The Nature of Compact Galaxies in the Hubble Deep Field. II. Spectroscopic Properties and Implications for the Evolution of the Star Formation Rate Density of the Universe

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Abstract
We present a spectroscopic study of 51 compact field galaxies with redshifts z < 1.4 and apparent magnitudes I814 < 23.74 in the flanking fields of the Hubble Deep Field. These galaxies are compact in the sense that they have small apparent half-light radii (r1/2 ≤ 05) and high surface brightnesses (μI814 ≤ 22.2 mag arcsec-2). The spectra, taken at the Keck telescope, show emission lines in 88% of our sample, and only absorption lines in the remaining 12%. Emission-line profiles are roughly Gaussian with velocity widths that range from the measurement limit of σ ~ 35 km s-1 to 150 km s-1. Rest frame [O II] λ3727 equivalent widths range from 5 to 94 Å, yielding star formation rates (SFRs) of ~0.1 to 14 M☉ yr-1. The analysis of various line diagnostic diagrams reveals that ~60% of compact emission-line galaxies have velocity widths, excitations, Hβ luminosities, SFRs, and mass-to-light ratios characteristic of young star-forming H II galaxies. The remaining 40% form a more heterogeneous class of evolved starbursts, similar to local starburst disk galaxies. We find that, although the compact galaxies at z > 0.7 have similar SFRs per unit mass to those at z < 0.7, they are on average ~10 times more massive. Our sample implies a lower limit for the global comoving SFR density of ~0.004 M☉ yr-1 Mpc-3 at z = 0.55, and ~0.008 M☉ yr-1 Mpc-3 at z = 0.85 (assuming Salpeter IMF, H0 = 50 km s-1 Mpc-1, and q0 = 0.5). These values, when compared to estimates for a sample of local compact galaxies selected in a similar fashion, support a history of the universe in which the SFR density declines by a factor ~10 from z = 1 to today. From the comparison with the SFR densities derived for magnitude-limited samples of field galaxies, we conclude that compact emission-line galaxies, though only ~20% of the general field population, may contribute as much as ~45% to the global SFR of the universe at 0.4 < z < 1.
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