There is a linear relation between the mass of dense gas, traced by the HCN(1-0) luminosity, and the star formation rate (SFR), traced by the far-infrared luminosity. Recent observations of galactic disks have shown some systematic variations. In order to explore the SFR-dense gas link at high resolution (∼ 4 ′′ , ∼ 150 pc) in the outer disk of an external galaxy, we have mapped a region about 5 kpc from the center along the northern spiral arm of M51 in the HCN(1-0), HCO + (1-0) and HNC(1-0) emission lines using the Northern Extended Millimeter Array (NOEMA) interferometer. The HCN and HCO + lines were detected in 6 giant molecular associations (GMAs) while HNC emission was only detected in the two brightest GMAs. One of the GMAs hosts a powerful HII region and HCN is stronger than HCO + there. Comparing with observations of GMAs in the disks of M31 and M33 at similar angular resolution (∼ 100 pc), we find that GMAs in the outer disk of M51 are brighter in both HCN and HCO + lines by a factor of 3 on average. However, the I HCN /I CO and I HCO + /I CO ratios are similar to the ratios in nearby galactic disks and the Galactic plane. Using the Herschel 70 µm data to trace the total IR luminosity at the resolution of the GMAs, we find that both the L IR -L HCN and L IR -L HCO + relations in the outer disk GMAs are consistent with the proportionality between the L IR and the dense gas mass established globally in galaxies within the scatter. The IR/HCN and IR/HCO + ratios of the GMAs vary by a factor of 3, probably depending on whether massive stars are forming or not.

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Identifying the young optically visible population in a star-forming region is essential for fully understanding the star formation event. In this paper, we identify 211 candidate members of the Perseus molecular cloud based on Gaia astrometry. We use LAMOST spectra to confirm that 51 of these candidates are new members, bringing the total census of known members to 856. The newly confirmed members are less extincted than previously known members. Two new stellar aggregates are identified in our updated census. With the updated member list, we obtain a statistically significant distance gradient of 4.84 pc deg−1 from west to east. Distances and extinction corrected color–magnitude diagrams indicate that NGC 1333 is significantly younger than IC 348 and the remaining cloud regions. The disk fraction in NGC 1333 is higher than elsewhere, consistent with its youngest age. The star formation scenario in the Perseus molecular cloud is investigated and the bulk motion of the distributed population is consistent with the cloud being swept away by the Per-Tau Shell.

We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of 12CO and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 μm emission and molecular gas tracers at subkiloparsec scales. We find that 12 μm luminosity is more tightly correlated with 12CO than it is with 13CO or dense gas tracers. Residuals between predicted and observed 12CO are only weakly correlated with molecular gas mass surface density (Σmol) in regions where Σmol is very low (∼10 M ⊙ pc−2). Above this limit, the 12CO residuals show no correlations with physical conditions of molecular gas, while 13CO residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the 12CO−12 μm relation is strong and statistically robust with respect to star-forming galaxies and active galactic nucleus hosts. These results suggest that WISE 12 μm emission can be used to trace total molecular gas instead of dense molecular gas, likely because polycyclic aromatic hydrocarbons (PAHs, a major contributor to WISE 12 μm emission) may be well mixed with the gas that is traced by 12CO. We propose that WISE 12 μm luminosity can be used to estimate molecular gas surface density for statistical analyses of the star formation process in galaxies.