A&A 475, 791-799 (2007)
DOI: 10.1051/0004-6361:20065218
N. Seymour1,2 - B. Rocca-Volmerange1,3 - V. de Lapparent1
1 - Institut d'Astrophysique de Paris, UMR7095 CNRS / Univ. Pierre & Marie Curie, 98 bis boulevard Arago, 75014 Paris, France
2 -
Spitzer Science Center, California Institute of Technology,
Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA
3 -
Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
Received 16 March 2006 / Accepted 24 May 2007
Abstract
We present a detailed reduction of a mid-infrared 12 m (LW10 filter)
ISOCAM open time observation performed on the ESO-Sculptor Survey field
(Arnouts et al. 1997, A&AS, 124, 163). A complete catalogue of 142 sources (120 galaxies and
22 stars), detected with high significance (equivalent to 5
),
is presented above an integrated flux density of 0.24
.
Star/galaxy
separation is performed by a detailed study of colour-colour diagrams.
The catalogue is complete
to 1
and, below this flux density, the incompleteness is corrected
using two independent methods. The first method uses stars and the second
uses optical counterparts of the ISOCAM galaxies;
these methods yield consistent results. We also apply
an empirical flux density calibration using stars in the field. For each
star, the 12
m flux density is derived by fitting optical colours
from a multi-band
to stellar templates (BaSel-2.0) and using empirical optical-IR
colour-colour relations. This article is
a companion analysis to our 2007 paper (Rocca-Volmerange et al. 2007, A&A, 475, 801) where the
faint galaxy
counts are presented and analysed per galaxy type with the evolutionary code PÉGASE.3.
Key words: infrared: galaxies - Galaxy: evolution - methods: data analysis - catalogs - galaxies: photometry
Deep infrared surveys performed with ISOCAM (Cesarsky et al. 1996) aboard ESA's ISO satellite (Kessler et al. 1996) have greatly increased our knowledge of the faint IR background (e.g. Aussel et al. 1999). From dust emission, the mid-infrared (MIR) is an ideal wavelength domain for studying the fundamental process of star formation at cosmological distances. There is also evidence of a strong evolution of sources in the MIR, including recent results from ISO and Spitzer satellites (Pozzi et al. 2004; Appleton et al. 2004). However, progress in this area has been complicated by technical difficulties in reducing the ISOCAM data. Despite the recent advances of Spitzer, there is still a great deal of information to be extracted from the ISOCAM data.
When examining faint sources with ISOCAM, to be confident in the reliability of a source, one must make certain to have removed all sources of flux variation above the background noise which are not due to astronomical objects. Principle amongst these are "glitches'' caused by cosmic ray impacts on the SiGa detector. There is additional transient behaviour comprising long-term transients that are effectively slow variations in the background and short-term transients that occur when a pixel moves on and off a source causing an upward or downward transient respectively. This memory effect is an unfortunate property of the type of detectors available when ISOCAM was developed. The temporal shape of this lagged response after a flux step has been corrected by a technique developed by Abergel et al. (1996), which successfully removes transients due to strong sources or changes in illumination. Then the software developed by Starck et al. (1999) known as PRETI (Pattern REcognition Technique for ISOCAM data) was designed to identify and remove the other artifacts due to cosmic rays effects and residual low-frequency variations.
The ultimate flux density calibration of ISOCAM has also been the
subject of much research.
Great care is not only needed to distinguish real astronomical
sources from other transient effects in the detectors, but also to
calculate the true flux density of these objects. A detailed analysis of
the behaviour of the detector was applied to ISO observations of
the Hubble Deep Field (HDF), which used the LW2 (
)
and LW3
(
)
filters (Aussel et al. 1999) and was found to be successful at
removing most types of transients significantly above the noise.
Although largely consistent at bright flux densities, the results
of this analysis are considerably
different at fainter flux densities from those previously derived by
Serjeant et al. (1997) from the same observations. (There is, however,
reasonable consistency with another analysis of
Désert et al. 1999.) The method of Aussel et al. (1999) involved
PRETI identifying and correcting the cosmic rays effects, as well as
removing the low-frequency variation of the background left after applying
the Abergel method; many simulations were
performed to test the completeness and flux density calibration of these
data. Another approach, which is empirical and does not depend on
simulations, is that of Efstathiou et al. (2000), who used stars to
calibrate their ISOPHOT
survey. Väisänen et al. (2002) and
Oliver et al. (2002) also used stars to calibrate their ISOCAM LW2 and
LW3 observations. Likewise, Clements et al. (1999) used
observations of stars to verify their flux density calibration.
