Gradients in the stellar populations (SP) of galaxies – e.g., in age, metallicity, stellar Initial Mass Function (IMF) – can result in gradients in the stellar mass to light ratio, M ∗ /L . Such gradients imply that the distribution of the stellar mass and light are diﬀerent. For old SPs, e.g., in early-type galaxies at z ∼ 0, the M ∗ /L gradients are weak if driven by variations in age and metallicity, but signiﬁcantly larger if driven by the IMF. A gradient which has larger M ∗ /L in the center increases the estimated total stellar mass ( M ∗ ) and reduces the scale which contains half this mass ( R e, ∗ ), compared to when the gradient is ignored. For the IMF gradients inferred from ﬁtting MILES simple SP models to the H β , (cid:104) Fe (cid:105) , [MgFe] and TiO 2SDSS absorption lines measured in spatially resolved spectra of early-type galaxies in the MaNGA survey, the fractional change in R e, ∗ can be signiﬁcantly larger than that in M ∗ , especially when the light is more centrally concentrated. The R e, ∗ − M ∗ correlation which results is oﬀset by 0.3 dex to smaller sizes compared to when these gradients are ignored. Comparisons with ‘quiescent’ galaxies at higher- z must account for evolution in SP gradients (especially age and IMF) and the light proﬁle before drawing conclusions about how R e, ∗ and M ∗ evolve. The implied merging between higher- z and the present is less contrived if R e, ∗ /R e at z ∼ 0 is closer to our IMF-driven gradient calibration than to unity.

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The half mass radius of MaNGA galaxies: Effect of IMF gradients

(Abridged) We study the galaxy mass-size relation in CARLA spectroscopically confirmed clusters at $1.4<z<2.8$, which span a total stellar mass $11.3<\mathrm{log}(M^c_*/M_{\odot})<12.6$ (halo mass $13.5 \lesssim \mathrm{log}(M^c_h/M_{\odot}) \lesssim 14.5$). Our main finding is that cluster passive ETG at $z \gtrsim 1.5$ with ${\rm log}(M/M_{\odot})>10.5$ are systematically $\gtrsim 0.2-0.3~{\rm dex}$ larger than field ETGs. The passive ETG average size evolution is slower at $1<z<2$ when compared to the field. This could be explained by differences in the formation and early evolution of galaxies in haloes of a different mass. Strong compaction and gas dissipation in field galaxies, followed by a sequence of mergers may have also played a significant role in the field ETG evolution, but not in the evolution of cluster galaxies. Our passive ETG mass-size relation shows a tendency to flatten at $9.6<{\rm log}(M/M_{\odot})<10.5$, where the average size is $\mathrm{log}(R_e/\mathrm{kpc}) = 0.05 \pm 0.22$. This implies that galaxies in the low end of the mass-size relation do not evolve much from $z\sim 2$ to the present, and that their sizes evolve in a similar way in clusters and in the field. BCGs lie on the same mass-size relation as satellites, suggesting that their size evolution is not different at redshift z $\gtrsim$ 2. Half of the active ETGs ($\sim 30\%$ of the ETGs) follow the field passive galaxy mass-size relation, and the other half follow the field active galaxy mass-size relation. These galaxies likely went through a recent merger or neighbor galaxy interaction, and would most probably quench at a later epoch and increase the fraction of passive ETGs in clusters. We do not observe a large population of compact galaxies, as is observed in the field at these redshifts, implying that the galaxies in our clusters are not observed in an epoch close to their compaction.

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The galaxy mass-size relation in CARLA clusters and proto-clusters at 1:4 < z < 2:8: Larger cluster galaxy sizes

In previous work we showed that a semi-empirical model in which galaxies in host dark matter haloes are assigned stellar masses via a stellar mass-halo mass (SMHM) relation and sizes ($R_e$) via a linear and tight $R_e-R_h$ relation, can faithfully reproduce the size function of local SDSS central galaxies and the strong size evolution of massive galaxies (MGs, $M_{\rm star}>10^{11.2}M_\odot$). In this third paper of the series, we focus on the population of satellite MGs. We find that without any additional calibration and irrespective of the exact SMHM relation, fraction of quenched galaxies or level of stellar stripping, the same model is able to reproduce the local size function of quiescent satellite MGs in SDSS. In addition, the same model can reproduce the puzzling weak dependence of mean size on host halo mass for both central and satellite galaxies. The model also matches the size function of starforming satellite MGs, after assuming that some of them transform into massive lenticulars in a few Gyr after infalling in the group/cluster environment. However, the vast majority of satellite lenticulars is predicted to form before infall. The $R_e-R_h$ appears to be fundamental to connect galaxies and their host haloes.

