1	

多相星間媒質における巨大分子雲質量関数の時間発展と分子雲衝突に誘起された星形成( Kobayashi+ 2017 in ApJ, 2018 in revision in PASJ, 2018 in progress.)

Masato I.N. KOBAYASHI(D3, Nagoya U)

Rironkon Symposium 2017, Dec. 26th 2017 at U of Tokyo

SKA-JP Collaborators; Shu-ichiro Inutsuka, Hiroshi Kobayashi,
Square Kilometre Array
Japanese Consortium

Team BISTRO-J

Yasuo Fukui, Kenji Hasegawa,
Tsuyoshi Inoue (Nagoya U)

2	Outline
ü Backgroundw Multiple Episodes of Compression in magnetized ISM

ü Time Evolution of GMCs on Galactic Scalesw Network of expanding shellsw Formulation of GMC MF evolution with Cloud-Cloud Collisions(CCC)ü CCC-driven (Massive) Star Formationw Rapid dispersal of GMCs

ü Towards Galactic-Scale Studiesw Shock propagation in the multiphase ISMü Summary

3	Backgroundsü Observed GMC MF
ü Multiphase Simulationsü Multiple Episodes of Compressions

4	

IRAM: GMC Distribution in M51
Sub-division in the galactic disk (Colombo+ 2014a)
1) Face-on view of identified GMCs
1507 GMCs

2) Cumulative mass function of GMCs
2

DEC (J2000)

1

IRAM 30m

RA (J2000)
IRAM PdBI

log n(>M) [/kpc2]

0-1
2
1InterArm

ArmInterArm

0-12
1Power-law slope variation:
shallower (<-2) in arm regions,steeper (>-2) in inter-arm regions

Arm0-156756756

Larger GMCs tend to reside along spirals. log GMC mass [Msun](Consistent trend from Koda+ 2009)7

ISM Simulations

Similar results (e.g., Heitsch+ 2009,Körtgen & Banerjee 2015, Valdivia+ 2016)

Magnetic fields retard cloud formation
Inoue & Inutsuka, 2012

Inoue & Inutsuka, 2008
Without B-fieldection
riddleifBWith B-field (2µG)

I flow
Supersonic H

2 pc

Supersonic HI flow

02 pc

12

log # [/cc]3log # [/cc]

4.0
3.2
2.1
1.0
0.2

Cloud formation take place
after supersonic compression with multiple times.

5	

6	

ISM Simulations
Typical time scale for GMC formation
Multiple episodes of compression is essential
to form molecular clouds from magnetized WNM,
which occupy most of the volume in galactic disks.

GMC

HI cloudA few 10 Myrs	
e.g., Inoue and Inutsuka, 2012
Inutsuka+ 2015

Goal: determine how this multiple compression governs: the observed variation in GMC mass functions. 	

7	Time Evolution of GMCs
on Galactic Scalesü Network of expanding shellsü Formulation of GMC MF evolution

GMC Formation & Evolution

8	Network of expanding shells	  	  	  	  	  	  	  	  	  	  Cloud-­‐Cloud	  Collisions	  on galactic scales	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  (CCCs)	  (Inutsuka+ 2015 A&A)

HII region/ SNR		  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Fukui+	  2014	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Nakamura+	  2012	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Torii+	  2015	  	Formulate the time evolution of GMC populations
coarse-grained on a few 10 to 100 pc scales (disk regions).

Formulation

9	(c.f., Levinson & Roberts 1981, Kwan 1979,Scoville & Hersch 1979, Cowie 1980, Tomisaka 1984): differential number density of GMCs with mass m

GMC self-dispersal
due to radiation by massive stars(Td ~ 14 Myr; c.f. radiation hydrodynamics
simulation: Inutsuka 2015)

GMC formation/growththrough compressions(Tf ~ 10 Myr;c.f. Inoue &Inutsuka 2012)

Gas Resurrection
Cloud-Cloud
Collisions (CCCs)(coagulation)(Replenishment of minimummass population)
DispersedCompressions

GMC

10	

Fate of Dispersed Gas
Dispersed gas may resurrect to replenish theminimum-mass population.
CO-dark
H2Total gas dispersal rate
due to stellar feedbackMinimum-massin the system

CO-dark
H2
Optically
thick HI
Furthercompressions

GMC

Resurrecting factor (0.01-1)
The rate to generate new generation
minimum-mass GMCs out of the total dispersed gas.
N.B.: In a steady state,
can be understood as a rateto accrete onto pre-existing GMCs out of the total dispersed gas.

