|Annu. Rev. Astron. Astrophys. 1999. 37:
Copyright © 1999 by . All rights reserved
4.1. Evidence Against Young Stars
UV imaging of early-type galaxies began in the 1970s. Early rocket and balloon experiments obtained low S/N images of the central bulge of M31 which showed that it was an extended source in the far-UV with r 4' (Deharveng et al 1976;, Carruthers et al 1978;, Deharveng et al 1980). This was sufficient to exclude the AGN interpretation (in this particular case) but could not readily distinguish between the old and young star models or other types of diffuse sources. A later rocket imaging experiment by Bohlin et al (1985) provided far- and mid-UV photometry of M31 with 20" resolution. The UV intensity profiles of the bulge were smooth and similar to Kent's (1983) R-band profile for r 1.1'. Although localized regions of massive star formation were readily detectable in the outer spiral arms, similar structures were absent in the bulge, nor could individual bright OB stars be detected there.
IUE observations of bright early-type galaxies (including M31, M32, NGC 3379, NGC 4472, M87, NGC 4552, and NGC 4649) confirmed the spatial extension of the far-UV light, even in the case of the prominent AGN of M87, and indicated that it paralleled the profile of the visible light, at least over the innermost 10" (Bertola et al 1980, Perola & Tarenghi 1980, Nørgaard-Nielsen & Kjærgaard 1981, Oke et al 1981, Bertola et al 1982, O'Connell et al 1986). Deharveng et al (1982), Welch (1982) used multiple IUE spectra to study the light distribution within the inner 15" of the M31 bulge. They obtained a smooth profile, unlike those of star-forming regions, but found that the FUV light was slightly more concentrated to small radii within this region than MUV or B band light, producing gradients of several 0.1 mags in colors. The smooth distribution of UVX light in these cases and its similarity to the optical band profile, where old stars dominate, strongly suggested that old stars produced the UVX.
In another rocket experiment, Onaka et al (1989), Kodaira et al (1990) obtained low-resolution, wide-field UV images of the Virgo cluster, extending the earlier photometry of Smith & Cornett (1982) at 2400 Å to 1600 Å. By comparing their total fluxes with IUE values for the nuclei, Kodaira and colleagues found evidence of large UV color gradients in five Virgo E galaxies. They attributed the blue nuclear excesses and the observed scatter in 1500-V colors to recent star formation from galactic cooling flows, though their observations were also consistent with gradients in low-mass populations.
The best available set of large area UV maps of early-type galaxies was obtained by the Astro UIT experiment during two Space Shuttle missions in 1990 and 1995 (Stecher et al 1997). Twenty-two ellipticals and early-type (S0-Sb) spiral bulges were imaged with good S/N at 3" resolution, and results for 10 of these have been published (O'Connell et al 1992, Ohl et al 1998). UIT images of two Fornax cluster elliptical galaxies are shown in Figure 2. In the best cases, it was possible to obtain UV surface brightness profiles to µ ~ 27 mag arcsec-2. All of the objects exhibit smooth UV profiles (except M87, in which the nonthermal jet is bright), with none of the clumpiness normally associated with recent massive-star formation, and the FUV contours are consistent in shape and orientation with optical-band isophotes. There is little evidence for dust lanes or clouds in the galaxy centers; such features should be readily detectable because of the high selective UV extinction of normal dust. In the M31 bulge, the point source detection threshold was m (2500 Å) ~ 18.4, which excluded the presence of individual main sequence stars hotter than B1 V (O'Connell et al 1992). Over 200 such objects would be expected in the central 2' of the bulge if massive stars formed with a normal initial mass function produced the FUV light. The FUV profiles of about half the sample are well fitted by de Vaucouleurs functions (µ ~ a + br0.25), which are characteristic of spheroids at optical wavelengths. However, the inner FUV profiles of several objects (NGC 3379, 4472, and 4649) are more consistent with an exponential function (Ohl et al 1998). Although exponentials are normally associated with disks, the FUV isophotal contours are congruent to the B-band contours, and the 3-dimensional FUV light distributions are therefore unlikely to be genuinely disklike. (Because of the large UV/optical color gradients discussed below, it is not necessarily expected that the UV profiles of objects that are true spheroids at optical wavelengths would be closely de Vaucouleurs in shape.)
Figure 2. Astro/UIT images of the Fornax cluster ellipticals NGC 1399 and 1404 in broad bands in the far-UV (1500 Å) and mid-UV (2500 Å) with spatial resolution of ~ 3". The mid-UV band is dominated by the main sequence turnoff. All of the far-UV light is from the UVX component. It is smooth, without evidence for massive stars, though is more concentrated than the mid-UV light. NGC 1399 has one of the strongest UVX components yet discovered. Note that the foreground stars have mostly vanished in the FUV band; this is a pictorial representation of how unusual are the objects which make up the UVX.
High-resolution UV imaging from HST (mainly of smaller 22" nuclear fields with the FOC) has confirmed the absence of massive stars in the centers of M31 and M32 (King et al 1992, Bertola et al 1995, King et al 1995, Cole et al 1998, Brown et al 1998a, Lauer et al 1998) and in most UVX sources in the nuclei of 56 early-type galaxies in the 2300 Å survey of Maoz et al (1996).
The collective evidence of all these structural studies is that the far-UV light in most early-type galaxies originates in a stellar component with dynamics characteristic of the bulk of the old stellar population. Active nuclei or young massive stars are not important in most cases.