A Herschel*-ATLAS study of dusty spheroids: Probing the minor-merger process in the local Universe

We use multiwavelength (0.12–500 μm) photometry from Herschel-ATLAS, WISE, UKIDSS, SDSS and GALEX to study 23 nearby spheroidal galaxies with prominent dust lanes (DLSGs). DLSGs are considered to be remnants of recent minor mergers, making them ideal laboratories for studying both the interstellar medium (ISM) of spheroids and minor-merger-driven star formation in the nearby Universe. The DLSGs exhibit star formation rates (SFRs) between 0.01 and 10 M_⊙ yr⁻¹, with a median of 0.26 M_⊙ yr⁻¹ (a factor of 3.5 greater than the average SG). The median dust mass, dust-to-stellar mass ratio and dust temperature in these galaxies are around 10^7.6 M_⊙, ≈0.05 per cent and ≈19.5 K, respectively. The dust masses are at least a factor of 50 greater than that expected from stellar mass loss and, like the SFRs, show no correlation with galaxy luminosity, suggesting that both the ISM and the star formation have external drivers. Adopting literature gas-to-dust ratios and star formation histories derived from fits to the panchromatic photometry, we estimate that the median current and initial gas-to-stellar mass ratios in these systems are ≈4 and ≈7 per cent, respectively. If, as indicated by recent work, minor mergers that drive star formation in spheroids with (NUV − r) > 3.8 (the colour range of our DLSGs) have stellar mass ratios between 1:6 and 1:10, then the satellite gas fractions arelikely ≥50 per cent.