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

¹ Purple Mountain Observatory, Chinese Academy of Sciences, No.10 Yuanhua Road, Qixia District, Nanjing 210023, China
² School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
³ Department of Astronomy, Xiamen University, 422 Siming South Road, Xiamen 361005, China
⁴ Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, China
⁵ Key Laboratory of Dark Matter and Space Astronomy, CAS, Nanjing 210023, China
⁶ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
⁷ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China
⁸ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
⁹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹⁰ DTU-Space, Technical University of Denmark, Elektrovej 327 2800 Kgs. Lyngby, Denmark
¹¹ School of Physics and Astronomy, China West Normal University, No. 1 Shida Road, Nanchong 637002, China
¹² Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada
¹³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁴ Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea
¹⁵ RAL Space, Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, UK
¹⁶ East Asian Observatory, 660 N. A‘ohōkū Place, Hilo, HI 96720, USA
¹⁷ School of Mathematical and Physical Sciences, Macquarie University, Balaclava Road, Sydney, NSW 2109, Australia
¹⁸ Astrophysics and Space Technologies Research Centre, Macquarie University, Balaclava Road, Sydney, NSW 2109, Australia
¹⁹ International Space Science Institute–Beijing, 1 Nanertiao, Zhongguancun, Hai Dian District, Beijing 100190, China
²⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
²¹ Department of Astronomy, School of Physics, Peking University, Beijing 100871, China
²² Institute of Astronomy and Astrophysics, Academia Sinica, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
²³ Astronomical Observatory Institute, Faculty of Physics and Astronomy, Adam Mickiewicz University, ul. Słoneczna 36 60-286, Poznań, Poland
²⁴ School of Space Research, Kyung Hee University, 1732 Deogyeong-daero, Yongin 17104, Korea
²⁵ National Astronomical Research Institute of Thailand, 260 Moo 4, Donkaew, Mae Rim, Chiang Mai 50180, Thailand
²⁶ Gemini Observatory/NSF NOIRLab, 670 N. A’ohoku Place, Hilo, Hawai’i 96720, USA
²⁷ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK

^⋆ Corresponding authors: jfwang@pmo.ac.cn, qhtan@pmo.ac.cn

Received: 23 January 2025
Accepted: 23 June 2025

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 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.