S7/S11

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S7/S11

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
Introduction
Interstellar Dust & AKARI IRC
Onaka et al. (2007)
!  InfraRed Camera
Draine & Li (2007)
!  NIR
!  N3: star + neutral PAH
!  N4: star
!  MIR
!  S7: ionized PAH
!  S11: PAH
!  L15: hot dust + PAH
!  L24: warm dust (~100K)
Dale et al. (2005)
2
Fumi Egusa (ISAS/JAXA)
dust
stellar
3
30
Introduction
M51
!  NGC5194/NGC5195
!  Nearby (D=8.4Mpc)
!  1”=41pc
!  Face on (i=24deg)
!  Grand-design spiral structure
!  Dust lanes along spiral arms
(upstream)
!  Active star formation in spiral
arms (downstream)
!  With AKARI IRC images
!  Dust distribution and
properties in arm/inter-arm
regions
3
Evolution of ISM across spiral arms
Data Reduction
Basic Data Reduction
Date
!  Datasets
!  Two pointed observations toward
M51
!  FoV covering almost the entire
M51 system
!  Data Reduction
!  Latest reduction toolkit
!  Neighbor dark frames
(Coadd dark frames from
neighbor (+/- 5 days)
pointings.)
4
Filters # of long
2006/12/16 N3, N4
4
S7, S11
12
L15
9
L24
12
2007/06/16
Super dark (left), self dark (middle), neighbor dark
(right) for MIR-L long exposure frames of M51.
Data Reduction
Additional Data Reduction
!  Earth-shine Light Removal for L24
L24-long images after prepipeline -> after EL removal
5
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.
6
L24 long image before (left) and after (right) PSF correction
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.
7
S11 before (top) and after (bottom) ghost masking
Data Reduction
Final Images
!  Four MIR Bands
!  Aligned
!  WCS registered
!  North-up
!  3.7’’/pix
!  Ghost masked
!  PSF corrected
!  FWHM=7.4’’~30
0pc
!  Flux calibrated
!  Tanabe et al.
(2008)
!  unit: MJy/sr
8
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)
9
Inter-arm 1
!  Definition
!  Center: r < 30’’
!  Arm
!  filtered amplitude > 1
!  Inter-arm
!  filtered amplitude < -1
Inter-arm 2
Results
Arm/Inter-arm Contrast
!  A/I Ratio
!  Highest in L24
!  tracing warm dust in star forming
regions in the arms
!  Lowest in S11
!  including ~11um bands from
ionized and neutral PAHs
!  well correlates with the total FIR
(Onaka et al. 2007), i.e.
represents total dust including
colder component
Average flux [MJy/sr]
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
(IRAS)
!  Reflects the temperature
distribution in the galactic disk
10
Onaka et al. (2007)
Results
Color Maps
!  Flux ratio based on S11
S7/S11
L15/S11
L24/S11
Arm
Inter-arm
Flux > 5σ was used to calculate the ratio.
11
Results
Radial Color Profiles
Arm 1 2
Inter-arm 1 2
!  Large scatter in each radius
!  No clear difference between the defined regions
!  S7/S11 slightly larger in arms than in inter-arm regions
!  No systematic trend with radius
!  slight increase of L24/S11 toward outer disk?
12
Results
Color Map
!  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
around 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
13
Results
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)
14
fixed: α=2 & Umax=1e6
Results
!  Color-color Diagram
!  DL07 model cannot
explain the
observational data.
!  variety of S7/S11
!  excess in S11
!  qPAH > 4.6%?
Observational data,
1pix=3.7’’
15
Arm/Inter-arm
Center
Models (color=qPAH): γ=0-1, Umin=0.1-3e5
Colors & Models
Results
Contamination?
!  Stellar Continuum
!  ~10% in S7 and less in S11
Center
Arm1
Inter-arm1
from 2MASS
!  [NeII] emission line
!  λ=12.81um, from
Spitzer IRS (Dale et al.
2009)
!  [NeII]/S11 = 1--3%
Blackbody with 5000K
Position where the
flux measured
16
Results
Another Model
!  PAH + Large Grains
!  Components
!  neutral and ionized PAHs
from DL07 (model spectra
provided by DustEM;
Compiègne et al. 2011)
!  LG: gray body with
T=100K, β=2
!  Parameters
!  ratio of ionized PAHs to
neutral ones
!  ratio of LGs to neutral PAHs
17
neutral PAH
ionized PAH
T=100K
SED for M51 arm region with model spectra.
Color indicates pixel number histogram.
Results
Colors & Models
gray dots: observations
blue: DL07 models
red: PAH+LG models
DL07
PAH+LG
!  S7/S11, L24/S11 well explained by PAH+LG models
!  L15/S11 excess: needs additional component, probably Very
Small Grains (VSGs) stochastically heated to higher temperature
18
Summary
!  M51 IRC Images
!  Dust distribution across
the disk at 300pc
resolution
!  Arm/Inter-arm contrast
!  largest in L24 & smallest in
S11
!  dust temperature distribution
!  Colors
!  no radial gradient
!  S7/S11>1 in arms suggests
large ionization fraction
around HII regions
20
!  Dust in M51
!  DL07 models cannot fully
explain the observational
data.
!  Key parameters
!  PAH ionization fraction (S7/
S11)
!  PAH/LG ratio (L24/S11)
!  hot VSGs (L15/S11)

S7/S11

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