Accepted for Publication in ApJS
An Optical Source Catalog of the North Ecliptic Pole Region1
Narae Hwang2,11, Myung Gyoon Lee2, Hyung Mok Lee2, Myungshin Im2, Taehyun Kim2,
Hideo Matsuhara3, Takehiko Wada3, Shinki Oyabu3, Soojong Pak4, Moo-Young Chun5,
Hidenori Watarai6, Takao Nakagawa3, Chris Pearson3,7, Toshinobu Takagi3,
Hitoshi Hanami8, and Glenn J. White9,10
ABSTRACT
We present a five (u∗,g′,r′,i′,z′) band optical photometry catalog of the sources
in the North Ecliptic Pole (NEP) region based on deep observations made with
MegaCam at CFHT. The source catalog covers about 2 square degree area cen-
tered at the NEP and reaches depths of about 26 mag for u∗, g′, r′ bands, about
25 mag for i′ band, and about 24 mag for z′ band ( 4 σ detection over an 1
′′aperture). The total number of cataloged sources brighter than r′ = 23 mag
1Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and
CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Re-
search Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de
la Recherche Scientifique (CNRS) of France, and the University of Hawaii.
2Astronomy Program, Department of Physic and Astronomy, Seoul National University, Seoul 151-747,
Korea
3Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kana-
gawa 229-8510, Japan
4Department of Astronomy and Space Science, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701,
Korea
5Korea Astronomy and Space Science Institute, 61-1, Hwaam-Dong, Yuseong-Gu, Daejeon 305-348, Korea
6Office of Space Applications, Japan Aerospace Exploration Agency, Tsukuba, Ibaraki 305-8505, Japan
7ISO Data Centre, ESA, Villafranca del Castillo, Madrid, Spain
8Iwate University, 3-18-8 Ueda, Morioka 020-8550, Japan
9Astrophysics Group, Department of Physics, The Open University, Milton Keynes, MK7 6AA, UK
10Space Science & Technology Department, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot,
Oxfordshire, OX11 0QX, UK
11e-mail: nhwang@astro.snu.ac.kr
http://arxiv.org/abs/0704.1182v1
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is about 56,000 including both point sources and extended sources. From the
investigation of photometric properties using the color-magnitude diagrams and
color-color diagrams, we have found that the colors of extended sources are mostly
(u∗ − r′) < 3.0 and (g′ − z′) > 0.5. This can be used to separate the extended
sources from the point sources reliably, even for the faint source domain where
typical morphological classification schemes hardly work efficiently. We have de-
rived an empirical color-redshift relation of the red sequence galaxies using the
Sloan Digital Sky Survey data. By applying this relation to our photometry
catalog and searching for any spatial overdensities, we have found two galaxy
clusters and one nearby galaxy group.
Subject headings: galaxies: general — galaxies: photometry — galaxies: clusters
— catalogs
1. Introduction
The North Ecliptic Pole (NEP) is an undistinguished region in the sky, located at
α = 18h00m00s, δ = +66◦33′38′′. It is, however, a very special region since many astronomical
satellites have accumulated a large number of exposures over this location since Earth-
orbiting satellites must point their fixed solar panels to the Sun and be in their orbits over
the ecliptic poles. Unlike the South Ecliptic Pole (SEP) region where the South Atlantic
Anomaly and the LMC prevent the clear view of the extragalactic sky at certain wavelengths,
the NEP region suffers very little or no obscuration by foreground Galactic sources. The
galactic coordinates of the NEP are l ≈ 96.4◦, b ≈ +29.8◦ and the foreground extinction
in this direction is E(B − V ) = 0.047 (Schlegel et al. 1998). Therefore, the NEP is a
good target for deep, unbiased, contiguous surveys for extragalactic objects such as galaxies,
galaxy clusters, and AGNs.
The ROSAT All-Sky Survey (RASS) is one of the most representative surveys of the
NEP region (Voges et al. 1999). The ROSAT was the first X-ray imaging satellite to survey
the entire sky. A source catalog of X-ray sources that were extracted from a large area
surrounding the NEP was constructed from this RASS data (Henry et al. 2001, 2006). From
the followup observations and optical counterpart investigations, Gioia et al. (2003) reported
that most X-ray sources in the NEP region are AGNs (∼ 49%), stars (∼ 34%), and galaxy
groups/clusters (∼ 14%). Mullis et al. (2001) also found a supercluster of galaxies in the
NEP region using the RASS data and suggested that some galaxy clusters in this region are
part of the supercluster at z = 0.087. The NEP region was also observed in the 1.5 GHz
band by Kollgaard et al. (1994). Using this radio source catalog, Brinkmann et al. (1999)
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investigated the correlation between the radio sources and the RASS X-ray sources, and
identified optical counterparts of the radio/X-ray sources. They found that a significant
number of radio loud sources are also bright in the X-ray band, and that X-ray selection is
an effective way for the search of galaxy clusters and groups.
While the X-ray luminosity is an efficient measure of the hot ionized gas in the galaxy
clusters, the star formation activities and the resultant stellar mass in galaxies can be esti-
mated from the optical and infrared flux. Recently, a new infrared space telescope, named
AKARI, was launched in February 2006. AKARI is expected to give exceedingly higher qual-
ity data than the previous infrared space missions such as Infrared Astronomical Satellite
(Neugebauer et al. 1984) and Infrared Space Observatory (Kessler et al. 1996). The AKARI
data will also complement the successful Spitzer Space Telescope (Werner et al. 2004) data
by providing better wavelength coverage over the 8−24µm band, which is not available from
the Spitzer. As one of its major science programs, AKARI is currently carrying out deep
near-to-mid infrared surveys over the wide area around the NEP through two major survey
programs, ‘NEP-Deep’ and ‘NEP-Wide.’ The key scientific objectives of these NEP surveys
are to unveil the dusty star formation history of the Universe, the mass assembly and large
scale structure evolution, and the nature of the cosmic infrared background (CIRB). For
further information on these NEP Programs, please refer to Matsuhara et al. (2006).
Combined with these space-borne infrared surveys, the optical survey of the NEP re-
gion constitutes the multifrequency dataset that is essential for the studies of cosmic star
formation history. Many researchers have investigated optical counterparts of radio and X-
ray sources in this region, but, for the imaging data, they mostly relied on the COSMOS
scan (e.g., Brinkmann et al. 1999) or Digitized Sky Survey images of the second Palomar
Observatory Sky Survey (POSS-II) plates, only complemented by some independent but
patchy observations (e.g., Gioia et al. 2003). Although the POSS-II data are excellent in
complete coverage of the whole sky, the usefulness of the data are still limited by the low
resolution with ∼ 1.0′′ per pixel scale and the shallow depth with the limiting magnitude of
rF ∼ 21.0 mag (Gal et al. 2004). In this study, therefore, we provide the first optical catalog
of sources in the NEP region based on deep observations in u∗, g′, r′, i′, z′ bands obtained
with MegaCam at CFHT. This optical catalog will be an important part of multifrequency
datasets of galactic and extragalactic sources in the NEP region. It will also serve as a basis
of the AKARI NEP survey mission by providing the information on the optical counterparts
of the infrared sources. This paper is composed as follows: We describe our observations in
Section 2 and data reduction procedures in Section 3. We present a bright source catalog
in Section 4.1, photometric properties of the sources in the NEP region in Section 4.2, and
galaxy number counts in Section 4.3. The result of galaxy cluster search using the color of
red sequence galaxies is given in Sections 4.4 and 4.5. Some photometric properties of other
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X-ray sources are presented in Section 4.6 and our primary results are summarized in the
last section.
2. Observation
The observation of the North Ecliptic Pole (NEP) region was carried out over nine photo-
metric nights between August 22 and September 22, 2004 using the 3.6m CFHT telescope lo-
cated at Mauna Kea, Hawaii. We used a wide field imager MegaCam at the telescope primary
focus MegaPrime. The MegaCam is composed of 36 2048 × 4612 CCD’s covering about 1◦×
1◦ area with 0.185 arc seconds resolution per pixel (Boulade et al. 2003). The u∗, g′, r′, i′, z′
filter system provided with MegaCam is basically the same as that used by the Sloan Digital
Sky Survey (SDSS) (York et al. 2000) except for the u∗ filter which is designed to take advan-
tage of the improved UV extinction condition at the CFHT site. Therefore, the photometric
data generated from this observation are presented in the CFHT unique u∗ and the SDSS
g′, r′, i′, z′ system. For further information on these filter systems used with MegaCam, see
the ‘Filter set’ section in http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/specsinformation.html
page.
As shown in Figure 1, the observed fields are composed of two fields separated by about
1◦ away from each other in the East-West direction, covering about two square degrees in
total. The NEP-E (East) field was observed with five filters, u∗, g′, r′, i′, z′ and the other
field, NEP-W (West), was observed with only four filters, g′, r′, i′, z′. Each field was covered
twice with a 15′′ offset for dithering in each filter with total exposure times of 3600 sec for
u∗ (1800 × 2), 2400 sec for g′ (1200 × 2), 3000 sec for r′ (1500 × 2), 3000 sec for i′ (1500 ×
2), and 3600 sec for z′ (1800 × 2). The raw images were processed using Elixir system by
CFHT staff. The Elixir is a collection of programs, databases, and other tools specialized
in processing and evaluation of the large mosaic data (Magnier & Cuillandre 2004). The
overall quality of the preprocessed images is good with a typical seeing of 0.7 ∼ 1.1′′. Detailed
information on the observation is listed in Table 1.
3. Data Reduction
3.1. Pre-Photometry Processing
For the reliable source detection and photometry, it is essential to prepare a master
image that satisfies the following two factors: (1) to obtain the required photometric depth
http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/specsinformation.html
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and (2) to give the complete areal coverage. We used a software package SWarp1 written
by E. Bertin at Terapix to transform each Multi Extension Fits (MEF) file containing 36
individual CCD frames into a single fits image data. SWarp also corrects different distortion
effects between input data through the resampling process, and then combines the resampled
images to create a deep output image after flux scaling and background subtraction.
To make a master image, we combined the g′, r′, i′, z′ four band data using SWarp for the
NEP-E and the NEP-W field respectively. During the SWarp run, BILINEAR resampling
method was used since this method was found to be effective in suppressing the discontinuity
effects around the chip boundaries, while other methods such as LANCZOS3 produced more
noticeable discontinuities. We also used weighted images during SWarp run to enhance the
quality of the output image. In the weight maps we assigned zero weights to pixels with
negative values. We excluded the u∗ band data from the master image construction since we
do not have the u∗ band data for the NEP-W and the S/N is relatively lower compared to
other bands. Finally, we created two master detection images with high S/N’s, one for the
NEP-E and the other for the NEP-W region.
Two MEF images for each band were processed using SWarp with the same parame-
ters to generate a final photometry image for the corresponding filter. These u∗, g′, r′, i′, z′
band images were made to have the same dimension and coordinates with the master de-
tection image of the corresponding field. The depth of photometry attained by combin-
ing two raw images was calculated as 4σ flux over a circular aperture with 1 ′′ diameter.
The measured limiting magnitudes are u∗ ∼ 26.0, g′ ∼ 26.1, r′ ∼ 25.6, i′ ∼ 24.7, and
z′ ∼ 23.7 mag, as listed in Table 1. After all these processings, two master detection im-
ages and nine photometry images were prepared for the source detection and photometry.
All the information on the reduced photometry images will be available from an web site
http://astro.snu.ac.kr/˜ nhwang/index.files/nep.html.
3.2. Source Detection & Photometry
We have used SExtractor (Bertin & Arnouts 1996) to detect sources from the master
detection images of the NEP-E and NEP-W fields. A source is confirmed if it has more than
five contiguous pixels above four times the background sky fluctuation. The signals from each
source were measured in the u∗, g′, r′, i′, z′ band photometry images over the isophotal area
previously defined during the source detection. The photometry was made using SExtractor
in dual mode operation. This scheme enables the detection and photometry of any source
1See http://terapix.iap.fr/rubrique.php?id rubrique=49 for further information
http://astro.snu.ac.kr/~~nhwang/index.files/nep.html
http://terapix.iap.fr/rubrique.php?id_rubrique=49
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that is registered at least once in any of the g′, r′, i′, z′ band images. The total number of
sources detected in our two observed NEP fields is about 130,000.
