Work shop on Galaxy Evolution@ Ehime University
Jun 6-8, 2018

Physical Properties of Molecular Gas
in Nearby Barred Spiral Galaxies
Yoshiyuki Yajima (M1, Hokkaido Univ.)
K. Sorai, S. Shibata (Hokkaido Univ.)
N. Kuno, Y. Watanabe, T. Tanaka, A. Yasuda (Univ. of Tsukuba)
Y. Miyamoto, H. Kaneko (NAOJ)
T. T. Takeuchi, M. Yoda (Nagoya Univ.)
K. Muraoka, M. Kuroda (Osaka Prefecture Univ.)& COMING members

Star Formation in a Barred Spiral Galaxy
spatially resolved K-S plot(Momose+ 2010)

In the bar,• plenty molecular gas→log ΣSFR [Msun yr-1 pc-2]• little newborn stars(lower star formationrate; SFR)
• difficult to form stars(lower star formationefficiency; SFE)
← contour of SFE
the definition of SFE
log ΣH2 [Msun pc-2]

Molecular Gas in Ridges of the Bar
diffuse cloud

clouds in Ncloudspiral arms

σcloud [km s-1]

σinter-cloud [km s-1]

Σgas [Msun pc-2]
moderate cloud

dense cloudgravitationallyunbound state

clouds inbar ridges

Sorai+ 2012

ü Molecular clouds in bar ridges may be gravitationally unboundü There are few studies derived physical properties of molecular gas
in the bar because of difficulty→ Does lower density condition of molecular clouds in the barsuppress star formation? (cause lower SFE?)

Our observation - COMING projectl CO Multi-line Imaging of Nearby Galaxies
Sorai+ 2018 in prepl A NRO Legacy Projectl Observation periodMar., 2014 – Apr., 2018

l Number of observed galaxies# > 140 with NRO 45-m telescope, FOREST receiverl Observed emission lines
12CO (J=1→0), 13CO (J=1→0), C18O (J=1→0)
with simultaneous observationl Spatial resolution
17”.4 (12CO (J=1→0)), 17”.9 (13CO (J=1→0), C18O (J=1→0))l Velocity resolution
10 km/sArchival data
Line, Band
12CO (J=3→2) lineTelescope, Instrument
James-Clerk-Maxwell Telescope(JCMT), HARP

Far UV (λ=1350-1750Å) band GALEX
24µm mid-IR band

JCMT

Spitzer Space Telescope, MIPS

Samples of barred spiral galaxies
NGC 4303

NGC 3627

ü 12CO(J=3→2) archival
mapping data is availableü easy to recognize the bar
ü gas rich in the bar→ make it easy todiscuss properties ofmolecular gas in the bar
spatial resolution 17”.4NE

images by HST→ 0.76 kpc for NGC 3627
1.6 kpc for NGC 4303

Region separation
NGC 4303

Arms, bar-ends & bar region are defined
We measured SFE, I12CO(1-0) /I13CO(1-0),
I12CO(3-2) /I12CO(1-0) for each region
NGC 3627

Stacking analysis
13CO(J=1→0) spectrum is

too noisy to measure I13CO(1-0)→
performed ‘stacking analysis’(Schruba+ 2011)

ü shifting velocity
according tointensity
mean velocity fieldvelocity

ü averaging spectramean velocity fieldof NGC 4303

ü S/N is increased

Deriving molecular gas density & temperature
integrated intensity ratios
I12CO(1-0) /I13CO(1-0)
I12CO(3-2) /I12CO(1-0)

iteratively solve• level population of the moleculeat static state…

a code ‘RADEX’(van der Tak+ 2007)
for non-LTE analysis

kinetic temperature Tkinof molecular gas(LTE: Local Thermodynamic Equilibrium)…

number density n(H2)level i+1
level ilevel i-1

• radiative transferTbg
TCorrection for ‘beam dilution’ effect
For RADEX calculation, 12CO column density of one cloud is necessary•

an usual method for deriving column density
Ωc << ΩbeamHowever,← so-called ‘beam dilution’cloud size
~ a few 10 pc

column density will be underestimatedHence,

beam size~ 1 kpc; beam filling factor

Considering σinter-cloud, the best value of ηf is determined whenbecomes 0 with given ηf → best beam filling factor

