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Extinction-free Census of AGNs in the AKARI/IRC
North Ecliptic Pole Field from 23-band Infrared
Photometry from Space Telescopes
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Wang, Ting-Wen; Goto, Tomotsugu; Kim, Seong Jin; Hashimoto, Tetsuya; Burgarella, Denis; Toba, Yoshiki;
Shim, Hyunjin; Miyaji, Takamitsu; Hwang, Ho Seong; Jeong, Woong-Seob; Kim, Eunbin; Ikeda, Hiroyuki; Pearson,
Chris; Malkan, Matthew; Oi, Nagisa; Santos, Daryl Joe D; Małek, Katarzyna; Pollo, Agnieszka; Ho, Simon C-C;
Matsuhara, Hideo; On, Alvina Y L; Kim, Helen K; Hsiao, Tiger Yu-Yang and Huang, Ting-Chi (2020). Extinction-free
Census of AGNs in the AKARI/IRC North Ecliptic Pole Field from 23-band Infrared Photometry from Space Telescopes.
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ORIGINAL UNEDITED MANUSCRIPT
AGN properties by CIGALE SED fitting 1
Extinction-free Census of AGNs in the AK ARI/IRC North
Ecliptic Pole Field from 23-band Infrared Photometry
from Space Telescopes
Ting-Wen Wang
1?
, Tomotsugu Goto
1
, Seong Jin Kim
1
, Tetsuya Hashimoto
1,2
,
Denis Burgarella
3
, Yoshiki Toba
4,5,6
, Hyunjin Shim
7
, Takamitsu Miyaji
8,9
†,
Ho Seong Hwang
10
, Woong-Seob Jeong
10,11
, Eunbin Kim
10
, Hiroyuki Ikeda
12
,
Chris Pearson
13,14,15
, Matthew Malkan
16
, Nagisa Oi
17
, Daryl Joe D. Santos
1
,
Katarzyna Ma lek
3,18
, Agnieszka Pollo
18,19
Simon C.-C. Ho
1
, Hideo Matsuhara
20,21
,
Alvina Y. L. On
1,2,22
, Helen K. Kim
16
, Tiger Yu-Yang Hsiao
5,23
, and Ting-Chi Huang
20,21
1
Institute of Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu City 30013, Taiwan
2
Centre for Informatics and Computation in Astronomy (CICA), National Tsing Hua University, 101, Section 2. Kuang-Fu Road,
Hsinchu, 30013, Taiwan (R.O.C.)
3
Aix Marseille Univ. CNRS, CNES, LAM Marseille, France
4
Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
5
Academia Sinica Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU
No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
6
Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
7
Department of Earth Science Education, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea
8
Instituto de Astrnom´ıa sede Ensenada, Universidad Nacinal Aut´onoma de M´exico (IA-UNAM-E) Km 107, Carret. Tij.-Ens., 22860,
Ensenada,BC, Mexico
9
Leibnitz Instituto f
¨
ur Astrophysik (AIP), An der Sternwarte 16, 14482, Potsdam, Germany
10
Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
11
Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
12
National Institute of Technology, Wakayama College, Gobo, Wakayama 644-0023, Japan
13
RAL Space, STFC Rutherford Appleton Laboratory, Didcot, Oxon, OX11 0QX, UK
14
The Open University, Milton Keynes, MK7 6AA, UK
15
University of Oxford, Keble Rd, Oxford, OX1 3RH, UK
16
Department of Physics and Astronomy, UCLA, 475 Portola Plaza, Los Angeles, CA 90095-1547, USA
17
Tokyo University of Science, 1-3, Kagurazaka Shinjuku-ku Tokyo 162-8601 Japan
18
National Centre for Nuclear Research, ul. Pasteura 7, 02-931 Warszawa
19
Astronomical Observatory of the Jagiellonian University, ul.Orla 171, 30-244 Krakow, Poland
20
Department of Space and Astronautical Science, Graduate University for Advanced Studies, SOKENDAI, Shonankokusaimura,
Hayama, Miura District, Kanagawa 240-0193, Japan
21
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara,
Kanagawa 252-5210, Japan
22
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, United Kingdom
23
Department of Atmospheric Science, National Central University, No.300, Zhongda Rd., Zhongli Dist., Taoyuan City 32001, Taiwan (R.O.C.)
