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Published July 20, 2006 | Published
Journal Article Open

The Stellar, Gas, and Dynamical Masses of Star-forming Galaxies at z ~ 2

Abstract

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z ~ 2. By combining the Hα spectra with photometric measurements from observed 0.3-8 μm, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The Hα line widths give a mean dynamical mass M_(dyn) = (6.9 ± 0.6) × 10^(10) M_⊙ within a typical radius of ~6 kpc, after excluding AGNs. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has M_(dyn)≫ M. These objects are best fit by young stellar populations and tend to have high Hα equivalent widths, W_(Hα) ≳ 200 Å, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 σ significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

Additional Information

© 2006 American Astronomical Society. Received 2006 February 3, accepted for publication 2006 March 31. Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. We thank Andrew Blain, Jonathan Bird, David Kaplan, and Shri Kulkarni for obtaining near-IR images of some of our targets; the staffs of the Keck and Palomar observatories for their assistance with the observations; and the anonymous referee for a thoroughly helpful report that significantly improved the paper. C. C. S., D. K. E., and N. A. R. have been supported by grant AST 03-07263 from the US National Science Foundation and by the David and Lucile Packard Foundation. A. E. S. acknowledges support from the Miller Institute for Basic Research in Science, and K. L. A. acknowledges support from the Carnegie Institution of Washington. Finally, we wish to extend special thanks to those of Hawaiian ancestry, on whose sacred mountain we are privileged to be guests. Without their generous hospitality, most of the observations presented here would not have been possible.

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August 19, 2023
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