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

  • Jian-Fa Wang
  • , Yu Gao
  • , Qing-Hua Tan
  • , Xue-Jian Jiang
  • , Li Ji
  • , Zhi-Yu Zhang
  • , Jun-Zhi Wang
  • , Jun-Feng Wang
  • , R. Thomas Greve
  • , Yan Jiang
  • , Ashley Bemis
  • , Elias Brinks
  • , Aeree Chung
  • , J. Malcolm Currie
  • , Richard de Grijs
  • , Taotao Fang
  • , C. Luis Ho
  • , Bumhyun Lee
  • , Satoki Matsushita
  • , Michał Michałowski
  • Soojong Pak, Panomporn Poojon, G. Mark Rawlings, Amelie Saintonge, Yi-Chen Sun, Jing Zhou

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Abstract

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.
Original languageEnglish
Article numberA72
Number of pages16
JournalAstronomy & Astrophysics
Volume700
Early online date6 Aug 2025
DOIs
Publication statusPublished - 31 Aug 2025

Keywords

  • astro-ph.GA

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