The instrumental magnitudes were transformed into standard magnitudes using the
transformation information provided and recorded in the header of the images by the CFHT
staff. During this transformation, only the calibrated magnitudes of sources that have avail-
able color information required for the calibration are calculated and kept in the catalog.
Otherwise, we assigned dummy values (99.000) to the magnitudes of sources in the final
source catalog.
4. Results
Figure 2 displays a source count histogram in the u∗, g′, r′, i′, z′ band along with the r′
band error and stellarity distributions. This shows that the number of sources in our data
increases up to u∗ ∼ 24.3, g′ ∼ 24.0, r′ ∼ 23.5, i′ ∼ 23.0, and z′ ∼ 22.2 mag before the
incompleteness effect starts to take place and the number of the detected sources starts to
decrease. The r′ band magnitude error is estimated to be about 0.1 mag or less at r′ ∼ 23.5
mag where the source count reaches its maximum. The stellarity distribution shows how
efficiently the star/galaxy classifier works in our dataset. From this distribution, it is clearly
seen that the stellarity index, which is calculated by SExtractor based on the isophotal areas,
peak intensity, and seeing information, separates the point sources (stellarity ∼ 1) and the
extended sources (stellarity ∼ 0) with high confidence for the sources with r′ < 22 mag. One
more point to be noted in this stellarity distribution is that sources with r′ < 16.5 mag and
stellarity > 0.7 are the results of the image saturation. Thus the stellarity index is most
relable in the magnitude range of 17 < r′ < 22 mag.
4.1. Source Catalog
Considering the source count and the stellarity distribution, we have decided the mag-
nitude range of the most reliable sources for the final bright source catalog entry, which is
r′ ≤ 23 mag. The number of sources compiled in the final bright source catalog is about
56,000. Table 2 lists a sample of the bright source catalog for reader’s guide and the full
source catalog will be available electronically from the online Journal and from an web site
http://astro.snu.ac.kr/˜ nhwang/index.files/nep.html. Followings are the short descriptions
of data columns in the catalog.
Column 1 is the identification number of an optical source.
http://astro.snu.ac.kr/~~nhwang/index.files/nep.html
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Column 2 & 3 list, respectively, the J2000.0 right ascension (RA) and declination (DEC)
of a source in degrees. The uncertainties of the astrometric solution derived by Elixir are
about ±0.5′′.
Column 4 ∼ 13 are the AB magnitudes and magnitude errors of a source in the
u∗, g′, r′, i′, z′ bands. These are given by MAG AUTO and MAGERR AUTO parameters
of SExtractor, which are Kron-like elliptical aperture magnitudes and their errors.
Column 14 is the extraction flag of a source given as a sum of powers of 2 by SExtractor.
If a source has neighbors (flag = 1) and is blended with another source (flag = 2) and some
pixels are saturated (flag = 4), then the final extraction flag given to the source is 7 (= 1
+ 4 + 2). Generally, a source with the extraction flag 0 gives the most reliable photometry.
For detailed information on this flag, see the SExtractor User’s Manual.
Column 15 is the stellarity index of a source calculated from a g′, r′, i′, z′ combined
detection image. This index has a value between 1 (point sources) and 0 (extended sources).
See Figure 2 for the distribution of this index.
Column 16 is the ellipticity of a source calculated by SExtractor using the second order
image moments.
Column 17 is the FWHM in arcseconds of a source calculated under the assumption
that the source has a Gaussian profile.
Column 18 is the effective radius or a half-light radius of a source in arcseconds. This
value is computed by setting the input parameter PHOT FLUXFRAC = 0.5 and is given
by the output parameter FLUX RADIUS.
Column 19 is the semi-major axis of a source in arcseconds.
Column 20 is the detected isophotal area of a source. This parameter may be used as a
measure of the object’s size for the case of extended sources.
Column 21 is the field number that the photometric data of a source comes from. The
field number 1 represents the NEP-E field and 2 represents the NEP-W field.
4.2. Color-Magnitude and Color-Color Diagrams
We have investigated the photometric properties of the NEP sources using several color-
magnitude and color-color diagrams. In each diagram, we use the stellarity index given by
SExtractor to distinguish between point sources and extended sources: stellarity > 0.95
for point sources (mostly stars) and stellarity < 0.2 for extended sources (mostly galaxies).
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They are found to be statistically very useful tools to separate stars and galaxies brighter
than r = 23 mag from all kinds of sources in the catalog.
The r′ vs (g′ − r′) color-magnitude diagram (CMD) and (g′ − r′) color histogram in
Figure 3 shows two prominent vertical sequences of point sources, i.e., stars at (g′ − r′) ∼
0.3 (G dwarf stars) and 1.4 (M giant stars), respectively. The extended sources, presumably
galaxies, are seen to be concentrated around the peak at (g′ − r′) ∼ 0.8 that corresponds
to the (g′ − r′) color of early type galaxies (Fukugita et al. 1995). The extended sources
start to dominate at r′ ≈ 22 mag and fainter. Some of these sources may be faint stars
that the SExtractor failed to classify as point sources. However, the different (g′ − r′) color
histograms of the extended and the point sources suggest that the majority of these faint
extended sources are galaxies.
Figure 4 displays the r′ vs (u∗− r′) CMD and (u∗− r′) color histogram of sources in the
NEP-E field. The CMD shows that there are many galaxies distributed around (u∗ − r′) ∼
1.0 as shown in the (u∗ − r′) color histogram of extended sources. This value is consistent
with the (u∗−r′) color of Scd type galaxies (Fukugita et al. 1995), whereas the (g′−r′) color
of most galaxies in Figure 3 is that of early type galaxies. It is also noted that there is a
very long redward tail in the (u∗− r′) color distribution of extended sources, reaching nearly
(u∗−r′) ≈ 4 or higher. The elliptical galaxies can be as red as (u∗−r′) ∼ 2.8 (Fukugita et al.
1995). Therefore, some of those faint and red sources with (u∗ − r′) > 3.0 could be distant
galaxies with redshift z ≥ 0.1.
Figures 5 through 7 show the characteristic distribution of point sources and extended
sources in three color-color diagrams: (r′ − i′) vs (g′ − r′), (i′ − z′) vs (r′ − i′), and (g′ − z′)
vs (u∗ − r′). The most prominent feature in these color-color diagrams (CCD’s) is very
distinguishable sequences of point sources, i.e. stars. In Figure 5, the stellar sequence in a
flipped ‘L’ shape has a very narrow width of about 0.2 dex in the (r′ − i′) vs (g′ − r′) color
space. This tight sequence is also well represented by a straight line in Figure 6. In these
diagrams, extended sources are found to populate in a relatively well constrained color space
centered at a certain color. It is still clear that we can not possibly separate extended sources
from point sources by simply constraining colors in the (r′− i′) vs (g′−r′) and the (i′−z′) vs
(r′ − i′) color-color space. However, Figure 7 shows that the (g′ − z′) vs (u∗ − r′) color-color
combination enables us to define the two exclusive spaces that are mostly populated by stars
and galaxies, respectively. The boundary of these regions can be drawn by combination
of three straight lines connecting (u∗ − r′, g′ − z′) = (−1.0, 0.5), (0.5, 0.5), (3.0, 1.7), and
(3.0, 5.0). This is a very useful tool for separating faint galaxies from stars to search for
distant galaxy clusters or to compute the 2D correlation function of galaxies.
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4.3. Galaxy Number Counts
We have investigated the galaxy number counts using the NEP optical source catalog. To
select galaxies efficiently from the photometry catalog, we adopted two different definitions of
galaxies: (1) ‘Galaxy I’ sources with stellarity < 0.2 and (2) ‘Galaxy II’ sources that belong
to the designated area where the extended sources appeared to occupy in the (u∗ − r′) vs
(g′ − z′) CCD as denoted by the dashed line in Figure 7. The results of number counts for
Galaxy I and Galaxy II sample, as shown in Figure 8, show a consistent rise from r′ = 17.5
mag up to about r′ = 23 mag and then a steep down turn due to the incompleteness at
r′ ≥ 23 mag. From the lower and mid panels of Figure 8, it is clear that Galaxy II sample
outnumbers Galaxy I sample in r′ > 23 mag. This difference between number counts is
mostly due to the deteriorating reliability of stellarity index in r′ ≥ 23 mag, as shown in the
upper panel of Figure 2. Therefore, the Galaxy I sample is likely to lose a large number of
faint galaxies compared to the Galaxy II sample.
In Figure 8, we also plotted the R-band galaxy number count of Kümmel & Wagner
(2001) for comparison after a simple transformation into r′ band. Kümmel & Wagner (2001)
derived the number count of extended sources by modeling the number count distribution
of point-like sources (star) and then subtracting the extrapolated model count of point-like
sources from the detected source count. On the other hand, we defined galaxies as a certain
part of all the detected sources that satisfy a given stellarity condition (Galaxy I) or a
given two-color criterion (Galaxy II). Nonetheless, it is clear from the plot that the number
count by Kümmel & Wagner (2001) is generally consistent with our result except for the
differences in the faint magnitude domain where the incompleteness becomes significant.
As shown in the mid panel of Figure 8, the logarithmic number counts defined as
Log(Count/0.5mag/deg2) also display very similar slopes over the range of r′ = 18 ∼ 22
mag. Simple linear square fits over that magnitude range returned the slope d(LogN)/dm =
0.387 for Galaxy I, 0.408 for Galaxy II, and 0.397 for Kümmel & Wagner (2001) data. The
errors of the fitted slopes are about ±0.010 for all cases. Therefore, the slopes of logarithmic
galaxy number counts are in good agreement with that of Kümmel & Wagner (2001) within
the errors.
However, there are some differences in several minor features between the logarithmic
profiles of galaxy number count. This is more clearly shown in the upper panel of Figure
8. It is apparent that there are two small dips in Galaxy II data: one at r′ ≈ 17.5 ∼ 18
mag and another smooth one at r′ ≈ 19 ∼ 20 mag. The first dip also appears to exist
in Kümmel & Wagner (2001) data but the second smooth and shallow dip could not be
identified from Kümmel & Wagner (2001) data in this plot. However, the magnitude of the
second dip is roughly coincident with the break point at R ≈ 19 ∼ 20 mag as shown in their
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Figure 4 2.
4.4. Galaxy Clusters
We have carried out a galaxy cluster search using our optical photometry catalog.
Among numerous galaxy cluster finding methods available, we adopted a simplified version
of the ‘C4 cluster finding algorithm’ by Miller et al. (2005) that utilizes a seven-dimensional
position and color space. In this study, we use only a three-dimensional color space con-
structed based on (g′− r′), (r′− i′), and (i′− z′) colors to select cluster member galaxies and
then we investigate the spatial distribution of the selected galaxies to find any overdensity
of these galaxies in small regions. Finally, the CMDs of any overdense region are consulted
to check whether the red sequence of galaxies is apparent before we identify the region as a
galaxy cluster.
This approach requires the definition of color ranges spanned by the cluster member
galaxies with various redshifts and richness classes before the actual application to the pho-
tometric data. We have used the Sloan Digital Sky Survey (SDSS) (York et al. 2000) data
for this purpose. The SDSS project provides a huge amount of multiband photometric and
spectroscopic data over a large sky area. This enables us to select cluster member galaxies
using the spectroscopy information and to reduce the possible contamination by field galax-
ies and stars. One more advantage of using the SDSS dataset to define color ranges occupied
by cluster member galaxies is that the SDSS filter system is the same as the MegaCam
filter system of CFHT except for the u∗ filter (see Section 2 for details). Therefore, we have
searched the SDSS database and retrieved photometric and spectroscopic data of nearby
galaxy clusters for the calibration of the red sequence colors in the SDSS filter system.
4.4.1. Nearby Galaxy Clusters in SDSS
We have searched the SDSS DR5 database (Adelman-McCarthy et al. 2007) and found
101 nearby Abell galaxy clusters whose u′, g′, r′, i′, z′ 3 photometric and spectroscopic data
2Figure 4 of Kümmel & Wagner (2001) is constructed using the galaxy number count made with 0.25
mag bin. But the number count data in a table published in the same paper, which we used for Figure 8, is
made with 0.5 mag bin. The use of 0.5 mag bin instead of 0.25 mag bin in the number count is considered
to cause the smoothing out the feature.
3Please note that the u′ filter is the Sloan system, which is different from the CFHT u∗ system. See
Section 2 for details.
