Spectroscopy of Massive Galaxy Cluster Members


Photo of Alexei Finoguenov
Alexis Finoguenov
University of Helsinki, Finland


Spectra obtained to find spectroscopic redshifts of massive galaxy clusters selected in x-ray imaging, with the goal of finding the three-dimensional structure of those clusters

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
CLUSTER_MEMBER 16 Member of a galaxy cluster selected from ROSAT All-Sky Survey data 2,823 2,757


Obtaining spectroscopic redshifts for galaxy clusters serves a broad range of cosmology applications. Spectroscopic confirmation removes the uncertainties and biases associated with photometric redshifts, and is the only method that measures the three-dimensional structure of the cluster and its surroundings. Confirmation of membership removes projection effects and improves the richness estimates of cluster finders. Velocity dispersion estimates for cluster galaxies provide a measurement of dynamical cluster mass that is independent of masses derived from gravitational lensing.

This cluster follow-up program is a prototype for spectroscopic programs that will target cluster candidates in the eROSITA survey. Sources selected for this ancillary target program were candidate member galaxies of clusters, selected as the optical counterparts to faint sources in the ROSAT All-Sky Survey (Voges et al. 1999, Voges et al. 2000).

This ancillary target program increases the number of spectroscopic cluster members from 5 to 20 for rich clusters and from none to a few for low-mass clusters.

Target Selection

Selected sources were optical counterparts to X-ray clusters selected as faint sources in the ROSAT All-Sky Survey (Voges et al. 1999, Voges et al. 2000), identified by applying the redMaPPer (Rykoff et al. 2014) cluster finding algorithm to the position of an X-ray source. The X-ray magnitude limit corresponds roughly to the brightest 30% of clusters that the X-ray satellite eROSITA will find within the BOSS survey area.

Objects denoted by the target flag CLUSTER_MEMBER are selected from the redMaPPer catalog with icmodel < 19.9 and ifib2 < 21.5 to ensure high redshift success rates.


Rykoff, E. S., et al. 2014,, ApJ, 785, 104

Voges, W., et al. 1999, A&A, 349, 389

Voges, W., et al. 2000, IAU Circ., 7432, 1