The SWAN view of dense gas in the Whirlpool -- A cloud-scale comparison of N2H+, HCO+, HNC and HCN emission in M51

TL;DR

This study uses the SWAN 125 pc-resolution map of HCN(1-0), HNC(1-0), HCO+(1-0), N2H+(1-0), and CO(1-0) in M51 to compare dense-gas tracers across the galaxy’s center, ring, and arm environments. By integrating ancillary maps of Σmol, ΣSFR, Σ⋆, and PDE, and applying mock-data tests and binning, the authors quantify how line intensities and line ratios depend on environment and physical conditions. They find that HCO+ tracks N2H+ emission more closely than HCN or HNC in most environments, while HCN/CO and HNC/CO are poor proxies for average cloud density at cloud-scales in M51, especially near the center. The results reveal pronounced environmental modulation, with the ring exhibiting a pronounced N2H+-bright region and the arms showing asymmetries in dense-gas line behavior, highlighting the need to account for galactic context when inferring dense gas properties from line emission.

Abstract

Tracing dense molecular gas, the fuel for star formation, is essential for the understanding of the evolution of molecular clouds and star formation processes. We compare the emission of HCN(1-0), HNC(1-0) and HCO+(1-0) with the emission of N2H+(1-0) at cloud-scales (125 pc) across the central 5x7 kpc of the Whirlpool galaxy, M51a, from "Surveying the Whirlpool galaxy at Arcseconds with NOEMA" (SWAN). We find that the integrated intensities of HCN, HNC and HCO+ are more steeply correlated with N2H+ emission compared to the bulk molecular gas tracer CO, and we find variations in this relation across the center, molecular ring, northern and southern disk of M51. Compared to HCN and HNC emission, the HCO+ emission follows the N2H+ emission more similarly across the environments and physical conditions such as surface densities of molecular gas, stellar mass, star-formation rate, dynamical equilibrium pressure and radius. Under the assumption that N2H+ is a fair tracer of dense gas at these scales, this makes HCO+ a more favorable dense gas tracer than HCN within the inner disk of M51. In all environments within our field of view, even when removing the central 2 kpc, HCN/CO, commonly used to trace average cloud density, is only weakly depending on molecular gas mass surface density. While ratios of other dense gas lines to CO show a steeper dependency on the surface density of molecular gas, it is still shallow in comparison to other nearby star-forming disk galaxies. The reasons might be physical conditions in M51 that are different from other normal star-forming galaxies. Increased ionization rates, increased dynamical equilibrium pressure in the central few kpc and the impact of the dwarf companion galaxy NGC 5195 are proposed mechanisms that might enhance HCN and HNC emission over HCO+ and N2H+ emission at larger-scale environments and cloud scales.

Figures (28)

• Figure 1: Integrated intensity maps of dense gas tracers HCN, HNC, HCO+ and N$_2$H$^+$ from SWAN at a common resolution of $3\arcsec$ (top row from left to right), as well as from $^{12}$CO (PAWS, bottom left). We divide the disk into a center and ring environment colombo_pdbi_2014, and the outer disk into northern and southern halves (bottom row, second panel from left). We add contours of integrated N$_2$H$^+$ emission of 0.75, 2 and 4 K km/s to the environment map. We show the pixel-based integrated intensity distribution (in K km/s) in various environments in the disk for all pixels in the FoV (colored shaded area), as well as for pixels where emission is detected (emission $>3\sigma$, light grey shaded area) in the bottom right panels. The area of each histogram is normalized to unity. We indicate the median of all pixels (black dashed grey line) and median of masked pixels (dotted grey line) of each environment. The median represents the median value of logarithmic emission (med$\left(\mathrm{log}_{10}\left( I \right) \right)$. Pixels with negative emission are excluded in the logarithmic scaling of the histograms. Since CO is detected across most of the FoV and is used as a prior in the creation of the moment-0 map (Section \ref{['sec:Data:SWANmoments']}), its masked histogram distribution agrees well with the unmasked one.
• Figure 2: Molecular gas mass surface densities $\Sigma_\mathrm{mol}$, star formation rate surface densities $\Sigma_\mathrm{SFR}$, stellar mass surface density $\Sigma_\ast$, dynamical equilibrium pressure P$_{DE}$ as well as CO velocity dispersion $\sigma_\mathrm{CO}$ at 3 resolution. We show contours of integrated N$_2$H$^+$ emission (0.75, 2,4 K km/s) on top.
• Figure 3: Left panel: Line ratios of integrated line emission from dense gas tracers HCN, HNC and HCO$^+$. For visual purpose, we only show line ratios for significant detected pixels ($>3\sigma$), but include non-detections in all calculations. We mark pixels in which CO is detected (grey points) and the center of the galaxy (green plus). The intensity scale is centered logarithmically on the average line ratios (log$_{10} R$, with $R$ from Table \ref{['tab:Averagelineratios']}) determined for all pixels in the FoV, including non-detections. The average line ratio (log$_{10} R$) and the total range of 1 dex covered by the color bar are indicated by the black dashed line and greyshaded area in the right panel. We show contours of integrated N$_2$H$^+$ emission (0.75, 3 K km/s) on top. Right panel: Histogram of line ratios per environment analogous to Figure \ref{['fig:Gallery']} but for HCN/HNC, HCN/HCO$^+$ and HNC/HCO$^+$ (colored histograms). We indicate the amount of pixels shown in the histogram (top right corner), which varies slightly, as values with negative noise can not be shown in the logarithmic scale.
• Figure 4: Same as Figure \ref{['fig:Gallery_linerationon2hp']} but for the HCN-to-N$_2$H$^+$, HNC-to-N$_2$H$^+$ and HCO$^+$-to-N$_2$H$^+$ line ratios. The colorbar spans the same 1 dex range as in Figure \ref{['fig:Gallery_linerationon2hp']}, and is centered logarithmically on the average line ratios (log$_{10} R$, with $R$ from Table \ref{['tab:Averagelineratios']}).
• Figure 5: Same as Fig. \ref{['fig:Gallery_linerationon2hp']} and \ref{['fig:Gallery_lineration2hp']} but for line ratios with CO. In contrast to the previous Figures, the colorbar spans a larger range of 1.5 dex, centered on the average line ratios (Table \ref{['tab:Averagelineratios']}) and we add line ratios in pixels with non-detections. Since we are showing the logarithmic line ratio, negative values arising due to negative noise can not be shown in either the spatial map or the histograms. We mark pixels where CO is significantly detected, but the line ratio can not be shown in logarithmic scaling in dark grey. The contours depict integrated N$_2$H$^+$ emission at 0.75 and 3 K km/s.