Spitzer-IRS Study of the Antennae Galaxies NGC 4038/39

Creators: Brandl, B. R.; Snijders, L.; den Brok, M.; Whelan, D. G.; Groves, B.; van der Werf, P. P.; Charmandaris, V.; Smith, J. D.; Armus, L.; Kennicutt, R. C., Jr.; Houck, J. R.

Abstract

Using the Infrared Spectrograph on the Spitzer Space Telescope, we observed the Antennae galaxies obtaining spectral maps of the entire central region and high signal-to-noise 5-38 μm spectra of the two galactic nuclei and six infrared-luminous regions. The total infrared luminosity of our six IR peaks plus the two nuclei is L_(IR) = 3.8 × 10^(10)L☉, with their derived star formation rates ranging between 0.2 and 2 M☉ yr^(–1), with a total of 6.6 M☉ yr^(–1). None of the typical mid-IR tracers of active galactic nucleus activity is detected in either nucleus of the system, excluding the presence of a dust-enshrouded accretion disk. The hardest and most luminous radiation originates from two compact clusters in the southern part of the overlap region, which also have the highest dust temperatures. Polycyclic aromatic hydrocarbon (PAH) emission and other tracers of softer radiation are spatially extended throughout and beyond the overlap region, but regions with a harder and more intense radiation field show a reduced PAH strength. The strong H_2 emission is rather confined around the nucleus of NGC 4039, where shocks appear to be the dominant excitation mechanism, and the southern part of the overlap region, where it traces the most recent starburst activity. The luminosity ratio between the warm molecular gas (traced by the H_2 lines) and the total far-IR emission is ~ 1.6 × 10^(–4), similar to that found in many starbursts and ultraluminous infrared galaxies. The total mass of warm H_2 in the Antennae is 2.5 × 10^7 M☉ , with a fraction of warm to total H_2 gas mass of about 0.35%. The average warm H_2 temperature is 302 ± 26 K and appears anticorrelated with the radiation field hardness, possibly due to an evolution of the photodissociation region (PDR) morphology. The previously reported tight correlation between the H_2 and PAH emission was not found but higher total PAH emission to continuum ratios were found in PDRs with warmer gas.

Additional Information

© 2009. The American Astronomical Society. Received 2008 July 30; accepted 2009 May 11; published 2009 June 26. We thank ZhongWang, who provided us with the proprietary IRAC images of the Antennae immediately after their observations, which enabled the optimal planning of our spectroscopic follow up. We also thank Henrik Spoon for stimulating discussions and Martin Haas for many critical comments, which, however, did not result in an agreement on the H2 emission. Finally, we thank the referee for many comments that helped to clarify the discussion. 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 NASA contract 1407. Support for this work was provided by NASA through Contract 1257184 issued by JPL/Caltech. V.C. would also like to acknowledge partial support from the EU ToK grant 39965.

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