Molecular Gas and Star Formation in Nearby Galaxies. Tony Wong Bolton Fellow. Australia Telescope National Facility. Outline. Observations of molecular gas in galaxies CO single-dish CO interferometry (Sub)millimetre dust emission UV absorption
Australia Telescope National Facility
Advantages of the CO molecule:
FCRAO Extragalactic CO Survey:
The baseline for our understanding of H2 in galaxies
4m NANTEN telescope (2.6’ ~ 40 pc)
Fukui et al. 1999, 2001
168 GMCs identified
30m IRAM (23” ~ 70 pc)
Neininger et al. 2001
Individual case studies (e.g. NGC 4736)
Wong & Blitz 2000, BIMA
E. Schinnerer, PdB
Helfer et al. 2003,
44 nearby spirals
Most maps extend to 100” radius or more
Single-dish data included
IRAM PdB NUGA
(García-Burillo et al. 2003)
(Sub)millimetre dust emission
Regan et al. (2001)
No central excess
No central excess
SAB/SBCO Profiles from BIMA SONG
Of 27 SONG galaxies for which reliable CO profiles could be derived, 19 show evidence of a central CO excess corresponding to the stellar bulge.
CO excesses are found in galaxies of all Hubble types, and preferentially in galaxies with some bar contribution (SAB-SB).
Thornley, Spohn-Larkins, Regan, & Sheth (2003)
Overlaid CO (KP 12m) and HI (VLA) images
Crosthwaite et al. 2001, 2002
Wong & Blitz (2002) found evidence for a strong dependence of the HI/H2 ratio on the hydrostatic midplane pressure.
Consistent with ISM modelling (e.g. Elmegreen 1993) & observations of star formation “edges.”
A difficulty with such studies is estimating SFRs from Ha fluxes, which are subject to extinction.
Kewley et al. (‘02) derive a correction factor of ~3 for Ha, and conclude that LIR is a better SFR indicator.
Within galaxies, the SFR surface density is roughly proportional to S(H2) but is poorly correlated with HI.
Wong & Blitz 2002
1. Stars form on dynamical timescale of gas:
2. Stars form on a constant timescale from H2 only:
Elmegreen (2002) derives the observed SF timescale from the fraction of gas above a critical density of ~105 cm–3, which in turn is determined by the density PDF resulting from turbulence.
See also Kravtsov (2003).
1.High-resolution observations of molecular gas in nearby galaxies, using the CO line as a tracer, are becoming available for large numbers of galaxies.
2. At high resolution, CO radial profile often shows a depression or excess relative to exponential.
3. The CO/HI ratio decreases strongly with radius, mainly due to decreasing interstellar pressure.
4. The SFR (traced by Ha or IR emission) is well-correlated with CO but not necessarily HI.
5. The ‘universality’ of the Schmidt law may be related to the generic nature of turbulence.