1,156 research outputs found
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 100 pc
and absolute 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
. 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
The Fornax Spectroscopic Survey --- Low Surface Brightness Galaxies in Fornax
The Fornax Spectroscopic Survey is a large optical spectroscopic survey of
ALL 14,000 objects with 16.5<Bj<19.7 in a 12 sq.deg area of sky centered on the
Fornax Cluster. We are using the 400-fibre Two Degree Field spectrograph on the
Anglo-Australian Telescope: the multiplex advantage of this system allows us to
observe objects conventionally classified as `stars' as well as `galaxies'.
This is the only way to minimise selection effects caused by image
classification or assessing cluster membership.
In this paper we present the first measurements of low surface brightness
(LSB) galaxies we have detected both in the Fornax Cluster and among the
background field galaxies. The new cluster members include some very low
luminosity (M_B approx -11.5 mag) dwarf ellipticals, whereas the background LSB
galaxies are luminous (-19.6<M_B<-17.0 mag) disk-like galaxies.Comment: To appear in "The Low Surface Brightness Universe", IAU Coll 171,
eds. J.I. Davies et al., A.S.P. Conference Series. 8 pages, LaTex, 6
encapsulated ps-figures, requires paspconf.st
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
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
Total focussing method for volumetric imaging in immersion non destructive evaluation
This paper describes the use of a 550 (25x22) element 2MHz 2D piezoelectric composite array in immersion mode to image an aluminum test block containing a collection of artificial defects. The defects included a 1mm diameter side-drilled hole, a collection of 1mm slot defects with varying degrees of skew to the normal and a flat bottomed hole. The data collection was carried out using the full matrix capture; a scanning procedure was developed to allow the operation of the large element count array through a conventional 64-channel phased array controller. A 3D TFM algorithm capable of imaging in a dual media environment was implemented in MATLAB for the offline processing the raw scan data. This algorithm facilitates the creation of 3D images of defects while accounting for refraction effects at material boundaries. In each of the test samples interrogated the defects, and their spatial position, are readily identified using TFM. Defect directional information has been characterized using VTFM for defect exhibiting angles up to and including 45o of skew
The phase shift of an ultrasonic pulse at an oil layer and determination of film thickness
An ultrasonic pulse incident on a lubricating oil film in a machine element will be partially reflected and partially transmitted. The proportion of the wave amplitude reflected, termed the reflection coefficient, depends on the film thickness and the acoustic properties of the oil. When the appropriate ultrasonic frequency is used, the magnitude of the reflection coefficient can be used to determine the oil film thickness. However, the reflected wave has both a real component and an imaginary component, and both the amplitude and the phase are functions of the film thickness. The phase of the reflected wave will be shifted from that of the incident wave when it is reflected. In the present study, this phase shift is explored as the film changes and is evaluated as an alternative means to measure oil film thickness. A quas i-static theoretical model of the reflection response from an oil film has been, developed. This model relates the phase shift to the wave frequency and the film properties. Measurements of reflection coefficient from a static model oil film and also from a rotating journal bearing have been recorded. These have been used to determine the oil film thickness using both amplitude and phase shift methods. In both cases, the results agree closely with independent assessments of the oil film thickness. The model of ultrasonic reflection is further extended to incorporate mass and damping terms. Experiments show that both the mass and the internal damping of the oil films tested in this work have a negligible effect on ultrasonic reflection. A potentially v ery useful application for the simultaneous measurement of reflection coefficient amplitude and phase is that the data can be used to negate the need for a reference. The theoretical relationship between phase and amplitude is fitted to the data. An extrapolation is performed to determine the values of amplitude and phase for an infinitely thick layer. This is equivalent to the reference signal determined by measuring the reflection coefficient directly, but importantly does not require the materials to be separated. This provides a simple and effective means of continuously calibrating the film measurement approach
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
Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups
We present the results of a search for ultra-compact dwarf galaxies (UCDs) in
six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and
NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except
NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates
in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs
were found in the other groups. We compared these results with predicted
luminosities of UCDs in the groups according to the hypothesis that UCDs are
globular clusters formed in galaxies. The theoretical predictions broadly agree
with the observational results, but deeper surveys are needed to fully test the
predictions.Comment: 8 pages, 2 figures, accepted for publication in MNRA
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
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