OTELO: The stellar component of the Groth
field
Pérez Garcı́a, A.M.1 , Alfaro, E.,2 , Cepa, J.,1,3 , Bongiovanni, A.1,4 ,
Castañeda, H.1 , Gallego, J.5 , González, J.J.,6 , González Serrano, J.I.7 ,
Sánchez-Portal, M.8 and the OTELO collaboration
1
2
3
4
5
6
7
8
Instituto de Astrofı́sica de Canarias, La Laguna, Spain
apg@iac.es
Instituto de Astrofı́sica de Andalucı́a, Granada, Spain
Departamento de Astrofı́sica, ULL, La Laguna, Spain
Centro de Investigaciones de Astronomı́a, Mérida, Venezuela
Departamento de Astrofı́sica, UCM, Madrid, Spain
Instituto de Astronomı́a, UNAM, México
IFCA(CSIC-U. de Cantabria), Santander, Spain
HSC,INSA/ESAC, Madrid, Spain
Summary. We present results of the analysis of the broad band images of the
Groth field. This broad band survey is a preparatory survey of the OTELO
project and was carried out with the 4.2m WHT at La Palma. In particular,
we analyse the stellar component of the Groth field, explaining the calibration
and selection of point sources methods used. Also, we compare with models
of stellar population synthesis of the Galaxy.
1 The OTELO project
OTELO, the key OSIRIS science project is a deep emission line object survey to be performed with the OSIRIS Tunable Filters in selected atmospheric
windows relatively free of sky emission lines (see [2],[3] for details). The observing strategy will allow studying a clearly defined volume of the Universe at a
known flux limit. The total survey area is about 1 square degree, distributed in
different low extinction fields with adequate angular separation. These include
a wide variety of astronomical sources: normal star-forming galaxies, starbusts
galaxies, emission –line ellipticals, AGNs, QSOs, Lyα emitters, peculiar stars,
etc. A number of scientific problems can be adressed, including chemical evolution of galaxies; chemical evolution of the Universe; star-formation rates
(SFR) and metallicity; AGN and QSO evolution. To address all these topics
a multi-wavelength study is needed.
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Pérez Garcı́a et al
An auxiliary broad band survey has been undertaken in order to provide morphological classification, photometric redshifts, environment of the
sources, fraction of emission line targets and approximate population synthesis.
2 Observations and reduction
The Groth strip is one of the fields selected for the OTELO project. It is
a commonly observed sky region for different scientific aims in extragalactic
astronomy. Optical broad band photometry has been obtained for this field
by other groups but reaching neither the depth nor the angular size required
for our purposes.
Three different pointings were observed in the direction of Groth, covering
a total area of 0.18 square degrees. Observations were carried out along several
runs using the Prime Focus Camera at the 4.2m William Herschel Telescope
(WHT) of the Observatorio del Roque de Los Muchachos (La Palma, Canary
Islands). The camera provides a field of view of 16’ x 16’ with a pixel size of
0.24 arcsec. Final exposure times range from 7000 to 9000 sec.
1.0
efficiency
0.8
0.6
0.4
0.2
0.0
18
20
22
magnitude
24
26
Fig. 1. Point sources detection efficiency. B (asterisks), V (triangles), R (crosses)
and I (diamonds) are shown.
Reduction sets followed standard steps using IRAF packages. The astrometry was carried out using data of USNO B1.0 astrometric catalogue [4] that
is on the International Celestial Reference System (ICRS). Each individual
CCD frame was calibrated independently by fitting more than one hundred
objects. The final residuals are tipically ∼0.25-0.3 arcsec and no systematic
trends with magnitudes and coordinates were observed.
OTELO: The stellar component of the Groth field
3
The instrumental magnitude for all frames were obtained using SExtractor v2.2 [1]. Source detection in SExtractor depending on two fundamental
parameters, minimum area and detection threshold. A minimum area of 16-20
pixels (seeing disk) and a detection threshold of 1.5σ above local background
were adopted.
3 Photometric calibration
Since photometric conditions were not met during the observing runs, we have
used stars from the Sloan Sky Digited Survey (SDSS) for flux calibration.
SExtractor classifies each object as a star or galaxy by means of the class star
parameter. The output value depends on the seeing, the peak intensity and
the isophotal area, ranging from between 0 (galaxy) to 1 (star). Intermediate
values give an indication of the uncertainty of the classification. In Section 4
we present an analysis of the goodness of this parameter to classify objects.
