A star-bursting proto-cluster in making associated to a radio galaxy at z=2.53 discovered by H_alpha imaging

TL;DR

This paper reports the discovery of a proto-cluster at z=2.53 associated with a radio galaxy, revealing vigorous star formation, spatial clustering, and insights into early galaxy formation processes through narrow-band H_alpha imaging.

Contribution

First wide-field H_alpha imaging study revealing a proto-cluster with multiple merging clumps and detailed spatial distribution of star-forming galaxies at high redshift.

Findings

Identified 68 H_alpha emitters with SFRs down to 8.6 Msun/yr

Detected three prominent clumps of star-forming galaxies

Observed extended H_alpha emission in the radio galaxy

Abstract

We report a discovery of a proto-cluster in vigorous assembly and hosting strong star forming activities, associated to a radio galaxy USS 1558-003 at z=2.53, as traced by a wide-field narrow-band H_alpha imaging with MOIRCS on Subaru Telescope. We find 68 H_alpha emitters with dust-uncorrected SFRs down to 8.6 Msun/yr. Their spatial distribution indicates that there are three prominent clumps of H_alpha emitters, one surrounding the radio galaxy and another located at ~1.5 Mpc away to the south-west, and the other located in between the two. These contiguous three systems are very likely to merge together in the near future and may grow to a single more massive cluster at later times. Whilst most H_alpha emitters reside in the "blue cloud" on the color--magnitude diagram, some emitters have very red colors with J-Ks>1.38(AB). Interestingly, such red H_alpha emitters are located towards the faint end of the red sequence, and they tend to be located in the high density clumps. We do not see any statistically significant difference in the distributions of individual star formation rates or stellar masses of the H_alpha emitters between the dense clumps and the other regions, suggesting that this is one of the notable sites where the progenitors of massive galaxies in the present-day clusters were in their vigorous formation phase. Finally, we find that H_alpha emission of the radio galaxy is fairly extended spatially over ~4.5 arcsec. However it is not as widespread as its Lya halo, meaning that the Lya emission is indeed severely extended by resonant scattering.