Galaxy evolution and large-scale structure in the far-infrared. II - The IRAS faint source survey

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Abstract

Using the new IRAS Faint Source Survey data base at |b^II^| > 50^deg^, we confirm the steep slope of the faint 60 micron source counts discovered by Hacking, Condon, and Houck. The anisotropy between the 60 micron source counts in the northern and southern Galactic caps, found by Rowan-Robinson et al., is also confirmed and found to extend almost to the faint limit of our sample. Neither result can be attributed to local satellite effects related to survey strategy and calibration techniques, to data analysis techniques, or to Galactic cirrus, and they are thus likely to be extragalactic in origin. The count anisotropy at bright flux densities is due to the Local Supercluster, but at the faintest levels it has a characteristic distance of > 100h^-1^ Mpc. If 60 micron luminosity traces galaxy mass, the simplest explanation is a large mass overdensity (supercluster) in the northern cap of comparable projected size to the cap itself (i.e., 100-150 h^-1^ Mpc), and/or an underdensity (void) of similar size in the southern cap. The two-dimensional sky distribution of the Faint Source Survey extragalactic 60 micron sources is considerably smoother than that of optically selected galaxies from the major catalogs. This is partly due to the greater median distance (factor of ~ 2) of Faint Source Survey galaxies, compared to optical catalogs, and also to a bias of the far- infrared toward late-type galaxies. The slope of the faint number counts requires some cosmological evolution. The rate of evolution is much higher than predicted by conventional models for passively evolving galaxies, supporting models in which starbursts are important elements of evolution. The range of luminosity evolution rates which are compatible with the 60 micron number count data is consistent with evolutionary rates of QSOs and radio sources. This result supports a connection between active nuclei and infrared-luminous events. Density evolution is also consistent with the infrared data. A scenario in which the percentage of galaxies undergoing an infrared luminous event is evolving, in turn caused by evolution of the galaxy-galaxy interaction rate, is then attractive.

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