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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

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molecular gas and star formation in nearby galaxies

Molecular Gas and Star Formation in Nearby Galaxies

Tony Wong

Bolton Fellow

Australia Telescope National Facility

  • Observations of molecular gas in galaxies
    • CO single-dish
    • CO interferometry
    • (Sub)millimetre dust emission
    • UV absorption
  • Current issues in relating H2 to star formation
    • Radial CO distributions, vs. HI and stellar light
    • The Schmidt law within galaxies
    • Triggered (sequential) star formation
co as a tracer of h 2
CO as a Tracer of H2

Advantages of the CO molecule:

  • Most abundant trace molecule: 10-5 of H2
  • Rotational lines easily excited: DE10/k = 5.5 K
  • Effective critical density quite low, due to high opacity: ncr/t ~ 300 cm-3


  • Optically thick in most regions
  • Not as self-shielding as H2
  • Expect low abundance in metal-poor regions
co single dish studies
CO Single-Dish Studies

FCRAO Extragalactic CO Survey:

  • 300 galaxies, incl. most bright northern ones
  • CO usually peaked toward galaxy centres (Young et al. 1995)
  • CO linearly related to star formation tracers (Rownd & Young 1996) except in merging or interacting galaxies (Young et al. 1996)
  • Molecular gas not easily stripped by intracluster medium (Kenney & Young 1986, 1989)

 The baseline for our understanding of H2 in galaxies

local group lmc
Local Group: LMC

CO (1-0)

4m NANTEN telescope (2.6’ ~ 40 pc)

Fukui et al. 1999, 2001

168 GMCs identified

local group m31
Local Group: M31

30m IRAM (23” ~ 70 pc)

Neininger et al. 2001

  • CO in narrow arms extending into inner disk
  • No structure comparable to Milky Way’s Molecular Ring
  • CO appears to trace H2 well (no dust extinction w/o CO)
co interferometry
CO Interferometry

Individual case studies (e.g. NGC 4736)

Wong & Blitz 2000, BIMA

E. Schinnerer, PdB

large scale mapping bima song
Large-Scale Mapping: BIMA SONG

Helfer et al. 2003,

ApJS 145:259

44 nearby spirals

6”-9” resolution

Most maps extend to 100” radius or more

Single-dish data included

high resolution towards nuclei
High Resolution Towards Nuclei



NGC 4826

(García-Burillo et al. 2003)

NGC 1068

(Baker 2000)

other probes of h 2
Other Probes of H2

(Sub)millimetre dust emission

  • Reveals cold dust not seen by IRAS
  • Conversion to NH depends on Td (but only linearly), grain parameters, and gas-to-dust ratio
  • Very good correlation with CO (Alton et al. 2002)
  • UV absorption towards continuum sources
  • Extremely sensitive tracer of diffuse H2
  • Tumlinson et al. 2002: diffuse H2 fraction in MCs very low (~1% vs. ~10% in Galaxy)
co profiles from bima song
CO Profiles from BIMA SONG

Regan et al. (2001)

co profiles from bima song1


Central excess

No central excess









Central excess


No central excess










CO 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)

co vs hi radial profiles
CO vs. HI Radial Profiles

Overlaid CO (KP 12m) and HI (VLA) images

Crosthwaite et al. 2001, 2002

co vs hi radial profiles1


IC 342



CO vs. HI Radial Profiles

Crosthwaite et al. 2001, 2002

atomic to molecular gas ratio
Atomic to Molecular Gas Ratio

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.”

the edge on spiral ngc 891


Wong, Howk, & van der Hulst

10 kpc

The Edge-On Spiral NGC 891


Swaters, Sancisi, & van der Hulst (1997)

the star formation law

Kennicutt 1998

The Star Formation Law

Various empirical “laws” have been devised to explain correlations between SFR and other quantities, the most popular being the Schmidt law:

rSFR (rgas)n

n=1.4 ± 0.15

determining the sfr
Determining the SFR

A difficulty with such studies is estimating SFRs from Ha fluxes, which are subject to extinction.

determining the sfr1
Determining the SFR

Kewley et al. (‘02) derive a correction factor of ~3 for Ha, and conclude that LIR is a better SFR indicator.

considering hi and h 2 separately
Considering HI and H2 Separately

Within galaxies, the SFR surface density is roughly proportional to S(H2) but is poorly correlated with HI.

Wong & Blitz 2002

origin of schmidt law index
Origin of Schmidt Law Index

1. Stars form on dynamical timescale of gas:

2. Stars form on a constant timescale from H2 only:

normalisation of the schmidt law
Normalisation of the Schmidt Law

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).

sequential star formation

Yamaguchi et al. 2001

Sequential Star Formation

Can pressures from one generation of stars compress surrounding gas to form a new generation?


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.