A3" class="html-table">Table A3</a> and <a href="#galaxies-08-00022-t0A4" class="html-table">Table A4</a>. Full article ">Figure 4
(a) The simulation for the model for T2010 with <math display="inline"><semantics> <mi>θ</mi> </semantics></math> = 1.1<math display="inline"><semantics> <mo>°</mo> </semantics></math>. (b) The simulation with the same parameters with the exception of the viewing angle, which has been changed to 1.4<math display="inline"><semantics> <mo>°</mo> </semantics></math>. Full article ">Figure 5
Simulated T1985 light curves with the addition of a weak helical magnetic field component. Panel (a) shows the simulated light curves for T1985 assuming a negligible helical magnetic field (an order multiple of 0.05; see text). As would be expected, this simulation is nearly identical to our adopted model (<a href="#galaxies-08-00022-t001" class="html-table">Table 1</a>) which did not include a helical magnetic field. Panel (b) shows the simulated light curves for a helical field with strength approaching that of the random component (an order multiple of 0.5). Time is shown in arbitrary units. Full article ">Figure 6
Simulated T1985 light curves with the addition of a strong helical magnetic field component. Panel (a) shows the result for a helical field of strength comparable to that of the random component (an order multiple of 1.0, see text) and panel (b) shows the result for a helical field with strength that dominates over the random component (an order multiple of 10). Time is shown in arbitrary units. Full article ">

10 pages, 471 KiB

Open AccessReview

(Sub)mm-Wavelength Observations of Pre-Planetary Nebulae and Young Planetary Nebulae

by Carmen Sánchez Contreras

Galaxies 2020, 8(1), 21; https://doi.org/10.3390/galaxies8010021 - 10 Mar 2020

Viewed by 3731

Abstract

This is a non-comprehensive review of observations of pre-Planetary Nebulae (pPNe) and young Planetary Nebulae (yPNe) at (sub)mm-wavelengths, a valuable window for probing multi-phased gas and dust in these objects. This contribution focuses on observations of molecular lines (from carbon monoxide—CO—and other species), [...] Read more.

This is a non-comprehensive review of observations of pre-Planetary Nebulae (pPNe) and young Planetary Nebulae (yPNe) at (sub)mm-wavelengths, a valuable window for probing multi-phased gas and dust in these objects. This contribution focuses on observations of molecular lines (from carbon monoxide—CO—and other species), and briefly at the end, on hydrogen radio recombination lines from the emerging H ii regions at the center of yPNe. The main goal of this contribution is to show the potential of (sub)mm-wavelength observations of pPNe/yPNe to help the community to devise and develop new observational projects that will bring us closer to a better understanding of these latest stages of the evolution of low-to-intermediate (∼0.8–8

M_{⊙}

) mass stars. Full article

(This article belongs to the Special Issue Workplans II: Workshop for Planetary Nebula Observations)

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27 pages, 5016 KiB

Open AccessReview

Luminous Blue Variables

by Kerstin Weis and Dominik J. Bomans

Galaxies 2020, 8(1), 20; https://doi.org/10.3390/galaxies8010020 - 29 Feb 2020

Cited by 44 | Viewed by 5421

Abstract

Luminous Blue Variables are massive evolved stars, here we introduce this outstanding class of objects. Described are the specific characteristics, the evolutionary state and what they are connected to other phases and types of massive stars. Our current knowledge of LBVs is limited [...] Read more.

Luminous Blue Variables are massive evolved stars, here we introduce this outstanding class of objects. Described are the specific characteristics, the evolutionary state and what they are connected to other phases and types of massive stars. Our current knowledge of LBVs is limited by the fact that in comparison to other stellar classes and phases only a few “true” LBVs are known. This results from the lack of a unique, fast and always reliable identification scheme for LBVs. It literally takes time to get a true classification of a LBV. In addition the short duration of the LBV phase makes it even harder to catch and identify a star as LBV. We summarize here what is known so far, give an overview of the LBV population and the list of LBV host galaxies. LBV are clearly an important and still not fully understood phase in the live of (very) massive stars, especially due to the large and time variable mass loss during the LBV phase. We like to emphasize again the problem how to clearly identify LBV and that there are more than just one type of LBVs: The giant eruption LBVs or

η

Car analogs and the S Dor cycle LBVs. Full article

(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)

