The Role of Neutral Hydrogen in the Evolution of Sprial and Irregular Galaxies

Extract

The Hubble classification suggested for the spiral galaxies, Sa–Sb–Sc–Sm–Irr, was considered by many astronomers as an evolution succession. The version of transformation from early spirals to the late spirals and vice versa was considered. The existence of several parameters, monotonically increasing or decreasing along this succession, together with the parameters which do not depend on the structural type of the galaxies of this succession, is a confirmation of this succession to be a succession of stages of evolution. Let us consider in detail both types of parameters. About the full masses and luminosities of galaxies we can state, that except Irr and maybe Sm-galaxies there is not a clear expressed difference of mean values of these parameters for galaxies of different structural types. Irr galaxies are the systems with low masses and luminosities, but on the (M-L) diagram a part of them is obviously mixed with the late spirals. There is a strong change of the M/L ratio with changing of the structural type, though the early type galaxies on average apparently have the least values of that ratio. As monotonically changing parameters we can suggest the following three: