Caltech Faint Galaxy Redshift Survey. XIII. Spectral Energy Distributions for Galaxies in the Region of the Hubble Deep Field North*

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Abstract
We introduce a new empirical function for modeling the spectral energy distributions (SEDs) of galaxies. We apply it to a sample of 590 galaxies in the region of the Hubble Deep Field (HDF) with z < 1.5 using multicolor photometry with wide wavelength coverage combined with spectroscopic redshifts from our 93% complete R-selected redshift survey there. We find the following: As expected, galaxies with strong signs of recent star formation (i.e., those that show emission lines) have bluer continua in both the rest-frame ultraviolet and optical/near-infrared. The redder galaxies tend to be more luminous. Although galaxies with strong absorption lines and no emission features are ~15% of the total sample with 0.25 < z < 0.8, they are ~50% of the 25 most luminous galaxies in the sample at rest-frame R. The SEDs of actively star-forming galaxies become bluer in the mean in the rest-frame UV at higher redshifts, a trend that might arise from SED modeling errors. Aside from this, we discern no change with redshift in the relationship between SED characteristics and galaxy spectral type based on the strength of narrow emission and absorption features. Combining with similar work at higher and lower redshift, the bluest galaxies have indistinguishable SEDs in the rest-frame UV over the redshift regime of 0–3. There is no evidence in our R-selected sample that supports the existence of a substantial population of very dusty star-forming galaxies at z 1.5. Our ability to predict the mid-IR flux using the UV, optical, and near-IR SEDs is limited. The potential accuracy of photometric redshifts is evaluated, bearing in mind that a break at 4000 Å must be detectable to within the errors of the photometry to assign a photo-z for galaxies in this redshift regime. The rest-frame K-band luminosity as a function of redshift clearly shows a gradual change in the population of various types of galaxies, with star-forming galaxies becoming both more luminous and a larger fraction of the total population at higher redshifts. The overall pattern of the L(K)-z relationship suggests that passive evolution at constant stellar mass is a good approximation to the actual behavior of at least the most luminous galaxies in this large sample of galaxies in the region of the HDF out to z ~ 1.5.