5.1 Observations of Individual Galaxies

Skillman et al. (1987) studied 4 dwarf irregular galaxies in Virgo. By comparing HI rotation curves and optical observations of these galaxies, they found that the mass models of 3 of these objects required significant DM halos. The central DM density in all 3 galaxies was around or above 0.01M pc^-3.

DDO 154 is a faint galaxy at M_B = -13.8 and is particularly gas-rich. Carignan and Freeman (1988) obtained its HI rotation curve out to 7.6 kpc, corresponding to a radius of about 15 ^-1, where ^-1 is the scale-length of the stellar component of the galaxy. A more detailed study, including a fuller discussion of the optical properties of this galaxy, was presented by Carignan and Beaulieu (1989). Within the last measured point of this rotation curve the mass ratio of dark to luminous material exceeds 10. The central density of the preferred mass model is 0.02 M pc^-3. More recently, Carignan (DMW) has pushed the rotation curve even further to 8.5 kpc. He reports clear evidence that the rotation curve is declining considerably, suggesting that the edge of the dark halo has been reached (see Figure 1).

Figure 1. The rotation curve of DDO 154. The solid line represents the best-fit model. The long dashed curve is the halo contribution for this model, whereas the dot-dash curve and lower dashed curve represent the velocities due to the HI and stellar disk, respectively. The upper dashed curve is a Keplerian decline for a disk mass of 5 x 108 M. Figure courtesy of C. Carignan.

GR8 is a galaxy of even lower luminosity with a B-magnitude around -11. HI and optical observations have been carried out by Carignan, Beaulieu and Freeman (1990). The stellar component is relatively normal with the exponential decline characteristic of disk galaxies. However, the rotation axis of the HI disk is parallel to the common major axis of the optical and HI distributions, rather than perpendicular as is normally the case. The gas dynamics indicate the presence of a dark halo with a core radius that exceeds the optical radius of the galaxy. The density of the halo in this core is estimated to be around 0.07 M pc^-3. It appears that even at such low luminosities, disk galaxies have dark halos.

Lake, Schommer and van Gorkom (1990) obtained HI and optical images of DDO 170. At a magnitude of M_B = -15.15 this galaxy is relatively bright for a dwarf irregular. Like DDO 154, the object is gas-rich, allowing the rotation curve to be followed to several optical scale-lengths. At the last measured point of the rotation curve, the dark-to-luminous mass ratio is around 6. The central DM density is again around 0.01M pc^-3. DDO 170 has a small companion galaxy. If the two objects are gravitationally bound, then much larger values for the DM fraction in DDO 170 are inferred.

Not all dwarf irregulars show such high DM fractions and central densities. Carignan, Sancisi and van Albada (1988) found that UGC 2259 had DM properties typical of bright spirals. However, while this galaxy is fairly faint (M_B -16), it exhibits ``grand design'' spiral structure. Well-defined spiral arms in a galaxy of such low luminosity are unusual. Moreover, it is possible that grand design spirals can only be produced when the disk mass is a significant fraction of the halo mass (Athanassoula, Bosma and Papaioannou 1987 and Section 6.1 below). The lack of well-defined spiral structure in the majority of low-luminosity disk galaxies may therefore suggest that DM fractions are usually high in these objects.

IC 1613 also appears to have a halo that is less dominant than those around other dwarfs. Lake and Skillman (1989) carried out HI observations of this dwarf irregular, which has a magnitude of M_B -14.3, and found a relatively low halo density. However, their HI measurements did not extend far enough to show a turnover in the rotation curve, so the resulting mass models are not strongly constrained.

Studies of late-type spirals also illustrate that there is a good deal of overlap in the properties of their dark halos and those of dwarf irregulars. NGC 7793 with a B-magnitude of -18.3 and NGC 5585 at M_B = -17.5 are both late-type spirals, considerably more luminous than most of the dwarf irregulars described so far. However, their DM properties are similar to the extreme dwarfs. For NGC 7793, Carignan and Puche (1990a) find for their best-fit model a central density of around 0.04 M pc^-3, considerably higher than normally found in spirals. In the case of NGC 5585, the best-fit central density is even higher at 0.054 M pc^-3, and within the last measured point of the rotation curve the dark-to-luminous mass ratio is around 10 (Côté, Carignan and Sancisi 1991). While model-fitting of this kind has some limitations (see Lake and Feinswog 1989 and Section 6.1 below), this illustrates that even some normal spirals have extreme DM properties.

Broeils (1992) has carried out HI and optical observations of the dwarf spiral NGC 1560 (M_B = -15.9) and finds a dark-to-luminous mass ratio of around five at the last measured point of the rotation curve. Mass models imply a central density ranging from 0.005 M pc^-3 to 0.04 M pc^-3, making this object intermediate between typical bright spirals and dwarf irregular galaxies.