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This figure taken from Burggraf (2015) [<a href="#B18-galaxies-08-00020" class="html-bibr">18</a>] shows a lightcurve spanning more than 100 years of the LBV and original Hubble Sandage Variable Var B in M 33. In addition to the B magnitudes upper section the spectral type if know for the same date is plotted in the lower section. Note the for S Dor cycles typical changes in the spectral type. Full article ">Figure 2
This figure shows the classical plot by Wolf [<a href="#B17-galaxies-08-00020" class="html-bibr">17</a>] (top) and in a new version (bottom) we plotted the luminosity L and change in Temperature <math display="inline"><semantics> <mo>Δ</mo> </semantics></math>T<math display="inline"><semantics> <msub> <mrow/> <mi>eff</mi> </msub> </semantics></math> for a new way to visualize the amplitude- luminosity-relation. Full article ">Figure 3
HRD with Galactic and LMC LBVs and LBV candidates. Circles are used for LBVs with an emission line (optical/NIR) nebulae, squares for all others. If an S Dor cycle has been observed both the cool (open symbol) and the hot phase (filled symbol) are marked. Otherwise an open grayish symbol is used. In color evolutionary tracks for different masses are added. The tracks are based on the data from the Geneva code for Z = 0.02 and v<math display="inline"><semantics> <msub> <mrow/> <mrow> <mi>r</mi> <mi>o</mi> <mi>t</mi> </mrow> </msub> </semantics></math> = 300 km/s, colors code the generally three different evolutionary scenarios, see text for details. Full article ">Figure 4
HST images of LBV nebulae sorted by morphology: hourglass AG Car [<a href="#B54-galaxies-08-00020" class="html-bibr">54</a>], R 127 with bipolar attachments, weakly bipolar He 3-519 [<a href="#B55-galaxies-08-00020" class="html-bibr">55</a>], spherical S 61 [<a href="#B52-galaxies-08-00020" class="html-bibr">52</a>] and last in row irregular R 143 [<a href="#B52-galaxies-08-00020" class="html-bibr">52</a>]. Full article ">Figure 5
An LBT LUCI AO [FeII] image of the inner nebula (or inner shell) of P Cygni (Weis et al., in prep). The images has pixel scale of 0.015”/pixel and resolve scales down to 85 AU. Full article ">Figure 6
The nebula around <math display="inline"><semantics> <mi>η</mi> </semantics></math> Carinae in the optical and X-ray. Left: An optical F658N HST image in greyscale, the Homunculus nebula additionally marked in contour to distinguish it from the outer ejecta, shown only in grey scale [<a href="#B58-galaxies-08-00020" class="html-bibr">58</a>]. Right: A CHANDRA Xray image with color coded energy regimes, green:0.2-0.6 keV, red: 0.6–1.2 keV and blue 1.2–12 keV color version of Figure 1 in [<a href="#B95-galaxies-08-00020" class="html-bibr">95</a>]. Full article ">Figure 7
Plot of the gas-phase metallicity of nearby galaxies versus their distance and spatial resolution. Only a selection of the galaxies in the Local volume are plotted, but the sample is complete for the significantly starforming galaxies in the Local Group. Metalicities of the inner disk are chosen for the spiral galaxies with metallicity gradients, the metalicities of stars in the outer disk of these galaxies can be a a few faction of tens solar lower. Galaxies with LBVs and/or LBV candidates are plotted as red dots, the other galaxies are plotted as blue dots. Plot was adapted and updated from [<a href="#B163-galaxies-08-00020" class="html-bibr">163</a>]. Full article ">

33 pages, 15972 KiB

Open AccessArticle

Density Profiles of 51 Galaxies from Parameter-Free Inverse Models of Their Measured Rotation Curves

by Robert E. Criss and Anne M. Hofmeister

Galaxies 2020, 8(1), 19; https://doi.org/10.3390/galaxies8010019 - 26 Feb 2020

Cited by 12 | Viewed by 5594

Abstract

Spiral galaxies and their rotation curves have key characteristics of differentially spinning objects. Oblate spheroid shapes are a consequence of spin and reasonably describe galaxies, indicating that their matter is distributed in gravitationally interacting homeoidal shells. Here, previously published equations describing differentially spinning [...] Read more.

Spiral galaxies and their rotation curves have key characteristics of differentially spinning objects. Oblate spheroid shapes are a consequence of spin and reasonably describe galaxies, indicating that their matter is distributed in gravitationally interacting homeoidal shells. Here, previously published equations describing differentially spinning oblate spheroids with radially varying density are applied to 51 galaxies, mostly spirals. A constant volumetric density (ρ, kg m⁻³) is assumed for each thin homeoid in these formulae, after Newton, which is consistent with RCs being reported simply as a function of equatorial radius r. We construct parameter-free inverse models that uniquely specify mass inside any given r, and thus directly constrain ρ vs. r solely from velocity v (r) and galactic aspect ratios (assumed as 1:10 for spirals when data are unavailable). Except for their innermost zones, ρ is proven to be closely proportional to rⁿ, where the statistical average of n for all 36 spirals studied is −1.80 ± 0.40. Our values for interior densities compare closely with independently measured baryon density in appropriate astronomical environments: for example, calculated ρ at galactic edges agrees with independently estimated ρ of intergalactic media (IGM). Our finding that central densities increase with galaxy size is consistent with behavior exhibited by diverse self-gravitating entities. Our calculated mass distributions are consistent with visible luminosity and require no non-baryonic component. Full article

(This article belongs to the Special Issue Debate on the Physics of Galactic Rotation and the Existence of Dark Matter)

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3 pages, 665 KiB

Open AccessEditorial

Radio Galaxies at TeV Energies: Preface

by Dorit Glawion

Galaxies 2020, 8(1), 18; https://doi.org/10.3390/galaxies8010018 - 22 Feb 2020

Cited by 1 | Viewed by 1999

Abstract

The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger [...] Read more.

