Issue |
A&A
Volume 509, January 2010
|
|
---|---|---|
Article Number | A64 | |
Number of page(s) | 7 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912863 | |
Published online | 19 January 2010 |
Near-infrared colours of active galactic nuclei
S. Kouzuma - H. Yamaoka
Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan
Received 10 July 2009 / Accepted 9 October 2009
Abstract
We propose near-infrared colour selection criteria to extract active
galactic nuclei (AGNs) using the near-infrared colour-colour diagram
(CCD) and predict near-infrared colour evolution with redshift. We
first cross-identify two AGN catalogues with the 2MASS Point Source
Catalogue, and confirm both the loci of quasars/AGNs in the
near-infrared CCD and redshift-colour relations. In the CCD, the loci
of over
of AGNs can be distinguished from the stellar locus.
To examine the colours of quasars, we simulate near-infrared
colours using the Hyperz code. Assuming a realistic quasar SED, we
derive simulated near-infrared colours of quasars with redshift
(up to
).
The simulated colours can reproduce not only the redshift-colour
relations but also the loci of quasars/AGNs in the near-infrared CCD.
We finally discuss the possibility of contamination by other types of
objects. We compare the locus of AGNs with an other four types of
objects (namely, microquasars, cataclysmic variables, low mass X-ray
binaries, and massive young stellar objects), which have a radiation
mechanism similar to that of AGNs. In the near-infrared CCD, each type
of object is located at a position similar to the stellar locus.
Accordingly, it is highly probable that we can differentiate
between the four types of objects on the basis of their locus in a
near-infrared CCD. We additionally consider contamination by distant
normal galaxies. The near-infrared colours of several types of galaxies
are also simulated using the Hyperz code. Although galaxies with
have near-infrared colours similar to those of AGNs, these galaxies are
unlikely to be detected because they are very faint. In other words,
few galaxies should contaminate the locus of AGNs in the near-infrared
CCD. Consequently, we can extract reliable AGN candidates on the basis
of the near-infrared CCD.
Key words: galaxies: active - quasars: general - catalogs
1 Introduction
Active galactic nuclei (AGNs) are tremendous energetic sources, where
vast amounts of energy are generated by gravitational accretion of mass
around a supermassive black hole. The radiation at nearly all
wavelengths enables us to detect AGNs in multiwavelength observations.
Hence, AGNs have been studied at various wavelengths. Past studies show
that their spectral energy distributions (SEDs) are roughly represented
by a power-law (i.e.,
),
whilst normal galaxies have an SED that peaks at
m and
resembles the composite black-body spectra of the stellar population.
Because the colours of an object provide us with rough but essential
information about its spectrum, colours are important tools for
differentiating AGNs from normal stars.
Colour selection is an efficient technique for distinguishing
AGNs from normal stars and have played an important role in extracting
AGN candidates without spectral observations. A classic method
is known as the UV-excess (;
Boyle
et al. 1990; Sandage 1965; Schmidt
& Green 1983). The
technique exploits
that quasars are relatively brighter than stars at shorter wavelengths
and therefore occupy a bluer locus in a CCD with respect to stars.
In addition, many AGN candidates have been selected
on the basis of colours at various wavelengths: optical (Richards et al. 2002),
optical and near-infrared (Glikman
et al. 2007), and mid-infrared (Stern
et al. 2005; Lacy et al. 2004).
These studies provide us with clues about the properties
of AGNs.
Target selection of high redshift quasars has also been performed using their colours, mainly at optical wavelengths (e.g., Fan et al. 2003,2000,2001). However, near-infrared properties are required when attempting to select targets such as higher redshift quasars, since the shift of the Lyman break to longer wavelengths makes observations difficult at optical wavelengths. Therefore, near-infrared selection should be a useful technique for extracting high-redshift quasars.
In this paper, we present a study of the near-infrared colours of AGNs and demonstrate, by using both observed and simulated colours, that near-infrared colours can help us to differentiate between AGNs and normal stars. In addition, we predict near-infrared colour evolution based on a Monte Carlo simulation. In Sect. 2, we introduce the catalogues of AGNs, which are used to investigate the observed colours. We confirm the near-infrared properties of spectroscopically confirmed AGNs on the basis of the near-infrared CCD and redshift-colour relations in Sect. 3. In Sect. 4, we simulate the near-infrared colours using the Hyperz code developed by Bolzonella et al. (2000) and demonstrate that AGNs reside in a distinct position in the near-infrared CCD. In Sect. 5, we consider other probable objects that are expected to have near-infrared colours similar to those of AGNs.
