The Colors and Sizes of Recently Quenched Galaxies: A Result of Compact Starburst before Quenching

We are interested in galaxies whose SFR is low but that was much higher in the recent past. Accordingly, our sample must satisfy two criteria: they must have had a rapid drop in star formation, as evidenced by enhanced Balmer absorption, and they must currently be quiescent.

We use the Hδ absorption as the probe for recent star formation activity. We require a fiducial cut of EW(Hδ) ≥ 4 Å after subtracting the contribution from the emission line derived from a joint fitting of the stellar continuum and emission lines using the Penalized Pixel-Fitting method (Cappellari & Emsellem 2004; Cappellari 2017). We select galaxies with S/N higher than 10 per pixel at rest-frame 4000  Å (SN_RF_4000 ≥ 10) and quality flags f_use = 1 in the LEGA-C DR2 catalog (Straatman et al. 2018) to minimize misclassification. The typical uncertainties in EW(Hδ) is ∼0.4  Å.

There are different methods to select quiescent galaxies. The lack of emission lines associated with star formation activity is commonly used as a criterion, for example, Balmer lines. However, Hα redshifts to the near-IR at z ≳ 0.5 and only less than half of the LEGA-C spectra cover both Hδ and Hβ lines. An alternative is the [O ii]λ37273729 emission. Nevertheless, recently quenched galaxies commonly have detectable [O ii]λ37273729 emission from ionizing sources other than star-forming regions (Yan et al. 2006). Requiring the lack of [O ii] emission leads to an incomplete sample and potential biases against active galactic nucleus (AGN) hosts (Lemaux et al. 2010; Wu et al. 2014; Lemaux et al. 2017).

Instead, we use the rest-frame U − V and V − J broadband colors to define quiescence. We derive the color by fitting the observed multiwavelength spectral energy distributions (SEDs) from the UltraVISTA catalog (Muzzin et al. 2013a) using the FAST code (Kriek et al. 2009). We adopt the demarcation between star-forming and quiescent galaxies proposed by Muzzin et al. (2013b). Recently, selections based on only broadband photometry have been extensively applied (e.g., Kriek et al. 2010; Whitaker et al. 2012). We will discuss the different methods in Section 5.

We derive the R_e of galaxies, the semimajor radius enclosing 50% of light, using the Hubble Space Telescope (HST) ACS F814W images from the COSMOS program (GO-9822, GO-10092, PI: N. Scoville Scoville et al. 2007), following the procedure of van der Wel et al. (2012) using the galfit program (Peng et al. 2010). Although galaxies do not necessarily have simple Sérsic profiles (e.g., Margalef-Bentabol et al. 2016), tests on simulated galaxy images suggest that as long as the S/N ratios of the images are reasonably high (S/N ≳ 100), R_e can generally be recovered with a ∼20% accuracy even for galaxies with double Sérsic profiles (Davari et al. 2014, 2016). While recently quenched galaxies may intrinsically consist of multiple components (Section 4), we choose to adopt the R_e from single Sérsic fits for all galaxies for consistency in methodology. The HST images of all our recently quenched galaxies have S/N ≳ 100; therefore, we expect the R_e is reliable.

We exclude two galaxies with catastrophic failures in the galfit fitting or highly disturbed morphologies. One of them exhibits three bright clumps and the other has a diffuse asymmetric low surface brightness feature extending toward a nearby faint object. However, their disturbed morphologies carry important clues for the formation of recently quenched galaxies as will be discussed in Section 5.2. We also exclude three galaxies with close companions that potentially affect broadband photometry and thus the estimate of stellar masses and rest-frame colors (f_int = 1). The final sample consists of 32 recently quenched galaxies (Table 1). Figure 1 shows their redshifts, stellar masses, U − V and V − J colors, and EW(Hδ). Figure 2 shows the HST images and spectra of three recently quenched galaxies with different colors as examples. Recently quenched galaxies in our sample reside mainly in the average density environment with local overdensity log(1 + δ) = 0.1 ± 0.4 (zero is the average) according to the estimate of Darvish et al. (2015). Our sample thus does not probe extreme environments like galaxy clusters.

