A classical approach to faint extragalactic source extraction from ISOCAM deep surveys. Application to the Hubble Deep Field
F. X. Désert, J. L. Puget, D. L. Clements, M. Pérault, A. Abergel, J. P. Bernard, C. J. Cesarsky;
AaA, 1999, 342, 363
ABSTRACT:We have developed a general data reduction technique for ISOCAM data coming from various deep surveys of faint galaxies.
In order to reach the
fundamental limits of the camera (due to the background photon noise and the readout
noise), we have devised several steps in the reduction processes that
transform the raw data to a sky map which is then used for point source and sligthly
extended source extraction.
The main difficulties with ISOCAM data are the
long-term glitches and transient effects which can lead to false source
detections or large photometric inaccuracies.
In many instances, redundancy is
the only way towards clear source count statistics.
A sky pixel must have
been ``seen'' by many different CAM pixels.
Our method is based on
least-squares fits to temporal data streams in order to remove the various
instrumental effects.
Projection onto the sky of the result of a ``triple-beam
method'' (ON -(OFF1 + OFF2)/2) obtained from the signal of a given pixel during
three consecutive raster positions leads to the removal of the low frequency
noise.
This is the classical approach when dealing with faint sources on top of a
high background.
We show illustrative examples of our present scheme by
using data taken from the publicly available Hubble Deep Field ISOCAM survey
in order to demonstrate its characteristics.
More than thirty sources
down to the 60 (resp.
100) mu Jy level are clearly detected above 4 sigma at
wavelengths of 7 (resp.
15) mu m , the vast majority at 15 mu m .
A large fraction of
these sources can be identified with visible objects of median magnitude 22
and K-band magnitude of 17.5 and redshifts between 0.5 and 1 (when
available).
A few very red objects could be at larger redshifts.
Based on
observations with ISO, an ESA project with instruments funded by ESA Member States
(especially the PI countries: France, Germany, the Netherlands and the United
Kingdom) and with participation of ISAS and NASA.
KEYWORDS: cosmology: observations, infrared: galaxies, galaxies: evolution, galaxies: photometry, galaxies: statistics, methods: data analysis
CODE: desert99