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

Abstract

This paper presents Australia Telescope Compact Array H I 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 H I 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 H I 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 1 parameters. Nevertheless it is clear that the core of the DM halo is unusually dense (ρ 0 ≈ 0.1 M ⊙ pc −3 ) and compact (R c ≈ 1 kpc). The neutral gas component, with mass M g = 1.27×10 9 M ⊙ 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 (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 (1989), when only the neutral gas is considered. A simple H I surface density threshold of Σ HI,crit ≈ 10 21 cm −2 adequately describes the location of current star formation. Although the H I properties of NGC2915 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.

H I Imaging Observations of Superthin Galaxies. Ii. Ic 2233 and the Blue Compact Dwarf NGC 2537

“…This flocculent appearance of the H i is not confined to the star-forming portion of the galaxy, implying that it is not linked to feedback processes or to the instabilities giving rise to star formation. The overall H i distribution and morphology of NGC 2537 are similar to other low-mass starburst and poststarburst galaxies observed with comparable spatial resolution (e.g., Israel & van Driel 1990;Meurer et al 1996;Wilcots & Miller 1998;Hunter et al 1999;Thuan et al 2004), although frequently BCDs show global H i morphologies that are much more disorganized and irregular than seen in NGC 2537 (cf. van Zee et al , 2001Thuan et al 2004).…”

The dark matter halo shape of edge-on disk galaxies

“…On the other hand, similar velocity dispersions are seen in regions of star formation and in regions where there is no visible star formation (e.g. Meurer et al 1996), and both low and high surface brightness galaxies seem to have similar gas velocity dispersion. Sellwood & Balbus (1999) offer a plausible alternative, suggesting that weak magnetic fields in galaxies allow energy to be extracted from differential rotation via MHD-driven turbulence.…”

Kk 246: A Dwarf Galaxy With an Extended H I Disk in the Local Void

“…KK 246 is marked by a cross, and it and other extended disks from the FIGGS sample are labeled. Also included are extended H i disks from the literature ESO 215-G?009 (Warren et al 2004), DDO 154 (Carignan & Purton 1998), and NGC 2915 (Meurer et al 1996). Fig.…”