Over the past few years, the Lari method (Lari et al. 2001) has been successfully applied to other surveys (Gruppioni et al. 2002; Pozzi et al. 2003; Vaccari et al. 2005). This technique involves a full analysis of the history of each pixel and much simulation. We did not use this method in our desire to avoid simulations, but we note that several of these papers (Gruppioni et al. 2002; Vaccari et al. 2005), as well as the more recent work of Rodighiero et al. (2004), use the well-known IR properties of normal stars to verify their flux density calibration.
Here, we present ISOCAM observations that were designed to cover
the portion of the ESO-Sculptor faint galaxy redshift survey
(ESS; Arnouts et al. 1997) that is least affected by the cirrus confusion
noise. The ISOCAM area was selected using the IPAC/IRSKY software
that measured a mean sky flux density of 18.78 mJy/arcmin2 at
and a rms of 5.42 mJy/arcmin2; these values were measured
in 1.5 arcmin pixels in a large field of view of 90'
90' centred
on the ESS field. We also estimate the colour excess as
0.014<EB-V<0.022 on the survey area (Schlegel et al. 1998).
The ESO-Sculptor Survey is located close to the South Galactic Pole
and covers a strip of approximately 0.24
deg2 with CCD photometry complete to Johnson-Cousins B=24.5, V=24.0, and
.
The ESS also provides a nearly complete redshift sample to
(de Lapparent et al. 2003,2004) over a sub-region of
0.24
deg2. The ISOCAM-ESS survey
thus provides a unique
complement to the other existing surveys in its combination of i)
700 square arcminute sky area, ii)
CCD photometry and NIR (DENIS and 2MASS) photometry, and iii) spectroscopic completeness to
.
In the following, we present the data reduction and calibration of
the ISOCAM-ESS survey. The observations were performed with the
LW10 ISOCAM filter, which was designed to have a similar passband to
the IRAS
band. This similarity allows us to take advantage of
the flux density calibration of the IRAS data. In Sect. 2, we first
describe the adopted procedure for source extraction (Sect. 2.1) and
astrometry (Sect. 2.2). This analysis is followed by the flux density
re-calibration in
Sect. 3. We provide the complete source catalogue in Sect. 4.
The ISOCAM observations consist of 10 overlapping raster
observations arranged in a 5
2 configuration centred on RA (J2000) 00
22
23.06
and Dec (J2000)
04'55.65''. Each of the 10 rasters is
composed of
pointings
(with M=N=8) of the long wavelength (LW) detector of ISOCAM,
each offset by
along the axis of the detector.
Table 1 gives the target name, coordinates, observation
number (ION), calculated on-target time (CoTT), number of stabilizing
exposures (
), and number of exposures (
)
for each raster pointing. Table 2 shows the
parameters, which were constant for all observations. Each pointing
of the 32
32 pixel detector used the 6'' pixel field of
view (PFOV) mirror so that the detector's view at each pointing was
a 192''
192'' area of the sky. The total field of view of the
ISOCAM survey is therefore approximately 0.3
0.8 deg2 intersecting over
of the ESS spectroscopic area. For
ISO observations, it was not possible to request a particular
orientation of the camera. In the event, our observations were
performed at
to the axes of the 5
2 arrangement
of the observations (see Fig. 1).
This arrangement left four very small patches of 20'' diameter
unobserved by ISOCAM, which are aligned at the middle declination of the ISO
pointings Dec(J2000)
and have the following
values of RA(J2000):
,
,
,
(these 4 patches are barely visible in Fig. 1).
Table 1: ISO observation log for 10 raster pointings.
Table 2: ISO observation parameters common to all rasters.
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Figure 1:
Signal-to-noise map (shown as contours) of the 10 ISOCAM
rasters overlaying a gray-scale image from the Digitized Sky Survey,
with the ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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We used the latest version of
the original raw data files obtained from the ISO Data
Archive,
corresponding to the 10 raster pointings with the most up-to-date
FITS header information. The data reduction was largely done with
the ISOCAM Interactive Users Analysis System, called CIA (Ott et al. 1997).
At this stage, PRETI (Starck et al. 1999) was run within the CIA/IDL
environment to remove the cosmic rays effects and residual variation
of the background, after subtracting the transients, to perform the flat correction and to
suppress the time-varying baseline. We used the transient correction for
the on-source detector response as described in Abergel et al. (1996). The
flux density of each pixel may be converted from ADUs to
using the
conversion given in CIA: 1 ADU =
.
(More details are given in
Sect. 3 on the flux density calibration.) The 64 independent pointings of each raster were then projected onto the sky allowing for
the known distortion of the sky due to the optics of ISOCAM.