The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger viewing angle have been detected in the TeV range. These objects also show fascinating properties, such as fast variability or spectral features and are called “radio galaxies”. The TeV radio galaxies provide a unique laboratory for studying key aspects of active galactic nuclei. This Special Issue of Galaxies targets these exciting objects. Full article

(This article belongs to the Special Issue Radio Galaxies at TeV Energies)

16 pages, 7299 KiB

Open AccessReview

From SN 2010da to NGC 300 ULX-1: Ten Years of Observations of an Unusual High Mass X-Ray Binary in NGC 300

by Breanna A. Binder, Stefania Carpano, Marianne Heida and Ryan Lau

Galaxies 2020, 8(1), 17; https://doi.org/10.3390/galaxies8010017 - 18 Feb 2020

Cited by 5 | Viewed by 5975

Abstract

In May 2010, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength observations of the misnamed “SN 2010da” have continually reshaped our understanding of this high [...] Read more.

In May 2010, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength observations of the misnamed “SN 2010da” have continually reshaped our understanding of this high mass X-ray binary system. In this review, we present an overview of the multi-wavelength observations and attempt to understand the 2010 transient event, and later, the reclassification of this system as NGC 300 ULX-1: a red supergiant + neutron star ultraluminous X-ray source. Full article

(This article belongs to the Special Issue High-Mass X-ray Binaries and Ultraluminous X-ray Sources Hosting Neutron Stars)

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25 pages, 466 KiB

Open AccessArticle

Frequency of Planets in Binaries

by Mariangela Bonavita and Silvano Desidera

Galaxies 2020, 8(1), 16; https://doi.org/10.3390/galaxies8010016 - 18 Feb 2020

Cited by 27 | Viewed by 3709

Abstract

The frequency of planets in binaries is an important issue in the field of extrasolar planet studies because of its relevance in the estimation of the global planet population of our galaxy and the clues it can give to our understanding of planet [...] Read more.

The frequency of planets in binaries is an important issue in the field of extrasolar planet studies because of its relevance in the estimation of the global planet population of our galaxy and the clues it can give to our understanding of planet formation and evolution. Multiple stars have often been excluded from exoplanet searches, especially those performed using the radial velocity technique, due to the technical challenges posed by such targets. As a consequence and despite recent efforts, our knowledge of the frequency of planets in multiple stellar systems is still rather incomplete. On the other hand, the lack of knowledge about the binarity at the time of the compilation of the target samples means that our estimate of the planet frequency around single stars could be tainted by the presence of unknown binaries, especially if these objects have a different behavior in terms of planet occurrence. In a previous work we investigated the binarity of the objects included in the Uniform Detectability sample defined by Fisher and Valenti (2005), showing how more than 20% of their targets were, in fact, not single stars. Here, we present an update of this census, made possible mainly by the information now available thanks to the second Gaia Data Release. The new binary sample includes a total of 313 systems, of which 114 were added through this work. We were also able to significantly improve the estimates of masses and orbital parameters for most of the pairs in the original list, especially those at close separations. A few new systems with white dwarf companions were also identified. The results of the new analysis are in good agreement with the findings of our previous work, confirming the lack of difference in the overall planet frequency between binaries and single stars but suggesting a decrease in the planet frequency for very close pairs. Full article

(This article belongs to the Special Issue Habitability of Planets in Stellar Binary Systems)

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Mass vs. semi-major axes of companions compatible with the observed <math display="inline"><semantics> <mrow> <mi mathvariant="sans-serif">Δ</mi> <mi>μ</mi> </mrow> </semantics></math> for HD 67458 (left panel) and HD 80913 (right panel), calculated using the COPAINS (Code for Orbital Characterization of Astrometrically Inferred New Systems; see [<a href="#B47-galaxies-08-00016" class="html-bibr">47</a>] for details) and assuming a uniform distribution for the eccentricity. The red curve shows Gaia’s sensitivity limits [<a href="#B27-galaxies-08-00016" class="html-bibr">27</a>,<a href="#B50-galaxies-08-00016" class="html-bibr">50</a>]. The blue dot in the left panel shows the position of the companion to HD 67458 retrieved in Gaia DR2. The blue solid line in the right panel shows the position of the companions compatible with the RV trend observed by [<a href="#B38-galaxies-08-00016" class="html-bibr">38</a>] for HD 80913. Full article ">Figure 2
Critical semi-major axis vs. mass ratio (<math display="inline"><semantics> <mrow> <msub> <mi>M</mi> <mi>A</mi> </msub> <mo>/</mo> <msub> <mi>M</mi> <mi>B</mi> </msub> </mrow> </semantics></math>) for the pairs in the Uniform Detectability (UD) sample with (stars) and without (filled circles) planetary companions. The new pairs not in BD07 are shown in light blue and orange, respectively. Full article ">Figure 3
Histogram of the projected separation for the pairs in the updated (dark blue) and the original (light blue) UD binary sample. The number of stars with planets (P-Stars) in the final sample is shown in light orange. The solid line corresponds to <math display="inline"><semantics> <mrow> <mi>ρ</mi> <mo>=</mo> <msup> <mn>2</mn> <mrow> <mo>″</mo> </mrow> </msup> </mrow> </semantics></math>. Full article ">Figure 4
Histogram of the critical semi-major axis (<math display="inline"><semantics> <msub> <mi>a</mi> <mrow> <mi>c</mi> <mi>r</mi> <mi>i</mi> <mi>t</mi> </mrow> </msub> </semantics></math>, left panel) and mass ratio (<math display="inline"><semantics> <mrow> <msub> <mi>M</mi> <mi>B</mi> </msub> <mo>/</mo> <msub> <mi>M</mi> <mi>A</mi> </msub> </mrow> </semantics></math>, right panel) for the pairs in the updated (dark blue) and the original (light blue) UD binary sample. The number of stars with planets (P-Stars) in the final sample is shown in light orange. Full article ">Figure 5
Left: Fraction of planet-host binaries as a function of <math display="inline"><semantics> <msub> <mi>a</mi> <mrow> <mi>c</mi> <mi>r</mi> <mi>i</mi> <mi>t</mi> </mrow> </msub> </semantics></math>. Right: Cumulative distribution of the <math display="inline"><semantics> <msub> <mi>a</mi> <mrow> <mi>c</mi> <mi>r</mi> <mi>i</mi> <mi>t</mi> </mrow> </msub> </semantics></math> for UD pairs with (red) and without (blue) planetary companions. Full article ">