2 Data
We examine the near-infrared properties of quasars/AGNs using 2MASS magnitudes. The samples of quasars/AGNs are extracted from the Sloan Digital Sky Survey Data Release 5 (SDSS-DR5) quasar catalog and the catalogue of Quasars and Active Galactic Nuclei (12th edn.), which are both briefly introduced below.
2.1 2MASS
The Two Micron All Sky Survey (2MASS;
Skrutskie et al. 2006)
is a project that observed 99.998% of the entire sky at the J (1.25
m), H (1.65
m), and Ks (2.16
m) bands, at
Mt. Hopkins, Arizona, USA (in the northern
hemisphere) and at CTIO, Chile (in the southern hemisphere)
between June 1997 and February 2001. The instruments
are both highly automated 1.3-m telescopes equipped with three-channel
cameras, each channel consisting of a 256
256 array of HgCdTe detectors. The 2MASS obtained
4 121 439 FITS images (pixel size
)
with 7.8 s of integration time. The
point-source
detection levels are fainter than 15.8, 15.1, and
14.3 mag at J, H, and
bands.
The Point Source Catalogue (PSC) was produced using these images and
catalogued 470 992 970 sources.
At the 2MASS website, the images and the PSC are easily
available to the public.
![]() |
Figure 1: a) The distribution of AGNs in the QA catalogue. b) The distribution of quasars in the SQ catalogue. The stellar locus (Bessell & Brett 1988), the CTTS locus (Meyer et al. 1997), and the reddening vector taken from Rieke & Lebofsky (1985) are also shown. |
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Table 1: The number of objects distributed in the regions I and II.
2.2 SDSS-DR5 quasar catalog
The Sloan Digital Sky Survey (SDSS) provides a photometrically and
astrometrically calibrated digital imaging survey of
sr above Galactic latitude
in five broad optical bands to a depth of
mag
(York et al. 2000).
Many astronomical catalogues have been produced by
this survey.
The SDSS quasar catalog IV (Schneider
et al. 2007, hereafter SQ) is the fourth edition of
the SDSS quasar catalog I (Schneider
et al. 2002), which is selected from the SDSS-DR5 (Adelman-McCarthy et al.
2007). The SQ catalogue consists of
77 429 quasars, the vast majority of which were
discovered by the SDSS. The area covered by the catalogue is deg2.
The quasar redshifts range from 0.08 to 5.41, with a
median value of 1.48. The positional accuracy of each object
is superior to
.
2.3 Quasars and active galactic nuclei (12th edn.)
The catalogue of quasars and active galactic nuclei
(12th edn.) (Véron-Cetty
& Véron 2006, hereafter QA) is the
12th edition of the catalogue of quasars first published in
1971 by De Veny et al. The QA catalogue consists of
85 221 quasars,
1122 BL Lac objects, and
21 737 active galaxies (including 9628
Seyfert 1), and includes position and redshift data as well as
optical brightness (U, B,
V) and 6 cm and 20 cm flux
densities when available. The positional accuracy is superior
to
.
3 Near-infrared properties of AGNs
3.1 Extraction of near-infrared counterparts
The sources in two of the above-mentioned AGN catalogues (SQ and QA) were cross-identified with the 2MASS PSC, and we extracted the near-infrared counterpart of each source. As mentioned in the previous section, the positional accuracies of both catalogues are superior to 1''. Therefore, we identified an near-infrared counterpart when a 2MASS source was located within 1'' of a SQ/QA position.
As a result of the extraction, we derived 9658
(SQ catalogue) and 14 078 (QA catalogue)
near-infrared counterparts. For investigating the
near-infrared properties using 2MASS magnitudes, we used only
2817 (SQ) and 7061 (QA) objects, where 2MASS
photometric quality flags are superior to B (signal-to-noise
ratio (S/N) > ).
3.2 Colour-colour diagram
The near-infrared
CCD is a powerful tool for investigating the properties of celestial
objects. We investigated the near-infrared properties of quasars/AGNs
using a near-infrared CCD.
Figure 1
shows the distributions of quasars/AGNs in a
CCD.
In previous studies, the intrinsic loci of stars and classical
T Tauri stars (CTTS) were clearly defined by Bessell & Brett (1988)
and Meyer et al. (1997).
