CON-quest II. Spatially and spectrally resolved HCN/HCO+ line ratios in local luminous and ultraluminous infrared galaxies
Astronomy & Astrophysics(2024)
摘要
Nuclear regions of ultraluminous and luminous infrared galaxies (U/LIRGs) are
powered by starbursts and/or active galactic nuclei (AGNs). These regions are
often obscured by extremely high columns of gas and dust. Molecular lines in
the submillimeter windows have the potential to determine the physical
conditions of these compact obscured nuclei (CONs). We aim to reveal the
distributions of HCN and HCO^+ emission in local U/LIRGs and investigate
whether and how they are related to galaxy properties. Using ALMA, we have
conducted sensitive observations of the HCN J=3–2 and HCO^+ J=3–2 lines
toward 23 U/LIRGs in the local Universe (z < 0.07) with a spatial resolution of
0.3" ( 50–400 pc). We detected both HCN and HCO^+ in 21 galaxies, only HCN
in one galaxy, and neither in one galaxy. The global HCN/HCO^+ line ratios,
averaged over scales of 0.5–4 kpc, range from 0.4 to 2.3, with an unweighted
mean of 1.1. These line ratios appear to have no systematic trend with
bolometric AGN luminosity or star formation rate. The line ratio varies with
position and velocity within each galaxy, with an average interquartile range
of 0.38 on a spaxel-by-spaxel basis. In eight out of ten galaxies known to have
outflows and/or inflows, we found spatially and kinematically symmetric
structures of high line ratios. These structures appear as a collimated bicone
in two galaxies and as a thin spherical shell in six galaxies. Non-LTE analysis
suggests that the high HCN/HCO^+ line ratio in outflows is predominantly
influenced by the abundance ratio. Chemical model calculations indicate that
the enhancement of HCN abundance in outflows is likely due to high-temperature
chemistry triggered by shock heating. These results imply that the HCN/HCO^+
line ratio can aid in identifying the outflow geometry when the shock velocity
of the outflows is sufficiently high to heat the gas.
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