The brightest UV-selected galaxies in protoclusters
at z〜4: Ancestors of Brightest Cluster Galaxies?

Kei Ito (SOKENDAI/NAOJ)
Nobunari Kashikawa, Jun Toshikawa, Roderik Overzier, Masayuki Tanaka, Mariko Kubo,
Takatoshi Shibuya, Shogo Ishikawa, Masafusa Onoue, Hisakazu Uchiyama, Yongming Liang,
Ryo Higuchi, Crystal L.Martin, Chien-Hsiu Lee, Yutaka Komiyama, and Song Huang

Ito et al. (2019) ApJ, In Press (arXiv:1904.01597)

Brightest Cluster Galaxies
Brightest Cluster Galaxies (BCG): The most massive and luminous cluster galaxy→Good example of galaxies affected by the environment

The formation of BCGs:
・Early stellar formation and minor merger (e.g., De Lucia & Blaizot 2007)~100 M◉ /yr
Stop Star formation at z~1-2

The all existed massSFR(M◉ /yr)

Fraction to BCG mass at z~0

・Continuing star formation even at low-z (e.g., Bonaventura et al. 2017)

The stellar mass emerged to main branch

Cosmic Age

De Lucia & Blaizot 2007

0.5z

1.5

Bonaventura et al. 2017

When and how BCG are formed?
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!2

The current protocluster studies
The difficulty of high-z BCGs research:
The sample number of protoclusters (PCs) is extremely limited.
Redshift Distribution of protoclusters

z=3(~20 @ z 3)

Overzier+16※Protocluster: a structure that will grow into Mhalo > 1014 M⊙ at z>0•

Some protoclusters are found through radio galaxies/QSOs→They are biased protoclusters?

•Various tracer for finding overdense regions (LBGs, LAEs,HAEs…)→ We need large and systematic PCs sample.
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!3

HSC Protocluster Survey at z~4
Toshikawa et al. (2018)•We selected 〜400K g-dropout sample.
•Through the over density of them, we select overdensity > 4σ

0.2

0.4

0.6

1.4

1.6

1

0.4
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0

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−2

1.8

224

222

220

218
216
R.A. (degree)

214

212

0.0

6
4
2
0

overdensity (σ)

Decl. (degree)

0.0

R.A. (comoving Gpc)
0.8
1.0
1.2

Decl. (comoving Gpc)

regions as protocluster candidates.

−2179 PCs from 121deg2 (〜10 times larger than the previous sample)
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!4HSC Protocluster Survey at z~4
Toshikawa et al. (2018)•We selected 〜400K g-dropout sample.
•Through the over density of them, we select overdensity > 4σ

0.2

0.4

0.6

1.4

1.6

1.8

1

0.3

0

0.2
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−1
−2

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224

222

220

218
216
R.A. (degree)214
212
Toshikawa et al.2018

0.06
4
20overdensity (σ)

Decl. (degree)

0.0

R.A. (comoving Gpc)
0.8
1.0
1.2

Decl. (comoving Gpc)

regions as protocluster candidates.

−2179 PCs from 121deg2 (〜10 times larger than the previous sample)
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!4

Aim of this Study
Selecting the UV-brightest galaxies in each z〜4 HSC protocluster- UV-brighter galaxies should be more massive (M*-SFR relation)→ Candidates of the progenitor of BCGs (proto-BCGs)

Investigating properties of proto-BCGs- color and radial profile- any difference with field gal. at z〜4?

•The 1st systematic search of proto-BCGs at z〜4•Using 10 times larger PC sample

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!5

proto-BCG Candidate Selection
proto-BCGs: uniquely brightest protocluster galaxy compared to other galaxies
1. Define PC members within 3 arcmin from the overdensity peak.
2. Select “proto-BCG” as the brightest objects that are 1 mag or morebrighter than the 5th brightest objects in each PC.
→Select 63 objects totally
1.25Protocluster
RandomNormalized N

1.00

Protocluster

0.75
0.50

Random Selected

0.25
0.00

0123
5th-1st [mag]45p~3.5 10-3 @AD-test
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!6

Subsamples• We construct 4 subsample- proto-BCGs- PC member w/ proto-BCGs- PC member w/o proto-BCGs- Field galaxies

101

LUV/L∗z=4100

pBCGs
10−1
Normalized NField
10−210PC members
w/ pBCGPC members
w/o pBCG−3Subsample 1
Subsample 2Subsample 3
Subsample 4

