[Paper Review] A search for clusters at high redshift - II. A proto cluster around a radio galaxy at z=2.16

L. Pentericci, J. Kurk+8 more|arXiv (Cornell University)|Aug 9, 2000

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy20 citations

TL;DR

This study confirms a massive, evolving galaxy proto-cluster around the high-redshift radio galaxy 1138–262 at z=2.16 using VLT spectroscopy, revealing 14 galaxies and one QSO within 1.5 Mpc, grouped into two substructures with velocity dispersions of ~500 km s⁻¹ and ~300 km s⁻¹, implying total dynamical masses exceeding 10¹⁴ M⊙ and supporting the formation of massive clusters in the early universe.

ABSTRACT

VLT spectroscopic observations of Ly alpha--excess objects in the field of the clumpy radio galaxy 1138-262 at z=2.16 have led to the discovery of 14 galaxies and one QSO at approximately the same distance as the radio galaxy. All galaxies have redshifts in the range 2.16 \pm 0.02, centered around the redshift of the radio galaxy, and are within a projected physical distance of 1.5 Mpc from it. The velocity distribution suggests that there are two galaxy subgroups having velocity dispersions of ~500 km/s and ~300 km/s and a relative velocity of 1800 km/s. If these are virialized structures, the estimated dynamical masses for the subgroups are ~9 and ~3x10E13 solar masses respectively, implying a total mass for the structure of more than 10E14 solar masses. The new observations, together with previous results, suggest that the structure of galaxies around 1138-262 is likely to be a forming cluster.

Motivation & Objective

• To investigate whether high-redshift radio galaxies like 1138–262 are located in overdense environments indicative of forming galaxy clusters.
• To determine the redshift distribution and kinematic structure of Lyα-emitting galaxies around 1138–262 to assess their dynamical state.
• To estimate the dynamical mass of the system and evaluate its potential to evolve into a virialized cluster.

Proposed method

• Conducted deep VLT FORS1 multi-object spectroscopy (MOS) with 600B grism to obtain redshifts of 75 candidate Lyα emitters in a 6.8′×6.8′ field around 1138–262.
• Used wavelength calibration from He and HgCd arc-lamps and flux calibration via standard star GD108 to ensure accuracy within 0.2 Å.
• Applied the gapper sigma estimator (Beers et al. 1990) to compute velocity dispersions for subgroups, improving robustness for small samples.
• Computed the angular two-point correlation function using the Landy-Szalay estimator to assess spatial clustering of galaxies relative to the radio galaxy.
• Excluded the QSO from velocity dispersion calculations to better isolate the galaxy group dynamics.
• Combined spectroscopic redshifts with prior narrow- and broad-band imaging (Paper I) to identify 15 galaxies at z≈2.16 within 1.5 Mpc.

Experimental results

Research questions

• RQ1 Is there a significant overdensity of galaxies at z≈2.16 around the radio galaxy 1138–262, consistent with a forming cluster?
• RQ2 Do the kinematic properties of the galaxies—particularly their velocity dispersion and spatial distribution—indicate virialized substructures or ongoing merging?
• RQ3 What is the dynamical mass of the system, and does it exceed the threshold required for cluster formation?
• RQ4 How do the spatial clustering patterns of the galaxies compare to random distributions, and what does this imply about the system’s structure?
• RQ5 Can the presence of hot X-ray-emitting gas and a massive central galaxy support the idea that this system is a proto-cluster?

Key findings

• Fourteen galaxies and one QSO were spectroscopically confirmed at redshifts within 2.16 ± 0.02, all within 1.5 Mpc of the radio galaxy 1138–262.
• The velocity distribution reveals two distinct subgroups with median redshifts of 2.145 ± 0.002 and 2.164 ± 0.002, separated by a relative velocity of 1800 km s⁻¹.
• The velocity dispersions of the subgroups are estimated at 520 ± 140 km s⁻¹ and 280 ± 70 km s⁻¹, respectively, using the gapper sigma estimator.
• The dynamical masses for the subgroups are estimated at ~9 × 10¹³ M⊙ and ~3 × 10¹³ M⊙, implying a total mass exceeding 10¹⁴ M⊙.
• The angular two-point correlation function shows significant clustering at ~25″ and ~150″ with 99.7% and 99.8% significance, respectively, indicating spatial clustering.
• Despite no significant spatial segregation between subgroups, the presence of substructures is consistent with hierarchical cluster formation models and observed in other high-redshift clusters.