The Rest‐Frame Optical Spectra of SCUBA Galaxies

We present near-infrared spectroscopy and narrow-band imaging at thewavelength of redshifted H-alpha for a sample of 30 high-redshift, far-infraredluminous galaxies. This sample is selected from surveys in the sub-millimeter,millimeter and radio wavebands and has complete redshift coverage with a medianredshift of z~2.4. Removing obvious AGN, we find that the predicted H-alphastar formation rates in this diverse population are suppressed (by ~10x)compared to those derived from their far-infrared luminosities. We estimatethat AGN are present in at least 40% of the galaxies in our sample. To furtherinvestigate this, we construct a composite rest-frame spectrum for both theentire sample and for those galaxies which individually show no signs ofnuclear activity. We find [NII]/H-alpha ratios for both composite spectra whichsuggest that the energy output of the galaxies is star-formation- rather thanAGN-dominated. The median H-alpha line width for our sample (removing obviousAGN) is 400+/-70km/s (FWHM), and the typical spatial extent of the H-alphaemission in our narrow-band observations is < 4-8kpc, which indicates adynamical mass of 1-2 x 10^11 Mo. This estimate is larger than recent estimatesof the dynamical masses of UV-selected galaxies at similar redshifts derived inan identical manner. We also compared our H-alpha and far-infrared luminositieswith deep Chandra observations of a subset of our survey fields and use thesedata to further assess their AGN content. We conclude that these high-redshift,far-infrared luminous galaxies represent a population of massive, metal-rich,merging systems with high instantaneous star formation rates, strong dustobscuration and actively-fueled AGN which are likely to be the progenitors ofmassive local elliptical galaxies. (abridged)Comment: ApJ in press. 18 pages, 10 figure