Although the original pixel size of the individual exposures was 6'', each raster image was finely re-sampled to 2'' in order to increase the accuracy of the sky projection. Finally, the 10 raster pointing images were combined on the sky plane producing an image map and a map of the associated rms error at each pixel (henceforth referred to as the "noise'' map). The final pixel size of these maps, from the re-projection of the ten raster images, was chosen to be 3'' as a compromise between retaining the accurate high resolution of the sky projection and possibly lightly over-sampling the PSF whose FWHM is of order of the original 6'' pixels. We note that some additional correlated noise and potential small positional errors may be introduced by a second re-sampling of the image.
ISOCAM observations require specifically designed source extraction algorithms, as the noise in raster data is correlated and varies across the field. This effect is especially noticeable near the edge of a raster where there are fewer readouts per sky position. As a result, the border regions of the survey are noisier than the central part. One must therefore use the corresponding "noise'' map to determine the significance level of a source and to avoid false detections on the border of the survey.
In order to extract the ISOCAM faint sources of the ESS field, we used the multi-scale vision model (MVM, Rué & Bijaoui 1996) as applied to ISOCAM data of the Hubble Deep Field by Starck et al. (1999). This code is implemented in their Multi-Resolution (MR) software. This method searches for objects on different scales in wavelet space using the so-called "à trous'' algorithm. We refer the reader to Starck et al. (1999) for full details.
We apply the Multi-Resolution software with a detection
threshold of
where
is the noise level in wavelet
space and is not directly equivalent to the dispersion
of Gaussian data, but gives a qualitative idea of the
significance of our detection. The ultimate limit to the detection
of sources is the rate at which false detections occur due to
residual glitches in the noise, which are too faint to be removed by
PRETI. Starck et al. (1999) performed simulations to assess the
reliability of their data and obtained a
false detection rate
at the completeness limit (
). Here we choose to perform the
simple test of applying the source extraction to the negative of our
sky image for different thresholds. We detect no sources in the
negative images above
.
Hence we believe our detections
to be quite robust, although we cannot quantify our false detection rate.
We note, though, that we find optical counterparts to all
our
sources covered by the ESS within 6'' and
with R<25 (see Sect. 3). If randomly distributed we would have expected
only
to have had optical counterparts within 6''.
Using the MRDETECT task from MR with a PSF model sampled at
3'' and a detection threshold of
,
we search for
objects down to the 4th wavelet scale and obtain 142 sources to a
detection limit of
.
This limit corresponds to the
integrated flux density, reconstructed by the wavelet detection program,
of the faintest object detected. The final source catalogue, after
astrometry and flux density re-calibration, is presented in
Sect. 4.
With the final goal of validating the adopted extraction method
suited to raster data with ISO, we also investigate the application of
SE XTRACTOR (Bertin & Arnouts 1996) to our field. For a high,
approximately equivalent detection threshold of ,
SE XTRACTOR finds far fewer sources than MR. Of those sources
found by SE XTRACTOR, only
are also found by MR. This
percentage then decreases rapidly with lower thresholds (i.e.
at
). Additionally, the consistent sources in both
catalogues are found to have a random rms offset of
1.5'' in their
position. These offsets are most likely due to the different strategies
of the two different codes, as MR searches for structure in wavelet space
and SE XTRACTOR searches the standard sky plane.
When compared to the list of ESS optical sources, we
find that the SE XTRACTOR positions are marginally more
accurate than those measured by MR (rms of 2.0'' instead of
2.75'').
This comparison thus confirms that a standard source extraction
algorithm such as SE XTRACTOR cannot compete with an extraction
technique specifically designed for ISOCAM data, such as MR.
A first check of the astrometry is obtained by cross-correlation of
our ISOCAM source list with the ESS bright objects
(
)
located within 6''. We obtain an rms offset
of 2'' with no systematic offset, thus indicating that the
absolute astrometry for both ISOCAM and ESS catalogues are
reliable.
To obtain an independent astrometric calibration, we also searched
for another infrared catalogue. The closest available data in
wavelength is the 2 Micron All Sky Survey (2MASS), which includes
J, H, and crucially K-band at
.
As the 2MASS and ISOCAM
observations were taken within a few years of each other, the proper
motion of stars (which make up most of the sources used for the astrometry)
is unlikely to be a problem. We cross-correlate our ISOCAM list
with the 2MASS Point Source Catalogue to search for objects within
3'' of each other. This correlation yields 34 ISOCAM objects with firm
detections and provisional flux densities above
,
which were also
detected in the
-band. We then use these sources with the XTRAN task in AIPS to modify the header of the FITS file of the
image and rms map. This modification leads to a maximum change of 0.3''
to the positions of the 142 final catalogue objects across the field and a
0.1'' improvement in the rms offset with respect to the ESS catalogue.
