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Published May 20, 2011 | Published
Journal Article Open

Hubble Space Telescope Morphologies of z ~ 2 Dust-obscured Galaxies. II. Bump Sources


We present Hubble Space Telescope 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 1.6 μm associated with stellar emission. These sources, which we call "bump DOGs," have star formation rates (SFRs) of 400-4000 M_⊙ yr^(–1) and have redshifts derived from mid-IR spectra which show strong polycyclic aromatic hydrocarbon emission—a sign of vigorous ongoing 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), submillimeter-selected galaxies, and other less-reddened ULIRGs from the Spitzer Extragalactic First Look Survey. Bump DOGs are larger than power-law DOGs (median Petrosian radius of 8.4 ± 2.7 kpc versus 5.5 ± 2.3 kpc) and exhibit more diffuse and irregular morphologies (median M _(20) of –1.08 ± 0.05 versus –1.48 ± 0.05). These trends are qualitatively consistent with expectations from simulations of major mergers in which merging systems during the peak SFR period evolve from M _(20) = –1.0 to M _(20) = –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 SFR period. Alternatively, it may indicate that the intense star formation in these less-obscured ULIRGs is not the result of a recent major merger.

Additional Information

© 2011 American Astronomical Society. Received 2010 June 25; accepted 2011 March 11; published 2011 April 29. The work is based primarily on observations made with the Hubble Space Telescope. This work also relies in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. We are grateful to the expert assistance of the staff Kitt Peak National Observatory where the Boötes field observations of the NDWFS were obtained. The authors thank NOAO for supporting the NOAO Deep Wide-Field Survey. In particular, we thank Jenna Claver, Lindsey Davis, Alyson Ford, Emma Hogan, Tod Lauer, Lissa Miller, Erin Ryan, Glenn Tiede, and Frank Valdes for their able assistance with the NDWFS data. We also thank the staff of the W. M. Keck Observatory, where some of the galaxy redshifts were obtained. We gratefully acknowledge the anonymous referee whose helpful suggestions have resulted in an improved manuscript. R.S.B. gratefully acknowledges financial assistance from HST grants GO10890 and GO11195, without which this research would not have been possible. Support for program numbers HST-GO10890 and HST-GO11195 were provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. The research activities of A.D. and B.T.J. are supported by NOAO, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Support for E. Le Floc'h was provided by NASA through the Spitzer Space Telescope Fellowship Program.

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