10−4
10−5−252019 6/7

Kei Ito−24−23−22i [mag]−21−20The brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!7

N norma

N nor

2.0

i-z color comparison (proto-BCGs)
1.5

1

1.0

020

0.5

2224

i-z distribution of proto-BCG and Field gals.
(i + z)/2 [mag]

4.0
0.0

subsample 31
subsample 4

3.5
3.5

2.5
2.5
2.0
2.0
1.5
1.0

p-value:p-value: 0.0603
0.0106
1.04
0.8
30.6
20.4
10.2
0.00

0.5
4.0
0.00.8

N normalized

N normalized

22240.6

subsample 2subsample 40.4

p-value: 0.0003

2.5
2.020

(i + z)/2 [mag]

3.5
3.0

pBCGsField

N normalized
N normalized

N normalized

3.0
3.0

proto-BCG
2Field gal.

0.2

i0.0

0.2

0.4

0.6

z [mag]median0.1771 0.0254 [mag]0.1423 0.0010[mag]1→proto-BCGs have1.5redder
color than Field gal.
… significant at 2σ 1.0
from Anderson-Darling test (p=0.01)
02019 6/7

Kei Ito2022240.5(i + z)/2[mag] in protoclusters at z~4 (arXiv:1904.01597)
The brightest
UV-selectedgalaxies!8

2.5

N normalized

N normalized

1.00.82

2.5

20.6

0.41

2.0
1.5

N normalized

1.5

N normalized
N normalized

i-z color comparison (PC member)
2.0

10.2

1.0

1.0020222224i-z 0.5color20 comparison
betweeneachPCmemberand 24field gal.
0.5
(i + z)/2 [mag](i + z)/2 [mag]0.00

PC members w/ pBCG

subsample 23
subsample 42.5
2.5
2.0
2.0
1.5

N normalized

N normalized

3.0
3.0

PC members

p-value: 0.0003
0.0603p-value:
4Field232
1100.5

20

222.5
2.0
1.5

32
10.5

0.8

0.6

subsample 3subsample 4

0.4

0.2

i0.0

0.2

0.4

0.6

0.0

20

22

p-value: 0.0603

24(i + z)/2 [mag]

0.8

0.6

0.40.2iz [mag]

0.0

N normalized

2.5
PC member w/ pBCG
3
Field gal.
2.0
2
1.5
11.002022
240.5(i + z)/2 [mag]0.20.40.6z [mag]median4N normalized

Field

4024

(i + z)/2 [mag]3.53.0w/o pBCG

p-value: 0.0603

1.01.00.03.0w/ pBCG

PC members

subsample 3subsample 4

3.5

N normalized

3.53.5PC members w/o pBCG

0.0

N normalized

0.0
4.0

median0.212 0.016 [mag]PC member w/o pBCG

0.183 0.019 [mag]0.154 0.001[mag]Field gal.

0.152 0.001mag]→PC member w/ pBCG are redder than field gal. (p=0.0003)
PC member w/o pBCG have the same color with Field gal. (p=0.06)
0.0
0.8
0.6
0.2
0.0
0.2
0.6
proto-BCG
and 0.4surrounding
gal. 0.4are redderthan Field gal. and other PC membersi2019 6/7

Kei Itoz [mag]The brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!9

The cause of the i-z difference
What causes the i-z difference?

•The factor determining i-z (UV slope): Dust, Age, Metallicity•Bouwens et al. (2009) evaluated the effect of the change of each factor
and conclude that the dust is the most prominent.

→The dust can be the primary cause for i-z difference
The cause of the dust enrichment
1. Star formation from the early period
2. The existence of the discrete Starburst phase→proto-BCG and surrounding gal. have experienced different SFH?

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!10

05r [kpc]
15

10

20

25-0.0052-0.00220.0009proto-BCG
63 objects

0.00390.00700.01010.01310.01620.01930.02

field galaxies
628 objects

Normalized Count100pBCGs
10 1

Field

PSF
10 2※ HSC i-band images

10 1
0.5

1.0

BCG
field galaxies
BCG PSF
field galaxies PSF

1.50

Compare proto-BCG’s size withthat of Field gal.
→Compare “average” profile by Stacking•proto-BCGs have more extended
radial profile than that ofField gal.