Accepted XXX. Received YYY; in original form ZZZ
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ORIGINAL UNEDITED MANUSCRIPT
2 Ting-Wen Wang et al.
ABSTRACT
In order to understand the interaction between the central black hole and the whole galaxy
or their co-evolution history along with cosmic time, a complete census of active galactic nuclei
(AGN) is crucial. However, AGNs are often missed in optical, UV and soft X-ray observations since
they could be obscured by gas and dust. A mid-infrared (mid-IR) survey supported by multiwave-
length data is one of the best ways to find obscured AGN activities because it suffers less from
extinction. Previous large IR photometric surveys, e.g., W ISE and Spitzer, have gaps between the
mid-IR filters. Therefore, star forming galaxy (SFG)-AGN diagnostics in the mid-IR were limited.
The AK ARI satellite has a unique continuous 9-band filter coverage in the near to mid-IR wave-
lengths. In this work, we take advantage of the state-of-the-art spectral energy distribution (SED)
modelling software, CIGALE, to find AGNs in mid-IR. We found 126 AGNs in the NEP-Wide field
with this method. We also investigate the energy released from the AGN as a fraction of the total
IR luminosity of a galaxy. We found that the AGN contribution is larger at higher redshifts for a
given IR luminosity. With the upcoming deep IR surveys, e.g., JW ST, we expect to find more AGNs
with our method.
Key words: galaxies: active – infrared: galaxies
1 INTRODUCTION
An active galactic nucleus (AGN) is a compact ultra-
luminous region at the centre of a galaxy. It is widely be-
lieved that a supermassive black hole (SMBH) resides in
every AGN, and the mass of a black hole is related to the
bulge mass of a galaxy (e.g., Kormendy & Ho 2013). In or-
der to study galaxy evolution, it is important to find AGNs
in the Universe, as the prescence of an AGN has a nonneg-
ligible impact on the main physical parameters of galaxies,
such as star formation rate (SFR), etc. There are several
ways to search for AGNs, e.g., based on optical, UV and
soft X-ray observations (e.g., Hickox & Alexander 2018).
However, AGNs may be obscured by gas and dust. The ob-
scured AGNs could be missed by these observations. Also,
Compton-thick AGNs (CTAGNs) are highly obscured, even
at hard X-rays with E
>
∼
10 keV. To avoid missing dust-
obscured AGNs in the Universe, mid-infrared (mid-IR) sur-
vey of AGNs is crucial.
Mid-IR surveys are sensitive to AGNs due to the warm
dust emission of AGNs. In general, AGNs show thermal
emission from warm dust at mid-IR wavelengths, which is
heated by strong radiation from the central accretion disks.
However, in the mid-IR wavelengths such as 3.3, 6.2, 7.7,
8.6, and 11.3 µm, star-forming galaxies (SFGs) also pro-
duce strong polycyclic aromatic hydrocarbon (PAH) emis-
sion (e.g., Feltre et al. 2013; Ohyama et al. 2018; Kim et al.
2019). Therefore, in order to find dust-obscured AGNs using
mid-IR surveys, we need to carefully examine the spectral
features in mid-IR wavelengths.
Many works have been conducted to search, identify and
characterise AGNs by using the data from Spitzer infrared
telescope and Wide field Infrared Survey Explorer (W ISE)
(e.g., Hwang et al. 2012; Toba et al. 2015; Toba & Nagao
2016). However, both W ISE and Spitz er have limited avail-
able filters (3.4, 4.6, 12 and 22 µm in WISE and 3.6, 4.5, 5.8,
8.0 and 24 µm in Spitzer) and there are wavelength gaps
within the mid-IR wavelengths such as 10-20 µm, making
?
