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Common Far-Infrared Properties of the Galactic Disk and Nearby Galaxies

MNRAS 379, 974 (2007)

Hiroyuki Hirashita Hiroyuki Hirashita (Univ. Tsukuba, Japan)Y. Hibi, H. Shibai (Nagoya Univ.), Y. Doi (Univ. Tokyo)

Abstract: A recent data analysis of the DIRBE far-infrared (FIR) map of the Galaxy and the Magellanic Clouds has shown that there is a tight correlation between two FIR colors: the 60 m – 100 m and 100 m – 140 m colors. This FIR color relation called ``main correlation'' can be interpreted as indicative of a sequence of various interstellar radiation fields with a common FIR optical property of grains. Here, we constrain the FIR optical properties of grains by comparing the calculated FIR colors with the observational main correlation. We show that neither of the ``standard'' grain species (i.e. astronomical silicate and graphite grains) reproduces the main correlation. However, if the emissivity index at 100 mm ~< ~< 200 m is changed to ~ 1 – 1.5 (not ~ 2 as the above two species), the main correlation can be successfully explained. Thus, we propose that the FIR emissivity index is ~ 1 – 1.5 for the dust in the Galaxy and the Magellanic Clouds at 100 mm ~< ~< 200 m. We also show that the main correlation also explains the FIR colors of nearby galaxies. This general applicability provides a possibility of unified understanding of the FIR SEDs of nearby galaxies.

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Désert et al. (1990)

Large grains (LGs) in radiative equilibrium with the interstellar radiation field

Excess by very small grains(VSGs)

Wavelength (m)

Inte

nsit

y 140 mm

1. Properties of Far-Infrared (FIR) Spectral Energy Distribution (SED)

The FIR SED can be used to constrain the emission properties of dust grains ranging from VSGs to LGs.

FIR

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FIR Color Relation of the Galactic Plane

Galactic Plane

|b| < 5°

Strong data concentration alongthis relation: main correlation

140 m – 100 m color

60

m –

100

m

col

or

　　　　　　 Hibi et al. (2006)DIRBE/ZSMA data at = 60, 100, and 140 m⇒Pixels with I(60 m) > 3MJy/sr are used to avoid the uncertainty caused by the zodiacal emission.

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Common Correlation between the Galaxy and the Magellanic Clouds

Hibi et al. (2006)

“Main correlation”

The LMC and SMC data: located at the extension of the main correlation defined by the Galactic-plane data.

Contours: Distribution of the Galactic plane data

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2. Properties of the Main Correlation

(1)Longitude (l) dependence

The data shift along the main correlation.Mean dust temperature T ~ 18 K (toward the Galactic center) T ~ 16 K (toward the anti-center),which reflect the difference in the radiation field intensity.

The main correlation is almost independent of l.⇒The main correlation is robust against the change of environment in the Galaxy.

Contours: Datatoward the Galacticcenter

Contours: Datatoward the anti-center

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(2) Galactic latitude (b)

The correlation is robust against the change of b.

Since the radiation field is more uniform in high b than in the Galactic plane, the main correlation should reflect the sequence of dust color illuminated by a uniform radiation field with various intensity.

Contours: Data ofthe Galactic plane

Contours: Data of high Galactic latitudes |b| > 5°

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3. Theoretical Analysis

Inputs:＊ Grain properties（ heat capacity, absorption coefficients）＊ Interstellar radiation field

Temperature distribution functionof each grain size: dP/dT

We adopt the FIR SED model developed by Li & Draine (2001).

Grain size distributionn(a) ∝ a–3.5

Output:FIR SED of dust

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= 0.3

= 1

= 3

= 10

= 0.3

= 1

= 3

= 10

Results

: Radiation field intensity normalized to the solar neighborhood value

Silicate from Draine & Lee (1984)

Graphite from Draine & Lee (1984)

The optical properties of silicate and graphite (Draine & Lee 1984) is not consistent with the observed FIR colors.

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Dependence on the FIR emissivity indexWe modify the FIR emissivity law:Q ∝

( > 100 m)[Emission∝QB(T)] is called emissivity index.For < 100 m, we adopt the optical constants of graphite in Draine & Lee (1984).

= 0.3

= 1

= 3

= 10

= 1 = 1.5

~ 1 is consistent with theobserved colors.cf. ~ 2 for Draine & Lee (1984). 9

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Sub-Correlation

A minor correlation: sub-correlation

Observationally, this correlation is associated with high radiation field.

= 1

= 1 contaminatedby 10% = 3

= 1 contaminatedby 10% = 10

The sub-correlation is explained if a region with high radiation field is contaminated in the line of sight.

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4. How about Other Nearby Galaxies?Hibi et al. (2006)

Main correlationMain correlation

Observational sample fromNagata et al. (2002): IRAS,KAO, and ISO data are used.

The main correlation also reproduce the FIR colors of The main correlation also reproduce the FIR colors of nearby galaxies! ( The FIR color is universal!?)⇒nearby galaxies! ( The FIR color is universal!?)⇒

Dust temperature derived from > 100 m

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5. Analysis of AKARI

Doi et al. (2007)： AKARI Observation of LMC at = 65, 90, and 140 m

(1)Higher spatial resolution than COBE

(2)More far-infrared bands than Spitzer

(3) face-on geometry of the LMC

⇒Analysis is ongoing.

LMC

Advantages:

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6. Summary(1)Observational Analysis (Hibi et al. 2006)

a. A common correlation between 60 m – 100 m color and 140 m – 100 m color is found for the Milky Way, the LMC, and the SMC.

b. This “main correlation” also explains the FIR colors of nearby galaxies, which suggests a universal nature of the FIR SEDs of nearby galaxies.

(2)Theoretical Analysis (Hirashita et al. 2007)a. The grain emissivities often assumed (Q ∝ –2) are

not successful in reproducing the main correlation.b. Our results strongly suggest that the FIR emissivity

index is ~ 1 (Q ∝ –1).