• 〜25% flux excess at r〜1”
2019 6/7

Kei ItofBCG/fLBG

0083

Radial Profile

1.25
1.00
0.75
0.50
012
r [arcsec]

The brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)

34!11

Size Estimation• Estimate the effective radius in Sersic Profile by GALFIT• n=1.5 in Sersic Index
10

LUV /L∗z=4100

1R 1/nI(r) / exp[ bn () ]Re↵• proto-BCG is 28% larger than

10−1
Shibuya+15
BCGs
field galaxies

pBCGs

field gal.

(re, BCG = 2.042+0.012−0.013 kpc)

Re [kpc]<latexit sha1_base64="pHf5Yj4ornhb05NcDO4IFUNquPE=">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</latexit>

Field
100

−24−23−22−21
MUV [mag]−20−19−18

The possible origin of the profile difference
1. Concentration of the dust at center
2. The existence of satellite gal. around the outer part of proto-BCGs
2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!12Suggestion to the BCG’s evolution
Size-Mass Growth•Stellar mass: estimated from SF main sequence (Song+16)
log M* /M⊙ = 10.87 ± 0.2

Re,maj [kpc]102

Minor merger

101
Major merger
Kubo+18
Major merger
Minor merger
This work

100
123M⇤ [1011M ]45•Estimate the growth-track from the toy-model (Bezanson+09)→Suggest mainly evolve through the minor merger→The possibility to evolve into M* ∼ 3 − 4 × 1011M⊙ at z〜0 from the minor merger

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!13

Suggestion to the BCG’s evolution
Size-Mass Growth•Stellar mass: estimated from SF main sequence (Song+16)
log M* /M⊙ = 10.87 ± 0.2

Re,maj [kpc]102

Minor merger

101
Major merger

z~2Kubo+18
Kubo+18
Major merger
Major
Minor merger
Minormerger
Zhao+16This work

100
123M⇤ [1011M ]45•Estimate the growth-track from the toy-model (Bezanson+09)→Suggest mainly evolve through the minor merger→The possibility to evolve into M* ∼ 3 − 4 × 1011M⊙ at z〜0 from the minor merger

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!13

Suggestion to the BCG’s evolution
Size-Mass Growth•Stellar mass: estimated from SF main sequence (Song+16)
log M* /M⊙ = 10.87 ± 0.2

Re,maj [kpc]

102

z~0

Minor merger

101
Major merger

z~2z~2100
12
3M⇤ [1011M ]Kubo+18
Major merger
Kubo+18
Minormerger
Kubo+18
Major merger
Zhao+15cD
Major
Minor merger
Furnell+18
Minor merger
Zhao+16This work45•Estimate the growth-track from the toy-model (Bezanson+09)→Suggest mainly evolve through the minor merger→The possibility to evolve into M* ∼ 3 − 4 × 1011M⊙ at z〜0 from the minor merger

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!13

Suggestion to the BCG’s evolution
Size-Mass Growth•Stellar mass: estimated from SF main sequence (Song+16)
log M* /M⊙ = 10.87 ± 0.2Re,maj [kpc]102z~0Minor merger

101
Major merger

z~2z~2100
123M⇤ [1011M ]Kubo+18
Major merger
Minormerger
Kubo+18
Kubo+18
Zhao+15cD
Major merger
Major
Furnell+18
Minor merger
Minor merger
Zhao+16This work45•Estimate the growth-track from the toy-model (Bezanson+09)→Suggest mainly evolve through the minor merger→The possibility to evolve into M* ∼ 3 − 4 × 1011M⊙ at z〜0 from the minor merger

2019 6/7

Kei ItoThe brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!13

SummarySurvey the UV brightest galaxies “proto-BCGs” from 179 z〜4 HSC PCs(Ito et al. ApJ 2019 arXiv:1904.01597)•63 brightest galaxies that are 1mag brighter than the 5th brightest are selected.
•Proto-BCGs are redder in i-z than Field gal. considering the luminosity.
•PC member w/ pBCG are also redder but PC member w/o pBCG have the same→proto-BCG and surrounding gal. are dustier→Different SFH in protocluster regions compared to blank field?

•We compare average radial profile from stacking analysis.
•proto-BCG’s radial profile are elongated than Field gal.
•proto-BCG’s effective radius is 28% larger than Field gal.
→The concentration of the dust at the center?
orThe existence of the satellite galaxies?

2019 6/7Kei Ito

The brightest UV-selected galaxies in protoclusters at z~4 (arXiv:1904.01597)!14