E-mail: tinattw0127@gapp.nthu.edu.tw
SFG-AGN diagnostics difficult. If the feature of a source
falls in the gap of the filters in W ISE or Spit zer, we may fail
to classify SFG and AGN correctly.
To avoid this type of failure, the best way is to
use an instrument with continuous filter system covering
these mid-IR gaps, such as the Infrared Camera (IRC,
Onaka et al. (2007)) installed in AK ARI space telescope.
AK ARI (Murakami et al. 2007), an IR space telescope
launched by ISAS/JAXA in 2006, successfully carried out
an all-sky survey at IR wavelengths. The North Ecliptic Pole
(NEP) survey (Matsuhara et al. 2006) was one of the dedi-
cated photometry surveys with the AK ARI’s pointing obser-
vations. AK ARI observed the sky region with its 9 continu-
ous passbands, from near- to mid-IR wavelengths. Detailed
information on AK ARI is in Section 2.
The combination of the AK ARI IR photometry and X-
ray data gives a strong tool to find Compton-thick AGNs
(Krumpe et al. 2015; Miyaji & Team 2018). Huang et al.
(2017) performed spectral energy distribution (SED) fitting
to select AGNs from data of the AK ARI NEP deep survey
(Matsuhara et al. 2006) covering a field of 0.57 deg
2
, con-
taining ∼ 5800 sources. By combining AK ARI’s 9 passbands,
W I SE ALLWISE 1, 2, 3, 4 and Spitzer Infrared Array Cam-
era (IRAC) 1, 2, 3, 4 and Multiband Imaging Photometer
for Spit zer (MIPS) 1 passbands, they fit 25 empirical models
by using an SED template fitting code, Le Phare (PHoto-
metric Analysis for Redshift Estimate; Arnouts et al. 1999;
Ilbert et al. 2006). Because of the continuous mid-IR cov-
erage of AK ARI, they recovered more X-ray selected AGNs
(Krumpe et al. 2015) than previous works (Lacy et al. 2007)
based on IR colour-colour diagrams by ∼20 percent.
Chiang et al. (2019) also used Le Phare to select AGNs
by taking advantage of the unique AK ARI NEP wide field
survey (NEP-Wide; Matsuhara et al. 2006; Lee et al. 2009;
Kim et al. 2012) sample with 18 IR bands of data, includ-
ing AK A RI’s 9 passbands, W ISE 1-4, Spitzer IRAC 1-4 and
MIPS 1 photometry. Their results indicate that AGN num-
ber fraction seems to show stronger IR luminosity depen-
dence than redshift dependence. They also examined the
fractions of SFGs and found mild decreasing trends at high
IR luminosities.
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ORIGINAL UNEDITED MANUSCRIPT
AGN properties by CIGALE SED fitting 3
Recently, Subaru Hyper Suprime-Cam (HSC;
Miyazaki et al. 2018) observations with g, r, i, z, and
Y bands were carried out to cover the entire field of the
AK ARI NEP Wide field (covering 5.4 deg
2
) (Goto et al.
2017). The new HSC observation allowed us to extend
AK ARI sources with photometric redshift to the full cover-
age of the NEP Wide field (Ho et al. 2020). Furthermore,
the advanced SED fitting code CIGALE
1
(Code Investigat-
ing GALaxy Emission; Burgarella et al. 2005; Noll et al.
2009; Boquien et al. 2019) can not only classify AGNs but
also obtain physical properties of our targets.
With the new HSC photometry and the state-of-the-art
SED-fitting software, CIGALE, our goal is to recover more
dust-obscured AGNs in the AK ARI NEP-Wide survey and
investigate the physical properties of the sources.
This work is organised as follows. We describe our sam-
ple selection and method in Section 2. The result of our AGN
selection and the physical properties of AGNs are described
in Section 3. Our discussion is shown in Section 4. Our con-
clusions are in Section 5. Throughout this paper, we use AB
magnitude system unless otherwise mentioned. We assume
a cosmology with H
0
= 70.4 km s
−1
Mpc
−1
, Ω
Λ
= 0.728, and
Ω
M
= 0.272 (Komatsu et al. 2011).
