here - Institute for Astronomy Umleitung

Title: Science Case
Reference: MUSE-MEM-SCI-052
Issue: 1.3
Date: 04/02/2004
Page: 49/100
The main issue for a MUSE survey is the limited field of view (1 arcmin 2 ). The comoving
scale corresponding to 1 arcmin does not change strongly with redshift at z > 2 and is
approximately 2.5 Mpc. For studying the assembly of individual galaxies, surveying scales of
only a few Mpc is sufficient. At turn-around, the 6×10 10 M
of matter which is currently
within the virialized halo of a ~L* galaxy such as the Milky Way is contained within a
volume of radius 750 kpc. Thus a survey field of side 2.5 Mpc should contain all the baryonic
material that will assemble into individual L* galaxies.
The other physical scale of interest is the clustering scale of galaxies, about 5-10 comoving
Mpc. This enters into the issue of sampling variance since it means that the galaxy population
within spheres of this size is highly correlated and the statistics of galaxies are far more noisy
than simple consideration of their numbers would indicate - put another way, the n galaxies
within a sample do not represent n statistically independent entities but rather n/m entities,
where m may be calculated from the correlation function knowing the survey geometry.
Surveys on arc-minute scales are dominated by this sampling variance: a good example is the
very different population of red galaxies in the HDF-N and HDF-S which, on their own would
lead to quite different interpretations of the global star-formation rate.
Given the limited field of view of MUSE, the two strategies for overcoming sampling
variance are (a) to observe adjacent contiguous fields to build up a larger area, or (b) to
observe widely separated fields. Of these,
the second is much more efficient: the
statistical weight of the survey builds up as
N 0.5 (where N is the number of MUSE
pointings) whereas in (a) the gain is more
like N 0.3 . Thus the optimum survey
strategy would be to observe multiple
widely spaced pointings.
A good feature of this is that this strategy
naturally accommodates the need for m ~
17–18 guide stars for the AO system.
Unfortunately, at present, most of the deep
extragalactic survey fields, and especially
those that have been observed with the
HST) consist of a handful of large
contiguous areas (e.g. GOODS-S 10×16
arcmin 2 ). However, within these, multiple
pointings around available stars could be
made (Fig. 2-26).
Figure 2-26: Potential guide-stars in the CDFS
region, each surrounded by a 90 arcsec radius
region. Axes are decimal degrees in RA and dec