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Register a new userResolving Quiescent Galaxies at $z gtrsim 2$: II. Direct Measures of Rotational Support
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Stellar kinematics provide insights into the masses and formation histories of galaxies. At high redshifts, spatially resolving the stellar kinematics of quiescent galaxies is challenging due to their compact sizes. Using deep near-infrared spectroscopy, we have measured the resolved stellar kinematics of four quiescent galaxies at z=1.95-2.64, introduced in Paper I, that are gravitationally lensed by galaxy clusters. Analyses of two of these have previously been reported individually by Newman et al. and Toft et al., and for the latter we present new observations. All four galaxies show significant rotation and can be classified as fast rotators. In the three systems for which the lensing constraints permit a reconstruction of the source, we find that all are likely to be highly flattened (intrinsic ellipticities of $approx0.75-0.85$) disk-dominated galaxies with rapid rotation speeds of $V_{rm max}=290-352$ km/s and predominantly rotational support, as indicated by the ratio $(V/sigma)_{R_e}=1.7-2.3$. Compared to coeval star-forming galaxies of similar mass, the quiescent galaxies have smaller $V/sigma$. Given their high masses $M_{rm dyn} gtrsim 2times10^{11} M_{odot}$, we argue that these galaxies are likely to evolve into slow rotator elliptical galaxies whose specific angular momentum is reduced by a factor of 5-10. This provides strong evidence for merger-driven evolution of massive galaxies after quenching. Consistent with indirect evidence from earlier morphological studies, our small but unique sample suggests that the kinematic transformations that produced round, dispersion-supported elliptical galaxies were not generally coincident with quenching. Such galaxies probably emerged later via mergers that increased their masses and sizes while also eroding their rotational support.
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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 present stellar rotation curves and velocity dispersion profiles for 104 quiescent galaxies at $z=0.6-1$ from the Large Early Galaxy Astrophysics Census (LEGA-C) spectroscopic survey. Rotation is typically probed across 10-20kpc, or to an average of 2.7${rm R_e}$. Combined with central stellar velocity dispersions ($sigma_0$) this provides the first determination of the dynamical state of a sample selected by a lack of star formation activity at large lookback time. The most massive galaxies ($M_{star}>2times10^{11},M_{odot}$) generally show no or little rotation measured at 5kpc ($|V_5|/sigma_0<0.2$ in 8 of 10 cases), while ${sim}64%$ of less massive galaxies show significant rotation. This is reminiscent of local fast- and slow-rotating ellipticals and implies that low- and high-redshift quiescent galaxies have qualitatively similar dynamical structures. We compare $|V_5|/sigma_0$ distributions at $zsim0.8$ and the present day by re-binning and smoothing the kinematic maps of 91 low-redshift quiescent galaxies from the CALIFA survey and find evidence for a decrease in rotational support since $zsim1$. This result is especially strong when galaxies are compared at fixed velocity dispersion; if velocity dispersion does not evolve for individual galaxies then the rotational velocity at 5kpc was an average of ${94pm22%}$ higher in $zsim0.8$ quiescent galaxies than today. Considering that the number of quiescent galaxies grows with time and that new additions to the population descend from rotationally-supported star-forming galaxies, our results imply that quiescent galaxies must lose angular momentum between $zsim1$ and the present, presumably through dissipationless merging, and/or that the mechanism that transforms star-forming galaxies also reduces their rotational support.
We examine the Fundamental Plane (FP) and mass-to-light ratio ($M/L$) scaling relations using the largest sample of massive quiescent galaxies at $1.5<z<2.5$ to date. The FP ($r_{e}, sigma_{e}, I_{e}$) is established using $19$ $UVJ$ quiescent galaxies from COSMOS with $Hubble$ $Space$ $Telescope$ $(HST)$ $H_{F160W}$ rest-frame optical sizes and X-shooter absorption line measured stellar velocity dispersions. For a very massive, ${rm{log}}(M_{ast}/M_{odot})>11.26$, subset of 8 quiescent galaxies at $z>2$, from Stockmann et al. (2020), we show that they cannot passively evolve to the local Coma cluster relation alone and must undergo significant structural evolution to mimic the sizes of local massive galaxies. The evolution of the FP and $M/L$ scaling relations, from $z=2$ to present-day, for this subset are consistent with passive aging of the stellar population and minor merger structural evolution into the most massive galaxies in the Coma cluster and other massive elliptical galaxies from the MASSIVE Survey. Modeling the luminosity evolution from minor merger added stellar populations favors a history of merging with dry quiescent galaxies.
We present ALMA CO(2-1) spectroscopy of 6 massive (log$_{10}$M$_{rm{*}}/rm{M}_odot>$11.3) quiescent galaxies at $zsim1.5$. These data represent the largest sample using CO emission to trace molecular gas in quiescent galaxies above $z>1$, achieving an average 3$sigma$ sensitivity of M$_{rm{H_{2}}}sim10^{10}rm{M}_odot$. We detect one galaxy at 4$sigma$ significance and place upper limits on the molecular gas reservoirs of the other 5, finding molecular gas mass fractions M$_{rm{H_{2}}}$/M$_{rm{*}}$=f$_{rm{H_{2}}}<2-6$% (3$sigma$ upper limits). This is 1-2 orders of magnitude lower than coeval star-forming galaxies at similar stellar mass, and comparable to galaxies at $z=0$ with similarly low sSFR. This indicates that their molecular gas reservoirs were rapidly and efficiently used up or destroyed, and that gas fractions are uniformly low ($<$6%) despite the structural diversity of our sample. The implied rapid depletion time of molecular gas (t$_{rm{dep}}<0.6$ Gyr) disagrees with extrapolations of empirical scaling relations to low sSFR. We find that our low gas fractions are instead in agreement with predictions from both the recent SIMBA cosmological simulation, and from analytical bathtub models for gas accretion onto galaxies in massive dark matter halos (log$_{10}M_{rm{halo}}/rm{M}_odotsim14$ at $z=0$). Such high mass halos reach a critical mass of log$_{10}M_{rm{halo}}/rm{M}_odot>12$ by $zsim4$ that halt the accretion of baryons early in the Universe. Our data is consistent with a simple picture where galaxies truncate accretion and then consume the existing gas at or faster than typical main sequence rates. Alternatively, we cannot rule out that these galaxies reside in lower mass halos, and low gas fractions may instead reflect either stronger feedback, or more efficient gas consumption.
We combine high-resolution HST/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (log(M*) > 10) galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10^8 yr). The cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary scenarios of QG formation: an early (z > 2), fast-formation path of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs without passing through a compact state.
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