AGB Connection and Ultraviolet Luminosity Excess in Elliptical Galaxies

Abstract

Relying on infrared surface brightness fluctuactions to trace AGB properties in a sample of elliptical galaxies in the Virgo and Fornax Clusters, we assess the puzzling origin of the ``UV upturn'' phenomenon, recently traced to the presence of a hot horizontal branch (HB) stellar component. The UV upturn actually signals a profound change in the galaxy stellar populations, involving both the hot stellar component and red giant evolution. In particular, the strengthening of the UV rising branch is always seen to correspond to a shortening in AGB deployment; this trend can be readily interpreted as an age effect, perhaps mildly modulated by metal abundance. Brightest stars in ellipticals are all found to be genuine AGB members, all the way, and with the AGB tip exceeding the RGB tip by some 0.5-1.5 mag. The inferred core mass of these stars is found to be 0.57 -->M☉ among giant ellipticals. This value accounts for the recognized deficiency of planetary nebulae in these galaxies, as a result of a lengthy transition time for the post-AGB stellar core to become a hard UV emitter and eventually ``fire up'' the nebula. The combined study of galaxy -->(1550 − V)0 color and integrated Hβ index points to a a bimodal temperature distribution for the HB with both a red clump and an extremely blue component, in a relative proportion -->[N(RHB) : N(BHB)] ~ [80 : 20]. For the BHB stellar population, [Fe/H] values of either –0.7 or +0.5 dex may provide the optimum ranges to feed the needed low-mass stars ( -->M* 0.58 -->M☉) that at some stage begin to join the standard red clump stars.