Metallicities of emission-line galaxies from HST ACS pears and HST wfc3 ERS grism spectroscopy at 0.6 < z < 2.4

Abstract

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 on the Hubble Space Telescope and in the near-infrared using Wide-Field Camera 3. Using a sample of 11 emission-line galaxies at 0.6 < z < 2.4 with luminosities of -22 ≲ M B ≲ -19 which have [O II], Hβ, and [O III] 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 (πMC 2) fitting of their spectral energy distribution, and span the mass range 8.1 < 10.1. These galaxies show mass-metallicity (M-Z) and luminosity-metallicity (L-Z) relations, which are offset by -0.6dex in metallicity at given absolute magnitude and stellar mass relative to the local Sloan Digital Sky Survey galaxies, as well as continuum-selected DEEP2 samples at similar redshifts. The emission-line-selected galaxies most resemble the local "green peas" galaxies and Lyα 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. © 2012. The American Astronomical Society. All rights reserved.