Star Formation in Emission-Line Galaxies between Redshifts of 0.8 and 1.6

December 2002 • 2002ApJ...581..205H

Authors • Hicks, Erin K. S. • Malkan, Matthew A. • Teplitz, Harry I. • McCarthy, Patrick J. • Yan, Lin

Abstract • Optical spectra of 14 emission-line galaxies representative of the 1999 NICMOS parallel grism Hα survey of McCarthy et al. are presented. Of the 14, 9 have emission lines confirming the redshifts found in the grism survey. The higher resolution of our optical spectra improves the redshift accuracy by a factor of 5. The [O II]/Hα values of our sample are found to be more than 2 times lower than expected from Jansen et al. This [O II]/Hα ratio discrepancy is most likely explained by additional reddening in our Hα-selected sample [on average, as much as an extra E(B-V)=0.6], as well as by a possible stronger dependence of the [O II]/Hα ratio on galaxy luminosity than is found in local galaxies. The result is that star formation rates (SFRs) calculated from [O II] λ3727 emission, uncorrected for extinction, are found to be on average 4+/-2 times lower than the SFRs calculated from Hα emission. Classification of emission-line galaxies as starburst or Seyfert galaxies based on comparison of the ratios [O II]/Hβ and [Ne III] λ3869/Hβ is discussed. New Seyfert 1 diagnostics using the Hα line luminosity, H-band absolute magnitude, and Hα equivalent widths are also presented. One galaxy is classified as a Seyfert 1 based on its broad emission lines, implying a comoving number density for Seyfert 1 galaxies of 2.5^+5.9_-2.1×10^-5 Mpc^-3. This comoving number density is a factor of 2.4^+5.5_-2.0 times higher than estimated by other surveys.

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Harry Teplitz

Senior Scientist