Near-infrared Spectroscopy of Five Ultra-massive Galaxies at 1.7 < z < 2.7

Abstract

We present the results of a pilot near-infrared spectroscopic campaign of five very massive galaxies ($\mathrm{log}({M}_{\star }/{M}_{\odot })\gt 11.45$) in the range of $1.7\lt z\lt 2.7$. We measure an absorption feature redshift for one galaxy at ${z}_{\mathrm{spec}}=2.000\pm 0.006$. For the remaining galaxies, we combine the photometry with the continuum from the spectra to estimate continuum redshifts and stellar population properties. We define a continuum redshift (${z}_{\mathrm{cont}}$ ) as one in which the redshift is estimated probabilistically from the combination of catalog photometry and the observed spectrum using EAZY. We derive the uncertainties on the stellar population synthesis properties using a Monte Carlo simulation and examine the correlations between the parameters with and without the use of the spectrum in the modeling of the spectral energy distributions. The spectroscopic constraints confirm the extreme stellar masses of the galaxies in our sample. We find that three out of five galaxies are quiescent (star-formation rate of $\lesssim 1{M}_{\odot }\,{\mathrm{yr}}^{-1}$) with low levels of dust obscuration (${A}_{{\rm{V}}}\lt 1$) , that one galaxy displays both high levels of star formation and dust obscuration ($\mathrm{SFR}\approx 300{M}_{\odot }\,{\mathrm{yr}}^{-1}$, ${A}_{{\rm{V}}}\approx 1.7$ mag), and that the remaining galaxy has properties that are intermediate between the quiescent and star-forming populations.