NGC 2915.II.A Dark Spiral Galaxy With a Blue Compact Dwarf Core

This paper presents Australia Telescope Compact Array HI synthesis observations of the weak blue compact dwarf (BCD) galaxy NGC 2915. It is shown that NGC 2915 has the H I properties of a late-type spiral galaxy (Sd-Sm), including a double horn global profile, and H I spiral arms. The HI extends out to over five times the Holmberg radius, and 22 times the exponential scale length in the B band. The optical counterpart corresponds to a central HI bar. The H I distribution and kinematics are discussed in detail. A rotation curve is derived and fitted with a mass model consisting of a stellar disk, a neutral gas disk, and a dark matter (DM) halo. The DM halo dominates at nearly all radii. The total mass to blue light ratio, M_T_/L_B_ = 76 within the last measured point. Thus NGC 2915 is one of the darkest disk galaxies known. The complex H I dynamics of the central region results in a high uncertainty of many of the fitted parameters. Nevertheless it is clear that the core of the DM halo is unusually dense (ρ_0_ ~ 0.1 M_sun_ pc^-3^) and compact (R_c_ ~ 1 kpc). The neutral gas component, with mass M_g_ = 1.27 x 10^9^ M_sun_ is probably more massive than the stellar disk. Split and broad H I lines (velocity dispersion ~35 km s^-1^) are seen in the central region. Pressure support is probably significant, and it is not clear whether the core is in equilibrium. Beyond the optical disk the average H I line of sight velocity dispersion is 8 km s^-1^, which is normal for disk galaxies. NGC 2915 does not obey the Tully-Fisher [A&A, 54,661(1977)] relation, being underluminous for its V_rot_ = 88 km s^-1^ by a factor of nine. It also does not obey the star-formation threshold model of Kennicutt [ApJ, 344,685(1989)], when only the neutral gas is considered. A simple H I surface density threshold of {SIGMA}_H I,crit_~ 10^21^ cm^- 2^ adequately describes the location of current star formation. Although the HI properties of NGC 2915 are extreme relative to normal galaxies they appear less extreme in comparison to other BCDs, which have similar radial profiles of H I density and velocity dispersion, and H I extending well beyond the optical disk.