Bls si n c e

2018uGDO

Discovery of blue-excess
dust-obscured galaxies
by using Subaru Hyper Suprime-Cam
Akatoki Noboriguchi,Tohru Nagao, Masaru Kajisawa, Yoshiki Matsuoka,
Yuichi Terashima, Takuji Yamashita, Mana Niida,
Shuhei Koyama, Nobuhara Kodai (Ehime U.),
Masafusa Onoue (MPIA),
Yoshihiro Ueda, Yoshiki Toba (Kyoto U.),
Yutaka Komiyama (NAOJ), Yu-Yen Chang (ASIAA),
Toshihiro Kawaguchi (Onomichi-city U.)

Noboriguchi et al. accepted to ApJ
2019ApJ...876..132N

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Summary・We searched for infrared-bright (IR-bright)
dust-obscured galaxies (DOGs) with a blue
excess in optical bands in 105 deg2.
・We found 8 blue-excess DOGs (BluDOGs).
・The possible origins of their blue-excess area leaked AGN light and/or stellar UV lightfrom starbursts.
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Introduction

Dust-ObscuredGalaxies
DOGsvery
redrarefar・definition
R - [24] ≥ 14.0 [vega mag]i - [22] ≥ 7.0 [AB mag]

Dey+08
Toba+15

・number density
logφ = − 6.59 ± 0.11 [Mpc -3 ] Toba+15
・redshift
z=1-2

Dey+08, Toba+153The major merger scenario
Dey+09DOGs
Dey+09 suggested
the DOGs are in the transition phase.
4Introduction

Hot DOGs
Hot DOGs are selected by only using the WISE bands.
(Eisenhardt+12; Wu+12)
Dust temperatures
DOGs

Hot DOGs

Tdust < 60 K

Tdust > 60 K

Melbourne+12

Eisenhardt+12; Wu+12

DOGsʼ feature +High dust temperatures
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Introduction

blue-excess Hot DOGs
Blue-excess Hot DOGs are discovered by Assef+16.
・Blue excess in optical bands
23-2・NH ~ 6x10 [cm ]from X-ray observation
They suggest thatthe blue excess of Hot DOGs
is thought to be a leaking
scattered AGN emission
into our line of sight.
The optical blue-excess of Hot DOGs may bean evidence of the evolution from DOGs to quasars.
6Introduction

The relation among
HotDOGs, DOGs, and quasars

However…
Do DOGs evolve into quasars via Hot DOGs?
If DOGs can directly evolve into quasars,DOGs should experience the blue excess phase.
Path A
Blue excess
DOGs
Hot DOGsquasars
Hot DOGs
Path B
DOGs

Blue excess
phasequasars

Hot DOGs

Blue excess
Hot DOGsquasars7

This work

Purpose of this work
Question
Are there not only blue-excess Hot DOGs,
but also blue-excess DOGs?

This work
We searched for blue-excess DOGs
with Subaru Hyper Suprime-Cam,
VIKING (near-IR), and WISE (mid-IR).
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Sample selection

Hyper Suprime-Cam
Hyper Suprime-Cam (HSC)
The properties of the HSC
SSP survey are the following:

ongoingcompletedi-band limiting mag
deeper
25.9 [AB mag]
2

Future 1400 [deg ]widerThis work 105 [deg2 ]http://hsc.mtk.nao.ac.jp/ssp/wp-content/uploads/2016/05/hsc̲ssp̲rv̲jan13.pdf

Best data for exploring DOGs
which are optically faint and rare.

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Data and sample selection

ALLWISE
Wide-field Infrared Survey Explorer (WISE)ALLWISE catalog
・All-sky survey catalog (Wright+10)
・WISE has 4 bands (3.4, 4.6, 12, and 22 µm)

Toba+1510

Sample selection

Sample selectionof IR-bright DOGs
Optical
HSC clean sample

Near-infrared

16,680,947

VIKING clean sample

r = 1” match

Mid-infrared

13,455,180

ALLWIES clean sample
9,439,990

1,534,327(i1.2707,924

r = 3” match
1,915571

HSC-WISE DOGsKs)AB(i[22])AB7.011

Sample selection

Sample selection ofblue-excess DOGs

Our deﬁnition ofblue-excess DOGs (BluDOGs)
is as follows:
αopt < 0.4,
where αopt is a slope oflog fopt = β+αopt x logλopt
for the 5 HSC bands.
We selected 8 BluDOGs.
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Results

Selection results
The averaged SED

The SEDsof BluDOGsof blue-excess Hot DOGs

31010

3HSC
VIKINGALLWISEHSC

VIKINGALLWISE22

νobsfobsν (10-13 erg s-1 cm-2)

1010

110101100100-11010-1-2-21010 0.1

111010Obs-frame wavelength (µm)
observed-frame wavelength [µm]100Assef+16

The shape of the optical SED of BluDOGs isvery similar to that of these blue-excess Hot DOGs.
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Discussion

Are there blue-excess DOGs?
Question
Are there not only blue-excess HotDOGs,
but also blue-excess DOGs?

AnswerYes, there are 8 BluDOGs.

We suggest thatDOGs do not necessarily evolve into quasars
through the Hot DOGs phase.
14Discussion

The lifetime of BluDOGs
From results
・8 BluDOGs out of 571 IR-bright DOGs
・The number fraction of BluDOGs = ~1%

The lifetime of BluDOGs
Assumption:
DOGs lifetime of 100 Myr (Narayanan+10),・The estimated lifetime of BluDOGs is a few Myr.
・If the blue-excess comes from the leaked AGN light,
the timescale of the outﬂow phase could be a few Myr.
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Discussion

What is the origin ofblue excess of BluDOGs?

Possibilities
・The leaked AGN light・Stellar UV light from starbursts

Future plan・An identiﬁcation of the origin
based on spectroscopic observations ofbload emission lines.
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Summary

Summary
There are not only blue-excess HotDOGs,
but also blue-excess DOGs.
Results / Discussion
・We found 8 BluDOGs out of 571 IR-bright DOGs.
・The timescale of BluDOGs
is estimated to be about a few Myr.
・The possible origins of their blue excess are
the leaked AGN light and/or stellar UV lightfrom starbursts.
Next step・We have to obtain their optical spectra.
-> A VLT proposal for BluDOGs was executed!
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