Published on August 29, 2007
Molecular Gas and Star Formation in Nearby Galaxies: Molecular Gas and Star Formation in Nearby Galaxies Tony Wong Bolton Fellow Australia Telescope National Facility Outline: 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 CO as a Tracer of H2: 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 CO Single-Dish Studies: CO Single-Dish Studies 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 andamp; Young 1996) except in merging or interacting galaxies (Young et al. 1996) Molecular gas not easily stripped by intracluster medium (Kenney andamp; Young 1986, 1989) The baseline for our understanding of H2 in galaxies FCRAO Extragalactic CO Survey: 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 andamp; Blitz 2000, BIMA E. Schinnerer, PdB Large-Scale Mapping: BIMA SONG: 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: High Resolution Towards Nuclei IRAM PdB NUGA NGC 1068 (Baker 2000) NGC 4826 (García-Burillo et al. 2003) OVRO MAIN Other Probes of H2: 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 SONG: 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. Thornley, Spohn-Larkins, Regan, andamp; 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: CO vs. HI Radial Profiles Overlaid CO (KP 12m) and HI (VLA) images Crosthwaite et al. 2001, 2002 CO vs. HI Radial Profiles: CO vs. HI Radial Profiles Crosthwaite et al. 2001, 2002 Atomic to Molecular Gas Ratio: Atomic to Molecular Gas Ratio Wong andamp; 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) andamp; observations of star formation 'edges.' The Edge-On Spiral NGC 891: The Edge-On Spiral NGC 891 WSRT HI Swaters, Sancisi, andamp; van der Hulst (1997) The Star Formation Law: 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 SFR: 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 H2 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 andamp; 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: Sequential Star Formation Can pressures from one generation of stars compress surrounding gas to form a new generation? Summary: 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.