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Register a new userHST/WFC3 grism observations of $z~mathtt{sim}~1$ clusters: The cluster vs. field stellar mass-size relation and evidence for size growth of quiescent galaxies from minor mergers
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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.
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Minor mergers have been proposed as the driving mechanism for the size growth of quiescent galaxies with decreasing redshift. The process whereby large star-forming galaxies quench and join the quiescent population at the large size end has also been suggested as an explanation for this size growth. Given the clear association of quenching with clusters, we explore this mechanism by studying the structural properties of 23 spectroscopically identified recently quenched (or poststarburst (PSB)) cluster galaxies at $zsim1$. Despite clear PSB spectral signatures implying rapid and violent quenching, 87% of these galaxies have symmetric, undisturbed morphologies in the stellar continuum. Remarkably, they follow a mass-size relation lying midway between the star-forming and quiescent field relations, with sizes $0.1$ dex smaller than $zsim1$ star-forming galaxies at log$(M_{*}/M_{odot})=10.5$. This implies a rapid change in the light profile without directly effecting the stellar distribution, suggesting changes in the mass-to-light ratio gradients across the galaxy are responsible. We develop fading toy models to explore how star-forming galaxies move across the mass-size plane as their stellar populations fade to match those of the PSBs. Outside-in fading has the potential to reproduce the contraction in size and increase in bulge-dominance observed between star-forming and PSB cluster galaxies. Since cluster PSBs lie on the large size end of the quiescent mass-size relation, and our previous work shows cluster galaxies are smaller than field galaxies, the sizes of quiescent galaxies must grow both from the quenching of star-forming galaxies and dry minor mergers.
We perform a comprehensive study of the stellar population properties of quiescent galaxies as a function of size and stellar mass to constrain the physical mechanism governing the stellar mass assembly and the likely evolutive scenarios that explain their growth in size. After selecting all the quiescent galaxies from the ALHAMBRA survey by the dust-corrected stellar mass$-$colour diagram, we built a shared sample of $sim850$ quiescent galaxies with reliable sizes from the HST. The stellar population properties were retrieved using the SED-fitting code MUFFIT with various sets of composite stellar population models. Age, formation epoch, metallicity, and extinction were studied on the stellar mass$-$size plane as function of size through a Monte Carlo approach. This accounted for uncertainties and degeneracy effects amongst stellar population properties. The stellar population properties of quiescent galaxies and their stellar mass and size since $zsim1$ are correlated. At fixed stellar mass, the more compact the quiescent galaxy, the older and richer in metals it is ($1$Gyr and $0.1$dex, respectively). In addition, more compact galaxies may present slight lower extinctions than their more extended counterparts at the same stellar mass ($<0.1$ mag). By means of studying constant regions of stellar population properties across the stellar mass$-$size plane, we obtained empirical relations to constrain the physical mechanism that governs the stellar mass assembly of the form $M_star propto r_mathrm{c}^alpha$, where $alpha$ amounts to $0.50-0.55 pm 0.09$. There are indications that support the idea that the velocity dispersion is tightly correlated with the stellar content of galaxies. The mechanisms driving the evolution of stellar populations can therefore be partly linked to the dynamical properties of galaxies, along with their gravitational potential.
Dust-enshrouded, starbursting, submillimeter galaxies (SMGs) at $z geq 3$ have been proposed as progenitors of $z geq 2$ compact quiescent galaxies (cQGs). To test this connection, we present a detailed spatially resolved study of the stars, dust and stellar mass in a sample of six submillimeter-bright starburst galaxies at $z sim 4.5$. The stellar UV emission probed by HST is extended, irregular and shows evidence of multiple components. Informed by HST, we deblend Spitzer/IRAC data at rest-frame optical finding that the systems are undergoing minor mergers, with a typical stellar mass ratio of 1:6.5. The FIR dust continuum emission traced by ALMA locates the bulk of star formation in extremely compact regions (median $r_{rm{e}} = 0.70 pm 0.29$ kpc) and it is in all cases associated with the most massive component of the mergers (median $log (M_{*}/M_{odot}) = 10.49 pm 0.32$). We compare spatially resolved UV slope ($beta$) maps with the FIR dust continuum to study the infrared excess ($rm{IRX} = L_{rm{IR}}/L_{rm{UV}}$)-$beta$ relation. The SMGs display systematically higher $rm{IRX}$ values than expected from the nominal trend, demonstrating that the FIR and UV emissions are spatially disconnected. Finally, we show that the SMGs fall on the mass-size plane at smaller stellar masses and sizes than cQGs at $z = 2$. Taking into account the expected evolution in stellar mass and size between $z = 4.5$ and $z = 2$ due to the ongoing starburst and mergers with minor companions, this is in agreement with a direct evolutionary connection between the two populations.
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.
We present and publicly release (https://www.gclasshst.com) the first spatially resolved H$alpha$ maps of star-forming cluster galaxies at $zsim1$, made possible with the Wide Field Camera 3 (WFC3) G141 grism on the Hubble Space Telescope (HST). Using a similar but updated method to 3D-HST in the field environment, we stack the H$alpha$ maps in bins of stellar mass, measure the half-light radius of the H$alpha$ distribution and compare it to the stellar continuum. The ratio of the H$alpha$ to stellar continuum half-light radius, $R[mathrm{H}alpha/mathrm{C}]=frac{R_{mathrm{eff, H}alpha}}{R_{mathrm{eff, Cont}}}$, is smaller in the clusters by $(6pm9)%$, but statistically consistent within $1sigma$ uncertainties. A negligible difference in $R[mathrm{H}alpha/mathrm{C}]$ with environment is surprising, given the higher quenched fractions in the clusters relative to the field. We postulate that the combination of high quenched fractions and no change in $R[mathrm{H}alpha/mathrm{C}]$ with environment can be reconciled if environmental quenching proceeds rapidly. We investigate this hypothesis by performing similar analysis on the spectroscopically-confirmed recently quenched cluster galaxies. 87% have H$alpha$ detections, with star formation rates $8pm1$ times lower than star-forming cluster galaxies of similar stellar mass. Importantly, these galaxies have a $R[mathrm{H}alpha/mathrm{C}]$ that is $(81pm8)%$ smaller than coeval star-forming field galaxies at fixed stellar mass. This suggests the environmental quenching process occurred outside-in. We conclude that disk truncation due to ram-pressure stripping is occurring in cluster galaxies at $zsim1$, but more rapidly and/or efficiently than in $zlesssim0.5$ clusters, such that the effects on $R[mathrm{H}alpha/mathrm{C}]$ become observable just after the cluster galaxy has recently quenched.
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