Cooperative galaxy formation and large-scale structure

We consider a model in which galaxy formation occurs at high peaks of the mass density field, as in the standard picture for biased galaxy formation, but is further enhanced by the presence of nearby galaxies. This modification is accomplished by assuming the threshold for galaxy formation to be modulated by large-scale density fluctuations rather than to be spatially invariant. We show that even a weak modulation can produce significant large-scale clustering. In a universe dominated by cold dark matter, a 2%-3% modulation on a scale exceeding 10 h^-1^ Mpc produces enough additional clustering to fit the angular correlation function of the APM galaxy survey. Such an effect might be detectable as a small (about 0.2 mag rms) spatial modulation of the characteristic scale in the galaxy luminosity function or as an apparent dependence of large-scale clustering on luminosity or distance in flux-limited samples. although such effects are not necessarily present in all simple versions of the model, they could provide an observational indicator for cooperative galaxy formation. We discuss several astrophysical mechanisms for which there are observational indications that cooperative effects could occur on the scale required. although the importance of such effects is highly uncertain, it would clearly be rash to exclude the possibility that large-scale power in the galaxy distribution is a consequence of the physics of galaxy formation. There is thus no compelling reason to reject the density fluctuation predictions of the simplest inflationary models.