Most present-day galaxies with stellar masses $geq10^{11}$ solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars at rates of hundreds solar masses per year.
It is debated how star formation ceased, on which timescales, and how this quenching relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with $sim1$ kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on timescales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores.
We present the analysis of HST $J$- and $H$-band imaging for 29 galaxies on the star-forming main sequence at $zsim2$, which have Adaptive Optics VLT SINFONI integral field spectroscopy from our SINS/zC-SINF program. The SINFONI H$alpha$ data resolve
the on-going star-formation and the ionized gas kinematics on scales of $1-2$ kpc; the near-IR images trace the galaxies rest-frame optical morphologies and distributions of stellar mass in old stellar populations at a similar resolution. The global light profiles of most galaxies show disk-like properties well described by a single Sersic profile with $nsim1$, with only $sim15%$ requiring a high $n>3$ Sersic index, all more massive than $10^{10}M_odot$. In bulge+disk fits, about $40%$ of galaxies have a measurable bulge component in the light profiles, with $sim15%$ showing a substantial bulge-to-total ratio $B/Tge0.3$. This is a lower limit to the frequency of $zsim2$ massive galaxies with a developed bulge component in stellar mass because it could be hidden by dust and/or outshined by a thick actively star-forming disk component. The galaxies rest-optical half-light radii range between $1-7$ kpc, with a median of 2.1 kpc, and lie slightly above the size-mass relation at these epochs reported in the literature. This is attributed to differences in sample selection and definitions of size and/or mass measurements. The $(u-g)_{rest}$ color gradient and scatter within individual $zsim2$ massive galaxies with $ge10^{11}M_odot$ are as high as in $z=0$ low-mass, late-type galaxies, and are consistent with the high star-formation rates of massive $zsim2$ galaxies being sustained at large galactocentric distances.
Gamma Ray Bursts (GRBs) and galaxies at high redshift represent complementary probes of the star formation history of the Universe. In fact, both the GRB rate and the galaxy luminosity density are connected to the underlying star formation. Here, we
combine a star formation model for the evolution of the galaxy luminosity function from z=0 to z=10 with a metallicity-dependent efficiency for GRB formation to simultaneously predict the comoving GRB rate. Our model sheds light on the physical origin of the empirical relation often assumed between GRB rate and luminosity density-derived star formation rate: Rgrb(z) = epsilon(z)*SFR_{obs}(z), with epsilon(z) (1+z)^{1.2}. At z<4, epsilon(z) is dominated by the effects of metallicity evolution in the GRB efficiency. Our best-fitting model only requires a moderate preference for low-metallicity, that is a GRB rate per unit stellar mass about four times higher for log(Z/Zsun)<-3 compared to log(Z/Zsun)>0. Models with total suppression of GRB formation at log(Z/Zsun)>0 are disfavored. At z>4, most of the star formation happens in low-metallicity hosts with nearly saturated efficiency of GRB production per unit stellar mass. However at the same epoch, galaxy surveys miss an increasing fraction of the predicted luminosity density because of flux limits, driving an accelerated evolution of epsilon(z) compared to the empirical power-law fit from lower z. Our findings are consistent with the non-detections of GRB hosts in ultradeep imaging at z>5, and point toward current galaxy surveys at z>8 only observing the top 15-20 % of the total luminosity density.
