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Casey Papovich (Steward Obs./STScI)
Present--day galaxy surveys now provide the resources to probe the nature of the stellar content in high--redshift galaxies. However, it remains unclear which processes govern the modes of star formation, and what fraction of these galaxies' baryonic material has been converted to stars over time. To resolve some of these issues, we have analyzed high--quality images of the Hubble Deep Field North (HDF-N), including WFPC2, NICMOS, and Ks data.
By comparing the galaxy photometry to a suite of spectral--template models, we have measured fairly robust estimates for the galaxies' total stellar masses, even with generally poor constraints for the stellar--population ages, star--formation history, and dust extinction. At z > 2, there is a stark absence of red, old galaxies with stellar--mass estimates comparable to present--day ``L*'' galaxies. Red, and presumably older galaxies appear for z < 1.5, but retain inferred stellar masses less than or equal to that of present--day ``L*'' galaxies. If such objects exist, then they should be present in the NICMOS data. By summing the volume--averaged stellar mass in the HDF-N as a function of redshift, we have constructed a global, star--formation history for the Universe, which is consistent with measures of the cosmic star--formation--rate density, and cosmological simulations of mass assembly.
We have examined the rest--frame FUV--to--Optical morphologies of HDF-N galaxies at z~1 and z~2.7. The high--redshift galaxies generally possess greater homogeneity in their distribution of FUV--to--Optical colors relative to those at moderate redshifts, and they exhibit a stronger degree of homogeneity in their individual internal colors. One interpretation of this evidence is that young stars dominate the FUV--to--Optical emission from all regions of high--redshift galaxies. By z~1, objects have developed greater diversity in the properties of their stellar populations, which contribute strongly to the light at optical and redder wavelengths.
Support for this work is provided by NASA through grant GO-07817.01-96A.
The author(s) of this abstract have provided an email address for comments about the abstract: papovich@as.arizona.edu