A View of the M81 Galaxy Group via the Hα Window

Abstract

We present images for 36 galaxies of the M81 Group obtained in the Hα line. Estimates of the Hα flux and star formation rate (SFR) are now available for all the known members of the group with absolute magnitudes down to MB = -10 mag. The character of distribution of the galaxies over three parameters, MB, SFR, and total hydrogen mass, permits us to draw the following conclusions as to the evolutionary status of the group population: (1) Spiral and irregular-type galaxies have time to generate their luminosity (baryon mass) during the cosmological time T0 = 13.7 Gyr, but dwarf spheroidal objects are capable of reproducing only ~5% of their observed luminosity. (2) S and Im/blue compact dwarf (BCD) galaxies possess a supply of gas sufficient to maintain their observed SFRs only during the next T0 yr, while dIrr and dSph populations have a mean gas depletion time of about 3T0. (3) There is indirect evidence that the star formation in Im/BCD and dIrr galaxies proceeds in a mode of vigorous burst activity rather than in the form of a sluggish process. We note that the dwarf tidal system near NGC 3077, the Garland, has the highest SFR per unit luminosity among 150 galaxies of the Local Volume with known SFRs. Being averaged over the local cell of homogeneity of 4 Mpc in diameter around M81, the rate of star formation of the group, SFR = 0.165 M⊙ yr-1 Mpc-3, proves to be 5-8 times higher than the average global rate at z = 0.