Recently Quenched Galaxies at z = 0.2 - 4.8 in the COSMOS UltraVISTA Field

TL;DR

This study analyzes recently-quenched galaxies across a broad redshift range, revealing their increasing number density over time and their role as transitional objects in galaxy evolution.

Contribution

First comprehensive analysis of recently-quenched galaxies from z=0.2 to 4.8 using COSMOS data, highlighting their evolving properties and transitional role.

Findings

Number density of RQGs increases with time at z>1.

Low-mass RQGs grow rapidly at z<1.

RQGs have intermediate morphology between star-forming and passive galaxies.

Abstract

We present a new analysis of the stellar mass function and morphology of recently-quenched galaxies (RQGs), whose star formation has been recently quenched for some reason. The COSMOS2015 catalog was exploited to select those galaxies at 0.2 < z < 4.8, over 1.5 deg2 of the Cosmic Evolution Survey (COSMOS) UltraVISTA field. This is the first time that RQGs are consistently selected and studied in such a wide range of redshift. We find increasing number density of RQGs with time in a broad mass range at z>1, while low-mass RQGs start to grow very rapidly at z < 1. We also demonstrate that the migration of RQGs may largely drive the evolution of the stellar mass function of passive galaxies. Moreover, we find that the morphological type distribution of RQGs are intermediate between those of star-forming and passive galaxies. These results indicate that RQGs represent a major transitional phase of galaxy evolution, in which star-forming galaxies turn into passive galaxies, accompanied by the build up of spheroidal component.

Tables

Table 1. Table 1 Best-fit parameters of the Schechter stellar mass functions

$\pm$   star-forming galaxies
redshift	${\Phi }^{\ast }$ (${10}^{-3}$/${\mathrm{Mpc}}^3$/dex)		log$M^{\ast }\left(M_{\odot }\right)$		$\alpha$		${\chi }_{reduced}^2$	Number111Number of galaxies in each redshift bin.
	0.860 $\pm$	0.070	10.81 $\pm$	0.03	-1.37 $\pm$	0.01	2.14	9750
	0.888 $\pm$	0.090	10.89 $\pm$	0.03	-1.37 $\pm$	0.03	6.65	23843
	0.605 $\pm$	0.036	10.93 $\pm$	0.02	-1.38 $\pm$	0.02	1.81	17956
	0.620 $\pm$	0.069	10.86 $\pm$	0.03	-1.30 $\pm$	0.04	5.93	12902
	0.302 $\pm$	0.055	10.94 $\pm$	0.05	-1.32 $\pm$	0.07	5.92	6846
	0.212 $\pm$	0.022	10.94 $\pm$	0.03	-1.50 $\pm$	0.04	0.67	4050
	0.088 $\pm$	0.026	10.99 $\pm$	0.07	-1.83 $\pm$	0.08	1.34	2694
	0.003 $\pm$	0.005	11.53 $\pm$	0.32	-2.30 $\pm$	0.12	1.74	1213
	0.000 $\pm$	0.001	13.71 $\pm$	23.79	-2.11 $\pm$	0.14	3.37	896
recently-quenched galaxies (RQGs)
redshift	${\Phi }^{\ast }$ (${10}^{-3}$/${\mathrm{Mpc}}^3$/dex)		log$M^{\ast }\left(M_{\odot }\right)$		$\alpha$		${\chi }_{reduced}^2$	Numbera
	1.373 $\pm$	1.275	9.15 $\pm$	0.20	-1.92 $\pm$	0.30	0.68	185
	0.009 $\pm$	0.014	11.45 $\pm$	0.53	-2.04 $\pm$	0.07	0.72	549
	0.222 $\pm$	0.041	10.80 $\pm$	0.07	-1.09 $\pm$	0.08	0.62	356
	0.194 $\pm$	0.054	10.77 $\pm$	0.11	-0.94 $\pm$	0.16	0.99	249
	0.283 $\pm$	0.030	10.37 $\pm$	0.11	0.08 $\pm$	0.39	1.47	185
	0.073 $\pm$	0.020	10.53 $\pm$	0.23	-0.26 $\pm$	0.64	0.60	44
	0.062 $\pm$	0.019	10.60 $\pm$	0.21	-0.26		1.54	44
	0.075 $\pm$	0.030	10.18 $\pm$	0.13	-0.26		1.09	25
	0.035 $\pm$	0.015	10.47 $\pm$	0.27	-0.26		1.45	27
passive galaxies
redshift	${\Phi }^{\ast }$ (${10}^{-3}$/${\mathrm{Mpc}}^3$/dex)		log$M^{\ast }\left(M_{\odot }\right)$		$\alpha$		${\chi }_{reduced}^2$	Numbera
	0.870 $\pm$	0.107	11.07 $\pm$	0.05	-0.85 $\pm$	0.04	5.04	2615
	1.565 $\pm$	0.057	10.86 $\pm$	0.02	-0.31 $\pm$	0.03	3.24	7686
	0.679 $\pm$	0.022	10.69 $\pm$	0.03	0.17 $\pm$	0.07	3.43	3949
	0.287 $\pm$	0.010	10.67 $\pm$	0.03	0.34 $\pm$	0.09	2.28	1888
	0.067 $\pm$	0.002	10.70 $\pm$	0.03	0.08 $\pm$	0.08	0.38	472
	0.033 $\pm$	0.003	10.58 $\pm$	0.06	0.43 $\pm$	0.26	0.83	219
	0.007 $\pm$	0.003	10.53 $\pm$	0.26	0.67 $\pm$	1.16	2.33	66
	0.002 $\pm$	0.003	11.16 $\pm$	0.78	-1.34 $\pm$	0.63	1.07	41
	0.001 $\pm$	0.001	11.91 $\pm$	1.44	-1.24 $\pm$	0.32	0.30	38

