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/DEIMOS spectroscopic follow-up of candidate galaxies of i-band-dropout protocluster candidate galaxies at $z\sim 6$ in the COSMOS field. We securely detect Lyman-$\hat{I}\pm$ 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 $z\approx 6$ 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$\hat{I}\pm$, resulting in a redshift range of $z=5.85-6.08$. Using the spectroscopic redshifts, we re-calculate the overdensity map for the COSMOS field and find the galaxies to be in a significant overdensity at the $4\ddot{I}ƒ$ level, with a peak overdensity of $\hat{I}´=11.8$ (compared to the previous value of $\hat{I}´=9.2$). The protocluster galaxies have stellar masses derived from Bagpipes SED fits of $10^{8.29}-10^{10.28}{\mathrm{M}}_{\odot }$ and star formation rates of $2-39{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, 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 $\sim 10^{12}{\mathrm{M}}_{\odot }$. 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.

26 pages, 14 figues, 5 tables, main text is 16 pages, appendix is 10 pages, published in MNRAS