Dark Matter Halo Properties

Figure 2 shows maximum disk mass model fits to NGCá 2915's curve. There are three components to the mass models: (1) a stellar disk parameterized by its mass to light ratio, here set at (the maximum allowed by the innermost measurement); (2) a neutral gaseous disk whose mass distribution is determined directly from the ATCA data; and (3) a spherical dark matter halo parameterized by a velocity dispersion , which determines the level of the flat part of the rotation curve, and the core radius , which determines the location of the knee in the rotation curve. These parameters yield the central DM density . Two types of DM halo were fitted (a) a non-singular isothermal sphere, and (b) an analytic DM density distribution:

Both forms produce flat curves at large R, but have different density distributions and different scaling relations between , , and the asymptotic at (Lake etá al.\ 1990; Binney & Tremaine, 1987). The parameters of the two fits are given in tableá 2. The isothermal halo does not fit our data as well as the analytic halo. However, many fits in the literature use this form to model the DM halo, so we include its fit for comparison.

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Table 2: DM halo fits

The important things to note about the mass model fits are: (1) The neutral gaseous disk is more massive than the maximum stellar disk, hence NGCá 2915 is not a very evolved galaxy. (2) DM dominates at nearly all radii. For the analytic model, within the last measured point, making NGCá 2915 along with DDO 154 (Carignan and Beaulieu, 1989), the darkest known disk galaxies. (3) The DM central velocity dispersion is equal to or greater than the in the center. Thus although the HI is violently energized by star formation, it is still in virial equilibrium with the DM halo. This suggests that the DM halo may be involved in regulating NGCá 2915's star formation: if the star formation rate gets too high it will over-energize the ISM so that , then the ISM will be blown out of the core and star formation will be halted. (4) The halo core is an order of magnitude denser than found in ``normal'' disk galaxies. This is illustrated in Fig.á 3 which plots versus for disk galaxies of various types.

á© Copyright Astronomical Society of Australia 1997