The `next generation' was the theme of the final session of the
conference. [Glazebrook] discussed new observational techniques
for redshift surveys. He described the application of the `nod and
shuffle' (also known as `va et vient') and `micro-slit' techniques,
which together allow near-optimal sky-subtraction at faint magnitudes
and a greatly increased multiplex gain. High-precision sky-subtraction
is achieved by simultaneously moving the electrons back and forth on the
CCD (charge-shuffling) in concert with moving the telescope back and
forth on the sky (nodding). This exact differential beam-switching
approach means that the object and the sky are observed through the same
part of the slit (or the same fibre) and with the same pixels of the
detector, but avoids the extra read-noise imposed by simply nodding
between exposures. It also offers more rapid time-sampling. With
near-perfect sky-subtraction, long slits are not needed, so one can use
small apertures (micro-slits) in the focal plane mask, and so observe
many more objects simultaneously. Glazebrook described how these
techniqes (and the introduction of a volume-phase holographic grating)
allowed the LDSS++ spectrograph on the 4-metre AAT to measure redshifts
in the Hubble Deep Field South to R
24 in a 12 hour exposure
(6 hours on-object). Further extensions to this technique include the
use of blocking filters to isolate small spectral ranges and so allow
more spectra to be crammed onto the detector. Alternatively, one can use
a `pseudo-slitless' mode in which the object spectra are allowed to
overlap (as in objective prism spectroscopy, but with a mask). The
ambiguity of which line belongs to which object can be resolved by
obtaining a second set of spectra with the grating rotated through
180°. Glazebrook outlined some case studies showing how these
techniques could significantly improve the grasp (in terms of both depth
and sample size) of the next generation of redshift surveys.
Large, deep redshift surveys, and particularly surveys of relatively rare objects, require deep, wide-field imaging to select the target samples. [Sutherland] described the plans for a next-generation survey telescope, the 4-metre Visible-Infrared Survey Telescope for Astronomy (VISTA). In the visible, VISTA will have a 1.5° x 1.5° field of view provided by a mosaic of 50 2048 x 4096 CCDs; in the near-infrared it will be able to cover 1.0° x 1.0° with 4 pointings of its 9 2048 x 2048 IR arrays. The observational strengths will be large surveys (e.g. weak lensing, photometric redshifts), searches for rare objects (e.g. very high redshift QSOs and galaxies, brown dwarfs) and studies of variable objects (supernovae, microlensing, Kuiper belt objects). VISTA will be located in Chile, possibly on Cerro Pachon, and the current target is to achieve first light in late 2003.
The Next Generation Space Telescope
(NGST) will be
an extraordinarily
potent probe of the origin of galaxies and cosmology.
[Christian] reviewed the scientific goals and likely
configuration of NGST. The baseline plan calls for an 8-metre telescope
with wavelength coverage from 0.6-10+µm to be launched in 2008. The
Design Reference
Mission, which
specifies the core science goals of NGST, includes deep imaging and
spectroscopic surveys of galaxy formation and evolution. A deep imaging
survey with NGST could in principle detect a 108
M
galaxy with an
old stellar population out to z > 5, and a galaxy forming stars at the
rate of 1 M
yr-1 to z > 10 (see
Figure 7a). A
spectroscopic survey at a resolving power of 1000 could measure the
star-formation rate in normal galaxies at z
2 and in starburst
galaxies at z > 5 (see Figure 7b). These
capabilities
will allow the direct observation of the processes of star-formation in
the very early universe.
|
|
Figure 7. The capabilities of the Next Generation Space Telescope for imaging and spectroscopy. (a) The broad-band flux limits, as a function of wavelength, reached by a proposed `Next Generation Deep Field'. (b) The spectroscopic flux limits, as a function of wavelength, achievable at a resolving power of 1000 by ground-based telescopes (with and without wide-field capability) and by NGST. |
I thank Luigi Guzzo for macros used in producing Figure 1, Scott Croom for some of data used in producing Figure 3, John Peacock for Figure 4, the SDSS team for Figure 5, Piero Madau for Figure 6, and Carol Christian and Simon Lilly for Figure 7