Study of the ISM in nearby
molecular cloud regions basedon the optically thick HI
Katsuhiro Hayashi (Nagoya University)
Ryuji Okamoto, Hiroaki Yamamoto, Tsunefumi Mizuno,Takahiro Hayakawa, Hidetoshi Sano, Kengo Tachihara
and Yasuo Fukui
Dec 16, 2016, CTA Conference@ICRR, University of Tokyo

Outline• Introduction of the local ISM• Total gas column density model• Target molecular clouds (Perseus and Chamaeleon)• Optically thick HI in the local ISM

Katsuhiro Hayashi

2Local Inter Stellar Medium

Warm HI gas (WNM)
T > ~1000 K
n ~ 1-10 cm-3
τHI << 1Cold HI gas (CNM)
T ~10-100 K
n ~ 10-100 cm-3
τHI > 1

Molecular clouds
T ~10 Kn > 100 cm-3

• HI: 21 cm line, NHI = 1.823 x 1018 x WHI (optically thin approximation)• H2: 2.6 mm CO line, gas mass inferred by using Xco = NH2/WCO
Katsuhiro Hayashi

3Dust Grains

Dust grain
C, Si, Fe…
r ~ 0.1um• Mixed with gas components• Promotes constructions of molecular clouds => evolution of the ISM• Correlation between gas and dust => total gas column density model• Many studies of dust properties (e.g., dust optical depth at 353 GHz: τ353)
by Planck(e.g., ; Planck Collaboration XIII, 2014; Planck collaboration XXVIII, 2015)

Katsuhiro Hayashi

4Diffuse γ-ray Emission
γ-rays 〜 CRs x ISM (or ISRF)
inverse Compton
Sun

e-
Gamma-rayBremsstrahlunge- CR source:

CRs SNR?
π0 decay

p+

• ISM distribution measured by other wavelengths => CR density and spectrum“measured” CRs => gas distribution, volume and property• Many studies of diffuse gamma-rays by Fermi-LAT(e.g., Ackermann+12, ApJ, 755, 22; Casandjian+15, ApJ, 806, 240;
Planck collaboration XXVIII, 2015)
Katsuhiro Hayashi

5Dark Gas
γ-raysWHIWCO?
Dust

Dark gas (DG)

e.g., Grenier+15, Sci, 307, 1292; Ackermann+12, 755, 22;
Planck Collaboration XIII, 2014

• Not traced well by
HI and CO surveys• Interface between
the HI/H2 phases• Similar gas mass
to that of traced byCO
CO-dark H2 or cold HI (optically thick HI)
Katsuhiro Hayashie.g., Wofire+10, ApJ, 716, 1191;
Fukui+15, ApJ, 798, 66τ353-WHI Relation
Fukui+14, ApJ 796, 56
Fukui+15, ApJ 798, 6

HI integrated intensity (WHI)

All-sky in the local ISM (|b| > 15°)

Dust optical depth (τ353 (x 10-5))• Larger scatter at lower Td– “Dust property evolution” or HI emission is saturated ? = “optically thick HI”• Assuming uniform dust property in the local ISM and the optically thin at higher Td– NH model with linear relation → ~2-2.5 times larger HI density
18 xN
H = 1.82x10– Alternative interpretation of DG (optically thick HI hypothesis)(1.15 x 108 x τ353)
Katsuhiro Hayashi

7Optically thick HI

21 cm absorption line(Heiles & Troland 03)

Optically thin
— Ts = 200K
— Ts = 100K
— Ts = 50K

100K
MHD simulation to form molecular
clouds (Inoue & Inutsuka 12)
WHI saturated

100K
Possible saturation of WHI→ HI gas with lower Ts (optically thick HI)
Katsuhiro Hayashi

8Non-linear Relation
Roy+13, ApJ, 763, 55(Herschel)

Orion A

τ1200 ∝ (NH)1.28±0.01(Reddening data obtained with 2MASS)• Non-linear relation b/w NH and dust optical depth– Evidence for grain evolution in dense gas by aggregation processes
Katsuhiro Hayashi

9Motivation
Accurate measurements of the local interstellar hydrogen
Total column density (NH) model based on dust optical depth (τ353)
τ353: transparent to the interstellar gas even in dense core regions => accurate
measurements of the interstellar gas
Explore possibility of the optically thick HI, taking into account dust evolution
effects
Correlation with gamma-rays=> CR density/spectrum and gas properties (Xco, gas mass) in the local ISM

Perseus(Okamoto)

MBM53,54,55 & Pegasus(Mizuno)Katsuhiro Hayashi

Orion(Yamamoto, Hidaka)
Chamaeleon(KH, Okamoto)10

Target Molecular Clouds
Chamaeleon

Perseus

Mon R2Perseus

Chamaeleon

Distance~300 pc~170 pc

Diameter~50 pc~30 pcMass (H2)~3x104 Msolar~1x104 Msolar

star-forming regions

IC348, NGC1333

Katsuhiro Hayashi

11Dataset / Physical properties / ROI
Td

τ353

WHI

PlanckAJ (NIR extinction)

2MASS(NICEST)

PlanckWCO

Dame+01 (12CO (J=(1-0)))

GALFA HI Survey

HαFinkibeiner+03

Masking: WCO > 3 σ, Hα > 10 R
Katsuhiro Hayashi

12Dataset / Physical properties / ROITdτ353Planck

AJ (NIR extinction)

