1. Yongmin Yoon, Myungshin Im, Gwang-ho Lee, Gu Lim, and Seong-kook Lee
Observational evidence for bar formation via cluster–cluster interactions
Yongmin Yoon, Myungshin Im, Gwang-ho Lee, Gu Lim, and Seong-kook Lee
CEOU, Seoul National University
Steward observatory, University of Arizona
Korea Astronomy and Space Science Institute
Hello, My name is Yongmin Yoon from Seoul National University. I am a Ph.D student under the direction of Professor Myungshin Im.
I am going to talk about observational evidence for bar formation via cluster-cluster interactions
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2. Introduction
Bars are commonly found in disk galaxies in the local universe.
In the local universe, barred disk galaxies are commonly found. It is known that roughly 30% of disk galaxies have bars, although the percentage is dependent on how we select the sample and wavelength window we observe.
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NGC 2217
NGC 1365
M95

3. Introduction
Broadly, there are two main bar formation mechanisms: (1) internal; (2) external.
Kwak et al. (2017)
Spontaneous disk instability
Broadly, there are two bar formation mechanisms
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One is an internal process like spontaneous disk instability without an external origin.
The other is an external origin such as tidal force from galaxy-galaxy interactions.
Tidal force from interactions
Lang et al. (2014)

4. No observational study has been conducted yet.
Introduction
Not very well recognized mechanism
: Bar formation by cluster-cluster interaction
Bekki (1999)
The time-dependent tidal gravitational field in interacting clusters induces a stellar bar.
No observational study has been conducted yet.
However, there is another, not very well recognized mechanism that can create bars.
20 years ago, Bekki conducted numerical simulation and found that the bars can be formed by cluster-cluster interactions.
This figure shows the two clusters in the numerical simulation before colliding.
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A disk galaxy is in a cluster.
In the simulation, as time goes by, the clusters begin to interact, and a bar is formed in the disk galaxy.
The simulation suggested that the time-dependent gravitational field in interacting clusters can induce a stellar bar.
However, no observational study on this bar formation mechanism has been conducted so far
So, we studied to find the observational evidence for this mechanism.

5. Bar fraction enhancement in interacting clusters?
Introduction
In this study...
Selecting clusters in pairs or with substructures
(Interacting clusters)
Finding galaxy clusters
Bar fraction enhancement in interacting clusters?
Comparison between interacting clusters with non-interacting clusters
Bar classifications for galaxies in clusters
In this study,
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we find galaxy clusters in the local universe using SDSS data.
Then, we select clusters in pairs or with substructures. We call them interacting clusters in this study.
Then, we classify which galaxies in clusters have bars
Then, we compare bar fraction of interacting clusters with that of non-interacting clusters
So, we determine whether the bar fraction in interacting clusters is enhanced compared to non-interacting, normal clusters.

6. Sample SDSS MPA-JHU catalog
0.010 < z < 0.065, log(Mstar/Msun) > 10.0
logM
Count
We used stellar mass information from MPA-JHU catalog. The redshift range of the sample is between 0.01 and We used galaxies more massive than 10 to the 10th solar mass, because as shown in the right panel, the completeness is very low under this mass cut.
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Redshift
logMstar

7. Friends-of-Friends (FoF) algorithm
Analysis
Finding overdense regions
Friends-of-Friends (FoF) algorithm
Galaxies in overdense region (2 σ)
Overdense region
Mpc
Before finding clusters, we found galaxies in overdense regions where galaxy surface number densities are above 2 sigma. The surface number density is calculated with an 1Mpc aperture and a redshift slice of +/-1000km/s.
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Then, the galaxies in overdense regions are grouped into a single overdense region using Friends-of-Friends algorithm.
The right figure is overdensity map around 10Mpc of a overdense region like that in the left panel.
Mpc

8. Analysis Measuring dynamical masses
105 clusters with log(M200/Msun) > are found in 0.015<z<0.060.
Count
Count
After that, we calculated dynamical masses of these overdense regions using a well known method.
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By doing so, we found 105 clusters that have dynamical masses heavier than 10 to the solar mass. The figures are the examples of radial velocity distributions of some clusters.
ΔV (km/s)
ΔV (km/s)
Demarco et al. (2010)
Kim et al. (2016)

9. Analysis Selecting clusters in pairs
7 cluster pairs (14 clusters) are found.
Criteria for
Velocity
&
Projected distance
Mpc
We selected clusters in pairs with proximity criteria in projected and radial distances.
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By this process, we found 7 cluster pairs among the 105 clusters.
Mpc
Mpc
Mpc

