Because EHB stars are the source of the UV upturn, the UV upturn is
expected to fade dramatically with increasing redshift as one looks to
younger elliptical galaxies
(Greggio & Renzini,
1990;
Tantalo et al., 1996).
We have been undertaking a series of observations with HST to map the
evolution of the UV upturn as a function of redshift, by observing
galaxy clusters at 0.3 < z < 0.6
(figure 4). Although early
measurements were consistent with a relatively flat evolution out to
z 
0.4 and a
rapid fading at higher redshifts
(Brown et al., 1998a;
Brown et al., 2000a),
recent observations
(Brown et al., 2003)
at z = 0.33 show a UV upturn as weak as that at z =
0.55. Because the earliest observations, at z = 0.375, were the
only ones that did not
use a solar-blind camera, they might be systematically in error;
setting aside the z = 0.375 measurements, the remaining observations
show a UV upturn that is weaker than that in the present epoch, but a
relatively flat evolution with increasing lookback time. Taking the
models at face value implies large variations in the formation epoch
of giant elliptical galaxies in clusters, which is implausible
(Brown et al., 2003).
Although the onset of the UV upturn occurs at ~ 6 Gyr
in these models, the formation of EHB stars is tied to a wide range of
poorly constrained parameters (mass loss, metallicity, binary
fraction, etc.), with the elliptical galaxies presenting the aggregate
behavior at any one epoch. The "floor" in the UV emission seen at
increasing redshift might indicate a wide dispersion in the parameters
that govern EHB formation, or it might be that another source of UV
emission is becoming dominant at increasing redshift as the EHB stars
disappear (e.g., residual star formation). With our current understanding
of EHB formation, the UV upturn remains a poorly calibrated age indicator.
However, if galaxy ages are determined by independent methods, these
surveys of the UV upturn could instead be used to constrain theories
of EHB formation.
 |
Figure 4. The evolution of the UV upturn as a function of redshift (triangles), with the expected evolution of this emission in giant elliptical galaxies (Tantalo et al., 1996), assuming three different formation redshifts (zf; labeled). |