SFE vs. Molecular Gas Density

barbar-end

open marker: NGC 3627
filled marker: NGC 4303

arm

Positive correlation
between SFE & n(H2)lower density ofmolecular gasdifficult to form stars

Gas Density & Inter-cloud Velocity Dispersion
bar-endobserver

arm𝑖inclination

positive correl.
(ΔV/sin i ≲ 100 km/s)

bar

ΔV/sin i~

negative correl.
(ΔV/sin i ≳ 100 km/s)

open: NGC 3627
filled: NGC 4303

σinter-cloud projectedgalactic plane(ΔV: FWHM of stacked spectrum)

Gas Density & Inter-cloud Velocity Dispersion
In positive correl. phase(from arms to bar-ends)

bar-endarm

due to moderate σinter-cloud• cloud-cloud collision
is promoted?

positive correl.
(ΔV/sin i ≲ 100 km/s)

In negative correl. phase(from bar-ends to the bar)barnegative correl.
(ΔV/sin i ≳ 100 km/s)

by too large σinter-cloud• shear becomes effective?
• internal cloud’s motion
is enhanced?
• clouds are broken?

open: NGC 3627
filled: NGC 4303

• cores can’t grow?

Summary & Conclusions• Barred spiral galaxies often show that it is difficult
to form stars in the bar•

We study about physical properties of molecular gas inbarred spiral galaxies NGC 3627, 4303•

Beam dilution correction method is developed
for deriving physical properties of molecular gas•

There is positive correlation between SFE & n(H2)•The relationships between n(H2) & σinter-cloud is changing
at the boundary ΔV/sin i ≈ 100 km/s

•Star formation in the bar is suppressed by lower densityof molecular gas &lower density is caused due to strong motion of gas

AppendixCO observation results (NGC 3627)
velocity-integrated intensity map
12CO(J=1→0)

13CO(J=1→0)

CO observation results (NGC 4303)

13CO(J=1→0)

12CO(J=1→0)

SFR & SFE (NGC 3627)

ΣSFRSFE

SFR & SFE (NGC 4303)
ΣSFRSFE

NGC 3627 as an interaction galaxy
NGC 3628

contour: HINGC 3623
NGC 3627
Haynes+ 1979

M1 presentation

Input parameters for ‘RADEX’ code
input parameters• I12CO(1-0) /I13CO(1-0) ≡ R12/13• I12CO(3-2) /I12CO(1-0) ≡ R3/1
• 12CO column density of a cloud N12CO• 13CO column density of a cloud N13CO• FWHM of one cloud’s spectrum dv
output parameters• number density of a molecular cloud n(H2)• kinetic temperature of a molecular cloud Tkin

Setting of input parameters for ‘RADEX’ code
l column density of 12CO, 13CO

introducing[12CO]/[H2] = 8.0 × 10-5 (Langer & Penzias 1993)[12CO]/[13CO] = 70 (Milam+ 2005)
l FWHM of one cloud’s spectrum dv
dv = 5 km/s for entire arms region as a standard
For other regions,dv = 5 km/s ×∆𝑉∆𝑉(entire arms)

local ISM value in MW

Calculation of non-LTE analysis
At first, assuming optical thin case → excitation is produced by only CMB

Solve the eq. of statistical equilibriumEiteration

β: escape probabilityE→Solve the eq. of radiative transfer→ifWe get

Example results of ‘RADEX’ calculation(e.g., NGC 4303)

R12/13

R3/1

the intersection of black & red lines corresponds tophysical properties of molecular gas

How to estimate beam filling factor ηf
If σinter-cloud = 0

However, σinter-cloud ~ a few 10 km/s

intensity

intensity

Tobs,peakobserved
spectrum

Tclouds × ηf<=Tclouds × ηfvelocity

Tobs,peakobserved
spectrumvelocity

ü observed peak intensity is no longer available for estimating ηfü observed integrated intensity I12CO(1-0),obs can be an alternative
to Tpeak,obs w/o depending on σinter-cloud
I by ensemble of clouds
on galactic plane in the beam(observed value)

I of a cloudcalculated by RADEX(theoretical value)

the best ηf value → Δηf = 0
we evaluate the best ηf value to 2 significant figures

Gas orbit in the barDownes+ 1996