Although selected to be far from the galactic plane, the relatively
large area of the ESS field provides a sample of stars of
various types sufficient to estimate the empirical flux density
calibration of the ISOCAM observations. This areal size has a significant
advantage compared to most other medium/deep ISOCAM surveys (e.g.
the ISO Hubble Deep Field North, which covers only 27 square
arcminutes of the sky and thus contains only a few calibrating
stars
).
The following analysis uses a series of colour-colour diagrams and
relationships involving optical, NIR and
colours. Due to the
high galactic latitude of our survey, the low extinction
hardly affects these colour-colour relations. For example, the B to
R-band flux density ratio varies by less than
,
and this percentage
is lower for flux density ratios of longer wavelength bands.
In the following sections, we describe the various stages of our
calibration procedure. We first obtain optical and NIR magnitudes
for the detected ISOCAM sources by cross-identification with the
ESS and various other existing catalogues (Sect. 3.1).
We then use colour-colour diagrams to identify stars among the
ISOCAM sources (Sect. 3.2). The core of the calibration
strategy uses a fitting procedure (D. Le Borgne, private communication)
to search for the best-fit stellar template from the PÉGASE
library (Sect. 3.3). We use the colours of the best-fit
templates to predict
flux densities from known IRAS
colour-colour relations (Waters et al. 1987, hereafter WCA) and then
convert to an ISO flux density.
The optical and NIR magnitudes are taken from a variety of sources:
The remaining 31 objects are correlated with the USNO B catalogue in a similar fashion. This correlation leads to 23 further optical counterparts, which include objects either outside the ESS field or cut out of the ESS due to saturation or masking. This list of 134 optical sources is then correlated with the 2MASS data using the optical position and a 1'' search radius, yielding 41 cross-identifications. The I-band magnitudes from DENIS are also found for 79 sources with the same search radius. The more accurate DENIS I-band is used in preference to the USNO photographic I-band when available. Seven objects that remain are too faint to be detected in USNO, 2MASS or DENIS catalogues: sources 100 and 110 are masked by diffraction spikes in the ESS and sources 12, 51, 81, 114, and 137 are not coincident with the ESS area. One source (44) is only detected in 2MASS. Hence all ISOCAM sources covered by ESS have optical counterparts with R<25.
We use the following colour/colour diagrams to separate stars from galaxies.
For all 41 objects with 2MASS counterpart, we select stars by
examining their NIR colours using the criteria
To check for MIR excess due to dust and circumstellar material, we
examine
versus
.
The IRAS Explanatory
Supplement
provides a zero-magnitude flux density of
for the 12
magnitude. However, this value is not technically correct as it was obtained
by assuming that Vega is a blackbody from 10.6
to 12
,
whereas
Vega has quite a large IR excess that affects both the 10.6
flux
density and the spectrum. Here we adopt a value of
from
Cohen et al. (1992). Therefore,
To extend the analysis to objects too faint to be detected by
2MASS (for which we have no NIR data) we examine the positions of
all objects in a optical-MIR colour-colour diagram, shown
in Fig. 4. On this diagram is marked the loci of
blackbodies with a temperature ranging from 3000 K (upper
right) to 104 K (lower left). The 14 objects near the blackbody
line are normal stars without IR excess; they define the
subsample that we analyse with the fitting procedure. All the
14 objects belong to the subset of 19 objects with
and
in Fig. 3. The remaining 5 objects are: i) 4 of the 5 S EXTRACTOR stars identified in the ESS, which lie immediately above the blackbody line, and ii) the misidentified galaxy to the right of the graph with
.
The fifth star in the row above the
blackbody line (with
)
has
and
hence does not satisfy the NIR/MIR colour selection criteria.
Note that there are no objects previously not identified as stars
in the lower part of Fig. 4.
Therefore, among all the objects with optical counterparts, we
identify only 22 stars from the colour-colour diagrams (i.e. all
and only those selected in Fig. 2).
With no further information available the 7 ISOCAM sources without
either an optical or NIR counterpart are assumed to be galaxies.
At this stage we change the DENIS i-band magnitudes from the ESC to the PSC for all 22 sources found to be stars.
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Figure 2:
NIR colour-colour diagram of 40/41 objects with 2MASS
counterparts (minus one object with extreme H-K colour, ![]() |
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Figure 3: NIR-MIR colour-colour diagram of 40 objects with 2MASS counterparts. The symbols are the same as in Fig. 2. |
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Figure 4:
Optical-MIR colour-colour diagram of all objects with optical
(ESS/USNO) counterparts. The symbols are the same as in
Figs. 2 and 3, but with the caveat that
R and B-band magnitudes with no stellarity index are from the USNO
catalogue (i.e. the crosses).
