Using The MAST Virtual Observatory To Study Light
Using The MAST Virtual Observatory To Study Light
Using The MAST Virtual Observatory To Study Light
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Using the MAST virtual observatory to study light profiles in elliptical galaxies
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Using the MAST virtual observatory to study light profiles in elliptical galaxies
F. Ávila-Castro and J. Saucedo-Morales
Departamento de Investigación en Fı́sica, Universidad de Sonora,
Apartado Postal 5-088, Hermosillo, Sonora, 83190, México,
e-mail: favilac@astro.uson.mx, jsaucedo@astro.uson.mx
Recibido el 15 de julio de 2008; aceptado el 29 de agosto de 2008
In this paper we use high quality images from the M.A.S.T. Virtual Observatory to elliptical galaxies with redshift z < 0.02 to explore the
differences between the classic de Vaucouleurs r1/4 and Sérsic r1/n surface light profiles. With the data obtained from the light profiles fits
for a sample of 20 elliptical galaxies, we study the fundamental plane of elliptical galaxies. The Fundamental Plane involves several physical
quantities of the galaxy such as luminosity, dispersion velocity and effective radius. While it is known that the Sérsic profile gives a better
physical description of an individual galaxy, it was surprising that this data does not fit the Fundamental Plane for a sample of galaxies as
well as the classical de Vaucouleurs fit does.
Keywords: Virtual observatory; elliptical galaxies; surface light profile; fundamental plane.
En este artı́culo usamos imágenes de alta calidad del Observatorio Virtual MAST, de galaxias elı́pticas con un corrimiento al rojo de z < 0.02
para explorar las diferencias en el ajuste al perfil de brillo superficial dados por de Vaucouleurs r1/4 y Sérsic r1/n . Con los datos obtenidos
por estos ajustes a una muestra de 20 galaxias elı́pticas, estudiamos el Plano Fundamental de galaxias elı́pticas. El Plano Fundamental
involucra varias cantidades fı́sicas de una galaxia como su luminosidad, velocidad de dispersión y radio efectivo. Aunque es conocido que
el perfil de Sérsic da una mejor descripción de una galaxia individual, fue una sorpresa encontrar que para la muestra completa los datos
obtenidos por el perfil de de Vaucouleurs se ajustó mejor al Plano Fundamental.
Descriptores: Observatorio virtual; galaxias elı́pticas; perfil superficial de luminosidad; plano fundamental.
PACS: 98.52.Eh
and the effective radius re we obtain from the surface light wavelenght of 0.8 microns. This filter is commonly used in
profile. Looking at the equation, we ask ourselves: Which galactic studies because it is not severely affected by dust ex-
effective radius should we use, the one that fits the de Vau- istence.
couleurs profile, or the more general r1/n ? Will there even
be a difference? We think there will be a difference for a
3. Results
sharper profile, where the light is more concentrated at the
center of the galaxy, the effective radius will be smaller. A The galaxies were selected with the next criteria:
more gentle slope in the profile will give us a larger effective
radius. With this reasoning, we see that the de Vaucouleurs 1. They are part of the New General Catalog (NGC),
profile sometimes will give values below or above the given 2. They should be true elliptical galaxy. In the prelim-
by Sérsic. We saw that using a small sample of high resolu- inary sample we found two that were the results of
tion images of elliptical galaxies is a quick way to find out if mergers and did not give a clean fit for the light profile
there are differences. To obtain this type of images in short (each previous core produced a bump in the profile),
time, a Virtual Observatory is the best way to do it. even when they appeared to be ellipticals,
The Multimission Archive at STScI is a NASA funded 3. The galaxy must be near to us so it presents a big angu-
project to support and provide to the astronomical commu- lar size in terms of pixels to get more resolution for the
nity a variety of astronomical data archives, with the primary light profile fitting. Most galaxies in the SDSS have
focus on scientifically related data sets in the optical, ultra- an angular size around 5 pixels in contrast to over 30
violet, and near-infrared parts of the spectrum. Of special pixels in MAST and will not show a big difference be-
interest to us, is the High Level Science Products (HLSP). tween the two profiles as there are too few data points,
These are community contributed images and spectra. This
data is fully processed (reduced, cleaned of cosmic rays, etc), 4. They are well known and there is a lot of information
so it is ready for scientific analysis. The access is simple. of them in Hyperleda (http://leda.univ-lyon1.fr/). From
Right in the webpage http://archive.stsci.edu/ is a search box. Hyperleda we used the apparent magnitude, absolute
We simply introduce the name of the object (for example, magnitude and velocity dispersion, and
NGC 1399) and it will tell us if there are HLSP available for 5. It must have a HLSP image, and it should not be over-
download. exposed.
The data used in this work came from the Hubble Space We started with over 30 galaxies, but the last criteria re-
Telescope (HST). The images were captured with the Wide duced the sample to 20 galaxies. It is expected that as times
Field Camera which has four CCDs. Three of this CCDs goes by, more and more data will be available as HLSP. One
are arranged in an L-shaped array with a resolution of 0.1” of the benefits of using images of the HLSP, is that they are
per pixel. The filter used is F814W with an approximate ready for analysis. Typical astroimaging reduction (cosmic
rays, flat sky, dark current subtraction, etc.) is already done
for us.
Once we obtained the images, we used the IRAF software
for the analysis. We used it because it has a special task to
fit elliptical isophotes to an image (Fig. 1), and construct a
table of information of several parameters including inten-
sity, radial distance from the center, ellipticity among many
others. The isophotes are regions that have the same pixel
values. It is possible using this table, to reconstruct a model
of the galaxy. This is useful in some cases to find inner struc-
ture such as multiple cores or bars and can be done by simple
substracting the model from the real image.
While doing the fitting, we observe that there are a couple
of problems. At the center of the image, the CCD sensor may
be saturated and this gives us a plateau in the profile. In the
outer part of the galaxy the problem is how to know where
the galaxy ends and where the background light starts. This
presents a catch-22 because if the image has a low exposure
time, then the center will be all right while the outer part will
be lost. If we increase the exposure to have a better grasp
on the real size of the object, then most of the center will be
F IGURE 1. Ellipse fitting using the ellipse task from IRAF, for saturated. From the HLSP we used images with a slight satu-
NGC 1427. Each ring has the same value in brightness (isophote). rated center. A python program was used to do the fittings in
Image from STScI. both light profiles. The program did a non-linear fit using a
TABLE I. Errors for the Fundamental Plane fit. From the values we
see that de Vaucouleurs was a better fit to the Fundamental Plane.
The R2 values the closer to 1.0 the better they are.
Std. Error R2 Adj. R2
Sérsic 0.303 0.632 0.588
de Vaucouleurs 0.269 0.709 0.675
1. de Vaucouleurs, Ann. Astrophysics 11 (1948) 247. 5. K.C. Freeman, Ap.J. 160 (1970) 811.
1
2. J.L. Sérsic, Generalized R 4 Law., Atlas de Galaxias Australes
6. S. Faber and B. Jackson, Ap. J. 204 (1976) 668.
(Córdova, Argentina: Observatorio Astronómico, 1968).
3. N. Caon, M. Capaccioli, and D’Onofrio, MNRAS 265 1993 7. M. Blanton et al., Ap. J. 594 (2002) 186.
10131.
4. R. Tully and J. Fisher, A&A 54 (1977) 661. 8. N. Padmanabhan, et al., New Astron. 9 (2004) 329.