[Paper Review] An Extreme Starburst in Close Proximity to the Central Galaxy of a Rich Galaxy Cluster at z=1.7

Tracy Webb, Allison Noble+20 more|arXiv (Cornell University)|Aug 20, 2015

Galaxies: Formation, Evolution, Phenomena78 references27 citations

TL;DR

This study identifies a rare, rich galaxy cluster at z=1.7089 hosting an extreme starburst in its central Brightest Cluster Galaxy (BCG), driven by a gas-rich interaction. Multiwavelength data reveal a star formation rate of ~860 M☉/yr, with a 66 kpc 'beads on a string' structure indicating recent wet mergers, offering direct evidence of early BCG mass assembly via wet mergers at high redshift.

ABSTRACT

We have discovered an optically rich galaxy cluster at z=1.7089 with star formation occurring in close proximity to the central galaxy. The system, SpARCS104922.6+564032.5, was detected within the Spitzer Adaptation of the red-sequence Cluster Survey, (SpARCS), and confirmed through Keck-MOSFIRE spectroscopy. The rest-frame optical richness of Ngal(500kpc) = 30+/-8 implies a total halo mass, within 500kpc, of ~3.8+/-1.2 x 10^14 Msun, comparable to other clusters at or above this redshift. There is a wealth of ancillary data available, including Canada-France-Hawaii Telescope optical, UKIRT-K, Spitzer-IRAC/MIPS, and Herschel-SPIRE. This work adds submillimeter imaging with the SCUBA2 camera on the James Clerk Maxwell Telescope and near-infrared imaging with the Hubble Space Telescope (HST). The mid/far-infrared (M/FIR) data detect an Ultra-luminous Infrared Galaxy spatially coincident with the central galaxy, with LIR = 6.2+/-0.9 x 10^12 Lsun. The detection of polycyclic aromatic hydrocarbons (PAHs) at z=1.7 in a Spitzer-IRS spectrum of the source implies the FIR luminosity is dominated by star formation (an Active Galactic Nucleus contribution of 20%) with a rate of ~860+/-30 Msun/yr. The optical source corresponding to the IR emission is likely a chain of of > 10 individual clumps arranged as "beads on a string" over a linear scale of 66 kpc. Its morphology and proximity to the Brightest Cluster Galaxy imply a gas-rich interaction at the center of the cluster triggered the star formation. This system indicates that wet mergers may be an important process in forming the stellar mass of BCGs at early times.

Motivation & Objective

• To investigate the role of wet mergers in the early formation of massive Brightest Cluster Galaxies (BCGs) at high redshift.
• To characterize the star formation activity and structural morphology of the central galaxy in a rare, optically rich cluster at z=1.7.
• To determine whether cooling flows or galaxy interactions dominate the fueling of extreme starbursts in cluster cores at early cosmic epochs.
• To assess the contribution of star formation versus AGN activity to the ultra-luminous infrared emission in the cluster's central galaxy.
• To evaluate the dynamical and structural evidence for a recent gas-rich interaction triggering the starburst.

Proposed method

• Spectroscopic confirmation of the cluster using Keck-MOSFIRE, yielding a redshift of z=1.7089 and identifying 27 member galaxies.
• Photometric richness estimation from 3.6 µm imaging (Spitzer/IRAC), yielding N_gal = 30 ± 8 within 500 kpc.
• Mass estimation via velocity dispersion (430+80/-100 km/s) and standard scaling relations, yielding M ≈ 3.8 ± 1.2 × 10^14 M☉ within 500 kpc.
• Multiwavelength analysis combining HST/WFC3 NIR imaging, SCUBA2 submillimeter, Spitzer/MIPS/IRAC, Herschel/SPIRE, and CFHT/UKIRT data to probe SED and morphology.
• Infrared spectral analysis using Spitzer-IRS to detect polycyclic aromatic hydrocarbon (PAH) features, confirming star formation dominance over AGN (20% AGN contribution).
• Morphological decomposition of the central region to identify the 'beads on a string' structure and its spatial correlation with mid/far-IR emission.

Experimental results

Research questions

• RQ1 What is the origin of the extreme star formation rate (~860 M☉/yr) observed in the central galaxy of this z=1.7 cluster?
• RQ2 Is the star formation in the BCG driven by a cooling flow or by a recent wet merger involving gas-rich galaxies?
• RQ3 What is the dynamical mass and structural richness of this cluster, and how does it compare to other z>1.5 clusters?
• RQ4 To what extent is the ultra-luminous infrared emission (L_IR = 6.2 ± 0.9 × 10^12 L☉) powered by star formation versus AGN activity?
• RQ5 How do the observed 'beads on a string' clumps in the central region relate to the star formation and merger history?

Key findings

• The cluster at z=1.7089 is spectroscopically confirmed with 27 member galaxies, confirming its richness and high redshift.
• The cluster's mass within 500 kpc is estimated at 3.8 ± 1.2 × 10^14 M☉, consistent with other massive clusters at z>1.5.
• The Brightest Cluster Galaxy hosts an ultra-luminous infrared source with L_IR = 6.2 ± 0.9 × 10^12 L☉, indicating extreme star formation activity.
• Spitzer-IRS spectroscopy detects PAH features at z=1.7, confirming that the FIR luminosity is dominated by star formation with a corrected SFR of ~860 ± 130 M☉/yr.
• HST/WFC3 imaging reveals a 66 kpc-long, clumpy structure resembling 'beads on a string', spatially coincident with the mid/far-IR emission, indicating ongoing or recent interaction-driven star formation.
• The morphology and proximity of the starburst to the BCG strongly suggest that the star formation was triggered by a gas-rich interaction, possibly involving the BCG itself, marking a potential new pathway for BCG mass assembly at high redshift.