The solid line represents the loci of blackbodies with a temperature
ranging from 3000 K to 10 000 K for the Johnson-Cousins system. We note
this line would vary by <0.1 dex for the USNO filter system.
Furthermore, there are 7 ISOCAM sources (three misidentified, open
stars, and four unknown, crosses) not in the ESS
with
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For the template fitting we used the stellar library from the PÉGASE.2
(http://www2.iap.fr/pegase) code. This library has a significant
coverage of the HR diagram. It is based on the Kurucz library
rearranged by Lejeune et al. (1997): BaSel-2.0 (see Fioc &
Rocca-Volmerange 1987, for details). The chi-squared fitting routine was
applied to the 14 objects of the selected subsample, all
have
magnitudes with the B, R and I-band magnitudes from
the USNO catalogue, not the ESS catalogue, as they were all
saturated or off the ESS field.
The 2MASS pass-bands come from the 2MASS
web-page
.
The USNO pass-bands (from the Palomar Sky Survey and approximately
equivalent to the Johnson B, R, and I-bands) were taken from
Reid et al. (1991). We remind the reader that the extinction is low enough
in this area of sky for its effect on the current analysis to be small,
in the optical and less at longer wavelengths.
The chi-squared fitting of the sample of 14 normal stars is carried out
in a 3-dimensional parameter space: effective temperature (
), surface gravity (g), and metallicity (Z/Z0). The fits are constrained by the
errors of each magnitude: 0.05 mag for 2MASS,
0.05 mag for
DENIS, and 0.25 mag for USNO. The
exhibit a sharp
minimum as a function of
and with a reasonable
dependence on the surface gravity. For 13 out of the 14 stars,
best-fit templates have
,
while the 14th star has
so we exclude it from our sample.
We show in Fig. 5 the stellar fit with the largest
(
). The input magnitudes of the 13 stars are
listed in Table 3 with the parameters values of the
best-fit stellar SED. The first column lists the source number
from Table 4. The next column contains the observed
I-band magnitude from USNO (further photometry of these sources
is presented in Table 4). The final 4 columns contain the
results of the template fitting: the log surface gravity (
), the log metallicity (Z/Z0), the effective temperature in Kelvin, and
of the fit.
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Figure 5:
Result of the stellar fitting to ISOCAM source 20 from
Table 4. The open diamonds with error bars are the
observation magnitudes (
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Figure 5 illustrates that the NIR region of the SED is
more constrained due to the higher accuracies of the NIR magnitudes
(
). This wavelength range corresponds to the region in which
the effective temperature is essentially defined, as is the Wien-tail of the
blackbody spectrum. From Fig. 5, one might expect that
there would be many other stellar templates consistent with this
fit, but with a different SED below
.
Although these different
optical SEDs would
probably not change the effective temperature by much, it could
considerably affect the optical colours, B-V and R-I, which the
WCA colour-colour relations use (see Sect. 3.4). To
investigate this effect, we reran the fitting procedure to all 14 stars without using the B and R-band magnitudes (i.e. without
constraining the fit below I-band wavelengths). In all cases but
one, the effective temperatures changes by only one step in
parameter space (200-250 K), and the SEDs below I-band are generally
consistent with those derived using the B and R-band magnitudes;
similar values of B-V and R-I colours are also obtained. The
one case with a considerably different effective temperature
and SED is found to be almost degenerate with 2 sharp troughs in
parameter space, the slightly higher of the
corresponding to the original 6 band fit. The other fit can easily
be discarded as it is inconsistent with the observed B and R-band magnitudes. Therefore, we conclude that the large errors of the
B and R-band magnitudes do not significantly deteriorate the
quality of the fits.
Table 3: Sub-sample of 13 stars selected for stellar fitting.
Although the BaSeL-2.0 stellar libraries do extend to MIR
wavelengths, they have not been widely tested, and then only at brighter
flux densities (e.g. Cohen et al. 2003, using the Kurucz templates for
certain A0-AV5 stars). We decided to use empirical stellar colours
to verify the calibration of our flux densities and the
long-wavelength part of the stellar template as a consistency check.
We use the two colour-colour relationships of WCA to derive
flux densities. These WCA relations are for IRAS
flux
densities of stars that specifically relate the V-[12] colour
with B-V and R-I:
Table 4:
First ten lines of the complete
source catalogue.
The full catalogue will be available in electronic form at the CDS.
