  We present a spectroscopic study of 51 compact field galaxies with redshifts
z < 1.4 and apparent magnitudes I < 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 (r_e < 0.5 arcsec) and high surface brightnesses
(SB_e < 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 sigma = 35 km s^-1 to 150 km s^-1.
Rest-frame [OII]3727 equivalent widths range from 5A to 94A , yielding star
formation rates (SFR) 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, Hbeta luminosities, SFRs, 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
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, Ho = 50 km s^-1 Mpc^-1, and qo = 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.
