Statistical Study ofStar Formation Law
in Nearby Galaxies
Moe Yoda (Nagoya University, M2)
Collaborators:
Tsutomu T. Takeuchi (Nagoya University)
COMINGers

Star Formation Activity
Gas converted to stars

HIH2StarsRelation between gas and SFR has beenstudied for a long time
01/14

Kennicutt-Schmidt Law
Logarithmic linear relation
𝑁ΣSFR ∝ Σgas

𝑁: 1 − 2

Found at various scales
among galaxies

in a galaxy

Resolved
K-S Law
Kennicutt, 1998b

Bigiel et al., 2008

02/14

Problems of K-S Law Studies
The meaning of K-S law is poorly understood
The physical meaning of the power-law index N is unclear

Many previous resolved K-S law works arecase studies
Observation of gas is time-consuming

03/14

Problems of K-S Law Studies
The meaning of K-S law is poorly understood
The physical meaning of the power-law index N is unclear

Many previous resolved K-S law works arecase studies
Observation of gas is time-consuming•
Howeverthese days …
COMING project enables us to study
resolved K-S law statistically
CO-Multiline Imaging of Nearby Galaxies
Obtained 140 galaxies’ CO maps using NRO45(Sorai et al. in prep)
03/14

Aim of the Study
Obtain resolved K-S laws for many galaxies
Reveal the physical meaning of K-S law
using statistical method
Key of this study
Uses the largest sample
compared with previous studies

04/14

MethodDataΣHIΣH2ΣSFR(FUV)ΣSFR(FIR)

VLA
21 cm line

NRO45
12CO(1 − 0)

GALEX
FUV

WISE
22 μm

Σgas

ΣSFR
05/14

Sample Selection

61

Molecular
K-S law

32

Total gas
K-S law

COMING sample
Observed by GALEX and WISE
Isolated galaxy
Enough data points at 1 kpc scale
06/14

ResultResolved Molecular Gas K-S Law

K-S law index vary from galaxy to galaxy
Mean resolved K-S law index
𝑁 = 1.01 ± 0.06

07/14

Discussion

Relations between
K-S Law and Galaxy’s Properties
K-S relations vary from galaxy to galaxy
Estimatelog Σgascorrelation coefficient log ΣSFRlog Σgas − log Σgas

2

Resolved K-S law index log ΣSFR − log ΣSFR
HI saturation threshold Dust extinction
Morphology

210/14

Index and Galaxy’s Properties
Index of molecular K-S law has significant
Molecular K-S law indexcorrelation with 𝐥𝐨𝐠 𝚺𝐒𝐅𝐑 − 𝐥𝐨𝐠 𝚺𝐒𝐅𝐑

Cor = 0.8
P-value < 0.05
Variance of SFR density
Strong correlation

𝟐P-value > 0.05
Variance of H2 density
No significant correlation
11/14

One Possible Model of K-S Law

12/14

One Possible Model of K-S Law

H2 dominant
HI – H2 transition
HI dominant

12/14

One Possible Model of K-S Law
Large ΣSFR variance
Large index

Small ΣSFR variance
Small index

13/14

Summary
Kennicutt-Schmidt law is a relation between
gas and SFR surface density
Obtain 61 galaxies resolved K-S law
Index of molecular K-S law has significant
correlation with variance of SFR density
Future Study
Explain the results physically

14/14

Appendix

Kennicutt-Schmidt Law
Global K-S law

Kennicutt, 1998b=🌀

Resolved K-S law

Bigiel et al., 2008

=🌀

Sample Morphology

otherotherEnough Data Points
Estimate correlation coefficient’s p-value
between Σgas and ΣSFR
If a galaxy does not have enough data points,
the p-value will be > 0.05
We adopted the galaxies which havep-value < 0.05

FormulaeHI𝑀 HI M⊙ = 2.356 × 105 𝑆HI Jy km s −1 𝐷 Mpc
H2
ΣH2 M⊙ kpc −2 = 3.21 × 106 × cos(𝑖)𝑋CO = 2.0 × 1020 cm−2 K km s −1 −1

2

𝐼12CO 1−0

K km s−1

SFR(FUV)ΣSFRFUV M⊙ yr −1 kpc −2 = 8.1 × 10−2 × 𝐼FUV MJy sr −1 × 1.59
SFR(22um)
ΣSFR22 M⊙ yr −1 kpc −2 = 3.2 × 10−3 × 𝐼22 MJy sr −1 × 1.59

Casasola et al., 2017

Analysis
1. Convolution
2. Making 1 kpc-grid maps
3. Fitting with least square methodlog ΣSFR = 𝑁 × log Σgas + 𝐴

Molecular K-S Law Index

Index

Molecular K-S Law’s Properties

Total Gas K-S Law’s Properties