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Figure 6:
Ratio of predicted ISOCAM
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The V magnitudes and B-V, R-I colours, which we used when
applying Eqs. (4)-(6) to the
13 stars listed in Table 3, are those derived directly
from best-fit template spectra. We apply both relationships for each
star, thus yielding 2 estimates of the
magnitude.
Equation (3) subsequently gives us 2 estimates of the flux
density, from which we derive an average flux density. The ratio of this
predicted ISOCAM flux density to the initially measured ISOCAM flux
density, i.e. the observed flux density, is plotted in
Fig. 6. We find a mean value of 1.01
0.17, which
we also mark in Fig. 6. This mean value of the flux density
ratios includes the factor of 1.11 due to the colour corrections.
The ratios presented in Fig. 6 indicate that, for our observational setup,
at least, the flux densities are systematically overestimated for the
normal stars, with no apparent systematic variation in the offset
value with
flux density. Due to the large PSF of these ISOCAM
observations, 6'', slightly extended sources (galaxies) have a
similar response to the detector as unresolved (stars); hence, we
apply this correction to our entire catalogue (see
Table 4). Furthermore, the 1.01 correction factor is not too dissimilar to the
combined flux density correction found by Rodighiero et al. (2004)
of 0.84 (from the combination of their projection bias, mosaic bias and
stellar flux density correction: 0.84
0.915
1.097=0.84).
We are confident we have successfully excluded all stars that exhibit
unusual properties (e.g. Be-stars or those with dust around
them). As a test, we also calculate the ratio of predicted to observed
flux density using the
flux density derived from the
Kurucz stellar template. The ratio for the Kurucz template flux density is
1.05
0.18. We note that the Kurucz value agrees with the
value from our template fitting procedure, given the error bars,
suggesting that the Kurucz models are not too far off at MIR wavelengths.
Table 4 lists the full flux density-calibrated
catalogue of 142 sources detected by ISOCAM above the minimum
integrated flux density of
.
All
available optical and NIR data are also given, as well as
classification (star/galaxy) and the source of the optical
data. This catalogue is used to compute the faint galaxy
counts at 12
m in the companion paper (Rocca-Volmerange et al. 2007). A
complementary paper on the luminosity function at 12
m is in
preparation.
The uncertainties in the 12 m flux density in Table 4 (Col. [16]) are obtained using
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(7) |
Only the first 10 lines of the catalogue are presented in
Table 4. The full catalogue is available in electronic form at the CDS. The first column contains the source number.
The second column contains the IAU designated naming with the prefix IES (ISO ESO-Sculptor). The third and fourth columns contain the
RA and Dec (J2000). The fifth column indicates the classification of
each source (G = galaxy, S = star). The sixth column indicates the source
of the magnitudes (ESS = ESS, USNO = USNO survey, OFFF =
off ESS area and not detected by USNO, SPIK = hidden by a
diffraction spike in ESS and not detected by USNO and SATD =
saturated in ESS and not detected by USNO). The following 10 columns are, respectively the
and the
flux
density and its uncertainty in mJy. The BU and RU magnitudes are
from USNO,
from ESS, I-band from DENIS and
from 2MASS.
In order to use our ISOCAM source catalogue to derive galaxy numbers
counts, one needs to evaluate the completeness of the catalogue as a
function of flux density.
We use two independent empirical methods. Firstly, we use the same method as
used in the previous section to determine the flux density of stars
(Sect. 3), which we extend to those stars not detected by ISOCAM.
The second method is based on the optical counterparts to galaxies
associated with ISOCAM sources, but detected with lower
significance, in the interval
to
.
The method is based on the following stages: i) selecting stars from
the 2MASS catalogue using their NIR colours, ii) confirming that
they are stars by template fitting (as in Sect. 3.3), and
iii) using the results of the fitting to predict the
flux
density from the optical colours.
We select stars from the 2MASS catalogue as all objects with H-K<0.3,
J-K<1 and K<14 within the area of the ISOCAM field. The two
colour criteria are the same as were used earlier (Eqs. (1)
and (2)) and are typical of normal stars (Allen 1976; Bessell & Brett 1988).
The limiting magnitude criterion
is intended to avoid selecting objects significantly fainter than the
detection limit of the ISOCAM survey (
is
approximately equivalent to
mag),
but is faint enough to allow for stars with some MIR excess to be included.
The resulting catalogue contains
51 stars. We discuss possible selection effects at the end of this
section, especially the impact of stars with infrared excess.
Using all available magnitudes from ESS, 2MASS, DENIS, and USNO for each of
the 51 stars, we find the best-fit template spectrum from the
library of Lejeune et al. (1997), in the same fashion as in
Sect. 3.3 for the 22 stars detected by ISOCAM.
