The evolution of post-starburst galaxies from z=2 to 0.5

MNRAS 463, 832–844 (2016) doi:10.1093/mnras/stw1996 Advance Access publication 2016 August 11 The evolution of post-starburst galaxies from z = 2 to 0.5 Vivienne Wild,1‹ Omar Almaini,2 Jim Dunlop,3 Chris Simpson, Kate Rowlands,1 Rebecca Bowler,4 David Maltby2 and Ross McLure3 1 SUPA†, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK of Nottingham, School of Physics and Astronomy, Nottingham NG7 2RD, UK 3 SUPA†, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 4 Subdepartment of Astrophysics, University of Oxford, The Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 2 University ABSTRACT We present the evolution in the number density and stellar mass functions of photometrically selected post-starburst galaxies in the UKIDSS Ultra Deep Survey, with redshifts of 0.5 < z < 2 and stellar masses log (M/M ) >10. We find that this transitionary species of galaxy is rare at all redshifts, contributing ∼5 per cent of the total population at z ∼ 2, to <1 per cent by z ∼ 0.5. By comparing the mass functions of quiescent galaxies to post-starburst galaxies at three cosmic epochs, we show that rapid quenching of star formation can account for 100 per cent of quiescent galaxy formation, if the post-starburst spectral features are visible for ∼250 Myr. The flattening of the low-mass end of the quiescent galaxy stellar mass function seen at z ∼ 1 can be entirely explained by the addition of rapidly quenched galaxies. Only if a significant fraction of post-starburst galaxies have features that are visible for longer than 250 Myr, or they acquire new gas and return to the star-forming sequence, can there be significant growth of the red sequence from a slower quenching route. The shape of the mass function of these transitory post-starburst galaxies resembles that of quiescent galaxies at z ∼ 2, with a preferred stellar mass of log (M/M ) ∼10.6, but evolves steadily to resemble that of star-forming galaxies at z < 1. This leads us to propose a dual origin for post-starburst galaxies: (1) at z  2 they are exclusively massive galaxies that have formed the bulk of their stars during a rapid assembly period, followed by complete quenching of further star formation; (2) at z  1 they are caused by the rapid quenching of gas-rich star-forming galaxies, independent of stellar mass, possibly due to environment and/or gas-rich major mergers. Key words: galaxies: evolution – galaxies: formation – galaxies: high-redshift – galaxies: luminosity function, mass function – galaxies: stellar content. 1 I N T RO D U C T I O N In the local Universe, the population of massive galaxies is bimodal in both structure and spectral type: the majority can be described as either being quiescent and elliptical, or star-forming and spiral. Understanding the origin of this bimodality in the galaxy population has been the topic of considerable research for many decades. Recently, large multiwavelength surveys have allowed us to measure the fraction of quiescent and star-forming galaxies over a large portion of cosmic time, providing direct observations of the emergence of galaxy bimodality. These have shown that both the number density of quiescent galaxies, as well as their total stellar mass den-  † E-mail: Scottish Universities Physics Alliance sity, increases steadily with time since at least z ∼ 3–4 (e.g. Bell et al. 2004; Faber et al. 2007; Ilbert et al. 2013; Muzzin et al. 2013), although details depend on the survey as well as the methods used to calculate photometric redshifts and stellar masses, and to identify quiescent galaxies. As quiescent galaxies are unable to form new stars in situ, the increase in the global stellar mass density of quiescent galaxies must arise from a cessation of star formation in star-forming galaxies and transfer of galaxies from the star-forming to quiescent populations. This process has come to be known as galaxy ‘quenching’, and one of the central questions in galaxy evolution research is which physical processes are responsible for this shut down in star formation in a portion of the galaxy population. The presence of quiescent galaxies out to redshifts of z ∼ 4 or more (e.g. Straatman et al. 2014), shows that star formation can be rapidly quenched following an early formation epoch. These early quiescent galaxies are found to be elliptical and highly compact (e.g.  C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Accepted 2016 August 8. Received 2016 August 2; in original form 2016 April 1 The evolution of post-starburst galaxies Ultimately, the assembly history of the quiescent galaxy population is unlikely to be simple, with different physical processes dominating at different stellar masses and redshifts (see e.g. Choi et al. 2014; Brennan et al. 2015). Given the number of free parameters, studying the evolution of the distribution of galaxies in star formation rate/stellar mass/morphology planes can only help to constrain the role of different processes in shutting down star formation in combination with detailed models which help to link cause and effect. A more direct method involves the identification of galaxies caught in the act of transition: either those thought to be in the final stage of a starburst (Barro et al. 2014; Nelson et al. 2014; van Dokkum et al. 2015), or galaxies that have recently quenched their star formation (Wild et al. 2009; Whitaker et al. 2012; Pattarakijwanich et al. 2014). One such class of transition galaxies are post-starburst (PSB) galaxies, which are identified by their (temporally resolved) recent star formation history. The presence of strong Balmer absorption lines, or a significant Balmer break, in their spectra indicates an increased fraction of A/F stars. When these spectral features are strong enough, simulations show that this implies a recent, short and strong burst of star formation, followed by rapid truncation (Wild et al. 2009, hereafter WWJ09; Snyder et al. 2011); weaker features may indicate truncation alone. At low redshift, PSB galaxies have been linked to merger events (Zabludoff et al. 1996; Blake et al. 2004; Goto 2005; Yang et al. 2008; Pawlik et al. 2016), and recently low-redshift PSB galaxies have been found to have significant residual cold gas reservoirs (Zwaan et al. 2013; French et al. 2015; Rowlands et al. 2015). Together this implies that multiple merger events may be required to fully quench star formation in low-redshift star-forming galaxies (Rowlands et al. 2015). The formation of quiescent galaxies in the present-day Universe may well follow a slower, less dramatic route than at high redshift where higher gas fractions and plentiful external gas supplies will lead to less stable discs, stronger starbursts and AGN accretion, and stronger associated outflows. Measuring the evolution in the number density, properties and (...truncated)