Annu. Rev. Astron. Astrophys. 1999. 37: 603-648 Copyright © 1999 by Annual Reviews. All rights reserved

6. CANDIDATE LOW MASS UVX SOURCES

On the basis of the evidence in the last two sections, old, low-mass stars are the primary sources of far-UV light in E galaxies. Interpretational effort during the last 10 years has therefore focused on the viability of various types of hot, low-mass stars and their relationship to the dominant populations of E galaxies. Since observational information on the UVX is still sparse, much of this work has been based on new generations of theoretical models for advanced stellar evolution. A fully satisfactory interpretation has not emerged, but there has been good progress in narrowing the range of possibilities. In this section and the next we review the main conclusions.

6.1. Globular Cluster-Type Populations

The presence of hot stars in old populations was, of course, not unprecedented because "blue horizontal branch" (BHB) stars had long been associated with metal-poor globular clusters (having [Fe / H] -1). The most natural old-star interpretation of the UVX was therefore that it arose from the low-metallicity tail of a stellar population with a large abundance range.

Surprisingly, however, observations quickly demonstrated that the UVX could not simply be the sum of globular cluster-type populations. The first quantitative comparison between clusters and a UVX source was made using ANS data for the bulge of M31 by Wu et al (1980). In order to fit the far-UV spectrum of M31, Wu et al were forced to add an additional high temperature component to the cluster M13, which has one of the hardest UV spectra among Galactic globulars. Models by Nesci & Perola (1985) likewise showed that normal cluster BHBs could not match the galaxy IUE spectra unless a second, hotter HB component was included. Oke et al (1981), Welch (1982), Bohlin et al (1985) all emphasized the dissimilarity between typical cluster and galaxy UV energy distributions. Compilations of ANS, IUE, HUT, and UIT data for globulars (van Albada et al 1981, Castellani & Cassatella 1987, Bica & Alloin 1988, Davidsen & Ferguson 1992, Dorman et al 1995) show that although clusters in general have total UV-bright star fractions exceeding those of galaxies (see Figure 6), galaxies can have steeper far-UV spectra than any cluster. The distinction between the two populations is quite clear in two-color diagrams such as Figure 7, where the color 1500-2500 is used to measure the slope of the UV upturn.

Figure 7. Broad-band UV colors for E galaxies and globular clusters compared. Data are mainly from IUE. The color 1500-2500 measures the slope of the UV-upturn component, while 1500-V measures its amplitude. Globular clusters and galaxies are clearly segregated in the diagram, with galaxies having steeper FUV spectral slopes at a given amplitude. The boxes enclose several fiducial model sets, with the upper one corresponding to Z Z and the lower to Z 0.04 Z. The shaded regions represent models in which the hot stellar component consists mainly of EHB stars. Most of the galaxies require an EHB contribution, but most of the clusters do not. From Dorman et al (1995).

The temperature distribution of UV-bright sources in the clusters is evidently cooler and broader than in the galaxies. The far-UV slope in the galaxies is equivalent to T_e 2000K, whereas BHBs in Galactic globulars normally do not extend beyond T_e ~ 10000-12000 K. Hotter stars can be found in some globulars on or above the horizontal branch, as Hills (1971) originally pointed out, but these are relatively uncommon (see de Boer 1985, 1987; and Section 6.2). Even where hotter stars are present, the mean integrated UV light of typical clusters is heavily influenced by cooler horizontal branch objects with a wide range of temperatures. Lower metallic line blanketing in the atmospheres of cluster stars on the warm HB and near the main sequence turnoff also produces larger mid- and near-UV emergent flux than for galaxies, tending to flatten the UV energy distributions.

Even if one ignores the distinctions between clusters and galaxies in UV spectral shape, the limits on metal-poor light at optical wavelengths in M31 and E galaxies are inconsistent with a large contribution from cluster-like populations in the UV (O'Connell 1976, 1980;, Rose 1985;, Bica & Alloin 1988;, Dorman et al 1995).

The fact that the galaxy UVX does not appear to arise from a metal-poor subpopulation similar to the Galactic globular clusters does not, of course, necessarily mean that the kinds of hot stars in the galaxies differ from those in the clusters - only that the mixture of these is different.