Kavli Affiliate: Sarah Church
| First 5 Authors: Jialu Li, Andrew I Harris, Erik Rosolowsky, Amanda Kepley, David Frayer
| Summary:
We report observations of the ground state transitions of $^{12}$CO,
$^{13}$CO C$^{18}$O, HCN, and HCO$^+$ at 88-115 GHz in the inner region of the
nearby galaxy IC 342. These data were obtained with the 16-pixel spectroscopic
focal plane array Argus on the 100-m Robert C. Byrd Green Bank Telescope (GBT)
at 6-9$^{primeprime}$ resolution. In the nuclear bar region, the intensity
distributions of $^{12}$CO(1-0) and $^{13}$CO(1-0) emission trace moderate
densities, and differ from the dense gas distributions sampled in
C$^{18}$O(1-0), HCN(1-0), and HCO$^+$(1-0). We observe a constant
HCN(1-0)-to-HCO$^+$(1-0) ratio of 1.2$pm$0.1 across the whole $sim$1 kpc bar.
This indicates that HCN(1-0) and HCO$^+$(1-0) lines have intermediate optical
depth, and that the corresponding $n_{textrm{H}_2}$ of the gas producing the
emission is of 10$^{4.5-6}$ cm$^{-3}$. We show that HCO$^+$(1-0) is thermalized
and HCN(1-0) is close to thermalization. The very tight correlation between
HCN(1-0) and HCO$^+$(1-0) intensities across the 1~kpc bar suggests that this
ratio is more sensitive to the relative abundance of the two species than to
the gas density. We confirm the angular offset ($sim$10$^{primeprime}$)
between the spatial distribution of molecular gas and the star formation sites.
Finally, we find a breakdown of the $L_textrm{IR}$-$L_textrm{HCN}$
correlation at high spatial resolution due to the effect of incomplete sampling
of star-forming regions by HCN emission in IC 342. The scatter of the
$L_textrm{IR}$-$L_textrm{HCN}$ relation decreases as the spatial scale
increases from 10$^{primeprime}$~to 30$^{primeprime}$ (170-510~pc), and is
comparable to the scatter of the global relation at the scale of 340 pc.
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