Equations

$$(NUV-r)<2.5\mathrm{or}(NUV-r)<3(r-J)+1$$

$$(NUV-r)>2.5$$

$$\Phi (M)\mathrm{d}M=\ln (10){\Phi }^{\ast }[10^{(M-M^{\ast })(\alpha +1)}]\exp [-10^{(M-M^{\ast })}]\mathrm{d}M.$$

Full text

1. INTRODUCTION

Galaxies are an important constituent of the universe, and are known to evolve throughout the cosmic time. As such, it is crucial to disentangle the process of their formation and evolution, in order to better understand the history of the universe. There are two major classes of galaxies, namely, star-forming and passive galaxies, but an evolutionary link between these two classes is still not well understand. A seminal work by Kauffmann et al. (2003) demonstrated that the two classes have distinct stellar mass distributions, with the star-forming class dominating low-mass galaxies (stellar mass $M\lesssim 10^{10.5}M_{\odot}$). Transition from star-forming to passive galaxies must be accompanied by quenching of star formation in any form, whose process(es) may depend on the properties of individual galaxies, such as stellar mass and surrounding environment (e.g., Peng et al., 2010).

A potentially important population to probe the above quenching process(es) is post-starburst galaxies, characterized by the spectra being dominated by A-type stars (Dressler & Gunn 1983). These galaxies are thought to be in a short period along the galaxy evolution, when the precedent active star formation was recently quenched . Their lifetime is estimated to be 0.1 - 0.6 Gyr (Wild et al., 2009; Snyder et al., 2011). Post-starburst galaxies are frequently observed with spectral signatures of active galactic nuclei (AGNs), which have been found in X-ray, optical, and/or mid-infrared (IR) wavelengths (Brown et al., 2009; Alatalo et al., 2016). This may suggest that the star-formation quenching may be driven, at least partly, by energy input from AGNs (see also Matsuoka et al., 2014, 2015). Cales et al. (2011) found that post-starburst quasars are split into an equal number of spiral and early-type host galaxies, and that the luminous objects tend to exhibit morphological disturbance, which may suggest the merger origin of these systems. High fractions of merger signatures were also reported for post-starburst galaxies by, e.g., Zabludoff et al. (1996), Yang et al. (2008), and Wong et al. (2012). On the other hand, the majority of post-starburst galaxies are found in field environment (Zabludoff et al., 1996), and no clear dependence of morphological evolution on the surrounding environment has been found for this class of galaxies (e.g., Vulcani et al. 2015).

Meanwhile, investigation of post-starburst galaxies is still limited compared to those of star-forming and passive galaxies. This is partly due to a much smaller number density of post-starburst galaxies than the other two classes; roughly 0.01 – 1 % of local galaxies are found in post-starburst phase, depending on the selection criteria (e.g., Zabludoff et al., 1996; Quintero et al., 2004; Goto, 2007). This problem may be alleviated by selecting post-starburst analogs with photometry data alone, which enables to construct a much larger sample of galaxies than would be possible with secure spectroscopic data. Indeed, such photometrically-selected “recently-quenched galaxies” (RQGs) have been studied at $z<2$, with the imaging data from, e.g., the NEWFIRM Medium-Band Survey (Whitaker et al., 2011, 2012) and the UKIDSS (Lawrence et al., 2007) Deep Survey (Wild et al., 2016).

In the present work, we exploit the exquisite multi-band photometry data from the Cosmic Evolution Survey (COSMOS) in order to study RQGs. The COSMOS field has been observed to very deep depths over $\sim$2 deg2, across the electromagnetic spectrum from X-ray to radio wavelengths (Scoville et al., 2007). In particular, deep near-IR data (see below) enable us to construct a robust sample of RQGs out to very high redshift; in this study, we investigate the stellar mass function and morphology of RQGs at $0.2<z<4.8$. We assume the cosmological parameters of $\Omega_{m}=0.3$, $\Omega_{\Lambda}=0.7$, and $H_{0}=70$ km s*-1* Mpc*-1* throughout this work. All magnitudes are presented in the AB system (Oke & Gunn, 1983), unless otherwise noted.