2 DATA AND ANALYSIS
2.1 Data
We selected samples from the AK ARI NEP Wide survey. The
NEP Wide survey was centered at (RA, Dec)=(18h00m00s,
+66
◦
33
0
38
00
), covering 5.4 deg
2
with the AK ARI Infrared
Camera (IRC). The AK ARI IRC has 3 channels: NIR, MIR-
S and MIR-L. There are 3 filters in each channel, so the IRC
has 9 filters in total: N2, N3, N4, S7, S9W, S11, L15, L18W,
and L24, corresponding to 2, 3, 4, 7, 9, 11, 15, 18, and 24 µm
of the reference wavelengths, respectively. Kim et al. (2012)
presented a photometric catalogue of IR sources from the
NEP-Wide survey using the nine photometric filters of the
IRC. In the near-IR bands, the N2 filter reaches a depth
of ∼20.9 mag, and the N3 and N4 bands reach ∼21.1 mag.
The mid-IR detection limits are much shallower than those
of near-IR bands: ∼19.5 (S7), 19.3 (S9W), and 19.0 (S11)
for the MIR-S bands, and ∼18.6, 18.7, and 17.8, correspond-
ing to L15, L18W and L24 for the MIR-L bands, respectively.
Recently, Oi et al. (2020, submitted) cross-matched the Sub-
aru Hyper Suprime-Cam (HSC) sources with the AK ARI
sources in the NEP Wide field and constructed an HSC-
AK ARI catalogue (see also Toba et al. (2020b) for AK A RI
sources without HSC counterparts). The HSC optical imag-
ing observations were carried out in five broad bands (g , r,
i, z, and Y) covering an almost entire field of the NEP-Wide
survey (∼5.4 deg
2
). This catalogue enables us to calculate
the photometric redshifts of the AK ARI sources.
In order to optimise the advantage of AK ARI pho-
tometry, Kim et al. (2020) constructed a multiwavelength
catalogue based on AK ARI data. They used the “match-
ing radii” determined by astrometry offset tests for each
matching procedure. The multiwavelength catalogue con-
tains UV to submillimetre counterparts of the AK ARI
1
https://cigale.lam.fr
18.2 18.0 17.8
R.A. (J2000)
65.0
65.5
66.0
66.5
67.0
67.5
68.0
Dec (J2000)
Figure 1. An overall map showing a variety of surveys around
the NEP. The red circular area shows the AK ARI’s NEP-Wide
field (Kim et al. 2012). The four blue circles represent the r-band
coverage of the Subaru/HSC survey (Oi et al. 2020, submitted).
The gray dashed line represents the optical surveys done with the
the Maidanak SNUCAM (B, R, I) (Jeon et al. 2010). The green
dashed rectangle represents the CFHT/MegaCam (u
∗
, g, r , i, z)
survey (Hwang et al. 2007). The yellow square shows an observa-
tion with MegaCam and WIRCam (Y, J, K
s
) on the NEP-Deep
field (Oi et al. 2014). The orange dotted shape indicates the H
band survey with KPNO/FLAMINGOS (Jeon et al. 2014). The
black dashed square inside the yellow box shows the area observed
by the Her schel/PACS (Pearson et al. 2019). The largest dark-
red rhombus shows the Herschel/SPIRE coverage (Pearson et
al. in prep). The g, i, z and y band observations by the Subaru
HSC (Oi et al. 2020, submitted) is not shown in this figure. The
u-band observation by the MegaPrime/MegaCam (Huang et al.
2020) is not shown in this figure.
sources in the NEP-Wide field. In this catalogue, there
are 91,861 sources in total. Among them, there are 2,026
sources with spectroscopic redshifts. Spectroscopic red-
shifts are collected from various published and unpub-
lished sources as detailed below. The spectroscopic data
are provided by several spectroscopic observations with
various telescopes/instruments in optical, Keck/DEIMOS
(Shogaki et al. 2018; Kim et al. 2018), MMT/Hectospec
(Shim et al. 2013), WIYN/Hydra (Shim et al. 2013), GTC
(Miyaji et al. in prep.; D´ıaz Tello et al. 2017; Krumpe et al.
