On the nature of the uv turnup in early-type galaxies

Abstract

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 very weak or absent. We study the relationships of the UV turn up intensity with absolute magnitude, X-ray and Hα luminosities. Galaxies in the strong UV turnup groups are systematically bright, and have high X-ray and Hα luminosities; however, some other galaxies with the latter properties do not exhibit a significant UV turn up. The spectral groups with an important intermediate age component are far-UV weak. The contribution from an intermediate age population with varying strength might explain why some red stellar population early-type galaxies present the UV turn up while others do not. The available spectral groups have been further coadded into one with strong UV turnup and another one without it, and we analyse their difference. We compare the 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, some nuclei of planetary nebulae or DO white dwarfs, have the proper slope 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 λ1400 Å and λ1600 Å ones, characteristic of moderately cool white dwarfs (DA 5), which indicates that these stars are dominant flux contributors between λ1300 - 2000 Å in the farUV weak groups.The cooling time of DA 5 stars together with the evolutionary time since they left the main sequence, imply that they evolved from low-mass stars possibly associated with the initial burst of star formation in early-type galaxies and/or merger events at intermediate ages. On the other hand, two possible scenari are discussed for the origin of the hot component which causes the UV turn up, one related to late stages of normal evolution of low-mass stars, and another related to past nuclear activity events and jets which might have blown away the atmospheres of red giants in the central parts of the galaxies, exposing the hot stellar cores and mimicking a spectral distribution like that of the hottest stars observed.