The H I structure of nine intrinsically faint dwarf galaxies

The neutral atomic hydrogen (H I) in nine dwarf irregular galaxies, with absolute magnitude fainter than MB= - 14^m: LGS-3, UGC 4483, DDO 69 (Leo A), C Vn dwA, DDO 155 (GR 8), DDO 187, Sag DIG, DDO 210, DDO 216 (Pegasus), was mapped with the Very Large Array (VLA) at a typical resolution of ≲18″ and velocity resolution of 6 km s^-1 during 1981.9 to 1983.2. These observations probe the internal motions and the interstellar medium in these faint galaxies, and provide a dynamical mass determination. The nine dwarf irregular galaxies, at the extreme faint end of the galaxy luminosity function, are H I rich, with high Script M signH/LB ratios ranging between 0.2 and 7 (except for LGS-3). The H I mass ranges between 2×10⁵ Script M sign⊙ to 8×10⁷Script M sign⊙. The H I column density distribution, which is clumpy on the one to three hundred parsec scale, is generally not well correlated with the stellar distribution. The clumps or clouds are similar to the giant molecular clouds in our Galaxy in terms of size and mass. The H I mean density (∼1 to 10 cm^-3) and column density (as high as 4×10²¹ cm^-2 or 12Script M sign⊙/pc²) are similar to those found in larger galaxies. Thus, unlike optical surface brightness, the H I column density of dwarf galaxies does not decrease with absolute luminosity. Only two galaxies of the sample show a velocity gradient that may be due to rotation in the H I gas system; the velocity fields of the H I gas in these dwarf galaxies are dominated by chaotic motion rather than by rotation (i.e., Vrot/σ≲1). The radial velocity dispersion of the H I, at one to a few hundred parsec scale, ranges between 5 and 9 km s^-1. The global linewidths, therefore, imply a clump-to-clump dispersion ranging between 6 to 11 km s^-1. Assuming the H I gas in the faint dwarf galaxies is in a steady state, we have derived Virial masses, Script M signVT, of the galaxies. For some of the faint dwarf galaxies, the derived total mass-to-light ratio, Script M signVT/LB, ranges between 7 and 26, indicating the possible presence of dark matter. The others have ratios smaller than 7. In the case of the smallest galaxy in the sample-LGS 3, the existence of a halo of massive neutrinos would imply a neutrino mass, mν≲200 eV, which conflicts with the upper limit to mν set by the critical density of the Universe. However, considerable uncertainties in Script M signVT are involved. Furthermore, the presence of substantial amount of undetected molecular hydrogen is a possibility and can account for the inferred total mass. These small galaxies also provide a setting for testing the proposed modified Newtonian dynamics (MOND) at low accelerations; the inferred mass according to MOND is smaller than the observed H I mass of seven of the galaxies. The presence of substantial amount of H I in the faint dwarf galaxies raises interesting questions about how star formation depends on the gas properties. Because of the inadequacy of rotational support, dissipation of the kinetic energy of the H I gas should lead to collapse of the H I gas and eventual star formation on a time scale short compared to the age of the galaxies. The low metal abundance in these low luminosity, low surface brightness, galaxies and the observed quiescence of star formation activities, pose limits on the number of episodes of star formation in the history of these galaxies. The support of the H I gas against collapse and star formation in such dwarf systems is not understood. It is a puzzle how H I gas with little rotational support can exist in these faint dwarf irregular galaxies until the present. Resolving the paradox would lead to a better understanding of the factors controlling star formation and the evolution of galaxies.

Duke Scholars

Author Ken H Young Pathology