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Published September 2010 | Published
Journal Article Open

Warm Molecular Hydrogen Emission in Normal Edge-on Galaxies NGC 4565 and NGC 5907

Abstract

We have observed warm molecular hydrogen in two nearby edge-on disk galaxies, NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph. The 0-0 S(0) 28.2 μm and 0-0 S(1) 17.0 μm pure rotational lines were detected out to 10 kpc from the center of each galaxy on both sides of the major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one side. This location is beyond the transition zone where diffuse neutral atomic hydrogen starts to dominate over cold molecular gas and marks a transition from a disk dominated by high surface-brightness far-infrared (far-IR) emission to that of a more quiescent disk. It also lies beyond a steep drop in the radio continuum emission from cosmic rays (CRs) in the disk. Despite indications that star formation activity decreases with radius, the H_2 excitation temperature and the ratio of the H_2 line and the far-IR luminosity surface densities, Σ(L_H_2)/Σ(L_(TIR)), change very little as a function of radius, even into the diffuse outer region of the disk of NGC 4565. This suggests that the source of excitation of the H_2 operates over a large range of radii and is broadly independent of the strength and relative location of UV emission from young stars. Although excitation in photodissociation regions is the most common explanation for the widespread H_2 emission, CR heating or shocks cannot be ruled out. At r = 15 kpc in NGC 4565, outside the main UV- and radio-continuum-dominated disk, we derived a higher than normal H_2 to 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission ratio, but this is likely due to a transition from mainly ionized PAH molecules in the inner disk to mainly neutral PAH molecules in the outer disk. The inferred mass surface densities of warm molecular hydrogen in both edge-on galaxies differ substantially, being 4(-60) M_⊙ pc^(–2) and 3(-50) M_⊙ pc^(–2) at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher values represent very unlikely point-source upper limits. The point-source case is not supported by the observed emission distribution in the spectral slits. These mass surface densities cannot support the observed rotation velocities in excess of 200 km s^(–1). Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous Infrared Space Observatory study of the nearby edge-on galaxy NGC 891.

Additional Information

© 2010 The American Astronomical Society. Received 2010 February 18; accepted 2010 July 4; published 2010 August 5. We are grateful to Tom Jarrett at IPAC for helping us to correct the rogue pixels with his custom-made IRAF script. We thank the anonymous referee for very helpful and detailed comments. We acknowledge stimulating discussions with Eric Murphy on the mid-IR and far-IR properties of nearby galaxies. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. SMART was developed by the IRS Team at Cornell University and is available through the Spitzer Science Center at Caltech. The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and Ames Research Center. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. Facility: Spitzer (IRAC, IRS, MIPS)

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August 22, 2023
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