UV radiation from galaxies

Far UV radiation from galaxies was detected in the late sixties by space satellites such as the OAO-2 and later the TD-1. It was the advent of the IUE however, that provided a large number of spectra for many galaxies, of different morphological types, that confirmed and greatly expanded the earlier UV data. Attempts to explain the UV emission from galaxies led to a flourishing of modeling work and to new views on star formation and chemical evolution. In spite of the enormous contribution of the IUE experiment, the information about UV emission from galaxies is very sparse and the astronomical community still lacks a large sample of a few thousand galaxies with good UV information, from which to perform adequate statistical studies. This information is very important in order to understand evolution in the Universe.

Significant information on selected objects, mainly on stellar populations and the nature of the ISM, was obtained from the early IUE spectra (O'Connell 1992). Similar observational data, combining UV with optical and near-IR spectrophotometry through matched apertures, were used to derive template spectral energy distributions (SEDs) for various types of galaxies (Storchi-Bergmann et al. 1994; McQuade et al. 1995). A combination of IUE observations and data from other UV imaging missions was used to extract ``total'' UV information on galaxies (Longo et al. 1991; Rifatto et al. 1995a, 1995b). Some of the latter information was included in the ULDA Guide to Normal Galaxies (Longo & Capaccioli 1992) and in a comparison of galaxy properties in the UV using synthetic photometry from the IUE spectra in seven photometric bands, five of which matched the ANS bands (Longo et al. 1991).

There is hope to derive the star-forming histories of galaxies through a combination of data from the UV to the near-IR, in the manner of the Storchi-Bergmann et al. (1994; SB2) templates. The significant UV data collected by IUE from normal galaxies is usually at $\lambda>$1400Å, except for the very young starburst galaxies and for some ellipticals. This spectral region, in late-type galaxies, contains mainly radiation from A-type stars and requires extrapolation of the stellar population to earlier types in order to account for Lyman continuum photons. The flux below 1400Å, observed in starburst galaxies, originates from stars earlier than type A, mainly B stars, although sometimes a contribution by field O-stars cannot be ruled out (Brosch et al. 1999). Some elliptical galaxies exhibit upturns of their spectral energy distributions below 2000Å (Burstein et al. 1988; Bonatto et al. 1996; O'Connell 1999). These are presumably produced by low-mass, helium-burning stars on the extreme horizontal-branch and in later stages of evolution.

It is impractical to rely only on the detailed modeling of spectral features in the optical region in order to understand large populations of galaxies in terms of stellar populations and star-formation histories. One should combine information from many spectral bands, covering as wide a spectral region as possible. In the absence of very deep UV surveys in more than a single spectral band, such as those expected to result from the GALEX all-sky two-band UV survey (Bianchi & Martin 1997), our information about significant numbers of galaxies measured in the UV originates from the SCAP-2000 (Donas et al. 1987), FOCA (Milliard et al. 1992), and FAUST (Deharveng et al. 1994; Bowyer et al. 1995; Brosch et al. 1995, 1997, 1999, et seq.) measurements. These consist of integrated photometry at 1650Å or 2000Å of a few hundred galaxies. UV spectral information for about two dozen galaxies was also obtained by the ASTRON observatory (Boyarchuk 1994). In the 2000Å band, and in the UV brightness range 16.5-18.5, galaxies apparently dominate the source counts (Milliard et al. 1992). The corresponding blue magnitudes of these galaxies are B=18-20 mag. and their typical color index is $[2000-V]\approx-$1.5. Comparing this color index with the template spectra of Kinney et al. (1996), the FOCA-detected galaxies seem to fit the SB2 template, i.e., a slightly reddened starburst galaxy. The claim by the FOCA group of a large contribution of UV galaxies in deep-UV sky observations is supported by theoretical arguments requiring a fast-evolving population of perhaps dwarf galaxies for z=0.2-1.0, in order to explain the faint source counts in other spectral domains (Ellis 1997).