The size function of massive satellites from the R e -R h and M star -M h relations: constraining the role of environment

The co-evolution between central supermassive black holes (BH), their host galaxies, and dark matter halos is still a matter of intense debate. Present theoretical models suffer from large uncertainties and degeneracies, for example, between the fraction of accreting sources and their characteristic accretion rate. In recent work we showed that Active Galactic Nuclei (AGN) clustering represents a powerful tool to break degeneracies when analysed in terms of mean BH mass, and that AGN bias at fixed stellar mass is largely independent of most of the input parameters, such as the AGN duty cycle and the mean scaling between BH mass and host galaxy stellar mass. In this paper we take advantage of our improved semi-empirical methodology and recent clustering data derived from large AGN samples at $z \sim 1.2$, demonstrate that the AGN bias as a function of host galaxy stellar mass is a crucial diagnostic of the BH--galaxy connection, and is highly dependent on the scatter around the BH mass--galaxy mass scaling relation and on the relative fraction of satellite and central active BHs. Current data at $z \sim 1.2$ favour relatively high values of AGN in satellites, pointing to a major role of disc instabilities in triggering AGN, unless a high minimum host halo mass is assumed. The data are not decisive on the magnitude/covariance of the BH-galaxy scatter at $z \sim 1.2$ and intermediate host masses $M_\mathrm{star} \lesssim 10^{11} \,\mathrm{M}_\odot$. However, future surveys like Euclid/LSST will be pivotal in shedding light on the BH--galaxy co-evolution.

The role of scatter and satellites in shaping the large-scale clustering of X-ray AGN as a function of host galaxy stellar mass

We use surveys covering the redshift range $0.05 < z < 3.8$ to explore quiescent galaxy scaling relations and the redshift evolution of the velocity dispersion, size, and dynamical mass at fixed stellar mass. For redshift $z < 0.6$ we derive mass limited samples and demonstrate that these large samples enhance constraints on the evolution of the quiescent population. The constraints include 2985 new velocity dispersions from the SHELS F2 survey (Geller et al. 2014). In contrast with the known substantial evolution of size with redshift, evolution in the velocity dispersion is negligible. The dynamical-to-stellar mass ratio increases significantly as the universe ages, in agreement with recent results that combine high redshift data with the SDSS. Like other investigators, we interpret this result as an indication that the dark matter fraction within the effective radius increases as a result of the impact of the minor mergers that are responsible for size growth. We emphasize that dense redshift surveys covering the range $0.07 < z < 1$ along with strong and weak lensing measurements could remove many ambiguities in evolutionary studies of the quiescent population. 

Quiescent Galaxy Size, Velocity Dispersion, and Dynamical Mass Evolution

The mean size (effective radius Re) of massive galaxies (MGs; Mstar > 1011.2M⊙) is observed to increase steadily with cosmic time. It is still unclear whether this trend originates from the size growth of individual galaxies (via, e.g. mergers and/or AGN feedback) or from the inclusion of larger galaxies entering the selection at later epochs (progenitor bias). We here build a data-driven, flexible theoretical framework to probe the structural evolution of MGs. We assign galaxies to dark matter haloes via stellar mass–halo mass (SMHM) relations with varying high-mass slopes and scatters σSMHM in stellar mass at fixed halo mass, and assign sizes to galaxies using an empirically motivated, constant and linear relationship between Re and the host dark matter halo radius Rh. We find that (1) the fast mean size growth of MGs is well reproduced independently of the shape of the input SMHM relation; (2) the numbers of compact MGs grow steadily until z ≳ 2 and fall off at lower redshifts, suggesting a lesser role of progenitor bias at later epochs; (3) a time-independent scatter σSMHM is consistent with a scenario in which compact star-forming MGs transition into quiescent MGs in a few 108 yr with a negligible structural evolution during the compact phase, while a scatter increasing at high redshift implies significant size growth during the star-forming phase. A robust measurement of the size function of MGs at high redshift can set strong constraints on the scatter of the SMHM relation and, by extension, on models of galaxy evolution.

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H. Fu

Papers

The evolution of compact massive quiescent and star-forming galaxies derived from the Re–Rh and Mstar–Mh relations