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Molecular Gas and Star Formation in Dwarf Galaxies

Molecular Gas and Star Formation in Dwarf Galaxies. Alberto Bolatto Research Astronomer UC Berkeley Adam Leroy* Josh Simon* Leo Blitz. * Hard working grad students. Why should you care?.

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Molecular Gas and Star Formation in Dwarf Galaxies

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  1. Molecular Gas and Star Formation in Dwarf Galaxies Alberto Bolatto Research Astronomer UC Berkeley • Adam Leroy* • Josh Simon* • Leo Blitz * Hard working grad students

  2. Why should you care? “…Extreme properties are often sought for in Astronomy as one way to sharpen our understanding of fundamental concepts…” Dwarf galaxies: • are the first structures to form in bottom-up ΛCDM cosmologies • have low heavy element abundances, just like primordial systems • are the simplest systems Local dwarfs are windows onto the high-z Universe

  3. A single-dish/interferometric survey • MIDGet • A CO survey of IRAS-detected, compact, nearby, northern dwarf galaxies out to VLSR=1000 km s-1, with rotational velocities under ~100 km s-1 • Observed 121 central pointings with the Kitt Peak 12m • Follow up of 30+ galaxies mapped using BIMA • Fabian Walter’s OVRO sample UASO 12m BIMA

  4. Two questions: • What global properties distinguish galaxies with and without CO? • Some of the best molecular gas predictors are surprising: LK, Mdyn/LK, B-K (B-V) • Are there any differences between large and dwarf galaxies in their molecular gas/star formation properties? • Remarkably very few, even where some where expected

  5. Distributions of detections/nondetections • Best predictors of CO: LK, LB, Hubble Type,… 1/5 Z

  6. Distributions of detections/nondetections • Best predictors of CO: LK, LB, Hubble Type, FIR luminosity, B-K color, K-band mass to light ratio

  7. One of the best predictors of CO in the survey… • M/L ~ 3 (B-band) and ~ 2 (K-band) • But the correlation is much tighter at the low end in B light… CO nondetections are systematically fainter in K-band!

  8. M M M What is the driving relationship? • LFIR, LK, LB, B-K, Hubble Type, Z, are all correlated • Can we identify a driving parameter? • Normalizing by LK removes trends and minimizes dispersion M

  9. What is the driving relationship? • LFIR, LK, LB, B-K, Hubble Type, Z, are all correlated • Can we identify a driving parameter? • Normalizing by LK removes trends and minimizes dispersion • Mmol/LK is the tightest correlation. Across all galaxy sizes Mmol/LK~0.075

  10. What does it mean? • Facts: • Tightest Mmol correlation is with LK, a proxy for M* and Σ* • Correlations with Mgas (HI) or Mdyn are considerably weaker Taken together, suggest that what matters in the HIH2 conversion is the amount of matter in the disk (Σ*), not just the amount of “stuff” • Correlations with B-K could arise from enhanced photodissociation/less dust in bluer systems… • …but systems with no CO tend to be underluminous (for their mass) in K-band, not overluminous in B-band Suggests that photodissociation plays only a secondary role in setting the global amount of H2 • This is indirect evidence in support of the local density (pressure) controlling HIH2

  11. Are large and dwarf galaxies different in their molecular gas/star formation properties?

  12. The SFR vs. H2 relationship… • 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SFSNCRsynchrotron?)

  13. ΣSFR=10-3.4±0.1ΣH21.3±0.1 ΣSFR=10-3.4±0.2ΣH21.4±0.2 The SFR vs. H2 relationship… • 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SFSNCRsynchrotron?) • MIDGet and large galaxies fall on the same SFR-H2 correlation

  14. The SFR vs. H2 relationship… is independent of Z! • 1.4 GHz flux traces star formation (e.g., Condon et al. 2002, Murgia et al. 2002; SFSNCRsynchrotron?) • MIDGet and large galaxies fall on the same SFR-H2 correlation using the Galactic Xco!

  15. Attempts to correct CO-H2 for metallicity fail • There is no segregation by inferred metallicity(using Richer & McCall 1995)

  16. Attempts to correct CO-H2 for metallicity fail • There is no segregation by inferred metallicity(using Richer & McCall 1995) • Corrections destroy the agreement!

  17. Ways out of a constant Xco… • Size-dependent corrections to RC-SFR (e.g. Bell 2003)? • Even then large changes in Xco are out of the question • A different SFR-H2 regime for dwarf galaxies?

  18. The sweet spot for star formation efficiency… • A maximum star formation efficiency at 1010 M? • To a first approximation galaxy-size / metallicity corrections to LFIR and Xco cancel • A large Xco(Z) makes the maximum more pronounced

  19. Summary • Mmol correlates very well with LK, not with MHI or Mdyn • Indirect support for a local density/pressure controlled HIH2 transition • Same SFR-H2 relationship for dwarfs and large galaxies, suggesting constant CO-H2 for star forming gas despite changing metallicity • A minimum H2 depletion time / maximum SF efficiency at 1010 M?

  20. CARMA is moving forward

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