Goal of the exercise is to learn how to recognize elliptical, S0 and spiral galaxies from photometric data and how to measure their luminosities. You will analyse the CCD (see Section 2) frames of 22 galaxies of the galaxy cluster A2593 taken with R and B filters (see Section 2) at the 0.9m telescope in Cerro Tololo, Chile. You should classify these galaxies taking into account their morphological appearance and luminosity profile, and measure their half-luminosity radii, total magnitudes and colours. As tools you will use an X-window terminal and the MIDAS and SUPERMONGO packages.
Abell 2593 (1950 coordinates 
, 
)
is a rich cluster of galaxies in the supercluster of
Pisces-Cetus, a cluster of cluster of galaxies between 160 and 300 Mpc
away from us. With a redshift of 0.04, A2593 has a distance of 240
Mpc.
The cluster has been observed
as part of a long-term scientific project which aims at measuring the
distances of 80 clusters of galaxies distributed in the two ``nearby''
superclusters of Pisces-Cetus and Herculis-Corona Borealis.
For a comprehensive description of the morphological and physical properties which point to the classification schemes of galaxies, you should refer to the Vorlesung on ``Galaxien und grossräumige Struktur'' by R. Bender. A general reference book is ``Extragalactic astronomy'', by Mihalas and Binney (1981). For a more advanced and deep insight into the dynamics of these objects you can read ``Galactic Dynamics'', by Binney and Tremaine (1987). What follows is a short description of the so called Hubble tuning-fork diagram (see front page).
Elliptical galaxies (see Fig. 1) are nearly featureless
tri-dimensional stellar systems with very little rotation and
flattened by anisotropic velocity distribution. This means that the
kinetic energy or mean squared velocity of the stars of an elliptical
galaxy can be different in different directions, in contrast to the
well known case of a thermalized gas, where the velocity distribution
of the gas molecules is a gaussian function of the squared
velocity. Ellipticals are dominated by an old, evolved stellar population,
where all
of the massive, bright and blue main sequence stars have died out, and have
therefore red colours. Their central surface brightness is very high.
Moreover, they possess very little cold gas but can have extended hot,
X-ray emitting gas. They appear as ellipses on the sky, with
flattening 
(where a and b are the semi-major and
minor axis of the galaxies) ranging from 0 (round) to 7 (very
flattened).
In contrast, spiral galaxies (see Fig. 2) are
two-dimensional disks, rotationally supported, with a large fraction
of cold (HI) and warm gas, on-going star formation. Their stellar population
is constantly refurnished by newly formed massive, bright and blue young main
sequence stars, so that spirals have blue
colours. Spirals have low central surface brightnesses compared to
ellipticals. In their central parts one can find a bulge, or
spheroidal component similar to an elliptical galaxy. As their name
says, spirals show often bright spiral arms and/or barred structures.
Spirals appear as circles if seen face-on (inclination angle i=0),
when ellipses more and more flattened as the inclination angle
increases, and edge-on when i=90, following the equation 
.
Between the two groups one finds S0 galaxies (see Fig. 3), which possess a large central bulge similar to an elliptical galaxy, a gasless disk and therefore no on-going star formation.
The galaxies you will have to classify are much more distant (more than a factor of 10!) than those shown in Fig. 1-3. Therefore, do not be astonished if you will not be able to see all of the detailed structure visible on these figures. One of the goals of the exercise is to show that galaxy classification is possible even for distant clusters.
The next pages are organized as follows. Section 2 gives a short introduction to the use of CCD in galaxy photometry (2.1) and discusses the issue of sky subtraction and absolute calibration (2.2). Then it describes the basic properties of the photometric profiles of elliptical and spiral galaxies (2.3), and how to derive total magnitudes and half-luminosity radii of galaxies (2.4). Section 3 describes how you should perform the exercise. You are supposed to answer to all of the questions indicated with A followed by a number. Some other questions, indicated with B followed by a number, are given to allow you a deeper understanding of the subject. Answer to these questions if you have time and will. The appendix lists the MIDAS and SUPERMONGO commands that you should use.