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of member galaxies are available. Figure 9 displays the redshift distribution of these cluster
galaxies. The redshift of the selected galaxy clusters runs from 0 to 0.2 with a peak at
z ∼ 0.07. After the data retrieval from the SDSS data archive, we have selected member
galaxies of each cluster based on the velocity distribution and the spatial separation from the
cluster center. The total number of the selected member galaxies for the 101 clusters is about
5,700. From the cluster member galaxies, only the early type galaxies were selected using the
‘fracDev’ and ‘eclass’ parameters provided by the SDSS archive: fracDev > 0.8 and eclass
< 0 (for further information on these constraints, see Bernardi et al. 2005). Application
of these parameters finally returned about 1,700 early type galaxies in 88 clusters and the
number of early type galaxies in each cluster runs from 4 (A2192) to 73 (A2199) depending
on the redshift and the richness class.
Figure 10 displays the CMDs of the cluster galaxies in the SDSS color system. From
these diagrams, it is seen that most early type galaxies in the clusters lie within a well-
defined and narrow color range with a width of ≤ 0.2 dex in (g′ − r′)0, (r
′ − i′)0, and
(i′ − z′)0. Although the (u
′ − r′)0 vs r
0 CMD shows a rather large dispersion, it still shows a
strong concentration at 2.0 ≤ (u′ − r′)0 ≤ 3.0. The colors of these narrow sequences of early
type galaxies have been used as indicators of the clusters’ redshifts (Gladders & Yee 2000).
We used the SDSS photometry data of member galaxies in four clusters (A2199, A1166,
A1349, and A775) to derive the relation between the clusters’ redshift and the (g′−r′)0 color
of the sequence as shown in Figure 11. From the linear fit after repeating one sigma clipping
twice, we derived a linear and empirical relation between the redshift and the (g′− r′)0 color
of the fitted sequences at r′0 = 18 mag (hereafter (g
′ − r′)r18,0) as follows:
Redshift [z] = (0.415± 0.044)× (g′ − r′)r18,0 − (0.286± 0.039) (1)
Therefore, the galaxies in nearby clusters occupy a certain space in a multi-color parameter
space, which can be used to separate galaxies in the clusters from field galaxies and to
estimate the approximate redshift of the cluster using the (g′− r′)r18,0 color of the sequence.
4.4.2. Galaxy Cluster Search in the NEP Field
To find galaxy clusters based on the results shown in Figure 10 and to estimate the red-
shift using Equation 1, the Galactic foreground reddening was corrected for our photometry
data using Schlegel et al. (1998), assuming Ar′ ≈ 2.751E(B−V ), E(g
′−r′) ≈ 1.042E(B−V ),
E(r′− i′) ≈ 0.665E(B−V ), and E(i′−z′) ≈ 0.607E(B−V ). After some tests on the CFHT
data, we have defined color ranges for the cluster galaxies as follows: 0.6 < (g′ − r′)0 < 1.1,
0.1 < (r′ − i′)0 < 0.4, 0.0 < (i
− z′)0 < 0.4. We did not use (u
− r′)0 color for this color
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space definition because of the unavailability of u∗ band data for the NEP-W field and the
general low S/N in the u∗ band in the NEP-E field. According to a test performed after
adopting the range 2.0 < (u∗ − r′)0 < 3.0 as the fourth constraining color, the efficiency of
finding the cluster galaxies turned out to be comparable to that of another test run using the
three color combination of (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0. This three color combination
approach also enables a homogeneous search of cluster galaxies over our two data fields.
Possible cluster galaxies were selected by constraining the colors of galaxies with the
predefined parameters of (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0. Using their RA and Dec in-
formation, we constructed spatial number density maps of the selected galaxies. Then we
identified 13 possibly overdense regions in the NEP field. Over these 13 regions, CMDs
of galaxies in (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0 color were constructed using the source
catalog to find any feature that resembles the red sequence of galaxies in a cluster. From
this investigation, we have identified two galaxy clusters and one nearby galaxy group, and
estimated their redshifts using the (g′− r′)0 colors of the red sequences. The CMDs of those
galaxy clusters and the group are presented in Figures 12 − 14. More details about them
are discussed below.
4.5. Galaxy Clusters and Groups in X-ray/Radio Source Catalogs
Some galaxy clusters or groups were found and reported in the previous studies made
by using the X-ray and the radio band data of the NEP region. Henry et al. (1995) found
several X-ray-selected groups of galaxies in the NEP region based on RASS data and this
work was revised and extended further by Henry et al. (2006). Brinkmann et al. (1999) also
found that many X-ray sources in the NEP region have counterparts in the radio bands that
were observed with VLA. From the X-ray and radio source catalogs provided by Henry et al.
(2006) and Brinkmann et al. (1999), we have found a few galaxy clusters or groups in our
data field. Among these clusters and groups, we will discuss two galaxy clusters and a galaxy
group that were photometrically identified with our catalog.
4.5.1. NEPX1/VLA 1801.5+6645
This is the richest galaxy cluster found in our data field, which is easily seen in Figure 12.
In the upper panel of Figure 12, the CMDs of galaxies show very strong red sequences running
from r′0 ≈ 16 mag (magnitude of the third brightest galaxy) in three color domains: (g
−r′)0,
(r′ − i′)0, and (i
′ − z′)0. Those galaxies belonging to the red sequences are concentrated in a
– 13 –
very compact region, as shown in the lower panel of Figure 12. The redshift estimated from
the red sequence’s (g′ − r′)r18,0 color and Equation 1 is about 0.072 ± 0.037. This cluster
was previously discovered by Burg et al. (1992) from ROSAT survey data and named as
NEPX1. They estimated the redshift of the cluster to be about 0.09 from the spectroscopic
observations, which is in good agreement with our estimate.
This cluster is considered as a part of the large supercluster structure that has been
reported to exist in the NEP region (Hasinger et al. 1991; Mullis et al. 2001). It was also
detected in the radio band from the VLA observations and was suggested as a possible coun-
terpart of the X-ray source RXS J180137.7+664526 by Brinkmann et al. (1999). Although
Brinkmann et al. (1999) listed this source as a galaxy group, we have reached a conclusion
based on the red sequence galaxies in the CMDs that it is a galaxy cluster rather than a
galaxy group.
4.5.2. RX J1754.7+6623
Henry et al. (2006) classified this source as a galaxy cluster with redshift z = 0.0879.
However, the CMDs shown in Figure 13 suggest that there are two kinds of galaxies: (1)
several bright galaxies that form the red sequence running from r′0 ≈ 16 to 18 mag and (2)
many faint galaxies in the background. The second component of galaxies are not brighter
than r′0 = 19 mag, which is about 1 mag fainter than the faintest galaxy of the brighter
component. This indicates that those bright galaxies may happen to be located in front
of the faint background galaxies by chance. Therefore, we suggest that these galaxies are
members of a galaxy group rather than a galaxy cluster. The red sequence’s (g′ − r′)r18,0
color is about 0.793, corresponding to an estimated redshift of 0.043 ± 0.048. The relative
large error may be due to the poorly determined slope of red sequence depends sensitively
on several bright galaxies with r′0 < 18 mag.
4.5.3. RX J1757.9+6609
There is a known X-ray source RX J1757.9+6609 at the similar position. It is listed as
a type 2 AGN with redshift z = 0.4865 in the catalog of Henry et al. (2006). A type 2 AGN
is a narrow emission line (FWHM < 2000 km s−1) object, while a type 1 AGN is a broad
emission line (FWHM ≥ 2000 km s−1) object (Gioia et al. 2003). The spatial number density
plot (lower panel) of the red sequence galaxies in Figure 14 shows some clustering in the 2D
projected space. This point was also noted by Gal et al. (2003) who carried out a cluster
– 14 –
search using the digitized Second Palomar Observatory Sky Survey (DPOSS) data. They
detected an overdensity of galaxies at this location and listed it as a galaxy cluster candidate
under the name of NSC J175751+660924, and also estimated its redshift to be z ≈ 0.1663
from their photometric redshift analysis. Our photometric data shows well developed red
sequences running from r′0 ≈ 17.5 mag in (g
′− r′)0, (r
′− i′)0, and (i
′− z′)0 colors, confirming
that it is a genuine galaxy cluster. The red sequence’s (g′ − r′)r18,0 color is about 0.811 and
this leads to the estimated redshift of z = 0.043 ± 0.025. Therefore, there appears to be
a galaxy cluster with redshift z < 0.2 and a type 2 AGN with redshift z = 0.4875 in the
background of the same projected area.
4.5.4. Other Galaxy Clusters
We have identified two galaxy clusters and one galaxy group by applying the simple
color cut method to our photometry catalog. However, there are two more galaxy clusters
that are listed in the literature in our observation field that we failed to confirm. One is RX
J1801.7+6637 (z = 0.57) reported by Bower et al. (1996) and the other is RX J1757.3+6631
(z = 0.6909) from the catalog of Henry et al. (2006). There appear to be some weak hints
of clustering around these clusters on the images. But our photometry is not deep enough
to identify any red sequence of galaxies in the CMD of the galaxies.
4.6. The Photometric Properties of Other X-ray Sources
The photometric properties of various X-ray sources such as stars, AGNs, BL Lac’s could
provide valuable information regarding their stellar populations and evolution. Comparison
of our photometry catalog with the X-ray source catalog of Henry et al. (2006) returned
about 25 possible matches. Based on the classification information of the X-ray sources by
Henry et al. (2006), we have investigated the photometric property of each object class using
several CMDs and CCDs. Figure 15 shows a (g′ − r′) vs r′ CMD and the three different
CCDs of various optical counterparts of X-ray sources. For a guideline in each diagram, the
distribution of point sources, which are mostly stars, is also plotted.
In Figure 15, it is easily seen that (1) generally, X-ray bright stars do not belong to the
well defined stellar sequence in each diagram, and (2) AGNs and BL Lac objects are not
readily separated from stars or other kind of objects in these CMDs and CCDs. Most X-ray
bright stars that are used in this comparison are very bright (r′ < 15 mag) and are mostly
saturated except for one faint giant star as shown in the upper-left panel of Figure 15. This
– 15 –
may explain why these X-ray bright stars appear to be in different color domains from other
generic stars. Even one faint star (marked by a star with a circle in Figure 15) is bluer in
(u∗− r′) color than normal stars. Although AGNs and BL Lac’s as a whole do not show any
distinct pattern in each color, the AGN1s with stellarity > 0.9, which are relatively free from
wrong identification, are found only in the blue domains of (g′ − r′), (r′ − i′), and (u∗ − r′).
The (g′ − r′) CMD also shows that any sources with (g′ − r′) < 0.0 and r′ > 15 ∼ 16 mag
are very likely to be AGN or BL Lac objects.
5. Summary and Conclusion
We have obtained u∗, g′, r′, i′, z′ optical band high resolution images of the two square
degrees area centered at the NEP with MegaCam/MegaPrime at CFHT. From the source
detection and the photometry using SExtractor, we have compiled about 56,000 sources with
r′ ≤ 23 mag, including point and extended sources, into the final optical photometry catalog
as listed in Table 2. The use of the color-magnitude diagrams and color-color diagrams
revealed strikingly different photometric characteristics of stars and galaxies. We have found
that the use of (u∗ − r′) vs (g′ − z′) color enables us to clearly separate galaxies from stars
and this separation does not suffer from the uncertainties involved in the morphological
classification of faint sources. The galaxy number counts constructed from the galaxies
selected based on the (u∗ − r′) vs (g′ − z′) color show a nearly monotonic increase up to
about r′ = 23 mag with a slope d(LogN)/dm ≈ 0.40 ± 0.01, which is in agreement with
the literature. However, there are some changes in the slope at r′ = 17.5 ∼ 18 mag and
r′ = 19 ∼ 20 mag, which needs further studies.
Using the SDSS DR5 data of the 101 nearby Abell galaxy clusters with redshift z < 0.2,
we have derived a relation between the redshift and the color of the red sequences in the
SDSS filter system which is compatible with the CFHT MegaCam filter system. Utilizing
the information derived from the nearby galaxy clusters, we have applied a simple color
cut method to find galaxy clusters in our data field, which returned two galaxy clusters
and one galaxy group. These galaxy clusters and group are also radio and X-ray sources,
which were reported by previous studies. We have also estimated the redshift of these galaxy
clusters and group using the linear relation between the (g′ − r′)r18,0 color and the redshift.