2. sample selection

This work exploits the COSMOS2015 catalog (Laigle et al. 2016), in which the source detection is based on the near-IR data taken from the UltraVISTA survey (McCracken et al. 2012) data release (DR) 2. The UltraVISTA covers 1.5 deg2 of the COSMOS field, with the DR2 limiting magnitudes (3$\sigma$ depth in an aperture of 2″diameter) of $\sim$25 mag in the $YJHK_{\rm S}$ bands. The effective area are 0.46 and 0.92 deg2 in the Ultra-deep (UD) and Deep fields, respectively, after removing masked regions around bright sources. Photometric redshifts of galaxies were measured with $LePhare$ (Arnouts et al. 2002; Ilbert et al. 2006), with the spectral templates taken from Polletta et al. (2007) and Bruzual & Charlot (2003, hereafter BC03). These templates do not explicitly include a spectrum of post-starburst galaxies (such as that from Kriek et al., 2010), but post-starburst galaxies may be accounted for partly by the above BC03 templates, with the assumed starburst ages ranging from 0.03 to 3 Gyr. Here we simply assume that the redshifts of our RQGs are reasonably determined with the adopted templates, and defer a more detailed analysis of photometric-redshift accuracy to a future work. The present work uses photometric redshifts for all the galaxies, and does not exploit spectroscopic information of any individual galaxies. Absolute magnitudes were derived with the best-fit spectral templates, while stellar masses were estimated with the BC03 models, assuming the Chabrier (2003) initial mass function. We removed X-ray sources in this work, given that their redshift, absolute magnitude, and stellar mass estimates are relatively uncertain.

The samples of galaxies used in this work are selected as follows. First, we extract all the galaxies from the COSMOS2015 catalog in the UltraVISTA fields, excluding those in masked regions or with matched X-ray detection. We then remove objects with absolute magnitudes $NUV>-25$ mag, since a small fraction of COSMOS2015 objects have catastrophically bright $NUV$ magnitudes for unknown reason (C. Laigle, private communication). This work makes use of the limiting masses of 90% completeness ($M^{\rm full}_{\rm lim}$) presented in Laigle et al. (2016), and selects galaxies more massive than $M^{\rm full}_{\rm lim}$ at each redshift. A discussion about possible selection effects is found in the following section.

Next, we classify the extracted galaxies into star-forming, passive, and RQG classes based on their colors. Broad-band selection of these classes of galaxies have been commonly performed (e.g., Fontana et al. 2009; Drory et al. 2009; Ilbert et al. 2010). In particular, the rest-frame $U$, $V$, and $J$-band colors have been frequently used for galaxy classification (e.g., Mendel et al. 2015). The spectral energy distributions (SEDs) of passive galaxies have a strong Balmer/4000 $\rm{\AA}$ break, which effectively separates star-forming and passive galaxies in a broad-band color space. In this work, we adopt the color selection with the rest-frame $NUV$, $r$, and $J$ bands (Ilbert et al. 2013). The COSMOS2015 $U$ band refers to the CFHT/MegaPrime $u^{*}$ band, whose filter transmission extends to a slightly longer wavelength than the 4000 $\rm{\AA}$ break; this makes the $NUV$ band a better tracer of the blue side of the 4000 $\rm{\AA}$ break, in a given spectrum.

In order to establish the color selection criteria, we calculated the typical colors of galaxies with the BC03 models. We assumed simple stellar populations with three different star-formation histories, i.e., a single starburst with 0.5 Gyr duration, an exponentially declining star formation rate ($\propto e^{-\tau}$) with $\tau$ = 0.1 Gyr, and a constant star formation rate. The results of these calculates are presented in Figure 1. The SEDs of RQGs have a strong Balmer/4000 $\rm{\AA}$ break contributed by A-type stars, which makes their $NUV-r$ colors redder than those of star-forming galaxies (e.g. Martin et al. 2007, Ilbert et al. 2013). In addition, this diagram allows to distinguish RQGs and passive galaxies, due to the bluer SEDs (bluer $r-J$ colors) of RQGs at $>$4000 Å compared to passive galaxies.

Figure 1 demonstrates that RQGs, which corresponds to the age around 1 Gyr in the single burst or the exponentially declining model, can be selected at the top left corner of the diagram. Our selection criteria of star-forming galaxies, passive galaxies, and RQGs are represented by the black solid lines, which are defined as:

[TABLE]

for star-forming galaxies,

[TABLE]

for passive galaxies, and

[TABLE]

for RQGs. The separation between star-forming and passive galaxies (the diagonal line in Figure 1) is identical to that used by Ilbert et al. (2013), which is nearly parallel to the direction of dust reddening. The separation between star-forming and RQGs (the horizontal line in Figure 1) corresponds to the epoch of 0.2 Gyr after the star formation is stopped in the single burst model. This is based on the hydrodynamic simulations of galaxy mergers presented by Snyder et al. (2011), who demonstrated that a galaxy may be selected as a post-starburst system after $\sim$0.2 Gyr of the merger (and starburst) event.