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Home All issues Volume 700 (August 2025) A&A, 700 (2025) A72 Full HTML
Open Access
Issue
A&A
Volume 700, August 2025
Article Number A72
Number of page(s) 16
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202553865
Published online 06 August 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

Molecular gas serves as a reservoir of raw material for star formation. Numerous studies have indicated that star formation in galaxies is more directly linked to dense gas rather than to diffuse atomic gas or low-density molecular gas components (e.g., Gao & Solomon 2004a;b; Gao et al. 2007; Baan et al. 2008; Bigiel et al. 2008; Liu & Gao 2010; Wang et al. 2011; García-Burillo et al. 2012; Kennicutt & Evans 2012; Zhang et al. 2014; Liu et al. 2016; Onus et al. 2018; Li et al. 2020; 2021).

M82 is a prototypical starburst galaxy, characterized by a galactic-scale outflow (initially observed by Lynds & Sandage 1963). With the properties listed in Table 1, it provides an excellent environment for investigating the connection between star formation and gas in a galaxy, owing to its proximity (D ≈ 3.5 Mpc; Dalcanton et al. 2009), elevated infrared (IR) luminosity (LIR ≈ 5.6 × 1010 L⊙; Sanders et al. 2003), and substantial star formation rate (∼9 M⊙ yr−1 over the last 100 Myr; e.g., Calzetti 2013; Strickland et al. 2004). Far-infrared (FIR) continuum observations have revealed a characteristic dust temperature of ∼30 K in the central starburst region of M82 (e.g., Pattle et al. 2023; Roussel et al. 2010). The molecular gas in M82 has been studied through a variety of CO-line observations across the millimeter (mm) to FIR (e.g., Loiseau et al. 1990; Weiß et al. 2001; Walter et al. 2002; Salak et al. 2013; Leroy et al. 2015; Chisholm & Matsushita 2016). The molecular gas disk exhibits a prominent double-lobed structure, with bright lobes located northeast (NE) and southwest (SW) of the galaxy’s center. This structure is clearly seen in low-J12CO (hereafter simply referred to as CO when not otherwise specified) and 13CO transitions, and is preserved in higher transitions up to at least J = 7−6 (e.g., Loenen et al. 2010). While the lobes are roughly symmetric in position, the intensity distribution is asymmetric (e.g., Kikumoto et al. 1998). Furthermore, the separation between the two lobes varies with the J number (e.g., Mao et al. 2000; Ward et al. 2003). These trends indicate the presence of two molecular gas components: low-excitation gas that is more extended and dominates low-J transitions and high-excitation gas that is more compact and dominates mid- to high-J transitions (e.g., Loenen et al. 2010). 13CO shows a similar overall distribution as CO but it is relatively weak and more centrally concentrated. The CO/13CO intensity ratio is generally high in M82, possibly due to enhanced UV radiation in the starburst region that photodissociates 13CO more efficiently (e.g., Loiseau et al. 1988; Neininger et al. 1998). It is also possible that elevated local temperatures cause the 13CO(1–0) to become optically thin by shifting the population distribution toward higher levels (e.g., Matsushita et al. 2010). These results suggest that the molecular gas in M82 is multi-phase and structurally complex.

Table 1.

Basic properties of M82.

Low-J CO transitions have low critical densities (∼300–1000 cm−3) and are often used to trace the total molecular gas as a proxy for H2, which lacks an electric dipole moment (e.g., Bolatto et al. 2013). Higher-J CO transitions (J ≥ 4–3) have higher critical densities, but are more sensitive to warm molecular gas, while dense-gas tracers such as HCN and HCO+ are used to trace cold and dense molecular gas (e.g., Shirley 2015). Combining different tracers is essential for understanding the distribution and excitation of molecular gas in galaxies. Existing observations of HCN and HCO+ in M82 have primarily focused on the J = 1−0 transitions (e.g., Salas et al. 2014; Kepley et al. 2014; Ginard et al. 2015; Li et al. 2022). While detections of high-J HCN and HCO+ lines have been reported in the galaxy’s center and its surrounding regions (e.g., Israel 2023; Seaquist et al. 2006), spatially resolved maps and systematic comparisons across multiple transitions remain scarce.

A major challenge in studying the distribution and excitation state of dense gas in extragalactic systems lies in the intrinsic faintness of its emission. In most regions of galaxies, HCN and HCO+ lines are significantly weaker than those of CO, typically by a factor of by 10–30 (e.g., Gao & Solomon 2004b). Benefiting from the 16-receptor array receiver Heterodyne Array Receiver Program (HARP, Buckle et al. 2009), the James Clerk Maxwell Telescope (JCMT) is capable of efficiently producing large-scale maps, enabling spatially resolved observations of dense molecular gas in nearby galaxies. The JCMT-MALATANG program aims to map HCN(4–3) and HCO+(4–3) emission in 23 nearby galaxies (Tan et al. 2018; Jiang et al. 2020). In this study, we combine MALATANG observations of M82, with HCN(1–0) and HCO+(1–0) data from Salas et al. 2014, along with additional archival datasets, to investigate the spatial distribution and excitation state of dense gas across the disk of M82.

In Sect. 2, we briefly describe the observations and data reduction. Section 3 presents the spectra, stacking results, velocity distributions, and line ratios. In Sect. 4, we analyze the trends in the line ratios, explore their possible physical origins, and compare our findings with previous studies. Our main results are summarized in Section 5.

