Sub-kpc ALMA imaging of compact star-forming galaxies at z~2.5: revealing the formation of dense galactic cores in the progenitors of compact quiescent galaxies

Abstract

We present spatially-resolved Atacama Large Millimeter/sub-millimeter Array (ALMA) 870 $\hat{I}¼$m dust continuum maps of six massive, compact, dusty star-forming galaxies (SFGs) at $z\sim 2.5$. These galaxies are selected for their small rest-frame optical sizes ($r_{\mathrm{e},F160W}\sim 1.6$ kpc) and high stellar-mass densities that suggest that they are direct progenitors of compact quiescent galaxies at $z\sim 2$. The deep observations yield high far-infrared (FIR) luminosities of L${}_{\mathrm{IR}}=10^{12.3-12.8}$ 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 $r\sim 5$ kpc, by a factor of over 100$\times$ from the unobscured UV-SFR. Furthermore, the effective radius of the mean SFR profile ($r_{\mathrm{e},\mathrm{SFR}}\sim 1$ 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 $\hat{I}”t=\pm 100$ 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.

Accepted to ApJ Letters