A Re-examination of Galactic Conformity and a Comparison with Semi-analytic Models of Galaxy Formation

The observed correlation between star-formation in central galaxies and in their neighbours (a phenomenon dubbed “galactic conformity”) is in need of a convincing physical explanation. To gain further insight, we use a volume-limited sample of galaxies with redshifts less than 0.03 drawn from the SDSS Data Release 7 to investigate the scale dependence of the effect and how it changes as a function of the mass of the central galaxy. Conformity extends over a central galaxy stellar mass range spanning two orders of magnitude. The scale dependence and the precise nature of the effect depend on the mass of the central. In central galaxies with masses less than 1010M⊙, conformity extends out to scales in excess of 4 Mpc, well beyond the virial radii of their dark matter halos. For low mass central galaxies, conformity with neighbours on very large scales is only seen when they have low star formation rate or gas content. In contrast, at high stellar masses, conformity with neighbours applies in the gas-rich regime and is clearly confined to scales comparable to the virial radius of the dark matter halo of the central galaxy. Our analysis of a mock catalogue from the Guo et al (2011) semi-analytic models shows that conformity-like effects arise because gas-poor satellite galaxies are sometimes misclassified as centrals. However, the effects in the models are much weaker than observed. Mis-classification only influences the low-end tail of the SFR/M∗ distribution of neighbouring galaxies at large distances from the primary. The median and the upper percentiles of the SFR/M∗ distribution remain almost unchanged, which is in contradiction with the data. We speculate that the conformity between low-mass, gas-poor central galaxies and their distant neighbours may be a signature of “pre-heating” of the intergalactic gas at an earlier epoch. The smaller-scale conformity between high-mass, gas-rich central galaxies and their close neighbours may be a signature of ongoing gas accretion onto central galaxies in a minority of massive dark matter halos.