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We examine the Hβ emission-line strengths of recently quenched galaxies to confirm their quiescence. The spectra of 12 galaxies in the sample cover the Hβ line. Eight of them have EW(Hβ)_em > −1 Å, consistent with being quiescent (Wu et al. 2018a). For the remaining four galaxies, two galaxies have EW(Hβ)_em consistent with −1  Å (ID: 211263, −1.20 ± 0.23  Å and ID: 230053, −1.04 ± 0.36  Å), one galaxy (ID: 232099, −3.92 ± 0.45  Å) has strong [Ne iii] and [O ii] emission, indicating an ionizing source other than star formation, and one galaxy (ID: 184250, −2.24 ± 0.58  Å) would be considered as weakly star-forming. Its Hβ emission strength is ∼0.3 dex lower than the median value of star-forming galaxies in the LEGA-C DR2 sample (−4.70  Å). We conclude that our sample has low SFRs.

We cross-check the SFH derived from the full-spectral fitting technique used by Chauke et al. (2018) and find that our selected galaxies all have an elevated period of star formation activity in the past ≲1 Gyr. The strong Hδ absorption feature ensures that these galaxies are truly "recently quenched."

Galaxies have a wide range of mass growth and chemical evolutionary histories. Recovering the true formation histories from observables remains a challenging task. In this paper, we use simple SFHs combined with stellar population synthesis models to illustrate how recent star formation events affect observables. We adopt the following analytical form for our fiducial analysis:

This star formation history consists of two components: a constant star formation phase followed by a subsequent burst with an exponential decay timescale τ with different strengths of bursts (Figure 3(a)).

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In this paper, we define the burst strength b as the ratio between the masses formed in the burst to the mass formed in the constant star formation period, b ≡ M_burst/M_const. We generate four sets of models, b = 3, 1, 0.3, and 0.1, with two different τ = 0.1 Gyr and 0.5 Gyr, respectively. The combination of b and τ determines the amount of stars formed right before galaxies become quiescent, which is the key feature for recently quenched galaxies. A shorter τ and larger b represent a more drastic decline of SFR. A sudden truncation of star formation corresponds to τ = 0. We will see later in Section 3 that 0.5 Gyr is roughly the longest τ that can produce the observation signatures of recently quenched galaxies with our assumed star formation history.

The b = 0.1 model only increases the SFR by ∼2 times, averaging over the first 100 Myr after the burst. The b ≥ 1 models would be considered as strong starburst events. More extreme bursts with b > 3 behave qualitatively similarly to the b = 3 model for the parameters that we are interested in. We make one more model with b = 0, mimicking a sudden truncation of star formation without forming new stars. Figure 3(a) shows the SFHs of the nine models used in this paper. Real recently quenched galaxies do not have to follow Equation (1). The parameterization, b and τ, provides insight into how the strength and the timescale of the last period of star formation affect the color evolution.

We then use the Python implementation of the Flexible Stellar Population Synthesis package (FSPS; Conroy et al. 2009; Conroy & Gunn 2010; Foreman-Mackey et al. 2014) to generate the spectra and broadband colors of each model for each 100 Myr time step, adopting the MILES spectral library (Sánchez-Blázquez et al. 2006), Padova isochrones (Girardi et al. 2000; Marigo & Girardi 2007; Marigo et al. 2008), and a Chabrier IMF (Chabrier 2003).

