Interstellar dust in M51 from IRC images

Fumi Egusa (1), I. Sakon (2), T. Onaka (2), H. Matsuhara (1), K.

Arimatsu (1&2), T. Suzuki (1), T. Wada (1), the IRC team

1: ISAS/JAXA, 2: University of Tokyo

Interstellar dust in M51 from IRC images

Outline

Fumi Egusa (ISAS/JAXA) 2

!  Introduction ! M51 -- nearby grand-design spiral galaxy !  Interstellar dust in MIR -- PAHs and warm dust

!  Data Reduction ! With the latest reduction toolkit ! Additional procedures -- PSF correction, etc.

!  Results ! Arm/Inter-arm ratio ! Colors & dust models

!  Summary

Introduction

3 Fumi Egusa (ISAS/JAXA)

M51!  NGC5194/NGC5195

! Nearby (D=8.4Mpc) !  Face on (i=24deg) ! Grand-design spiral

structure ! Active star formation in

spiral arms ! Dust lanes along spiral

arms and in inter-arm regions


Interstellar Dust & AKARI IRC

5 Fumi Egusa (ISAS/JAXA) Draine & Li (2007)

!  InfraRed Camera ! NIR

!  N3: star + neutral PAH !  N4: star

! MIR !  S7: ionized PAH !  S11: PAH !  L15: hot dust + PAH !  L24: warm dust (~100K)

Models from GRASIL (Silva et al. 1998)

Data Reduction


Basic Data Reduction!  Datasets

! Two pointed observations toward M51

!  FoV covering almost the entire M51 system

!  Data Reduction !  Latest reduction toolkit ! Neighbor dark frames

Date Filters # of long 2006/12/16 N3, N4 4

S7, S11 12 2007/06/16 L15 9

L24 12

Super dark (left), self dark (middle), neighbor dark (right) for MIR-L long exposure frames of M51.

(Coadd dark frames from neighbor (+/- 5 days) pointings.)


Additional Data Reduction!  Earth-shine Light Removal for L24

L24-long images after prepipeline -> after EL removal 8 Fumi Egusa (ISAS/JAXA)

Additional Data Reduction!  PSF Correction

!  PSFs for MIR-S and -L provided by Arimatsu et al. (2011)

!  FFT was used to correct the extended PSFs with different FWHM to a common 2D Gaussian PSF. !  FWHM=7.4’’ was

adopted. L24 long image before (left) and after (right) PSF correction


Additional Data Reduction!  Ghost Masking

!  In MIR-S bands ! Cannot be fully removed

by the ghost model from Arimatsu et al. (2011) !  probably due to dependence

on color and position

!  15’’ around the ghost was masked.

10 Fumi Egusa (ISAS/JAXA) S11 before (top) and after (bottom) ghost masking

Final Images!  Four MIR Bands

! Aligned !  WCS registered !  North-up !  3.7’’/pix

! Ghost masked !  PSF corrected

!  FWHM=7.4’’ !  Flux calibrated

!  Tanabe et al. (2008)

!  unit: MJy/sr

S7 S11

L15 L24


Results


Arm/Inter-arm Definition!  Filter N3 image

! N3: shortest wavelength !  traces the stellar distribution

most among the IRC bands !  Procedures

!  deproject to face-on view !  median filter to remove stars !  mask M51b out !  subtract azimuthal average at

each radius !  apply Wavelet analysis to

subtract components with 83’’ scale (c.f. Dumas et al. 2011)

!  Definition ! Center: r < 30’’ ! Arm

!  filtered amplitude > 1

!  Inter-arm !  filtered amplitude < -1


Inter-arm 2

Inter-arm 1

Flux Distribution!  Histogram for Each Region

! Center peaks at large flux ! No significant difference between

Arm1&2 or Inter-arm1&2 ! Arm and Inter-arm peak at similar flux

but Arm has a tail in bright end

!  Histogram for Each Band ! Highest contrast in L24 !  Lowest contrast in S11

Center Arm 1 2 Inter-arm 1 2


Arm/Inter-arm Contrast


!  A/I Ratio !  Highest in L24

!  tracing warm dust around star forming region in the arms

!  Lowest in S11 !  including ~11um bands from

PAHs !  well correlates with the total FIR

(Onaka et al. 2007), i.e. represents total dust including colder component

!  Reflects the temperature distribution in the galactic disk

Region S7 S11 L15 L24 Center 25.26 24.54 29.69 40.58 Arm 1 4.67 4.25 4.85 6.19 Arm 2 4.14 3.74 4.50 6.10 Inter-arm 1 1.32 1.38 1.53 1.64 Inter-arm 2 2.13 2.15 2.31 2.49 A/I 2.64 2.35 2.52 3.09