17 pages, 316 KiB

Open AccessArticle

A Multi-Wavelength View of OJ 287 Activity in 2015–2017: Implications of Spectral Changes on Central-Engine Models and MeV-GeV Emission Mechanism

by Pankaj Kushwaha

Galaxies 2020, 8(1), 15; https://doi.org/10.3390/galaxies8010015 - 14 Feb 2020

Cited by 17 | Viewed by 3103

Abstract

A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We [...] Read more.

A diverse range of observational results and peculiar properties across the domains of observation have made OJ 287 one of the best-explored BL Lac objects on the issues of relativistic jets and accretion physics as well as the strong theory of gravity. We here present a brief compilation of observational results from the literature and inferences/insights from the extensive studies but focus on the interpretation of its ∼12-yr quasi-periodic optical outbursts (QPOOs) and high energy emission mechanisms. The QPOOs in one model are attributed to the disk-impact related to dynamics of the binary SMBHs while alternative models attribute it to the geometrical effect related to the precession of a single jet or double jets. We discuss implications of the new spectral features reported during the 2015–2017 multi-wavelength high activity of the source—a break in the NIR-optical spectrum and hardening of the MeV-GeV emission accompanied by a shift in the location of its peak, in the context of the two. The reported NIR-optical break nicely fits the description of a standard accretion disk emission from an SMBH of mass

\sim 10^{10} M_{⊙}

while the time of its first appearance at the end of May, 2013 (MJD 56439) is in close coincidence with the time of impact predicted by the disk-impact binary SMBH model. This spectral and temporal coincidence with the model parameters of the disk-impact binary SMBH model provides independent evidence in favor of the model over the geometrical models which argue for a total central-engine mass in the range of

10^{7 - 9} M_{⊙}

. On the other hand, the MeV-GeV spectral change is naturally reproduced by the inverse Compton scattering of photons from the broad-line region and is consistent with the detection of broad emission lines during the previous cycles of quasi-periodic outbursts. Combining this with previous SED studies suggests that in, OJ 287, the MeV-GeV emission results from external Comptonization. Full article

(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)

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8 pages, 711 KiB

Open AccessArticle

Circular Geodesics Stability in a Static Black Hole in New Massive Gravity

by Andrés Aceña, Ericson López and Franklin Aldás

Galaxies 2020, 8(1), 14; https://doi.org/10.3390/galaxies8010014 - 14 Feb 2020

Cited by 3 | Viewed by 2294

Abstract

We study the existence and stability of circular geodesics in a family of asymptotically AdS static black holes in New Massive Gravity theory. We show that the mathematical sign of the hair parameter determines the existence of such geodesics. For a positive hair [...] Read more.

We study the existence and stability of circular geodesics in a family of asymptotically AdS static black holes in New Massive Gravity theory. We show that the mathematical sign of the hair parameter determines the existence of such geodesics. For a positive hair parameter, the stability regions follow the usual pattern, with the innermost geodesic being null, unstable, and separated from the horizon, followed by a region of unstable timelike geodesics and then a region of stable timelike geodesics, which extends in the asymptotic region. Full article

29 pages, 11021 KiB

Open AccessReview

Massive Star Formation in the Ultraviolet Observed with the Hubble Space Telescope

by Claus Leitherer

Galaxies 2020, 8(1), 13; https://doi.org/10.3390/galaxies8010013 - 9 Feb 2020

Cited by 12 | Viewed by 4817

Abstract

Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 [...] Read more.