2. Observations and data reduction

2.1. JCMT HCN(4–3) and HCO+(4–3) observations

The first phase of the JCMT Large Program MALATANG (project code: M16AL007) conducted a total of approximately 390 hours of observations from December 2015 to July 2017, mapping HCN(4–3) and HCO+(4–3) line emission in 23 nearby IR-bright star-forming galaxies (Tan et al. 2018; Jiang et al. 2020). For M82, observations of HCN(4–3) and HCO+(4–3) were conducted across the central 2′×2′ region, which is centered at RA (J2000.0) = 09h55m52 . s $ \overset{\text{ s}}{.} $4 and Dec (J2000.0) = + 69 ° 40 ′ 46 . ″ 9 $ +69{{\circ}}40{{\prime}}46{{{\overset{\prime\prime}{.}}}}9 $. The total integration times were 150 minutes for HCN(4–3) and 100 minutes for HCO+(4–3), including the time spent integrating on both the source and the reference position. The HARP array was used to carry out 3 × 3 jiggle-mode observations with grid spacing of 10″. Figure 1 shows the mapped positions for M82, while the incompleteness of observation coverage was due to two adjacent receptors (H13 and H14) at the edge of the array not working. The receiver backend is the Auto-Correlation Spectral Imaging System (ACSIS) spectrometer with a bandwidth of 1 GHz and a resolution of 0.488 MHz, corresponding to 840 km s−1 and 0.41 km s−1 at 354 GHz, respectively. The FWHM beam width of each receiver at 350 GHz is about 14″, corresponding to a physical scale of 245 pc at M82’s distance of 3.5 Mpc. All the MALATANG observation parameters of M82 are shown in Table 2.

thumbnail Fig. 1.

JCMT MALATANG observed positions for M82 overlaid on HST Hα emission (NASA, ESA, The Hubble Heritage Team, Mutchler et al. 2007). The yellow circles indicate to the spatial areas covered by the JCMT HARP receivers using jiggle mode, and the galaxy’s center is marked by a red circle. The 9 × 9 JCMT MALATANG observation of HCN(4–3) and HCO+(4–3) is represented by the blue box. In this study, we define the region enclosed by the red rectangle as the area dominated by the disk of M82, which is slightly larger than the CO molecular disk described in Walter et al. 2002 and Salas et al. 2014. Following the definition of the outflow regions in Leroy et al. 2015 based on CO, Hα, FUV, and X-ray emission, we represent the regions associated with outflow as a conical structure with a base of 300 pc and an opening angle of 20°, as indicated by the red dashed lines.

Table 2.

Summary of MALATANG observational parameters for M82.

2.2. GBT HCN(1–0), HCO+(1–0) and ancillary data

The HCN(1–0) and HCO+(1–0) data were obtained from Kepley et al. 2014 and Salas et al. 2014, observed with the 4-mm (W band) receiver and spectrometer of the Green Bank Telescope (GBT). Observations were conducted under excellent weather conditions for approximately 15 hours. The beam size of the calibrated cubes is 9 . ″ $ \overset{\prime \prime }{.} $2. In order to compare these with the HCN(4–3) and HCO+(4–3) data, we convolved the GBT data to a 14″ resolution, following the method of Aniano et al. 2011.

We obtained the M82 12CO(1–0) data from the Nobeyama COMING project1 (Sorai et al. 2019). At 115 GHz, the beamsize of the Nobeyama 45-m radio telescope is approximately 14″, which is consistent with the MALATANG data. The CO(3–2) data were obtained from the JCMT Nearby Galaxies Survey (Wilson et al. 2012).

2.3. Data reduction

This work used the GILDAS/CLASS2 software package for spectral reduction throughout the entire process. These JCMT NDF (N-Dimensional Data Format) files were converted into a format compatible with the GILDAS/CLASS software package, retaining all the raw spectra. We filtered each individual raw spectrum instead of the averaged results at a given position, allowing for more spectral data to be retained. First, we manually inspected and removed spectra with obvious issues. Then, we developed an automated script to mask the 10% of spectra with the highest noise levels. Next, we averaged the spectra at each position and combined data from different scans using noise-weighted averaging. After these steps, we performed low-order-polynomial baseline subtraction and used the velocity range over which line emission was detected in CO(3–2) as a reference for where we would expect emission to be found for the other observed lines, as well as to calculate upper limits. Finally, we smoothed the spectra to a velocity resolution of approximately 13 km s−1. Figure A.1 presents a comparison of selected spectra from this work and Tan et al. 2018.

2.4. Measurements of flux density and line luminosities

We adopted the same method as proposed by Gao 1996 to identify the emission lines, based on the criterion that the velocity-integrated line intensity ≥3σ, where the uncertainty, σ, is obtained from the following formula:

σ I = T rms Δ v line Δ v res 1 + Δ v line Δ v base · $$ \begin{aligned} \sigma _I = T_{\text{rms}} \sqrt{\Delta v_{\text{line}} \Delta v_{\text{res}}} \sqrt{1+ \frac{\Delta v_{\text{line}}}{\Delta v_{\text{base}}}}\cdot \end{aligned} $$(1)

Here, Trms denotes the root-mean-square (RMS) main-beam temperature of the line data, considering a spectral velocity resolution of Δvres, Δvline represents the velocity range of the emission line, while Δvbase is the velocity range employed to fit the baseline. For the JCMT HCN(4–3), HCO+(4–3) and CO(3–2) data, the flux density is calculated using S/Tmb = 15.6/ηmb = 24.4 Jy K−1 (Tan et al. 2018 for more details). For the GBT data, we adopt the main-beam efficiency ηmb = 0.26 (Kepley et al. 2014) and S/Tmb3 = 1 Jy K−1. For the NRO 45 m COMING data, we adopt the ηmb of 0.45 at 115 GHz4, and the S/Tmb calculated for 12CO(1–0) is 2.125.

