Transcript Molecular Gas in Nearby 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?
“…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 bottomup 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

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:

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

Best
predictors
of CO: LK,
LB, Hubble
Type,…
1/5 Z
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…


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!

M
M

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

M
What is the driving relationship?
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?
 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
Are large and dwarf
galaxies different in their
molecular gas/star
formation properties?
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. H2 relationship…

1.4 GHz flux traces star formation

MIDGet and large galaxies fall on the same SFRH2 correlation
(e.g., Condon et al.
2002, Murgia et al. 2002; SFSNCRsynchrotron?)
ΣSFR=10-3.4±0.1ΣH21.3±0.1
ΣSFR=10-3.4±0.2ΣH21.4±0.2
The SFR vs. H2 relationship… is independent of Z!

1.4 GHz flux traces star formation

MIDGet and large galaxies fall on the same SFRH2 correlation using the Galactic Xco!
(e.g., Condon et al.
2002, Murgia et al. 2002; SFSNCRsynchrotron?)
Attempts to correct CO-H2 for metallicity fail

There is no segregation by inferred metallicity
Richer & McCall 1995)
(using
Attempts to correct CO-H2 for metallicity fail

There is no segregation by inferred metallicity
Richer & McCall 1995)

Corrections destroy the agreement!
(using
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 SFRH2 regime for
dwarf galaxies?

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
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?

CARMA is moving forward