Spectroscopic observations of a massive star formation in the ultraviolet and their interpretation are reviewed. After a brief historical retrospective, two well-studied resolved star clusters and the surrounding H II regions are introduced: NGC 2070 in the Large Magellanic Cloud and NGC 604 in M33. These regions serve as a training set for studies of more distant clusters, which can no longer be resolved into individual stars. Observations of recently formed star clusters and extended regions in star-forming galaxies in the nearby universe beyond the Local Group are presented. Their interpretation relies on spectral synthesis models. The successes and failures of such models are discussed, and future directions are highlighted. I present a case study of the extraordinary star cluster and giant H II region in the blue compact galaxy II Zw 40. The review concludes with a preview of two upcoming Hubble Space Telescope programs: ULLYSES, a survey of massive stars in nearby galaxies, and CLASSY, a study of massive star clusters in star-forming galaxies. Full article

(This article belongs to the Special Issue Star Formation in the Ultraviolet)

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16 pages, 556 KiB

Open AccessArticle

Entropy and Mass Distribution in Disc Galaxies

by John Herbert Marr

Galaxies 2020, 8(1), 12; https://doi.org/10.3390/galaxies8010012 - 8 Feb 2020

Cited by 6 | Viewed by 3722

Abstract

The relaxed motion of stars and gas in galactic discs is well approximated by a rotational velocity that is a function of radial position only, implying that individual components have lost any information about their prior states. Thermodynamically, such an equilibrium state is [...] Read more.

The relaxed motion of stars and gas in galactic discs is well approximated by a rotational velocity that is a function of radial position only, implying that individual components have lost any information about their prior states. Thermodynamically, such an equilibrium state is a microcanonical ensemble with maximum entropy, characterised by a lognormal probability distribution. Assuming this for the surface density distribution yields rotation curves that closely match observational data across a wide range of disc masses and galaxy types and provides a useful tool for modelling the theoretical density distribution in the disc. A universal disc spin parameter emerges from the model, giving a tight virial mass estimator with strong correlation between angular momentum and disc mass, suggesting a mechanism by which the proto-disc developed by dumping excess mass to the core or excess angular momentum to a satellite galaxy. The baryonic-to-dynamic mass ratio for the model approaches unity for high mass galaxies, but is generally <1 for low mass discs, and this discrepancy appears to follow a similar relationship to that shown in recent work on the Radial Acceleration Relation (RAR). Although this may support Modified Newtonian Dynamics (MOND) in preference to a Dark Matter (DM) halo, it does not exclude undetected baryonic mass or a gravitational DM component in the disc. Full article

(This article belongs to the Special Issue Debate on the Physics of Galactic Rotation and the Existence of Dark Matter)

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Figure 1
NGC 2366’s (a) Rotational velocity for the LN model (solid line) and observations ([<a href="#B38-galaxies-08-00012" class="html-bibr">38</a>]). (b) Radial distribution of density (<math display="inline"><semantics> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </semantics></math>/kpc<math display="inline"><semantics> <msup> <mrow/> <mn>2</mn> </msup> </semantics></math>) for the LN model (solid line) and Freeman exponential disc model (dashed line). Full article ">Figure 2
Rotational Curve (RC) for the massive galaxy M31, with rotational velocity for the LN model (solid line) and observations with error bars ([<a href="#B39-galaxies-08-00012" class="html-bibr">39</a>,<a href="#B40-galaxies-08-00012" class="html-bibr">40</a>,<a href="#B41-galaxies-08-00012" class="html-bibr">41</a>]). Full article ">Figure 3
Log-log plot of angular momentum vs. disc mass for the LN model galaxies (squares) and the theoretical slope 5/3 (purple solid line). Idealised evolutionary changes are shown for a disc with excess mass dumping mass into the core while conserving angular momentum (R–S ); and a disc with excess angular momentum shedding mass and angular momentum to a satellite galaxy, leaving a less massive disc (P–Q ). Full article ">Figure 4
Log-log plot of the modelled lognormal dynamic disc mass vs. the estimated observational baryonic masses (<math display="inline"><semantics> <mrow> <msub> <mi>M</mi> <mi>g</mi> </msub> <mo>+</mo> <msup> <mi>M</mi> <mo>*</mo> </msup> </mrow> </semantics></math>) for a wide range of galaxy masses and types. The solid line is the line of mass equality; the dashed line is for <math display="inline"><semantics> <mrow> <msub> <mi>M</mi> <mrow> <mi>d</mi> <mi>y</mi> <mi>n</mi> <mi>a</mi> <mi>m</mi> <mi>i</mi> <mi>c</mi> </mrow> </msub> <mo>=</mo> <mi>f</mi> <mrow> <mo>(</mo> <msub> <mi>M</mi> <mrow> <mi>b</mi> <mi>a</mi> <mi>r</mi> <mi>y</mi> <mi>o</mi> <mi>n</mi> <mi>i</mi> <mi>c</mi> </mrow> </msub> <mo>)</mo> </mrow> </mrow> </semantics></math>, with <math display="inline"><semantics> <mrow> <msub> <mi>M</mi> <mo>†</mo> </msub> <mo>=</mo> <mn>3</mn> <mo>.</mo> <mn>98</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>10</mn> </msup> <mspace width="3.33333pt"/> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </mrow> </semantics></math> (see the text). Full article ">

7 pages, 1113 KiB

Open AccessEditor’s ChoiceArticle

The Microvariable Activity of BL Lacertae

by Alberto C. Sadun, Masoud Asadi-Zeydabadi, Lauren Hindman and J. Ward Moody

Galaxies 2020, 8(1), 11; https://doi.org/10.3390/galaxies8010011 - 7 Feb 2020

Cited by 2 | Viewed by 2610

Abstract

We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of [...] Read more.

