12.7.3. HI in Active Galaxies
Some of the most luminous galaxies in the universe are those that are undergoing an enhanced phase of activity. The term "active" is applied to galaxies that are undergoing a variety of processes that increase the total luminosity of the host galaxy. Most often, the activity is confined to the nucleus or the inner few kiloparsecs of the galaxy. Seyfert galaxies contain bright, starlike nuclei and exhibit broad emission lines. The survey performed by the Infrared Astronomical Satellite (IRAS) in the wavelength range from 10 to 100 ym has revealed a new category of objects that are ultraluminous in the far-infrared. bands. More often than not, these objects show signs of interaction with companions; and their enhanced infrared luminosity results from the heating of dust by ultraviolet photons emitted in a burst of star formation [see the review by Soifer et al. (1987)]. Even the Milky Way harbors energetic phenomena in its own nucleus,'and it is likely that many galaxies will be active for some portion of their lifetime.
The Seyfert phenomenon occurs most often in early-type spirals and is proposed to result from the powering of nuclear activity by the accretion of gas onto a massive object in the center of the galaxy (Weedman 1986). HI surveys of Seyfert galaxies as have been conducted by Heckman et al. (1978) and Mirabel and Wilson (1984) help to establish the nature of the galaxy containing the Seyfert nucleus. While for the majority of Seyfert galaxies, the ratio of hydrogen mass to optical luminosity, MH / L, is similar to that expected for galaxies of similar optical morphology, some Seyferts appear to be usually gas rich. The redshifts obtained from the 21-cm line observations are systematically larger than those derived from the nuclear optical emission lines, suggesting a net outflow of gas in the narrow-emission line regions. A significant percentage, 40% or so, of detected Seyfert galaxies show highly asymmetric HI profiles, not the characteristic two-homed ones expected from quiescent spiral disks. It is not yet known whether these deviations arise from interactions with companions, ionization of significant HI in selected regions of the disk, or blending of more than one emitting galaxy within the single-dish beam.
Because active galactic nuclei are often the hosts of compact radio continuum sources, 21-cm absorption as well as emission may often be seen both in Seyfert galaxies and in others that are radio sources. As illustrated in Equation (12.1), an HI cloud of total optical depth which is bathed in a radiation field with a significant flux at 21-cm wavelength will produce a spectrum that is the combination of the emission by (and self-absorption within) the cloud and the absorption of the continuum radiation originating behind the absorbing cloud. Because of the magnetic dipole nature of the 21-cm line transition, and of the fact that a significant correction of stimulated emission must be applied to , the 21-cm line optical depth is normally very low. Because of this small opacity, the detection of HI in absorption is possible only where the line of sight to a source of radio continuum emission passes through an HI region with a large column density, NH, of neutral hydrogen. The resulting opacity is determined by the ratio NH / Ts, where Ts is the spin temperature, as described in Section 12.1.2. If NH can be derived from measurements at other spectral regimes, can then provide an estimate of the excitation characteristics of the gas. Furthermore, the ability of a source to appear in absorption, given some value of optical depth for the intervening HI region, depends only on its continuum flux density, not on its distance. Thus, HI absorption can be detected in objects at very high redshift; such objects need not be particularly massive in HI, but only optically thick along the line of sight and favorably positioned in front of a relatively strong continuum source.
Obviously, absorption lines are produced against sources which emit strongly in the radio continuum, and so information is gleaned only about the line of sight to such sources. HI absorption has been detected in some two dozen galaxies, with typical optical depths of a few hundredths found. Most typically, absorption is seen against a continuum source which is located in the galaxy's center. Absorption arises in clouds contained in the same galaxy, perhaps in its disk. Optical depths are derived on the assumption that the absorbing cloud (or clouds) covers the continuum source completely. If the cloud actually covers only a portion of the illuminating source, the optical depths will be underestimated.
In fact, only a small fraction of radio galaxies, including spirals and ellipticals with prominent dust lanes, exhibit HI absorption. Figure 12.11 shows the HI emission and absorption observed in the lenticular galaxy NGC 1052; the absorption profile is obtained in the direction of its central continuum source. The geometry of the source relative to potential absorbers plays a critical role. While narrow HI absorption features are seen in many active galaxies, those observed in others are broad and resemble features seen in lines of the OH radical. The HI absorption is often found offset from the galaxian systemic velocity, as would be expected if the absorbing material is infalling into the nuclear region. However, exceptions do occur; several cases of HI absorption show features that are blue-shifted by 100 km s-1 or more with respect to the galaxy's line emission centroid (Dickey 1982). Detailed maps of the absorption and (when detectable) emission are required in order to establish the location and kinematics of the gas.
Figure 12.11. Top: HI distribution in the early-type galaxy NGC 1052, obtained with the very Large Array by van Gorkom et al. (1986). Contours correspond to HI column densities of 0.15, 0.6, 1.07 and 1.83 × 1020 cm-2. Bottom: absorption profile obtained against the galaxy's nuclear continuum source.
The far-infrared emission detected by IRAS arises from both normal and peculiar galaxies. The dominant population of galaxies that are bright in the far infrared are spirals. The infrared emission is likely to arise from two dust components in the disk: a component closely associated with star formation activity in HII regions and molecular cloud complexes, and a second that is heated by the diffuse inter-stellar radiation field. The latter is identified with the so-called "infrared cirrus" seen in the Milky Way. The exact correlation of these two components with HI in our own galaxy and external galaxies is still under investigation.
Some of the galaxies that are overluminous in the far infrared and particularly identified to have very high star formation rates earn the designation of "starburst" galaxies. The high occurrence of multiplicity and peculiar morphology in these systems suggests that the current burst of star formation is somehow triggered by the interaction process between close companions. Observations of atomic hydrogen in such systems identify them as spirals because their hydrogen masses are generally large, typical of spirals but not earlier-type systems. In a significant fraction, the shape of the HI profile indicates the presence of global disturbances. In the most peculiar of these systems, such as the merger candidate Arp 220, there is a striking absence of atomic gas in locations where molecular hydrogen is found, indicating an enhancement of the atomic-to-molecular conversion process that must ultimately then lead to a higher star formation rate.
Low-optical-luminosity blue compact dwarf (BCD) galaxies, sometimes referred to as "extragalactic HII regions," also seem to be undergoing active star formation. Like the blue star light, the HI emission is patchy and the HI clumps actually avoid the brightest optical features. Like the normal dI's, the HI usually extends well, beyond the optical image. It is not at all clear what triggers star formation in some dwarfs and not in others, even though the HI column densities in both types may remain in excess of NH = 1020 cm-2 even beyond the optical radius.