New Structure In The Shapley Supercluster
M.J.Drinkwater, D.Proust, Q.A.Parker, H.Quintana, E.Slezak, PASA, 16 (2), in press.

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Observations

Although a large body of galaxy velocity data is available in the literature for the SSC, the existing samples of redshifts in each cluster are highly incomplete, even at the bright end of the luminosity function. We have therefore started a campaign to obtain complete samples down to the same magnitude below L_* for each cluster. Each selected cluster has a projected diameter of 2.5 to 3.0 degrees, so the FLAIR-II system on the UKST with a deg² field is an ideal facility for this project. The very wide field also permits us to probe the regions between the dominant clusters neglected in previous observations. In this paper we emphasise our results from these regions.

We selected targets from red ESO/SRC sky survey plates scanned by the MAMA machine at Paris Observatory (as described in Paper II; see also Infante et al. 1996). The fields observed (listed in Table 1) were the standard survey fields nearest to the centre of the cluster (13:25:00 -31:00:00 B1950). These covered an area of 77 deg² allowing us to probe the limits of the SSC out to radii as large as 8 deg.

We defined a sample of galaxies to a limit of R<16, corresponding (assuming a mean B-R=1.5) to B<17.5, the nominal galaxy limiting magnitude of the FLAIR-II system. This corresponds to an absolute magnitude of M_B=-19 at the Shapley distance of 200

. This gave samples of 600-1000 galaxies per field. We then removed any galaxies with published measurements in the NED database or measured by H. Quintana and R. Carrasco (private communication, 1997): 46 galaxies for F382, 81 for F383 and 200 for F444. For each observing run we then selected random subsamples of about 110 targets per field from the unobserved galaxies. When preparing each field for observation at the telescope we made a further selection of 80 targets to observe (10 fibres being reserved for measurement of the sky background). This final selection was essentially random, but we did reject any galaxies too close (less than about 1 arcminute) to another target already chosen or a bright star.

We observed a total of 3 fields with the FLAIR-II spectrograph in 1997 May and two more in 1998 April. The details of the observations are given in Table 1. In 1997, out of 6 allocated nights we were only able to observe 3 FLAIR fields successfully due to poor weather and the first of these was repeated over 3 nights. Field F444 was observed in particularly poor weather resulting in a much lower number of measured redshifts. In 1998 we again had poor weather, and were only able to observe two fields in an allocation of 8 half-nights.

The data were reduced as in Drinkwater et al. (1996) using the dofibers package in IRAF (Tody 1993). We measured the radial velocities with the RVSAO package (Kurtz & Mink 1998) contributed to IRAF.

Redshifts were measured for absorption-featured spectra using the cross-correlation task XCSAO in RVSAO. We decided to adopt as the absorption velocity the one associated with the minimum error from the cross-correlation against the templates. In the great majority of cases, this coincided also with the maximum R parameter of Tonry & Davis (1979). The redshifts for the emission line objects were determined using the EMSAO task in RVSAO. EMSAO finds emission lines automatically, computes redshifts for each identified line and combines them into a single radial velocity with error. Spectra showing both absorption and emission features were generally measured with the two tasks XCSAO and EMSAO and the result with the lower error used. In two spectra with very poor signal (13:05:19.9 -33:00:31 and 13:23:22.9 -36:47:09) the emission lines were measured manually and a conservative error of 150

assigned. We measured velocities successfully for 306 galaxies in the sample: these are presented in Table 3.

We have compared the distributions of the galaxies we measured to the input samples to check they are fair samples. There is no significant difference in the distributions of the coordinates but there is a small difference in the magnitude distributions in the sense that the measured sample does not have as many of the faintest galaxies as the input sample. This is to be expected as these would have the lowest signal in the FLAIR-II spectra, but this should not affect our study of the spatial distribution significantly.

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Next Section: Results
Title/Abstract Page: New Structure In The
Previous Section: Introduction
Contents Page: Volume 16, Number 2

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© Copyright Astronomical Society of Australia 1997