The clustering of warm and cool IRAS galaxies

We use a series of statistical techniques to compare the clustering of samples of IRAS galaxies selected on the basis of their far-infrared emission temperature, to see whether a temperature-dependent eeect, such as might be produced by interaction-induced star formation, could be responsible for the increase in clustering strength with redshift in the QDOT redshift survey that has been reported by several authors. The temperature-luminosity relation for IRAS galaxies means that warm and cool samples drawn from a ux-limited sample like QDOT will sample quite diierent volumes of space. To overcome this problem, and to distinguish truly temperature-dependent results from those depending directly on the volume of space sampled, we consider a pair of samples of warmer and cooler galaxies with matched redshift distributions, as well as pairs of samples selected using a simple temperature cut. We nd that the redshift space autocorrelation function of warm QDOT galaxies is signiicantly stronger than that of cool galaxies on large scales, but that this diierence disappears when we come to consider the warmer and cooler samples with matched redshift distributions. A counts-in-cells analysis reveals no signiicant diierence between the clustering of the warm and cool QDOT samples, while the use of a new, symmetric estimator reveals that the cross-correlations of warm and cool IRAS galaxies with Abell clusters do not diier signiicantly. A higher signal-to-noise test is provided by computing the projected cross-correlations of the matched samples with the parent two-dimensional catalogue from which QDOT is drawn and this does yield a marginal detection of greater large scale power for warmer galaxies. A direct comparison of the distributions of the warmer and cooler samples, using a new technique which tests the null hypothesis that they are drawn from the same population, reveals that the two classes of galaxy do cluster diierently on small scales in redshift-space, while their (;) plots suggest that the apparent concentration of more warm IRAS galaxies into richer environments reeects the fact they sample richer volumes of space within the QDOT survey, rather than illustrating a correlation between temperature and richness. We conclude that there may be a temperature-dependent component to the observed increase in the clustering strength of QDOT galaxies with redshift, but that it is less important than a sampling eeect, which reeects the local cosmography, rather than the physical properties of the galaxies and their environment. We discuss the implications of this work for the …