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.

Abstract:

We present a revised model for the infrared emission of the hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent photometric data from Spitzer and Herschel that sample the peak of its spectral energy distribution. We first present and discuss a grid of smooth active galactic nucleus (AGN) torus models computed with the method of Efstathiou & Rowan-Robinson and demonstrate that the combination of these models and the starburst models of Efstathiou and coworkers, while able to give an excellent fit to the average spectrum of Seyfert 2s and spectra of individual type 2 quasars measured by Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724. This is mainly due to the fact that the νSν distribution of the galaxy falls very steeply with increasing frequency (a characteristic that is usually indicative of heavy absorption by dust) but shows a silicate feature in emission. Such emission features are not expected in sources with optical/near-infrared type 2 AGN spectral signatures. The Herschel data show that there is more power emitted in the rest-frame 20-50 μm wavelength range compared with the model presented by Efstathiou which assumes three components of emission: an edge-on torus, clouds (at a temperature of 610 and 200 K) that are associated with the narrow-line region (NLR) and a highly obscured starburst that dominates in the submillimetre. We present a revised version of that model that assumes an additional component of emission which we associate with NLR clouds at a temperature of 100 K. The 100 K dust component could also be explained by a highly obscured hot starburst. The model suggests that the NLR of IRAS F10214+4724 has an unusually high covering factor (≥17 per cent) or more likely the magnification of the emission from the NLR clouds is significantly higher than that of the emission from the torus. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.