THE REST-FRAME OPTICAL SPECTROSCOPIC PROPERTIES OF LYα-EMITTERS AT z ∼ 2.5: THE PHYSICAL ORIGINS OF STRONG LYα EMISSION*

Abstract

We present the rest-frame optical spectroscopic properties of 60 faint (R AB ∼ 27; L ∼ 0.1L * ) Lyα-selected galaxies (LAEs) at z ≈ 2.56. These LAEs also have rest-UV spectra of their Lyα emission line morphologies, which trace the effects of interstellar and circumgalactic gas on the escape of Lyα photons. We find that the LAEs have diverse rest-optical spectra, but their average spectroscopic properties are broadly consistent with the extreme low-metallicity end of the populations of continuumselected galaxies selected at z ≈ 2 − 3. In particular, the LAEs have extremely high [O III] 4 K), low oxygen abundances (12 + log(O/H) ≈ 8.04, Z neb ≈ 0.22Z ), and high excitations with respect to their more luminous continuum-selected analogs. Several of our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) ≈ 6.9−7.4 (Z neb ≈ 0.02 − 0.05Z ). We interpret these observations in light of new models of stellar evolution (including binary interactions) that have been shown to produce long-lived populations of hot, massive stars at low metallicities. We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionizationera galaxies. Furthermore, we investigate the physical trends accompanying Lyα emission across the largest current sample of combined Lyα and rest-optical galaxy spectroscopy, including both the 60 KBSS-Lyα LAEs and 368 more luminous galaxies at similar redshifts. We find that the net Lyα emissivity (parameterized by the Lyα equivalent width) is strongly correlated with nebular excitation and ionization properties and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content.

Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population

“…Ly Å) has been identified in RXC J2248-ID3 by VLT/VIMOS (Balestra et al 2013), VLT/FORS (Boone et al 2013), and the HST WFC3/IR grism (Schmidt et al 2016), consistent with expectations for young, metal-poor galaxies (e.g., Cowie et al 2011;Trainor et al 2016).…”

The VIMOS Ultra Deep Survey

“…However, the outflow velocity seems to mainly contribute to EW(Lyα) and the outflow velocity, together with N HI , also explain the Lyα line shape and large-scale spatial extension. Combined with recent results in the literature, the low NHI together with low metallicity and highionization capability would make a galaxy a strong Lyα emitter (Trainor et al 2016).…”

“…Kornei et al 2010;Guaita et al 2011;Hathi et al 2016). Also, there is evidence that the largest equivalent width LAEs present highionization state and low oxygen abundances Nakajima et al 2013;Trainor et al 2016;Nakajima et al 2016;Amorín et al 2017). In fact, high-ionization absorption, such as the CIVλλ1548,1550 doublet, and strong nebular emission lines, such as the collisionally excited CIIIλλ1907,1909 doublet, are detected in their spectra.…”

ALMA Reveals Metals yet No Dust within Multiple Components in CR7

“…A large number of early sources observed by the ALMA were selected based on their Lyα emission needed to measure redshifts. Strong Lyα emission is typically associated with a high-ionization state and/or lower metallicity (e.g., Nakajima et al 2016;Trainor et al 2016;Matthee et al 2017a;Stark et al 2017), which should result in a deficit in [C II] luminosity due to photodissociation (e.g., Vallini et al 2015). This can be tested with deeper [C II] observations of sources that span a wider parameter space (e.g., Bradač et al 2017;Knudsen et al 2017).…”