|Annu. Rev. Astron. Astrophys. 1999. 37:
Copyright © 1999 by . All rights reserved
Although the UVX produces a ubiquitous extended light background in old populations, it is at a low level and is coincident with a "dark window" in the natural sky background centered at about 2000 Å, where the sky is about 40× fainter than at any other wavelength in the optical-IR region (O'Connell 1987). The faint UV backgrounds permit isolation of other interesting phenomena that are either unique to the UV or are drowned out in the visible bands by the glare of the main sequence and giant branch stars. This includes low-luminosity active nuclei, recent star formation, blue straggler populations, gas in the 105-106 K temperature range, scattered light from dust grains, and H2 fluorescence features near 1600 Å. As noted in Section 3, hot continuum sources that contribute as little as 0.1% of the V-band light of a galaxy can be readily detected in the UV region. In this section we briefly discuss UV observations relevant to massive star formation and nonthermal nuclei.
Recent Massive Star Formation. Identification of a minority component of massive stars in an old population depends on detection of spectral or color distortions in integrated light or on imaging of individual stars or concentrations of stars that stand out against the smooth background light. The vacuum UV is about 30-50× more sensitive to such effects than the optical/IR bands (McNamara & O'Connell 1989). Based on spectral synthesis models for constant star formation with a normal IMF (e.g. Bruzual & Charlot 1993, Cornett et al 1994), the star formation rate per unit V-band luminosity is related to far-UV color as follows:
The 1500 Å flux used to compute the color here is the part of the total far-UV flux that is attributed to young stars. The coefficient in this expression is almost independent of the period over which the star formation is assumed to have persisted, for periods over 50 Myr.
If all of the UV light in the strongest UV upturn cases (1500-V ~ 2) were attributed to massive star formation, the implied normalized rate would be / LV ~ 1.3 × 10-11, or a total rate of ~ 0.25 M for a typical gE galaxy with MV = -21. This is obviously a strong upper limit to massive star formation in a normal E galaxy since only a small fraction of the far-UV light can be produced by massive stars, as discussed in detail in Sections 4 and 5. If we take 20% as the upper limit on the contribution of young starlight at 1500 Å in a galaxy with a more typical UV upturn with 1500-V = 3.5, then the maximal total rate becomes 0.01 M. This is a very stringent limit on the amount of continuing star formation in a typical gE galaxy.
These values can be compared with the estimated total mass loss from stars evolving up the giant branch. The "evolutionary rate" in an old population (i.e. the number of stars evolving off the main sequence per unit time) is ~ 4 × 10-11 yr-1 LV, -1 (e.g. Renzini & Buzzoni 1986, DOR). If each star sheds 0.3-0.5 M, then the total estimated normalized mass loss rate is / LV ~ 1-2 × 10-11 M yr-1 LV, -1, or 0.2-0.4 M yr-1 for a galaxy with MV = -21. The maximal continuing star formation rate derived from far-UV data is some 20-40× smaller. Clearly, most of the material produced by giant branch mass loss is not being recycled into new stars in normal E galaxies, at least not with a normal IMF. The UV is the key to this conclusion, since high S/N optical-band studies generally cannot exclude complete recycling (e.g. O'Connell 1980, Gunn et al 1981).
The ultimate fate of the lost red giant envelopes remains unclear. At early times the material is probably removed from galaxy interiors by high-temperature, supernovae-driven winds. In more massive galaxies, the gas forms a hot corona, which is detectable at X-ray wavelengths (e.g. Forman et al 1985). Some fraction of the corona is returned to the interior by a cooling flow (e.g. Sarazin & White 1988, David et al 1991), but the final repository of the material from the flow remains to be identified. One interesting example of young stars in a normal old population is the remarkable source P2, which is coincident with the dynamical center of M31 (King et al 1995, Lauer et al 1998). This is slightly extended and considerably bluer than the surrounding UVX population. It has the characteristics of an intermediate-age star cluster, but with MV = -5.7, it can account for only a tiny fraction of recent mass loss by the bulge giants. Its massive stars may have been formed through stellar collisions. The second concentrated nuclear source in M31, denoted P1, is not at the dynamical center and is brighter at optical wavelengths. However, its UV properties are similar to those of the inner bulge. It has been suggested that this is a cannibalized galaxy nucleus in the final stages of consumption by M31. If so, it has managed to clothe itself with a UVX population similar to the bulge stars in M31.
The minority of nearby early-type galaxies that do exhibit evidence for recent star formation (including NGC 205, 5102, and 5253) have probably mostly suffered gas transfer during a recent interaction. UV observations in these cases provide a much improved picture of the massive star population and its history than do optical data (e.g. BBBFL, Wilcots et al 1990, Deharveng et al 1997, Calzetti et al 1997). By contrast with normal E galaxies, recent star formation is often found in early-type galaxies associated with massive cluster cooling flows (reviewed in Fabian 1994). Systems with UV observations include M87, Abell 2199, and NGC 1275 (Perola & Tarenghi 1980, Bertola et al 1982, Bertola et al 1986, BBBFL, McNamara & O'Connell 1989, Smith et al 1992, Dixon et al 1996). The UV is important here in placing better limits on star formation rates (always much smaller than X-ray estimates of total accretion rates) and in exploring possible anomalies in the initial mass function.
Active Nuclei . The flat energy distributions of nuclear nonthermal sources imply that the contrast of an AGN against its surroundings in an E galaxy can improve by a factor up to ~ 100 in the UV compared with the optical-IR. This permits better study of known nuclei and searches for very low-luminosity activity. A number of identifications of nuclear point sources have recently been made by UV imaging either of complete samples of nearby galaxies (Maoz et al 1995, 1996) or of samples of objects with Low Ionization Nuclear Emission Region (LINER) optical spectra (Barth et al 1998). Only about 30% of the known LINERs are detected this way, and Maoz and Barth and their respective colleagues suggest that obscuration by dust reduces the visibilty of the other nuclei, at least in the disk galaxies in their samples. However, the UV brightnesses of the nonthermal nuclei support photoionization (rather than shock excitation) models for the LINER emission lines.
In the case of E galaxies with known bright nuclei (e.g. M87), the AGN contributes only a small part of the FUV light within the IUE aperture. From the UIT images of Ohl et al (1998), we find that the nucleus and jet in M87 produce only 10% of the FUV light within a radius of 10". A similar situation applies to NGC 4278, whose nonthermal nucleus was recently detected by Moller et al (1995). These amounts are, however, sufficient to shift the active galaxies such as M87, NGC 4278, and NGC 1052 slightly in 1500-V vs Mg2 diagrams such as Figure 6 (as first remarked by BBBFL).
The most interesting case of UV-facilitated observations of an E galaxy AGN is that of NGC 4552. This object has conspicuous radio and infrared signatures of an active nucleus and was originally observed with IUE for that reason (O'Connell et al 1986). Aside from a strong, spatially extended UV-upturn, however, there were no nuclear anomalies obvious until HST imaging was obtained by Renzini et al (1995), Cappellari et al (1998). The HST observations show a time-variable, unresolved (r 0.07") spike of UV light which brightened by a factor of 4.5× between 1991 and 1993. Without the resolution of HST and the improved contrast offered by the UV, it would have been impossible to detect this source, which is currently the least luminous known AGN, having an H luminosity of only 6 × 1037 erg s-1. The outburst probably corresponds to the accretion of material stripped from a single star during a close fly-by of the nuclear black hole (Cappellari et al 1998).