Star formation activity of intermediate redshift cluster galaxies out to the infall regions

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 Vand 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 Hα 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 Hα 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.