The estimated redshift is in agreement with the known value of 0.09 for galaxies in NEPX1
but it is relatively lower than the spectroscopic redshift in the literature for galaxies in
RX J1754.7+6623 and the photometrically derived redshift for RX J1757.9+6609. For RX
J1754.7+6623, this underestimation of redshift may be due to the poorly determined slope
of red sequence since there are only a few bright galaxies. For RX J1757.9+6609, the galaxy
– 16 –
cluster seems to be overlayed on the background type 2 AGN with redshift of 0.4875.
We have compared our photometry catalog with an X-ray source catalog in the literature
to investigate the photometric properties of other X-ray sources. The result of comparison
implies that the sources with (g′− r′) < 0.0 could be classified as candidates of AGNs or BL
Lac objects.
N.H. and T.K. are in part supported by the BK21 program of the Korean Government.
This work was in part supported by the ABRL (R14-2002-058-01000-0).
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This preprint was prepared with the AAS LATEX macros v5.2.
– 18 –
Table 1. Observation Log
Field RA DEC Filter Total Exp Time Seeing Depth Observation Date (UT)
(hh:mm:ss) (dd:mm:ss) (sec) (arcsec) (AB mag)a (yyyy-mm-dd)
NEP-E 18:04:31.51 66:33:38.60 u∗ 1800 × 2 1.13 25.98 2004-09-13/14
g′ 1200 × 2 1.08 26.12 2004-08-22
r′ 1500 × 2 0.99 25.58 2004-08-23
i′ 1500 × 2 0.69 24.70 2004-08-23/09-12
z′ 1800 × 2 0.73 24.03 2004-09-07
NEP-W 17:55:28.49 66:33:38.60 g′ 1200 × 2 1.01 26.12 2004-08-22
r′ 1500 × 2 0.87 25.58 2004-09-19/20
i′ 1500 × 2 0.99 24.85 2004-09-13/14
z′ 1800 × 2 0.85 23.71 2004-09-22
aThe depth of each filter data was measured as 4 σ flux over a circular aperture with a diameter of 1 ′′.
Table 2. NEP Optical Source Cataloga
ID RA (J2000) Dec (J2000) u∗ err(u∗) g′ err(g′) r′ err(r′) i′ err(i′) z′ err(z′) flag stellarity ellipticity fwhm r eff sma area field
[deg] [deg] [mag] [mag] [mag] [mag] [mag] [arcsec] [arcsec] [arcsec] [arcsec2]
3 271.1438293 66.0613251 99.000 99.000 25.438 0.098 22.822 0.020 22.608 0.031 26.193 1.738 0 0.08 0.295 1.611 0.620 0.423 1.677 1
5 271.4050293 66.0613251 99.000 99.000 24.105 0.033 22.895 0.020 22.200 0.019 99.000 99.000 0 0.76 0.042 0.990 0.460 0.347 1.814 1
6 272.0104980 66.0599136 99.000 99.000 99.000 99.000 18.853 0.001 18.484 0.000 99.000 99.000 4 1.00 0.162 0.773 0.326 0.156 0.274 1
17 270.6894836 66.0615005 99.000 99.000 24.819 0.066 22.994 0.025 21.750 0.014 99.000 99.000 0 0.28 0.056 1.373 0.562 0.389 1.814 1
20 270.6847839 66.0614319 99.000 99.000 26.160 0.182 22.875 0.022 21.472 0.010 99.000 99.000 0 0.18 0.133 2.218 0.580 0.398 1.882 1
24 271.0955811 66.0624542 23.651 0.040 23.783 0.026 22.803 0.019 22.544 0.028 99.000 99.000 0 0.76 0.110 1.117 0.531 0.334 1.437 1
25 270.9777527 66.0611649 99.000 99.000 20.694 0.002 19.145 0.001 18.382 0.001 99.000 99.000 0 0.99 0.132 1.025 0.407 0.481 6.640 1
26 270.6968384 66.0618744 23.781 0.050 25.055 0.061 22.911 0.018 22.476 0.022 99.000 99.000 0 0.88 0.129 1.175 0.484 0.318 1.369 1
35 271.8403320 66.0610962 24.433 0.072 24.490 0.041 22.855 0.018 22.677 0.028 99.000 99.000 0 0.88 0.160 1.032 0.468 0.333 1.506 1
37 270.7434998 66.0618744 99.000 99.000 24.161 0.036 22.042 0.011 20.852 0.006 99.000 99.000 0 0.98 0.090 0.995 0.506 0.408 2.772 1
aThe complete version of this table is in the electronic edition of the Journal. The printed edition contains only a sample.
– 20 –
Fig. 1.— The observation field map around the North Ecliptic Pole (NEP) (dashed line
boxes). Squares and asterisks represent, respectively, the known radio (Brinkmann et al.
1999) and X-ray (Henry et al. 2006) sources. Circles with an asterisk represent galaxy groups
or clusters classified by Gioia et al. (2003) and cataloged in Henry et al. (2006). The large
cross at the center shows the location of NEP. There are about 40 radio and/or X-ray sources
found in our two fields of observation, NEP-E (East) and NEP-W (West).
– 21 –
Fig. 2.— The distribution of the stellarity index (upper panel), the r′ band magnitude error
(mid panel), and the source number counts in u∗, g′, r′, i′, z′ bands (lower panel). The number
of detected sources reaches its maximum at about 24.3 mag for u∗, 24.0 mag for g′, 23.7 mag
for r′, 23.0 mag for i′, and 22.5 mag for z′ band.
– 22 –
Fig. 3.— Upper panel: (g′ − r′) vs r′ color magnitude diagram (CMD) of NEP sources.
Sources selected with r′ ≤ 23 mag and (g′ − r′) color error < 0.1 mag criteria are plotted
in different colors: point sources with stellarity > 0.95 in red and extended sources with
stellarity < 0.2 in blue. Black dots with r′ < 17 mag mostly represent saturated stars.
Lower panel: (g′ − r′) color distribution of those selected sources: point sources in solid
line and extended sources in dashed line. Point sources and extended sources show different
distributions in (g′ − r′) color: two peaks at (g′ − r′) ≈ 0.3 (G dwarf stars) and 1.4 (M giant
stars) for point sources and a single peak at (g′ − r′) ≈ 0.8 for extended sources.
– 23 –
Fig. 4.— Upper panel: (u∗−r′) vs r′ color magnitude diagram (CMD) of sources in the NEP-
E field. Sources selected with r′ ≤ 23 mag and (u∗ − r′) color error < 0.1 mag criteria are
plotted in different colors: point sources with stellarity > 0.95 in red and extended sources
with stellarity < 0.2 in blue. Lower panel: (u∗ − r′) color distribution of those selected
sources: point sources in solid line and extended sources in dashed line.
– 24 –
Fig. 5.— Panel A: (g′ − r′) vs (r′ − i′) color-color diagram (CCD) of NEP sources. Sources
selected with r′ ≤ 23 mag, (g′ − r′) color error < 0.1 mag, and (r′ − i′) color error < 0.1
mag criteria are plotted: point sources with stellarity > 0.95 in crosses and extended sources
with stellarity < 0.2 in dots. The color distribution in (g′ − r′) color (Panel B) and (r′ − i′)
color (Panel C) of those selected sources: point sources in solid line and extended sources in
dashed line. Please note that the (r′− i′) and (g′− r′) color sequence of point sources is very
narrow and clear while the feature of extended sources is very diffuse and broad centered at
about (g′ − r′) ∼ 0.8 and (r′ − i′) ∼ 0.3.
– 25 –
Fig. 6.— Panel A: (r′ − i′) vs (i′ − z′) color-color diagram (CCD) of NEP sources. Sources
selected with r′ ≤ 23 mag, (r′ − i′) color error < 0.1 mag, and (i′ − z′) color error < 0.1
mag criteria are plotted: point sources with stellarity > 0.95 in crosses and extended sources
with stellarity < 0.2 in dots. The color distribution in (r′ − i′) color (Panel B) and (i′ − z′)
color (Panel C) of those selected sources: point sources in solid line and extended sources
in dashed line. In this plot, point sources are distributed in a narrow straight line running
from (r′ − i′, i′ − z′) ≈ (0.1, 0.1) to (1.8, 0.8).
– 26 –
Fig. 7.— Panel A: (u∗ − r′) vs (g′ − z′) color-color diagram (CCD) of NEP sources in the
NEP-E field. Selected sources with r′ ≤ 23 mag, (u∗−r′) color error < 0.1 mag, and (g′−z′)
color error < 0.1 mag criteria are plotted: point sources with stellarity > 0.95 in crosses and
extended sources with stellarity < 0.2 in dots. The dashed line in Panel A separates point
sources from extended sources. See text for details. The color distribution in (u∗ − r′) color
(Panel B) and (g′ − z′) color (Panel C) of those selected sources: point sources in solid line
and extended sources in dashed line.
– 27 –
Fig. 8.— The number counts of galaxies in the NEP region, normalized to one square
degree area. The Kümmel & Wagner (2001) (KW01) data in R band are also shown in all
the three panels for comparison with our data (Galaxy I & II) assuming a simple relation
of r′ ≃ R + 0.24. Galaxy I is defined to be sources with stellarity < 0.2 while Galaxy
II is selected based on its position on the (u∗ − r′) vs (g′ − z′) CCD. It is clear that the
overall shapes and patterns of galaxy number counts are generally in good agreement with
Kümmel & Wagner (2001) result.
– 28 –
Fig. 9.— The redshift distribution of 101 galaxy clusters whose photometric and spectro-
scopic data of member galaxies were retrieved from the SDSS DR5 data archive.
– 29 –
Fig. 10.— The color-magnitude diagrams (CMD) of galaxies in 101 nearby (z < 0.2) galaxy
clusters in the SDSS data archive. All the photometric data used in these diagrams are
foreground reddening corrected. Early type galaxies are plotted in crosses. Please note the
relatively narrow and well-defined red sequence of early type galaxies in each CMD.
– 30 –
Fig. 11.— The representative (g′ − r′)0 color red sequences of four galaxy clusters: A2199
(inverted triangle, z = 0.0302), A1169 (square, z = 0.0586), A1346 (triangle, z = 0.0975),
and A775 (circle, z = 0.1334). The color of the red sequence was derived by fitting the
colors of early type galaxies in each cluster marked by the filled symbols while the open
symbols were rejected from the fit. The red sequence of a galaxy cluster shifts redward as
the corresponding redshift increases.
Fig. 12.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of NEPX1. In each CMD, only the extended
sources are plotted and the magnitude limit adopted for the analysis is indicated with an
arrow. The color ranges used to find the cluster galaxies are shown in short dashed lines and
the red sequence line fitted in each color is displayed in long dashed lines. The estimated
redshift from the (g′ − r′) color of the red sequence is indicated in the head as ‘ZCMR’. The
galaxies satisfying the color cut criteria and being located within a solid circle in the lower
left panel are represented by filled circles in the CMDs and the spatial distribution plot.
The size of the circles in the spatial distribution plot are proportional to luminosity: the
larger, the brighter. Galaxies that satisfies only the color cut criteria are plotted in open
circles while other galaxies are in open squares. The dashed line in the density plot displays
the number density profile of all galaxies around the selected area and the solid line shows
the profile of ten times the number density of those selected galaxies and the corresponding
errors.
– 32 –
Fig. 13.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of RX J1754.7+6623. There are several bright
galaxies in the foreground over the many faint background galaxies in this region. See Figure
12 for the legend.
– 33 –
Fig. 14.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of RX J1757.9+6609. See Figure 12 for the
legend.
– 34 –
Fig. 15.— The photometric properties of X-ray sources. Stellar symbols for stars, triangles
for type 1 AGN (AGN1), reversed triangles for type 2 AGN (AGN2), asterisks for BL Lac’s,
squares for galaxies, filled circles for galaxy clusters, and dots for point sources from our
photometry catalog. The classification information of X-ray sources are from Henry et al.
(2006). The distribution of point sources is presented in each CMD or CCD for references.
Stars marked by stellar symbols in these diagrams are suspected to suffer from saturation
effect except for one faint star represented by a stellar symbol within a circle. Please note
that the point-like AGN1 sources (filled triangles) are bluer than (g′ − r′) ∼ 0.5.
