12.5.2. Content of Early-Type Galaxies
Elliptical galaxies are usually not detected in the HI line, even at sensitivity levels of a few mJy. While this may not be totally unexpected, as their stellar content is old and their angular momentum low, some gas should be present as a result of mass loss from aging stars and perhaps even more so in the fraction of ellipticals that appear to have dust lanes, central HII regions, or active nuclear sources that might result from gas inflow. In fact, some detections of HI in ellipticals have been obtained. Figure 12.11 illustrates one such detection, that of NGC 1052. Sanders (1980) proposed that the HI content of E's is bimodally distributed: most E's contain little or no gas, while a few are gas rich. Weighing carefully the information content even of non-detections, Knapp et al. (1985a) have analyzed a much larger sample - approximately 150 objects, of which 23 were detected - and have concluded that the ratio k = (MH / LB) is distributed for ellipticals roughly as
 |
(12.11) |
where N(k) dk is the number of objects with (MH / LB) between k and k + dk. Note this distribution is not bimodal. This behavior is very different from that of spirals, which are generally distributed rather in Gaussian fashion than according to a power law. The concept of a "typical" value of MH / LB for ellipticals may thus be meaningless. The distribution of HI content for spirals and its correlation with disk properties indicates a tight relationship between the gas and the stellar population; on the other hand, the light of the elliptical bulges and the HI content of E's have little regard for each other. Knapp and coworkers suggest that the HI in E's has an external origin, obtained by accretion of a surrounding envelope or of a gas-rich companion. This proposal is also supported by the tendency for the HI, when found, to be located well outside the optical image of the galaxy and to be characterized by peculiar kinematics.
While it is generally accepted that in the more luminous ellipticals, the HI gas has an external origin and is not produced by evolutionary stellar mass loss, the origin of HI disks in low-luminosity galaxies has been more controversial. Further evidence that the cool gas seen in all ellipticals comes from a source other than mass loss from the stars seen in the optical image is offered by VLA observations of the HI distribution and kinematics in four low-luminosity ellipticals (Lake et al. 1987). The velocity field of these galaxies is roughly regular, implying the presence of a rotating disk. In the brighter two galaxies, the HI distribution is annular; in the fainter two, it is centrally concentrated. In all four cases, however, the HI emission extends to twice the optical (Holmberg) radius. It is hard to see how such large HI disks could accumulate from stellar mass loss over a volume much large than the stellar component.
In the very nearby galaxies, even small amounts of atomic gas can be detected. VLA observations of the HI in the dwarf elliptical companions to M31, NGC 185 and NGC 205, show the presence of a few times 105 solar masses of atomic gas in contrasting configurations (Johnson and Gottesman 1983). Both galaxies show evidence of dust patches near their nuclei and small populations of blue, presumably young stars. In NGC 185, the centroid of the HI distribution is not coincident with either the nucleus or the dust clouds. On the other hand, the HI distribution in NGC 205, while elongated, reveals a rotating disk roughly coincident with the dust and blue star distribution. It seems that ellipticals in fact do contain small but measurable quantities of not only the atomic gas but also the expected associated quantities of dust derived from far-infrared observations (Jura et al. 1987).
The HI distribution in lenticular (S0) galaxies, recently reviewed by Wardle and Knapp (1985), appears to be intermediate between the cases of ellipticals and spirals. While S0a's appear to resemble spirals, in the sense that some properties of the stellar population are correlated with the gas content and that it appears sensible to estimate mean values of MH / LB and MH / D2, S0's are more like ellipticals. As for ellipticals, the values of k for S0's cover a very wide range and appear to be unrelated to the stellar population.
Knapp and Wardle propose that the HI in S0's may also have an external origin. Perhaps the strongest arguments in favor of such a hypothesis come from the special examples of the polar ring galaxies. These objects are otherwise-normal S0's, viewed edge-on and surrounded by tilted rings of luminous material. The current coexistence of two nearly orthogonal orbital planes suggests the formation of the ring by an unusual event, probably the capture of a gas-rich companion. Observations of the rotation curves of the ring material allow one to probe the shape of the dark halo surrounding these S0 galaxies (Schweizer et al. 1983). In combination with clues derived from optical spectroscopy and photometry, the derivation of the HI distribution and velocity field from synthesis maps determines an estimate of relatively young ages, about 1 × 109 to 3 × 109 years, for the observed polar rings. VLA maps are now available for several polar ring systems (van Gorkom et al. 1987). These objects appear to be relatively rich in HI, which is generally aligned with the ring rather than the S0 disk. Furthermore, the HI distribution is sometimes asymmetric and extends well beyond the optical image, suggesting that there has not been enough time for the gas to settle into the ring or for the induced star formation to be completed. The S0's with neutral hydrogen rings but no optical counterpart may be more extreme examples of the same phenomenon (Knapp et al. 1985b). Although ellipticals and S0's are indeed found typically in regions of higher galaxy density than spirals, where the accretion or merger rate is expected to be similarly elevated, it is not yet clear that accretion of gas-rich companions can explain all seemingly anomalous cases of HI in early-type systems.