HI Morphology and Kinematics

One interesting difference between low-luminosity galaxies (

-19 > M_B > -16) compared to more luminous galaxies (

-22 > M_B > -19) is that the range of morphology and kinematics observed is different for the two groups. From our data, together with data in the literature (e.g. Lake et al. 1987) there are now about 10 HI data cubes available for low-luminosity early-type galaxies. Almost without exception, the HI in these galaxies is in a disk with a regular morphology and kinematics. In some galaxies there is evidence that (part of) the HI may have been accreted recently, but in several galaxies the structure of the HI is very regular and there is no evidence from the kinematics that a recent accretion has occurred. To illustrate this, in figure 2 we give the total HI image and the velocity field for the galaxy NGC 802 (M_B = -18). The HI in this galaxy is very centrally concentrated. The velocity field shows however that the gas is rotating around the optical major axis, like in polar-ring galaxies. This suggests that the HI in NGC 802 has been accreted after the main stellar body had formed. Another low-luminosity galaxy that shows a very similar HI configuration is NGC 855 (Walsh et al. 1990).

Figure 3: left: Total HI image of NGC 2328 as obtained with ATCA. The optical image is taken from the Digital Sky Survey. right: Position-velocity map taken along the major axis of NGC 2328

An example of a regular HI disk in a low-luminosity galaxy is given in figure 3, where we show the total HI image and a position-velocity map taken along the major axis NGC 2328. Also in this galaxy the HI is quite centrally concentrated. The position-velocity map in figure 3 shows that the HI in NGC 2328 is in a regularly rotating disk, aligned with the optical body.

In contrast to the low-luminosity galaxies, the range in HI morphology in the more luminous galaxies (

-22 > M_B > -19) is much broader. For this group, in most galaxies the HI shows an irregular morphology, indicating that the gas is accreting onto the galaxy, or is left over from a recent merger event. A good example of this is NGC 5266 (figure 4; Morganti et al. 1997a). This is a minor-axis dust-lane elliptical with a large amount of HI (10¹⁰ ,

). Almost all the HI is in an elongated structure parallel to the optical major axis. Most of this gas is rotating in a reasonably regular fashion, although several subsystems can be identified that are not in stable circular rotation. Interestingly, this large-scale HI structure is perpendicular to the inner, minor axis, dust lane, and some HI associated with this dust lane is also detected (a few percent of the HI mass). Clearly, NGC 5266 is a system where a large amount of HI has been accreted recently, or is a remnant of a recent merger, and the HI is still settling in the galaxy.

Interestingly, there are now a few luminous early-type galaxies known that do have very regular HI structures. A very good example is the E4 galaxy NGC 807 (figure 5). Deep HI observations reveal a low-surface brightness HI disk that shows no signs that this HI has been accreted recently. Figure 5 gives the position-velocity map of this HI disk, clearly showing the regular rotation of this disk. The evolution of this disk is very slow, and this HI disk can be quite old. Often, these regular HI structures have a depression or hole in the centre that is filled up with a disk of ionized gas that has very similar kinematics as the HI disk. A good example of this is the dust-lane galaxy NGC 3108.

Figure 4: Contours of the total HI image of NGC 5266 superposed on an optical image obtained from the Digital Sky Survey

Figure 5: Position-velocity plot of NGC 807, taken along the kinematical major axis (data taken from the VLA archive, original data taken by Dressel in 1985)

Another striking difference between the HI structures seen in low-luminosity early-type galaxies and in more luminous galaxies, is that the central surface brightnesses are quite different. In the low-luminosity galaxies, the HI is quite centrally concentrated with central HI surface densities of at least 4 pc^-2. These densities are high enough for star formation to occur on a reasonable large scale, and indeed star formation is observed in the centres of these galaxies. Outside the centre, the surface densities of the HI are below 1 pc^-2 and perhaps only sporadic star formation could occur. In contrast, the surface densities in the more luminous galaxies are much lower, even in the galaxies with a regular HI disk or ring. The peak surface densities are typically around 1 pc^-2, too low for large scale star formation to occur. Figure 6 shows the HI density profile of a low-luminosity elliptical and of a more luminous E4 galaxy. The difference between these profiles is quite typical for what is observed in most galaxies.

© Copyright Astronomical Society of Australia 1997