The calculation of line luminosity, L′gas, is conducted using the equation provided in Solomon et al. 1997:

L ′ gas = 3.25 × 10 7 ( S Δ v 1 Jy km s − 1 ) ( ν obs 1 GHz ) − 2 × ( D L 1 Mpc ) 2 ( 1 + z ) − 3 [ K km s − 1 pc 2 ] . $$ \begin{aligned} L\prime _{\text{gas}}&= 3.25 \times 10^7 \left(\frac{S\Delta v}{{1\,\text{ Jy} \text{ km} \text{ s}^{-1}}}\right) \left(\frac{\nu _{\text{obs}}}{1\,\text{ GHz}}\right)^{-2} \nonumber \\&\quad \times \left(\frac{D_{\text{L}}}{1\,\text{ Mpc}}\right)^2 \left(1+z\right)^{-3}\,[\text{ K} \text{ km} \text{ s}^{-1}\,\text{ pc}^2]. \end{aligned} $$(2)

3. Results

3.1. Spectra and stacking results

Figures A.3–A.5 show the spectra of all gas tracers. Given that the intensities of HCN(4–3) and HCO+(4–3) are significantly lower than those of other lines, we applied scaling factors to CO(1–0), CO(3–2), HCN(1–0), and HCO+(1–0) for clearer comparison. In most regions, different tracers exhibit similar spatially integrated line centers and line widths. However, in certain areas, such as on the southwest (SW) side of the major axis at offset (−10,0) and (−20,0), the line center of CO(1–0) deviates by ∼50 km s−1.

We performed spectral stacking for HCN(4–3), HCO+(4–3), HCN(1–0), and HCO+(1–0) in the disk regions. We calculated the weights using the RMS of HCO+(1–0): weight = 1/σ2, and the same weight distribution was applied to all dense gas tracers. The global rotation and local dynamics of M82 cause velocity shifts in emission lines at different regions. To achieve a higher S/N in the stacked results, we performed velocity corrections before stacking. We assumed that the velocities of dense-gas signals potentially hidden in the noise are the same as or close to the CO(3–2) velocities at the same regions. Under this assumption, we shifted the velocity centers of dense-gas lines to 0 km s−1 based on Gaussian fits to the CO(3–2) line. We also conducted spectral stacking without velocity corrections to preserve the local dynamical characteristics across different regions. Figure 2 presents both the velocity-corrected (top panel) and uncorrected (bottom panel) stacking results. In the velocity-corrected result, the velocity-integrated intensity of HCN(4–3) is lower than HCO+(4–3) with a lower ratio compared to that of HCN/HCO+(1–0) ratio, indicating that the excitation of high-J HCN emission is significantly suppressed within M82. The HCN(4–3) and HCO+(4–3) spectral profiles are more alike to each other than to their respective J = 1–0 transitions. However, slight shifts in peak velocity are observed between the J = 1−0 and J = 4−3 lines. In the uncorrected result, these velocity offsets mainly appear on the blueshifted side, corresponding to the SW lobe shown in Fig. 3.

thumbnail Fig. 2.

Top: RMS-weighted spectral stacking results for the disk of M82. The velocities of dense-gas tracers were corrected according to the line center of CO(3–2) before stacking. A scaling factor of 6 was applied to HCN(1–0) and HCO+(1–0). The velocities at the centers of the Gaussian fits for HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3) are 14.3 ± 0.5, 13.6 ± 0.6, –0.6 ± 2.5, and –1.8 ± 1.9 km s−1, respectively. Bottom: RMS-weighted spectral stacking results for the disk of M82 without velocity correction.

thumbnail Fig. 3.

Position-velocity diagrams along the major axis. Velocity 0 corresponds to the systemic velocity of the galaxy, Vsys(LSR) = +225 km s−1 (Neininger et al. 1998), and the position (0,0) is marked by a star. The contour levels are at 30%, 50%, 70%, 90%, and 95% of the peak intensities. The white contours represent HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3), while the black contours correspond to the CO(3–2). Color scale shows the normalized intensity of CO(1–0).

Additionally, we performed spectral stacking for two specified regions to search for potential J = 4−3 signals. Region A includes areas where HCO+(1–0) was detected, but HCO+(4–3) was not; region B includes areas where HCN(1–0) was detected but HCN(4–3) was not. The results of stacked spectra, as well as the spatial definitions of regions A and B, are presented in Fig. A.2. The integrated velocity flux densities obtained from the stacking are listed in the last columns of Table A.2. Following stacking, HCO+(4–3) emission with S/N ≥ 3 was detected in both regions, whereas HCN(4–3) emission with S/N ≥ 3 was detected only in region B.

3.2. Velocity distribution

In Fig. 3, we present the position-velocity (p–v) diagram along the major axis. The systemic velocity of M82, V sys ( LSR ) $ V_\mathrm{{sys}}(\rm{LSR}) $ = + 225 km s−1 (Neininger et al. 1998) has been subtracted, and the dynamical center is marked at (V − Vsys = 0, offset = 0). All tracers exhibit a characteristic “figure-eight” pattern, with intensity offset from the center by ±10 arcsec.

On the NE side of the dynamical center (i.e., the upper left of each panel in Fig. 3), the contour shapes and peak positions of the four dense gas tracers are broadly consistent, although HCN(4–3) appears more spatially concentrated. CO(3–2) and CO(1–0) also show good spatial and kinematic agreement. In contrast, on the SW side of the dynamical center (i.e., lower right of each subpanel in Fig. 3), the peaks and contour shapes diverge significantly between different lines. In addition to the normal behaviour following the HCN(1–0) and HCO+(1–0) pattern, HCN(4–3) and HCO+(4–3) exhibit an additional bright region, which results in their peak velocities being about −110 km s−1, offset by ∼40 km s−1 from those of HCN(1–0) and HCO+(1–0).

