Multi-Wavelength View of Kiloparsec-Scale Clumps in Star-Forming
  Galaxies at z~2
  This paper studies the properties of kiloparsec-scale clumps in star-forming
galaxies at z~2 through multi-wavelength broad band photometry. A sample of 40
clumps is identified through auto-detection and visual inspection from 10
galaxies with 1.5<z<2.5 in the Hubble Ultra Deep Field, where deep and
high-resolution HST/WFC3 and ACS images enable us to resolve structures of z~2
galaxies down to kpc scale in the rest-frame UV and optical bands as well as to
detect clumps toward the faint end. The physical properties of clumps are
measured through fitting spatially resolved seven-band (BVizYJH) spectral
energy distribution to models. On average, the clumps are blue and have similar
median rest-frame UV--optical color as the diffuse components of their host
galaxies, but the clumps have large scatter in their colors. Although the star
formation rate (SFR)--stellar mass relation of galaxies is dominated by the
diffuse components, clumps emerge as regions with enhanced specific SFRs,
contributing individually ~10% and together ~50% of the SFR of the host
galaxies. However, the contributions of clumps to the rest-frame UV/optical
luminosity and stellar mass are smaller, typically a few percent individually
and ~20% together. On average, clumps are younger by 0.2 dex and denser by a
factor of 8 than diffuse components. Clump properties have obvious radial
variations in the sense that central clumps are redder, older, more extincted,
denser, and less active on forming stars than outskirts clumps. Our results are
broadly consistent with a widely held view that clumps are formed through
gravitational instability in gas-rich turbulent disks and would eventually
migrate toward galactic centers and coalesce into bulges. Roughly 40% of the
galaxies in our sample contain a massive clump that could be identified as a
proto-bulge, which seems qualitatively consistent with such a bulge-formation
scenario.
