DEIMOS spectroscopy of z = 6 protocluster candidate in COSMOS - a massive protocluster embedded in a large-scale structure?

Abstract

We present the results of our Keck/DEep Imaging Multi-Object Spectrograph spectroscopic follow-up of candidate galaxies of i-band-dropout protocluster candidate galaxies at z ∼6 in the COSMOS field. We securely detect Lyman α emission lines in 14 of the 30 objects targeted, 10 of them being at z = 6 with a signal-to-noise ratio of 5-20; the remaining galaxies are either non-detections or interlopers with redshift too different from z = 6 to be part of the protocluster. The 10 galaxies at z ≈ 6 make the protocluster one of the riches at z > 5. The emission lines exhibit asymmetric profiles with high skewness values ranging from 2.87 to 31.75, with a median of 7.37. This asymmetry is consistent with them being Ly α, resulting in a redshift range of z = 5.85-6.08. Using the spectroscopic redshifts, we recalculate the overdensity map for the COSMOS field and find the galaxies to be in a significant overdensity at the 4σ level, with a peak overdensity of δ= 11.8 (compared to the previous value of δ= 9.2). The protocluster galaxies have stellar masses derived from Bagpipes spectral energy distribution fits of and star formation rates of, placing them on the main sequence at this epoch. Using a stellar-to-halo-mass relationship, we estimate the dark matter halo mass of the most massive halo in the protocluster to be. By comparison with halo mass evolution tracks from simulations, the protocluster is expected to evolve into a Virgo- or Coma-like cluster in the present day.