HST photometry: Can UV colors probe the galaxy ages?

We present grids of magnitudes and colours for the WFPC2 of the refurbished HST using passbands from the far UV to the near infrared, i.e. the filters F170W, F218W, F255W, F300W, F336W, F439W, F450W, F555W, F606W, F702W, F814W, and F850LP. In addition to this, we consider also the filters F150W + F130LP, F175W, F220W and F342W of the old HST in the FOC mode in which a great deal of observations have been taken. The theoretical magnitudes and colours are calculated convolving spectral energy distributions with different metallicity (Z), effective temperature (T-eff), and gravity with the pass-band transmission, response of the camera, and telescope assembly. In order to enable an easy usage of the present results we provide tabulations of Bolometric Corrections BCDelta lambdaHST as function of gravity, T-eff, and chemical composition for each pass-band under consideration. As already pointed out by Yi et al. (1995), the UV filters suffer from the visible/red-leak problem, which destroys the monotonic relation between colours and T-eff and causes serious difficulties to analyze the data. With the aid of the above technique we calculate isochrones in the observational colour - magnitude diagram (CMD) and integrated magnitudes and colours of single stellar populations (SSP). The ages span the range from 18 Gyr to 0.01 Gyr. For the sake of brevity, results are described here in some detail limited to the solar chemical composition [Y = 0.28 Z = 0.020]. In addition to this, rye present synthetic CMDs for a composite sample of stars containing both young and old objects as they would appear in various pass-bands. Finally, we address the question whether the variation of the UV colours of elliptical galaxies as a function of the red-shift shows signatures from which one can infer the age and the type of source emitting the UV flux. The goal is achieved looking at the red-shift dependence of colours obtained from the pass-bands F170W, F210W, and F555W at varying the Hubble constant Ho and deceleration parameter go defining the model of the Universe. It is found that those colours run differently according to underlying source, i.e either P-AGB or P-AGB plus H-HB and AGB-manque stars (see below for details) and the cosmological parameters. If the UV flux is mainly generated by P-AGB plus H-HB and AGB-manque stars a marked discontinuity in suitable UV colours is expected at the red-shift corresponding to the age of about 5.6 Gyr.