3.3. Radial distribution

Figure 4 shows the velocity-integrated intensity maps of CO(1–0), CO(3–2), HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3) emission. The pixels with S/N ≥ 3 are highlighted, while lower-significance pixels are marked as hatched pixels. Contours based on peak brightness temperature reveal an obvious double-peaked structure in all tracers. Table 3 summarizes the line ratios measured at the two peaks. Both the HCN(4–3)/HCN(1–0) and HCO+(4–3)/HCO+(1–0) ratios are higher at the NE peak, whereas the remaining ratios are comparable between the NE and SW peaks.

Table 3.

Luminosity ratios at NE and SW lobes.

thumbnail Fig. 4.

Maps of the entire area observed of various molecular-line emissions, color-coded by the velocity-integrated line intensity. Regions where the velocity-integrated line intensity is lower than 3σ are marked by blank pixels, while the typical RMS is represented by the lowest values of each map. Each panel in the figure corresponds to a different molecular line, as indicated by the title. The galaxy’s center is marked by a red star and the gray squares denote positions with no available observational data. Contours are drawn at 50%, 60%, 70%, 80%, and 90% of the maximum of the peak main-beam temperature (Tpeak) for each line.

The CO(3–2)/CO(1–0) luminosity ratio is greater than unity near the galaxy’s center, but declines below unity at larger radii, consistent with the result of Salak et al. 2013. This radial variation is clearly illustrated in Fig. 5, which displays the line luminosities profiles of molecular gas along both the major and minor axes. The profiles also reveal a steeper decline along the minor axis than along the major axis. The luminosity ratios HCN(4–3)/(1–0), HCO+(4–3)/(1–0), HCO+(1–0)/CO(1–0), and HCN(1–0)/CO(1–0) all exhibit decreasing trends with increasing projected galactic radius (Fig. 6). However, the decreasing trend differs within a radius of ∼400 pc. The HCN(4–3)/(1–0) and HCO+(4–3)/(1–0) ratios decrease more rapidly on the SW side, while HCO+(1–0)/CO(1–0) and HCN(1–0)/CO(1–0) decrease more gradually in that same direction.

thumbnail Fig. 5.

Top: Distribution of the line luminosity of various molecular gas tracers along the major axis. A dashed line is drawn to mark offset = 0. Bottom: Similar to the upper figure, but along the minor axis. Given that the inclination angle of the outflow is quite small (∼10°, Leroy et al. 2015), we assume that 1″ along the minor axis associated with the outflow corresponds to a projected distance of about 17 pc, the same as along the major axis.

thumbnail Fig. 6.

Luminosity ratios of HCO+(4–3)/(1–0) and HCN(4–3)/(1–0) are represented by the dashed lines, and the luminosity ratios of HCN(1–0)/CO(1–0) and HCO+(1–0)/CO(1–0) are represented by the solid lines. Both are measured along the major axis of M82.

The luminosity ratios HCN(4–3)/(1–0), HCO+(4–3)/(1–0), HCN(4–3)/CO(1–0), HCO+(4–3)/CO(1–0), HCN/HCO+(4–3), and HCN/HCO+(1–0) across all observed positions are shown in Fig. 7. Among these, only the HCN(4–3)/CO(1–0) and HCO+(4–3)/CO(1–0) ratios peak at the galaxy’s center. Interestingly, the HCN(4–3)/(1–0) and HCO+(4–3)/(1–0) ratios peak in the northwest side region, where strong multiphase outflows have been detected (e.g., Leroy et al. 2015). Besides, the HCN/HCO+(4–3) and HCN/HCO+(1–0) ratios also show relatively high values in the same area. In all regions where J = 4−3 was detected, the HCN(4–3)/(1–0) ratio is consistently lower than HCO+(4–3)/(1–0), and HCN/HCO+(4–3) is also lower than HCN/HCO+(1–0). The details are given in Table A.3.

thumbnail Fig. 7.

Distribution of the luminosity ratios HCN (4–3)/(1–0), HCO+(4–3)/(1–0), HCN(4–3)/CO(1–0), HCO+(4–3)/CO(1–0), HCN/HCO+(4–3), and HCN/HCO+(1–0). Here, only regions with velocity-integrated line intensities exceeding 3σ for both lines are included, with the remaining pixels shown as blank. For the maps involving HCN(4–3) or HCO+(4–3), we applied a selection where both HCN(4–3) and HCO+(4–3) have S/N ≥ 3, with the remaining points marked as hatched. For the HCN/HCO+(1–0) map, we used a similar S/N ≥ 3 filter for both HCN(1–0) and HCO+(1–0). Similar to Fig. 1, we use red rectangles to indicate the disk-dominated region and a cone with a base of 300 pc and an opening angle of 20° to represent the regions associated with outflow.

4. Analysis and discussion

4.1. Line ratios

Along the galaxy’s major axis, the line ratios between different transitions of the same molecule clearly decrease with increasing distance from the center. Figure 6 shows a significant decline in the HCO+(4–3)/(1–0) ratio around 0.2–0.5 kpc SW side of the center, while the HCN(4–3)/(1–0) ratio remains consistently low along the major axis. The HCN(4–3)/(1–0) ratio reaches a peak of ∼0.2 at the center and decreases to ∼0.1 at an offset of 20″. In contrast, the HCO+(4–3)/(1–0) ratio declines from ∼0.4 at the center to about 0.2–0.3 (see Table A.3). Compared with the average values of 0.27 ± 0.04 for HCN(4–3)/(1–0) and 0.29 ± 0.07 for HCO+(4–3)/(1–0), derived from 22″-beam observations toward the centers of galaxies including starbursts, AGNs, and LIRG/ULIRG Israel 2023, M82 exhibits a lower HCN(4–3)/(1–0) ratio and a higher HCO+(4–3)/(1–0) ratio.

