DUST IN DWARF GALAXIES

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DUST IN DWARF GALAXIES

• a rare and intriguing ingredient....

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DWARF GALAXIES

• Low metallicity
• Small size
• Low mass
• Subset Gas-rich
  Blue

• lt 0.4 Solar
• lt 0.1 Milky Way
• lt0.01 Milky Way
• homogeneous!
• 20 50 HHe B-V lt 0.5, U-B lt 0.1

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Interstellar Dust

• Radiative heating/cooling dominant, depends on
• Dust grain composition
• Dust grain size (distribution)
• Ambient radiation field (UV, B)
• (function of environment)

Size distribution affected by erosion and
coagulation processes in the ISM i.e. function
of environment
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Interstellar Dust - II

• Dust properties from analysis of
• Extinction curve
• Polarization curve
• Emission spectrum (SED)
• How does this apply to dwarf galaxies?

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NGC 6822 FIR emission

• Israel, Bontekoe Kester, 1996

• IRAS 60 microns
• I

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Dust mass - I

• Dust column densities from reddening
• (advantage no temperature dependence)
• ND / E(B-V) 3.1 x 10-5 R Mgas / Mdust
• (c.g.s. units)
• selective-to-total extinction ?
• dust-to-gas ratio ?

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Dust-to-gas ratio

• Dependent on metallicity, but how ?
• log O/H a log Mdust / Mgas cst
• Issa et al. 1990
  a 0.85
• Schmidt Boller 1993 a
  0.63
• Lisenfeld Ferrara 1998 a
  0.52
• Dwek 1998 (model) a
  0.77

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Extinction

• Nebular spectroscopy
• Area-integrated stellar photometry
• dominated by intrisically brightest emitters
• dominated by least-extincted emitters
• reddening law dependent on relative geometries of
  emitters and extinctors
• Note R can take very large values!

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Grey extinction - I

• 
  Skillman Israel 1988

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Grey extinction - II

• Even with high extinction only little reddening
• Limit to reddening depends on wavelengths used
• Example
• 21 BCDGs observed by Thuan Izotov 2005
• c(Hß) ranges from 0.05 to 0.25 E(B-V) lt
  0.35 (exception IIZw40)
• Extinction/reddening not very useful tools

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Dust mass - II

• Dust column densities from emission spectrum
• SED reflects
• Big Grains 5-250 nm (MRN, thermal)
• Very Small Grains (nonthermal)
• Polycyclic Aromatic Hydrocarbons (PAHs)
• at various temperatures
• with potentially varying size distributions

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NGC 1569 ISO SCUBA

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Cold dust? Lisenfeld et al. 2002, 2005

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Same observations, different views

• Galliano et al. 2003
• most dust cold 5-7 K
• 40-70 of all dust
• in small clumps
• gas/dust ratio 320-680
• (740-1600)

• Lisenfeld et al. 2002/2005
• most dust warm 35 K
• hard, intense UV field
• extensive dust processing
• VSG enhanced 7-12 times
• gas/dust ratio 1500-2900

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Other dwarfs ...

• SCUBA He2-10, IIZw40, NGC 1140, 4214, 5253
• very similar SEDs,
• dust mass differs by factor 5-10
• Galliano et al. 2005,
  Hunt et al. 2005, Kiuchi 2004
• ISO many dwarfs observed, but no real
  constraints e.g. Hunter et al., 2001,
  Popescu et al. 2002, O'Halloran et al. 2005
• Popescu et al.
• unexpected high masses very cold
  dust in Virgo BCDGs
  and Ims

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More evidence for processing

• Spitzer PAHs present but depleted in BCDGs
• weak relation radiation field hardness
• strong relation effective energy density
• Wu et al. 2006, Rosenberg et al 2006, Higdon et
  al 2006
• IRAS PAH depletion sequence from f25 / f12
• Im 4.5 Sm 2.9 Sc
  1.8
• Melisse Israel 1994a,
  b
• ANS-UV BCDGs, Ims very weak 2175A bumps
• evidence for PAH processing by
  UV

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H2 from FIR or submm

• in dwarf galaxies abundances constant
• across dwarf galaxies dust temperatures vary
  little
• Ingredients
• FIR or subm maps tracing dust column densities
• Flux ratios tracing dust temperatures
• HI maps tracing atomic gas
• Assumption of constant dust-to-gas ratio

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Estimates of H2 and X

• Position A HI, FIR, no CO (no H2)
• Position B HI, FIR, CO (and H2)
• correct FIR for small DT
  FIR'
• Take N(HI)A / FIRA (NH )A /FIRA
• Apply to FIR' B to determine (NH)B
• Subtract N(HI)B , divide by 2 to obtain N(H2)B
• Divide N(H2)B by I(CO) to obtain X

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X-factor as function of metallicity

• Filled symbols large beam
• Open symbols resolved
• log X -a log O/H c
• a -2.3 (/-0.3)
• Israel 1997, 2000

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CO shielding

• Abundances C/H O/H1.5-2.0
• Mdust
  / Mgas O/H1.0-2.0
• CO column density Primary shielding
• high CO
  selfshielding
• intermediate CO Dust
  shielding
• low H2
  shielding

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X is not a constant ...

• but varies depending on
• metallicity
• ambient radiation field
• where and how you look
• measuring X in CO-emitting clouds selects those
  high-density volumes in space that are least
  affected by low metallicity and strong radiation
• such measurements reproduce the standard'
  X-factor and do this better the closer you stick
  to the outline of the CO emission!

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What to do?

• If you want to know the molecular gas mass of a
  specific CO cloud complex, go ahead and use the
• standard X-factor
• However, if you seek to use CO measurements to
  determine the molecular gas mass in an arbitrary
  fraction of a galaxy, be sure to use a
• metallicity-dependent X-factor
• calibrated by any means not involving CO!

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Molecular gas in dwarf galaxies

• Results on gas-rich starforming dwarf galaxies
• More H2 than expected from CO strength
• GMCs are comparable to those in spirals
• Locally, H2 mass exceeds HI mass
• Globally, molecular gas is 10-20 of the total
  gas mass

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Conclusions

• Main result of extinction in dwarf galaxies UV
  extinction curves indicate dust processing
• FIR/submm continuum emission in BCDGs lots of
  very cold dust or significant dust grain erosion
• mm/submm continuum and CO spectroscopy suggest CO
  photodestruction in dwarf galaxies
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  Bottom line
• Both CO and dust are processed in the dynamical
  high-energy, low-shielding environments of
  starforming gas-rich dwarf galaxies

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