The MALATANG survey: Dense gas distribution on sub-kiloparsec scales across the disk of M82

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 detected 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⁻¹ 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, 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 have also been 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 photodissociation region in M82.