Also, we calculated the ratios of HCN(4–3) to HCO+(4–3) where the intensity was greater than 3σ (see Table A.3), finding them to range from 0.11 ± 0.02 to 0.79 ± 0.16, with an average value of 0.36 ± 0.06. The mean value of HCN/HCO+(1–0) across all detected locations is 0.66 ± 0.09, and 0.70 ± 0.04 in the central area where the J = 4–3 transition was detected in both species. In comparison, the average intensity ratio of HCN/HCO+(1–0) of the nine EMPIRE galaxies in Jiménez-Donaire et al. 2019 is 1.43 ± 0.41, while in the 43 nearby galaxies studied by Israel 2023, this ratio of HCN/HCO+(1–0) is 1.11 ± 0.06. Meanwhile, Israel 2023 also reported an average HCN/HCO+(4–3) ratio of 0.70 ± 0.10 for 15 galaxies with HCN(4–3) and HCO+(4–3) detections. Both the HCN/HCO+(1–0) and HCN/HCO+(4–3) line ratios of M82 are lower than the average values reported in the literature.

The spatial distributions of HCN/HCO+ in the NE and SW lobes are comparable, with no significant asymmetry. In the central starburst region (within ∼0.5 kpc), both sides show similar HCN/HCO+(1–0) values of ∼0.65–0.70. At a higher excitation, the HCN/HCO+(4–3) ratio remains low (∼0.25–0.30). Although the ratios are generally uniform, localized departures are observed in specific regions. For example, around 0.2–0.5 kpc SW side of the center, the HCO+(4–3) intensity drops significantly (see Fig. 5), leading to a slight increase in the HCN/HCO+(4–3) ratio there. However, the ratio remains low overall, suggesting that HCO+ emission dominates over HCN, especially at higher-J transitions.

It has been suggested that supernova explosions and photodissociation regions (PDRs) can significantly increase the abundance of HCO+ and decrease the abundance of HCN (e.g., Wild et al. 1992; Schilke et al. 2001), which is consistent with the pure-starburst property of M82. The gas disk of M82 contains a giant PDR that is 650 pc in size, which may significantly increase the abundance of HCO+ in M82 (e.g., García-Burillo et al. 2002; Fuente et al. 2005; Krips et al. 2008).

Theoretical models also show that in PDRs with densities below 105 cm−3, the HCN/HCO+ ratio drops below unity (e.g., Meijerink et al. 2007; Yamada et al. 2007). Multi-component PDR models by Mao et al. 2000 and Loenen et al. 2010 suggest that most molecular gas in M82 is in low-excitation diffuse components with densities around n(H2) = 103–103.7 cm−3, and only a small fraction (∼1%) is present in dense gas with n(H2) = 106 cm−3. Under optically thin conditions and at 20 K, the critical densities (ncrit) of HCO+(1–0), HCN(1–0), HCO+(4–3), and HCN(4–3) are 4.5 × 104, 3.0 × 105, 3.2 × 106, and 2.3 × 107 cm−3, respectively Shirley 2015. Given its higher critical density, HCN(4–3) is likely not fully excited in M82’s relatively low average-density environment, while the excitation of HCO+ could be maintained at a higher level under the influence of PDRs. The combined effect of these two factors may contribute to the relatively low HCN/HCO+ ratio observed in M82 and its further decrease with increasing rotational transition.

4.2. Comparison with previous measurements

Our data reveal that the molecular emission in M82 exhibits a characteristic double-lobed structure, with prominent peaks located to the NE and SW of the nucleus, consistent with previous CO observations (e.g., Loiseau et al. 1990; Kikumoto et al. 1998; Neininger et al. 1998; Mao et al. 2000; Weiß et al. 2001; Walter et al. 2002; Ward et al. 2003; Seaquist et al. 2006; Loenen et al. 2010). As shown in Fig. A.3, the spectral line widths at the central position (0,0) are broader than those at the NE lobe (10,0) and the SW lobe (−10,0), while the peak line temperatures at the center are lower than those in the lobes. This trend is also evident in Fig. 4: although the velocity-integrated intensities reach a maximum at the (0,0) position, the contour maps of Tpeak show a double-peaked structure. This is likely due to the central beam encompassing partial emission from both lobes, resulting in spatial blending that broadens the line profile and reduces the observed peak brightness temperature.

Table A.4 summarizes the integrated intensities of various emission lines measured at the NE and SW lobes, along with their corresponding SW-to-NE intensity ratios. In terms of absolute values, the CO line intensities decrease with increasing rotational quantum number in both lobes, with a steeper decline observed from CO(5–4) onward. It is important to note that these data were obtained with different telescopes and beam sizes; therefore, the intensity ratios among the various transitions might not directly reflect the physical conditions. Comparisons of the NE and SW lobes with the same observation are more reliable. For most molecular lines, the SW lobe shows slightly stronger emission than the NE lobe, with SW/NE intensity ratios typically ranging from 1.1 to 1.3. A similar trend is observed in the 13CO lines, where transitions from J = 1−0 to 5−4 exhibit SW/NE ratios of about 1.2–1.5, consistent with those of the CO results. An extreme case is 13CO(6–5), which shows a SW/NE intensity ratio of ∼2.8 ± 0.88, likely due to the very low intensity at the NE lobe (Loenen et al. 2010). In contrast, our HCN(4–3) and HCO+(4–3) lines, along with CO(10–9) from Loenen et al. 2010, show SW/NE ratios below unity. This may suggest an enhanced fraction of dense gas toward the NE lobe.

