HST Morphologies of z~2 Dust Obscured Galaxies II. Bump Sources

We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared galaxies (ULIRGs) at z ≈ 2 with extremely red R − [24] colors (called dust-obscured galaxies, or DOGs) which have a local maximum in their spectral energy distribution (SED) at rest-frame 1.6μm associated with stellar emission. These sources, which we call “bump DOGs”, have star-formation rates of 400−4000M⊙ yr −1 and have redshifts derived from mid-IR spectra which show strong polycyclic aromatic hydrocarbon emission — a sign of vigorous on-going star-formation. Using a uniform morphological analysis, we look for quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are more typical of obscured active galactic nuclei than starbursts), sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than powerlaw DOGs (median Petrosian radius of 8.4± 2.7 kpc vs. 5.5± 2.3 kpc) and exhibit more diffuse and irregular morphologies (median M20 of −1.08± 0.05 vs. −1.48± 0.05). These trends are qualitatively consistent with expectations from simulations of major mergers in which merging systems during the peak star-formation rate period evolve from M20 = −1.0 to M20 = −1.7. Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally peaked, single-object morphologies rather than diffuse and irregular morphologies. This distinction in morphologies may imply that less obscured ULIRGs sample the merger near the end of the peak star-formation rate period. Alternatively, it may indicate that the intense star-formation in these less-obscured ULIRGs is not the result of a recent major merger. Subject headings: galaxies: evolution — galaxies: fundamental parameters — galaxies: high-redshift