Kavli Affiliate: Luis Ho
| First 5 Authors: Jian-Fa Wang, Jian-Fa Wang, , ,
| Summary:
We present observations of HCN J=4-3 and HCO^+ J=4-3 lines obtained with the
James Clerk Maxwell Telescope as part of the MALATANG survey, combined with
archival HCN J=1-0 and HCO^+ J=1-0 data from the Green Bank Telescope, to study
the spatial distribution and excitation conditions of dense molecular gas in
the disk of M82. We detect HCN J=4-3 and HCO^+ J=4-3 emission within the
central region (< 500 pc) of the galaxy, while the J=1-0 emission lines exhibit
a more extended spatial distribution (> 700 pc). The dense gas shows a clear
double-lobed structure in both spatial distribution and kinematics, with the
HCN and HCO^+ J=4-3 lines in the southwest lobe blueshifted by ~ 40 km/s
relative to the J=1-0 lines. The HCN J=4-3/1-0 and HCO^+ J=4-3/1-0
line-luminosity ratios range from 0.09 to 0.53 and from 0.14 to 0.87,
respectively, with mean values of 0.18 +/- 0.04 and 0.36 +/- 0.06. The HCN
ratio is lower than the typical average observed in nearby star-forming
galaxies, whereas the HCO^+ ratio is comparatively higher, suggesting that the
high-J HCN emission in M82 is significantly sub-thermally excited. Spatially,
the peak values of the J=4-3/1-0 ratios are found in the northwest region of
M82, coinciding with the galaxy-scale outflow. Elevated HCN/HCO^+ ratios are
also detected in roughly the same area, potentially tracing local excitation
enhancements driven by the outflow. The HCN/HCO^+ J=4-3 ratio across all
detected regions ranges from 0.19 to 1.07 with a mean value of 0.41 +/- 0.11,
which is significantly lower than the average J=1-0 ratio of 0.76 +/- 0.08.
Both ratios are significantly lower than the average values observed in nearby
star-forming galaxies, which could be related to the relatively low gas density
and the presence of an extended photo-dissociation region in M82.
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