2MASS(NICEST)
Katsuhiro Hayashi

WHIPlanck

WCO

NANTEN (12CO (J=(1-0)))

GASS (Parkes Galactic All SkySurvey)

Masking:– WCO > 3 σ– IVA: intermediate
clouds may not be
associated withlocal clouds
13τ353-WHI (Perseus and Chamaeleon)
Perseusτ353 (x 10-5)

Chamaeleon

τ353 (x 10-5)

Scattering characterized by Td distribution
Linear relation b/w NH and τ353 in all sky (Fukui+15)

Katsuhiro Hayashi

14τ353-WHI (Perseus and Chamaeleon)
Perseusτ353 (x 10-5)

Chamaeleon

τ353 (x 10-5)

Scattering characterized by Td distribution
Linear relation b/w NH and τ353 in all sky (Fukui+15)
Possible non-linear relation b/w NH and τ353e.g., α ~1.3 in Orion (Roy+13)
If dust grows with density in time, what is α?
Correlation between τ353 and AJ based on the assumption that AJ represents NH
Katsuhiro Hayashi

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AJ vs. τ353
Perseus

α = 1.25+/-001

Katsuhiro Hayashi

Chamaeleon

α = 1.18+/-002

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AJ vs. τ353
Perseus

α = 1.25+/-001

Chamaeleon

α = 1.18+/-002

Uncertainty due to– poor efficiency in lower AJ– possible variation of dust property in high density region
Perseus: α = 1.3, Chamaeleon: α = 1.2

Katsuhiro Hayashi

17τ353-WHI (Perseus)
α = 1.0 α = 1.3

Katsuhiro Hayashi

18τ353-WHI (Perseus)

dot line: α = 1.0

solid line: α = 1.3• Correlation of α = 1.3 is much better than that of α = 1.0• Possible large amount of HI with higher τHI (>~1) characterized by lower Td
Katsuhiro Hayashi

19τ353-WHI (Perseus)

Possible large amounts of HI with lower Ts (<100 K) characterized by lower Td
Katsuhiro Hayashi

20τ353-WHI (Chamaeloen)

dot line: α = 1.0

solid line: α = 1.2• Correlation of α = 1.2 is much better than that of α = 1.0• Possible large amount of HI with high τHI (>1) characterized by lower Td
Katsuhiro Hayashi

21τ353-WHI (Chamaeloen)

Possible large amount of HI with lower Ts (<100 K) characterized by lower Td
Katsuhiro Hayashi

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HI Optical Depth (τHI)
Mass (x104Msolar)

Perseusave. 1.1

Mass (x103Msolar)

Chamaeleon

Katsuhiro Hayashi

ave. 0.9

τHI23

Gas Temperature (Ts)
Mass (x104Msolar)

Perseusave. 80K

Mass (x103Msolar)

Chamaeleon

ave. 101K

Ts (K)
Katsuhiro Hayashi

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HI Column Density (NHI)
Mass (x104Msolar)

Perseusave. 1.8x1021 cm-2

Mass (x103Msolar)

Chamaeleon

Katsuhiro Hayashi

ave. 1.5x1021 cm-2

NHI (x 1021 cm-2)

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Ratio of NHI/NHI*
Mass (x104Msolar)

Perseusave. 1.7

Mass (x103Msolar)Chamaeleon

Katsuhiro Hayashi

ave. 1.6

NHI/NHI*26

Uncertainty of Recent Gamma-ray Studies
γ-ray

NHI= qHI(E) x
WCO+ qCO(E) x
DG+ qDG(E) x
Grenier+15, ARAA, 53, 199+ other gamma-ray background• Interstellar gas divided into HI, H2 and DG• Uniform Ts (> 100 K) or optically thin approximation in NHI map• Uniform and linear gas-to-dust ratio in each phase (HI, H2, DG)• Our results show possible large amount of the optically thick HI→ Re-consider local CR density and gas properties (Xco, gas mass)
Katsuhiro Hayashi

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Conclusion• Infer the HI column density in the local molecular cloud regions (Perseusand Chamaeleon) using dust optical depth (τ353).
• Non-linear relation (α ~ 1.25) between gas and dust (AJ and τ353) possibly
due to dust evolution effects.
• Even if we take account of the non-linear effects, a large amount of HI gas(~1.6 times larger that that of the optical thin approximation) with higherτHI (>~1) can be suggested.

Future work
• More detailed evaluation of uncertainty in the HI column density model due
to dust property effects• Studies of local CRs and gas properties using diffuse gamma-rayemission, taking into account the possible uncertainty in the column
density model
Katsuhiro Hayashi

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Back up

Katsuhiro Hayashi

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Gas Tracer
γ-raysWHIWCO• CRs x ISM• high energy
photos transparent
to interstellar gas
Dark gas (DG)
• Good gas tracer
under the uniform
CR densityDust• τ353: dust opticaldepth at 353 GHz• Small opticaldepth: ~10-5transparent to theinterstellar gas• Mixed with gas• Good gas tracer

Good spatial correlation between γ-rays and dust emission
Katsuhiro Hayashi

30Masking

WHI: GASS (Parkes Galactic All Sky Survey)
Katsuhiro Hayashi

WCO: NANTEN (12CO (J=(1-0)))31

NH model (t353,ref/NHref)

Katsuhiro Hayashi

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