	Introduction
	Observation
	Data Reduction
	Pre-Photometry Processing
	Source Detection & Photometry
	Results
	Source Catalog
	Color-Magnitude and Color-Color Diagrams
	Galaxy Number Counts
	Galaxy Clusters
	Nearby Galaxy Clusters in SDSS
	Galaxy Cluster Search in the NEP Field
	Galaxy Clusters and Groups in X-ray/Radio Source Catalogs
	NEPX1/VLA 1801.5+6645
	RX J1754.7+6623
	RX J1757.9+6609
	Other Galaxy Clusters
	The Photometric Properties of Other X-ray Sources
	Summary and Conclusion

  We present a five (u*,g',r',i',z') band optical photometry catalog of the
sources in the North Ecliptic Pole (NEP) region based on deep observations made
with MegaCam at CFHT. The source catalog covers about 2 square degree area
centered at the NEP and reaches depths of about 26 mag for u*, g', r' bands,
about 25 mag for i' band, and about 24 mag for z' band (4 sigma detection over
an 1 arcsec aperture). The total number of cataloged sources brighter than r'=
23 mag is about 56,000 including both point sources and extended sources. From
the investigation of photometric properties using the color-magnitude diagrams
and color-color diagrams, we have found that the colors of extended sources are
mostly (u*-r') < 3.0 and (g'-z') > 0.5. This can be used to separate the
extended sources from the point sources reliably, even for the faint source
domain where typical morphological classification schemes hardly work
efficiently. We have derived an empirical color-redshift relation of the red
sequence galaxies using the Sloan Digital Sky Survey data. By applying this
relation to our photometry catalog and searching for any spatial overdensities,
we have found two galaxy clusters and one nearby galaxy group.

Introduction
The North Ecliptic Pole (NEP) is an undistinguished region in the sky, located at
α = 18h00m00s, δ = +66◦33′38′′. It is, however, a very special region since many astronomical
satellites have accumulated a large number of exposures over this location since Earth-
orbiting satellites must point their fixed solar panels to the Sun and be in their orbits over
the ecliptic poles. Unlike the South Ecliptic Pole (SEP) region where the South Atlantic
Anomaly and the LMC prevent the clear view of the extragalactic sky at certain wavelengths,
the NEP region suffers very little or no obscuration by foreground Galactic sources. The
galactic coordinates of the NEP are l ≈ 96.4◦, b ≈ +29.8◦ and the foreground extinction
in this direction is E(B − V ) = 0.047 (Schlegel et al. 1998). Therefore, the NEP is a
good target for deep, unbiased, contiguous surveys for extragalactic objects such as galaxies,
galaxy clusters, and AGNs.
The ROSAT All-Sky Survey (RASS) is one of the most representative surveys of the
NEP region (Voges et al. 1999). The ROSAT was the first X-ray imaging satellite to survey
the entire sky. A source catalog of X-ray sources that were extracted from a large area
surrounding the NEP was constructed from this RASS data (Henry et al. 2001, 2006). From
the followup observations and optical counterpart investigations, Gioia et al. (2003) reported
that most X-ray sources in the NEP region are AGNs (∼ 49%), stars (∼ 34%), and galaxy
groups/clusters (∼ 14%). Mullis et al. (2001) also found a supercluster of galaxies in the
NEP region using the RASS data and suggested that some galaxy clusters in this region are
part of the supercluster at z = 0.087. The NEP region was also observed in the 1.5 GHz
band by Kollgaard et al. (1994). Using this radio source catalog, Brinkmann et al. (1999)
– 3 –
investigated the correlation between the radio sources and the RASS X-ray sources, and
identified optical counterparts of the radio/X-ray sources. They found that a significant
number of radio loud sources are also bright in the X-ray band, and that X-ray selection is
an effective way for the search of galaxy clusters and groups.
While the X-ray luminosity is an efficient measure of the hot ionized gas in the galaxy
clusters, the star formation activities and the resultant stellar mass in galaxies can be esti-
mated from the optical and infrared flux. Recently, a new infrared space telescope, named
AKARI, was launched in February 2006. AKARI is expected to give exceedingly higher qual-
ity data than the previous infrared space missions such as Infrared Astronomical Satellite
(Neugebauer et al. 1984) and Infrared Space Observatory (Kessler et al. 1996). The AKARI
data will also complement the successful Spitzer Space Telescope (Werner et al. 2004) data
by providing better wavelength coverage over the 8−24µm band, which is not available from
the Spitzer. As one of its major science programs, AKARI is currently carrying out deep
near-to-mid infrared surveys over the wide area around the NEP through two major survey
programs, ‘NEP-Deep’ and ‘NEP-Wide.’ The key scientific objectives of these NEP surveys
are to unveil the dusty star formation history of the Universe, the mass assembly and large
scale structure evolution, and the nature of the cosmic infrared background (CIRB). For
further information on these NEP Programs, please refer to Matsuhara et al. (2006).
Combined with these space-borne infrared surveys, the optical survey of the NEP re-
gion constitutes the multifrequency dataset that is essential for the studies of cosmic star
formation history. Many researchers have investigated optical counterparts of radio and X-
ray sources in this region, but, for the imaging data, they mostly relied on the COSMOS
scan (e.g., Brinkmann et al. 1999) or Digitized Sky Survey images of the second Palomar
Observatory Sky Survey (POSS-II) plates, only complemented by some independent but
patchy observations (e.g., Gioia et al. 2003). Although the POSS-II data are excellent in
complete coverage of the whole sky, the usefulness of the data are still limited by the low
resolution with ∼ 1.0′′ per pixel scale and the shallow depth with the limiting magnitude of
rF ∼ 21.0 mag (Gal et al. 2004). In this study, therefore, we provide the first optical catalog
of sources in the NEP region based on deep observations in u∗, g′, r′, i′, z′ bands obtained
with MegaCam at CFHT. This optical catalog will be an important part of multifrequency
datasets of galactic and extragalactic sources in the NEP region. It will also serve as a basis
of the AKARI NEP survey mission by providing the information on the optical counterparts
of the infrared sources. This paper is composed as follows: We describe our observations in
Section 2 and data reduction procedures in Section 3. We present a bright source catalog
in Section 4.1, photometric properties of the sources in the NEP region in Section 4.2, and
galaxy number counts in Section 4.3. The result of galaxy cluster search using the color of
red sequence galaxies is given in Sections 4.4 and 4.5. Some photometric properties of other
– 4 –
X-ray sources are presented in Section 4.6 and our primary results are summarized in the
last section.
2. Observation
The observation of the North Ecliptic Pole (NEP) region was carried out over nine photo-
metric nights between August 22 and September 22, 2004 using the 3.6m CFHT telescope lo-
cated at Mauna Kea, Hawaii. We used a wide field imager MegaCam at the telescope primary
focus MegaPrime. The MegaCam is composed of 36 2048 × 4612 CCD’s covering about 1◦×
1◦ area with 0.185 arc seconds resolution per pixel (Boulade et al. 2003). The u∗, g′, r′, i′, z′
filter system provided with MegaCam is basically the same as that used by the Sloan Digital
Sky Survey (SDSS) (York et al. 2000) except for the u∗ filter which is designed to take advan-
tage of the improved UV extinction condition at the CFHT site. Therefore, the photometric
data generated from this observation are presented in the CFHT unique u∗ and the SDSS
g′, r′, i′, z′ system. For further information on these filter systems used with MegaCam, see
the ‘Filter set’ section in http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/specsinformation.html
page.
As shown in Figure 1, the observed fields are composed of two fields separated by about
1◦ away from each other in the East-West direction, covering about two square degrees in
total. The NEP-E (East) field was observed with five filters, u∗, g′, r′, i′, z′ and the other
field, NEP-W (West), was observed with only four filters, g′, r′, i′, z′. Each field was covered
twice with a 15′′ offset for dithering in each filter with total exposure times of 3600 sec for
u∗ (1800 × 2), 2400 sec for g′ (1200 × 2), 3000 sec for r′ (1500 × 2), 3000 sec for i′ (1500 ×
2), and 3600 sec for z′ (1800 × 2). The raw images were processed using Elixir system by
CFHT staff. The Elixir is a collection of programs, databases, and other tools specialized
in processing and evaluation of the large mosaic data (Magnier & Cuillandre 2004). The
overall quality of the preprocessed images is good with a typical seeing of 0.7 ∼ 1.1′′. Detailed
information on the observation is listed in Table 1.
3. Data Reduction
3.1. Pre-Photometry Processing
For the reliable source detection and photometry, it is essential to prepare a master
image that satisfies the following two factors: (1) to obtain the required photometric depth
http://www.cfht.hawaii.edu/Instruments/Imaging/MegaPrime/specsinformation.html
– 5 –
and (2) to give the complete areal coverage. We used a software package SWarp1 written
by E. Bertin at Terapix to transform each Multi Extension Fits (MEF) file containing 36
individual CCD frames into a single fits image data. SWarp also corrects different distortion
effects between input data through the resampling process, and then combines the resampled
images to create a deep output image after flux scaling and background subtraction.
To make a master image, we combined the g′, r′, i′, z′ four band data using SWarp for the
NEP-E and the NEP-W field respectively. During the SWarp run, BILINEAR resampling
method was used since this method was found to be effective in suppressing the discontinuity
effects around the chip boundaries, while other methods such as LANCZOS3 produced more
noticeable discontinuities. We also used weighted images during SWarp run to enhance the
quality of the output image. In the weight maps we assigned zero weights to pixels with
negative values. We excluded the u∗ band data from the master image construction since we
do not have the u∗ band data for the NEP-W and the S/N is relatively lower compared to
other bands. Finally, we created two master detection images with high S/N’s, one for the
NEP-E and the other for the NEP-W region.
Two MEF images for each band were processed using SWarp with the same parame-
ters to generate a final photometry image for the corresponding filter. These u∗, g′, r′, i′, z′
band images were made to have the same dimension and coordinates with the master de-
tection image of the corresponding field. The depth of photometry attained by combin-
ing two raw images was calculated as 4σ flux over a circular aperture with 1 ′′ diameter.
The measured limiting magnitudes are u∗ ∼ 26.0, g′ ∼ 26.1, r′ ∼ 25.6, i′ ∼ 24.7, and
z′ ∼ 23.7 mag, as listed in Table 1. After all these processings, two master detection im-
ages and nine photometry images were prepared for the source detection and photometry.
All the information on the reduced photometry images will be available from an web site
http://astro.snu.ac.kr/˜ nhwang/index.files/nep.html.
3.2. Source Detection & Photometry
We have used SExtractor (Bertin & Arnouts 1996) to detect sources from the master
detection images of the NEP-E and NEP-W fields. A source is confirmed if it has more than
five contiguous pixels above four times the background sky fluctuation. The signals from each
source were measured in the u∗, g′, r′, i′, z′ band photometry images over the isophotal area
previously defined during the source detection. The photometry was made using SExtractor
in dual mode operation. This scheme enables the detection and photometry of any source
1See http://terapix.iap.fr/rubrique.php?id rubrique=49 for further information
http://astro.snu.ac.kr/~~nhwang/index.files/nep.html
http://terapix.iap.fr/rubrique.php?id_rubrique=49
– 6 –
that is registered at least once in any of the g′, r′, i′, z′ band images. The total number of
sources detected in our two observed NEP fields is about 130,000.
The instrumental magnitudes were transformed into standard magnitudes using the
transformation information provided and recorded in the header of the images by the CFHT
staff. During this transformation, only the calibrated magnitudes of sources that have avail-
able color information required for the calibration are calculated and kept in the catalog.
Otherwise, we assigned dummy values (99.000) to the magnitudes of sources in the final
source catalog.
4. Results
Figure 2 displays a source count histogram in the u∗, g′, r′, i′, z′ band along with the r′
band error and stellarity distributions. This shows that the number of sources in our data
increases up to u∗ ∼ 24.3, g′ ∼ 24.0, r′ ∼ 23.5, i′ ∼ 23.0, and z′ ∼ 22.2 mag before the
incompleteness effect starts to take place and the number of the detected sources starts to
decrease. The r′ band magnitude error is estimated to be about 0.1 mag or less at r′ ∼ 23.5
mag where the source count reaches its maximum. The stellarity distribution shows how
efficiently the star/galaxy classifier works in our dataset. From this distribution, it is clearly
seen that the stellarity index, which is calculated by SExtractor based on the isophotal areas,
peak intensity, and seeing information, separates the point sources (stellarity ∼ 1) and the
extended sources (stellarity ∼ 0) with high confidence for the sources with r′ < 22 mag. One
more point to be noted in this stellarity distribution is that sources with r′ < 16.5 mag and
stellarity > 0.7 are the results of the image saturation. Thus the stellarity index is most
relable in the magnitude range of 17 < r′ < 22 mag.