Similarly to the results shown in Fig. 3, previous studies have also reported a prominent double-lobed structure in the p–v diagrams of various emission lines. The two lobes are roughly symmetric in spatial position, located about 10–15 arcsec on either side of the center, but their velocity distributions show subtle differences. The NE lobe exhibits consistent velocities across different lines, with no clear evidence of velocity stratification. In contrast, the SW lobe shows a truncation in velocity: the velocities of low- to mid-J CO lines (J ≤ 4) and low-J (J = 1) HCN (HCO+) lines are concentrated around 140–170 km s−1 (e.g., Mao et al. 2000; Ward et al. 2003), while the peak velocities of high-J CO and high-J HCN and HCO+ transitions are closer to ∼110 km s−1 (e.g., Loenen et al. 2010; Ward et al. 2003; Mao et al. 2000; Wild et al. 1992). This suggests that the SW lobe contains multiple gas components with distinct excitation conditions and kinematic properties.

Mao et al. 2000 reported that the spatial separation between the NE and SW emission peaks in their velocity-integrated intensity maps decreases significantly with an increasing rotational quantum number, J: from 26″ in CO(1–0) and CO(2–1) to 15″ in CO(7–6). These authors argued that this difference exceeds the beam size and positional uncertainties and is therefore unlikely to be caused by resolution effects. However, Mao et al. 2000 noted that this truncation is not clearly apparent in p–v diagrams. By incorporating additional emission lines, particularly our new HCN(4–3) and HCO+(4–3) observations, we confirm the presence of excitation-dependent velocity truncation. Interestingly, our results reveal an opposite trend in the p–v diagrams compared to the velocity-integrated maps: on the SW side, high-excitation lines peak at lower velocities, resulting in a larger velocity separation between the NE and SW lobes. This feature is also clearly seen in Fig. 4 of Ward et al. 2003. In addition, Loenen et al. 2010 reported a secondary velocity component near 100 km s−1 in the SW region using Herschel high-J CO spectra. In the CO J = 9−8 and higher transitions, this secondary component is even stronger than the main peak around 160 km s−1, with the intensities scaled to a common beam size. This result is consistent with the lower-velocity emission we observe in high-excitation lines on the SW side. These findings suggest that while high-excitation lines are more spatially concentrated toward the galaxy’s center, the observed velocity offset in the SW lobe may reflect a localized dynamical feature, possibly associated with non-circular motions or feedback-driven structures that deviate from the overall disk rotation.

5. Summary

We have presented HCO+ J = 4−3 and HCN J = 4−3 observations of the central 90″ × 90″ region of M82 obtained as part of the JCMT MALATANG program. By combining these data with the HCO+ J = 1−0 and HCN J = 1−0 data from the GBT, as well as CO data from the JCMT NGLS survey and Nobeyama 45 m COMING project, we investigated the gas properties on sub-kpc scales. The main results and conclusions of this study are summarized as follows:

  1. All of the emission lines analyzed, CO(1–0), CO(3–2), HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3) peak near the center of M82 and exhibit declining intensities with the increasing distance from the center along both the major and minor axes of the galaxy. In M82, the emission from HCN(4–3) and HCO+(4–3) is primarily concentrated within ∼500 pc of the galaxy’s center. To probe low-level emission in two fainter regions, we applied spectral stacking, which revealed a weak HCO+(4–3) signal and provided an upper limit on the velocity-integrated flux density of HCN(4–3).

  2. The HCN(4–3)/(1–0) line ratio ranges from 0.09 to 0.54, with a mean value of 0.18 ± 0.04, while the HCO+(4–3)/(1–0) line ratio ranges from 0.14 to 0.87, with a mean of 0.32 ± 0.06. For both tracers, the highest ratios are found on the northwest side of the galaxy, spatially coinciding with M82’s well-known outflow region. The low-J transitions, J = 1−0 of HCN and HCO+, are brighter on the SW side of the major axis, while the high-J transitions, J = 4−3 of HCN and HCO+ are stronger on the NE side. In all regions where J = 4−3 transitions are detected, the HCN(4–3)/(1–0) ratio is consistently lower than the corresponding HCO+(4–3)/(1–0) ratio. The HCN/HCO+(4–3) ratio ranges from 0.19 ± 0.03 to 1.07 ± 0.31, with a mean value of 0.39 ± 0.11, which is lower than the average HCN/HCO+ (1–0) ratio of 0.74 ± 0.08. This difference might be attributed to the relatively low excitation of high-J HCN transitions.

  3. The position-velocity diagram along the major axis reveals the characteristic “figure-eight” pattern of M82’s rotating molecular disk, with the NE and SW lobes symmetrically distributed on either side of the nucleus. In the NE lobe, the velocities traced by different lines are closely aligned, indicating a coherent kinematic structure. In contrast, the SW lobe exhibits a ∼40 km s−1 blueshift in the peak velocities of high-excitation lines relative to those of the low-J transitions. This velocity offset may indicate the presence of gas components with distinct excitation and kinematic properties in the SW region.

The analysis of dense gas line ratios reveals significant variations in physical conditions across different regions of M82. Future multi-transition observations of dense gas with higher resolution and sensitivity, such as those enabled by ALMA, will help to uncover the physical state of star-forming gas.

