At the low-mass end

Abstract

The dwarf irregular galaxy GR 8, which is at the extreme faint end of the luminosity and mass functions, is studied using optical photometry and 21 cm H I line observations. For the light distribution, we find an exponential decline (as usual for more massive systems) with an extrapolated central surface brightness of B(0)c = 22.6 mag arcsec-2 and a scale length of α-1 = 76 pc. The colors are typical of Im galaxies with (B - V) = 0.38. The H I is found to be clumpy with most of the gas just outside the optical image. Its radial distribution is well approximated by a Gaussian. From the kinematics, it is found that, contrary to what is normally observed, the rotation axis of the gas is parallel to the common major axis of the optical and H I distribution and not perpendicular. It is shown that rotation is only important to the gravitational support of the system in the inner parts (r<250 pc). GR 8 is one of the very few nonelliptical systems known (with M81 dwA) where the random motions provide essentially all the support in the outer parts (r≥500 pc). The Gaussian nature of the H I distribution and the isothermal distribution of the H I velocity dispersion implies μ α R3 in the outer regions of GR 8 (i.e., the stellar disk and the H I lie in the approximately uniform density core of the dark halo). These results show that, even at the low-mass end, galaxies like GR 8 are embedded in the usual dark matter halo.