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Neutral atomic carbon in centers of galaxies

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
² Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, Hawaii, 96720, USA

Corresponding author: F. P. Israel, israel@strw.leidenuniv.nl

Received: 27 June 2001
Accepted: 30 November 2001

Abstract

We present measurements of the emission from the centers of fifteen spiral galaxies in the– fine-structure transition at 492 GHz. Observed galaxy centers range from quiescent to starburst to active.The intensities of neutral carbon, the 2–1 transition of and the 4–3 transition of are compared in matchedbeams. Most galaxy centersemit more strongly in [CI] than in, completely unlike the situation pertaining to Galacticmolecular cloud regions. [CI] intensities are lower than, but nevertheless comparable to 4–3 intensities, again rather different fromGalactic sources. The ratio of [CI] to increaseswith the central [CI] luminosity of a galaxy; it is lowest forquiescent and mild starburst centers, and highest for strong starburstcenters and active nuclei. Comparison with radiative transfer modelcalculations shows that most observed galaxy centers haveneutral carbon abundances close to, or exceeding, carbon monoxide abundances, rather independent from the assumed model gas parameters. The same modelssuggest that the emission from neutral carbon and carbon monoxide, if assumed to originate in the same volumes,arises from a warm anddense gas rather than a hot and tenuous, or a cold and very dense gas.The observed [CI] intensities together with literature [CII] lineand far-infrared continuum data likewise suggest that a significantfraction of the emission originates in medium-density gas (–), subjected to radiation fields of various strengths.