The effect of cluster environment on galaxy evolution in the Pegasus I Cluster.

摘要

We present 21 cm and optical wavelength observations of 54 galaxies in the Pegasus I Cluster. The 21 cm observations include single dish HI measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as HI images, obtained with the VLA for 10 of these. The optical wavelength observations include broadband B and R, as well as narrowband Halpha imaging, obtained with the SOAR telescope for 8 of these galaxies, and B-band, I-band, and Halpha imaging of 2 of these, obtained with the MDM telescope. The Arecibo profiles reveal an overall HI deficiency in the cluster, with ∼40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2--3. The HI morphology of some galaxies shows that the HI disk is smaller than the optical disk and slightly offset from the stars. We find a correlation between HI deficiency and the ratio of the HI disk size to optical disk size, with the more HI deficient galaxies having relatively smaller HI disks. The Halpha imaging indicates the HI deficient galaxies have reduced star formation rates (SFR) and smaller Halpha disks, with normal stellar disks. These configurations are usually attributed to an interaction between the interstellar medium (ISM) of the galaxy and the hot intracluster medium (ICM). Such a result is surprising since the Pegasus Cluster has a low level of X-ray emission, and a low velocity dispersion. The low velocity dispersion, coupled with the lack of a dense hot ICM indicate that ram pressure stripping should not play a significant role in this environment. In addition, two of the galaxies, NGC7604 and NGC7648, are morphologically peculiar. Their peculiarities indicate contradictory scenarios of what is triggering their unusual star formation. Halpha imaging, along with long-slit spectroscopy of NGC7648 reveal morphological features which point to a recent tidal interaction. On the other hand, Halpha imaging of NGC7604 reveals a strong episode of star formation concentrated into an asymmetric arc, preferentially located on one side of the galaxy. VLA HI mapping shows the HI also highly concentrated into that region, suggestive of a ram pressure event. Our data indicate that ISM-ICM interactions may play a role in a wider variety of environments than suggested by simple ram pressure arguments.