Ultra Deep Catalogue of Galaxy Structures

Ultra Deep Catalogue of Galaxy Structures in the Cosmic Evolution Survey field
Monthly Notices of the Royal Astronomical Society (2012), 423, 2436.

Ultra Deep Catalogue of Galaxy Structures in the Cosmic Evolution Survey field

Ilona K. Söchting1, Georgina V. Coldwell2,3, Roger G. Clowes4, Luis E. Campusano5, Matthew J. Graham6

1 University of Oxford, Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH
2 Instituto de Ciencias Astronómicas, de la Tierra y del Espacio, CONICET, San Juan, Argentina
3 Univeridad Nacional de San Juan, San Juan, Argentina
4 Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE
5 Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
6 CACR, California Institute of Technology, Pasadena, CA 91125, USA

This paper presents a large sample of intermediate- to high-redshift galaxy groups and clusters that have been detected using a fully automated search in the Cosmic Evolution Survey field. The detection algorithm is based on density-peak extraction from a density distribution that is sampled using Voronoi tessellation, within overlapping slices of photometric redshifts.

The cluster catalogue contains 1780 structures in the redshift range 0.2 < z < 3.0. The structures span three orders of magnitude in luminosity (108 < L4 < 5 × 1011 Lsun), and they have richness from eight to hundreds of galaxies. The catalogue is published as supporting information with the online version of the paper.

All clusters at z > 0.4, and many even below this threshold, show very prominent substructure, which indicates that z ∼ 0.4 marks the slow emergence of virialised clusters in this field, in agreement with published findings for other regions of the sky.

The redshift distribution of the structures shows strong variations, with prominent peaks that suggest the presence of large-scale structures across the whole range of redshifts. Supercluster candidates have been identified at redshifts z = 0.35, 0.72, 0.94, 1.12, 1.27, 1.45, 2.0 and 2.52.

At z = 2.9 we have identified a compact agglomeration of galaxy groups and clusters, suggesting the presence of another supercluster-like structure. It is the highest redshift candidate currently known.

Out of the nine supercluster candidates found in this catalogue, six are new detections.

Figure 1.

Figure 1.

Figure 1 illustrates the Voronoi Tessellation (VT) method for finding groups and clusters of galaxies. It is for real data (not simulated) for a slice in photometric redshift, 2.667 < z < 2.816. The black dots represent the galaxies within this slice. VT partitions the field into convex cells around each galaxy. All of the points within a cell are closer to its galaxy than to any other of the galaxies. The reciprocal of the area of each cell gives the local number-density of galaxies at the location of that cell. The eight asterisks represent a set of galaxies that has been identified as a poor cluster, and the surrounding polygon represents the boundary of the cluster. This cluster is at the limit of the detection criteria.

The VT method has been applied to data from the 2 deg2 COSMOS field (Scoville et al. 2007) , with 30-band photometric redshifts from Ilbert et al. (2009). The candidates for galaxy structures are detected as density peaks in narrow slices in photometric redshift. The slices have Δz=0.02×(1+z), with 50% overlap between them. A candidate structure is required to have a density peak that exceeds twice the mean density for the slice and at least eight connected cells that exceed a density threshold.

groups_clusters_zdist

Figure 2.

Figure 2 shows the photometric redshift distribution for the catalogue of galaxy structures. The catalogue contains 1780 structures ranging from galaxy groups to rich galaxy clusters.

Figure 3.

Figure 3.

Figure 3 shows the large-scale distribution of the luminous galaxy clusters as a function of RA (left panel) and Dec (right panel), for photometric redshift z < 1.5. The luminosity criterion is log10 ( Σi=4 Li / Lsun ) > 10.7.

Figure 4.

Figure 4.

Figure 4 illustrates the candidate supercluster at z = 2.9. The left-hand panel shows the distribution of all galaxies that are members of groups and clusters in the redshift range 2.70 < z < 3.00. The centre and right-hand panels show the RA and Dec distributions of groups and clusters as functions of redshift. The size of the plotting symbols in the centre and right-hand panels is proportional to log10 ( Σi=4 Li / Lsun ) of the structures.

Ilbert O. et al., 2009, ApJ, 690, 1236
Scoville N. Z. et al., 2007, ApJS, 172, 38

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