|Annu. Rev. Astron. Astrophys. 1999. 37:
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5.2. Amplitude and Correlations with Other Properties
The most striking feature of the UVX phenomenon is its large variation from object to object. As measured by the color 1500-V, the amplitude of the UVX in the centers of bright E-Sb galaxies varies from ~ 4.5 to ~ 2.0, which is a factor of 10 in the ratio of far-UV to visible flux. No optical-IR photometric or spectral index for normal old stellar populations exhibits a comparable range; in fact, most do not vary more than ± 30% (e.g. Sandage & Visvanathan 1978, Peletier et al 1990, Trager et al 1998). The UV variations are not confined to the nuclear regions observed with IUE; they are also present in all the large aperture data sets cited above (OAO, ANS, UIT, HUT, FAUST, and the various rocket/balloon experiments).
Large excursions of this kind are usually associated with an incidental component rather than with the aggregate population of a galaxy. To what extent does the UVX convey useful information about the fundamental properties of galaxies?
A key insight was provided by Faber and her colleagues (Faber 1983, BBBFL): the UVX appears to be stronger in more metal-rich galaxies. Faber (1983) found in a small sample of early-type galaxies that nuclear UV colors became bluer as the nuclear spectral line index Mg2, which measures the Mg I + MgH absorption features near 5170 Å, increased. The Mg features are produced by the dominant old stellar population, and this correlation simultaneously links the UVX to the bulk of the galaxy while further weakening the case for massive stars (since there is no obvious reason why recent star formation would be related to metal abundance). Interestingly, the correlation is reversed in sense from the well-known dependences of (U-B) or (B-V) colors on metal abundance in old populations. Driven mainly by opacity effects in stellar envelopes and atmospheres, these become redder as abundance increases.
The correlation was confirmed by the larger sample of BBBFL. They also found significant, if weaker, correlations between 1500-V and central velocity dispersion or luminosity. A later version of the Mg2 correlation, including data on Galactic globular clusters, is shown in Figure 6. The figure emphasizes the lack of continuity between the clusters and galaxies (see Section 6.1). Clusters with a wide range of Mg2 index have large, if scattered, FUV fluxes. Some strong-lined clusters, e.g. 47 Tuc, are faint in the FUV while others, e.g. NGC 6388 and 6441, are bright. The galaxies with line strengths comparable to the strong-lined clusters are relatively faint in the FUV, but galaxy 1500-V colors rapidly become bluer as Mg2 increases. The apparent correlation between 1500-V and Mg2 is much stronger for the galaxies than the clusters. FUV behavior is only one of a number of basic spectrophotometric distinctions that show that globulars and E galaxies do not form a simple population continuum (e.g. Burstein et al 1984, Rose 1985, Ponder et al 1998).
Figure 6. Amplitude of the UVX in old stellar populations, as measured by the color 1500-V, as a function of the Mg2 line index, which measures absorption from Mg I + MgH near 5170 Å. The E galaxy data are from IUE, mostly from the study by BBBFL. The globular cluster data are from several sources, as indicated in the legend. The clusters and galaxies are clearly distinct kinds of populations. From Dorman et al (1995).
Using the recent compilation of data for the Lick Observatory E galaxy spectral survey by Trager et al (1998), one can explore correlations between the UVX and other absorption line indices. There are good correlations, similar to that in Figure 6, between 1500-V and the Na I D lines or the CN bands at 4150 Å. But there is no correlation with a composite Fe index based on features at 5270 and 5335 Å. It is now clear that the abundances of certain light elements (N, Mg, Na) are decoupled from those of the iron peak in more luminous E galaxies (see reviews by McWilliam 1997, Worthey 1998), although there is no clear understanding of the nucleosynthetic origin of these abundance ratio variations. The behavior of the UVX is evidently linked to that of the lighter elements.
The scatter in 1500-V at a given Mg2 is appreciable, especially among the most metal-rich galaxies. This may indicate the influence of parameters other than metal abundance (see Section 6.3). It is also possible that the apparent correlation between UV colors and Mg2 may not reflect smoothly varying properties but instead might arise from several discrete classes of galaxies, as discussed by DOR. There is a suggestion of grouping in Figure 6 (though this is less pronounced in the correlations with Na I and CN). Most of the galaxies have colors in the range 3.5 ± 0.5; within this group there is only a mild UV-Mg correlation. A few objects, including M32, have significantly redder colors. At the other extreme, there are four strong-lined objects with 1500-V < 3 which stand out as a distinct group.
Interestingly, Longo et al (1989) have pointed out that the strongest UV upturns occur in objects with "boxy" isophotes. Most of the systems in the middle group of Figure 6 have "disky" isophotes. The isophotal distinctions between the two groups are now known to correlate with a wider set of morphological and kinematic properties (e.g. Jaffe et al 1994, Faber et al 1997). The boxy galaxies are probably merger products (Bender 1988). It is therefore possible that the UVX is influenced by the dynamical environment of galaxies. Alternatively, all of these characteristics may be related independently to the mass of galaxies.
In low resolution photometry of 40 early-type galaxies in the Virgo cluster, Smith and Cornett (1982) detected the effects of the long-wavelength tail of the UVX component in the integrated mid-UV colors (2400-V) of E galaxies. These, however, had a significantly different color-luminosity relation than the S0 galaxies in the sample. Such potential morphological dependencies have not been carefully investigated.
The UV-Mg2-Na-N correlation and the large internal UV color gradients (Section 4.2) remain the most suggestive clues linking the UVX to the global properties of galaxies. Interpretations of the UVX must accommodate such correlations, but with the caveat that we do not yet really understand the nucleosynthetic drivers of E galaxy chemistry.