Fig. 2. Magnitude distribution for our catalogue of stars (black) and Besançon
model stars (orange)
For each filter, we have selected all stars (stellarity > 0.9) with instrumental magnitude > 17 (linearity range) that are common in all frames. Then, we
have matched these stars with the SDSS stars catalogue. The transformation
equations between Sloan and Harris filters used for selected SDSS stars are
(Lupton 2005):
B = g + 0.3130(g − r) + 0.2271; σ = 0.0107
V = g − 0.5784(g − r) − 0.0038; σ = 0.0054
R = r − 0.2936(r − i) − 0.1439; σ = 0.0072
I = r − 1.2444(r − i) − 0.3820; σ = 0.0078
(1)
(2)
(3)
(4)
Once we have made transformations, we have fitted the following equation
to each frame:
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Pérez Garcı́a et al
Mi − MSDSSi = am1 − am2 (MSDSSi − MSDSSj )
(5)
We have combined all images with similar coefficients and then repeated the
fit, obtaining only transformation coefficients for each pointing. From inverse
transformations we calculate colours and finally apparent magnitudes for all
the objects in the catalogue. Final average zero point is estimated comparing
common objects between pointings (three pointings of Groth overlap two by
two).
4 Stars catalogue
The final catalogue of the three pointings contains ∼ 45000 objects, from
which ∼850 are stars, i.e., approximately 2% of total sources. Again, stars
have been separated by selecting objects whose stellarity is >0.9 in addition
to be present in all filters. The catalogue includes RA, DEC, and aperture
magnitude and error in each broad band filter.
Fig. 3. Colour distributions for our catalogue of stars (black) and Besançon model
stars (orange)
In order to test if the stellarity parameter is a good criterion to separate
point from extended sources, we have made some simulations introducing
artificial point sources in our science images. After that, we have applied
SExtractor with the same input parameters and we have counted how many
objects are classified as stars by SExtractor. Results are shown in Figure 1.
For B and V filters, between 19 and 24.5 only 1% of sources are misclassified,
while for R and I filters significative differences are only found for magnitudes
over 23.
4.1 Besançon models
The Besançon models [5] are based on a population synthesis scheme. They
include four distinct populations: a thin disk, a thick disk, a bulge and a
OTELO: The stellar component of the Groth field
5
spheroid. Each population is described by a SFR history, a IMF, an age or age–
range, a set of evolutionary tracks, kinematics, metallicity characteristics and
include white dwarf population. Density laws for the thin disk are constrained
self–consistently by the potential via the Boltzmann equation and are age
dependent. Finally, the extinction is modelled by a diffuse thin disk, which
is adequate in fields of high galactic latitude. The resulting model can be
used for simulations of the galactic stellar populations in any direction in the
UBVRIZJHKL photometric bands.
Fig. 4. Colour–magnitude and colour–colour diagrams for our catalogue of stars
(white) and for the Besançon catalogue (red).
Input parameters are galactic coordinates, field size, intervals of apparent
magnitude in each band and coefficients for photometric errors equations. Selected stars limiting magnitudes have been 25.0, 24.5, 24.5 and 24.0 for B, V,
R and I filter respectively, in order to cover similar ranges than our observed
stellar magnitudes. The equations for photometric errors are exponential and
their coefficients have been calculated by fitting Sextractor photometirc errors. Figures 2 and 4 show the number counts distribution of magnitudes and
colours, respectively, in each photometric band compared to those derived
from the Besançon models.
As shown in Figures 2 and 4 both the observed magnitudes and colour
distributions are well reproduced by the model. Therefore, there is a good
agreement between our observations and the model predictions, giving confidence on the accuracy of our photometric calibration and the completness of
the catalogue.
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Pérez Garcı́a et al
1.0
efficiency
0.8
0.6
0.4
0.2
0.0
18
20
22
B magnitude
24
26
28
Fig. 5.
5 Completness
We have performed two different simulations to determine the limiting magnitude in our data. First, we have introduced a distribution of artificial objects
similar to the real one in background real images. Second, we have included
in each image a set of three hundred objects (that is approximately the total
number of objects detected in each image) per magnitude interval. In both
cases, we have extracted the artificial sources with Sextractor using the same
parameters as for real sources and compared the results with the input catalogue of artificial point sources. The results of both simulations are shown in
Figure 5. It can be seen that results are not significantly different. Limiting
magnitudes at 50% efficiency are 26.0 in B and V bands, 25.5 in R band and
25.0 in I band.
References
1.
2.
3.
4.
5.
E. Bertin & S. Arnouts : A&AS 117, 393 (1996)
J. Cepa et al. : RMxAA, 42, 33 (2005)
J. Cepa et al. : RMxAA, in press (2006)
D.G. Monet et al. : AJ, 125, 984 (2003)
A.C. Robin et al. : A&A, 409, 323 (2003)