Far-infrared emission from rich clusters of galaxies.

Abstract

We have examined the frequency with which central dominant galaxies are sources of far-infrared (FIR) emission in a complete sample of clusters of galaxies. Previous searches using IRAS 60$\mu m$ and 100$\mu m$ measurements of small, inhomogeneous samples of dominant cluster galaxies found 20% to 50% of clusters to have significant FIR emission. In this new study, we analyze the $60\mu m$ and $100\mu m$ emission properties of cD galaxies in a complete sample of 163 Abell Clusters. For comparison, a control sample of 207 blank fields was analyzed to determine the distribution of spurious detections, which is greater than expected from Gaussian statistics. The contribution of Galactic cirrus at 60 $\mu m$ and 100 $\mu m$ to non-Gaussian noise is clearly demonstrated by the correspondence of a 98% confidence level to a signal to noise of 4 or 4.5 rather than to a signal to noise of 2 as expected from Gaussian statistics. After correcting for contaminated fields and spurious detections, we find that about 10% of cD galaxies in rich clusters are sources of FIR emission. Typical detected cDs have FIR luminosities of about $10\sp{44}$ erg $\rm sec\sp{-1}$, which is comparable to the blue luminosities from the dominant galaxy and the X-ray luminosities produced in the core of clusters. Dust masses derived from the 60$\mu m$ and 100 $\mu m$ fluxes are ${\sim}10\sp7 \rm M\sb\odot.$ Because only about 10% of the clusters have high FIR luminosities, such strong emission is probably a transient state for an individual cluster. Although one usually thinks of dust as being heated by starlight, in these clusters of galaxies we do not observe the large amounts of extinction we might expect from the FIR luminosities observed. The dust could also be collisionally heated by the energetic electrons in the hot intra-cluster medium (ICM). To test this model of gas to dust energy transfer, we have engaged in a program of X-ray and optical imaging of the FIR-bright clusters identified in our survey. We observed three clusters in the X-ray with the ROSAT satellite, and found one to have an electron density of 0.05 cm$\sp{-3},$ high enough to plausibly provide the heat source for the dust. This cluster, Abell 1991, has a number of characteristics that are suggestive of the stripping of gas from cluster galaxies, indicating that perhaps the dust is being injected into the ICM where it can be heated by the energetic electrons. Our optical data reveal only one of ten observed clusters to possess a dust feature visible in $B{-}I$ images. This cluster, Abell 1126, contains a weak dust feature detected at the 1.7 $\sigma$ level, with a dust mass of about one third that detected in the FIR. We find no evidence for dust heating by absorption of starlight sufficient to power the observed FIR emission any of the observed clusters.