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Session 62 - Elliptical Galaxies: Dynamics and the UV Upturn.
Oral session, Tuesday, January 14
Harbour A,

[62.03] The Origin of the UV Upturn in Elliptical Galaxies


The unexpectedly high ultraviolet flux in the spectra of giant elliptical galaxies, known as the UV upturn, has been one of the most controversial topics ever since space observations with UV capability became available. This thesis presents a population synthesis and analysis which concludes that this phenomenon is a natural result of the advanced stellar evolution of a metal-rich population of stars. All three important empirical discoveries about the UV upturn - the presence of a UV upturn only in giant elliptical galaxies, the positive UV upturn-metallicity relationship, and the narrow range of the T_eff of UV sources - are precisely reproduced. The achievements are as follows:

The origin of the UV bright phase of metal-rich core helium burning stars has been understood. More metal-rich core helium-burning stars become UV bright more easily when Z \stackrel\textstyle>\raisebox-0.7ex\sim\, Z_ødot because their lifetime is longer due to their smaller mass and their hot helium-burning core gets exposed earlier as the hydrogen shell burns up more quickly. I provide an analytic derivation of the origin.

(2) This first detailed sensitivity study of the resulting UV spectrum to various input parameters shows that the UV flux is very sensitive not only to age and metallicity, but also to the treatments of the mass loss efficiency and the HB mass distribution.

(3) The origin of the UV upturn has been uncovered. Under the conventional assumptions, only old, metal-rich model galaxies develop a sufficient number of UV bright HB and evolved HB stars to generate UV upturns of the observed properties introduced above within a Hubble time. Accordingly, important clues to the evolution of giant elliptical galaxies are discussed.

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