AAO Newsletter July 1996 - Page 6


close pairing. The field consists of a sample of moderately bright (B=14-18 mag) galaxies and 14-15th magnitude guide stars in the region of the Centaurus cluster, one of the richest nearby concentrations of galaxies in the southern sky. Positions were measured from recent UK Schmidt plates using the SuperCOSMOS measuring machine at ROE. The observations were obtained as part of a PATT approved programme (A/96A/44) to obtain velocity dispersions of the brighter galaxies in Centaurus in order to investigate the unusually large scatter in the Dn- sigma relation for this cluster and to obtain redshifts for the fainter galaxies to study the extent of its bimodal velocity substructure. Of the 175 useable fibres on the plate, 4 were assigned to guide stars and 138 to target galaxies; the remaining 36 fibres were used to monitor the sky spectrum.

Figure 2: (a) Comparison of velocity dispersions of Centaurus galaxies measured from 2dF spectra with values taken from the literature. The rms scatter is ~ 10 km s-1.

(b) Dn- sigma plot for E/SO galaxies in the Centaurus data. Open symbols are the Cen30 group at cz = 3000 km s-1, filled symbols are for the Cen45 group at cz = 4500 km s-1, confirming that the two subclusters are at essentially the same distance.

Technical limitations during this first run meant that the field was only accessible for 4x 1200 sec exposures, but even with this short integration time, excellent spectra have been obtained for a subset of the field. Initial data reduction using the IRAF DoFibers scripts has concentrated on deriving velocity dispersions for the brightest ~ 30 galaxies for comparison with previous studies. Fig. 2(a) shows the excellent agreement (< 8%) with a compendium of literature data from Lucey & Carter, Dressler & Faber and the Seven Samurai, which extends to velocity dispersions under 100 km s-1 even though the spectrograph focus was not optimal. The Dn- sigma diagram for this data is shown in Fig. 2(b). Given that the calibration frames for this field were very limited (for example, the arcs and flat field spectra had to be taken at prime focus access) and that the Tektronix CCD inside the spectrograph is currently an engineering grade device with several cosmetic defects, the quality of this early data is impressive and reflects the continuing dedicated efforts of the whole 2dF instrument team. It also augurs well for the time (hopefully not too distant) when 2dF is fully operational with 400 fibres (double the complexity of Fig. 1!). two spectrographs and an automatic data, reduction pipeline.

Ray Sharples (AAO) & John Lucey (Durham)


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