HIGH-REDSHIFT GALAXIES IN THE HUBBLE DEEP FIELD - COLOR SELECTION ANDSTAR-FORMATION HISTORY TO Z-SIMILAR-TO-4

The Lyman decrement associated with the cumulative effect of H I in QS O absorption systems along the line of sight provides a distinctive fe ature for identifying galaxies at z greater than or similar to 2.5. Co lour criteria, which are sensitive to the presence of a Lyman continuu m break superposed oil an otherwise flat UV spectrum, have been shown, through Keck spectroscopy, to successfully identify a substantial pop ulation of star-forming galaxies at 3 less than or similar to z less t han or similar to 3.5. Such objects have proven to be surprisingly elu sive in field galaxy redshift surveys; quantification of their surface densities and morphologies is crucial for determining how and when ga laxies formed. The Hubble Deep Field (HDF) observations offer the oppo rtunity to exploit the ubiquitous effect of intergalactic absorption a nd obtain useful statistical constraints on the redshift distribution of galaxies to considerably fainter limits than the current spectrosco pic limits. We model the H I cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lyman continuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents and redshifts to derive colour selection criteria that provide a robust separation bet ween high-redshift and low-redshift galaxies. From the HDF images we c onstruct a sample of star-forming galaxies at 2 less than or similar t o z less than or similar to 4.5. While none of the similar to 60 objec ts in the HDF having known Keck/Low-Resolution Imaging Spectrograph (L RIS) spectroscopic redshifts in the range 0 less than or similar to z less than or similar to 1.4 is found to contaminate our high-redshift sample, our colour criteria are able to efficiently select the 2.6 les s than or similar to z less than or similar to 3.2 galaxies identified by Steidel et al. The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early Universe. We set a lower limit to the ejection rate of he avy elements per unit comoving volume from Type II supernovae at [z]=2 .75 of approximate to 3.6 x 10(-4) M. yr(-1) Mpc(-3) (for q(0)=0.5 and H-0=50 km s(-1) Mpc(-1)), which is 3 times higher than the local valu e but still 4 times lower than the rate observed at z approximate to 1 . At [z]=4, our lower limit to the cosmic metal ejection rate is appro ximate to 3 times lower than the [z]=2.75 value. We discuss the implic ations of these results on models of galaxy formation, and on the chem ical enrichment and ionization history of the intergalactic medium.