11	Resultsü Slope of GMC MF
ü Gas Resurrection

12	

Steady State with various Tf
CCC affects only the massive-end evolution…

Arm: Tf = 4.2 Myr,

Observed shallow slope	-2-2-4-4-5-544

Time [Myr]204060801201602005566log(GMC Mass [M ])log GMC mass [Msun]=0.45

Kobayashi+ 201711log(ncl [/kpc /M ])log(ncl [/kpc3/M ])Tf ,fid = 4.2 [Myr]Td = 14.0 [Myr]"res = 0.0123

log dn/dM [ / kpc3 3 / Msun]

Kobayashi+ 2017

-1-1-3-3= 0.012, Inter-arm: Tf = 22.4 Myr,

Tf ,fid = 22.4 [Myr]Td = 14.0 [Myr]"res = 0.45

Observed steep slope	

-1-1Time [Myr]20406080120160200-3-3-5-54455log(GMC Mass [M ])66log GMC mass [Msun]

Arm regions: short Tf due to many massive stars/supernovae,
little resurrection due to many massive (i.e. large) GMCs.

13	

Optimal Steady State Resurrection
Coagulation equation for mass evolution

Net mass flux(in case of)Outgoing flux
Incoming flux

Define the resurrecting factor
mResurrecting factor depends on α

14	Optimal Steady State Resurrection
Log (Resurrecting
log("res ) factor)

Relation between resurrection and GMCMF slope
Kobayashi+ 201700

Inter-arm regions
CCC mode-1-1
Decay mode-2-2

Arm regions"res,stdminmax

1.2 1.4
1.4 1.6
2.2 2.4
2.0 2.2
2.6
1.2
2.4 2.6
1.8 2.0
1.6 1.8
GMCMF↵Slope

Large surveys may put unique constraints on GMC
formation/dispersal timescales and the resurrecting
factor by measuring GMCMF slopes.

15	

Optimal Steady State Resurrection

N.B.

Log (Resurrectinglog("res ) factor)

Relation between resurrection and GMCMF slope
Kobayashi+ 2017

00

Inter-arm regions
CCC mode-1-1
Decay mode

(We suggest what resurrecting factor properly reproduces observations.)

Resurrecting -2
processesof opticallythick HI and"-2Arm regions
minCO-dark H2 should be confirmed/understoodin themax
context of physics
by massive simulations.	
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
res,std

1.2 1.4

1.6 1.8 2.0 2.2 2.4 2.6
GMCMF↵Slope

Large surveys may put unique constraints on GMC
formation/dispersal timescales and the resurrecting
factor by measuring GMCMF slopes.

Controversy on GMC lifetimelog (Growth Time [Myr])

GMC self-growth time starting with 100 Msun
121086Growth Suppressed

Exponential Growth42023456789log GMC mass [Msun]

Massive GMCs are long-lived;
Typical “Age” and typical “Lifetime” is different!

16	17	CCC-driven
Star Formationü Rapid Massive Star Formation
ü Star Formation Rateü Collision Frequency

Star Formation at CCC sites

18	O(B) stars and YSOs are often observed.
Ex.) RCW38 (Fukui+ 2016)

O(B) star candidates form at the intersection.

CO velocity	

1) Complementary distribution on the sky and2) bridging feature in CO position-velocity diagram
indicate a recent CCC event ~0.1 Myr ago.

Star Formation at CCC sites

19	O(B) star formation possibly triggered by CCC.
Ex.) NGC3603 Similar trends observed in different sites (Westerlund2, W51, M17, NGC6334, NGC6357,M16, W33, M42, RCW166, S116, S117, S118, M43, RCW36, M20, RCW120, NGC2024, RCW34, … etc.)