4.1. Source Catalog
Considering the source count and the stellarity distribution, we have decided the mag-
nitude range of the most reliable sources for the final bright source catalog entry, which is
r′ ≤ 23 mag. The number of sources compiled in the final bright source catalog is about
56,000. Table 2 lists a sample of the bright source catalog for reader’s guide and the full
source catalog will be available electronically from the online Journal and from an web site
http://astro.snu.ac.kr/˜ nhwang/index.files/nep.html. Followings are the short descriptions
of data columns in the catalog.
Column 1 is the identification number of an optical source.
http://astro.snu.ac.kr/~~nhwang/index.files/nep.html
– 7 –
Column 2 & 3 list, respectively, the J2000.0 right ascension (RA) and declination (DEC)
of a source in degrees. The uncertainties of the astrometric solution derived by Elixir are
about ±0.5′′.
Column 4 ∼ 13 are the AB magnitudes and magnitude errors of a source in the
u∗, g′, r′, i′, z′ bands. These are given by MAG AUTO and MAGERR AUTO parameters
of SExtractor, which are Kron-like elliptical aperture magnitudes and their errors.
Column 14 is the extraction flag of a source given as a sum of powers of 2 by SExtractor.
If a source has neighbors (flag = 1) and is blended with another source (flag = 2) and some
pixels are saturated (flag = 4), then the final extraction flag given to the source is 7 (= 1
+ 4 + 2). Generally, a source with the extraction flag 0 gives the most reliable photometry.
For detailed information on this flag, see the SExtractor User’s Manual.
Column 15 is the stellarity index of a source calculated from a g′, r′, i′, z′ combined
detection image. This index has a value between 1 (point sources) and 0 (extended sources).
See Figure 2 for the distribution of this index.
Column 16 is the ellipticity of a source calculated by SExtractor using the second order
image moments.
Column 17 is the FWHM in arcseconds of a source calculated under the assumption
that the source has a Gaussian profile.
Column 18 is the effective radius or a half-light radius of a source in arcseconds. This
value is computed by setting the input parameter PHOT FLUXFRAC = 0.5 and is given
by the output parameter FLUX RADIUS.
Column 19 is the semi-major axis of a source in arcseconds.
Column 20 is the detected isophotal area of a source. This parameter may be used as a
measure of the object’s size for the case of extended sources.
Column 21 is the field number that the photometric data of a source comes from. The
field number 1 represents the NEP-E field and 2 represents the NEP-W field.
4.2. Color-Magnitude and Color-Color Diagrams
We have investigated the photometric properties of the NEP sources using several color-
magnitude and color-color diagrams. In each diagram, we use the stellarity index given by
SExtractor to distinguish between point sources and extended sources: stellarity > 0.95
for point sources (mostly stars) and stellarity < 0.2 for extended sources (mostly galaxies).
– 8 –
They are found to be statistically very useful tools to separate stars and galaxies brighter
than r = 23 mag from all kinds of sources in the catalog.
The r′ vs (g′ − r′) color-magnitude diagram (CMD) and (g′ − r′) color histogram in
Figure 3 shows two prominent vertical sequences of point sources, i.e., stars at (g′ − r′) ∼
0.3 (G dwarf stars) and 1.4 (M giant stars), respectively. The extended sources, presumably
galaxies, are seen to be concentrated around the peak at (g′ − r′) ∼ 0.8 that corresponds
to the (g′ − r′) color of early type galaxies (Fukugita et al. 1995). The extended sources
start to dominate at r′ ≈ 22 mag and fainter. Some of these sources may be faint stars
that the SExtractor failed to classify as point sources. However, the different (g′ − r′) color
histograms of the extended and the point sources suggest that the majority of these faint
extended sources are galaxies.
Figure 4 displays the r′ vs (u∗− r′) CMD and (u∗− r′) color histogram of sources in the
NEP-E field. The CMD shows that there are many galaxies distributed around (u∗ − r′) ∼
1.0 as shown in the (u∗ − r′) color histogram of extended sources. This value is consistent
with the (u∗−r′) color of Scd type galaxies (Fukugita et al. 1995), whereas the (g′−r′) color
of most galaxies in Figure 3 is that of early type galaxies. It is also noted that there is a
very long redward tail in the (u∗− r′) color distribution of extended sources, reaching nearly
(u∗−r′) ≈ 4 or higher. The elliptical galaxies can be as red as (u∗−r′) ∼ 2.8 (Fukugita et al.
1995). Therefore, some of those faint and red sources with (u∗ − r′) > 3.0 could be distant
galaxies with redshift z ≥ 0.1.
Figures 5 through 7 show the characteristic distribution of point sources and extended
sources in three color-color diagrams: (r′ − i′) vs (g′ − r′), (i′ − z′) vs (r′ − i′), and (g′ − z′)
vs (u∗ − r′). The most prominent feature in these color-color diagrams (CCD’s) is very
distinguishable sequences of point sources, i.e. stars. In Figure 5, the stellar sequence in a
flipped ‘L’ shape has a very narrow width of about 0.2 dex in the (r′ − i′) vs (g′ − r′) color
space. This tight sequence is also well represented by a straight line in Figure 6. In these
diagrams, extended sources are found to populate in a relatively well constrained color space
centered at a certain color. It is still clear that we can not possibly separate extended sources
from point sources by simply constraining colors in the (r′− i′) vs (g′−r′) and the (i′−z′) vs
(r′ − i′) color-color space. However, Figure 7 shows that the (g′ − z′) vs (u∗ − r′) color-color
combination enables us to define the two exclusive spaces that are mostly populated by stars
and galaxies, respectively. The boundary of these regions can be drawn by combination
of three straight lines connecting (u∗ − r′, g′ − z′) = (−1.0, 0.5), (0.5, 0.5), (3.0, 1.7), and
(3.0, 5.0). This is a very useful tool for separating faint galaxies from stars to search for
distant galaxy clusters or to compute the 2D correlation function of galaxies.
– 9 –
4.3. Galaxy Number Counts
We have investigated the galaxy number counts using the NEP optical source catalog. To
select galaxies efficiently from the photometry catalog, we adopted two different definitions of
galaxies: (1) ‘Galaxy I’ sources with stellarity < 0.2 and (2) ‘Galaxy II’ sources that belong
to the designated area where the extended sources appeared to occupy in the (u∗ − r′) vs
(g′ − z′) CCD as denoted by the dashed line in Figure 7. The results of number counts for
Galaxy I and Galaxy II sample, as shown in Figure 8, show a consistent rise from r′ = 17.5
mag up to about r′ = 23 mag and then a steep down turn due to the incompleteness at
r′ ≥ 23 mag. From the lower and mid panels of Figure 8, it is clear that Galaxy II sample
outnumbers Galaxy I sample in r′ > 23 mag. This difference between number counts is
mostly due to the deteriorating reliability of stellarity index in r′ ≥ 23 mag, as shown in the
upper panel of Figure 2. Therefore, the Galaxy I sample is likely to lose a large number of
faint galaxies compared to the Galaxy II sample.
In Figure 8, we also plotted the R-band galaxy number count of Kümmel & Wagner
(2001) for comparison after a simple transformation into r′ band. Kümmel & Wagner (2001)
derived the number count of extended sources by modeling the number count distribution
of point-like sources (star) and then subtracting the extrapolated model count of point-like
sources from the detected source count. On the other hand, we defined galaxies as a certain
part of all the detected sources that satisfy a given stellarity condition (Galaxy I) or a
given two-color criterion (Galaxy II). Nonetheless, it is clear from the plot that the number
count by Kümmel & Wagner (2001) is generally consistent with our result except for the
differences in the faint magnitude domain where the incompleteness becomes significant.
As shown in the mid panel of Figure 8, the logarithmic number counts defined as
Log(Count/0.5mag/deg2) also display very similar slopes over the range of r′ = 18 ∼ 22
mag. Simple linear square fits over that magnitude range returned the slope d(LogN)/dm =
0.387 for Galaxy I, 0.408 for Galaxy II, and 0.397 for Kümmel & Wagner (2001) data. The
errors of the fitted slopes are about ±0.010 for all cases. Therefore, the slopes of logarithmic
galaxy number counts are in good agreement with that of Kümmel & Wagner (2001) within
the errors.
However, there are some differences in several minor features between the logarithmic
profiles of galaxy number count. This is more clearly shown in the upper panel of Figure
8. It is apparent that there are two small dips in Galaxy II data: one at r′ ≈ 17.5 ∼ 18
mag and another smooth one at r′ ≈ 19 ∼ 20 mag. The first dip also appears to exist
in Kümmel & Wagner (2001) data but the second smooth and shallow dip could not be
identified from Kümmel & Wagner (2001) data in this plot. However, the magnitude of the
second dip is roughly coincident with the break point at R ≈ 19 ∼ 20 mag as shown in their
– 10 –
Figure 4 2.
4.4. Galaxy Clusters
We have carried out a galaxy cluster search using our optical photometry catalog.
Among numerous galaxy cluster finding methods available, we adopted a simplified version
of the ‘C4 cluster finding algorithm’ by Miller et al. (2005) that utilizes a seven-dimensional
position and color space. In this study, we use only a three-dimensional color space con-
structed based on (g′− r′), (r′− i′), and (i′− z′) colors to select cluster member galaxies and
then we investigate the spatial distribution of the selected galaxies to find any overdensity
of these galaxies in small regions. Finally, the CMDs of any overdense region are consulted
to check whether the red sequence of galaxies is apparent before we identify the region as a
galaxy cluster.
This approach requires the definition of color ranges spanned by the cluster member
galaxies with various redshifts and richness classes before the actual application to the pho-
tometric data. We have used the Sloan Digital Sky Survey (SDSS) (York et al. 2000) data
for this purpose. The SDSS project provides a huge amount of multiband photometric and
spectroscopic data over a large sky area. This enables us to select cluster member galaxies
using the spectroscopy information and to reduce the possible contamination by field galax-
ies and stars. One more advantage of using the SDSS dataset to define color ranges occupied
by cluster member galaxies is that the SDSS filter system is the same as the MegaCam
filter system of CFHT except for the u∗ filter (see Section 2 for details). Therefore, we have
searched the SDSS database and retrieved photometric and spectroscopic data of nearby
galaxy clusters for the calibration of the red sequence colors in the SDSS filter system.
4.4.1. Nearby Galaxy Clusters in SDSS
We have searched the SDSS DR5 database (Adelman-McCarthy et al. 2007) and found
101 nearby Abell galaxy clusters whose u′, g′, r′, i′, z′ 3 photometric and spectroscopic data
2Figure 4 of Kümmel & Wagner (2001) is constructed using the galaxy number count made with 0.25
mag bin. But the number count data in a table published in the same paper, which we used for Figure 8, is
made with 0.5 mag bin. The use of 0.5 mag bin instead of 0.25 mag bin in the number count is considered
to cause the smoothing out the feature.
3Please note that the u′ filter is the Sloan system, which is different from the CFHT u∗ system. See
Section 2 for details.
– 11 –
of member galaxies are available. Figure 9 displays the redshift distribution of these cluster
galaxies. The redshift of the selected galaxy clusters runs from 0 to 0.2 with a peak at
z ∼ 0.07. After the data retrieval from the SDSS data archive, we have selected member
galaxies of each cluster based on the velocity distribution and the spatial separation from the
cluster center. The total number of the selected member galaxies for the 101 clusters is about
5,700. From the cluster member galaxies, only the early type galaxies were selected using the
‘fracDev’ and ‘eclass’ parameters provided by the SDSS archive: fracDev > 0.8 and eclass
< 0 (for further information on these constraints, see Bernardi et al. 2005). Application
of these parameters finally returned about 1,700 early type galaxies in 88 clusters and the
number of early type galaxies in each cluster runs from 4 (A2192) to 73 (A2199) depending
on the redshift and the richness class.
