Wide field imaging of distant clusters

Wide field imaging is key to understanding the build-up of distant clusters and their galaxy population. By focusing on the so far unexplored outskirts of clusters, where infalling galaxies first hit the cluster potential and the hot intracluster medium, we can help separate cosmological field galaxy evolution from that driven by environment. I present a selection of recent advancements in this area, with particular emphasis on Hubble Space Telescope wide field imaging, for its superior capability to deliver galaxy morphologies and precise shear maps of distant clusters.Comment: Invited contribution. To appear in "Outskirts of galaxy clusters: intense life in the suburbs", A. Diaferio et al. eds. 7 pages, 5 figures. Refereed versio

The Galaxy UV Luminosity Function Before the Epoch of Reionization

We present a model for the evolution of the galaxy ultraviolet (UV) luminosity function (LF) across cosmic time where star formation is linked to the assembly of dark matter halos under the assumption of a mass dependent, but redshift independent, efficiency. We introduce a new self-consistent treatment of the halo star formation history, which allows us to make predictions at $z>10$ (lookback time $\lesssim500$ Myr), when growth is rapid. With a calibration at a single redshift to set the stellar-to-halo mass ratio, and no further degrees of freedom, our model captures the evolution of the UV LF over all available observations ($0\lesssim z\lesssim10$). The significant drop in luminosity density of currently detectable galaxies beyond $z\sim8$ is explained by a shift of star formation toward less massive, fainter galaxies. Assuming that star formation proceeds down to atomic cooling halos, we derive a reionization optical depth $\tau = 0.056^{+0.007}_{-0.010}$, fully consistent with the latest Planck measurement, implying that the universe is fully reionized at $z=7.84^{+0.65}_{-0.98}$. In addition, our model naturally produces smoothly rising star formation histories for galaxies with $L\lesssim L_*$ in agreement with observations and hydrodynamical simulations. Before the epoch of reionization at $z>10$ we predict the LF to remain well-described by a Schechter function, but with an increasingly steep faint-end slope ($\alpha\sim-3.5$ at $z\sim16$). Finally, we construct forecasts for surveys with \JWST~and \WFIRST and predict that galaxies out to $z\sim14$ will be observed. Galaxies at $z>15$ will likely be accessible to JWST and WFIRST only through the assistance of strong lensing magnification.Comment: Accepted for publication in ApJ. 11 pages, 13 figures. Corrected mislabelled redshifts in Figure

Spheroids scaling relations over cosmic time

I report on recent measurements of two scaling relations of spheroids in the distant universe: the Fundamental Plane, and the relation between lensing velocity dispersion and stellar velocity dispersion. The joint analysis of the two scaling relations indicates that the most massive (above 10^11.5 solar masses) spheroids are consistent with no evolution since z~1 both in terms of star formation and internal structure. Furthermore their total mass density profile is on average well described by an isothermal sphere with no evidence for redshift evolution. At smaller masses the picture appears to be substantially different, as indicated by evidence for substantial recent star formation (as much as 20-40% of stellar mass formed since z~1), and by hints of a reduced dark matter content at smaller masses. A larger sample of lenses extending to velocity dispersions below 200 km/s, and to redshifts above >0.5 is needed to verify these trends.Comment: invited paper at the 235th IAU symposium Galaxy evolution across the Hubble Tim

Dry mergers and the formation of early-type galaxies: constraints from lensing and dynamics

Dissipationless (gas-free or "dry") mergers have been suggested to play a major role in the formation and evolution of early-type galaxies, particularly in growing their mass and size without altering their stellar populations. We perform a new test of the dry merger hypothesis by comparing N-body simulations of realistic systems to empirical constraints provided by recent studies of lens early-type galaxies. We find that major and minor dry mergers: i) preserve the nearly isothermal structure of early-type galaxies within the observed scatter; ii) do not change more than the observed scatter the ratio between total mass M and "virial" mass R_e*sigma/2G (where R_e is the half-light radius and sigma the projected velocity dispersion); iii) increase strongly galaxy sizes [as M^(0.85+/-0.17)] and weakly velocity dispersions [as M^(0.06+/-0.08)] with mass, thus moving galaxies away from the local observed M-R_e and M-sigma relations; iv) introduce substantial scatter in the M-R_e and M-sigma relations. Our findings imply that, unless there is a high degree of fine tuning of the mix of progenitors and types of interactions, present-day massive early-type galaxies cannot have assembled more than ~50% of their mass, and increased their size by more than a factor ~1.8, via dry merging.Comment: ApJ, accepted. 16 pages, 11 figure