Average flux [MJy/sr]

Color Maps


!  Flux ratio based on S11 S7/S11 L15/S11 L24/S11

Flux > 5σ was used.

Color Maps


!  S7/S11 !  Sakon et al. (2007)

found S7/S11 > 1 in the spiral arm of NGC6946.

!  Similar trend found in M51! !  also high ratio in feathers !  mildly correlates with

L24 !  higher ionization fraction

in HII regions?

S7/S11 C 1.03 ± 0.09 A1 1.04 ± 0.14 A2 1.09 ± 0.14 I1 0.95 ± 0.13 I2 0.96 ± 0.13

Color Maps


!  S7/L15 ! No global trend !  ISO LW2 (7um) & LW3

(15um) !  Sauvage et al. (1996) report

small LW2/LW3 in the arm !  only for the brightest HII

regions? ISO LW3

S7/L15

S7/L15 C 0.86 ± 0.08 A1 0.98 ± 0.20 A2 0.90 ± 0.23 I1 0.91 ± 0.20 I2 0.89 ± 0.16

Dust Models


!  Draine & Li (2007) ! Composition

!  carbonaceous (incl. PAH) !  silicate

! Abundance !  PAH fraction=0.5--4.6% !  ionization fraction fixed

! Radiation field !  U=1 for solar neighborhood

from Mathis et al. (1983)

fixed: α=2 & Umax=1e6

Colors & Models


!  S7/S11 !  Large qPAH, large S7/S11 !  Large γ, small S7/S11 (large

contribution from large U) !  S7/S11 ~ const. for Umin <

100 (and smaller for larger Umin)

!  S7/S11>1 for arm !  Large qPAH and/or small γ in

arm? !  Large ionization fraction in arm?

S7/S

11

increasing qPAH

Dust model γ Umin

0.5% 2.5% 4.6%


Observational data Models with γ=0

incr

easin

g q PA

H

increasing Umin

increasing Um

in

increasing qPAH

Colors & Models


!  Color-color Diagram ! Observational data

and models not consistent?

!  Excess in S11 (& S7 perhaps)

!  PAH fraction need to be > 4.6%? !  but the ratio is large

in inter-arm regions

Observational data, 1pix=3.7’’

Models (color=qPAH): γ=0-1, Umin=0.1-25

Color Maps


!  S11 Excess !  In inter-arm regions !  PAH formation?

L24/S11

increasing Umin

increasing qPAH

L15/S11

S11/

L15

Contamination?


!  Stellar Continuum ! ~10% in S7 and less in S11

!  [NeII] emission line ! λ=12.81um, from

Spitzer IRS (Dale et al. 2009)

!  [NeII]/S11 = 1--3%

Center Arm1 Inter-arm1

Blackbody with 5000K from 2MASS

Position where the flux measured

Summary


!  M51 IRC Images ! Arm/Inter-arm contrast

!  Smallest in S11 !  Largest in L24

! Colors !  S7/S11>1 in arms

!  large qPAH, small γ !  large ionization fraction

!  S11/L15>1 in inter-arm !  even larger qPAH? !  low ionization fraction?

!  Dust in M51 !  PAH ionization

!  fixed models cannot explain the observational data.

!  more ionized PAHs in arms and more neutral PAHs in inter-arm regions

Future Works


!  Dust Model ! With larger qPAH and

varying ionization fraction

!  Neutral PAH !  Strong emission at 3.3um ! N3 (or IRAC1) can be

used? !  foreground star !  stellar continuum !  elongated PSF for N3

N3 IRAC1

Dale et al. (2005)

stellar continuum

ionization fraction v.s. dust size (Li & Draine 2001)

Interstellar dust in M51 from IRC images

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