2015), and in near-IR, Subaru/FMOS (Oi et al. 2017). For
the sources without spectroscopic redshifts, photometric
reshifts of our sample are calculated by using Le Phare
(Ho et al. 2020). Also, Kim et al. (2020) obtained the X-
ray catalogue from Chandra NEP deep survey and the
spectra of 1,796 sources were obtained from (Shim et al.
2013). Shim et al. (2013) identified 1,128 star-forming or
absorption-line-dominated galaxies, 198 Type-1 AGNs, 8
Type-2 AGNs, 121 Galactic stars, and 190 spectra of un-
known sources due to low signal-to-noise ratio. Most of the
sources with spectroscopic redshifts are already classified by
Shim et al. (2013) and Krumpe et al. (2015).
There are 42 bands in total in the multiwavelength cat-
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ORIGINAL UNEDITED MANUSCRIPT
4 Ting-Wen Wang et al.
36 bands from the band-merged catalogue (Kim et al. submitted,
91861 sources in total) + selection criteria:
(1) Spec-z >0 or Photo-z >0
(2) Detection at AKARI L18W (5𝛔)
(3) Detection at Herschel SPIRE 250 𝛍m or PACS 100 𝛍m
→446 spec-z sources & 1397 photo-z sources
reduced
𝛘
2
<10
→404 spec-z sources (spec-z sample)
& 1267 photo-z sources (photo-z sample)
CIGALE SED fitting
Combine spec-z & photo-z sample
1671 all-z sample
Figure 2. Flowchart of the sample selection process.
alogue (Kim et al. 2020). We used 36 bands from the cat-
alogue in this work, including data in the UV band from
CFHT MegaCam (u
∗
) and CFHT MegaPrime (u), 5 optical
bands from Subaru HSC (g, r, i, z, Y ), 3 bands from Maid-
anak SNUCAM (B, R, I), 1 band from FLAMINGOS (H),
9 bands from AK ARI IRC (2.4, 3.2, 4.1, 7, 9, 11, 15, 18, and
24 µm), 3 bands from CFHT WIRCam (Y , J, K
s
), 4 mid-IR
bands from W ISE ALLWISE 1-4 (3.4, 4.6, 12 and 22µm),
4 mid-IR bands from Spitzer IRAC 1-4 (3.6, 4.5, 5.8 and
8.0µm), 2 bands from Her schel PACS (100 and 160 µm),
and 3 bands from Her schel SPIRE (250, 350 and 500 µm).
An overall map showing a variety of surveys around the NEP
is shown in Fig. 1. Information on the filters is given below.
Aside from the AK ARI IRC observational data, we also use
other near to mid-IR data in this work.
The H-band catalogue (Jeon et al. 2014) of the NEP
region is obtained from the Florida Multi-object Imaging
Near-IR Grism Observational Spectrometer (FLAMINGOS;
Elston et al. 2006) on the Kitt Peak National Observatory
(KPNO). KPNO has a 2.1 m telescope covering a 5.1 deg
2
area down to a 5σ depth of 21.3 mag for the H-bands with
an astrometric accuracy of 0.14 and 0.17 for 1σ in the R.A.
and Dec. directions, respectively.
We obtain the W ISE 4-band data from Jarrett et al.
(2011). W ISE is a NASA-funded Medium-Class Explorer
mission, consisting of a 40-cm space-based infrared telescope
whose science payload consists of mega-pixel cameras, cooled
with a two-stage solid hydrogen cryostat. W ISE mapped the
entire sky at 3.4µm (W1), 4.6µm (W2), 12µm (W3), and 22
µm (W4) with 5σ depth of 18.1, 17.2, 18.4, and 16.1 mag,
respectively.
The CFHT WIRCam data are obtained from Oi et al.
(2014). WIRCam uses four 2048 × 2048 HAWAII2RG CCD
arrays. The 5σ limiting magnitudes are 23.4, 23.0, and 22.7
for WIRCam Y, J, and K
s
-bands, respectively.
