AAO Newsletter - Science Highlights 2

Probing the depths of the Fornax Cluster and beyond

Background: FLAIR survey of compact dwarf galaxies
Although nearby clusters like Fornax and Virgo provide us with excellent samples of galaxies that have defined our current understanding of dwarf galaxy populations and formation (Binggeli, Sandage & Tammann 1985, ARA&A, 26, 509), the existing samples all suffer from some uncertainty as to cluster membership. Most of the galaxies do not have measured redshifts, so there is always the doubt that a galaxy classified as a dwarf cluster member might actually be a giant background galaxy and vice versa. The very wide field spectroscopy facilities at the AAO now make this problem tractable as it is possible to quickly measure the redshifts of hundreds of galaxies over fields of several degrees.

Over the last 4 years a group of us (Drinkwater, Gregg & Sadler) have been been using the FLAIR-II spectrograph on the UK Schmidt Telescope (UKST) to search for blue compact dwarf (BCD) and M32-like dwarf galaxies in the Fornax cluster. As a result of measuring nearly 500 galaxies in the direction of the cluster we have shown that none of the possible M32-like dwarfs in the Fornax Cluster Catalog (FCC; Ferguson 1989, AJ, 98, 367) are cluster members. The fact that M32-like dwarfs are rare in the cluster is strong argument against the tidal-stripping hypothesis of their formation. From our observations we find that the velocity dispersion of dwarf members of the cluster is 490 km/s, significantly larger than that of the giants (310 km/s). In addition we have found 4 new dwarf members of the cluster (see below).

The 2dF Complete Spectroscopic Survey of the Fornax Area
We are now continuing this project with a complete spectroscopic survey of objects in the central area of the Fornax cluster using 2dF. We will obtain spectra of all objects with B_J=16.5-20 in a 3.5x3.5 degree² area centred on the cluster. The objects will include dwarf cluster galaxies (both low surface brightness [LSB] and putative normal surface brightness dwarfs), a comparison sample of background field galaxies, as well as unresolved or nearly resolved objects including stars, quasars and any previously unrecognised compact galaxies. Our scientific objectives are as follows.

To verify the cluster membership and obtain spectral signatures for the LSB galaxies in the cluster. These are conventionally assumed to be cluster members, but this has never actually been proven via redshift measurements.
To obtain cluster membership for our complete sample. For higher (``normal'') surface brightness faint galaxies, the situation regarding membership is even less clear than for LSB galaxies: the question of whether cluster dwarfs of normal surface brightness even exist is highly controversial.
To obtain a sample of BCDs; evolutionary models of dwarf galaxies often include a BCD phase, and the length of this phase might be estimated through the number of cluster dwarfs currently observed in this phase.
To determine the cluster luminosity function for different morphological types (cf. Binggeli et al. 1985) and different surface brightness ranges, and to look for any possible variation with position in the cluster.
We will also have, as a by-product, a large sample of stellar and near stellar objects. This can be used to obtain (a) an unbiased sample of compact galaxies not identifiable by morphology alone, (b) a serendipitous survey for quasars, (c) a similar survey for white dwarfs, and (d) a large complete sample of disc and halo stars for studies of the stellar luminosity function and spatial distribution. The survey will represent a major improvement on existing ``all object'' surveys like that of Morton et al. (1984, MNRAS, 212, 325) who surveyed 0.31 square degrees.

First 2dF Results
In November 1996 we obtained our first 2dF data for the project: a 2 hour exposure of 150 targets in the centre of the cluster. Most of the targets were previously unmeasured galaxies in the magnitude range B_J=16.5-19.5 but we also observed some low surface brightness galaxies and some radio sources detected on the (University of Sydney) Molonglo Observatory Synthesis Telescope at 843MHz with optical counterparts on the UKST sky survey.

We reduced the spectra using IRAF. The spectra mostly had good signal-to-noise (s/n approx 20 at 5000Å for B_J=18 galaxies in 2 hours) but a significant fraction (about 35%) were worse - although these probably included the low-surface-brightness targets. We were able to measure redshifts of 87 of the targets observed including two of the radio sources which were background quasars, one new one at redshift z=1.6, B_J=19.9, and PKS0335-364 at z=1.5, B_J=18.5.

Figure: Histogram of all previously measured velocities of galaxies in the Fornax field with the new 2dF-measured velocities highlighted. The peak at 1000 km/s is the Fornax cluster.

The impact 2dF will have on our knowledge of the Fornax cluster and field galaxies is shown in the Figure where we compare the velocity data obtained from FLAIR (and earlier publications) over a period of years with the new 2dF data obtained in two hours. This histogram also indicates the considerable inhomogeneity in the large-scale distribution of galaxies behind the cluster which we will investigate with further 2dF data.

New Cluster Members
Using the 2dF data we detected two new dwarf cluster members as well as confirming several FCC classifications. This gives a total of 6 new dwarf members of the cluster when combined with the FLAIR results above: they have absolute magnitudes of M_B=-14.5 to -13.0 and therefore are at the very faint end of the known luminosity function of the cluster which we may now have to revise. In particular, fainter than B_J=17, there are only about twenty possible late-type galaxies in the FCC, none of which are BCDs, whereas three of these new members appear to be BCDs.

M.J. Drinkwater (UNSW), J.I. Davies (Cardiff), R.J. Dickens (Bristol), M.D. Gregg (Livermore), Q.A. Parker (AAO), S. Phillipps (Bristol), E.M. Sadler (Sydney) & R.M. Smith (Cardiff)

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