Near-infrared Spectroscopy and HST Imaging of a Dusty Starburst ERO

We present near-IR spectroscopy and HST imaging of EROJ164023, an Extremely Red Object (ERO) with R-K=5.9 at z=1.05. EROJ164023 is a disk galaxy, with an optical/IR spectral energy distribution which is strongly reddened by dust (L_FIR/L_B<~200; A_V~5). The narrow emission lines (~300km/s) and the high [NII]/Halpha line ratio indicate that this is a ``composite'' starburst-Seyfert galaxy. Assuming that star formation dominates, we constrain the SFR to be 10-700Mo/yr from a variety of indicators. We compare EROJ164023 with the only other spectroscopically identified dusty EROs: HR10 (z=1.44) and ISOJ1324-2016 (z=1.50). EROJ164023 and HR10 have similar disk-like morphologies, and both exhibit a variation in the apparent dust obscuration depending upon the diagnostic used, suggesting that there is a complex spatial mix of stellar populations and dust in these galaxies. In contrast, the compact morphology and spectral properties of ISOJ1324-2016 indicate that it is a dusty quasar. Our results demonstrate that dusty galaxies identified using photometric ERO criteria include pure starbursts, composite systems such as EROJ164023 and dusty quasars. We suggest that the classification of EROs into these sub-classes cannot be reliably achieved from optical/near-IR photometry and instead requires mid/far-IR or sub-mm photometry and near-IR spectroscopy. The advent of efficient multi-object spectrographs working in the near-IR as well as the imminent launch of SIRTF therefore promise the opportunity of rapid progress in our understanding of the elusive ERO population. [Abridged]