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Register a new userQuiescent galaxies in a virialized cluster at redshift 2: Evidence for accelerated size-growth
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We present an analysis of the galaxy population in XLSSC 122, an X-ray selected, virialized cluster at redshift $z=1.98$. We utilize HST WFC3 photometry to characterize the activity and morphology of spectroscopically confirmed cluster members. The quiescent fraction is found to be $88^{+4}_{-20}$ per cent within 0.5$r_{500}$, significantly enhanced over the field value of $20^{+2}_{-2}$ per cent at $zsim2$. We find an excess of bulge-like quiescent cluster members with Sersic index $n>2$ relative to the field. These galaxies are found to be larger than their field counterparts at 99.6 per cent confidence, being on average $63^{+31}_{-24}$ per cent larger at a fixed mass of $M_star = 5times10^{10} M_odot$. This suggests that these cluster member galaxies have experienced an accelerated size evolution relative to the field at $z>2$. We discuss minor mergers as a possible mechanism underlying this disproportionate size growth.
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The origin of the correlations between mass, morphology, quenched fraction, and formation history in galaxies is difficult to define, primarily due to the uncertainties in galaxy star-formation histories. Star-formation histories are better constrained for higher redshift galaxies, observed closer to their formation and quenching epochs. Here we use non-parametric star-formation histories and a nested sampling method to derive constraints on the formation and quenching timescales of quiescent galaxies at $0.7<z<2.5$. We model deep HST grism spectroscopy and photometry from the CLEAR (CANDELS Lyman$-alpha$ Emission at Reionization) survey. The galaxy formation redshifts, $z_{50}$ (defined as the point where they had formed 50% of their stellar mass) range from $z_{50}sim 2$ (shortly prior to the observed epoch) up to $z_{50} simeq 5-8$. editone{We find that early formation redshifts are correlated with high stellar-mass surface densities, $log Sigma_1 / (M_odot mathrm{kpc}^{-2}) >$10.25, where $Sigma_1$ is the stellar mass within 1~pkpc (proper kpc). Quiescent galaxies with the highest stellar-mass surface density, $logSigma_1 / (M_odot mathrm{kpc}^{-2}) > 10.25$, } show a textit{minimum} formation redshift: all such objects in our sample have $z_{50} > 2.9$. Quiescent galaxies with lower surface density, $log Sigma_1 / (M_odot mathrm{kpc}^{-2}) = 9.5 - 10.25$, show a range of formation epochs ($z_{50} simeq 1.5 - 8$), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold $logSigma_1/(M_odot mathrm{kpc}^{-2})>10.25$ uniquely identifies galaxies that formed in the first few Gyr after the Big Bang, and we discuss the implications this has for galaxy formation models.
Minor mergers are thought to be responsible for the size growth of quiescent field galaxies with decreasing redshift. We test this hypothesis using the cluster environment as a laboratory. Satellite galaxies in clusters move at high velocities, making mergers between them rare. The stellar mass-size relation in ten clusters and in the field is measured and compared at $z~mathtt{sim}~1$. Our cluster sample contains 344 spectroscopically-confirmed cluster members with Gemini/GMOS and 182 confirmed with HST WFC3 G141 grism spectroscopy. On average, quiescent and star-forming cluster galaxies are smaller than their field counterparts by ($0.08pm0.04$) dex and ($0.07pm0.01$) dex respectively. These size offsets are consistent with the average sizes of quiescent and star-forming field galaxies between $1.2leqslant zleqslant1.5$, implying the cluster environment has inhibited size growth between this period and $z~mathtt{sim}~1$. The negligible differences measured between the $z~mathtt{sim}~0$ field and cluster quiescent mass-size relations in other works imply that the average size of quiescent cluster galaxies must rise with decreasing redshift. Using a toy model, we show that the disappearance of the compact cluster galaxies might be explained if, on average, $mathtt{sim}40%$ of them merge with their brightest cluster galaxies (BCGs) and $mathtt{sim}60%$ are tidally destroyed into the intra-cluster light (ICL) between $0leqslant zleqslant1$. This is in agreement with the observed stellar mass growth of BCGs between $0leqslant zleqslant1$ and the observed ICL stellar mass fraction at $z~mathtt{sim}~0$. Our results support minor mergers as the cause for the size growth in quiescent field galaxies, with cluster-specific processes responsible for the similarity between the field and cluster quiescent mass-size relations at low redshift.
It is not well understood whether the growth of early-type cluster galaxies proceeds inside-out, outside-in, or at the same pace at all radii. In this work we measured the galaxy size, defined by the radius including 80% of the galaxy light, non-parametrically. We also determined a non-parametric estimate of galaxy light concentration, which measures the curvature of the surface brightness profile in the galaxy outskirts. We used an almost random sampling of a mass-limited sample formed by 128 morphologically early-type galaxies in clusters with $log M/M_{odot} ga 10.7$ spanning the wide range $0.17<z<1.81$. From these data we derived the size-mass and concentration-mass relations, as well as their evolution. At 80% light radius, early-type galaxies in clusters are about 2.7 times larger than at 50% radius at all redshifts, and close to de Vaucouleurs profiles in the last 10 Gyr. While between $z=2$ and $z=0$ both half-light and 80% light sizes increase by a factor of $1.7$, concentration stays constant within $2$%, that is to say the size growth of early-type galaxies in cluster environments proceeds at the same pace at both radii. Existing physical explanations proposed in the literature are inconsistent with our results, demonstrating the need for dedicated numerical simulations to identify the physical mechanism affecting the galaxy structure.
We examine the fraction of massive ($M_{*}>10^{10} M_{odot}$), compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at $zsim2$. These cSFGs are likely the direct progenitors of the compact quiescent galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that $39.2^{+3.9}_{-3.6}$% (65/166) of cSFGs at $1.4<z<3.0$ host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the $6.2sigma$ level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive, quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.
We quantify the presence of Active Galactic nuclei (AGN) in a mass-complete (M_* >5e10 M_sun) sample of 123 star-forming and quiescent galaxies at 1.5 < z < 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41+/-7% of the galaxies are detected directly in X-rays, 22+/-5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGN (L_0.5-8keV > 3e42 ergs/s). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGN are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGN. Among the quiescent galaxies, the excess suggests that as many as 70-100% of these contain low- or high-luminosity AGN, while the corresponding fraction is lower among star-forming galaxies (43-65%). The ubiquitous presence of AGN in massive, quiescent z ~ 2 galaxies that we find provides observational support for the importance of AGN in impeding star formation during galaxy evolution.
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