The local environment of flat galaxies

Abstract

The existence of flat or bulgeless galaxies poses a challenge to the hierarchical structure formation scenario advocated by modern cosmology. We determine the geometrical environment of a sample of 315 flat galaxies from the Revised Flat Galaxy Catalog (RFGC) using 'local dimension' D, which, on a given length scale, quantifies the dimension of the cosmic structure in which a galaxy is embedded. For galaxies residing in filaments, nodes and sheets, D ∼ 1, D ∼ 1.5 and D ∼ 2 respectively; D ∼ 3 represents field galaxies. We also determine the local dimensions of a sample of 15,622 non-flat galaxies identified in the Galaxy Zoo project from the Sloan Digital Sky Survey (SDSS). We find that the median values of D for the flat and the non-flat galaxies are 2.2 and 1.8 respectively, implying that flat galaxies are located in a relatively sparser environment compared to non-flat galaxies; a Kolmogorov-Smirnov (KS) test indicates that their geometrical environments are different at > 99% confidence level. Further, using a group finding algorithm, we study the local environment of a subset of 779 flat galaxies with major-to-minor axes ratio a/b > 10 identified as superthin galaxies. We find that the median clusterization index k min for superthin flat galaxies ∼ 2.3 while ∼ 1.7 for other flat galaxies, confirming that the superthins reside in an under-dense environment compared to other flat galaxies at > 98% confidence level. Our results may therefore have important implications for the formation and evolution models of flat galaxies in the universe.