The Future

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 R24 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+m 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 10⁸M galaxy with an old stellar population out to z>5, and a galaxy forming stars at the rate of 1Myr^-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 z2 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'. The two sets of spectral energy distributions shown correspond to a galaxy dominated by a young stellar population with a star-formation rate of 1Myr^-1 (at z=2, 5, 10, 20 and 40), and a 10⁸M galaxy with an old (1 Gyr) stellar population (at z=2, 5 and 10). (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. The spectra of galaxies with young and old stellar populations are indicated schematically.

© Copyright Astronomical Society of Australia 1997