We choose solar metallicity as the fiducial value (Gallazzi et al. 2014). For the dust attenuation, we use the Cardelli et al. (1989) dust reddening curve and a two-component dust model. Young stellar populations (<10⁷ yr) have and the rest have . Therefore, when the SFR is low, like the recently quenched galaxies, the total of the models approach A_v ≃ 0.5, which is motivated by the constraints from broadband SED fitting, as well as spectral-photometric fitting of galaxies at similar and slightly higher redshifts (Belli et al. 2019; Carnall et al. 2019). A different makes negligible difference. We use these fiducial values in Section 3 and Section 4 and discuss the assumptions in Section 5.

Figure 3 shows the evolution of EW(Hδ) and the V − J and U − V colors of the nine different SFHs with our fiducial choice of metallicity and dust attenuation. The thick segments indicate the period where model galaxies would be classified as recently quenched galaxies based on our selection criteria.

Galaxies with shorter τ can be identified as recently quenched galaxies for longer periods of time. We will compare these observables to our recently quenched galaxy sample in Section 3 and discuss scenarios that deviate from the fiducial parameters in Sections 4 and 5.

In general, the recently quenched galaxies have bluer V − J and U − V colors than the rest of the quiescent galaxies, reflecting their younger stellar population (Figure 1(a)). On the other hand, they spread over a wide range in the color–color space between 0.7 ≲ V − J ≲ 1.3 and 1.4 ≲ U − V ≲ 1.9.

This sample of recently quenched galaxies cannot be separated based on rest-frame U − V and V − J colors alone. The distributions of colors are much wider than prevalent photometric selections, which usually select UVJ quiescent galaxies with V − J ≲ 0.9 (e.g., Whitaker et al. 2012; Belli et al. 2019); only ∼30% of the sample fulfill the color selection. We will discuss the different color distributions in Section 5.

Figure 4(a) shows the recently quenched galaxies and the evolutionary tracks of different SFHs on the UVJ diagram. The model galaxies evolve from the bottom left of the diagram, become redder, and move to the top right. The thick segments indicate the period when the galaxy is classified as a recently quenched galaxy.

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On the UVJ diagram, the model tracks cover the observed distribution of recently quenched galaxies reasonably well. The wide range of color can be partially due to different decay timescales τ. For the same burst fraction b, recently quenched galaxies with τ = 0.1 Gyr (solid lines) are bluer than galaxies with τ = 0.5 Gyr (dashed lines) in the V − J color.

In addition, the V − J color also depends on the burst fraction b. Recently quenched galaxies with weaker bursts are on average redder (see also Figure 3). Even with a short decaying timescale of τ = 0.1 Gyr, a recently quenched galaxy with b = 0.3 is hardly ever bluer than V − J ≃ 0.9. Recently quenched galaxies with even bluer colors are predominantly those that experienced strong bursts, which have at least doubled their stellar masses in the starburst events.

Figure 4(b) shows the EW(Hδ) and V − J colors of recently quenched galaxies. In our sample, EW(Hδ) ranges from 4  Å to ≃8  Å. However, the τ = 0.5 Gyr models only produce recently quenched galaxies with  Å. About half of the sample must have shorter quenching timescales in order to produce the strong Hδ absorption.

The model tracks evolve from the top left to the bottom right in Figure 4(b) and are in broad agreement with the majority of the sample, suggesting that our fiducial parameter set is reasonable. However, at the reddest V − J colors, several recently quenched galaxies have high EW(Hδ) that are not well explained by the fiducial models. These galaxies possibly have much larger A_v than the fiducial value. We will discuss our assumptions in Section 5.

Wu et al. (2018a) have observed that the R_e of recently quenched galaxies at z ∼ 0.8 are on average smaller than the quiescent population and attributed it to central, compact starbursts that happened in the centers of their progenitor galaxies before being quenched. This hypothesis leads to a prediction that recently quenched galaxies with stronger bursts should have on average smaller R_e.

As we see in Figures 3 and 4, the burst strengths correlate with V − J colors. Recently quenched galaxies with blue V − J colors can only be the products of strong starburst events that roughly doubled the stellar masses of galaxies. On the other hand, recently quenched galaxies with red V − J colors can be a combination of galaxies with weak bursts, longer decaying timescale, and are observed later after the starburst. Therefore, we should expect that recently quenched galaxies with blue V − J colors have on average smaller R_e.

