MALATANG: MApping the dense moLecular gAs in the sTrongest stAr-formiNg Galaxies

A long-standing issue that has plagued our understanding of the dense (> 104cm-3) star-forming gas phase in nearby galaxies has been the lack of spatially resolved observations of its molecular line tracers such as HCN and HCO+. The vast majority of studies have relied on pointed single-dish observations of the central nuclei of galaxies, which are bright enough to ensure robust detections of the aforementioned lines. In a few cases, sparsely sampled maps at off-nuclear positions in the disks of nearby galaxies have been made, but given the typically low column densities in these regions, the lines are incredibly weak and require long integration time. While millimeter interferometers such as ALMA and NOEMA provide both the necessary sensitivity and exquisite angular resolution to detect and resolve the HCN and HCO+ lines, their small fields of view imply that in practice an exorbitant amount of observing time is required to map an extended nearby galaxy, let alone a statistically significant sample of them. As a result of this dearth in the number of resolved HCN and HCO+ studies of nearby galaxies, the picture we have of the dense gas and its star formation capabilities is bound to be biased as it is based on the high pressure excitation conditions typical of galaxy nuclear regions. How this picture changes in quiescent conditions on the SF spiral disks is something we presently are in the dark about. There is, therefore, a significant potential for discovery waiting to be realized, and the MALATANG program is designed to do exactly that.

The MALATANG Legacy Survey is a 390 hr campaign, using the heterodyne array HARP-B on the JCMT to map the HCN and HCO+ J = 4 − 3 line emission in 23 of the nearest, IR-brightest galaxies beyond the Local Group. The observations will reach a sensitivity of 0.3 K km s−1(∼ 4.5 × 106 M⊙) at linear resolutions of 0.2 − 2.8 kpc. It is the first survey to systematically map the distribution and potentially excitation of dense gas out to large galactocentric distances in a statistically significant sample of nearby galaxies. MALATANG will bridge the gap, in terms of physical scale and luminosity, between extragalactic (i.e., galaxy-integrated) and Galactic (i.e., single giant molecular clouds) observations. A primary goal of the survey is to delineate for the first time the spatially resolved relationships between dense gas, as traced by the HCN and HCO+ J = 4 − 3, and star formation, on scales of ∼ 1 kpc across our targets (see figure). Exploring the behavior of these star formation relations in low surface density regions found in the disks as well as in the nuclear regions where surface densities are high, will shed new light on whether such environments are host to fundamentally different star formation modes. The MALATANG data products of resolved HCN and HCO+ J = 4−3 maps of 23 IR-brightest local galaxies, will be of great value to the star formation community (both Galactic and extragalactic) and, in and of themselves, carry significant legacy value.

For more information see the MALATANG web pages.

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MALATANG in a nutshell: here illustrated by a study of M51 (Chen et el 2015). a) Moment 0 map of the HCN J= 1 – 0 emission towards M 51 (contours at: 0.1, 0.6, 1.9, 3.4, 4.9, 5.4 K km/s on the Tmb scale). b) Herschel/PACS 70 μm image tracing the IR dust continuum (contours at: 3, 9, 27, 81 mJy/pixel. c) The resolved LIR – L’HCNJ=1-0 relation observed towards M 51, with each symbol representing a region ~1 kpc in size. The solid and dashed lines show the best log-linear fits to the nuclear (filled triangles) and disk (open triangles) regions combined and to the disk regions only, respectively. The combined correlation is seen to be shallower than the galaxy-integrated linear relation observed by Gao & Solomon (2004) (illustrated by the dashed line). d) Schematic of a HARP-B jiggle mode observations of a MALATANG target (NGC 253). With a beam spacing of 1000 , the shown 3 x 3 jiggle pattern will result in fully sampled HCN and HCO+ J = 4 -3 maps that probe dense molecular gas across a range of environments, from inter-arm regions to the central starburst nuclei.

Coordinators: Yu Gao (Purple Mountain Observatory), Zhiyu Zhang (The Royal Observatory, Edinburgh and European Southern Observatory), and Thomas R. Greve (University College London).

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