We present Keck-I MOSFIRE spectroscopy in the Y and H bands of GDN-8231, a massive, compact, star-forming galaxy (SFG) at a redshift $zsim1.7$. Its spectrum reveals both H$_{alpha}$ and [NII] emission lines and strong Balmer absorption lines. The H$_
{alpha}$ and Spitzer MIPS 24 $mu$m fluxes are both weak, thus indicating a low star formation rate of SFR $lesssim5-10$ M$_{odot}$ yr$^{-1}$. This, added to a relatively young age of $sim700$ Myr measured from the absorption lines, provides the first direct evidence for a distant galaxy being caught in the act of rapidly shutting down its star formation. Such quenching allows GDN-8231 to become a compact, quiescent galaxy, similar to 3 other galaxies in our sample, by $zsim1.5$. Moreover, the color profile of GDN-8231 shows a bluer center, consistent with the predictions of recent simulations for an early phase of inside-out quenching. Its line-of-sight velocity dispersion for the gas, $sigma^{rm{gas}}_{!_{rm LOS}}=127pm32$ km s$^{-1}$, is nearly 40% smaller than that of its stars, $sigma^{star}_{!_{rm LOS}}=215pm35$ km s$^{-1}$. High-resolution hydro-simulations of galaxies explain such apparently colder gas kinematics of up to a factor of $sim1.5$ with rotating disks being viewed at different inclinations and/or centrally concentrated star-forming regions. A clear prediction is that their compact, quiescent descendants preserve some remnant rotation from their star-forming progenitors.
We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift $2leq z leq2.5$ with star formation rates of SFR$sim$100M$_{odot}$ y$^{-1}$ and masses of log(M/M$_{odot}$)$sim10.8$. Their high
integrated gas velocity dispersions of $sigma_{rm{int}}$=230$^{+40}_{-30}$ km s$^{-1}$, as measured from emission lines of H$_{alpha}$ and [OIII], and the resultant M$_{star}-sigma_{rm{int}}$ relation and M$_{star}$$-$M$_{rm{dyn}}$ all match well to those of compact quiescent galaxies at $zsim2$, as measured from stellar absorption lines. Since log(M$_{star}$/M$_{rm{dyn}}$)$=-0.06pm0.2$ dex, these compact SFGs appear to be dynamically relaxed and more evolved, i.e., more depleted in gas and dark matter ($<$13$^{+17}_{-13}$%) than their non-compact SFG counterparts at the same epoch. Without infusion of external gas, depletion timescales are short, less than $sim$300 Myr. This discovery adds another link to our new dynamical chain of evidence that compact SFGs at $zgtrsim2$ are already losing gas to become the immediate progenitors of compact quiescent galaxies by $zsim2$.
We analyze the star-forming and structural properties of 45 massive (log(M/Msun)>10) compact star-forming galaxies (SFGs) at 2<z<3 to explore whether they are progenitors of compact quiescent galaxies at z~2. The optical/NIR and far-IR Spitzer/Hersch
el colors indicate that most compact SFGs are heavily obscured. Nearly half (47%) host an X-ray bright AGN. In contrast, only about 10% of other massive galaxies at that time host AGNs. Compact SFGs have centrally-concentrated light profiles and spheroidal morphologies similar to quiescent galaxies, and are thus strikingly different from other SFGs. Most compact SFGs lie either within the SFR-M main sequence (65%) or below (30%), on the expected evolutionary path towards quiescent galaxies. These results show conclusively that galaxies become more compact before they lose their gas and dust, quenching star formation. Using extensive HST photometry from CANDELS and grism spectroscopy from the 3D-HST survey, we model their stellar populations with either exponentially declining (tau) star formation histories (SFHs) or physically-motivated SFHs drawn from semi-analytic models (SAMs). SAMs predict longer formation timescales and older ages ~2 Gyr, which are nearly twice as old as the estimates of the tau models. While both models yield good SED fits, SAM SFHs better match the observed slope and zero point of the SFR-M main sequence. Some low-mass compact SFGs (log(M/Msun)=10-10.6) have younger ages but lower sSFRs than that of more massive galaxies, suggesting that the low-mass galaxies reach the red sequence faster. If the progenitors of compact SFGs are extended SFGs, state-of-the-art SAMs show that mergers and disk instabilities are both able to shrink galaxies, but disk instabilities are more frequent (60% versus 40%) and form more concentrated galaxies. We confirm this result via high-resolution hydrodynamic simulations.
We investigate the causes of the different shape of the $K$-band number counts when compared to other bands, analyzing in detail the presence of a change in the slope around $Ksim17.5$. We present a near-infrared imaging survey, conducted at the 3.5m
telescope of the Calar Alto Spanish-German Astronomical Center (CAHA), covering two separated fields centered on the HFDN and the Groth field, with a total combined area of $sim0.27$deg$^{2}$ to a depth of $Ksim19$ ($3sigma$,Vega). We derive luminosity functions from the observed $K$-band in the redshift range [0.25-1.25], that are combined with data from the references in multiple bands and redshifts, to build up the $K$-band number count distribution. We find that the overall shape of the number counts can be grouped into three regimes: the classic Euclidean slope regime ($dlog N/dmsim0.6$) at bright magnitudes; a transition regime at intermediate magnitudes, dominated by $M^{ast}$ galaxies at the redshift that maximizes the product $phi^{ast}frac{dV_{c}}{dOmega}$; and an $alpha$ dominated regime at faint magnitudes, where the slope asymptotically approaches -0.4($alpha$+1) controlled by post-$M^{ast}$ galaxies. The slope of the $K$-band number counts presents an averaged decrement of $sim50%$ in the range $15.5<K<18.5$ ($dlog N/dmsim0.6-0.30$). The rate of change in the slope is highly sensitive to cosmic variance effects. The decreasing trend is the consequence of a prominent decrease of the characteristic density $phi^{ast}_{K,obs}$ ($sim60%$ from $z=0.5$ to $z=1.5$) and an almost flat evolution of $M^{ast}_{K,obs}$ (1$sigma$ compatible with $M^{ast}_{K,obs}=-22.89pm0.25$ in the same redshift range).
