Molecular Gas and Star Formation in Nearby Galaxies Tony Wong Bolton Fellow Australia Telescope National Facility

Outline 1.Observations of molecular gas in galaxies –CO single-dish –CO interferometry –(Sub)millimetre dust emission –UV absorption 2.Current issues in relating H 2 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 Advantages of the CO molecule: 1.Most abundant trace molecule: of H 2 2.Rotational lines easily excited:  E 10 /k = 5.5 K 3.Effective critical density quite low, due to high opacity: n cr /  ~ 300 cm -3 Disadvantages: 1.Optically thick in most regions 2.Not as self-shielding as H 2 3.Expect low abundance in metal-poor regions

CO Single-Dish Studies galaxies, incl. most bright northern ones 2.CO usually peaked toward galaxy centres (Young et al. 1995) 3.CO linearly related to star formation tracers (Rownd & Young 1996) except in merging or interacting galaxies (Young et al. 1996) 4.Molecular gas not easily stripped by intracluster medium (Kenney & Young 1986, 1989)  The baseline for our understanding of H 2 in galaxies FCRAO Extragalactic CO Survey:

Local Group: LMC CO (1-0) 4m NANTEN telescope (2.6’ ~ 40 pc) Fukui et al. 1999, GMCs identified

Local Group: M31 30m IRAM (23” ~ 70 pc) Neininger et al CO in narrow arms extending into inner disk No structure comparable to Milky Way’s Molecular Ring CO appears to trace H 2 well (no dust extinction w/o CO)

CO Interferometry Individual case studies (e.g. NGC 4736) Wong & Blitz 2000, BIMA E. Schinnerer, PdB

Large-Scale Mapping: BIMA SONG 44 nearby spirals 6”-9” resolution Most maps extend to 100” radius or more Single-dish data included Helfer et al. 2003, ApJS 145:259

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

Other Probes of H 2 (Sub)millimetre dust emission Reveals cold dust not seen by IRAS Conversion to N H depends on T d (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 H 2 Tumlinson et al. 2002: diffuse H 2 fraction in MCs very low (~1% vs. ~10% in Galaxy)

CO Profiles from BIMA SONG Regan et al. (2001)

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 SA SAB/SB Central excess No central excess (5) (6) (14) (2) Sab/SbSbc Sc/Scd Central excess No central excess Thornley, Spohn-Larkins, Regan, & Sheth (2003) CO excesses are found in galaxies of all Hubble types, and preferentially in galaxies with some bar contribution (SAB-SB).

CO vs. HI Radial Profiles Overlaid CO (KP 12m) and HI (VLA) images Crosthwaite et al. 2001, 2002

CO vs. HI Radial Profiles IC 342 M83 Crosthwaite et al. 2001, 2002 HI CO

Atomic to Molecular Gas Ratio Wong & Blitz (2002) found evidence for a strong dependence of the HI/H 2 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 WSRT HI Swaters, Sancisi, & van der Hulst (1997) BIMA CO 10 kpc Wong, Howk, & van der Hulst

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:  SFR  (  gas ) n Kennicutt 1998 n=1.4 ± 0.15

Determining the SFR A difficulty with such studies is estimating SFRs from H  fluxes, which are subject to extinction.

Determining the SFR Kewley et al. (‘02) derive a correction factor of ~3 for H , and conclude that L IR is a better SFR indicator.

Considering HI and H 2 Separately Within galaxies, the SFR surface density is roughly proportional to  (H 2 ) but is poorly correlated with HI. Wong & Blitz 2002

Origin of Schmidt Law Index 1.Stars form on dynamical timescale of gas: 2.Stars form on a constant timescale from H 2 only:

Normalisation of the Schmidt Law Elmegreen (2002) derives the observed SF timescale from the fraction of gas above a critical density of ~10 5 cm –3, which in turn is determined by the density PDF resulting from turbulence. See also Kravtsov (2003).

Sequential Star Formation Can pressures from one generation of stars compress surrounding gas to form a new generation? Yamaguchi et al. 2001

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.