ON THE NATURE OF THE UV TURNUP IN EARLY-TYPE GALAXIES

We study the UV turnup in early-type galaxies from coadded IUE spectra reaching an unprecedented signal to noise ratio. Some spectral groups resulted with strong or moderate UV turnup, while in others it is ver y weak or absent. We study the relationships of the UV turnup intensit y with absolute magnitude, X-ray and Her luminosities. Galaxies in the strong UV turnup groups are systematically bright, and have high X-ra y and H alpha emission luminosities; however, some other galaxies with the latter properties do not exhibit a significant UV turnup. The spe ctral groups with an important intermediate age component are far-UV w eak. The contribution from an intermediate age population with varying strength might explain why some red stellar population early-type gal axies present the UV turnup while others do not. The available spectra l groups have been further coadded into one with strong UV turnup and another one without it, and we analyse their difference. We compare th e spectral slope of the isolated UV turnup with IUE spectra of various hot sources which had been proposed in previous studies to explain it . We conclude that only very hot stars like sdO and sdB subdwarfs, som e nuclei of planetary nebulae or DO white dwarfs, have the proper slop e to explain the UV turnup. The UV turnup as detected through the IUE aperture is not featureless: it presents absorptions similar to those observed in the galaxies with weak far-UV flux. These features appear to be the lambda 1400 Angstrom and lambda 1600 Angstrom ones, characte ristic of moderately cool white dwarfs (DA 5), which indicates that th ese stars are dominant flux contributors between lambda 1300 - 2000 An gstrom in the far-UV weak groups. The cooling time of DA 5 stars toget her with the evolutionary time since they left the main sequence, impl y that they evolved from low-mass stars possibly associated with the i nitial burst of star formation in early-type galaxies and/or merger ev ents at intermediate ages. On the other hand, two possible scenari are discussed for the origin of the hot component which causes the UV tur nup, one related to late stages of normal evolution of low-mass stars, and another related to past nuclear activity events and jets which mi ght have blown away the atmospheres of red giants in the central parts of the galaxies, exposing the hot stellar cores and mimicking a spect ral distribution like that of the hottest stars observed.