Two categories of UV-upturn galaxies revealed by semi-analytic models

UV-upturn galaxies are characterized by unusually excessive flux in the far-ultraviolet band, observed in some elliptical galaxies and the bulges of disc galaxies. We examine UV-upturn galaxies within the semi-analytic model gabe, which embeds the formation of extreme horizontal branch (EHB) stars – proposed as key candidates responsible for the UV-upturn phenomenon. We have analysed all related physical processes, including stellar evolution, initial mass functions (IMFs), dust attenuation, galaxy age, metallicity, and binary fractions, in an effort to determine which processes play significant roles. Our findings reveal two categories of UV-upturn galaxies in the semi-analytic model, each with distinct formation channels: old metal-rich quenched elliptical galaxies, which are intrinsic UV-upturn galaxies induced by EHB stars within their old stellar populations, and dusty star-forming galaxies, which are relatively young and may also be photometrically identified as UV-upturn galaxies when accounting for dust attenuation. Dust attenuation contributes to $20~{{\ \rm per\ cent}}\!-\!60~{{\ \rm per\ cent}}$ of the UV-upturn galaxies, depending on the specific dust attenuation models adopted. With the binary star formation model of EHB stars, both of these formation channels exhibit strong preferences for high stellar metallicity. The high-mass end slope of the IMFs is found to have a marginal effect, indicating that a universal IMF is adequate for studying the UV-upturn phenomenon.