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Abstract:
A randomly chosen star in today’s universe is most likely to live in a galaxy with stellar mass between the Milky Way and Andromeda. It remains uncertain, however, how the structural evolution of these bulge-disk systems proceeded. Most of the unobscured star formation we observe by building Andromeda progenitor s at 0.7 < z < 1.5 occurs in disks, but ≳90% of their star formation is reprocessed by dust and remains unaccounted for. Here we map rest-500 μm dust continuum emission in an Andromeda progenitor at z = 1.25 to probe where it is growing through dust-obscured star formation. Combining resolved dust measurements from the NOthern Extended Millimeter Array interferometer with Hubble Space Telescope Hα maps and multicolor imaging (including new data from the Hubble Deep UV Legacy Survey, HDUV), we find a bulge growing by dust- obscured star formation: while the unobscured star formation is centrally suppressed, the dust continuum is centrally concentrated, filling the ring-like structure that is evident in the Hα and UV emission. Reflecting this, the dust emission is more compact than the optical/UV tracers of star formation with r e (dust) = 3.4 kpc, r e (Hα)/r e (dust) = 1.4, and r e (UV)/r e (dust) = 1.8. Crucially, however, the bulge and disk of this galaxy are building simultaneously; although the dust emission is more compact than the rest-optical emission (r e (optical)/r e (dust) = 1.4), it is somewhat less compact than the stellar mass (r e (M *)/r e (dust) = 0.9). Taking the rest-500 μm emission as a tracer, the expected structural evolution can be accounted for by star formation: it will grow in size by ∆r e /∆M * ̃ 0.3 and in central surface density by ∆Σcen/∆M * ̃ 0.9. Finally, our observations are consistent with a picture in which merging and disk instabilities drive gas to the center of galaxies, boosting global star formation rates above the main sequence and building bulges.
