anomalous dust in late type galaxies...urgent need for better insight in dust grain properties...
TRANSCRIPT
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Frank IsraelSterrewacht Leiden
ANOMALOUS DUST IN LATE TYPE
GALAXIES
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COSMIC DUST:
EASY TO SEE, HARD TO GAUGE ...
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SED (Continuum Spectrum) Analysis
Dust parameters from dust emission assuming(large) dust grains in thermal equilibrium and
emitting as modified blackbodies
Iv
= Nd
(c / )- Bv(T)
==> , composition (opacity, emissivity),
temperature, amount--- important degeneracies: N- and -T
dust emits at 10-300 microns (MIR, FIR)
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IRAS (1983) SURVEY FIRST GLOBAL IR(12–100 μm) FLUXES FROM ~11000
GALAXIES
a. large amounts of warm dust ubiquitous
b. missing IR peak intensity
c. no information on cold dust (<15 K)
d. potentially large dust mass unsampled
Need coverage >100 μ m
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ISO (1995-1998): up to 175 (240) μm
Spitzer (2003-2009): up to 160 μm
Herschel (2009-): up to 500 μm
a. define peak (temperature Td)
b. suggest RJ slope (emissivity )
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Dust emissivity ?
Crystalline Silicate: = 2.0Amorphous Graphite: = 1.0
change with temperature, frequency range(Jones 2002; Meny et al. 2007)
Empirical Milky Way dust:(Dupac et al. 2003; Desert et al. 2008)
T = 13 K ==> = 2T = 30 K ==> = 1
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Dust emissivity
Empirical MW dust:(Dupac et al. 2003; Desert et al. 2008)
(cold dust) is not 2!
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More sophisticated dust emission models(Zubko, Dwek, Draine, Li and others)
MAJOR COMPLICATION # 1
Degeneracy of long-wavelength SEDs!
Herschel Heritage Program LMC
Meixner et al. 2010Herschel Hermes Program M33
Kramer et al. 2010
MW vs LMC and M33similar FIR SED
different dust
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MAJOR COMPavelength SEDs!
Herschel Heritage Program LMCGalliano 2010
SED fits identical up to 500 μmbut dust mass differs by factor ~4
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Degeneracy of long-wavelength SEDs
MW, M33, and LMC similar FIR SEDdifferent metallicities:
MW = 1, M33 = 0.5; LMC = 0.4
Inferred dust-to-gas ratios:
MW (silicate, graphite): 1/150M33 (silicate, graphite): 1/40LMC (silicate, graphite): 1/65
LMC (silicate, amorphous carbon): 1/287
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Add submm data points: drops with approaching unity longwards 500 μm
GC 1569 starburst dwarf galaxy SED:very cold or very small dust grains?
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W
LMC and SMC: WMAP and COBE data added (Israel et al. 2010 )
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mm emission excess:
Israel et al. 2010 (ArXiv1006.2232)
Bot et al. 2010 (ArViv1008.2875)
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MAJOR COMPLICATION # 2
In SMC and LMC, cold dust cannot explainobserved (sub)millimeter wavelength SED
Occurrence of anomalous emission:
Very Small Grains (VSG) (Lisenfeld et al. 2002)
Disordered Charge Distribution (DCD)and/or Two-Level Systems (TLS) in Amorphous Dust Grains (Paradis 2007)
Spinning Dust (Draine & Lazarian 1998)
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LMC (Bot et al. 2010) SMC
In SMC and LMC, anomalous mmemission is best explained by
spinning dust grains.
Emission enhanced but no mass contribution!
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Other galaxies?
Adequate coverage of cm to submm (10-500
GHz) range is rare
WMAP: NGC 253,M82,NGC 4945
Groundbased: a fewarchetypes such asArp 220, IIZw40
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Anomalous Dust Emission:
mm fluxes lacking
FIR/submm slopes:
β = 1.50 +/- 0.06 (30 galaxies) β = 1.08 +/- 0.17 (6 dwarfs)
'excess' in all magellanic irregulars?
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Anomalous Dust Emission:
Milky Way: 'Cold' component(Reach et al. 1995, Draine & Lazarian 1998)
SMC and LMC(Israel et al. 2010, Bot et al. 2010)
NGC 6946(Murphy et al. 2010)
M82? Irregular Dwarfs? M33?
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SUMMARY
opacity and emissivity still poorly known
Dust models degenerate at λ < 500 μm FIR
No 'universal' dust
Anomalous emission at 500μm<λ<2cm(sub)mm – cm
SED degeneracy and contamination rule out identification of cold dust (T<15K)
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CONCLUSIONS
in other galaxies:mass of warm dust uncertain
mass of cold dust undetermined
dust- to-gas ratio and dust mass presently undetermined
urgent need for better insight indust grain properties
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Workshop
Herschel and the Nature of Dust
Lorentz Center
Leiden University, Netherlands
February 28 th to March 4th