We then use the optical-
relations of WCA in Eqs. (5)
and (6) to estimate the
flux density. Three of the
51 stars with predicted
have fits with
.
For these 3 stars, we instead use the relation
Of the 51 stars, 18 (including the 3 stars with )
have predicted
flux density above the detection limit, but were not found
by the MR source extraction software.
This sample, including the 22 flux-density calibrated, ISOCAM-detected
stars is then used to determine the fraction of 2MASS stars detected
by ISOCAM at
as a function of flux density. This result provides
us with a first estimate of the completeness, presented in
Fig. 7. The bins in Fig. 7 have
equal sizes in log flux density space and the errors for each bin are
assumed to be Poisson.
We now consider the possible selection effects. It is unlikely that
we have missed stars due to the colour criteria (Eqs. (1)
and (2)), which are robust for normal stars, even those
with an MIR excess. It is also unlikely that we
have included any galaxies, as they would be poorly fitted by the
stellar templates. But it is possible that some of the selected
stars have infrared excess, so their true
flux density is
higher than that predicted. An excess of
would be
equivalent to an increase of flux density equivalent to
.
Hence, if our field contains
stars with infrared excesses, then the completeness estimated here
represents an upper limit. We note, however, that only 3 out of the 22 stars detected by ISOCAM have
excesses. Therefore, we estimate that the completeness may be overestimated by 15% at most, which is well inside the Poisson error bars.
Because the pixels are relatively large, 6'', most galaxies appear unresolved to ISOCAM. We can then assume that the sensitivity of the detector is the same for unresolved objects, i.e. stars, as for resolved objects like galaxies. Hence the completeness in galaxies is likely to be similar to that of stars and can be used as such. If the completeness for galaxies is however affected by the non-detection of low surface-brightness objects, then again the estimate plotted in Fig. 7 is an upper limit. We also suspect that this effect is smaller than the plotted Poisson errors.
The second method provides an independent correction to the source
counts. It is based on the association of ISOCAM sources detected
at low significance with optically detected galaxies. A priori, some
of the ISOCAM sources detected with a detection threshold located
in the interval
to
may be real. By examining
their association with the ESS optical sources, we can evaluate
their reality in a statistical way. We use PRETI as in
Sect. 2, but with a
detection threshold in wavelet space of
.
This threshold leads to
detection of 328 sources, to be compared with the
list of
142 objects presented in Table 4. After
correcting the flux density of each object by the correction found in
Sect. 3.4, and removing the sources with a flux density below
the detection limit of
,
we end up with 292 potential
sources (i.e. the 142 sources of the original catalogue and 150 new
sources of lower significance).
![]() |
Figure 7:
Completeness of the ISOCAM catalogue as derived from both methods:
stellar template fitting to the 2MASS stars in the ESS field and the
inclusion of lower significance ISOCAM sources with a
detection threshold between
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We then cross-correlate the 150 new
ISOCAM
sources with the ESS catalogue. Using a 3'' (6'') search radius,
we find 74 (110) optical counterparts. From
f12/fR versus fB/fR
diagrams, like Fig. 4, all these sources are found to
be galaxies. As some of
these matches are by chance, due to the size of our search radius
and the space density of optical sources, we evaluate the excess
matches by offsetting the positions of the 2 data sets by increasing
multiples of half an arcsecond in RA and Dec. At large offsets
(
6''), the number of matches becomes roughly constant with a
value around 20 (55) for the 3'' (6'') search radius. These numbers are
comparable with the theoretical numbers of 17 (70) sources expected
within the 3'' (6'') search radius if one uses the sky density
of objects and assumes a random distribution.
We now assume that adding the sources from either the 3'' or 6''list to the high-significance list of 142 sources, yields a nearly
"complete'' source catalogue. This assumption is reasonable because
there is a large excess of ISOCAM coincidences with the ESS above
the theoretical and empirical random values. A "real'' object,
detected with [12]<13 (corresponding to the
detection
limit), must have V<22.5 if it is a star-forming galaxy at
.
This selection is derived from typical, maximum
colours (
5,
see Fig. 3) and
typical, maximum
colour (
4.5). The possibility
that some sources are not real is taken into account in the errors
on the source counts which are derived in Rocca-Volmerange et al. (2007).
The resulting completeness, defined as the ratio of the uncorrected
number counts from the
list to the counts from the "complete''
list, is plotted as a function of flux density in Fig. 7
(solid line). The bins again have equal sizes in log flux
density space, albeit smaller than before, and the errors
are simply Poisson. Note that the curve corresponding to a search
radius of 3'' is indistinguishable from that with 6'' plotted in
Fig. 7.
