Dust Observations With Scuba-2 In Nearby Galaxies

DOWSING is a new Large Program on the JCMT to observe a sample of nearby galaxies that have large angular sizes on the sky. For all objects we will obtain deep SCUBA-2 images at 450 and 850μm, getting close to the confusion limit at 850μm. The project will start in 2020 and has been awarded approximately 240 hours of band 1/2 weather to observe a selection of our 21 galaxies that we proposed. With the SCUBA-2 data we will investigate dust, the ISM and star formation across a wide range of environments both within and between galaxies. With a resolution of approximately 100-250 parsecs, we will be able to study our sample with five times better resolution than studies based on just Herschel data. The figure below shows the Herschel SPIRE 250μm images for the 21 galaxies in the DOWSING proposal.

The Herschel-SPIRE 250μm images for the sample of 21 galaxies in the DOWSING proposal. The SPIRE 250μm images have a resolution (FWHM) of 18″, when our SCUBA-2 observations will have a resolution of 7″ at 450μm and 13″ at 850μm. Our proposed PONG observations are shown by the white circles (note that this the prime area, the observations probably will have significant coverage outside the circles). All of the circles have a 15′ diameter with the exception of IC0342 which is a 30′ pong and the two edge-on objects (NGC891 and NGC4631) which are made of 4′ daisy observations.

These objects are some of the biggest and best-studied galaxies in the nearby universe, with distances between 3-16 Mpc (median 7 Mpc), and span a wide range of physical properties and environments. The data from this survey will be used to investigate whether the relations our projects to observe M31 and M33 on ~25pc scales hold over a larger range of environment. Unlike M31 and M33 a much larger range of interferometric follow-up (e.g., PHANGS) is available to investigate other gas properties in comparable detail.

The main science themes of DOWSING are:

  • The Physics of Dust: In M31 and other nearby galaxies current observations pose a challenge for dust models, with variations in beta, different temperature and beta relationships, and a ratio of dust emissivity to dust extinction that is 2.5 times greater than the value predicted by the most widely used models. By using our 5 times improvement in resolution compared to traditional studies with Herschel, we will look for the causes of the variation by looking for empirical relationships with other galaxy properties predicted by models for different processes that maybe responsible for the variation, such as aromatisation, coagulation, fragmentation and evolution in chemical composition.
  • The Interstellar Medium: We will study the relation between dust, atomic and molecular gas in our sample. For example, we will measure the variation in the CO X-factor and gas-to-dust ratio within individual galaxies, and make the first search for CO-dark gas in high-mass galaxies outside the Local group.
  • Sub-mm Transients: In the Milky Way with JCMT a range of sub-mm transients have been found. However, outside the Milky Way there have only been 14 bright transient events detected by Spitzer in the shorter wavelengths. We will investigate to see if any of our observations detect a sub-mm transient, potentially finding heavily dust obscured transients with luminosities a factor 100 less than the Spitzer

Coordinators: T. Brown (Canada), Y. Gao (China), E. J. Chung (Korea), A. Kong (Taiwan), M. Smith (UK)

– JCMT program code: M20AL011

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