DeGaS-MC: Dense Gas Survey in the Magellanic Clouds

Abstract

Context. Understanding the star-forming processes is key to understanding the evolution of galaxies. Investigating star formation requires precise knowledge of the properties of the dense molecular gas complexes where stars form and a quantification of how they are affected by the physical conditions to which they are exposed. The proximity, low metallicity, and wide range of star formation activity of the Large and Small Magellanic Clouds (LMC and SMC) make them prime laboratories to study how local physical conditions impact the dense gas reservoirs and their star formation efficiency. Aims. The aim of the Dense Gas Survey for the Magellanic Clouds (DeGaS-MC) project is to expand our knowledge of the relation between dense gas properties and star formation activity by targeting the LMC and SMC observed in the HCO+(2−1) and HCN(2−1) transitions. Methods. We carried out a pointing survey targeting two lines toward ∼30 LMC and SMC molecular clouds using the SEPIA180 instrument installed on the Atacama Pathfinder EXperiment (APEX) telescope. We performed a follow-up mapping campaign of the emission in the same transition in 13 star-forming regions. This first paper provides line characteristic catalogs and integrated line-intensity maps of the sources. Results. HCO+(2−1) is detected in 20 and HCN(2−1) in 8 of the 29 pointings observed. The dense gas velocity pattern follows the line-of-sight velocity field derived from the stellar population. The three SMC sources targeted during the mapping campaign were unfortunately not detected in our mapping campaign but both lines are detected toward the LMC 30Dor, N44, N105, N113, N159W, N159E, and N214 regions. The HCN emission is less extended than the HCO+ emission and is restricted to the densest regions. The HCO+(2−1)/HCN(2−1) brightness temperature ratios range from 1 to 7, which is consistent with the large ratios commonly observed in low-metallicity environments. A larger number of young stellar objects are found at high HCO+ intensities and lower HCO+/HCN flux ratios, and thus toward denser lines of sight. The dense gas luminosities correlate with the star formation rate traced by the total infrared luminosity over the two orders of magnitude covered by our observations, although substantial region-to-region variations are observed.

Dense Gas and Star Formation in Nearby Infrared Bright Galaxies: APEX survey of HCN and HCO+ J=2-1

“…Comparing with the HCN J = 2 → 1 and HCO + J = 2 → 1 data from star-forming clouds of the Magellanic Clouds (Galametz et al 2020), we find that the low-metallicity environment not only slightly deviates the data from the overall surface-density correlations, but also would bias the HCN J = 2 → 1/HCO + J = 2 → 1 line ratio to low ratios. This is consistent with previous findings in low-metallicity galaxies (e.g., Braine et al 2017).…”

“…Unfortunately, it is difficult to find systematic observations of the J = 2 → 1 transition towards our Milky Way, while such observations do exist towards the Magellanic Clouds. Therefore, we compare the galaxy survey with the Magellanic Clouds data from Galametz et al (2020), who presented HCN J = 2 → 1 and HCO + J = 2 → 1 observations towards ∼30 LMC and SMC molecular clouds using APEX.…”

“…As shown in Figure 7, after including molecular clouds of the Magellanic Clouds into our galaxy sample, the correlations between L HCN J=2→1 -L IR and L HCO + J=2→1 -L IR still hold with semi-linear slopes of 1.021 and 1.088, respectively. The upper limits of non-detections seem off the correlation, possibly due to different detection criteria in Galametz et al (2020), who consider a 3-σ peak Gaussian fitting (instead of integrated flux) as the detection threshold. We also include data of Magellanic Clouds into the correlations of surface densities, by adopting the area from (e.g., Wong et al 2011;Muller et al 2010).…”

“…The IR emission is in general extinction-free and often covered by multi-IR-wavelength space telescopes such as Herschel (Pilbratt et al 2010), Spitzer (Werner et al 2004), Infrared Astronomical Satellite (IRAS; Neugebauer et al 1984), etc. Therefore, the total IR luminosity is widely adopted as a star-formation rate tracer for gas-rich galaxies (e.g., Galametz et al 2016Galametz et al , 2020.…”

An ALMA Study of the Massive Molecular Clump N159W-North in the Large Magellanic Cloud: A Possible Gas Flow Penetrating One of the Most Massive Protocluster Systems in the Local Group

“…Using the same telescope, Nishimura et al (2016) reported marginal detections of N 2 H + emission in N159W and N113 despite a significant deficit of N-bearing molecules in the LMC (Millar & Herbst 1990). Measurements at millimeter/submillimeter windows (Paron et al 2016;Galametz et al 2020) detected several high-density gas tracers, HCO + , HCN, CS, and (tentative) C 18 O, toward the N159W region. Ott et al (2010) performed an NH 3 survey on such CO prominent regions, including N159W and N113, using the Australia Telescope Compact Array (ATCA).…”