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Published August 1, 2006 | Submitted + Published
Journal Article Open

The opaque nascent starburst in NGC 1377: Spitzer SINGS observations

Abstract

We analyze extensive data on NGC 1377 from the Spitzer Infrared Nearby Galaxies Survey (SINGS). Within the category of nascent starbursts that we previously selected as having infrared-to-radio continuum ratios in large excess of the average and containing hot dust, NGC 1377 has the largest infrared excess yet measured. Optical imaging reveals a morphological distortion suggestive of a recent accretion event. Infrared spectroscopy reveals a compact and opaque source dominated by a hot, self-absorbed continuum (τ~ 20 in the 10μm silicate band). We provide physical evidence against nonstellar activity being the heating source. H II regions are detected through the single [Ne II] line, probing <1% of the ionizing radiation. Not only is the optical depth in different gas and dust phases very high, but >85% of ionizing photons are suppressed by dust. The only other detected emission features are molecular hydrogen lines, arguably excited mainly by shocks, besides photodissociation regions, and weak aromatic bands. The new observations support our interpretation in terms of an extremely young starburst (<1 Myr). More generally, galaxies deficient in radio synchrotron emission are likely observed within a few Myr of the onset of a starburst and after a long quiescence, prior to the replenishment of the interstellar medium with cosmic rays. The similar infrared-radio properties of NGC 1377 and some infrared-luminous galaxies suggest that NGC 1377 constitutes an archetype that will be useful to better understand starburst evolution. Although rare locally because observed in a brief evolutionary stage, nascent starbursts may represent a nonnegligible fraction of merger-induced starbursts that dominate deep infrared counts. Since they differ dramatically from usual starburst templates, they have important consequences for the interpretation of deep surveys.

Additional Information

© 2006 The American Astronomical Society. Received 2006 March 5; accepted 2006 April 11. We wish to thank Marc Sauvage for fruitful discussions, Mark Wolfire for tirelessly running photodissociation region models to help interpret the data, and Nancy Silbermann for the timely scheduling of the Spitzer IRS observations. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through an award issued by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.

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