12CO contour + JHK image
Fukui+ 2014

Probability of stellar ages
(with assumed isochrones).
Kudryavtseva+ 2012

10 pc	

Multiple O stars might form ~0.5pc scale within 1Myr!

Is this process important for galactic star formation...?

20	

Two GMC Populations
Short Td for GMCs undergoing CCC
1) Normal Population

2) CCC Population
Dispersed Gas

Dispersed Gas

Normal GMCs

CCC / Normal GMCs

Normal GMCs

Normal GMCs

CCC GMCs

CCC GMCs

CCC / Normal GMCs

Growth through HI accretion

CCC

CCC GMCs

CCC GMCs

CCC GMCs

Dispersal due to massive starsü GMCs join the CCC population once they experience CCC.
ü CCC population has a short Td ~ 5Myr
representing observed rapid massive-star formation.

Two GMC Populations

21	Time evolution equation and star formation rate (SFR)
nacc: normal
population
ncol: CCC
population
Td = 14 Myr
Td,col = 5Myr

εSFE = 1%
CCC-driven star formation

22	

Two GMC Populations
GMCMF and CCC-driven star formation rate
Tf ,fid = 10.0 [Myr]Td = 14.0 [Myr]εres = 0.15

Typical power-law ~ -1.7	

-1-1-2-2-3-3Time [Myr]

-4-4

20
40
60
80

-5-54455

Mass[Msun][M⊙ ])
loglog(GMC
GMC mass

66SFR(>m) m)[ Msun/ kpc33/Myr]/ Myr]
SFR(>[M /kpc

3 / Msun]
3log
dn/dM
[ / kpclog(ncl [/kpc /M⊙ ])00x10 5⇥105Kobayashi+ 2018 in revision

5544

Kobayashi+ 2018 in revision= 10.0 [Myr]Solid: total TTf ,fid= 14.0
[Myr]d"res = 0.15 alone
Dotted: CCC-driven
Time [Myr]3302040
60
80

22
1100

4455log(GMCMass[Msun]
[M ])
log
GMC mass

6610kpc x 10kpc x 100pc disk (like Milky Way), this SFR = a few Msun per year.

CCC-driven star formation may amount to a few 10 percent of thetotal star formation in the Milky Way and nearby galaxies,
which is mostly driven by GMCs > 105 Msun.

Which mass pair can we observe?

23	Observability: Duration over which CCC
can be identified as CCC: Survey volume	Let us optimistically assume= 1Myr, and survey the entire MW disk.

log
GMC masslog(GMCMass[Msun][M⊙ ])Kobayashi+ 2018 in revision
6.0
6 log (# of possibly

observable events)

2.4
1.6
0.8

5.5
5.5

0.0
−0.8

5.0
5

−1.6

4.5
4.5
4
4.0
4.0
4−2.4
−3.2

4.5
5.0
5.5
6.065.55
4.5log(GMCMass [M⊙ ])log
GMC mass[Msun]

Most of the CCC pairs that current observations can probe must
consist of GMCs ~104 Msun; we need to push observations further tothe pairs >105 Msun, which are important for galactic star formation!

24	SummaryüBackgrounds: Multiple Episodes of Compressionw Variation of GMC MF on galactic scalesw Magnetic fields retard molecular cloud formation

üFormulation: Coagulation Equation with CCCw GMC MF slope is characterized by Tf/Td

whereas its massive end is governed by CCCw CCC-driven SF may amount to a few 10 per centof total SF in the Milky Way galaxy.

üFuture Prospects:

0Tf ,fid = 10.0 [Myr]Td = 14.0 [Myr]log(ncl [/kpc3/M ])log(ncl [/kpc3/M ])w The column density ofstar formation rate (SFR)w Transition from arm regions
to inter-arm regions

With CCC

Without CCC
0-2
Time [Myr]-420406080120160200-6-8456log(GMC Mass [M ])7

Tf ,fid = 10.0 [Myr]Td = 14.0 [Myr]-2Time [Myr]-420406080120160200-6-8456log(GMC Mass [M ])7