Figure 10 displays the CMDs of the cluster galaxies in the SDSS color system. From
these diagrams, it is seen that most early type galaxies in the clusters lie within a well-
defined and narrow color range with a width of ≤ 0.2 dex in (g′ − r′)0, (r
′ − i′)0, and
(i′ − z′)0. Although the (u
′ − r′)0 vs r
0 CMD shows a rather large dispersion, it still shows a
strong concentration at 2.0 ≤ (u′ − r′)0 ≤ 3.0. The colors of these narrow sequences of early
type galaxies have been used as indicators of the clusters’ redshifts (Gladders & Yee 2000).
We used the SDSS photometry data of member galaxies in four clusters (A2199, A1166,
A1349, and A775) to derive the relation between the clusters’ redshift and the (g′−r′)0 color
of the sequence as shown in Figure 11. From the linear fit after repeating one sigma clipping
twice, we derived a linear and empirical relation between the redshift and the (g′− r′)0 color
of the fitted sequences at r′0 = 18 mag (hereafter (g
′ − r′)r18,0) as follows:
Redshift [z] = (0.415± 0.044)× (g′ − r′)r18,0 − (0.286± 0.039) (1)
Therefore, the galaxies in nearby clusters occupy a certain space in a multi-color parameter
space, which can be used to separate galaxies in the clusters from field galaxies and to
estimate the approximate redshift of the cluster using the (g′− r′)r18,0 color of the sequence.
4.4.2. Galaxy Cluster Search in the NEP Field
To find galaxy clusters based on the results shown in Figure 10 and to estimate the red-
shift using Equation 1, the Galactic foreground reddening was corrected for our photometry
data using Schlegel et al. (1998), assuming Ar′ ≈ 2.751E(B−V ), E(g
′−r′) ≈ 1.042E(B−V ),
E(r′− i′) ≈ 0.665E(B−V ), and E(i′−z′) ≈ 0.607E(B−V ). After some tests on the CFHT
data, we have defined color ranges for the cluster galaxies as follows: 0.6 < (g′ − r′)0 < 1.1,
0.1 < (r′ − i′)0 < 0.4, 0.0 < (i
− z′)0 < 0.4. We did not use (u
− r′)0 color for this color
– 12 –
space definition because of the unavailability of u∗ band data for the NEP-W field and the
general low S/N in the u∗ band in the NEP-E field. According to a test performed after
adopting the range 2.0 < (u∗ − r′)0 < 3.0 as the fourth constraining color, the efficiency of
finding the cluster galaxies turned out to be comparable to that of another test run using the
three color combination of (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0. This three color combination
approach also enables a homogeneous search of cluster galaxies over our two data fields.
Possible cluster galaxies were selected by constraining the colors of galaxies with the
predefined parameters of (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0. Using their RA and Dec in-
formation, we constructed spatial number density maps of the selected galaxies. Then we
identified 13 possibly overdense regions in the NEP field. Over these 13 regions, CMDs
of galaxies in (g′ − r′)0, (r
′ − i′)0, and (i
′ − z′)0 color were constructed using the source
catalog to find any feature that resembles the red sequence of galaxies in a cluster. From
this investigation, we have identified two galaxy clusters and one nearby galaxy group, and
estimated their redshifts using the (g′− r′)0 colors of the red sequences. The CMDs of those
galaxy clusters and the group are presented in Figures 12 − 14. More details about them
are discussed below.
4.5. Galaxy Clusters and Groups in X-ray/Radio Source Catalogs
Some galaxy clusters or groups were found and reported in the previous studies made
by using the X-ray and the radio band data of the NEP region. Henry et al. (1995) found
several X-ray-selected groups of galaxies in the NEP region based on RASS data and this
work was revised and extended further by Henry et al. (2006). Brinkmann et al. (1999) also
found that many X-ray sources in the NEP region have counterparts in the radio bands that
were observed with VLA. From the X-ray and radio source catalogs provided by Henry et al.
(2006) and Brinkmann et al. (1999), we have found a few galaxy clusters or groups in our
data field. Among these clusters and groups, we will discuss two galaxy clusters and a galaxy
group that were photometrically identified with our catalog.
4.5.1. NEPX1/VLA 1801.5+6645
This is the richest galaxy cluster found in our data field, which is easily seen in Figure 12.
In the upper panel of Figure 12, the CMDs of galaxies show very strong red sequences running
from r′0 ≈ 16 mag (magnitude of the third brightest galaxy) in three color domains: (g
−r′)0,
(r′ − i′)0, and (i
′ − z′)0. Those galaxies belonging to the red sequences are concentrated in a
– 13 –
very compact region, as shown in the lower panel of Figure 12. The redshift estimated from
the red sequence’s (g′ − r′)r18,0 color and Equation 1 is about 0.072 ± 0.037. This cluster
was previously discovered by Burg et al. (1992) from ROSAT survey data and named as
NEPX1. They estimated the redshift of the cluster to be about 0.09 from the spectroscopic
observations, which is in good agreement with our estimate.
This cluster is considered as a part of the large supercluster structure that has been
reported to exist in the NEP region (Hasinger et al. 1991; Mullis et al. 2001). It was also
detected in the radio band from the VLA observations and was suggested as a possible coun-
terpart of the X-ray source RXS J180137.7+664526 by Brinkmann et al. (1999). Although
Brinkmann et al. (1999) listed this source as a galaxy group, we have reached a conclusion
based on the red sequence galaxies in the CMDs that it is a galaxy cluster rather than a
galaxy group.
4.5.2. RX J1754.7+6623
Henry et al. (2006) classified this source as a galaxy cluster with redshift z = 0.0879.
However, the CMDs shown in Figure 13 suggest that there are two kinds of galaxies: (1)
several bright galaxies that form the red sequence running from r′0 ≈ 16 to 18 mag and (2)
many faint galaxies in the background. The second component of galaxies are not brighter
than r′0 = 19 mag, which is about 1 mag fainter than the faintest galaxy of the brighter
component. This indicates that those bright galaxies may happen to be located in front
of the faint background galaxies by chance. Therefore, we suggest that these galaxies are
members of a galaxy group rather than a galaxy cluster. The red sequence’s (g′ − r′)r18,0
color is about 0.793, corresponding to an estimated redshift of 0.043 ± 0.048. The relative
large error may be due to the poorly determined slope of red sequence depends sensitively
on several bright galaxies with r′0 < 18 mag.
4.5.3. RX J1757.9+6609
There is a known X-ray source RX J1757.9+6609 at the similar position. It is listed as
a type 2 AGN with redshift z = 0.4865 in the catalog of Henry et al. (2006). A type 2 AGN
is a narrow emission line (FWHM < 2000 km s−1) object, while a type 1 AGN is a broad
emission line (FWHM ≥ 2000 km s−1) object (Gioia et al. 2003). The spatial number density
plot (lower panel) of the red sequence galaxies in Figure 14 shows some clustering in the 2D
projected space. This point was also noted by Gal et al. (2003) who carried out a cluster
– 14 –
search using the digitized Second Palomar Observatory Sky Survey (DPOSS) data. They
detected an overdensity of galaxies at this location and listed it as a galaxy cluster candidate
under the name of NSC J175751+660924, and also estimated its redshift to be z ≈ 0.1663
from their photometric redshift analysis. Our photometric data shows well developed red
sequences running from r′0 ≈ 17.5 mag in (g
′− r′)0, (r
′− i′)0, and (i
′− z′)0 colors, confirming
that it is a genuine galaxy cluster. The red sequence’s (g′ − r′)r18,0 color is about 0.811 and
this leads to the estimated redshift of z = 0.043 ± 0.025. Therefore, there appears to be
a galaxy cluster with redshift z < 0.2 and a type 2 AGN with redshift z = 0.4875 in the
background of the same projected area.
4.5.4. Other Galaxy Clusters
We have identified two galaxy clusters and one galaxy group by applying the simple
color cut method to our photometry catalog. However, there are two more galaxy clusters
that are listed in the literature in our observation field that we failed to confirm. One is RX
J1801.7+6637 (z = 0.57) reported by Bower et al. (1996) and the other is RX J1757.3+6631
(z = 0.6909) from the catalog of Henry et al. (2006). There appear to be some weak hints
of clustering around these clusters on the images. But our photometry is not deep enough
to identify any red sequence of galaxies in the CMD of the galaxies.
4.6. The Photometric Properties of Other X-ray Sources
The photometric properties of various X-ray sources such as stars, AGNs, BL Lac’s could
provide valuable information regarding their stellar populations and evolution. Comparison
of our photometry catalog with the X-ray source catalog of Henry et al. (2006) returned
about 25 possible matches. Based on the classification information of the X-ray sources by
Henry et al. (2006), we have investigated the photometric property of each object class using
several CMDs and CCDs. Figure 15 shows a (g′ − r′) vs r′ CMD and the three different
CCDs of various optical counterparts of X-ray sources. For a guideline in each diagram, the
distribution of point sources, which are mostly stars, is also plotted.
In Figure 15, it is easily seen that (1) generally, X-ray bright stars do not belong to the
well defined stellar sequence in each diagram, and (2) AGNs and BL Lac objects are not
readily separated from stars or other kind of objects in these CMDs and CCDs. Most X-ray
bright stars that are used in this comparison are very bright (r′ < 15 mag) and are mostly
saturated except for one faint giant star as shown in the upper-left panel of Figure 15. This
– 15 –
may explain why these X-ray bright stars appear to be in different color domains from other
generic stars. Even one faint star (marked by a star with a circle in Figure 15) is bluer in
(u∗− r′) color than normal stars. Although AGNs and BL Lac’s as a whole do not show any
distinct pattern in each color, the AGN1s with stellarity > 0.9, which are relatively free from
wrong identification, are found only in the blue domains of (g′ − r′), (r′ − i′), and (u∗ − r′).
The (g′ − r′) CMD also shows that any sources with (g′ − r′) < 0.0 and r′ > 15 ∼ 16 mag
are very likely to be AGN or BL Lac objects.
5. Summary and Conclusion
We have obtained u∗, g′, r′, i′, z′ optical band high resolution images of the two square
degrees area centered at the NEP with MegaCam/MegaPrime at CFHT. From the source
detection and the photometry using SExtractor, we have compiled about 56,000 sources with
r′ ≤ 23 mag, including point and extended sources, into the final optical photometry catalog
as listed in Table 2. The use of the color-magnitude diagrams and color-color diagrams
revealed strikingly different photometric characteristics of stars and galaxies. We have found
that the use of (u∗ − r′) vs (g′ − z′) color enables us to clearly separate galaxies from stars
and this separation does not suffer from the uncertainties involved in the morphological
classification of faint sources. The galaxy number counts constructed from the galaxies
selected based on the (u∗ − r′) vs (g′ − z′) color show a nearly monotonic increase up to
about r′ = 23 mag with a slope d(LogN)/dm ≈ 0.40 ± 0.01, which is in agreement with
the literature. However, there are some changes in the slope at r′ = 17.5 ∼ 18 mag and
r′ = 19 ∼ 20 mag, which needs further studies.
Using the SDSS DR5 data of the 101 nearby Abell galaxy clusters with redshift z < 0.2,
we have derived a relation between the redshift and the color of the red sequences in the
SDSS filter system which is compatible with the CFHT MegaCam filter system. Utilizing
the information derived from the nearby galaxy clusters, we have applied a simple color
cut method to find galaxy clusters in our data field, which returned two galaxy clusters
and one galaxy group. These galaxy clusters and group are also radio and X-ray sources,
which were reported by previous studies. We have also estimated the redshift of these galaxy
clusters and group using the linear relation between the (g′ − r′)r18,0 color and the redshift.
The estimated redshift is in agreement with the known value of 0.09 for galaxies in NEPX1
but it is relatively lower than the spectroscopic redshift in the literature for galaxies in
RX J1754.7+6623 and the photometrically derived redshift for RX J1757.9+6609. For RX
J1754.7+6623, this underestimation of redshift may be due to the poorly determined slope
of red sequence since there are only a few bright galaxies. For RX J1757.9+6609, the galaxy
– 16 –
cluster seems to be overlayed on the background type 2 AGN with redshift of 0.4875.
We have compared our photometry catalog with an X-ray source catalog in the literature
to investigate the photometric properties of other X-ray sources. The result of comparison
implies that the sources with (g′− r′) < 0.0 could be classified as candidates of AGNs or BL
Lac objects.