10. 16 clusters in pairs or with substructures  Interacting clusters
Analysis
Selecting clusters having substructures
5 cluster are found (3 clusters are in pairs).
Probability having substructure
>99.99%
DS test (Dressler & Shectman 1988)
16 clusters in pairs or with substructures
 Interacting clusters
Mpc
Count
Among the 105 clusters, we selected clusters having substructures using the DS test. This test can find clusters that have locally deviated velocity distribution compared to the velocity distribution of the whole members of the cluster.
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Using this method, we selected clusters with high probabilities of having substructures higher than 99.99%. The left figure shows the radial velocity distribution of member galaxies of a cluster with substructure. When the member galaxies are divided into two groups, these two groups have different spatial distributions as shown in the right figure.
We found 5 clusters with substructures, and among them, 3 clusters overlap with clusters in pairs.
To sum up, We found 16 clusters in pairs or with substructures. In this study, we defined them as interacting clusters and the others as non-interacting clusters.
ΔV (km/s)
Mpc

11. Analysis Classifying bars for cluster member galaxies
using the Ellipse task & visual inspection
Ellipticity
By visual inspection
Correcting false detection
&
Reclassifying obvious bars missed in the automatic process
arcsec
Bars are identified by a quantitative method with visual inspection.
This bar classification is conducted on the face-on galaxies with axis ratio higher than 0.5.
First, we used IRAF ellipse task on those galaxies,
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And extracted ellipticity and position angle profiles.
Barred galaxies generally have ellipticity profiles that increase
Then suddenly decrease
And show position angle profiles that are nearly constant over bars, but generally change at the end of bars. I classified bars using such criteria.
After that, we visually inspected all the galaxies and corrected false detections and reclassified obvious bars missed in the automatic process.
PA(degree)
arcsec

12. Analysis Comparing the bar fractions of 16 interacting clusters with
Comparison with Lee+12 catalog (only visual inspection):
89% of classifications are consistent.
Barred galaxies
Comparing the bar fractions of
16 interacting clusters with
those of 89 non-interacting clusters
This figure is examples of the barred galaxies and non-barred galaxies.
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We compared these classifications with a barred galaxy catalog of SDSS and found that 89% of classifications are consistent.
Then we compared the bar fractions of 16 interacting clusters with those of 89 non-interacting clusters.
Non-barred galaxies

13.  Disk-dominated galaxies
Results
Bars are more abundant in galaxies with lower B/T.
Bar fraction is enhanced (~1.5 times) in interacting clusters.
Bar Fraction
Count
From here, I present the results of this study.
This figure shows bar fraction as a function of bulge-to-total-light ratio.
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In this figure, bar fraction is higher at lower B/T values. This fact is consistent with other studies showing that bar is difficult to be formed in bulge-dominated galaxies.
Interestingly, this trend is more severe for interacting clusters.
Therefore, in interacting clusters, galaxies with lower B/T show higher bar fraction than those in non-interacting clusters.
From now on, we will show the results based on disk-dominated galaxies with B/T less than 0.5.
This figure shows distributions of bar fraction for 105 clusters. This figure shows that bar fraction is about 1.5 times enhanced in interacting clusters compared to that in non-interacting clusters. The KS test probability that the two distributions are not from the same distribution was %.
B/T
Bar Fraction
B/T<0.5
 Disk-dominated galaxies
KS test: % (4.5σ)

14. Results Bar Fraction logMstar
Bars are more abundant in galaxies with higher stellar masses.
Bar fraction is enhanced in interacting clusters.
Bar Fraction
And we examined bar fraction as a function of stellar mass.
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And found that bars are more abundant in galaxies with higher stellar masses.
Bar fraction is enhanced in interacting clusters in all the mass bins.
logMstar

15. Non-Interacting Clusters
Results
Bars are more abundant in galaxies
having higher stellar masses and lower B/T at the same time.
Non-Interacting Clusters
Interacting Clusters
B/T
B/T
This time, I show bar fraction in B/T and stellar mass plane divided into two-dimensional bins.
The left figure is for non-interacting clusters, while the right figure is for interacting clusters.
This figure shows that bars are more abundant
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in galaxies having higher masses and lower B/T at the same time.
logMstar
logMstar

16. Results Bar Fraction logM200
Bar fraction is not clearly dependent on the cluster mass.
Bar fraction is enhanced in interacting clusters.
Bar Fraction
This figure shows bar fraction as a function of cluster mass.
Bar fraction is not clearly dependent on the cluster mass. However
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Bar fraction is enhanced in interacting clusters in most of the mass bins.
logM200

17. Conclusion
Our results stand as the first observational evidence for a bar formation mechanism, cluster-cluster interaction.
In conclusion, these results stand as the first observational evidence for a much ignored bar formation mechanism, cluster-cluster interaction, and suggest that a large scale environment such as cluster-cluster interaction can affect the structure of individual galaxies.
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Bekki (1999)

18. Summary
105 clusters with log(M200/Msun) > are found in 0.015<z<0.060.
Among the cluster, we find that 16 clusters are in pairs or with substructures and they are defined as interacting clusters.
We detected bars using the Ellipse task & visual inspection
Bars are more abundant in galaxies having higher stellar masses and lower B/T at the same time.
Bar fraction is enhanced (~1.5 times) in interacting clusters compared with non-interacting clusters.
This is summary of this study.
This study was submitted to Nature Astronomy at the end of the last year, and now in review.
Thank you for your listening to my talk.