The Sun, stellar-population models and the age estimation of high-redshiftgalaxies

Given sufficiently deep optical spectroscopy, the age estimation of high-re dshift (z > 1) galaxies has been claimed to be a relatively robust process due to the fact that, for ages <5 Gyr, the near-ultraviolet light of a stel lar population is expected to be dominated by 'well-understood' main-sequen ce (MS) stars. Recently, however, the reliability of this process has been called into question by Yi et al., who claim to have developed models in wh ich the spectrum produced by the main sequence reddens much more rapidly th an in the models of Jimenez et al., leading to much lower age estimates for the reddest known high-redshift ellipticals. In support of their revised a ge estimates, Yi et al. cite the fact that their models can reproduce the s pectrum of the Sun at an age of 5 Gyr, whereas the solar spectrum is not re produced by the Jimenez et al. models until <similar or equal to>10Gyr. Her e we confirm this discrepancy, but point out that this is in fact a strengt h of the Jimenez et al. models and indicative of some Raw in the models of Yi et al. (which, in effect, imply that the Sun will turn into a red giant any minute now). We have also explored the models of Worthey (which are known to differ grea tly from those of Jimenez et al. in the treatment of post-MS evolution) and find that the main-sequence component of Worthey's models also cannot repr oduce the solar spectrum until an age of 9-10 Gyr. We conclude that either the models of Yi et al. are not as main-sequence dominated at 4-5 Gyr as cl aimed, or the stellar evolutionary time-scale in these models is in error b y a factor possibly as high as two. Our current best estimate of the age of the oldest galaxies at z similar or equal to 1.5 thus remains 3-4 Gyr, as we confirm with a new analysis of the existing data using the updated solar -metallicity models of both Jimenez et al. and Worthey. Finally, by fitting a mixed metallicity model to the Sun, we demonstrate th at, given rest-frame ultraviolet data of sufficient quality, it should be p ossible to break the age-metallicity degeneracy when analysing the spectra of high-redshift galaxies.