Modelling the spectral energy distribution of ULIRGs

Abstract

Aims. By using the spectral energy distribution (SED) from the near-infrared to the radio of a statistically significant number of luminous infrared galaxies we determine important physical parameters for this population of objects. In particular we constrain the optical depth towards the luminosity source, the star formation rate, the star formation efficiency and the AGN fraction. Methods. We fit the near-infrared to radio spectral energy distributions of a sample of 30 luminous and ultra-luminous infrared galaxies with pure starburst models or models that include both starburst and AGN components. Results. We find that although about half of our sample have best-fit models that include an AGN component, only 30% (9/30) have an AGN that accounts for more than 10% of the infrared luminosity from 8 to 1000 μm, whereas all have an energetically dominant starburst. Our derived AGN fractions are generally in good agreement with measurements of the mid-infrared line ratios, Ne[V]/Ne [II] and O[IV]/Ne[II] by Spitzer IRS, but much lower than those derived from PAH equivalent widths or the mid-infrared spectral slope. Our models determine the mass of dense molecular gas within which active star formation takes place via the extinction required to reproduce the infrared part of the SED. Assuming that this mass is that traced by the HCN molecule, we reproduce the observed linear relation between HCN flux and infrared luminosity found previously. We also find that the star formation efficiency, as defined by the current star formation rate per unit molecular gas mass, falls as the starburst ages. Conclusions. If the evolution of ULIRGs includes a phase in which an AGN contributes an important fraction to the infrared luminosity, this phase should last an order of magnitude less time than the starburst phase. However, we find no convincing evidence that an energetically important AGN is associated with a particular phase of the starburst. Because the mass of dense molecular gas that we derive is consistent with observations of the HCN molecule, it should be possible to estimate the mass of dense, star-forming molecular gas in such objects when molecular line data are not available.

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…Considering the observed 2-10 keV luminosity, the ratio is L L log 4.14 Using the relationship of Mullaney et al (2011) it is possible to convert the 2-10 keV luminosity into the expected IR ( ( ) ) = -we find that the IR luminosity of the AGN is between 0.1% and 0.6% of the total IR luminosity. This value is in disagreement with that obtained by Vega et al (2008) using spectral decomposition, who found that the contribution of the AGN to the total IR luminosity is about one order of magnitude larger. A 5% contribution to the total IR luminosity would imply that 9.6 Intrinsic 2-10 keV flux (F 2 10 -) (10 −12 erg s −1 cm −2 ) 2.6 intrinsic 10-50 keV flux (F 10 50 -)(10 −12 erg s −1 cm −2 ) 3.1 2-10 keV luminosity (L 2 10 -)(erg s −1 ) 2.01×10 42 10-50 keV luminosity (L 10 50 , also an order of magnitude larger than predicted by our X-ray spectral analysis.…”

“…It has been shown indeed that for AGNs the absorption-corrected 2-10 keV and the 6 and 12 μm luminosities are well-correlated (e.g., Gandhi et al 2009;Asmus et al 2015;Stern 2015) so that deviations from the correlation might imply the presence of heavy obscuration. Vega et al (2008) report that at 6 μm about 58% of the flux is produced by the AGN. This would imply that the ratio between the IR and observed X-ray AGN luminosity is very low: …”

“…The XMMNewton can in fact be well represented by a model taking into account only a collisionally ionized plasma and an unabsorbed power-law component, representing thermal emission from the starburst and X-ray radiation produced by X-ray binaries, respectively. Possible evidence of very faint AGN activity has been found by studying the near-IR to radio spectral energy distribution (SED; Vega et al 2008) and could be inferred by the detection of [Ne V] lines, although the detection of these features has been questioned by Inami et al (2013) and due to their weakness they might also have been produced by a young starburst.…”

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…This is not likely to be the case if the mechanism driving the FIR emission of AGN hosts was disparate from that of the SPIRE population with dormant nuclei (though see discussion in Hill & Shanks 2012 and references therein for a possible alternative viewpoint). Another line of evidence comes from a more complex treatment of local ULIRGS involving multi-component SED fitting by Vega et al (2008). In that study it was found that the bandpass spanning the rest-frame wavelengths 40 μm-500 μm was the least sensitive to the presence of a radio AGN, such that only ∼5% of their sample needed AGN contributions to the TIR luminosity in excess of 10% to properly reproduce the SEDs of their sample.…”

The hard X-ray view of bright infrared galaxies

“…Figure 2 presents the re-normalized average spectral energy distributions of each subsamples, together with the average infrared-optical spectra obtained for a sample of 30 ULIRGs (Vega et al 2008) for illustration. The spectra of the galaxy subsamples were re-normalized at 60 μm to the average obtained for the 30 ULIRGs.…”