MAGAZ3NE: Massive, Extremely Dusty Galaxies at z∼2 Lead to Photometric Overestimation of Number Densities of the Most Massive Galaxies at 3<z<4

We present rest-frame optical spectra from Keck/MOSFIRE and Keck/NIRES of 16 candidate ultramassive galaxies targeted as part of the Massive Ancient Galaxies at z>3 Near-Infrared (MAGAZ3NE) Survey. These candidates were selected to have photometric redshifts 3≲ z_ phot<4, photometric stellar masses log(M/M_⊙)>11.7, and well-sampled photometric spectral energy distributions (SEDs) from the UltraVISTA and VIDEO surveys. In contrast to previous spectroscopic observations of blue star-forming and post-starburst ultramassive galaxies, candidates in this sample have very red SEDs implying significant dust attenuation, old stellar ages, and/or active galactic nuclei (AGN). Of these galaxies, eight are revealed to be heavily dust-obscured 2.03, one is a z∼1.2 dusty galaxy, and four galaxies do not have a confirmed spectroscopic redshift. In fact, none of the sample has |z_ spec-z_ phot|<0.5, suggesting difficulties for photometric redshift programs in fitting similarly red SEDs. The prevalence of these red interloper galaxies suggests that the number densities of high-mass galaxies are overestimated at z≳3 in large photometric surveys, helping to resolve the `impossibly early galaxy problem' and leading to much better agreement with cosmological galaxy simulations. A more complete spectroscopic survey of ultramassive galaxies is required to pin down the uncertainties on their number densities in the early universe.