Their loci are also shown in the CCD. Bessell
& Brett (1988) and the Caltech (CIT) systems are
transformed into the 2MASS photometric system by the method introduced
by Carpenter (2001).
The reddening vector, taken from Rieke
& Lebofsky (1985), is also shown in the diagram.
Because the stellar and CTTS loci can only shift along the
reddening vector, most of these types of stars fundamentally should not
be located in the region described by the following equations:
Equation (1) represents the lower-limit line to where normal stars can reside and Eq. (2) represents the lower-limit line to where CTTS can reside. Both lines are also shown in Fig. 1. Below, we call the region enclosed by Eqs. (1) and (2) ``region II'' and all other regions ``region I''. In Fig. 1, we can see that most of the quasar/AGNs are located in clearly different areas than the stellar loci. The distributions of the quasar/AGNs are on the right side of the stellar loci in the CCD, i.e., they have a (J-H) colour similar to that of normal stars but have a






This colour property was proposed to be caused by
a K-excess by Warren
et al. (2000), who developed a KX method
where quasars with a (V-J) colour
similar to that of stars would be redder in (J-K) colour.
In other words, this KX method can
separate quasars and stars on the basis of their colours. This
technique was used for selecting quasar candidates (e.g., Nakos
et al. 2009; Jurek et al. 2008;
Smail
et al. 2008). The present work is a variant of the
original KX technique, using the (J-H) versus
diagram.
3.3 Colours versus redshift
![]() |
Figure 2: Colours versus redshift for SDSS quasars. The redshifts are taken from the SQ catalogue. The red solid lines show the average colour evolution with respect to redshift. |
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In Fig. 2, we plot the SDSS quasars, in terms of three colours versus redshift, and their average colour evolution with redshift. The redshifts are taken from the SQ catalogue.
Each colour experiences only a small change or dispersion with redshift. This is probably due to the variety of spectral shapes and/or extinctions. In the near-infrared CCD, this small colour change causes a small variation in the AGN locus. These properties can be reproduced by the simulation as mentioned below.
4 Simulating the near-infrared colours of quasars
We demonstrate that the locus of quasars is well separated from that of normal stars on the basis of a simulation using a realistic SED of quasars.
To simulate the near-infrared colours of quasars, we performed
a Monte Carlo simulation with Hyperz code (Bolzonella et al. 2000).
The Hyperz code calculates photometric redshifts using spectral
template library, which identifies the best-fit SED by minimizing the derived
by comparing the observed SED with the expected SEDs. The reddening
effects are taken into account according to a selected reddening law.
Although this code is usually used for estimating photometric
redshifts, we use it to derive the near-infrared colours at various
redshifts.
We first made a magnitude list containing randomly generated J,
H, magnitudes,
ranging from 8 to 16 mag (roughly coincident with a reliable
range of 2MASS magnitude) and produced
100 000 data sets. These data sets were fitted by
various model SED using the Hyperz code. A realistic SED of
quasars was taken from Polletta
et al. (2007) (i.e., QSO1 in Polletta et al. 2007).
According to Polletta
et al. (2007), the SED of the QSO1 is derived by
combining the SDSS quasar composite spectrum and rest-frame IR
data of a sample of 3 SDSS/SWIRE quasars (Hatziminaoglou et al.
2005). We used the reddening law from Calzetti et al. (2000),
which is included by default in the Hyperz code. After inputting the
data sets into the Hyperz code, we derived photometric redshifts with
the probabilities associated with the value of
.
We only used objects with probabilities of
.
![]() |
Figure 3:
Simulated colours versus redshift. The curves represent the simulated
colour evolution with |
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Figure 3
shows the simulated colour evolution with redshift. The curves in each
diagram represent the simulated colours with
(from bottom to top), respectively. To find the best-fits to
the average colour curves, we performed Kolmogorov-Smirnov (KS) tests
between the average colour curves and each simulated colour curve.
Table 2
shows the result of the KS tests. In all three colours, the
colour evolution with
is
the best-fit to each of the five
values.
In addition, the redshift-colour relations of
SQ quasars can be roughly reproduced by simulated curves with
.
A variety of extinctions probably generate the dispersion in
the colours. It should be noted that both the (J-H)
and (
) colours steeply
increase above redshift
.
This is due to shifting of the Lyman break into the J-band
wavelength range. This property can be useful for extracting
high-redshift quasars.
