Statistical evidence that galaxy companions trigger bars and change the spiral Hubble type

Abstract

The number of barred (SB-type) galaxies relative to the total number of spiral galaxies in binary and group systems and in the field has been examined. Among field and group galaxies, 0.32 ± 0.09 and 0.28 ± 0.02 of all early Hubble-type spirals are SB, whereas in binary systems, 0.50 ± 0.05 are SB. There is no obvious correlation like this for intermediate and late-type galaxies. This result implies that close galaxy companions are associated with bar formation, but primarily for early Hubble-types. In addition, the relative fraction of early-type galaxies (Sa-Sb) among all types Sa-Scd is examined. Among binary SB galaxies, the early-type fraction is 0.77 ± 0.05 whereas among all SA types in any environment the early-type fraction is only 0.44 ± 0.02. Thus binary galaxies tend to be both barred and early type. Considering recent N-body simulations, we suggest that galaxy interactions produce strong spirals, which, because of their torques, lead to bar formation and a significant mass inflow in the disk. For a prolonged encounter, the mass inflow can apparently change a nonbarred galaxy of intermediate Hubble-type, Sbc-Scd, to a barred galaxy of early Hubble-type, SBa-SBb, by making the galaxy density larger in the inner region and by removing a significant amount of the gas mass and, therefore star formation, from the outer disk.