The sources counts extended to
are subject to several
biases: they probably include some sources that are false and might
still miss some real sources, which would be above our
detection
threshold. The former would lead to an over-estimation of the source
counts (although we include it in our error), and the
latter to an under-estimation. It is nevertheless significant that
the source counts from both the low-significance lists (with 3''and 6'' search radii) yield a incompleteness consistent with what
is derived from stars in the previous section (see Fig. 7).
Although difficult to quantify, the agreement of these 2 independent
methods for estimating the completeness indicates that these 2 selection effects affecting the low-significance sources cancel out to a certain extent.
Therefore both the full and dotted lines in Figs. 7 show
that our ISOCAM catalogue is complete to 1 mJy, with a linearly
decreasing completeness in log flux density down to the our flux
density detection
limit of 0.24 mJy. A complete analysis of the ISOCAM-ESS
galaxy number-counts along with the fitting of theoretical models
are presented in the companion paper (Rocca-Volmerange et al. 2007).
We present the data reduction of ISOCAM observations performed with
the LW10 filter centred near
,
mostly in the field of the optical
ESO-Sculptor survey (Arnouts et al. 1997; de Lapparent et al. 2003). The data reduction of
the ISOCAM rasters was performed as in Aussel et al. (1999): we used the
multi-scale vision model of Rué & Bijaoui (1996) implemented into the MR
software (Starck et al. 1999), along with the PRETI algorithm aimed at
removing all image artifacts above the noise level. Using a
detection threshold of
where
is the noise level
in wavelet space, we reach a detection limit of
.
The final catalogue contains 142 ISOCAM sources with optical
counterparts in the ESO-Sculptor survey, which we complement by
optical and NIR magnitudes from the USNO B catalogue and the 2MASS
and DENIS surveys. Optical, near-infrared, and mid-infrared
colour-colour diagrams
subsequently allow us to identify 22 sources as stars and 120 as
galaxies, which dominate at faint flux densities. By template fitting of 13
of these stars, we derived their predicted
flux density using the
template optical colours combined with the optical-mid-infrared colour
relations of Waters et al. (1987). By comparison with the observed ISOCAM
flux densities normalized using the IRAS
zero-point, we found
that the observed flux densities systematically overestimate the theoretical
flux densities by a factor of 1.16 (1/0.86). We used this offset to correct
all flux densities (i.e. stars and galaxies). We used a similar method
(predicting the
flux density
of stars) to determine the completeness of our survey as a function
of flux density. This completeness function is found to be in good agreement
with that from a statistical study of the coincidence of low-significance
ISOCAM sources with the ESO-Sculptor optical sources.
The ISOCAM-ESO-Sculptor-Survey catalogue obtained here is used to analyse
the mid-infrared galaxy number counts in Rocca-Volmerange et al. (2007). By using
the available ESO-Sculptor redshifts (de Lapparent et al. 2003), we will also
derive the
luminosity function, which is a valuable tool for
interpreting the deep mid-infrared source counts and performing a
detailed study of the evolution of galaxies at
,
a wavelength
range unavailable to the MIPS and IRAC instruments
aboard Spitzer (Gallagher et al. 2003).
Acknowledgements
We thank the referee for the many constructive comments improving the quality of this paper. We give thanks to René Gastaud, Jean-Luc Starck, David Elbaz (SAp/CEA), Carlos del Burgo (Heidelberg), and Emmanuel Bertin (IAP). We are also grateful to Damien Le Borgne (CEA) for aiding us in the use of his software. We thank Hervé Aussel for useful discussions. Part of this work (N.S.) was supported by the Probing the Origin of the Extragalactic background (POE), European Network number HPRN-CT-2000-00138. 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. Additionally this research uses the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station (http://www.nofs.navy.mil/data/fchpix/). We also thank Gary Mamon for kind permission to use data from the DENIS survey. The DENIS project has been partly funded by the SCIENCE and the HCM plans of the European Commission under grants CT920791 and CT940627. It is supported by INSU, MEN, and CNRS in France, by the State of Baden-Württemberg in Germany, by DGICYT in Spain, by CNR in Italy, by FFwFBWF in Austria, by FAPESP in Brazil, by OTKA grants F-4239 and F-013990 in Hungary, and by the ESO C&EE grant A-04-046. The ISOCAM data presented in this paper were analysed using "CIA'', a joint development by the ESA Astrophysics Division and the ISOCAM Consortium. The ISOCAM Consortium was led by the ISOCAM PI, C. Cesarsky.