Figure 5(a) shows the R_e and the stellar masses of recently quenched galaxies, color-coded by their V − J colors. The R_e range from 0.7 to 4.9 kpc, with 16th, 50th, 84th percentiles of 1.2, 2.2, and 4.0 kpc, respectively. We see in general that recently quenched galaxies with blue V − J colors tend to have smaller R_e. Next, in order to take into account the intrinsic correlation between the sizes and the stellar masses, we compare the R_e of recently quenched galaxies with only galaxies of similar masses.

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We first fit a linear relation between the median sizes in each 0.1 dex mass bin and the mass in log−log space as

where (a, b) are (, ) and (, ) for quiescent and star-forming galaxies, respectively (dashed lines in Figure 5a). The 1σ dispersion around the median is ∼0.2 dex for both quiescent and star-forming populations in the stellar mass range of interest (also see Wu et al. 2018a). We then calculate , the logarithmic size difference between recently quenched galaxies and the best-fit mass−size relations of quiescent and star-forming galaxies, respectively. We find the V − J colors correlate strongly with when comparing either to quiescent or star-forming galaxies (Figures 5(b) and (c)). Blue recently quenched galaxies are relatively smaller than red recently quenched galaxies. The null hypothesis that the correlation is due to random uncertainty can be rejected with high confidence. The clear correlation between colors and R_e indicates that the SFHs and the structures of recently quenched galaxies are coupled.

5.1.1. Effect of Dust Attenuation

Our models assume an , which is close to the result of broadband SED fitting, as well as quiescent galaxies at z ∼ 1–2.5 (Belli et al. 2019; Carnall et al. 2019). On the other hand, low-redshift massive ellipticals have on average lower (Tojeiro et al. 2013; Citro et al. 2016; López Fernández et al. 2018). Assuming a lower brings τ = 0.5 Gyr models closer to the observations. We have tested models with different and find that if recently quenched galaxies have ranging from 0.1 to 0.5, the τ = 0.5 Gyr models would be able to reproduce the wide range of V − J color. A range of burst strength b is still required to reproduce the distribution of R_e.

Several recently quenched galaxies in our sample have both strong Hδ absorption and red colors (Figure 4(b)), indicating that the A_v of these galaxies is likely much higher. Our assumption of a single A_v value provides a simplified picture that allows us to investigate the interplay of SFHs, colors, and structures. However, better estimates on the dust attenuation for individual galaxies will still be needed to improve the constraints on SFHs.

Our models also assume that dust attenuation curves of the disk and the burst components have the same dependence on SFR. While the shapes of attenuation curves can vary (Kong et al. 2004; Kriek & Conroy 2013; Salim et al. 2018) and the dust property can be different in the burst component (Smercina et al. 2018), exploring its effect on structures over the entire parameter space is beyond the scope of this paper.

Instead, we show an illustrative case on how spatially nonuniform attenuation may affect the color and size. Figure 8 shows three models with b = 0.3, τ = 0.5 Gyr, and . For two of them, we artificially increase the A_v at the center 1 kpc to 0.5 and 1.0, respectively. With heavier attenuation in the center, model recently quenched galaxies are not only redder but also have larger R_e because a larger fraction of light from the central compact component attenuate, causing the light profile to become less compact. Nonuniform attenuation can qualitatively create the correlation between the color and the size, but a large variation will be needed to produce the observed distributions of color and R_e simultaneously.

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5.1.2. Aperture Effect

5.2.1. Should there be a Structural Transformation?

5.2.2. What Mechanisms Quench Galaxies?

The term post-starburst galaxy implies that these quiescent galaxies with strong Balmer absorption had experienced periods of elevated SFRs before becoming quiescent. However, such a star formation history is not required. A sudden quenching without a starburst can also produce quiescent galaxies with strong Balmer absorption (Le Borgne et al. 2006).

