The Nature of Compact Galaxies at z ~ 0.2–1.3: Implications for Galaxy Evolution and the Star Formation History of the Universe

We study the global scaling-laws of 51 compact field galaxies with redshifts z ~ 0.2 –1.3 and apparent magnitudes I814 < 23.74 in the flanking fields of the Hubble Deep Field. Roughly 60% of the 45 compact emission-line galaxies have sizes, surface brightnesses, luminosities, velocity widths, excitations, star formation rates (SFR), and mass-to-light ratios characteristic of young star-forming HII galaxies. The remaining 40% form a more heterogeneous class of evolved starbursts, similar to local disk starburst galaxies. Without additional star formation, HII-like distant compacts will most likely fade to resemble today’s spheroidal galaxies such as NGC 205. 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. These values, when compared to a similar sample of local galaxies, 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 from previous data sets, 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.