Efstathiou, Andreas

We present Herschel observations of 6 fine-structure lines in 25 ultraluminous infrared galaxies at z < 0.27. The lines, [O III]52 μm, [N III]57 μm, [O I]63 μm, [N II]122 μm, [O I]145 μm, and [C II]158 μm, are mostly single Gaussians with widths <600 km s-1 and luminosities of 107-109 LO. There are deficits in the [O I]63/L IR, [N II]/L IR, [O I]145/L IR, and [C II]/L IR ratios compared to lower luminosity systems. The majority of the line deficits are consistent with dustier H II regions, but part of the [C II] deficit may arise from an additional mechanism, plausibly charged dust grains. This is consistent with some of the [C II] originating from photodissociation regions or the interstellar medium (ISM). We derive relations between far-IR line luminosities and both the IR luminosity and star formation rate. We find that [N II] and both [O I] lines are good tracers of the IR luminosity and star formation rate. In contrast, [C II] is a poor tracer of the IR luminosity and star formation rate, and does not improve as a tracer of either quantity if the [C II] deficit is accounted for. The continuum luminosity densities also correlate with the IR luminosity and star formation rate. We derive ranges for the gas density and ultraviolet radiation intensity of 101 < n < 102.5 and 102.2 < G 0 < 103.6, respectively. These ranges depend on optical type, the importance of star formation, and merger stage. We do not find relationships between far-IR line properties and several other parameters: active galactic nucleus (AGN) activity, merger stage, mid-IR excitation, and SMBH mass. We conclude that these far-IR lines arise from gas heated by starlight, and that they are not strongly influenced by AGN activity.

We examine the power source of 41 local ultraluminous infrared galaxies (ULIRGs) using archival infrared (IR) and optical photometry. We fit the observed spectral energy distributions with starburst and active galactic nucleus (AGN) components, each component being drawn from a family of templates. We find that all of the sample require a starburst, whereas only half require an AGN. In 90 per cent of the sample the starburst provides over half the IR emission, with a mean fractional luminosity of 82 per cent. When combined with other galaxy samples we find that starburst and AGN luminosities correlate over six decades in IR luminosity, suggesting that a common factor governs both luminosities, plausibly the gas masses in the nuclear regions. We find no trend for increasing fractional AGN luminosity with increasing total luminosity, contrary to previous claims. We find that the mid-IR F7.7/C 7.7 line-continuum ratio is no indication of the starburst luminosity, or the fractional AGN luminosity, and therefore that F 7.7/C7.7 is not a reliable diagnostic of the power source in ULIRGs. The radio flux correlates with the starburst luminosity, but shows no correlation with the AGN luminosity, in line with previous results. We propose that the scatter in this correlation is due to a skewed starburst initial mass function and/or relic relativistic electrons from a previous starburst, rather than contamination from an obscured AGN. We show that most ULIRGs undergo multiple starbursts during their lifetime, and by inference that mergers between more than two galaxies may be common amongst ULIRGs. Our results support the evolutionary model for ULIRGs proposed by Farrah et al., where they can follow many different evolutionary paths of starburst and AGN activity in transforming merging spiral galaxies into elliptical galaxies, but that most do not go through an optical quasi-stellar object phase. The lower level of AGN activity in our local sample compared with z ∼ 1 hyperluminous infrared galaxies implies that the two samples are distinct populations. We postulate that different galaxy formation processes at high z are responsible for this difference.

We present optical to far-infrared photometry of 31 reddened QSOs that show evidence for radiatively driven outflows originating from active galactic nuclei (AGNs) in their rest-frame UV spectra. We use these data to study the relationships between the AGN-driven outflows, and the AGN and starburst infrared luminosities. We find that FeLoBAL QSOs are invariably IR-luminous, with IR luminosities exceeding 1012 L ⊙ in all cases. The AGN supplies 76% of the total IR emission, on average, but with a range from 20% to 100%. We find no evidence that the absolute luminosity of obscured star formation is affected by the AGN-driven outflows. Conversely, we find an anticorrelation between the strength of AGN-driven outflows, as measured from the range of outflow velocities over which absorption exceeds a minimal threshold, and the contribution from star formation to the total IR luminosity, with a much higher chance of seeing a starburst contribution in excess of 25% in systems with weak outflows than in systems with strong outflows. Moreover, we find no convincing evidence that this effect is driven by the IR luminosity of the AGN. We conclude that radiatively driven outflows from AGNs can have a dramatic, negative impact on luminous star formation in their host galaxies. We find that such outflows act to curtail star formation such that star formation contributes less than ∼ 25% of the total IR luminosity. We also propose that the degree to which termination of star formation takes place is not deducible from the IR luminosity of the AGN.