The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

Abstract

We present the first large‐scale study of E+A galaxies that incorporates photometry in the ultraviolet (UV) wavelengths. E+A galaxies are ‘post‐starburst’ systems, with strong Balmer absorption lines indicating significant recent star formation, but without [O ii] and Hα emission lines which are characteristic of ongoing star formation. The starburst that creates the E+A galaxy typically takes place within the last Gyr and creates a high fraction (20–60 per cent) of the stellar mass in the remnant over a short time‐scale (<0.1 Gyr). We find a tight correlation between the luminosity of our E+A galaxies and the implied star formation rate (SFR) during the starburst. While low‐luminosity E+As [M(z) > −20] exhibit implied SFRs of less than 50 M⊙ yr−1, their luminous counterparts [M(z) < −22] show SFRs greater than 300 and as high as 2000 M⊙ yr−1, suggesting that luminous and ultra‐luminous infrared galaxies in the low‐redshift Universe could be the progenitors of massive nearby E+A galaxies. We perform a comprehensive study of the characteristics of the quenching that truncates the starburst in E+A systems. We find that, for galaxies less massive than 1010 M⊙, the quenching efficiency decreases as the galaxy mass increases. However, for galaxies more massive than 1010 M⊙, this trend is reversed and the quenching efficiency increases with galaxy mass. Noting that the mass threshold at which this reversal occurs is in excellent agreement with the mass above which active galactic nuclei (AGN) become significantly more abundant in nearby galaxies, we use simple energetic arguments to show that the bimodal behaviour of the quenching efficiency is consistent with AGN and supernovae (SN) being the principal sources of negative feedback above and below M∼ 1010 M⊙, respectively. The arguments assume that quenching occurs through the mechanical ejection or dispersal of the gas reservoir and that, in the high‐mass regime (M > 1010 M⊙), the Eddington ratios in this sample of galaxies scale as Mγ, where 1 < γ < 3. Finally, we use our E+A sample to estimate the time it takes for galaxies to migrate from the blue cloud to the red sequence. We find migration times between 1 and 5 Gyr, with a median value of 1.5 Gyr.

Evidence of ongoing AGN-driven feedback in a quiescent post-starburst E+A galaxy

“…5.1.1 Mass outflow rate E+A galaxies are thought to be systems where a significant star formation episode has been quenched abruptly (Goto 2004;Yang et al 2004Yang et al , 2006Kaviraj et al 2007;Cales et al 2013;Cales & Brotherton 2015). Some studies suggest an evolutionary path in which gas-rich major mergers result in ULIRGs, which then evolve to become post starburst galaxies (e.g., Kaviraj et al 2007, Cales & Brotherton 2015, Wild et al 2016.…”

“…In this scenario, a gas-rich major merger triggers a powerful starburst, which can be observed as an ultra luminous infrared galaxy (ULIRG), with SFRs ranging between 100 and 1000 M /yr. The system then evolves into a post starburst galaxy (e.g., Kaviraj et al 2007;Cales & Brotherton 2015) which, in turn, evolves to a quiescent elliptical galaxy (Yang et al 2004(Yang et al , 2006Wild et al 2009Wild et al , 2016. Yesuf et al (2014), Rowlands et al (2015), and Alatalo et al (2016b) studied the dust and molecular gas content in samples of E+A galaxies and generally found significantly more molecular gas and dust, compared to quiescent galaxies.…”

“…Such systems show little or no contribution from younger stars (O and B type), with evidence for an abrupt termination of the star formation (SF) process Poggianti et al 1999;Goto 2004;Dressler et al 2004). The estimated star formation rates (SFRs) during the starburst range from 50 to 300 M /yr (Poggianti & Wu 2000;Kaviraj et al 2007), and the mass fractions forming in the burst are very high, 30-80% of the total stellar mass in the galaxy (Liu & Green 1996;Bressan, Poggianti & Franceschini 2001;Norton et al 2001;Yang et al 2004;Kaviraj et al 2007). …”

“…Various studies suggest that post starburst galaxies provide an evolutionary link between the gas-rich star forming population ('blue cloud') and gas poor quiescent galaxies ('red sequence'; e.g., Yang et al 2004Yang et al , 2006Kaviraj et al 2007;Wild et al 2009;Cales et al 2011Cales et al , 2013Yesuf et al 2014;Cales & Brotherton 2015;Alatalo et al 2016a,b;Wild et al 2016). In this scenario, a gas-rich major merger triggers a powerful starburst, which can be observed as an ultra luminous infrared galaxy (ULIRG), with SFRs ranging between 100 and 1000 M /yr.…”

“…Cales & Brotherton (2015) found that the more luminous PSQs (those hosted by ellipticals and earlytypes) display similar spectral properties to post starburst galaxies, but with older starburst ages. Kaviraj et al (2007) performed the first comprehensive observational and theoretical study of the quenching process that terminates the starburst in E+A galaxies. They found a clear bimodal behaviour.…”