Low Surface-Brightness Gas-Rich Galaxies

Low surface brightness (LSB) HI-rich galaxies will probably be discovered in significant numbers in the multibeam survey. I believe that these gas-rich LSB galaxies are astrophysically very interesting and, in this section, I will attempt to intepret them in terms of an argument due to Fall (see Fall 1985).

The collapse of a disk within a dark corona

In simulations of the growth of density fluctuations in the expanding universe (eg Zurek et al. 1988), blobs of matter acquire angular momentum from each other by tidal torques. Say a blob of matter (dark + gas) has a mass M, energy E and angular momentum J. The parameter is dimensionless and is a measure of the ratio (rotational velocity)/(virial velocity) for the blob. For disks in centrifugal equilibrium, . These cosmological simulations give mean values of for identifiable galaxy-sized objects, almost independent of the mass of the blob, its density, initial overdensity, and the adopted fluctuation spectrum.

In a simplistic version of the present view of disk galaxy formation, the gas in the blob settles by dissipation into centrifugal equilibrium within the dark matter distribution. Let the gas and the dark matter be initially well mixed and have similar specific angular momentum J/M. Then the gas needs to dissipate (conserving its J) to form an equilibrium disk with , having started with . We can estimate the collapse factor in terms of the initial . Say the dark matter has an density distribution, truncated at some radius , and let its constant circular velocity be V. From the virial theorem, it follows that the halo has . Now assume that the gas settles to an exponential disk with scalelength , in centrifugal equilibrium in the potential of the halo, so the disk has . Then if , it follows that the collapse factor

For our Galaxy, kpc (eg Freeman 1987), so the truncation radius of the halo should be about 120 kpc: this is entirely consistent with the observed extent of the galactic dark halo (eg Freeman 1996).

High galaxies

The observed distribution of the values in simulations has a mean of about 0.05 with a tail extending up to about (e.g Zurek et al. 1988). From equation (3), the collapse factor , so the mean surface density of the gas in the equilibrium disk . A typical galactic disk now has a surface density M pc at , so a high disk with say would have a much lower surface density, M pc. The threshold surface density for massive star formation is believed to be about 5 to 10 M pc (e.g. Kennicutt 1989), so one might expect high galaxies to have low surface density, with star formation suppressed and confined to the higher density inner regions. The typical HI surface densities in gas-rich LSB galaxies (giants and dwarfs) are in the range (2 to 5) M pc.

Some low surface brightness HI-rich galaxies

Here are some examples of LSB HI-rich galaxies, both giants and dwarfs. Table 1 gives dynamical and structural parameters (radial velocity in km or distance in Mpc estimated from the brightness of resolved objects in the nearby dwarfs NGC 2915 and DDO 154; central surface brightness in B mag arcsec; HI line width in km ; absolute magnitude; exponential scalelength in kpc for the luminous component). NGC 5084 is a recent addition to the list of likely LSB gas-rich giants. It is an edge-on galaxy with a faint, extended disk and a very large HI line width. Its disk would be about 3 mag fainter in surface brightness if it were seen face-on, and would probably be undetected. We note that

1. all of these galaxies were discovered optically, by their brighter central regions.
2. the Tully-Fisher law breaks down for these systems, as one might expect.
3. the two dwarfs have very large HI diameters: their (HI diameter)/(Holmberg diameter) .

TABLE 1: DYNAMICAL AND STRUCTURAL PARAMETERS FOR LSB GALAXIES


Table 2 gives the integrated mass and blue M/L ratio out to the outermost rotation point, and the total HI mass and HI mass to light ratio (all in solar units). We note that

1. the total M/L ratios for these systems are much larger than the usual M/L = 3 to 5 for spiral disks. Although their low surface brightness disks may give them a fragile appearance, they are in fact no more fragile than other disk galaxies, because the dark matter dominates in their outer regions.
2. the giant LSB gas rich galaxies are detectable with the multibeam system out to redshifts of 7000 to 14000 km with 5 minute integrations.
3. dwarf systems like NGC 2915 are detectable out to about 3000 km in 5 minutes.

TABLE 2: MASSES AND M/L RATIOS FOR LSB GALAXIES


Briggs (1990) has argued that there could be giant LSB galaxies with M M nearer than the prototype object Malin 1. Although the LSB galaxies in Tables 1 and 2 were discovered optically because they have bright central regions, it is possible that there are undiscovered giant LSB galaxies which are dark or almost dark. If they exist and contain HI in masses similar to the giants in the Tables, then the multibeam survey should detect them.

© Copyright Astronomical Society of Australia 1997