We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of minutes for each night. A relativistic beaming model was used in our analysis, to infer changes to the line of sight angles for the motion of the different relativistic components. This model has very few free parameters. The light curves we generated show both high and moderate frequency cadence to the variable behavior of BL Lac itself, in addition to the well documented long-term variability. Full article

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29 pages, 1376 KiB

Open AccessReview

Radiation-Driven Stellar Eruptions

by Kris Davidson

Galaxies 2020, 8(1), 10; https://doi.org/10.3390/galaxies8010010 - 5 Feb 2020

Cited by 13 | Viewed by 3243

Abstract

Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor [...] Read more.

Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature. Full article

(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)

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15 pages, 1195 KiB

Open AccessReview

Rotating Disk Galaxies without Dark Matter Based on Scientific Reasoning

by James Q. Feng

Galaxies 2020, 8(1), 9; https://doi.org/10.3390/galaxies8010009 - 1 Feb 2020

Cited by 6 | Viewed by 4983

Abstract

The most cited evidence for (non-baryonic) dark matter has been an apparent lack of visible mass to gravitationally support the observed orbital velocity of matter in rotating disk galaxies, yet measurement of the mass of celestial objects cannot be straightforward, requiring theories derived [...] Read more.

The most cited evidence for (non-baryonic) dark matter has been an apparent lack of visible mass to gravitationally support the observed orbital velocity of matter in rotating disk galaxies, yet measurement of the mass of celestial objects cannot be straightforward, requiring theories derived from the known physical laws along with some empirically established semi-quantitative relationship. The most reliable means for determining the mass distribution in rotating disk galaxies is to solve a force balance equation according to Newton’s laws from measured rotation curves, similar to calculating the Sun’s mass from the Earth’s orbital velocity. Another common method to estimate galactic mass distribution is to convert measured brightness from surface photometry based on empirically established mass-to-light ratio. For convenience, most astronomers commonly assumed a constant mass-to-light ratio for estimation of the so-called “luminous” or “visible” mass, which would not likely be accurate. The mass determined from a rotation curve typically exhibits an exponential-like decline with galactrocentric distance, qualitatively consistent with observed surface brightness but often with a larger disk radial scale length. This fact scientifically suggests variable mass-to-light ratio of baryonic matter in galaxies without the need for dark matter. Full article

(This article belongs to the Special Issue Debate on the Physics of Galactic Rotation and the Existence of Dark Matter)

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3 pages, 170 KiB

Open AccessEditorial

Acknowledgement to Reviewers of Galaxies in 2019

by Galaxies Editorial Office

Galaxies 2020, 8(1), 8; https://doi.org/10.3390/galaxies8010008 - 27 Jan 2020

Viewed by 1438

Abstract

The editorial team greatly appreciates the reviewers who have dedicated their considerable time and expertise to the journal’s rigorous editorial process over the past 12 months, regardless of whether the papers are finally published or not [...] Full article

5 pages, 974 KiB

Open AccessArticle

Investigating Multiwavelength Lognormality with Simulations—Case of Mrk 421

by Nachiketa Chakraborty

Galaxies 2020, 8(1), 7; https://doi.org/10.3390/galaxies8010007 - 16 Jan 2020

Cited by 5 | Viewed by 2238

Abstract

Blazars are highly variable and display complex characteristics. A key characteristic is the flux probability distribution function or flux PDF whose shape depends upon the form of the underlying physical process driving variability. The BL Lacertae Mrk 421 is one of the brightest [...] Read more.

Blazars are highly variable and display complex characteristics. A key characteristic is the flux probability distribution function or flux PDF whose shape depends upon the form of the underlying physical process driving variability. The BL Lacertae Mrk 421 is one of the brightest and most variable blazars across the electromagnetic spectrum. It has been reported to show hints of lognormality across the spectrum from radio to gamma-ray histograms of observed fluxes. This would imply that the underlying mechanisms may not conform to the “standard” additive, multi-zone picture, but could potentially have multiplicative processes. This is investigated by testing the observed lightcurves at different wavelengths with time-series simulations. We find that the simulations reveal a more complex scenario, than a single lognormal distribution explaining the multiwavelength lightcurves of Mrk 421. Full article

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28 pages, 737 KiB

Open AccessReview

Applications of Stellar Population Synthesis in the Distant Universe

by Elizabeth R. Stanway

Galaxies 2020, 8(1), 6; https://doi.org/10.3390/galaxies8010006 - 8 Jan 2020

Cited by 6 | Viewed by 4793

Abstract

Comparison with artificial galaxy models is essential for translating the incomplete and low signal-to-noise data we can obtain on astrophysical stellar populations to physical interpretations which describe their composition, physical properties, histories and internal conditions. In particular, this is true for distant galaxies, [...] Read more.

