SPITZER QUASAR AND ULIRG EVOLUTION STUDY (QUEST). IV. COMPARISON OF 1 Jy ULTRALUMINOUS INFRARED GALAXIES WITH PALOMAR-GREEN QUASARS

Abstract

We report the results from a comprehensive study of 74 ultraluminous infrared galaxies (ULIRGs) and 34 Palomar-Green (PG) quasars within z ∼ 0.3 observed with the Spitzer Infrared Spectrograph (IRS). The contribution of nuclear activity to the bolometric luminosity in these systems is quantified using six independent methods that span a range in wavelength and give consistent results within ∼ ±10−15% on average. This agreement suggests that deeply buried AGN invisible to Spitzer IRS but bright in the far-infrared are not common in this sample. The average derived AGN contribution in ULIRGs is ∼35−40%, ranging from ∼ 15 − 35% among "cool" (f 25 /f 60 ≤ 0.2) optically classified HIIlike and LINER ULIRGs to ∼50 and ∼75% among warm Seyfert 2 and Seyfert 1 ULIRGs, respectively. This number exceeds ∼80% in PG QSOs. ULIRGs fall in one of three distinct AGN classes: (1) objects with small extinctions and large PAH equivalent widths are highly starburst-dominated; (2) systems with large extinctions and modest PAH equivalent widths have larger AGN contributions, but still tend to be starburst-dominated; and (3) ULIRGs with both small extinctions and small PAH equivalent widths host AGN that are at least as powerful as the starbursts. The AGN contributions in class 2 ULIRGs are more uncertain than in the other objects, and we cannot formally rule out the possibility that these objects represent a physically distinct type of ULIRGs. A morphological trend is seen along the sequence (1) − (2) − (3), in general agreement with the standard ULIRG − QSO evolution scenario and suggestive of a broad peak in extinction during the intermediate stages of merger evolution. However, the scatter in this sequence, including the presence of a significant number of AGN-dominated systems prior to coalesence and starburst-dominated but fully merged systems, implies that black hole accretion, in addition to depending on the merger phase, also has a strong chaotic/random component, as in local AGN.

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

“…The H 2 rotational line fluxes for QSO PG 1440+356 were taken from the Spitzer Quasar and ULIRG Evolution Study (QUEST) sample of Veilleux et al (2009), and those for the 19 ULIRGs were obtained from Higdon et al (2006). The fluxes of H 2 rotational lines for the NGC 6240 and other LIRGs in the sample are from Armus et al (2006) and Pereira-Santaella et al (2010), respectively.…”

“…Hence, the H 2 gas that comprises the bulk of the molecular interstellar medium (ISM) is commonly believed to be too cold to be visible. And yet, despite these observational disadvantages, the advent of the Infrared Space Observatory and in particular Spitzer's Infrared Spectrograph (IRS, Houck et al 2004) revealed pure rotational H 2 emission from a rich variety of extragalactic sources, including normal star-forming galaxies (Roussel et al 2007), ultraluminous and luminous infrared galaxies (U/LIRGs Lutz et al 2003;Veilleux et al 2009;PereiraSantaella et al 2010;Stierwalt et al 2014), galaxy mergers (Appleton et al 2006), radio-loud active galactic nuclei (AGNs) (Ocaña Flaquer et al 2010), UV-selected galaxies (O'Dowd et al 2009), quasar hosts (Evans et al 2001), cooling-flow cluster systems (Egami et al 2006), sources with extreme shockdominated energetics (Ogle et al 2010), and even star-forming sources at redshifts 2 ).…”

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

“…This has been done by exploiting 5-8 μm spectroscopy (e.g., Nardini et al 2010) and the characteristics of several features in the L (3-4 μm) and M (4-5 μm) bands (Imanishi & Dudley 2000;Risaliti et al 2006;Sani et al 2008;Risaliti et al 2010): the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature, the bare carbonaceous 3.4 μm absorption feature, and the slope of the continuum. The 6.2 μm (e.g., Stierwalt et al 2013Stierwalt et al , 2014 and 7.7 μm PAH features (e.g., Veilleux et al 2009), the presence of highexcitation MIR lines (e.g., [Ne V] 14.32 μm, Veilleux et al 2009), or the radio properties (e.g., Parra et al 2010;Romero-Cañizales et al 2012a;Vardoulaki et al 2015) have also been used to infer the presence of a buried AGN.…”

The Circumnuclear Environment of IRAS 20551-4250: A Case Study of AGN/Starburst Connection forJWST

“…Another line of investigation into the physical properties of the circumnuclear medium relies on the rotation diagram of the warm molecular hydrogen, from which we can derive the temperature, column density, and mass of the gas. To this purpose, the observed fluxes listed in Table 1 are converted into column densities of the Jth state (N j ) assuming the LTE regime, an ortho-to-para ratio of 3, a point-like source, and no extinction [23] (see also [33]). While Higdon and collaborators [23] construct the rotation diagram for IRAS 20551-4250 using only the H 2 S(1) and S(3) transitions detected in the low-resolution mode, here we make use of high-resolution detections and add the S(0) and S(2) lines.…”