The sizes and colors together suggest that a pure truncation scenario may contribute to the recently quenched population but cannot account for the full sample. Most recently quenched galaxies are significantly smaller than star-forming galaxies (Figure 5). If the quenching mechanisms only shut down star formation without producing new stars in the center, the R_e would be similar to the progenitor star-forming galaxies. In addition, without a newly formed stellar component, the colors of recently quenched galaxies would be too red for at least half of the sample (the gray area in Figure 7(b)). We thus conclude that most recently quenched galaxies had at least a low-level starburst event before turning into quiescence.

Spatially resolved information will further verify this scenario. The central compact starburst makes the stellar ages younger in the center. Recently quenched galaxies may appear to have blue centers in multiwavelength HST images if the dust attenuation is not more severe in the center. While spectroscopy can provide a better estimate of stellar ages, large ground-based telescopes with AO or the James Webb Space Telescope (JWST) will be needed for good S/N and spatial resolution.

Traditionally, the Balmer absorption lines are used to select recently quenched galaxies, or post-starburst galaxies as they are often called in the literature, because they are one of the key features of A-type stars and thus are sensitive to recent star formation activities of galaxies. However, beyond the nearby universe, high-S/N spectra of galaxies are observationally expansive thus other techniques have been explored.

In recent years, deep multiwavelength photometry has become a popular alternative. Regardless of details, these photometric selections either explicitly or implicitly select the bluest quiescent galaxies mainly with rest-frame V − J ≲ 0.9 (Kriek et al. 2011; Whitaker et al. 2012; Wild et al. 2014; Yano et al. 2016; Belli et al. 2019). However, we have demonstrated in the paper that spectroscopically identified recently quenched galaxies span a wide range of color: 0.7 ≲ V − J ≲ 1.3 and only ∼30% (10/32) of the sample fulfill the prevalent color cut.

While the majority of recently quenched galaxies in this paper have V − J > 0.9, they present only a small fraction of quiescent galaxies at given U − V and V − J colors (Figure 1(a)). The photometric-only selections may not be able to effectively identify these redder recently quenched galaxies without introducing severe contamination to the sample. Based on our LEGA-C DR2 mother sample (Figure 1(a)), a more generous color cut of V − J < 1.0 selects 35 quiescent galaxies and 18 of them have EW(Hδ) < 4  Å. If we adopt an even more generous color cut of V − J < 1.3 and U − V < 1.9 to include all but one recently quenched galaxy, the contamination reaches ∼70%: 112 out of 143 quiescent galaxies that fulfill the color cut have EW(Hδ) < 4  Å.

On the other hand, by selecting extremely blue quiescent galaxies, the photometric selections can obtain clean samples. The blue colors ensure that most of the stars must have formed recently. Therefore, the SFRs have to be high in the recent past. These blue quiescent galaxies also have strong Balmer absorption lines. Among 13 UVJ quiescent galaxies with V − J ≤ 0.9 in Figure 1(a), all of them have EW(Hδ) ≥ 3 Å and 10 of them have EW(Hδ) ≥ 4  Å (also see Maltby et al. 2018). Nevertheless, this selection is very incomplete.

Under the framework of Section 4, this incompleteness also introduces a selection bias. The starbursts need to produce stellar masses nearly comparable to the preexisting masses to make the whole galaxies blue enough to be distinguishable from other quiescent galaxies in the color space. Recently quenched galaxies with V − J < 0.9 are predominantly galaxies with strong starbursts (b ≥ 1). Current photometric selections serve as effective alternatives when good spectra are not available, but they may only pick up those that had at least doubled their stellar masses in the starburst events. Recently quenched galaxies with weaker bursts may be missing from the sample, even though they can be produced by the same mechanism. This potential bias should be kept in mind when interpreting the results.