The Progenitors of the Compact Early-Type Galaxies at High Redshift

Abstract

We use GOODS and CANDELS images to identify progenitors of massive (M > 10 10 M ⊙ ) compact "early-type" galaxies (ETGs) at z ∼ 1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z ∼ 3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z ∼ 1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z ∼ 1.6.

Morphology Dependence of Stellar Age in Quenched Galaxies at Redshift ∼1.2:Massive Compact Galaxies Are Older than More Extended Ones

“…Over the last several years, efforts have been made to identify thestar-forming progenitors of soon-to-be QGs at z>2. These efforts have relied on the fortuitous observation that the first galaxies to quench are compact, making it relatively easy to identify their immediate star-forming progenitors among compact star-forming galaxies (Barro et al 2013;Patel et al 2013;Stefanon et al 2013;Nelson et al 2014;Williams et al 2014;van Dokkum et al 2015). It is interesting to note that various studies have come to very different conclusions about the nature of feedback in compact star-forming galaxies.…”

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…At z 2  , SFGs with the highest central densities are remarkably compact and have high Sérsic indices and spheroidal morphologies, lacking any signature of an underlying disk (Wuyts et al 2011b;Barro et al 2013Barro et al , 2014aBarro et al , 2014bPatel et al 2013;Stefanon et al 2013;Nelson et al 2014;Williams et al 2014). These galaxies resemble the quiescent population at the same redshift but are radically different from other SFGs that have irregular and clumpy appearances (Elmegreen et al 2004;Genzel et al 2008;Guo et al 2015).…”

Evolution of galaxy shapes from prolate to oblate through compaction events

“…This leads to an inside-out quenching process into a compact, passive "red nugget", the likely progenitor of the centre of an early-type galaxy today. The observational basis for this picture is becoming solid, both for the red-nugget phenomenon (Trujillo et al 2006a,b;van Dokkum et al 2008;Damjanov et al 2009Damjanov et al , 2011Newman et al 2010;Bruce et al 2012;Whitaker et al 2012;van Dokkum et al 2014) and for their potential bluenugget progenitors (Barro et al 2013(Barro et al , 2014aBruce et al 2014;Nelson et al 2014;Williams et al 2014;Tacchella et al 2015a). The theory, partly based on the same simulations that we analyse here, is also becoming robust (Dekel & Burkert 2014;Tacchella et al 2015b,c;Zolotov et al 2015).…”