Influence of Cooling Flow and Galactic Motion on the Iron Distribution in Clusters of Galaxies

Iron abundance distribution is now known for 12 clusters of galaxies. For some clusters (e.g. Centaurus) the observed abundance increases toward the cluster center, while for the others (e.g. Coma and Hydra-A) no significant inhomogeneity was observed. In order to understand this difference, we investigate the influence of cooling flow and turbulence produced by galactic motion on the iron abundance distribution by simple spherical models. We show that the cooling flow has a significant effect to flatten the iron abundance distribution if the flow velocity is sufficiently large. Further, by applying our analysis to the above clusters we show that we can give a systematic account for the observed variety of the iron abundance distribution qualitatively.Comment: 30 pages, uuencoded compressed postscript with figures, YITP/U-94-2

A Possible Origin of Magnetic Fields in Galaxies and Clusters: Strong Magnetic fields at z~10?

We propose that strong magnetic fields should be generated at shock waves associated with formation of galaxies or clusters of galaxies by the Weibel instability, an instability in collisionless plasmas. The strength of the magnetic fields generated through this mechanism is close to the order of those observed in galaxies or clusters of galaxies at present. If the generated fields do not decay rapidly, this indicates that strong amplification of magnetic fields after formation of galaxies or clusters of galaxies is not required. This mechanism could have worked even at a redshift of ~10, and therefore the generated magnetic fields may have affected the formation of stars in protogalaxies. This model will partially be confirmed by future observations of nearby clusters of galaxies. Mechanisms that preserve the magnetic fields for a long time without considerable decay are discussed.Comment: Accepted for publication in MNRA

The Detection Rate of Molecular Gas in Elliptical Galaxies: Constraints on Galaxy Formation Theories

In order to constrain parameters in galaxy formation theories, especially those for a star formation process, we investigate cold gas in elliptical galaxies. We calculate the detection rate of cold gas in them using a semi-analytic model of galaxy formation and compare it with observations. We show that the model with a long star formation time-scale (~20 Gyr) is inconsistent with observations. Thus, some mechanisms of reducing the mass of interstellar medium, such as the consumption of molecular gas by star formation and/or reheating from supernovae, are certainly effective in galaxies. Our model predicts that star formation induced when galaxies in a halo collide each other reduces the cold gas left until the present. However, we find that the reduction through random collisions of satellite (non-central) galaxies in mean free time-scale in a halo is not required to explain the observations. This may imply that the collisions and mergers between satellite galaxies do not occur so often in clusters or that they do not stimulate the star formation activity as much as the simple collision model we adopted. For cD galaxies, the predicted detection rate of cold gas is consistent with observations as long as the transformation of hot gas into cold gas is prevented in halos whose circular velocities are larger than 500 km s^-1. Moreover, we find that the cold gas brought into cDs through captures of gas-rich galaxies is little. We also show that the fraction of galaxies with observable cold gas should be small for cluster ellipticals in comparison with that for field ellipticals.Comment: 6 pages, accepted by PAS