In Fig. 4,
the simulated colours with
are shown in the
CCD, tracked by redshift evolution. An important point is that
the simulated position is separated well from the stellar locus, that
is, it is consistent with the loci of quasars/AGNs shown in
Fig. 1.
A variety of types of extinction causes a dispersion in the
simulated position and this can probably reproduce the dispersion in
the loci of quasars/AGNs in Fig. 1. It is also
consistent with the quasars with
having relatively redder colours in (
)
than the quasars with
.
Although it is difficult to identify high-redshift quasars at ,
we can extract high-redshift quasar candidates at
on the basis of a
diagram because the
(J-H) colour steeply
increases above
.
5 Discussion
5.1 Other probable objects
Table 2: Results of a KS test between average colour evolution and simulated colour evolution.
Although the locus of AGNs in the near-infrared CCD differs from that of normal stars, other types of objects might be distributed in the locus with properties similar to those of AGNs. If a position in the CCD depends on the radiation mechanism, other objects with radiation mechanisms similar to AGNs are also expected to be located at the same position. Below, we examine further the loci of four types of objects that emit non-thermal radiation or are considered to be bright at both near-infrared and X-ray wavelengths: microquasars, cataclysmic variables (CVs), low mass X-ray binaries (LMXBs), and massive young stellar objects (MYSOs).
Sample objects are extracted from three catalogues, namely
microquasar candidates (microquasars; Combi
et al. 2008), cataclysmic binaries, LMXBs, and
related objects (CVs and LMXBs; Ritter
& Kolb 2003b), and catalogue of massive young stellar
objects (MYSOs; Chan
et al. 1996). First, we cross-identified each
catalogue with 2MASS PSC, and extracted the near-infrared counterparts.
Combi et al. (2008)
cross-identified their catalogue with the 2MASS catalogue by adopting a
cross-identification of 4''. The positional accuracy in the
catalogue of Ritter & Kolb is
(Ritter & Kolb
2003a). The objects in the MYSO catalogue were selected from
the Infrared Astronomical Satellite (IRAS) PSC, whose typical position
uncertainties are between about 2'' and 6'' (Helou
et al. 1988; Beichman et al. 1988).
Therefore, we set positional criteria for the cross-identification to
be
(CV and LMXB catalogues) and
(microquasar and MYSO catalogues). We used objects with
a 2MASS photometric quality superior to B
(i.e., S/N >
). Using
their 2MASS magnitudes, they were plotted in a
diagram.
![]() |
Figure 4:
Simulated colour evolution with redshift in the |
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![]() |
Figure 5: The distribution of four types of objects: microquasars ( upper left), cataclysmic variables ( upper right), low mass X-ray binaries ( lower left), and massive young stellar objects ( lower right). The stellar locus and the reddening vector are the same as in Fig. 1. |
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Figure 5
shows the CCD of each object. In each case, a few objects were
distributed around the locus of the AGNs, although most objects were
distributed around the stellar locus or reddened region of the stellar
locus. Table 3
lists the number and percentage of objects distributed in each region.
Although the percentages of both CVs and LMXBs that reside in
region II are relatively higher than those of the other two
types of objects, they are not greater than .
In addition, few objects have
in the region II, although most quasars/AGNs have this colour
(see Fig. 1).
Accordingly, contamination by these four types of objects should be a
small fraction.
This means that the dominant radiation of the four objects should be thermal radiation. The AGNs also radiate thermal radiation, but it represents a very small fraction compared to the non-thermal component produced by accretion around supermassive black holes. Therefore, AGNs should be well separated by these four objects using the near-infrared colours.
Table 3: The number and percentage of objects distributed in each region.
5.2 Contamination by normal galaxies
![]() |
Figure 6:
Simulated colour evolution for seven spiral galaxies. Redshift ranges
from 0.0 to 3.0 with |
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Distant galaxies that appear as point-like sources might contaminate the AGN locus in the near-infrared CCD. We confirmed the locus of normal galaxies in the near-infrared CCD by performing a Monte Carlo simulation as in Sect. 4. The SED templates we used are seven spiral galaxies in Polletta et al. (2007): spirals range from early to late types (S0-Sd).
Figure 6
shows the simulated intrinsic colours (i.e., )
of the seven galaxies. Galaxies with
have intrinsic colours similar to those of normal stars (i.e., they are
in the region I). Galaxies with
are distributed
around the reddened region of either normal stars and/or CTTS.
Therefore, they should not be mistaken for AGN candidates. On
the other hand, simulated colours with
are located in the region II. A fraction of AGN in
the region II are possibly mistaken for galaxies with
.
