Abstract
We present a spectroscopic analysis of two galaxy clusters at z 0.2, out to ∼4 Mpc. The two clusters VMF73 and VMF74 as identified by Vikhlinin et al. (1998) were observed with multiple object spectroscopy using MOSCA at the Calar Alto 3.5 m telescope. Both clusters lie in the ROSAT Position Sensitive Proportional Counter field R285 and were selected from the X-ray Dark Cluster Survey (Gilbank et al. 2004) that provides optical V- and I-band data. VMF73 and VMF74 are located at respective redshifts of z = 0.25 and z = 0.18 with velocity dispersions of 671 km s -1 and 442 km s -1 , respectively. Both cluster velocity dispersions are consistent with Gaussians. The spectroscopic observations reach out to ∼2.5 virial radii. Line strength measurements of the emission lines H α and [O II]λ3727 are used to assess the star formation activity of cluster galaxies which show radial and density dependences. The mean and median of both line strength distributions as well as the fraction of star forming galaxies increase with increasing clustercentric distance and decreasing local galaxy density. Except for two galaxies with strong H α and [O II] emission, all of the cluster galaxies are normal star forming or passive galaxies. Our results are consistent with other studies that show the truncation in star formation occurs far from the cluster centre.
Highlights
Galaxy properties such as colour, morphology and spectral characteristics are functions of redshift as well as of galaxy environment
Further evidence for an evolution of galaxy properties with redshift is provided by studies which show that the universal average star formation rate (SFR) has been rapidly declining since z ∼ 1 (e.g. Lilly et al 1996; Madau et al 1996; Blain et al 1999; Somerville et al 2001)
We have observed three sky fields selected from the X-ray Dark Cluster Survey (XDCS) (Gilbank et al 2004), each containing two galaxy clusters, with multiobject spectroscopy
Summary
Galaxy properties such as colour, morphology and spectral characteristics are functions of redshift as well as of galaxy environment. B. Gerken et al.: Star formation activity out to the cluster infall regions cluster galaxies on the environment has been studied based on the clustercentric radius e.g. by Whitmore & Gilmore (1991, 1993) and based on local galaxy density by Dressler (1980); Dressler et al (1997) and others. Gerken et al.: Star formation activity out to the cluster infall regions cluster galaxies on the environment has been studied based on the clustercentric radius e.g. by Whitmore & Gilmore (1991, 1993) and based on local galaxy density by Dressler (1980); Dressler et al (1997) and others It has been discussed which parameter is more appropriate, i.e. whether galaxy morphology is influenced rather by global properties, characterized by the clustercentric distance, or by local properties like the local galaxy density.
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