Isothermal dust models of Herschel-ATLAS★ galaxies

Abstract

We use galaxies from the Herschel-ATLAS survey, and a suite of ancillary simulations based on an isothermal dust model, to study our ability to determine the effective dust temperature, luminosity and emissivity index of 250 µm selected galaxies in the local Universe (z < 0.5). As well as simple far-infrared SED fitting of individual galaxies based on χ 2 minimisation, we attempt to derive the best global isothermal properties of 13,826 galaxies with reliable optical counterparts and spectroscopic redshifts. Using our simulations, we highlight the fact that applying traditional SED fitting techniques to noisy observational data in the Herschel Space Observatory bands introduces artificial anti-correlation between derived values of dust temperature and emissivity index. This is true even for galaxies with the most robust statistical detections in our sample, making the results of such fitting difficult to interpret. We apply a method to determine the best-fit global values of isothermal effective temperature and emissivity index for z < 0.5 galaxies in H-ATLAS, deriving T eff = 22.3±0.1 K and β = 1.98±0.02 (or T eff = 23.5 ± 0.1 K and β = 1.82 ± 0.02 if we attempt to correct for bias by assuming that T eff and β eff are independent and normally distributed). We use our technique to test for an evolving emissivity index, finding only weak evidence. The median dust luminosity of our sample is log 10 (L dust /L ⊙ ) = 10.72 ± 0.05, which (unlike T eff ) shows little dependence on the choice of β used in our analysis, including whether it is variable or fixed. In addition, we use a further suite of simulations based on a fixed emissivity index isothermal model to emphasize the importance of the H-ATLAS PACS data for deriving dust temperatures at these redshifts, even though they are considerably less sensitive than the SPIRE data. Finally, we show that the majority of galaxies detected by H-ATLAS are normal star-forming galaxies, though with a substantial minority (∼ 31 per cent) falling in the Luminous Infrared Galaxy category.

Herschel-ATLAS: the connection between star formation and AGN activity in radio-loud and radio-quiet active galaxies

“…Table 2 shows the best-fitting temperatures for each type of AGN in the sample, together with uncertainties derived by the bootstrap method. The results show that the mean temperatures of the populations in the sample are around 25 K (Identical temperature estimates for star-forming galaxies were also reported by Smith et al 2013). This indicates that the far-IR measurements of our sources are not strongly affected by cold dust heated by the old stellar populations.…”

Should we believe the results of ultraviolet–millimetre galaxy spectral energy distribution modelling?

“…Although the agreement was generally good in the comparatively quiescent phases of the simulation, the most significant discrepancy was that the IR luminosity and SFR were underestimated by ∼ 0.5 dex during the starburst that occurs at first passage (but it is possible that this underestimate could be corrected by modifying the priors; E. da Cunha, private communication). This test further highlights the importance of the available photometry sampling the peak of the temperature-dependent SED for the purposes of recovering the true dust luminosity, in agreement with the investigation by Smith et al (2013) which used isothermal models to fit the dust SEDs of H-ATLAS galaxies.…”

“…the shift of the IR SED peak to shorter wavelength; e.g. Smith et al 2013) that is caused in starbursts primarily by the simultaneous sharp increase in luminosity and decrease in dust mass . 16 Although it is encouraging that the evolution of the MAGPHYS dust temperatures reflects this physical effect, this result should be interpreted with caution because of the significant error bars associated with T ISM C and T BC W and the proximity of the median-likelihood values to the bounds on the temperature priors (dotted grey horizontal lines in panels k and l).…”

“…Finally, treating the FIR emission as one or more modified blackbodies may be problematic if one's goal is to infer physical quantities rather than simply describe the SED (e.g. Shetty et al 2009a,b;Hayward et al 2012;Kelly et al 2012;Smith et al 2013), but more sophisticated models, such as those of Dale & Helou (2002) and , may be able to yield physical insight.…”

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…Using a range 1.4 < β < 2.2 (see e.g. Dye et al 2010;Hardcastle et al 2010;Smith et al 2013) we find that the best-fitting model returns lower χ 2 values for a fixed β = 1.6 dust emissivity for all AGN populations in the sample. The selection of β = 1.6 is consistent with the work of Dye et al (2010).…”