anomalous dust in late type galaxies...urgent need for better insight in dust grain properties...
TRANSCRIPT
Frank IsraelSterrewacht Leiden
ANOMALOUS DUST IN LATE TYPE
GALAXIES
COSMIC DUST:
EASY TO SEE, HARD TO GAUGE ...
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)
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
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 )
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
Dust emissivity
Empirical MW dust:(Dupac et al. 2003; Desert et al. 2008)
(cold dust) is not 2!
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
MAJOR COMPavelength SEDs!
Herschel Heritage Program LMCGalliano 2010
SED fits identical up to 500 μmbut dust mass differs by factor ~4
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
Add submm data points: drops with approaching unity longwards 500 μm
GC 1569 starburst dwarf galaxy SED:very cold or very small dust grains?
W
LMC and SMC: WMAP and COBE data added (Israel et al. 2010 )
mm emission excess:
Israel et al. 2010 (ArXiv1006.2232)
Bot et al. 2010 (ArViv1008.2875)
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)
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!
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
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?
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?
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)
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
Workshop
Herschel and the Nature of Dust
Lorentz Center
Leiden University, Netherlands
February 28 th to March 4th