JCMT Large Programs

MAJORS: Massive. Active, JCMT-Observed Regions of Star formation

Determining the role of dense gas in star formation

We are observing a large, mass-selected sample of dust-continuum traced, star-forming molecular clouds in HCN J=3-2 and HCO+ J=3-2 with ‘Ū’ū. This sample includes clouds in the Central Molecular Zone (CMZ), the Inner Galaxy, and the Outer Galaxy. Dense gas is vital to the star-formation process, and high-resolution observations of this dense gas in a large sample of resolved star-forming sources is crucial to understanding its exact role in regulating star-formation efficiency.

Predictive, empirical relationships of star formation, such as the Kennicutt-Schmidt law, are able to link the scaling of the star-formation rate surface density with the surface density of the gas. However, this relationship only hold for normal and dwarf galaxies, and becomes super-linear in starburst systems and breaks down on the smallest scales of individual giant molecular clouds. However, when dense-gas observations are used, these relationships survive, once again indicating the apparent importance of dense gas in the star-formation process.

The key science outcomes and goals of this project are:

  1. Understand the impact of Galactic environment on the physics of dense gas, allowing for an understanding of how dense gas is produced and intrinsically linked to star formation.
  2. Distinguish between star-formation theories, and whether the star-formation rate is controlled by the free-fall time within bound structures or the amount of dense gas available for star formation.
  3. Produce LIR – Lgas relationships linking resolved Galactic clumps, Galactic molecular clouds, extragalactic systems and ULIRGS to study the universality of the star-formation process.
  4. Determine the cause of variations of the HCN/HCO+ ratio, and how it is linked to the physical conditions caused by Galactic environment.
  5. Find a sample of extreme star-forming sources using maps of dense-gas mass fraction and a sample of Galactic mini-starbursts using a LIR – Lgas relationship produced using CO maps.
  6. Link the clump-mass fraction to the star-formation efficiency and clump-formation efficiency.
  7. Identify outflows and active regions of star formation and determine the infall rates of the gas into individual clumps
  8. Provide a legacy sample matching those of extragalactic studies for future studies.
LIR – LHCN relationship from Tan et al. (2018) spanning ten orders of magnitude in luminosity from Galactic clumps to high-redshift ULIRGs.

Coordinators: David Eden (UK), Xue-Jian Jiang (EAO), James Di Francesco (Canada), Kee-Tae Kim (South Korea), Xue-Jian Jiang & Tie Liu (China), Mana Imanishi (Japan), Raffaele Rani (Taiwan)

CLOGS: CO Large Outer-Galaxy Survey

Determining the nature of Outer Galaxy star formation

This project has been awarded 416 hours to extend and complement the CHIMPS, CHIMPS2 and COHRS surveys by completing 12CO J=3-2 observations in the longitude range ℓ = 198°-236°. This data set will produce the most sensitive, highest-resolution molecular cloud survey of the Outer Galaxy, which will be an invaluable data set for the JCMT and Galactic astronomy.

These data will also allow the following science goals to be achieved:

  1. Determine the nature of star formation in the Outer Galaxy by producing a new, large sample of molecular clouds which are at large Galactocentric distances, but low heliocentric distances.
  2. Analyse the power spectra of turbulence within molecular clouds, the ratio of solenoidal to compressive turbulence and the relation of these to the large variations in star-formation efficiency from cloud to cloud and as a function of Galactocentric radius.
  3. Extend the study of the relationship between Galactic environment and molecular cloud properties into the Outer Galaxy where the molecular cloud conditions, such as metalilicity, are much different.
  4. Determine Galactic structure as traced by molecular gas and star formation in the Outer Galaxy. Galactic structure is not well defined in this region of the Galaxy, and the high-resolution dense molecular gas observed at the JCMT has previously shown to be very efficient at tracing Galactic structure.
12CO J=3-2 intensity from a patch of the CHIMPS2 Outer Galaxy data.

Coordinators: David Eden (UK), Harriet Parsons (EAO), Erik Rosolowsky (Canada), Kee-Tae Kim (South Korea), Yang Su (China), Tetsuhiro Minamidani (Japan), Vivien Chen (Taiwan)