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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

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
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

a single dish interferometric survey
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

two questions
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
distributions of detections nondetections
Distributions of detections/nondetections
  • Best predictors of CO: LK, LB, Hubble Type,…

1/5 Z

distributions of detections nondetections1
Distributions of detections/nondetections
  • Best predictors of CO: LK, LB, Hubble Type, FIR luminosity, B-K color, K-band mass to light ratio
one of the best predictors of co in the survey
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!
what is the driving relationship

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

what is the driving relationship1
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
what does it mean
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
the sfr vs h 2 relationship
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?)
the sfr vs h 2 relationship1

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
the sfr vs h 2 relationship is independent of z
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!
attempts to correct co h 2 for metallicity fail
Attempts to correct CO-H2 for metallicity fail
  • There is no segregation by inferred metallicity(using Richer & McCall 1995)
attempts to correct co h 2 for metallicity fail1
Attempts to correct CO-H2 for metallicity fail
  • There is no segregation by inferred metallicity(using Richer & McCall 1995)
  • Corrections destroy the agreement!
ways out of a constant xco
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?
the sweet spot for star formation efficiency
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
summary
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?