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 Way’s 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 145:259 44 nearby spirals 6”-9” resolution Most maps extend to 100” radius or more Single-dish data included

9. High Resolution Towards Nuclei OVRO MAIN IRAM PdB NUGA NGC 4826 (García-Burillo et al. 2003) NGC 1068 (Baker 2000)

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. 10 Central excess No central excess 8 6 4 2 Sc/Scd Sab/Sb Sbc 15 Central excess (14) No central excess 12 9 6 (6) (5) 3 (2) SA SAB/SB 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. M83 IC 342 CO HI 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. BIMA CO Wong, Howk, & van der Hulst 10 kpc The Edge-On Spiral NGC 891 WSRT HI Swaters, Sancisi, & van der Hulst (1997)

17. 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

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 Within galaxies, the SFR surface density is roughly proportional to S(H2) but is poorly correlated with HI. Wong & Blitz 2002

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 cm–3, which in turn is determined by the density PDF resulting from turbulence. See also Kravtsov (2003).

23. Yamaguchi et al. 2001 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.