We have obtained spectra with the 10-m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 < ∼ z < ∼ 4.5 in the Hubble Deep Field (HDF). Eleven of these galaxies are confirmed to be at high redshift (z med = 3.0), one is at z = 0.5, and the other 12 have uncertain redshifts but have spectra consistent with their being at z > 2. The spectra of the confirmed high-redshift galaxies show a diversity of features, including weak Lyα emission, strong Lyα breaks or damped Lyα absorption profiles, and the stellar and interstellar rest-UV absorption lines common to local starburst galaxies and high-redshift star-forming galaxies reported recently by others. The narrow profiles and low equivalent widths of C IV, Si IV, and N V absorption lines may imply low stellar metallicities. Combined with the 5 high-redshift galaxies in the HDF previously confirmed with Keck spectra by Steidel et al. (1996b), the 16 confirmed sources yield a comoving volume density of n ≥ 2.5 × 10 −4 h 3 50 Mpc −3 for q 0 = 0.05, or n ≥ 1.2 × 10 −3 h 3 50 Mpc −3 for q 0 = 0.5. These densities are 3 − 4 times higher than the recent estimates of 1 Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the

The integrated bolometric effective surface brightness S e distributions of starbursts are investigated for samples observed in 1. the rest frame ultraviolet (UV), 2. the farinfrared and Hα, and 3. 21cm radio continuum emission. For the UV sample we exploit a tight empirical relationship between UV reddening and extinction to recover the bolometric flux. Parameterizing the S e upper limit by the 90th percentile of the distribution, we find a mean S e,90 = 2.0 × 10 11 L ⊙ kpc −2 for the three samples, with a factor of three difference between the samples. This is consistent with what is expected from the calibration uncertainties alone. We find little variation in S e,90 with effective radii for R e ∼ 0.1 − 10 kpc, and little evolution out to redshifts z ≈ 3. The lack of a strong dependence of S e,90 on wavelength, and its consistency with the pressure measured in strong galactic winds, argue that it corresponds to a global star formation intensity limit (Σ e,90 ∼ 45 M ⊙ kpc −2 yr −1 ) rather than being an opacity effect. There are several important implications of these results: 1. There is a robust physical mechanism limiting starburst intensity. We note that starbursts have S e consistent with the expectations of gravitational instability models applied to the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral bulges can plausibly be built with maximum intensity bursts, while normal spiral disks can not. 3. The UV extinction of high-z galaxies is significant, implying that star formation in the early universe is moderately obscured. After correcting for extinction, the observed metal production rate at z ∼ 3 agrees well with independent estimates made for the epoch of elliptical galaxy formation.

We have used the Submillimeter Array to image a flux limited sample of seven submillimeter galaxies, selected by the AzTEC camera on the JCMT at 1.1 mm, in the COSMOS field at 890 µm with ∼ 2 ′′ resolution. All of the sources-two radio-bright and five radio-dim-are detected as single point-sources at high significance (> 6σ), with positions accurate to ∼ 0.2 ′′ that enable counterpart identification at other wavelengths observed with similarly high angular resolution. All seven have IRAC counterparts, but only two have secure counterparts in deep HST/ACS imaging. As compared to the two radio-bright sources in the sample, and those in previous studies, the five radio-dim sources in the sample (1) have systematically higher submillimeter-to-radio flux ratios, (2) have lower IRAC 3.6-8.0 µm fluxes, and (3) are not detected at 24µm. These properties, combined with size constraints at 890 µm (θ ∼ < 1.2 ′′), suggest that the radio-dim submillimeter galaxies represent a population of very dusty starbursts, with physical scales similar to local ultraluminous infrared galaxies, and an average redshift higher than radio-bright sources.

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We present a spectroscopic study of 51 compact field galaxies with redshifts z < 1.4 and apparent magnitudes I 814 < 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 1/2 ≤ 0.5 arcsec) and high surface brightnesses (µ I814 ≤ 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[OII]λ3727 equivalent widths range from 5 Å to 94 Å, 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, Hβ 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, H 0 = 50 km s −1 Mpc −1 , and q 0 = 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.

We present results from a continuing interferometric survey of high-redshift submillimeter galaxies with the Submillimeter Array, including high-resolution (beam size ∼ 2 arcsec) imaging of eight additional AzTEC 1.1mm selected sources in the COSMOS Field, for which we obtain six reliable (peak S/N > 5 or peak S/N > 4 with multiwavelength counterparts within the beam) and two moderate significance (peak S/N > 4) detections. When combined with previous detections, this yields an unbiased sample of millimeter-selected SMGs with complete interferometric followup. With this sample in hand, we (1) empirically confirm the radio-submillimeter association, (2) examine the submillimeter morphology -including the nature of submillimeter galaxies with multiple radio counterparts and constraints on the physical scale of the far infrared -of the sample, and (3) find additional evidence for a population of extremely luminous, radio-dim submillimeter galaxies that peaks at higher redshift than previous, radio-selected samples. In particular, the presence of such a population of high-redshift sources has important consequences for models of galaxy formationwhich struggle to account for such objects even under liberal assumptions -and dust production models given the limited time since the Big Bang.

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