Galactic Extinction Curves
and Interstellar Dust Models
Nozawa, T. and Fukugita, M.
(Kavli IPMU, University of Tokyo)

2012/05/08

1. Introduction
Extinction curve: wavelength-dependence ofinterstellar extinction by dust
・ necessary for correcting the SEDs of stars/galaxies➔ especially, extragalactic objects whose appearances
are disturbed by the Galactic interstellar extinction
・ depends on the physical and optical properties of dust
➔ provides information on the composition and size
distribution of interstellar dust on the line of sight➔ holds important clues to the origin and evolutionhistory of interstellar dust

2. Average interstellar extinction curves in MW
Far-UV rise
2175 A bump➔ small graphite or PAHs

BV
〇 CCM relation (Cardelli, Clayton, & Mathis 1989)
・ Aλ/AV = a(x) + b(x) / RV, where x = 1 / λ・ RV : ratio of total-to-selective extinction
RV = AV / (AB – AV) cf. RV,ave = 3.05–3.10

3. Interstellar dust models in MW〇 MRN dust model (Mathis, Rumpl, & Nordsieck 1977)
・ dust composition : silicate (MgFeSiO4) & graphite (C)・ size distribution : power-law distributionn(a)da ∝ a^{-q}da with q=3.5, 0.005 μm ≤ a ≤ 0.25 μm〇 WD01 model(Weingartner & Draine2001)

・ dust composition :silicate + graphite +PAHs

・ size distribution :power-law withexponential decay +lognormal
0.3 nm ≤ a ≤ 1 μm

4. Variety of interstellar extinction curves
・ There are a large variety of interstellar extinction curves
・ How much can the properties of interstellar dust change?
gray curves:
328 extinction curves
derived by Fitzpatrick& Massa (2007)
red bars:
1σ ranges including
224 data
blue bars:
2σ ranges including
312 data

5. Comparison to CCM formula and WD curve
UV-through-IR extinction curves

Close-up of IR extinction curves

black: 1σ range of the FM07 data
red: CCM curve with Rv = 2.75
blue: CCM curve with Rv = 3.60
green: extinction curve for thecase of Rv=3.1 by WD01
fully consistent in UV region

Results from CCM formula withRv = 2.75-3.60 are 0.02-0.06 maghigher than the 1σ range in JHK
WD01 model is based on resultsby Fitzpatrick (1999), which arequite similar to the CCM results

6. What causes the difference in IR extinction?
α = 1.61

NIR extinction is interpolated bypower-law formula Aλ/Av ∝ λ^-αCCM : α = 1.61
from Rieke & Lebofsky (1985)
FM07 : α = 1.84
from Martin & Whittet (1990)

α = 1.84

the value of α is controversial !!
α = 1.7 (He et al. 1995)
α = 1.8 (Froebrich et al. 2007)
α = 2.0 (Nishiyama et al. 2006)
α = 2.3 (Larson & Whittet 2005)
α = 2.6 (Gosling et al. 2009)

7.Demonstration of contour plots
1σ range of FM07 data

Contour plots for graphite

UVUB
JHK

The 1σ ranges from FM07 data
are classified into three groups
UV: UV bump (0.22 μm), FUV dip(0.16 μm), FUV rise (0.125 μm)
UB: U band and B band
JHK: J band, H band, K band

A contour plot is depicted foreach of the groups defined inthe left panel
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-1. Contour plots for fgra/fsil = 1.0
Case of 1σ data, fgra/fsil = 1.0

Case of 1σ data, fgra/fsil = 1.0contour plots of amax and q that
fulfill the 1σ range of FM07 data
for fgra/fsil = 1.0 (Mgra/Msil = 0.78)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 1.0 (Mgra/Msil = 0.78)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-2. Contour plots for fgra/fsil = 0.5
Case of 1σ data, fgra/fsil = 0.5

Case of 1σ data, fgra/fsil = 0.5contour plots of amax and q that
fulfill the 1σ range of FM07 data
for fgra/fsil = 0.5 (Mgra/Msil = 0.39)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 0.5 (Mgra/Msil = 0.39)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

8-3. Contour plots for fgra/fsil = 0.2
Case of 1σ data, fgra/fsil = 0.2

Case of 1σ data, fgra/fsil = 0.2

contour plots of amax and q that
fulfill the 1σ range of FM07 datafor fgra/fsil = 0.2 (Mgra/Msil = 0.16)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

contour plots of amax and q that
fulfill the 1σ range of CCM result
for fgra/fsil = 0.2 (Mgra/Msil = 0.16)
blue: constraint from UV/FUV
green: constraint from UB band
red: constraint from JHK band

9. Summary
・ The extinction ranges in UV regions from FM07 aredescribed by the CCM formula with Rv = 2.75-3.60
・ The observed range of NIR extinction from FM07
does not match with the results from CCM relationdifference : 0.02-0.06 mag➔ The average interstellar extinction curve is notnecessarily universal
・ For the power-law grain-size distribution,－ The values of q and amax that satisfy the observed
1σ ranges of FM07 are confined to narrow ranges－ There is no combination of q and amax that satisfy
the observed ranges when CCM results are adopted