Warm Molecular Hydrogen in theSpitzerSINGS Galaxy Sample

Abstract

Results on the properties of warm molecular hydrogen in 57 normal galaxies are derived from measurements of H 2 rotational transitions, obtained as part of SINGS. This study extends previous extragalactic surveys of emission lines of H 2 to fainter and more common systems (L FIR ¼ 10 7 6 ; 10 10 L ). The 17 m S(1) transition is securely detected in the nuclear regions of 86% of galaxies with stellar masses above 10 9.5 M . The derived column densities of warm H 2 (T ! 100 K ), although averaged over kiloparsec-scale areas, are commensurate with values observed in resolved photodissociation regions. They amount to between 1% and >30% of the total H 2 . The power emitted in the three lowest energy transitions is on average 30% of the power of the bright [Si ii] cooling line (34.8 m) and about 4 ; 10 À4 of the total infrared power for star-forming galaxies, which is consistent with excitation in PDRs. The fact that the H 2 line intensities scale tightly with the aromatic band emission, even though the average radiation field intensity varies by a factor of 10, can also be understood if both tracers originate predominantly in PDRs, either dense or diffuse. Many of the 25 LINER /Seyfert targets strongly depart from the rest of the sample, in having warmer excited H 2 and excess H 2 rotational power with respect to the dust emission. We propose a threshold in H 2 -to-aromatic band power ratios, allowing the identification of low-luminosity AGNs by an excess H 2 excitation. A dominant contribution from shock heating is favored in these objects. Finally, we detect in nearly half the star-forming targets nonequilibrium orthoto-para ratios, consistent with the effects of FUV pumping combined with incomplete ortho-para thermalization, or possibly nonequilibrium photodissociation fronts.

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

“…The SINGS sample consists of LINERs, Seyferts, dwarfs, and galaxies dominated by star formation in their nuclei. We adopted the H 2 rotational line fluxes for the four lowest rotational lines (S(0) to S(3)) in SINGS galaxies from Roussel et al (2007). The targets were observed in spectral mapping mode and the details of the observing strategy are described in Kennicutt et al (2003) and Smith et al (2004).…”

“…For this purpose, we utilize the cold H 2 gas masses for the SINGS sample compiled by Roussel et al (2007), which are derived from aperture-matched CO velocity integrated intensities. The observed line intensities of the 2.6 mm 12 CO(1-0) line within the measured CO beam size was chosen to match the aperture of the Spitzer/IRS spectroscopic observations.…”

“…The observed line intensities of the 2.6 mm 12 CO(1-0) line within the measured CO beam size was chosen to match the aperture of the Spitzer/IRS spectroscopic observations. Roussel et al (2007) assumed a CO intensity to molecular gas conversion factor (a CO ) of 5.0…”

“…H 2 present in photo-dissociation regions (PDRs) in the outer layers of the molecular cloud can reach temperatures of a few hundred K (Hollenbach & Tielens 1997. For the SINGS galaxies, Roussel et al (2007) concluded that the mass of H 2 greater than 100 K contributes 1%-30% of the total H 2 gas mass. In the MOlecular Hydrogen Emission Galaxies (MOHEGs) of Ogle et al (2010), M(H 2 )<1500 K contributes only 0.01% of the total H 2 .…”

“…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 ).…”