865 research outputs found
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 origin of intracluster stellar objects
We numerically investigate dynamical evolution of non-nucleated dwarf
elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of
galaxies in order to understand the origin of intracluster stellar objects,
such as intracluster stars (ICSs), GCs (ICGCs), and ``ultra-compact dwarf''
(UCDs) recently discovered by all-object spectroscopic survey centred on the
Fornax cluster of galaxies. We find that the outer stellar components of a
nucleated dwarf are removed by the strong tidal field of the cluster, whereas
the nucleus manages to survive as a result of its initially compact nature. The
developed naked nucleus is found to have physical properties (e.g., size and
mass) similar to those observed for UCDs. We also find that the UCD formation
processes does depend on the radial density profile of the dark halo in the
sense that UCDs are less likely to be formed from dwarfs embedded in dark
matter halos with central `cuspy' density profiles. Our simulations also
suggest that very massive and compact stellar systems can be rapidly and
efficiently formed in the central regions of dwarfs through the merging of
smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be
stripped to form ICGCs.Comment: 6 pages and 3 figures (JPG file for Fig. 1), in the proceedings of
IAU 217 ``Recycling intergalactic and interstellar matter'
Observations of QSO J2233-606 in the Southern Hubble Deep Field
The Hubble Deep Field South (HDF-S) HST observations are expected to begin in
October 1998. We present a composite spectrum of the QSO in the HDF-S field
covering UV/optical/near IR wavelengths, obtained by combining data from the
ANU 2.3m Telescope with STIS on the HST. This intermediate resolution spectrum
covers the range 1600-10000A and allows us to derive some basic information on
the intervening absorption systems which will be important in planning future
higher resolution studies of this QSO.Comment: 9 pages and 2 figures, submitted to ApJ
Ultra-compact dwarf galaxies: a new class of compact stellar system discovered in the Fornax Cluster
We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain
a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5
to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of
galaxies. Among the unresolved targets are five objects which are members of
the Fornax Cluster. They are extremely compact stellar systems with scale
lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known
type of stellar system, being more compact and significantly less luminous than
other compact dwarf galaxies, yet much brighter than any globular cluster.Comment: To appear in IAU Symposium 207: Extragalactic Star Cluster
Unravelling Active Galactic Nuclei
A complete flat-spectrum radio-loud sample of AGN includes a significant
fraction of Seyfert-like AGN including a NLS1. Analysis of their optical
spectra suggests that the reddest continuum colours are either associated with
AGN in nearby resolved galaxies, or distant quasars showing relatively narrow
permitted emission lines.Comment: Poster contribution presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Black Hole Mass Estimates of Radio Selected Quasars
The black hole (BH) mass in the centre of AGN has been estimated for a sample
of radio-selected flat-spectrum quasars to investigate the relationship between
BH mass and radio properties of quasars. We have used the virial assumption
with measurements of the H FWHM and luminosity to estimate the central
BH mass. In contrast to previous studies we find no correlation between BH mass
and radio power in these AGN. We find a range in BH mass similar to that seen
in radio-quiet quasars from previous studies. We believe the reason that the
low BH mass radio-loud quasars have not been measured in previous studies is
due to optical selection effects which tend to miss the less optically luminous
radio-loud sources.Comment: 22 pages, 7 figures, accepted for publication in Ap
New Limits on the Possible Variation of Physical Constants
Recent detections of high-redshift absorption by both atomic hydrogen and
molecular gas in the radio spectra of quasars have provided a powerful tool to
measure possible temporal and spatial variations of physical `constants' in the
Universe.
We have compared the frequency of high-redshift Hydrogen 21 cm absorption
with that of associated molecular absorption in two quasars to place new (1
sigma) upper limits on any variation in y = g_p.alpha^2 (where alpha is the
fine structure constant and g_p is the proton g-factor) of |Delta(y)/y| <
5.10^{-6} at redshifts z=0.25 and z=0.68. These quasars are separated by a
comoving distance of 3000 Mpc (H_0=75 km/s/Mpc and q_0=0). We also derive
limits on the time rates of change of |(d.g_p/d.t)/g_p| < 10^{-15}/y and
|(d.alpha/d.t)/\alpha| < 5.10^{-16}/y between the present epoch and z=0.68.
These limits are more than an order of magnitude smaller than previous results
derived from high-redshift measurements.Comment: Latex document using mn style file; 6 pages and 3 figures. Accepted
for publication in Monthly Notices of the Royal Astronomical Societ
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