The Distribution of HCO+ J=4-3 and HCN J=4-3 in the Nuclear Region of M82

Details The Distribution of HCO+ J=4-3 and HCN J=4-3 in the Nuclear Region of M82 The Distribution of HCO+ J=4-3 and HCN J=4-3 in the Nuclear Region of M82

Sep 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...540..765s&link_type=abstract

The Astrophysical Journal, Volume 540, Issue 2, pp. 765-770.

Astronomy and Astrophysics

Astronomy

15

Galaxies: Individual: Messier Number: M82, Galaxies: Ism, Galaxies: Nuclei, Galaxies: Starburst, Radio Lines: Galaxies

Scientific paper

We present the first detection of HCN J=4-3 in the starburst galaxy M82 and a fully sampled grid of the distribution of its emission in the nuclear region. The angular resolution is 14" and the velocity resolution 6.3 km s-1. The distribution of HCN emission is compared with that of HCO+ J=4-3 made with the same resolution, and the conclusion is that the HCO+/HCN brightness ratio is higher at J=4-3 than at lower transitions, with a median ratio of 3.5. There is evidence for a variation in this ratio across the nuclear region, with higher values toward the edge of the nuclear disk. This variation is consistent with a variation in gas density, with higher densities associated with regions closer to the nucleus of M82. However, some of this variation could also be produced by a variation in the HCO+/HCN abundance ratio. A large-scale velocity gradient (LVG) analysis of three transitions in each molecule centered at the nucleus shows that the line ratios within each species and between the two species are consistent with a common set of physical conditions for the emitting regions, namely, a kinetic temperature of 50 K, gas density 105 cm-3, and gas column density 1022 cm-2. Although it has been suggested that HCO+ may be subject to mechanisms of excitation other than collisions with H2, such as electron collisions, our analysis shows that there is no reason to invoke any such mechanisms to account for the line ratios present. The HCO+/HCN intensity ratio is known to vary significantly from one galaxy to another, with M82 at the high end of the spectrum of variation. This effect is briefly discussed in the context of a bistable chemical equilibrium model in which the HCO+ abundance is a sensitive function of the cosmic-ray ionization rate and gas density in the cloud cores. Within the framework of this picture, a high cosmic-ray flux could be inversely related to HCO+ abundance, contrary to what is generally assumed.

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