THE ROLE OF STAR FORMATION AND AN AGN IN DUST HEATING OFz= 0.3–2.8 GALAXIES. I. EVOLUTION WITH REDSHIFT AND LUMINOSITY

Abstract

We characterize infrared spectral energy distributions of 343 (Ultra) Luminous Infrared Galaxies from z = 0.3 − 2.8. We diagnose the presence of an AGN by decomposing individual Spitzer mid-IR spectroscopy into emission from star-formation and an AGN-powered continuum; we classify sources as star-forming galaxies (SFGs), AGN, or composites. Composites comprise 30% of our sample and are prevalent at faint and bright S 24 , making them an important source of IR AGN emission. We combine spectroscopy with multiwavelength photometry, including Herschel imaging, to create three libraries of publicly available templates (2-1000 µm). We fit the far-IR emission using a two temperature modified blackbody to measure cold and warm dust temperatures (T c and T w ). We find that T c does not depend on mid-IR classification, while T w shows a notable increase as the AGN grows more luminous. We measure a quadratic relationship between mid-IR AGN emission and total AGN contribution to L IR . AGN, composites, and SFGs separate in S 8 /S 3.6 and S 250 /S 24 , providing a useful diagnostic for estimating relative amounts of these sources. We estimate that >40% of IR selected samples host an AGN, even at faint selection thresholds (S 24 > 100 µJy). Our decomposition technique and color diagnostics are relevant given upcoming observations with the James Webb Space Telescope.

A Total Molecular Gas Mass Census in Z ∼ 2–3 Star-Forming Galaxies: Low-J Co Excitation Probes of Galaxies’ Evolutionary States

“…Since this cold dust heated by star formation dominates at the longest wavelengths, FIR luminosities determined from small numbers of photometric data points on the Rayleigh-Jeans tail of the cold dust peak more accurately trace SFRs than dosingle-component fits to the full dust SED (which does not account for the warm/hot dust emission from the AGN; see also quasars without FIR detections in Leipski et al 2014). Kirkpatrick et al (2015) break down the IR SEDs of low-and high-z U/LIRGs into warm and cold componentsand find that thewarm dust component gets hotter and contributes a larger fraction to the total infrared luminosity with increasing AGN activity (as diagnosed by mid-IR polycyclic aromatic hydrocarbonspectro-scopy). Therefore, SED fits that do not account for two dust components will likely attribute too much IR flux to star formation.…”

“…The higher SFEs appearto be due to larger average FIR luminosities for the AGN host galaxies over the same range of molecular gas masses in the SMGs, which may be due to a dusty torus contribution to the FIR luminosity that is not actually tracing star formation (e.g., Weiß et al 2003;Beelen et al 2006;Leipski et al 2013;Kirkpatrick et al 2015). If we force the linear fit between the FIR and CO luminosities in the Schmidt-Kennicutt relation to have the same index (N = 1) for both SMGs and AGN host galaxies (in order to control for additional uncertainties in the index of the power law), the difference between the offsets measures the fraction of the FIR luminosity for the AGN host galaxies that is in excess of what would be predicted for pure starburst systems (the SMGs).…”

Reaching the peak of the quasar spectral energy distribution – II. Exploring the accretion disc, dusty torus and host galaxy

“…In this respect, our approach is similar to that employed by Mor & Trakhtenbrot (2011), who modelled a single, hot graphitic dust component in a large sample of AGN and Landt et al (2011), who used blackbody models of the hot dust in their sample of AGN. Kirkpatrick et al (2015) also modelled combined blackbody components to represent the warm and cold dust in their sample of luminous IR star-forming galaxies and AGN.…”

The Intrinsic Far-infrared Continua of Type-1 Quasars

“…Several groups have attempted to remove this contamination from the AGN template (e.g., Netzer et al 2007;Mullaney et al 2011;Kirkpatrick et al 2015;Xu et al 2015;Symeonidis et al 2016). However, there is still no consensus.…”

“…However, there is still no consensus. These AGN "intrinsic" IR templates range from (1) the minimalist farinfrared output derived by Netzer et al (2007) by subtracting a starburst-dominated ULIRG template from the average quasar IR SED under the assumption that the vast majority of the 50-100μm emission is due to star formation, to (2) versions with substantially more far-infrared emission obtained, for example, by Kirkpatrick et al (2015) who fitted the data with a combination of a galaxy mid-infrared spectrum, a power law in the same region for the AGN, and two black bodies at longer wavelengths. Most recently, based on a similar low-z sample of opticallyselected bright quasars, Symeonidis et al (2016) reported an "intrinsic" AGN IR template with more far-IR emission than that predicted by the classic quasar template from Elvis et al (1994), where the latter did not even correct the contamination by host galaxy star formation.…”