Acknowledgments

This paper is dedicated to the memory of late Professor Yu Gao, co-PI of the MALATANG project, R.I.P. We thank the anonymous referee for the constructive and helpful suggestions that improved the paper. This work was supported by National Natural Science Foundation of China (NSFC) grant No. 12033004 and National Key R&D Program of China grant No. 2017YFA0402704. We thank Pedro Salas for kindly providing the GBT data. This publication made use of data from COMING, CO Multi-line Imaging of Nearby Galaxies, a legacy project of the Nobeyama 45-m radio telescope. The Nobeyama 45-m radio telescope is operated by Nobeyama Radio Observatory, a branch of National Astronomical Observatory of Japan. We are very grateful to the EAO/JCMT staff for their help during the observations and data reduction. The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan; Academia Sinica Institute of Astronomy and Astrophysics; the Korea Astronomy and Space Science Institute; the National Astronomical Research Institute of Thailand; Center for Astronomical Mega-Science (as well as the National Key R&D Program of China with No. 2017YFA0402700). Additional funding support is provided by the Science and Technology Facilities Council of the United Kingdom and participating universities and organizations in the United Kingdom and Canada. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. MALATANG is a JCMT Large Program with project code M16AL007 and M20AL022. We are grateful to P. P. Papadopoulos for his generous help and support with the JCMT observations. Z.Y.Z. acknowledges the support of the NSFC under grants No. 12173016, 12041305, the science research grants from the China Manned Space Project with NOs.CMS-CSST-2021-A08 and CMS-CSST-2021-A07, and the Program for Innovative Talents, Entrepreneur in Jiangsu. A.C. acknowledges support by the National Research Foundation of Korea (NRF), grant Nos. 2022R1A2C100298213 and 2022R1A6A1A03053472. B.L. acknowledges support by the National Research Foundation of Korea (NRF), grant Nos. 2022R1A2C100298213. LCH was supported by the National Science Foundation of China (11991052, 12233001), the National Key R&D Program of China (2022YFF0503401), and the China Manned Space Project (CMS-CSST-2021-A04, CMS-CSST-2021-A06). The work of MGR is supported by the international Gemini Observatory, a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation, on behalf of the Gemini partnership of Argentina, Brazil, Canada, Chile, the Republic of Korea, and the United States of America. M.J.M. acknowledges the support of the National Science Centre, Poland through the SONATA BIS grant 2018/30/E/ST9/00208. This research was funded in part by the National Science Centre, Poland (grant number: 2023/49/B/ST9/00066). T.F. was supported by the National Natural Science Foundation of China under Nos. 11890692, 12133008, 12221003, and the science research grant from the China Manned Space Project with No. CMS-CSST-2021-A04.

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Appendix A: Spectra and measurements

thumbnail Fig. A.1.

Selection of spectra in units of TA (K) from this work and Tan et al. 2018 are compared using solid black lines and red dotted lines, respectively. Each panel is labeled with the tracer, and the position pointed at is provided as an offset along the major and minor axes.

thumbnail Fig. A.2.

RMS-weighted stacked spectra for region A and region B are shown in the left panel. Region A represents areas where HCN(1–0) is detected but HCN(4–3) is not, while region B corresponds to areas where HCO+(1–0) is detected but HCO+(4–3) is not. To minimize interference from high-noise spectra, data points at offsets along the minor axis of ±40 have been excluded. The specific locations of regions A and B are shown in the right panel.

thumbnail Fig. A.3.

Spectra of CO(1–0), CO(3–2), HCO+(1–0), HCN(1–0), HCO+(4–3), and HCN(4–3) are shown from top to bottom in each grid, in units of TMB (K). Spectra are referenced to the LSR velocity reference system and the radio definition for the Doppler shift is adopted. The spectra shown here correspond exactly to the red-rectangle region in Fig. 1. The offset from the center position in units of arcsec is annotated in the top corner of each panel. The offset (0,0) corresponds to the galaxy’s center, marked by an orange circle in Fig. 1. Offsets are positive toward the northeast along the major axis and toward the northwest along the minor axis, while negative offsets represent the opposite directions. The legend at top of figure shows the scaling factors for CO, HCN(1–0), and HCO+(1–0). The velocity resolutions for HCN(4–3), HCO+(4–3), HCN(1–0), and HCO+(1–0) are ∼13 km s−1. For CO(1–0) and CO(3–2), the velocity resolutions are 10 km s−1 and 19 km s−1, respectively. All data are re-sampled to a pixel size of 14 arcsec, corresponding to ∼240 pc.

thumbnail Fig. A.4.

Continuation of Fig. A.3, offset along the minor axis at 20, 30, and 40 arcsec.

thumbnail Fig. A.5.

Continuation of Fig. A.3, offset along the minor axis at −20, −30, and −40 arcsec.

Table A.1.

Integrated intensities and luminosities

Table A.2.

Integrated intensities and luminosities

Table A.3.

Luminosity ratios at positions with detected J = 4−3 emission

Table A.4.

Velocity-integrated intensities of emission lines at the SW and NE lobes, along with their corresponding SW-to-NE intensity ratios

All Tables

Table 1.

Basic properties of M82.

In the text
Table 2.

Summary of MALATANG observational parameters for M82.

In the text
Table 3.

Luminosity ratios at NE and SW lobes.

In the text
Table A.1.

Integrated intensities and luminosities

In the text
Table A.2.

Integrated intensities and luminosities

In the text
Table A.3.

Luminosity ratios at positions with detected J = 4−3 emission

In the text
Table A.4.

Velocity-integrated intensities of emission lines at the SW and NE lobes, along with their corresponding SW-to-NE intensity ratios

In the text

All Figures

thumbnail Fig. 1.

JCMT MALATANG observed positions for M82 overlaid on HST Hα emission (NASA, ESA, The Hubble Heritage Team, Mutchler et al. 2007). The yellow circles indicate to the spatial areas covered by the JCMT HARP receivers using jiggle mode, and the galaxy’s center is marked by a red circle. The 9 × 9 JCMT MALATANG observation of HCN(4–3) and HCO+(4–3) is represented by the blue box. In this study, we define the region enclosed by the red rectangle as the area dominated by the disk of M82, which is slightly larger than the CO molecular disk described in Walter et al. 2002 and Salas et al. 2014. Following the definition of the outflow regions in Leroy et al. 2015 based on CO, Hα, FUV, and X-ray emission, we represent the regions associated with outflow as a conical structure with a base of 300 pc and an opening angle of 20°, as indicated by the red dashed lines.
In the text
thumbnail Fig. 2.

