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Register a new userSpatially resolved kinematics in the central 1 kpc of a compact star-forming galaxy at z=2.3 from ALMA CO observations
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We present high spatial resolution (FWHM$sim$0.14) observations of the CO($8-7$) line in GDS-14876, a compact star-forming galaxy at $z=2.3$ with total stellar mass of $log(M_{star}/M_{odot})=10.9$. The spatially resolved velocity map of the inner $rlesssim1$~kpc reveals a continous velocity gradient consistent with the kinematics of a rotating disk with $v_{rm rot}(r=1rm kpc)=163pm5$ km s$^{-1}$ and $v_{rm rot}/sigmasim2.5$. The gas-to-stellar ratios estimated from CO($8-7$) and the dust continuum emission span a broad range, $f^{rm CO}_{rm gas}=M_{rm gas}/M_{star}=13-45%$ and $f^{rm cont}_{rm gas}=50-67%$, but are nonetheless consistent given the uncertainties in the conversion factors. The dynamical modeling yields a dynamical mass of$log(M_{rm dyn}/M_{odot})=10.58^{+0.5}_{-0.2}$ which is lower, but still consistent with the baryonic mass, $log$(M$_{rm bar}$= M$_{star}$ + M$^{rm CO}_{rm gas}$/M$_{odot}$)$=11.0$, if the smallest CO-based gas fraction is assumed. Despite a low, overall gas fraction, the small physical extent of the dense, star-forming gas probed by CO($8-7$), $sim3times$ smaller than the stellar size, implies a strong concentration that increases the gas fraction up to $f^{rm CO, 1rm kpc}_{rm gas}sim 85%$ in the central 1 kpc. Such a gas-rich center, coupled with a high star-formation rate, SFR$sim$ 500 M$_{odot}$ yr$^{-1}$, suggests that GDS-14876 is quickly assembling a dense stellar component (bulge) in a strong nuclear starburst. Assuming its gas reservoir is depleted without replenishment, GDS-14876 will quickly ($t_{rm depl}sim27$ Myr) become a compact quiescent galaxy that could retain some fraction of the observed rotational support.
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We present first results from the SXDF-ALMA 1.5 arcmin^2 deep survey at 1.1 mm using Atacama Large Millimeter Array (ALMA). The map reaches a 1sigma depth of 55 uJy/beam and covers 12 Halpha-selected star-forming galaxies at z = 2.19 or z=2.53. We have detected continuum emission from three of our Halpha-selected sample, including one compact star-forming galaxy with high stellar surface density, NB2315-07. They are all red in the rest-frame optical and have stellar masses of log (M*/Msun)>10.9 whereas the other blue, main-sequence galaxies with log(M*/Msun)=10.0-10.8 are exceedingly faint, <290 uJy (2sigma upper limit). We also find the 1.1 mm-brightest galaxy, NB2315-02, to be associated with a compact (R_e=0.7+-0.1 kpc), dusty star-forming component. Given high gas fraction (44^{+20}_{-8}% or 37^{+25}_{-3}%) and high star formation rate surface density (126^{+27}_{-30} Msun yr^{-1}kpc^{-2}), the concentrated starburst can within less than 50^{+12}_{-11} Myr build up a stellar surface density matching that of massive compact galaxies at z~2, provided at least 19+-3% of the total gas is converted into stars in the galaxy centre. On the other hand, NB2315-07, which already has such a high stellar surface density core, shows a gas fraction (23+-8%) and is located in the lower envelope of the star formation main-sequence. This compact less star-forming galaxy is likely to be in an intermediate phase between compact dusty star-forming and quiescent galaxies.
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We present spatially-resolved Atacama Large Millimeter/sub-millimeter Array (ALMA) 870 $mu$m dust continuum maps of six massive, compact, dusty star-forming galaxies (SFGs) at $zsim2.5$. These galaxies are selected for their small rest-frame optical sizes ($r_{rm e, F160W}sim1.6$ kpc) and high stellar-mass densities that suggest that they are direct progenitors of compact quiescent galaxies at $zsim2$. The deep observations yield high far-infrared (FIR) luminosities of L$_{rm IR}=10^{12.3-12.8}$ L$_{odot}$ and star formation rates (SFRs) of SFR$=200-700$ M$_{odot}$yr$^{-1}$, consistent with those of typical star-forming main sequence galaxies. The high-spatial resolution (FWHM$sim$0.12-0.18) ALMA and HST photometry are combined to construct deconvolved, mean radial profiles of their stellar mass and (UV+IR) SFR. We find that the dusty, nuclear IR-SFR overwhelmingly dominates the bolometric SFR up to $rsim5$ kpc, by a factor of over 100$times$ from the unobscured UV-SFR. Furthermore, the effective radius of the mean SFR profile ($r_{rm e, SFR}sim1$ kpc) is $sim$30% smaller than that of the stellar mass profile. The implied structural evolution, if such nuclear starburst last for the estimated gas depletion time of $Delta t=pm100$ Myr, is a 4$times$ increase of the stellar mass density within the central 1 kpc and a 1.6$times$ decrease of the half-mass radius. This structural evolution fully supports dissipation-driven, formation scenarios in which strong nuclear starbursts transform larger, star-forming progenitors into compact quiescent galaxies.
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