Molecular Gas and Star Formation in Nearby Galaxies

1
Molecular Gas and Star Formation in Nearby
Galaxies

• Tony Wong
• Bolton Fellow

Australia Telescope National Facility
2
Outline

• 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

3
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

• Disadvantages
• Optically thick in most regions
• Not as self-shielding as H2
• Expect low abundance in metal-poor regions

4
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

5
Local Group LMC

• CO (1-0)
• 4m NANTEN telescope (2.6 40 pc)
• Fukui et al. 1999, 2001
• 168 GMCs identified

6
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 Ways Molecular
  Ring
• CO appears to trace H2 well (no dust extinction
  w/o CO)

7
CO Interferometry

• Individual case studies (e.g. NGC 4736)

Wong Blitz 2000, BIMA
E. Schinnerer, PdB
8
Large-Scale Mapping BIMA SONG
Helfer et al. 2003, ApJS 145259
44 nearby spirals 6-9 resolution Most maps
extend to 100 radius or more Single-dish data
included
9
High Resolution Towards Nuclei

• IRAM PdB NUGA

OVRO MAIN
NGC 1068 (Baker 2000)
NGC 4826 (García-Burillo et al. 2003)
10
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)

11
CO Profiles from BIMA SONG

• Regan et al. (2001)

12
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)
13
CO vs. HI Radial Profiles
Overlaid CO (KP 12m) and HI (VLA) images
Crosthwaite et al. 2001, 2002
14
CO vs. HI Radial Profiles
Crosthwaite et al. 2001, 2002
15
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.
16
The Edge-On Spiral NGC 891
WSRT HI
Swaters, Sancisi, van der Hulst (1997)
17
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
n1.4 0.15
18
Determining the SFR

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

19
Determining the SFR

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

20
Considering HI and H2 Separately
Wong Blitz 2002

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

21
Origin of Schmidt Law Index

• 1. Stars form on dynamical timescale of gas

2. Stars form on a constant timescale from H2
only
22
Normalisation of the Schmidt Law

• Elmegreen (2002) derives the observed SF
  timescale from the fraction of gas above a
  critical density of 105 cm3, which in turn is
  determined by the density PDF resulting from
  turbulence.
• See also Kravtsov (2003).

23
Sequential Star Formation

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

24
Summary

• 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.