Stellar population characterizations in nearby, dusty early-type galaxies

ABSTRACT Dust in early-type galaxies (ETGs) may originate from internal or external sources. In this paper, we study the stellar populations of particularly dusty ETGs to search for evidence of the dust’s origin. Using the Southern African Large Telescope (SALT), we obtained long-slit optical spectra within the effective radius ($R_\mathrm{ e}$), along the major axis of 15 nearby ETGs, selected from the Galaxy and Mass Assembly (GAMA) and Herschel Astrophysical Terahertz Large Area Survey (Herschel-ATLAS) surveys for their high levels of interstellar dust. Using full spectrum fitting and Lick index fitting we analysed their major axis kinematics and stellar population characteristics. We used stellar population models from the newly developed semi-empirical Medium-resolution Isaac Newton Telescope library of empirical spectra (sMILES) library and from the empirical MILES library. Kinematic results show that most of our sample of dusty ETGs are rotationally supported and there are no detectable kinematic discontinuities. 12 of our sample of 15 dusty ETGs show evidence of young/intermediate age stellar population components suggesting ongoing/recent star formation. Using simulations, we show that these recent (≈1 Gyr) populations are not artefacts of the fitting process or data. As a check with a control sample we use stacked Sloan Digital Sky Survey spectra and find that dusty ETGs show a component with intermediate age, whereas non-dusty ETGs do not. Age, metallicity, and $\alpha$-element abundance ratio increase with increasing central velocity dispersion in the SALT spectra, as seen in previous studies of ETGs, but with larger scatter in our sample. Given our stellar population findings, we discuss formation scenarios that might cause or rule out a high dust/molecular gas content.