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We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infraredgalaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscuredgalaxies, or DOGs) which have a local maximum in their spectral energydistribution (SED) at rest-frame 1.6um associated with stellar emission. Thesesources, which we call "bump DOGs", have star-formation rates of 400-4000Msun/yr and have redshifts derived from mid-IR spectra which show strongpolycyclic aromatic hydrocarbon emission --- a sign of vigorous on-goingstar-formation. Using a uniform morphological analysis, we look forquantifiable differences between bump DOGs, power-law DOGs (Spitzer-selectedULIRGs with mid-IR SEDs dominated by a power-law and spectral features that aremore typical of obscured active galactic nuclei than starbursts),sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs fromthe Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger thanpower-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc)and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/-0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent withexpectations from simulations of major mergers in which merging systems duringthe peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7.Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrallypeaked, single-object morphologies rather than diffuse and irregularmorphologies. This distinction in morphologies may imply that less obscuredULIRGs sample the merger near the end of the peak star-formation rate period.Alternatively, it may indicate that the intense star-formation in theseless-obscured ULIRGs is not the result of a recent major merger.

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