Project B3

Project leader: Bigiel, Frank (AIfA)

Probing molecular gas distributions, physical properties and their relation to star formation (SF) among and across galaxies is a key topic of galaxy evolution and a key process in the baryonic matter cycle (e.g., Hopkins and Beacom, ApJ 651, 142, 2006; Tacconi et al., ARA&A 58, 157, 2020). An accurate characterisation of the content and distribution of star forming, molecular gas across galaxies requires robust and broadly applicable tracers, calibrated to work in a wide range of physical conditions. Likewise, a thorough understanding of key extragalactic scaling relations (e.g. the Kennicutt-Schmidt relation, Kennicutt, ApJ 498, 541, 1998; Bigiel et al., AJ 136, 2846, 2008) critically rely on an accurate census of the amount and, more broadly, the physical conditions of the molecular gas.

Generally, low-J CO rotational lines are the gold standard tracing the bulk molecular medium in galaxies near and far. This is largely rooted in their brightness and their low critical density (∽102cm3) so that they are easily excited across large parts of galaxies. At intermediate and high redshift, often only mid-J CO lines are easily accessible (e.g., Carilli and Walter, ARA&A 51, 105, 2013; Hodge and da Cunha, Royal Society Open Science 7, 200556, 2020). Hence, H2 estimates critically rely on gauging molecular gas physical conditions (excitation, density) driving the CO-to- H2 conversion factor and CO line ratios. Also, the same gas physics ultimately regulate the density structure of the ISM and hence the efficiency of this gas to form stars. Deriving accurate constraints on the molecular gas physics across local galaxies is thus the key science objective of this sub- project. Specifically, in this sub-project we will explore 1) C I as an alternative molecular gas tracer, 2) CO excitation and 3) the density structure via density sensitive molecular line ensembles.

References

  1. Barnes, Kauffmann, Bigiel, Brinkmann, and 19 co-authors including, Menten, Weiss, and Wyrowski, “LEGO – II. A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way disc”, MNRAS 497, 1972 (2020).
  2. Bešlic ́, Barnes, Bigiel, Puschnig, Pety, Herrera Contreras, Leroy, Usero, et al., “Dense molecular gas properties on 100 pc scales across the disc of NGC 3627”, MNRAS 506, 963 (2021).
  3. Bigiel, de Looze, Krabbe, Cormier, Barnes, Fischer, Bolatto, Bryant, et al., “SOFIA/FIFI-LS Full-disk [C II] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946”, ApJ 903, 30 (2020).
  4. Bigiel, Leroy, Walter, Brinks, de Blok, Madore, and Thornley, “The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales”, AJ 136, 2846 (2008).
  5. Bigiel, Leroy, Jiménez-Donaire, Pety, Usero, Cormier, Bolatto, Garcia-Burillo, et al., “The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-disk Mapping of M51”, ApJ Letters 822, L26 (2016).
  6. den Brok, Chatzigiannakis, Bigiel, Puschnig, Barnes, Leroy, Jiménez-Donaire, Usero, et al., “New constraints on the 12CO(2-1)/(1-0) line ratio across nearby disc galaxies”, MNRAS 504, 3221 (2021).
  7. den Brok, Bigiel, Sliwa, Saito, Usero, Schinnerer, Leroy, Jiménez-Donaire, et al., “A CO isotopologue Line Atlas within the Whirlpool galaxy Survey (CLAWS)”, A&A 662, A89 (2022).
  8. Eibensteiner, Barnes, Bigiel, Schinnerer, Liu, Meier, Usero, Leroy, et al., “A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946”, A&A 659, A173 (2022).
  9. Gallagher, Leroy, Bigiel, Cormier, Jiménez-Donaire, Hughes, Pety, Schinnerer, et al., “Do Spectro- scopic Dense Gas Fractions Track Molecular Cloud Surface Densities?”, ApJ Letters 868, L38 (2018).
  10. Jiménez-Donaire, Bigiel, Leroy, Usero, Cormier, Puschnig, Gallagher, Kepley, et al., “EMPIRE: TheIRAM 30 m Dense Gas Survey of Nearby Galaxies”, ApJ 880, 127 (2019).