Do galaxy morphologies
really affect the efficiency of star formation
during the phase of galaxy transition?
Shuhei KOYAMA
Ehime University
Y. Koyama (Subaru Telescope), T. Yamashita (Ehime U.),M. Hayashi(NAOJ), T. L. Suzuki (Tohoku U.), S. Namiki (SOKENDAI),
H. Matsuhara, T. Nakagawa (ISAS/JAXA)SF quenching

2Galaxies migrate from MS to RS by “quenching of their star formation”.
What triggers SF quenching? How galaxies migrate?
• Disk dominated• Star-forming• Bulge dominated• QuiescentMorphological quenching

3ETGs (bulge) have lower star formation efficiency than LTGs (disk).
-> Galaxy morphology may control star formation within galaxies.
Martig et al. 2013

Does morphology really affect SF quenching?
However, previous studies compare LTGs in MS and ETGs in RS.
-> Have we seen the real effect of MQ?
MS galaxies

RS galaxies

To assess the effect of galaxy morphologies on SF quenching,
we have to compare the galaxies in the process of SF quenching,
but having completely different morphology.

4Comparison of green-valley galaxies
green-valley galaxies are in the phase of galaxy transition.
Comparison between SFE of LTGs and ETGs in green-valley is fair.

5This study

6Construct two galaxy sample:• located on green-valley + disk-dominated• located on green-valley + bulge-dominatedCompare their SFEs to judge MQ really affect SF quenching.
disk-dominatedbulge-dominatedSample selection

7SDSS DR7 spec catalog with GALEX-WISE-SDSS catalog
selection criteria••••

0.025 < z < 0.05
10.8 < log M* < 11.2-11 < specific SFR < -10C-index (= R90/R50 at r band)• C-index < 2.2 for disk
• C-index > 2.8 for bulge

Disk + green sample: 13 galaxies

Bulge + green sample: 15 galaxies

CO(J=1-0) data

8CO(1-0)• 115 GHz
• Useful tracer of molecular gas mass.
NRO 45m

CO(1-0) observation at Nobeyama 45m••

2017 semester (2017/12, 2018/1, total: 150hr)
FOREST (ON-ON mode)CO(J=1-0) data

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CO(J=1-0) observation at Nobeyama 45m••

2017 semester (2017/12, 2018/1, total: 150hr)
FOREST (ON-ON mode)+ xCOLD GASS (Saintonge et al. 2017)•••••

extra galactic CO survey with IRAM 30m
Total: 532 galaxies
5 disk
21 bulge
22 intermed (2.2 < C < 2.8)

for all sample,NRO 45m
IRAM 30m

M(H2) – SFR relation

10Do disk+green and bulge+green galaxies have same relation?

Preliminary result

Statistical test

11KS test-> p-value = 0.37
Both disk+green and bulge+green galaxies follow
the same M(H2) and SFR relation.

Preliminary result

How we interpret an inconsistency of results?
Due to the bias?

Preliminary result

12Does morphology really affect SF quenching?
However, previous studies compare LTGs in MS and ETGs in RS.
-> Have we seen the real effect of MQ?
MS galaxies

RS galaxies

To assess the effect of galaxy morphologies on SF quenching,
we have to compare the galaxies in the process of SF quenching,
but having completely different morphology.

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C-index vs SFE

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SFE (=SFR/M(H2)) increase with sSFR(=SFR/M*).
(similar trend to Saintonge et al. (2012).)
However, it is independent of morphological types.

Saintonge et al. 2012

Preliminary resultSummary• We performed the CO observation to investigate themorphological dependence of SFE.
• We found that both disk+green and bulge+green
galaxies follow the same M(H2) and SFR relation.
• SFE increases with sSFR, and this relation is independentof galaxy morphology.

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