NGC 2915's oddness and how to put it in its place

NGC 2915 is an interesting galaxy because of its extremely odd qualities: its large DM content, the contrast between its optical and HI properties, and the high of its DM halo. It is also recalcitrant compared to normal galaxies because: (1) it does not obey the Tully-Fisher relationship; (2) its neutral gas disk is under dense for it to have the star formation observed in the core (Kennicutt, 1989); (3) similarly, the outer disk is underdense for it to have the spiral arms that are observed. This suggests that some of its DM is in the disk, perhaps in a molecular form. These properties are discussed more fully in Meurer et al. (1996).

 
Figure 3: DM halo central density compared to absolute magnitude, where both quantities have been corrected (to first order) for the different distance scales. Here is the dimensionless Hubble constant used in the individual studies. Circles represent fits to dI and spiral galaxies, while the squares represent the fits to NGC 2915 presented here. Solid symbols show isothermal sphere halo model fits, hollow symbols shows analytic halo model fits. Connected points represent different halo models fit to the same object. The dI and spiral data were taken from Puche & Carignan (1991), Begeman et al. (1991) and Sanders (1996 and references therein).

NGC 2915 is an important galaxy because it is not that odd compared to other BCDs which often have very extended HI and similar radial profiles of HI surface density (often centrally peaked) and velocity dispersion, as well as flat rotation curves at large radii (Taylor et al. 1994; 1995; Hunter et al. 1994; Meurer 1994; Brinks & Klein, 1988; Viallefond & Thuan, 1983). This suggests BCDs as a class have fundamental structural differences from dI and spiral galaxies, and NGC 2915 is only odd because it is the first BCD examined in such detail.

It is therefore important to do further detailed HI studies of BCDs. There are two issues in particular that need to be addressed. Firstly, if the abnormally high of NGC 2915 is common to BCDs, then transformations between BCD and dI morphologies could not be accomplished by turning starbursts on and off. Fundamental structural changes to the DM distributions would also have to occur, and this seems hardly likely. Such a result would make it hard or impossible to reconcile BCDs and dIs to some common evolutionary scenario. Secondly, the near equality between and of the DM in NGC 2915 suggests that the star formation is held in equilibrium with the DM halo. This is contrary to the common perception of explosive starbursts dominating the star formation and dynamics of BCDs. Further large sample comparisons of starburst dynamics and halo properties are needed to determine if there is a feedback mechanism operating between the two structures.

There may be a more mundane explanation for NGC 2915's peculiarities. Meurer et al. (1994) determined D from the colour-magnitude diagram of NGC 2915's bright stars and star-clusters. If it is more distant by a factor of two (i.e. larger than their stated D uncertainty) many of its odd properties would disappear. A better D determination and a search for faint stellar light associated with the HI disk (both could be done with the Hubble Space Telescope) may reduce NGC 2915's odd qualities to its peculiar motion relative to the Hubble flow.

So what is the relevance of NGC 2915 to the multibeam survey? NGC 2915 was painstakingly mapped by Becker et al. (1988) with the Parkes radio telescope, pointing at 19 positions plus sky, and requiring over three hours of telescope time. It is these observations that first indicated NGC 2915's huge HI extent. The multibeam survey is bound to uncover similar galaxies with extended HI disks using integration times of only a few minutes. This study illustrates what follow-up observations with the ATCA or Very Large Array may reveal about the DM distribution and dynamics of such galaxies.

© Copyright Astronomical Society of Australia 1997