Molecular gas conditions in the nuclear region of NGC 1614

Molecular gas conditions in the nuclear
region of NGC 1614 revealed with ALMA
Misaki Ando (SOKENDAI)
Working with: Daisuke Iono, Toshiki Saito, Junko Ueda, Tomonari Michiyama
Introduction
Merger and star formation Simulation
SFR (Msun/yr)
Star Formation Rate
increase via merger
First collision
Second
collision
Time (Myr)
Final
merger
gas-rich mergers trigger
intense star formation
merging galaxies become
very luminous
Teyssier+10
Introduction
Merger and star formation Observation
Ø  Survey of most luminous
infrared galaxies in the
local universe
Ø  Most of the ULIRGs in
this project are
interacting galaxies
U/LIRG: (Ultra) Luminous Infrared Galaxy
- 
- 
GOALS: Great Observatories All-sky LIRG Survey
ULIRG:
LIRG:
LIR > 1012 L¤
LIR > 1011 L¤
Armus+09
Science Goal
Global
12CO(1-0)/13CO(1-0)
Ø  Each point is one galaxy
filled symbols:
luminous
mergers
12CO(1-0)/13CO(1-0)
Ø  The ratio in luminous
merging galaxies ( 30)
is higher than normal
spiral galaxies ( 10)
Open symbols:
normal spiral galaxies
colder
f60/f100
line ratio
warmer
Ø  The differences of the
intensity ratio are due
to the differences in the
excitation conditions
and/or 12CO/13CO
relative abundance ratio
Aalto+95
Solve radiative transfer model
ü Solve radiative transfer model under the
assumption of LVG using RADEX code (van der
Tak et al. 2007)
ü To infer physical and chemical parameters
such as temperature, density and molecular
abundances
ü Investigate the condition of gas at each
region in merging galaxy
ü Consider the effect of merger event
Our Target
HST image
NGC 1614
Starburst ring
Paschenα
1
310 pc
SB ring
3 kpc
ü  Late-stage merger
ü  Starburst-dominant
DL 64 Mpc (1
310pc)
LIR 4 1011 L¤
SFR 50 M¤/yr
R. Herrero-Illana+14
Observation
ALMA cycle2 (2014) observation (PI: T. Saito)
Molecule
1
Noise level @ 30 km/s Spatial resolution
12CO(1-0)
1.6 mJy/beam
0.9
0.6
12CO(2-1)
1.4 mJy/beam
1.6
1.0
13CO(1-0)
0.5 mJy/beam
1.0
0.9
13CO(2-1)
0.4 mJy/beam
0.7
0.6
300 pc resolution at the distance of NGC 1614
Results
Intensity map
12CO(1-0)
12CO(2-1)
13CO(1-0)
13CO(2-1)
Ø  Black contour is the
starburst ring of Paα
Ø  The differences in the
distributions are due to
the differences in the
excitation conditions
and/or relative
abundance ratio
Ø  Tmb, CO(1-0) = 14 K
2
620 pc
Results
Inferred temperature and density
at the center of the star burst ring
ü Settings
[12CO]/[13CO] abundance ratio = 130
Velocity dispersion is 90 km/s
ü Results
nH2 = 7.9 105 cm-3
Tkin = 15 K
Discussion
ü  The density is very high at the center (nH2 = 7.9
à CO(2-1)/(1-0) will be thermalized
à Tkin=Tex (=15K)
105 cm-3 )
12CO(1-0)
ü Tmb, CO(1-0) = 14 K
ü  Tmb = Tex
filling factor
à filling factor = Tmb / Tkin
1 Beam size for analysis
However, this value is not appropriate for filling factor in
such condition
The kinetic temperature should be high
Discussion
ü  Typical filling factor from previous studies
is 0.3 at 100 pc resolution (Scoville+98)
ü  now, 300 pc resolution
-> filling factor should be smaller than
(and may be 0.03 )
0.3
ü  CO(6-5) high resolution image support that
the distribution of CO around the center of
NGC 1614 is clumpy and filling factor
should be less than 1
12CO(6-5)
high
resolution image
à  These evidences show the first setting may
be wrong
the first setting: [12CO]/[13CO] abundance ratio = 130
Velocity dispersion is 90 km/s
Discussion
Observational results of high [12CO]/[13CO] abundance ratio in starforming mergers are reported (e.g.: Sliwa+14, Israel+15 etc.)
due to 13CO selective dissociation by OB stars
ü  In the Milky Way, [12CO]/[13CO] abundance ratio is
70
ü  In our setting, [12CO]/[13CO] abundance ratio is 130 (by Sliwa+14)
However, the abundance ratio can be more larger value in some
papers
Discussion & Future Work
The estimated kinetic temperature from the model become higher
when we set the abundance ratio higher
30K
15K
setting abundance ratio: 70
setting abundance ratio: 130
Higher Tkin value bay be consistent with the reasonable filling factor
à We will parameterize the abundance ratio in the future work
Summary
ü  Gas-rich galaxy mergers trigger intense star formation
ü  We observed the
12CO(2-1)
and
13CO(2-1)
in merging starburst NGC 1614
with ALMA
ü  We will estimate the physical value in NGC 1614 using
13CO(1-0/2-1)
12CO(1-0/2-1)
&
and RADEX
ü  Current the [12CO]/[13CO] abundance ratio setting may be wrong
ü  We will also estimate the [12CO]/[13CO] abundance ratio in the future work
Thank you

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