The Spit zer two-band catalogue of the NEP field IRAC1
(3.6 µm) and IRAC2 (4.5 µm) is presented by Nayyeri et al.
(2018). The observations covered 7 deg
2
, with the 5σ depths
of 21.8 and 22.4 mag in the IRAC1 and IRAC2 bands, re-
spectively. The IRAC3(5.8 µm) and IRAC4(8 µm) data are
obtained from Jarrett et al. (2011). The observations cov-
ered 0.40 deg
2
, with the 5σ depths of 16.6 and 15.4 mag in
the IRAC3 and IRAC4 bands, respectively.
We used the data from Subaru HSC (Goto et al. 2017,
Oi et al. 2020, submitted). The deep HSC g, r, i, z and Y
imaging of the AK ARI NEP wide field can provide accurate
photometric redshift of the AK ARI sources. The 5σ detec-
tion depths of HSC (over a 1 arcsec aperture) at g, r, i, z,
and Y -bands are 28.6, 27.3, 26.7, 26.0, and 25.6 mag, respec-
tively.
The u
∗
imaging data of Canada-France-Hawaii Tele-
scope (CFHT; Iye & Moorwood 2003) MegaCam is col-
lected from Hwang et al. (2007) and Oi et al. (2014). The
data of u imaging data of CFHT MegaPrime is obtained
from Goto et al. (2019) and Huang et al. (2020).
We also obtained data from Maidanak’s Seoul National
University 4K × 4K Camera (SNUCAM; Jeon et al. 2010).
The SNUCAM contains B, R, and I imaging, where the B,
R, and I- band data reach the depths of ∼23.4, ∼23.1, and ∼
22.3 mag at 5σ, respectively.
We also used far-IR data from Her schel/PACS
(Pearson et al. 2019) and Herschel/SPIRE (Pearson et al.
in prep) observations at the NEP region. The PACS obser-
vation covers ∼0.44 deg
2
overlapping with the NEP region,
which contains the 100 µm and 160 µm imaging data. SPIRE
covers 9 deg
2
of the NEP, containing 250 µm, 350 µm and
500 µm imaging data.
2.2 SED fitting using CIGALE
We used CIGALE version 2018.0. to perform SED fitting
and model the optical to far-IR emission of each source.
The model is based on the energy balance principle: the
UV-to-optical energy absorbed by dust is self-consistently
re-emitted in the mid- to far-IR. CIGALE allows many pa-
rameters to be varied in order to fit an observed SED, such
as star formation history (SFH), single stellar population
(SSP), attenuation law, AGN emission, and dust thermal
emission.
We assumed a SFH delayed with optional exponential
burst. We fixed e-folding times of the main stellar pop-
ulation (τ
main
) and the late starburst population (τ
burst
),
while the age of the main stellar population in the galaxy
is parameterised. We adopted the stellar templates from
Bruzual & Charlot (2003), by assuming the initial mass
function (IMF) introduced by Salpeter (1955), and the
standard default nebular emission model in CIGALE (see
also Inoue 2011). Dust attenuation is modelled follow-
ing Charlot & Fall (2000) with additional flexibility. The
Charlot & Fall (2000) attenuation law recipe is described
by two power laws: one for birth cloud (BC), and the other
for the interstellar medium (ISM). According to the original
law, both power law slopes are equal to −0.7. In our anal-
ysis, we introduce slopes flexibility (then the law becomes
more similar to the Double Power Law, see for example
Lo Faro et al. (2017) and Buat et al. (2018). We also sep-
arately parameterised the V-band attenuation in the ISM
(A
ISM
V
). We do not introduce any flexibility for the µ param-
eter which is defined as the ratio of the attenuation in the
V-band experienced by old and young stars. While earlier
works on nearby starburst galaxies obtained µ = 0.3 (e.g.
Charlot & Fall 2000), recent works found slightly higher
values of µ = 0.44 (e.g. Ma lek et al. 2018) and µ = 0.5
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