Metallicities of Emission-Line Galaxies from HST ACS PEARS and HST WFC3 ERS Grism Spectroscopy at 0.6 < z < 2.4

Lifang Xia (1), Sangeeta Malhotra (1), James Rhoads (1), Nor Pirzkal (2), Amber Straughn (3), Steven Finkelstein (4), Seth Cohen (1), Harald Kuntschner (5), Martin Kümmel (5), Jeremy Walsh (5), Rogier A. Windhorst (1) and Robert O’Connell (6) ((1) School of Earth and Space Exploration, Arizona State University, AZ, 85287-1404; (2) Space Telescope Science Institute, Baltimore, MD 21218, USA; (3) Astrophysics Science Division, Goddard Space Flight Center, Code 665, Greenbelt, MD 20771, USA; (4) George P. and CynthiaWoods Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A & M University, College Station, TX 77843, USA; (5) European Southern Observatory, Karl Schwarzschild Str. 2, D 85748 Garching, Germany; (6) Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, USA)

Galaxies selected on the basis of their emission line strength show low metallicities, regardless of their redshifts. We conclude this from a sample of faint galaxies at redshifts between 0.6 < z < 2.4, selected by their prominent emission lines in low-resolution grism spectra in the optical with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) and in the near-infrared using Wide-Field Camera 3 (WFC3). Using a sample of 11 emission line galaxies (ELGs) at 0.6 < z < 2.4 with luminosities of -22 < M_B < -19, which have [OII], H\beta, and [OIII] line flux measurements from the combination of two grism spectral surveys, we use the R23 method to derive the gas-phase oxygen abundances: 7.5 < 12+log(O/H) < 8.5. The galaxy stellar masses are derived using Bayesian based Markov Chain Monte Carlo (\piMC^2) fitting of their Spectral Energy Distribution (SED), and span the mass range 8.1 < log(M_*/M_\sun) < 10.1. These galaxies show a mass-metallicity (M-L) and Luminosity-Metallicity (L-Z) relation, which is offset by –0.6 dex in metallicity at given absolute magnitude and stellar mass relative to the local SDSS galaxies, as well as continuum selected DEEP2 samples at similar redshifts. The emission-line selected galaxies most resemble the local “green peas” galaxies and Lyman-alpha galaxies at z~0.3 and z~2.3 in the M-Z and L-Z relations and their morphologies. The G-M_{20} morphology analysis shows that 10 out of 11 show disturbed morphology, even as the star-forming regions are compact. These galaxies may be intrinsically metal poor, being at early stages of formation, or the low metallicities may be due to gas infall and accretion due to mergers.

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