Inter-cluster Filaments of Galaxies Programme: Abundance and Distribution of Filaments in the 2dFGRS Catalogue

Filaments of galaxies are known to stretch between galaxy clusters at all redshifts in a complex manner. In this Letter, we present an analysis of the frequency and distribution of inter-cluster galaxy filaments selected from the 2dF Galaxy Redshift Survey. Out of 805 cluster-cluster pairs, we find at least 40 per cent have bone-fide filaments. We introduce a filament classification scheme and cast the filaments into several types according to their visual morphology: straight (lying on the cluster-cluster axis; 37 per cent), warped or curved (lying off the cluster-cluster axis; 33 per cent), sheets (planar configurations of galaxies; 3 per cent), uniform (1 per cent) and irregular (26 per cent). We find that straight filaments are more likely to reside between close cluster pairs and they become more curved with increasing cluster separation. This curving is toward a larger mass concentration in general. We also show that the more massive a cluster is, the more likely it is to have a larger number of filaments. Our results are found to be consistent with a Lambda cold dark matter cosmology.Comment: 6 pages, 3 figures, accepted for publication in MNRAS letter

Galaxy threshing and the formation of ultra-compact dwarf galaxies

Recent spectroscopic and morphological observational studies of galaxies around NGC 1399 in the Fornax Cluster (Drinkwater et al. 2000b) have discovered several `ultra-compact dwarf' galaxies with intrinsic sizes of $\sim$ 100 pc and absolute $B$ band magnitudes ranging from -13 to -11 mag. In order to elucidate the origin of these enigmatic objects, we perform numerical simulations on the dynamical evolution of nucleated dwarf galaxies orbiting NGC 1399 and suffering from its strong tidal gravitational field. Adopting a plausible scaling relation for dwarf galaxies, we find that the outer stellar components of a nucleated dwarf are totally removed. This is due to them being tidally stripped over the course of several passages past the central region of NGC 1399. The nucleus, however, manages to survive. We also find that the size and luminosity of the remnant are similar to those observed for ultra-compact dwarf galaxies, if the simulated precursor nucleated dwarf has a mass of $\sim$ $10^8$ $M_{\odot}$. These results suggest that ultra-compact dwarf galaxies could have previously been more luminous dwarf spheroidal or elliptical galaxies with rather compact nuclei.Comment: 9 pages 4 figures,2001, ApJL, 552, 10

2MASS Galaxies in the Fornax Cluster Spectroscopic Survey

The Fornax Cluster Spectroscopic Survey (FCSS) is an all-object survey of a region around the Fornax Cluster of galaxies undertaken using the 2dF multi-object spectrograph on the Anglo-Australian Telescope. Its aim was to obtain spectra for a complete sample of all objects with 16.5 < b_j < 19.7 irrespective of their morphology (i.e. including `stars', `galaxies' and `merged' images). We explore the extent to which (nearby) cluster galaxies are present in 2MASS. We consider the reasons for the omission of 2MASS galaxies from the FCSS and vice versa. We consider the intersection (2.9 square degrees on the sky) of our data set with the infra-red 2 Micron All-Sky Survey (2MASS), using both the 2MASS Extended Source Catalogue (XSC) and the Point Source Catalogue (PSC). We match all the XSC objects to FCSS counterparts by position and also extract a sample of galaxies, selected by their FCSS redshifts, from the PSC. We confirm that all 114 XSC objects in the overlap sample are galaxies, on the basis of their FCSS velocities. A total of 23 Fornax Cluster galaxies appear in the matched data, while, as expected, the remainder of the sample lie at redshifts out to z = 0.2 (the spectra show that 61% are early type galaxies, 18% are intermediate types and 21% are strongly star forming).The PSC sample turns out to contain twice as many galaxies as does the XSC. However, only one of these 225 galaxies is a (dwarf) cluster member. On the other hand, galaxies which are unresolved in the 2MASS data (though almost all are resolved in the optical) amount to 71% of the non-cluster galaxies with 2MASS detections and have redshifts out to z=0.32.Comment: 5 pages, accepted by A&A, resubmitted due to missing reference

The evolution and star formation of dwarf galaxies in the Fornax Cluster

We present the results of a spectroscopic survey of 675 bright (16.5<Bj<18) galaxies in a 6 degree field centred on the Fornax cluster with the FLAIR-II spectrograph on the UK Schmidt Telescope. We measured redshifts for 516 galaxies of which 108 were members of the Fornax Cluster. Nine of these are new cluster members previously misidentified as background galaxies. The cluster dynamics show that the dwarf galaxies are still falling into the cluster whereas the giants are virialised. Our spectral data reveal a higher rate of star formation among the dwarf galaxies than suggested by morphological classification: 35 per cent have H-alpha emission indicative of star formation but only 19 per cent were morphologically classified as late-types. The distribution of scale sizes is consistent with evolutionary processes which transform late-type dwarfs to early-type dwarfs. The fraction of dwarfs with active star formation drops rapidly towards the cluster centre. The star-forming dwarfs are concentrated in the outer regions of the cluster, the most extreme in an infalling subcluster. We estimate gas depletion time scales for 5 dwarfs with detected HI emission: these are long (of order 10 Gyr), indicating that active gas removal must be involved if they are transformed into gas-poor dwarfs as they fall further into the cluster. In agreement with our previous results, we find no compact dwarf elliptical (M32-like) galaxies in the Fornax Cluster.Comment: To appear in Monthly Notices of the Royal Astronomical Societ

UCD Candidates in the Hydra Cluster

NGC 3311, the giant cD galaxy in the Hydra cluster (A1060), has one of the largest globular cluster systems known. We describe new Gemini GMOS (g',i') photometry of the NGC 3311 field which reveals that the red, metal-rich side of its globular cluster population extends smoothly upward into the mass range associated with the new class of Ultra-Compact Dwarfs (UCDs). We identify 29 UCD candidates with estimated masses > 6x10^6 solar masses and discuss their characteristics. This UCD-like sequence is the most well defined one yet seen, and reinforces current ideas that the high-mass end of the globular cluster sequence merges continuously into the UCD sequence, which connects in turn to the E galaxy structural sequence.Comment: 5 pages, 3 figures. Accepted for publication in ApJ Letter

Public Release of 2dF data from the Fornax Cluster Spectroscopic Survey

Thanks to the 2dF spectrograph on the Anglo-Australian Telescope, we have recently completed the first stage of a complete spectroscopic survey more than one order of magnitude larger than any previous study, measuring 7000 spectra in a 6 sq.deg. area as part of our study of the Fornax Cluster. In this article we describe the public release of 3600 spectra from our first field. We hope that this public release will encourage colleagues making surveys for rare objects to choose these fields, as much of the follow-up spectroscopy that might be required is available from our data.Comment: To appear in the AAO Newsletter. Data online at http://astro.ph.unimelb.edu.au/data

Quasar-galaxy associations

There is controversy about the measurement of statistical associations between bright quasars and faint, presumably foreground galaxies. We look at the distribution of galaxies around an unbiased sample of 63 bright, moderate redshift quasars using a new statistic based on the separation of the quasar and its nearest neighbour galaxy. We find a significant excess of close neighbours at separations less than about 10 arcsec which we attribute to the magnification by gravitational lensing of quasars which would otherwise be too faint to be included in our sample. About one quarter to one third of the quasars are so affected although the allowed error in this fraction is large.Comment: uuencoded Postscript file (including figures and tables), SUSSEX-AST 94/8-