The conformity of HI galaxies in ALFALFA-SDSS sample
  The conformity effect, indicating the evolution of a galaxy is related to its
surrounding neighbour galaxies as far as a few Mpc, is an interesting
phenomenon in the modeling of galaxy and evolution. Here we study the
conformity effect of HI galaxies in a matched galaxy sample between SDSS DR7
and ALFALFA surveys. By checking the probability difference for the detected HI
galaxies as a function of distance around a normal or an HI galaxy, we find
that this effect is significant out to 5 Mpc. It also shows a dependence on the
stellar mass of galaxies, with the strength the strongest in the stellar mass
range of 10^10~10^10.5. However, when the sample is confined to central
galaxies in groups with virial radii smaller than 1 Mpc, the 1-halo conformity
signal is still evident, while the 2-halo conformity signal is reduced to a
very weak amplitude. Our results confirm the previous study in the optical band
that the 2-halo term is possibly caused by the bias effect in the sample
selection. Furthermore, we confirm the existence of the 1-halo conformity
discovered in the optical and radio band in previous lectures. Our results
provide another viewpoint of the conformity effect and hope to shed the light
on the co-evolution of the galaxies and their neighbours in the current galaxy
formation models.