However, galaxies at
should not have enough brightness to be detected with mid-scale
telescopes. Even the brightest galaxy has no more than
mag
at SDSS r-band (Baldry et al. 2004;
Blanton
et al. 2001). If such a galaxy were located
at
,
its apparent magnitude would be
mag at J-band.
In addition, the apparent magnitude would be even fainter
because most galaxies have M>-23 mag
and the apparent brightness suffers extinction. Accordingly, only
large-scale telescopes can observe these galaxies. Hence, few galaxies
should contaminate the AGN locus in the near-infrared CCD with
respect to the data where limiting magnitude is
below 20 mag.
6 Summary and conclusion
We have confirmed the existence of a loci of catalogued quasars/AGNs in
a
diagram, of which
above
are clearly separated from the stellar locus. In addition, we
have simulated the near-infrared colours of quasars on the basis of
a Monte Carlo simulation with Hyperz code, and
demonstrated that the simulated colours can reproduce both the
redshift-colour relations and the locus of quasars in the near-infrared
CCD. We have also predicted the colour evolution with redshift
(up to
).
Finally, we have discussed the possibility of contamination by other
types of objects. The locus of AGNs also differs from those of the
other four probable types of objects (namely, microquasars, CVs, LMXBs,
MYSOs) that are expected to be located within similar loci. We
also demonstrated with a Monte Carlo simulation that
normal galaxies are unlikely to contaminate the locus of AGNs in the
near-infrared CCD.
Hewett et al. (2006) investigated near-infrared colours of quasars using an artificial SED, but we have proposed near-infrared colour selection criteria for extracting AGNs and studied both observed and simulated colours, presenting quantitative discussions. An important point is that our selection criteria require only near-infrared photometric data, although some previous studies (e.g., Glikman et al. 2008,2007) adopted colour selections based on colours between near-infrared and optical wavelengths. In other words, our selection criteria make the extraction of candidates easier because only near-infrared colours are required. This technique should also be useful when searching for high-redshift quasars, since they become very faint at optical wavelength due to the shift in their Lyman break.
This paper demonstrates that near-infrared colours can be
useful for selecting AGN candidates. If an additional
constraint is imposed, more reliable candidates can be extracted. When
using the near-infrared colour selection with an additional constraint
on near-infrared catalogues containing sources distributed across a
large area (e.g., 2MASS, DENIS, UKIDSS, and future surveys),
many AGN samples (possibly over 10 000) are expected to be derived in a
region over
10 000 deg2.
Kouzuma & Yamaoka
(2009a) (see also Kouzuma
& Yamaoka 2009b) cross-identified the 2MASS PSC with
the ROSAT catalogue, and extracted AGN candidates across the
entire sky using the near-infrared colour selection in this paper.
These large number of samples may provide us with clues about such an
evolution of AGNs and X-ray background. Additionally, in our
simulation, quasars with
can be extracted on the basis of near-infrared colours. This property
might be helpful for searching for high-redshift quasars in
the future.
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. We thank the anonymous referee for useful comments to improve this paper.
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Footnotes
- ...(2MASS
- 2MASS website (http://www.ipac.caltech.edu/2mass/).
All Tables
Table 1: The number of objects distributed in the regions I and II.
Table 2: Results of a KS test between average colour evolution and simulated colour evolution.
Table 3: The number and percentage of objects distributed in each region.
All Figures
![]() |
Figure 1: a) The distribution of AGNs in the QA catalogue. b) The distribution of quasars in the SQ catalogue. The stellar locus (Bessell & Brett 1988), the CTTS locus (Meyer et al. 1997), and the reddening vector taken from Rieke & Lebofsky (1985) are also shown. |
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In the text |
![]() |
Figure 2: Colours versus redshift for SDSS quasars. The redshifts are taken from the SQ catalogue. The red solid lines show the average colour evolution with respect to redshift. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Simulated colours versus redshift. The curves represent the simulated
colour evolution with |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Simulated colour evolution with redshift in the |
Open with DEXTER | |
In the text |
![]() |
Figure 5: The distribution of four types of objects: microquasars ( upper left), cataclysmic variables ( upper right), low mass X-ray binaries ( lower left), and massive young stellar objects ( lower right). The stellar locus and the reddening vector are the same as in Fig. 1. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Simulated colour evolution for seven spiral galaxies. Redshift ranges
from 0.0 to 3.0 with |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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