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Register a new userMorphological evolution in situ: Disk-dominated cluster red sequences at z ~ 1.25
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R. De Propris
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We have carried out a joint photometric and structural analysis of red sequence galaxies in four clusters at a mean redshift of z ~ 1.25 using optical and near-IR HST imaging reaching to at least 3 magnitudes fainter than $M^*$. As expected, the photometry and overall galaxy sizes imply purely passive evolution of stellar populations in red sequence cluster galaxies. However, the morphologies of red sequence cluster galaxies at these redshifts show significant differences to those of local counterparts. Apart from the most massive galaxies, the high redshift red sequence galaxies are significantly diskier than their low redshift analogues. These galaxies also show significant colour gradients, again not present in their low redshift equivalents, most straightforwardly explained by radial age gradients. A clear implication of these findings is that red sequence cluster galaxies originally arrive on the sequence as disk-dominated galaxies whose disks subsequently fade or evolve secularly to end up as high Sersic index early-type galaxies (classical S0s or possibly ellipticals) at lower redshift. The apparent lack of growth seen in a comparison of high and low redshift red sequence galaxies implies that any evolution is internal and is unlikely to involve significant mergers. While significant star formation may have ended at high redshift, the cluster red sequence population continues to evolve (morphologically) for several Gyrs thereafter.
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Galaxies arrive on the red sequences of clusters at high redshift ($z>1$) once their star formation is quenched and evolve passively thereafter. However, we have previously found that cluster red sequence galaxies (CRSGs) undergo significant morphological evolution subsequent to the cessation of star formation, at some point in the past 9-10~Gyr. Through a detailed study of a large sample of cluster red sequence galaxies spanning $0.2<z<1.4$ we elucidate the details of this evolution. Below $z sim 0.5-0.6$ (in the last 5-6 Gyr) there is little or no morphological evolution in the population as a whole, unlike in the previous 4-5 Gyrs. Over this earlier time (i) disk-like systems with S{e}rsic $n < 2$ progressively disappear, as (ii) the range of their axial ratios similarly decreases, removing the most elongated systems (those consistent with thin disks seen at an appreciable inclination angle), and (iii) radial colour gradients (bluer outwards) decrease in an absolute sense from significant age-related gradients to a residual level consistent with the metallicity-induced gradients seen in low redshift cluster members. The distribution of their effective radii shows some evidence of evolution, consistent with growth of {it at most} a factor $<1.5$ between $zsim 1.4$ and $z sim 0.5$, significantly less than for comparable field galaxies, while the distribution of their central ($<1$kpc) bulge surface densities shows no evolution at least at $z<1$. A simple model involving the fading and thickening of a disk component after comparatively recent quenching (after $zsim 1.5$) around an otherwise passively evolving older spheroid component is consistent with all of these findings.
Substantial numbers of morphologically regular early-type (elliptical and lenticular) galaxies contain molecular gas, and the quantities of gas are probably sufficient to explain recent estimates of the current level of star formation activity. This gas can also be used as a tracer of the processes that drive the evolution of early-type galaxies. For example, in most cases the gas is forming dynamically cold stellar disks with sizes in the range of hundreds of pc to more than one kpc, although there is typically only 1% of the total stellar mass currently available to form young stars. The numbers are still small, but the molecular kinematics indicate that some of the gas probably originated from internal stellar mass loss while some was acquired from outside. Future studies will help to quantify the role of molecular gas (dissipational processes) in the formation of early-type galaxies and their evolution along the red sequence.
We use HST/WFC3 imaging to study the red population in the IR-selected, X-ray detected, low-mass cluster Cl J1449+0856 at z=2, one of the few bona-fide established clusters discovered at this redshift, and likely a typical progenitor of an average massive cluster today. This study explores the presence and significance of an early red sequence in the core of this structure, investigating the nature of red sequence galaxies, highlighting environmental effects on cluster galaxy populations at high redshift, and at the same time underlining similarities and differences with other distant dense environments. Our results suggest that the red population in the core of Cl J1449+0856 is made of a mixture of quiescent and dusty star-forming galaxies, with a seedling of the future red sequence already growing in the very central cluster region, and already characterising the inner cluster core with respect to lower density environments. On the other hand, the color-magnitude diagram of this cluster is definitely different from that of lower-redshift (z<1) clusters, as well as of some rare particularly evolved massive clusters at similar redshift, and it is suggestive of a transition phase between active star formation and passive evolution occurring in the proto-cluster and established lower-redshift cluster regimes.
The high abundance of disk galaxies without a large central bulge challenges predictions of current hydrodynamic simulations of galaxy formation. We aim to shed light on the formation of these objects by studying the redshift and mass dependence of their 3D shape distribution in the COSMOS galaxy survey. This distribution is inferred from the observed distribution of 2D shapes, using a reconstruction method which we test using hydrodynamic simulations. We find a moderate bias for the inferred average disk circularity and relative thickness with respect to the disk radius, but a large bias on the dispersion of these quantities. Applying the 3D shape reconstruction method on COSMOS data, we find no significant dependence of the inferred 3D shape distribution on redshift. The relative disk thickness shows a significant mass dependence which can be accounted for by the scaling of disk radius with galaxy mass. We conclude that the shapes of disk dominated galaxies are overall not subject to disruptive merging or feedback events below redshift $z=1.0$. This favours a scenario where these disks form early and subsequently undergo a tranquil evolution in isolation. In addition, our study shows that the observed 2D shapes of disk dominated galaxies can be well fitted using an ellipsoidal model for the galaxy 3D morphology combined with a Gaussian model for the 3D axes ratio distribution, confirming findings from similar work reported in the literature. Such an approach allows to build realistic mock catalogs with intrinsic galaxy shapes that will be essential for the study of intrinsic galaxy alignment as a contaminant of weak lensing surveys.
Recent simulations and observations of massive galaxy cluster evolution predict that the majority of stellar mass build up happens within cluster members by $z=2$, before cluster virialization. Protoclusters rich with dusty, star-forming galaxies (DSFGs) at $z>3$ are the favored candidate progenitors for these massive galaxy clusters at $zsim0$. We present here the first study analyzing stellar emission along with cold dust and gas continuum emission in a spectroscopically confirmed $z=4.002$ protocluster core rich with DSFGs, the Distant Red Core (DRC). We combine new HST and Spitzer data with existing Gemini, Herschel, and ALMA observations to derive individual galaxy-level properties, and compare them to coeval field and other protocluster galaxies. All of the protocluster members are massive ($>10^{10}$ M$_odot$), but not significantly more so than their coeval field counterparts. Within uncertainty, all are nearly indistinguishable from galaxies on the star-forming vs. stellar mass main-sequence relationship, and on the star formation efficiency plane. Assuming no future major influx of fresh gas, we estimate that these gaseous DSFGs will deplete their gas reservoirs in $sim300$ Myr, becoming the massive quiescent ellipticals dominating cluster cores by $zsim3$. Using various methodologies, we derive a total $z=4$ halo mass of $sim10^{14}$ M$_odot$, and estimate that the DRC will evolve to become an ultra-massive cluster core of mass $gtrsim10^{15}$ M$_odot$ by $z=0$.
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