Galaxy formation and evolution

Abstract:

We combine far-infrared photometry from Herschel (PEP/HerMES) with deep mid-infrared spectroscopy from Spitzer to investigate the nature and the mass assembly history of a sample of 31 luminous and ultraluminous infrared galaxies ((U)LIRGs) at z &thk 1 and 2 selected in GOODS-S with 24 μm fluxes between 0.2 and 0.5 mJy. We model the data with a self-consistent physical model (GRASIL) which includes a state-of-the-art treatment of dust extinction and reprocessing. We find that all of our galaxies appear to require massive populations of old (>1 Gyr) stars and, at the same time, to host a moderate ongoing activity of star formation (SFR ≤ 100M yr-1). The bulk of the stars appear to have been formed a few Gyr before the observation in essentially all cases. Only five galaxies of the sample require a recent starburst superimposed on a quiescent star formation history. We also find discrepancies between our results and those based on optical-only spectral energy distribution (SED) fitting for the same objects; by fitting their observed SEDs with our physical model we find higher extinctions (by ΔAV &thk 0.81 and 1.14) and higher stellar masses (by Δlog(M) &thk 0.16 and 0.36 dex) for z &thk 1 and z &thk 2 (U)LIRGs, respectively. The stellar mass difference is larger for the most dust-obscured objects. We also find lower SFRs than those computed from LIR using the Kennicutt relation due to the significant contribution to the dust heating by intermediate-age stellar populations through "cirrus" emission (&thk73% and &thk66% of the total LIR for z &thk 1 and z &thk 2 (U)LIRGs, respectively). © 2013. The American Astronomical Society. All rights reserved..

Abstract:

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 thenearby Universe. The DLSGs exhibit star formation rates (SFRs) between 0.01and 10M⊙ yr-1 with a median of 0.26M⊙ yr-1 (a factor of 3.5 greater thanthe average SG). The median dust mass, dust-to-stellar mass ratio and dust temperature in these galaxies are around 107.6M⊙, ≈0.05 per cent and ≈19.5K, 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 correlationwith galaxy luminosity, suggesting that both the ISM and the star formationhave 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 gasto- 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 are likely =50 per cent. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.