Comparison with artificial galaxy models is essential for translating the incomplete and low signal-to-noise data we can obtain on astrophysical stellar populations to physical interpretations which describe their composition, physical properties, histories and internal conditions. In particular, this is true for distant galaxies, whose unresolved light embeds clues to their formations and evolutions, and their impacts on their wider environs. Stellar population synthesis models are now used as the foundation of analysis at all redshifts, but are not without their problems. Here we review the use of stellar population synthesis models, with a focus on applications in the distant Universe. Full article

(This article belongs to the Special Issue Star Formation in the Ultraviolet)

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14 pages, 901 KiB

Open AccessArticle

Constraints to Dark Matter Annihilation from High-Latitude HAWC Unidentified Sources

by Javier Coronado-Blázquez and Miguel A. Sánchez-Conde

Galaxies 2020, 8(1), 5; https://doi.org/10.3390/galaxies8010005 - 30 Dec 2019

Cited by 9 | Viewed by 3502

Abstract

The

Λ

CDM cosmological framework predicts the existence of thousands of subhalos in our own Galaxy not massive enough to retain baryons and become visible. Yet, some of them may outshine in gamma rays provided that the dark matter is made of weakly [...] Read more.

The

Λ

CDM cosmological framework predicts the existence of thousands of subhalos in our own Galaxy not massive enough to retain baryons and become visible. Yet, some of them may outshine in gamma rays provided that the dark matter is made of weakly interacting massive particles (WIMPs), which would self-annihilate and would appear as unidentified gamma-ray sources (unIDs) in gamma-ray catalogs. Indeed, unIDs have proven to be competitive targets for dark matter searches with gamma rays. In this work, we focus on the three high-latitude (

| b | \geq 10

) sources present in the 2HWC catalog of the High Altitude Water Cherenkov (HAWC) observatory with no clear associations at other wavelengths. Indeed, only one of these sources, 2HWC J1040+308, is found to be above the HAWC detection threshold when considering 760 days of data, i.e., a factor 1.5 more exposure time than in the original 2HWC catalog. Other gamma-ray instruments, such as Fermi-LAT or VERITAS at lower energies, do not detect the source. Also, this unID is reported as spatially extended, making it even more interesting in a dark matter search context. While waiting for more data that may shed further light on the nature of this source, we set competitive upper limits on the annihilation cross section by comparing this HAWC unID to expectations based on state-of-the-art N-body cosmological simulations of the Galactic subhalo population. We find these constraints to be particularly competitive for heavy WIMPs, i.e., masses above ∼25 (40) TeV in the case of the

b \bar{b}

(

τ^{+} τ^{-}

) annihilation channel, reaching velocity-averaged cross section values of

2 \times 10^{- 25}

(

5 \times 10^{- 25}

) cm

^{3}

·s

^{- 1}

. Although far from testing the thermal relic cross section value, the obtained limits are independent and nicely complementary to those from radically different DM analyses and targets, demonstrating once again the high potential of this DM search approach. Full article

(This article belongs to the Special Issue The Role of Halo Substructure in Gamma-Ray Dark Matter Searches)

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45 pages, 1834 KiB

Open AccessReview

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

by Rainer Beck, Luke Chamandy, Ed Elson and Eric G. Blackman

Galaxies 2020, 8(1), 4; https://doi.org/10.3390/galaxies8010004 - 21 Dec 2019

Cited by 74 | Viewed by 4981

Abstract

Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival [...] Read more.

Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Furthermore, we measure the velocity dispersion of H i gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges of dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory. Full article

(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

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29 pages, 1228 KiB

Open AccessArticle

ConvoSource: Radio-Astronomical Source-Finding with Convolutional Neural Networks

by Vesna Lukic, Francesco de Gasperin and Marcus Brüggen

Galaxies 2020, 8(1), 3; https://doi.org/10.3390/galaxies8010003 - 20 Dec 2019

Cited by 25 | Viewed by 3232

Abstract

Finding and classifying astronomical sources is key in the scientific exploitation of radio surveys. Source-finding usually involves identifying the parts of an image belonging to an astronomical source, against some estimated background. This can be problematic in the radio regime, owing to the [...] Read more.