Top: RMS-weighted spectral stacking results for the disk of M82. The velocities of dense-gas tracers were corrected according to the line center of CO(3–2) before stacking. A scaling factor of 6 was applied to HCN(1–0) and HCO+(1–0). The velocities at the centers of the Gaussian fits for HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3) are 14.3 ± 0.5, 13.6 ± 0.6, –0.6 ± 2.5, and –1.8 ± 1.9 km s−1, respectively. Bottom: RMS-weighted spectral stacking results for the disk of M82 without velocity correction.
In the text
thumbnail Fig. 3.

Position-velocity diagrams along the major axis. Velocity 0 corresponds to the systemic velocity of the galaxy, Vsys(LSR) = +225 km s−1 (Neininger et al. 1998), and the position (0,0) is marked by a star. The contour levels are at 30%, 50%, 70%, 90%, and 95% of the peak intensities. The white contours represent HCN(1–0), HCO+(1–0), HCN(4–3), and HCO+(4–3), while the black contours correspond to the CO(3–2). Color scale shows the normalized intensity of CO(1–0).
In the text
thumbnail Fig. 4.

Maps of the entire area observed of various molecular-line emissions, color-coded by the velocity-integrated line intensity. Regions where the velocity-integrated line intensity is lower than 3σ are marked by blank pixels, while the typical RMS is represented by the lowest values of each map. Each panel in the figure corresponds to a different molecular line, as indicated by the title. The galaxy’s center is marked by a red star and the gray squares denote positions with no available observational data. Contours are drawn at 50%, 60%, 70%, 80%, and 90% of the maximum of the peak main-beam temperature (Tpeak) for each line.
In the text
thumbnail Fig. 5.

Top: Distribution of the line luminosity of various molecular gas tracers along the major axis. A dashed line is drawn to mark offset = 0. Bottom: Similar to the upper figure, but along the minor axis. Given that the inclination angle of the outflow is quite small (∼10°, Leroy et al. 2015), we assume that 1″ along the minor axis associated with the outflow corresponds to a projected distance of about 17 pc, the same as along the major axis.
In the text
thumbnail Fig. 6.

Luminosity ratios of HCO+(4–3)/(1–0) and HCN(4–3)/(1–0) are represented by the dashed lines, and the luminosity ratios of HCN(1–0)/CO(1–0) and HCO+(1–0)/CO(1–0) are represented by the solid lines. Both are measured along the major axis of M82.
In the text
thumbnail Fig. 7.

Distribution of the luminosity ratios HCN (4–3)/(1–0), HCO+(4–3)/(1–0), HCN(4–3)/CO(1–0), HCO+(4–3)/CO(1–0), HCN/HCO+(4–3), and HCN/HCO+(1–0). Here, only regions with velocity-integrated line intensities exceeding 3σ for both lines are included, with the remaining pixels shown as blank. For the maps involving HCN(4–3) or HCO+(4–3), we applied a selection where both HCN(4–3) and HCO+(4–3) have S/N ≥ 3, with the remaining points marked as hatched. For the HCN/HCO+(1–0) map, we used a similar S/N ≥ 3 filter for both HCN(1–0) and HCO+(1–0). Similar to Fig. 1, we use red rectangles to indicate the disk-dominated region and a cone with a base of 300 pc and an opening angle of 20° to represent the regions associated with outflow.
In the text
thumbnail Fig. A.1.

Selection of spectra in units of TA (K) from this work and Tan et al. 2018 are compared using solid black lines and red dotted lines, respectively. Each panel is labeled with the tracer, and the position pointed at is provided as an offset along the major and minor axes.
In the text
thumbnail Fig. A.2.

RMS-weighted stacked spectra for region A and region B are shown in the left panel. Region A represents areas where HCN(1–0) is detected but HCN(4–3) is not, while region B corresponds to areas where HCO+(1–0) is detected but HCO+(4–3) is not. To minimize interference from high-noise spectra, data points at offsets along the minor axis of ±40 have been excluded. The specific locations of regions A and B are shown in the right panel.
In the text
thumbnail Fig. A.3.

Spectra of CO(1–0), CO(3–2), HCO+(1–0), HCN(1–0), HCO+(4–3), and HCN(4–3) are shown from top to bottom in each grid, in units of TMB (K). Spectra are referenced to the LSR velocity reference system and the radio definition for the Doppler shift is adopted. The spectra shown here correspond exactly to the red-rectangle region in Fig. 1. The offset from the center position in units of arcsec is annotated in the top corner of each panel. The offset (0,0) corresponds to the galaxy’s center, marked by an orange circle in Fig. 1. Offsets are positive toward the northeast along the major axis and toward the northwest along the minor axis, while negative offsets represent the opposite directions. The legend at top of figure shows the scaling factors for CO, HCN(1–0), and HCO+(1–0). The velocity resolutions for HCN(4–3), HCO+(4–3), HCN(1–0), and HCO+(1–0) are ∼13 km s−1. For CO(1–0) and CO(3–2), the velocity resolutions are 10 km s−1 and 19 km s−1, respectively. All data are re-sampled to a pixel size of 14 arcsec, corresponding to ∼240 pc.
In the text
thumbnail Fig. A.4.

Continuation of Fig. A.3, offset along the minor axis at 20, 30, and 40 arcsec.
In the text
thumbnail Fig. A.5.

Continuation of Fig. A.3, offset along the minor axis at −20, −30, and −40 arcsec.
In the text

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