Coma Cluster

Abstract: First results are presented of a program of two-color photometry of very distant (z greater than or equal to 0.4) clusters of galaxies, using the KPNO ISIT vidicon. It is demonstrated that the ISIT system is capable of accurate photometry of faint (V approx. 23.5) galaxies in exposure times of about 1 hour on a 2.1-m telescope. Photometry is presented for two clusters, Cl 0024 + 1654 (z = 0.39) and the cluster around 3C 295 (z = 0.46). Both clusters are rich, centrally condensed systems similar to the Coma cluster. However, in contrast to nearby clusters of this type, which contain only elliptical and S0 galaxies, between one-third and one-half of the galaxies in these two distant clusters have the colors of spiral galaxies. The possible implications of this result for the evolutionary history of S0 galaxies are discussed. 17 figures, 3 tables.

Abstract: We report the discovery of 47 low surface brightness objects in deep images of a 3° × 3° field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness μ(g, 0) ranging from 24-26 mag arcsec–2 and effective radii r eff = 3''-10'', as measured from archival Canada-France-Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r eff = 1.5-4.6 kpc their sizes are similar to those of L * galaxies even though their median stellar mass is only ~6 × 107 M ☉. The galaxies are relatively red and round, with g – i = 0.8 and b/a = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope Advanced Camera for Surveys (ACS) observation. The ACS imaging shows a large spheroidal object with a central surface brightness μ475 = 25.8 mag arcsec–2, a Sersic index n = 0.6, and an effective radius of 7'', corresponding to 3.4 kpc at the distance of Coma. The galaxy is not resolved into stars, consistent with expectations for a Coma cluster object. We speculate that these "ultra-diffuse galaxies" may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.

Abstract: The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions. We propose a two-phase model in which the relativistic electrons injected in the Coma cluster by some processes (starbursts, AGNs, shocks, turbulence) during a first phase in the past are systematically reaccelerated during a second phase for a relatively long time (∼1 Gyr) up to the present time. We show that for reacceleration time-scales of ∼0.1 Gyr this hypothesis can well account for the radio properties of Coma C. For the same range of parameters which explain Coma C we have calculated the expected fluxes from the inverse Compton scattering of the Cosmic Microwave Background (CMB) photons, finding that the hard X-ray tail discovered by BeppoSAX may be accounted for by the stronger reacceleration allowed by the model. The possibility of extending the main model assumptions and findings to the case of the other radio haloes is also discussed, the basic predictions being consistent with the observations.

Abstract: Gavazzi and Jaffe found that late-type spiral galaxies in rich clusters are radio sources 10 times more powerful than spirals outside clusters. Gavazzi and Jaffe propose that ram pressure from the cluster medium causes collapse of dense molecular clouds, star formation, and other activity in the spirals disks. Bothun and Dressler similarly explain blue H I poor disk galaxies found in the central regions of the Coma Cluster via ram pressure effects. With computer simulations, it is shown in the present work that the tidal field of the cluster as a whole is physically sufficient to trigger cluster spiral nuclear and disk activity. It is shown that tidally induced collisions of disk gas clouds should be much more effective than ram pressure in causing activity. The observed tendency for disk galaxies in centers of rich clusters to be barred and S0 is explained as due to the cluster tidal field. Metals in the cluster medium would be enhanced by the expulsion of spirals enriched gas via tidally induced activity. 36 refs.