Finding and classifying astronomical sources is key in the scientific exploitation of radio surveys. Source-finding usually involves identifying the parts of an image belonging to an astronomical source, against some estimated background. This can be problematic in the radio regime, owing to the presence of correlated noise, which can interfere with the signal from the source. In the current work, we present ConvoSource, a novel method based on a deep learning technique, to identify the positions of radio sources, and compare the results to a Gaussian-fitting method. Since the deep learning approach allows the generation of more training images, it should perform well in the source-finding task. We test the source-finding methods on artificial data created for the data challenge of the Square Kilometer Array (SKA). We investigate sources that are divided into three classes: star forming galaxies (SFGs) and two classes of active galactic nuclei (AGN). The artificial data are given at two different frequencies (560 MHz and 1400 MHz), three total integration times (8 h, 100 h, 1000 h), and three signal-to-noise ratios (SNRs) of 1, 2, and 5. At lower SNRs, ConvoSource tends to outperform a Gaussian-fitting approach in the recovery of SFGs and all sources, although at the lowest SNR of one, the better performance is likely due to chance matches. The Gaussian-fitting method performs better in the recovery of the AGN-type sources at lower SNRs. At a higher SNR, ConvoSource performs better on average in the recovery of AGN sources, whereas the Gaussian-fitting method performs better in the recovery of SFGs and all sources. ConvoSource usually performs better at shorter total integration times and detects more true positives and misses fewer sources compared to the Gaussian-fitting method; however, it detects more false positives. Full article

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15 pages, 1363 KiB

Open AccessArticle

ASASSN-13db 2014–2017 Eruption as an Intermediate Luminosity Optical Transient

by Amit Kashi, Amir M. Michaelis and Leon Feigin

Galaxies 2020, 8(1), 2; https://doi.org/10.3390/galaxies8010002 - 19 Dec 2019

Cited by 9 | Viewed by 2826

Abstract

The low mass star ASASSN-13db experienced an EXor outburst in 2013, which identified it as a Young Stellar Object (YSO). Then, from 2014 to 2017 it had another outburst, longer and more luminous than the earlier. We analyze the observations of the second [...] Read more.

The low mass star ASASSN-13db experienced an EXor outburst in 2013, which identified it as a Young Stellar Object (YSO). Then, from 2014 to 2017 it had another outburst, longer and more luminous than the earlier. We analyze the observations of the second outburst, and compare it to eruptions of Intermediate Luminosity Optical Transients (ILOTs). We show that the decline of the light curve is almost identical to that of the V838 Mon, a prototype of a type of ILOT known as Luminous Red Nova (LRN). This similarity becomes conspicuous when oscillations that are associated with rotation are filtered out from the light curve of ASASSN-13db. We suggest that the eruption was the result of accretion of a proto-planet of a few Earth masses. The proto-planet was shredded by tidal forces before it was accreted onto the YSO, releasing gravitational energy that powered the outburst for

\approx 800 days

, and ended in a

\approx 55 days

decline phase. When the accretion material started depleting the accretion rate lowered and the eruption light curve declined for almost two months. Then it exhausted completely, creating a sharp break in the light curve. Another possibility is that the mass was a result of an instability in the proto-planetary disk that lead to a large episode of accretion from an inner viscous disk. We find that the variation of the temperature of the outburst is consistent with the surface temperature expected from a depleted viscous accretion disk. The 2014–2017 outburst of ASASSN-13db may be the least energetic ILOT to have been discovered to date, with an energy budget of only

\approx 10^{42} erg

. Full article

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28 pages, 3043 KiB

Open AccessReview

Relativistic Jets from AGN Viewed at Highest Angular Resolution

by Kazuhiro Hada

Galaxies 2020, 8(1), 1; https://doi.org/10.3390/galaxies8010001 - 18 Dec 2019

Cited by 19 | Viewed by 8715

Abstract

Accreting supermassive black holes in active galactic nuclei (AGN) produce powerful relativistic jets that shine from radio to GeV/TeV γ-rays. Over the past decade, AGN jets have extensively been studied in various energy bands and our knowledge about the broadband emission and rapid [...] Read more.

Accreting supermassive black holes in active galactic nuclei (AGN) produce powerful relativistic jets that shine from radio to GeV/TeV γ-rays. Over the past decade, AGN jets have extensively been studied in various energy bands and our knowledge about the broadband emission and rapid flares are now significantly updated. Meanwhile, the progress of magnetohydrodynamic simulations with a rotating black hole have greatly improved our theoretical understanding of powerful jet production. Nevertheless, it is still challenging to observationally resolve such flaring sites or jet formation regions since the relevant spatial scales are tiny. Observations with very long baseline interferometry (VLBI) are currently the only way to directly access such compact scales. Here we overview some recent progress of VLBI studies of AGN jets. As represented by the successful black hole shadow imaging with the Event Horizon Telescope, the recent rapid expansion of VLBI capability is remarkable. The last decade has also seen a variety of advances thanks to the advent of RadioAstron, GMVA, new VLBI facilities in East Asia as well as to the continued upgrade of VLBA. These instruments have resolved the innermost regions of relativistic jets for a number of objects covering a variety of jetted AGN classes (radio galaxies, blazars, and narrow-line Seyfert 1 galaxies), and the accumulated results start to establish some concrete (and likely universal) picture on the collimation, acceleration, recollimation shocks, magnetic field topology, and the connection to high-energy flares in the innermost part of AGN jets. Full article

(This article belongs to the Special Issue Jet Physics of Accreting Super Massive Black Holes)

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