Near‐Infrared Spectroscopy andHubble Space TelescopeImaging of a Dusty Starburst Extremely Red Object

We present near-IR spectroscopy and HST imaging of EROJ164023, an ExtremelyRed Object (ERO) with R-K=5.9 at z=1.05. EROJ164023 is a disk galaxy, with anoptical/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-Seyfertgalaxy. Assuming that star formation dominates, we constrain the SFR to be10-700Mo/yr from a variety of indicators. We compare EROJ164023 with the onlyother spectroscopically identified dusty EROs: HR10 (z=1.44) and ISOJ1324-2016(z=1.50). EROJ164023 and HR10 have similar disk-like morphologies, and bothexhibit a variation in the apparent dust obscuration depending upon thediagnostic used, suggesting that there is a complex spatial mix of stellarpopulations and dust in these galaxies. In contrast, the compact morphology andspectral properties of ISOJ1324-2016 indicate that it is a dusty quasar. Ourresults demonstrate that dusty galaxies identified using photometric EROcriteria include pure starbursts, composite systems such as EROJ164023 anddusty quasars. We suggest that the classification of EROs into thesesub-classes cannot be reliably achieved from optical/near-IR photometry andinstead requires mid/far-IR or sub-mm photometry and near-IR spectroscopy. Theadvent of efficient multi-object spectrographs working in the near-IR as wellas the imminent launch of SIRTF therefore promise the opportunity of rapidprogress in our understanding of the elusive ERO population. [Abridged]Comment: 5 pages, 4 figures, Accepted for publication in Ap