1999/01/28 by Padeli P. Papadopoulos, Padeli. P. Papadopoulos, E. R. Seaquist · 5 citations
Physics and Astronomy · #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/307090
published as Astrophys.J. 516 (1999) 114-126 · 41 pages, 5 figures, to appear in Astrophysical Journal 1999, May 1
arxiv created 1999/01/28 · openalex publication_date 1999/05/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
We present fully sampled 12 CO, 13 CO J =2-1, 3-2 maps of the inner ~1' × 1' region of NGC 1068. We combine these measurements with an existing interferometric map of 12 CO J =1-0 that includes single dish data and thus contains all the flux present. This allows a reliable estimate of the 12 CO ( J =3-2)/( J =1-0) ratio at the highest angular resolution currently possible and the use of this sensitive line ratio to probe the physical condition of the molecular gas. We also present two measurements of the faint C 18 O J =2-1 emission that confirm earlier measurements of a high C 18 O/ 13 CO intensity ratio in this galaxy. The ratios of the 12 CO, 13 CO isotopes can only be reproduced for small to moderate optical depths of 12 CO J =1-0 (τ ~1-2), which is incompatible with the high C 18 O/ 13 CO ratios observed. A simple two-phase model for the gas can account for all the observed line ratios if the C 18 O emission and part of the 13 CO emission arise in a dense spatially concentrated component, where C 18 O J =1-0 has optical depths of τ ≳ 1. The 12 CO emission originates from a warmer, diffuse gas phase with τ ~1-2 for J =1-0. The dense gas phase contains the bulk of the molecular gas mass, whereas the diffuse phase may not be virialized, leading to an overestimate of molecular gas mass when deduced from the luminosity of the 12 CO J =1-0 line and a standard galactic conversion factor. This suggests that, since type 2 Seyferts harbor a central starburst more often than type 1, the higher average 12 CO J =1-0 luminosity of type 2 hinted by earlier studies may simply reflect a difference in molecular gas excitation rather than in gas mass.