R. G. Sharp, R. G. McMahon, S. Hodgkin and C. D. Mackay (Institute of Astronomy,
University of Cambridge)
The Isaac Newton
Telescope has been used in conjunction with the Cambridge InfraRed Survey
Instrument (CIRSI), to undertake a wide area deep IR survey in the J and
H bands. This article gives a brief introduction to the survey and presents
some initial results. In the spirit of the INT Wide Field Camera Survey
et al., 2001; http://www.ing.iac.es/Astronomy/science/wfs/;
we are making reduced data products publicly available. A preliminary data
release is planned for April 2002 with a complete release planned in the
Summer 2002. The survey observations have been used in conjunction with optical
CCD data from the INT Wide Angle Survey to undertake a survey for low and
intermediate redshift quasars (z<3) free from the potential biasing effect
of dust absorption. The results of these observations are reported to illustrate
the utility of the survey data for combined optical-IR survey projects.
CIRSI-INT IR Survey
With a field of 4×7.80'×7.80' at the prime focus of the 2.5m
Isaac Newton Telescope, the Cambridge InfraRed Survey Instrument, CIRSI
et al., 2000), is currently the largest field of view IR imager in operation.
The camera, a mosaic of 4 Rockwell HgCdTe HAWAII IR arrays, is capable of
observing in the J and H bands at the INT. The physical construction of
the detector arrays prevents them being butted together in close proximity
as is normal for optical CCD mosaic cameras. There is a 90% spacing between
the elements of the mosaic. Sequential observations are offset to fill the
gaps in the mosaic. A 4 pointing tile of observations covers an area of 29.6'×29.6'.
Figure 2 demonstrates the camera layout.
Figure 1. CIRSI at the prime focus of the 2.5m
INT. [ JPEG | TIFF ]
Figure 2. The image to the left is a 29.6'×29.6'
mosaic of four pointings of the CIRSI camera. To the right the weight map
associated with the mosaic is shown. [ JPEG | TIFF ]
The nominal survey depths attained are J<20.0, H<19.0 roughly three
magnitudes deeper than the level attained by the 2MASS project. Details
of the fields observed are given in Table 1. The survey observing strategy
consists of ~350sec exposures. Depending on sky brightness conditions during
observations the exposure is built up from a sequence of 4 (or 5) dither
positions with 4 (or 3) exposures of 22s (or 30s) duration at each position.
Approximately 5Gb of data are obtained a night. Data processing is performed
using pipeline processing software developed in Cambridge (Sabbey
et al., 2001).
RA Dec (J2000)
INT WAS ISO ELAIS N1
INT WAS ISO ELAIS N2
INT WAS NGC
INT WAS SGC
Table 1. Survey regions and observational data available. The North and
South Galactic Cap regions (NGC/SGC) are coincident with the recently released
SDSS zero declination strip observations.
INT WAS Data
The fields targeted by the CIRSI-INT survey program are chosen to be coincident
with observations from the INT Wide Angle Survey project (McMahon
et al., 2001). The INT WAS fields surrounding the ISO ELAIS N1 and N2
fields at 1610+54 and 1637+41 have been extensively observed in the J and
H bands and observations have also been undertaken in the J band of the
zero declination strips at RAs of 14:55 and 22:08, coincident with the recent
data release from the Sloan Digital Sky Survey (SDSS).
Pixel scale (arcsec/pixel)
FOV per chip
7.8' × 7.8'
FOV for 2×2 mosaic
29.6' × 29.6'
J, 5min 5σ in 1.2'' seeing
H, 5min 5σ in 1.2'' seeing
Table 2. CIRSI Characteristics
on 2.5-m Isaac Newton Telescope.
Example Science: Reddening Independent Quasar Selection
It has long been known that absorption by dust, if present either along
the line of sight to quasars or within the quasar host galaxies themselves,
could bias samples of quasars selected primarily on the bases of ultraviolet
excess (UVX). Figure 3 demonstrates the predicted effects of dust reddening
on UVX selection of quasars. Two of the quasars identified in the CIRSI-INT
sample for which U band observations are currently available do not show a
UV excess. There are a number of reasons to endeavor to construct a quasar
sample free from a bias against dusty quasars. Two examples are the following:
There is an apparent lack of high column density (logN(HI)>21cm–2)
damped Lyman-α (DLA) quasar absorption systems observed with high metallicity
et al., 1998). If the dust within DLA absorption systems themselves is
responsible for reddening quasars then more evolved DLA hosts with higher
metallicities could well be expected to exhibit higher dust content and
greater dust obscuration of the background quasar, preventing the most evolved
DLA systems from being discovered.
Dust may bias quasar lensing statistics (Kochanek,
1996). If the identification of quasars is biased against objects lensed
by dusty lenses then any model associated with the population of lensed quasars
is also biased. Recently multiply imaged lensed quasars have been used to
study the structure of distant galaxies by detailed studies of quasar absorption
line systems. If lensed quasar samples are biased against dusty lens then
these studies will only investigate systems with little dust.
Figure 3. A comparison is shown of a sample of
previously known quasars identified in the SDSS. The new quasars identified
by gzH selection for which ugr data is available from the INT WAS are marked
with filled circles. The horizontal line at u–g= –0.3 represents a UVX selection
boundary analogous to that used in the 2dF quasar survey (2QZ Boyle
et al., 2000). The stellar locus, computed from a spectral atlas, is
shown along with stellar objects from a field of the INT WAS. The locus of
quasar colour as a function of redshift is indicated by the solid line. The
onset of absorption in the g band, due to the Lyman-a forest, is evident
for quasars with z>3. Reddening vectors for dust models based on the Galaxy
and the Small Magellanic Cloud are shown over a range of redshifts. The 2175Å
feature in the galactic law passes through the g band over the range 1<z<2.
[ JPEG | TIFF ]
Figure 4 illustrates the principle of the gzH colour selection technique
developed for defining quasar candidates. The principle is similar to the
VJK selection method proposed by Warren,
Hewett & Foltz (2000), however, observations are required in only
one IR band. A preliminary quasar identification has been undertaken by Sharp
et al. (2002). 68 candidate quasar are identified in data taken from a
subset of 0.7deg2 of the available survey area. Spectroscopic
observations of 32 targets have been obtained confirming 22 quasars, a success
rate of 65%. Observations are currently available across the ugrizJ and
H bands for a sub set of the quasars identified. The ugr colour diagram,
analogous to the traditional UVX quasar colour selection scheme, is shown
in Figure 3. Two of the newly discovered quasars show no UV excess at all.
Figure 4. The candidate selection diagram for a
0.17deg2 region of the survey is shown with the full quasar sample
overlaid. The selection boundary, chosen based on model quasar colours, is
indicated by the broken line offset from the linear fit to the stellar locus.
[ JPEG | TIFF ]
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