Near-IR Spectroscopy of a Galaxy During Reionization

We present Keck/MOSFIRE observations of galaxies at redshifts z ~ 5-8 targeting UV metal lines, and studying how these lines constrain the physical conditions in these most distant galaxies. While Ly-α (λ1216Å) emission is the most common rest-frame UV feature used in detecting galaxies at higher redshifts (z ≥ 2), it can be heavily attenuated by neutral Hydrogen, pushing emission hundreds of km/s redwards of a galaxy's systemic (or true) redshift. Alternatively, UV metal lines should be strong, measure the location of the systemic redshifts in galaxies, and contain a wealth of information about the state of the gas (ionization, pressure, metallicity) and the ionizing sources (including the properties of the initial mass function and presence of AGN). We describe our observing program to study these lines in distant galaxies. We highlight MOSFIRE H-band data showing the detection of CIII] (λλ1907,1909Å) in a galaxy at z = 7.5078, that was previously identified via its Lyα emission (Finkelstein et al. 2013). The CIII] line flux of this galaxy is (2.63 ± 0.52) e-18 erg/s/cm² with a systemic redshift for this galaxy of z = 7.5032 ± 0.0003 and a small velocity offset to Lyα of Δ(v)_Lyα = 88 km/s. The ratio of CIII]/Lyα is 0.30-0.45, one of the highest values measured for any z > 6 galaxy. To match the CIII] equivalent width and [3.6]-[4.5] color of this galaxy requires high-mass stars with photospheres consistent with those in the BPASS binary models, with extremely high ionization (likely requiring an IMF that extends to 300 Solar masses) and sub-Solar metallicity (10-20% Solar). We also discuss the possibility of AGN ionization and possible AGN + stellar population ionization. Finally, we investigate predictions for spectroscopy with the James Webb Space Telescope using these different models, which will ultimately test the nature of the ionizing radiation in this source. // This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. This work benefited from generous support from George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University. TAH acknowledges support from the NSF Graduate Research Fellowship.