Near-infrared spectroscopy and Hubble Space Telescope imaging of a dusty starburst extremely red object

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of ERO J164023+ 4644, an Extremely Red Object (ERO) with (R - K) = 5.9 at z = 1.05 that has been detected by Infrared Space Observatory (ISO) at 15 μm. ERO J164023 resembles a disk galaxy, with an optical/infrared spectral energy distribution that is strongly reddened by dust (L_FIR/L_B ≲ 200; A_V ∼ 5). The combination of the narrow width of the emission lines in our spectra (∼ 300 km s^-1) and the relatively high [N II]/Hα line ratio indicate that this is a "composite" starburst-Seyfert galaxy. Assuming that star formation dominates the energy output, we constrain the star formation rate to lie in the broad range ∼ 10-700 M_⊙ yr^-1 from a variety of star formation indicators. We compare ERO J164023 with the only other spectroscopically identified dusty EROs: HR10 (z = 1.44) and ISO J1324-2016 (z = 1.50). ERO J164023 and HR10 have similar disklike morphologies in the rest-frame UV, and both exhibit a variation in the apparent dust obscuration depending upon the diagnostic used, which suggests that there is a complex spatial mix of stellar populations and dust in these galaxies. In contrast, the compact morphology and spectral properties of ISO J1324-2016 indicate that it is a dusty quasar. Overall, our results demonstrate that the population of dusty galaxies identified using photometric ERO criteria includes systems ranging from pure starbursts through transition systems, such as ERO J164023, to dusty quasars. We suggest that the classification of EROs into these subclasses, necessary for the detailed modeling of the population, cannot be reliably achieved from optical/near-infrared photometry and instead requires mid/far-infrared or submillimeter photometry and near-infrared spectroscopy. The advent of efficient multiobject spectrographs working in the near-infrared, as well as the imminent launch of SIRTF, therefore promises the opportunity of rapid progress in our understanding of the elusive ERO population.