N.H. and T.K. are in part supported by the BK21 program of the Korean Government.
This work was in part supported by the ABRL (R14-2002-058-01000-0).
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This preprint was prepared with the AAS LATEX macros v5.2.
– 18 –
Table 1. Observation Log
Field RA DEC Filter Total Exp Time Seeing Depth Observation Date (UT)
(hh:mm:ss) (dd:mm:ss) (sec) (arcsec) (AB mag)a (yyyy-mm-dd)
NEP-E 18:04:31.51 66:33:38.60 u∗ 1800 × 2 1.13 25.98 2004-09-13/14
g′ 1200 × 2 1.08 26.12 2004-08-22
r′ 1500 × 2 0.99 25.58 2004-08-23
i′ 1500 × 2 0.69 24.70 2004-08-23/09-12
z′ 1800 × 2 0.73 24.03 2004-09-07
NEP-W 17:55:28.49 66:33:38.60 g′ 1200 × 2 1.01 26.12 2004-08-22
r′ 1500 × 2 0.87 25.58 2004-09-19/20
i′ 1500 × 2 0.99 24.85 2004-09-13/14
z′ 1800 × 2 0.85 23.71 2004-09-22
aThe depth of each filter data was measured as 4 σ flux over a circular aperture with a diameter of 1 ′′.
Table 2. NEP Optical Source Cataloga
ID RA (J2000) Dec (J2000) u∗ err(u∗) g′ err(g′) r′ err(r′) i′ err(i′) z′ err(z′) flag stellarity ellipticity fwhm r eff sma area field
[deg] [deg] [mag] [mag] [mag] [mag] [mag] [arcsec] [arcsec] [arcsec] [arcsec2]
3 271.1438293 66.0613251 99.000 99.000 25.438 0.098 22.822 0.020 22.608 0.031 26.193 1.738 0 0.08 0.295 1.611 0.620 0.423 1.677 1
5 271.4050293 66.0613251 99.000 99.000 24.105 0.033 22.895 0.020 22.200 0.019 99.000 99.000 0 0.76 0.042 0.990 0.460 0.347 1.814 1
6 272.0104980 66.0599136 99.000 99.000 99.000 99.000 18.853 0.001 18.484 0.000 99.000 99.000 4 1.00 0.162 0.773 0.326 0.156 0.274 1
17 270.6894836 66.0615005 99.000 99.000 24.819 0.066 22.994 0.025 21.750 0.014 99.000 99.000 0 0.28 0.056 1.373 0.562 0.389 1.814 1
20 270.6847839 66.0614319 99.000 99.000 26.160 0.182 22.875 0.022 21.472 0.010 99.000 99.000 0 0.18 0.133 2.218 0.580 0.398 1.882 1
24 271.0955811 66.0624542 23.651 0.040 23.783 0.026 22.803 0.019 22.544 0.028 99.000 99.000 0 0.76 0.110 1.117 0.531 0.334 1.437 1
25 270.9777527 66.0611649 99.000 99.000 20.694 0.002 19.145 0.001 18.382 0.001 99.000 99.000 0 0.99 0.132 1.025 0.407 0.481 6.640 1
26 270.6968384 66.0618744 23.781 0.050 25.055 0.061 22.911 0.018 22.476 0.022 99.000 99.000 0 0.88 0.129 1.175 0.484 0.318 1.369 1
35 271.8403320 66.0610962 24.433 0.072 24.490 0.041 22.855 0.018 22.677 0.028 99.000 99.000 0 0.88 0.160 1.032 0.468 0.333 1.506 1
37 270.7434998 66.0618744 99.000 99.000 24.161 0.036 22.042 0.011 20.852 0.006 99.000 99.000 0 0.98 0.090 0.995 0.506 0.408 2.772 1
aThe complete version of this table is in the electronic edition of the Journal. The printed edition contains only a sample.
– 20 –
Fig. 1.— The observation field map around the North Ecliptic Pole (NEP) (dashed line
boxes). Squares and asterisks represent, respectively, the known radio (Brinkmann et al.
1999) and X-ray (Henry et al. 2006) sources. Circles with an asterisk represent galaxy groups
or clusters classified by Gioia et al. (2003) and cataloged in Henry et al. (2006). The large
cross at the center shows the location of NEP. There are about 40 radio and/or X-ray sources
found in our two fields of observation, NEP-E (East) and NEP-W (West).
– 21 –
Fig. 2.— The distribution of the stellarity index (upper panel), the r′ band magnitude error
(mid panel), and the source number counts in u∗, g′, r′, i′, z′ bands (lower panel). The number
of detected sources reaches its maximum at about 24.3 mag for u∗, 24.0 mag for g′, 23.7 mag
for r′, 23.0 mag for i′, and 22.5 mag for z′ band.
– 22 –
Fig. 3.— Upper panel: (g′ − r′) vs r′ color magnitude diagram (CMD) of NEP sources.
Sources selected with r′ ≤ 23 mag and (g′ − r′) color error < 0.1 mag criteria are plotted
in different colors: point sources with stellarity > 0.95 in red and extended sources with
stellarity < 0.2 in blue. Black dots with r′ < 17 mag mostly represent saturated stars.
Lower panel: (g′ − r′) color distribution of those selected sources: point sources in solid
line and extended sources in dashed line. Point sources and extended sources show different
distributions in (g′ − r′) color: two peaks at (g′ − r′) ≈ 0.3 (G dwarf stars) and 1.4 (M giant
stars) for point sources and a single peak at (g′ − r′) ≈ 0.8 for extended sources.
– 23 –
Fig. 4.— Upper panel: (u∗−r′) vs r′ color magnitude diagram (CMD) of sources in the NEP-
E field. Sources selected with r′ ≤ 23 mag and (u∗ − r′) color error < 0.1 mag criteria are
plotted in different colors: point sources with stellarity > 0.95 in red and extended sources
with stellarity < 0.2 in blue. Lower panel: (u∗ − r′) color distribution of those selected
sources: point sources in solid line and extended sources in dashed line.
– 24 –
Fig. 5.— Panel A: (g′ − r′) vs (r′ − i′) color-color diagram (CCD) of NEP sources. Sources
selected with r′ ≤ 23 mag, (g′ − r′) color error < 0.1 mag, and (r′ − i′) color error < 0.1
mag criteria are plotted: point sources with stellarity > 0.95 in crosses and extended sources
with stellarity < 0.2 in dots. The color distribution in (g′ − r′) color (Panel B) and (r′ − i′)
color (Panel C) of those selected sources: point sources in solid line and extended sources in
dashed line. Please note that the (r′− i′) and (g′− r′) color sequence of point sources is very
narrow and clear while the feature of extended sources is very diffuse and broad centered at
about (g′ − r′) ∼ 0.8 and (r′ − i′) ∼ 0.3.
– 25 –
Fig. 6.— Panel A: (r′ − i′) vs (i′ − z′) color-color diagram (CCD) of NEP sources. Sources
selected with r′ ≤ 23 mag, (r′ − i′) color error < 0.1 mag, and (i′ − z′) color error < 0.1
mag criteria are plotted: point sources with stellarity > 0.95 in crosses and extended sources
with stellarity < 0.2 in dots. The color distribution in (r′ − i′) color (Panel B) and (i′ − z′)
color (Panel C) of those selected sources: point sources in solid line and extended sources
in dashed line. In this plot, point sources are distributed in a narrow straight line running
from (r′ − i′, i′ − z′) ≈ (0.1, 0.1) to (1.8, 0.8).
– 26 –
Fig. 7.— Panel A: (u∗ − r′) vs (g′ − z′) color-color diagram (CCD) of NEP sources in the
NEP-E field. Selected sources with r′ ≤ 23 mag, (u∗−r′) color error < 0.1 mag, and (g′−z′)
color error < 0.1 mag criteria are plotted: point sources with stellarity > 0.95 in crosses and
extended sources with stellarity < 0.2 in dots. The dashed line in Panel A separates point
sources from extended sources. See text for details. The color distribution in (u∗ − r′) color
(Panel B) and (g′ − z′) color (Panel C) of those selected sources: point sources in solid line
and extended sources in dashed line.
– 27 –
Fig. 8.— The number counts of galaxies in the NEP region, normalized to one square
degree area. The Kümmel & Wagner (2001) (KW01) data in R band are also shown in all
the three panels for comparison with our data (Galaxy I & II) assuming a simple relation
of r′ ≃ R + 0.24. Galaxy I is defined to be sources with stellarity < 0.2 while Galaxy
II is selected based on its position on the (u∗ − r′) vs (g′ − z′) CCD. It is clear that the
overall shapes and patterns of galaxy number counts are generally in good agreement with
Kümmel & Wagner (2001) result.
– 28 –
Fig. 9.— The redshift distribution of 101 galaxy clusters whose photometric and spectro-
scopic data of member galaxies were retrieved from the SDSS DR5 data archive.
– 29 –
Fig. 10.— The color-magnitude diagrams (CMD) of galaxies in 101 nearby (z < 0.2) galaxy
clusters in the SDSS data archive. All the photometric data used in these diagrams are
foreground reddening corrected. Early type galaxies are plotted in crosses. Please note the
relatively narrow and well-defined red sequence of early type galaxies in each CMD.
– 30 –
Fig. 11.— The representative (g′ − r′)0 color red sequences of four galaxy clusters: A2199
(inverted triangle, z = 0.0302), A1169 (square, z = 0.0586), A1346 (triangle, z = 0.0975),
and A775 (circle, z = 0.1334). The color of the red sequence was derived by fitting the
colors of early type galaxies in each cluster marked by the filled symbols while the open
symbols were rejected from the fit. The red sequence of a galaxy cluster shifts redward as
the corresponding redshift increases.
Fig. 12.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of NEPX1. In each CMD, only the extended
sources are plotted and the magnitude limit adopted for the analysis is indicated with an
arrow. The color ranges used to find the cluster galaxies are shown in short dashed lines and
the red sequence line fitted in each color is displayed in long dashed lines. The estimated
redshift from the (g′ − r′) color of the red sequence is indicated in the head as ‘ZCMR’. The
galaxies satisfying the color cut criteria and being located within a solid circle in the lower
left panel are represented by filled circles in the CMDs and the spatial distribution plot.
The size of the circles in the spatial distribution plot are proportional to luminosity: the
larger, the brighter. Galaxies that satisfies only the color cut criteria are plotted in open
circles while other galaxies are in open squares. The dashed line in the density plot displays
the number density profile of all galaxies around the selected area and the solid line shows
the profile of ten times the number density of those selected galaxies and the corresponding
errors.
– 32 –
Fig. 13.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of RX J1754.7+6623. There are several bright
galaxies in the foreground over the many faint background galaxies in this region. See Figure
12 for the legend.
– 33 –
Fig. 14.— The CMDs (the three upper panels), the spatial distribution (lower left), and the
number density plot (lower right) of galaxies of RX J1757.9+6609. See Figure 12 for the
legend.
– 34 –
Fig. 15.— The photometric properties of X-ray sources. Stellar symbols for stars, triangles
for type 1 AGN (AGN1), reversed triangles for type 2 AGN (AGN2), asterisks for BL Lac’s,
squares for galaxies, filled circles for galaxy clusters, and dots for point sources from our
photometry catalog. The classification information of X-ray sources are from Henry et al.
(2006). The distribution of point sources is presented in each CMD or CCD for references.
Stars marked by stellar symbols in these diagrams are suspected to suffer from saturation
effect except for one faint star represented by a stellar symbol within a circle. Please note
that the point-like AGN1 sources (filled triangles) are bluer than (g′ − r′) ∼ 0.5.
	Introduction
	Observation
	Data Reduction
	Pre-Photometry Processing
	Source Detection & Photometry
	Results
	Source Catalog
	Color-Magnitude and Color-Color Diagrams
	Galaxy Number Counts
	Galaxy Clusters
	Nearby Galaxy Clusters in SDSS
	Galaxy Cluster Search in the NEP Field
	Galaxy Clusters and Groups in X-ray/Radio Source Catalogs
	NEPX1/VLA 1801.5+6645
	RX J1754.7+6623
	RX J1757.9+6609
	Other Galaxy Clusters
	The Photometric Properties of Other X-ray Sources
	Summary and Conclusion
