The UV upturn has been a mystery ever since it was first found by the OAO-2 space telescope (Code & Welch 1979). According to the opacity effect more metal-rich populations show redder colours, and hence giant elliptical galaxies were not expected to contain any substantial number of hot stars to show a UV upturn. Yet, it was confirmed by subsequent space missions, ANS (de Boer 1982), IUE (Bertola et al. 1982) and HUT (Brown et al. 1997). Figure 1 shows an example spectrum of the giant elliptical galaxy NGC 4552 mosaicked from multi-band measurements.
Figure 1. The composite spectrum of the giant elliptical galaxy NGC 4552 shows a classic example of the UV upturn. The mosaic spectrum is originated from HUT (FUV), IUE (NUV), and ground-based telescope (optical). Excerpted from Yi, Demarque, & Oemler (1998).
Some of the observational findings based on the nearby bright elliptical galaxies are particularly noteworthy. The positive correlation between the UV-to-optical colour (i.e., the strength of the UV upturn) and the Mg2 line strength found by Burstein et al. (1988) through IUE observations has urged theorists to construct novel scenarios in which old ( a few Gyr) metal-rich ( Z) populations become UV bright (Greggio & Renzini 1990; Horch et al. 1992; Dorman et al. 1995). Also interesting was to find using HUT that, regardless of the UV strength, the UV spectral slopes at 1000-2000 Å in the six UV bright galaxies were similar suggesting a very small range of temperatures of the UV sources in these galaxies (Brown et al. 1997), which corresponds to Teff 20,000 ± 3,000 K. In fact, the characteristic temperature of the UV sources seems strangely somewhat lower in a stronger UV-upturn galaxy (Yi et al. 1998).