STARS
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BL LAC OBJECTS
BL
Lac objects comprise the most enigmatic class of quasi-stellar objects.
They are characterised by highly compact, variable, polarized continuum
emission, and in most cases show no features in their optical spectra.
Their broad-band continuum is modelled reasonably well by emission from
a relativistic jet oriented close to the line of sight. Together with the
observation of low-level extended radio emission surrounding the compact
core, these properties have led to the hypothesis that BL Lac objects are
double radio sources seen end-on, and are thus the most highly beamed objects
in the "unified scheme" of active galactic nuclei.
Although about 120 BL Lac objects
have been catalogued, very little is known about the nature of the underlying
host galaxies. In the few well-studied cases these are ellipticals of moderate
luminosity, which is in agreement with the predictions of the unified scheme.
Reseachers imaged a sample of 23 BL Lac objects using the WHT and TAURUS
optics in a search for the underlying hosts. They were able to determine
the morphology of three others. Two of the newly classified systems are
not ellipticals, but disc galaxies. Since no powerful radio double sources
have yet been associated with disc-dominated galaxies, these observations
cause difficulties for the currently accepted unified scheme. They also
find that the BL Lac cores are all located exactly in the centres of the
host galaxies. This finding argues strongly against the alternative model
for BL Lac objects, namely that they are background quasars microlensed
by stars in the foreground galaxies.
More
information
ING facilities involved:
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William Herschel Telescope
with TAURUS
Some references:
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Abraham, R.G. et al.,
1991, "Optical imaging of BL Lac host galaxies" MNRAS,252,
482
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GALAXIES
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GALACTIC MASS
Researchers
used the INT Intermediate Dispersion Spectrograph to measure stellar radial
velocities and velocity dispersions in the edge-on spiral galaxy NGC 891.
NGC 891 is very similar to our own Galaxy, the Hubble types are both Sb,
and their sizes and luminosities are very close. Spectra were taken at
three different positions on the galaxy with the slit perpendicular to
the major axis and radial velocities and velocity dispersions were determined
as a function of height above the galactic plane. The observations were
translated into dispersion values as a function of radius inside the galaxy
using a model with a disc, bulge and absorbing layer. Assuming an exponential
radial velocity dispersion decreasing with a scalelength twice the photometric
scalelength, a central dispersion of 120±20kms-1
is derived. Thiss compares with 100±10kms-1
for our Galaxy. This makes an independant estimate of the mass contained
within the disc possible. A mass-to-light ratio of between 6 and 10 is
deduced, which compares well with an earlier value of 7 obtained by measuring
the thickness of the hydrogen layer.
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More
information
ING facilities involved:
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Isaac Newton Telescope,
using IDS
Some references:
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Bottema, R. et al,
1991 "The stellar velocity dispersion of the edge-on spiral galaxy NGC
891", A&A, 247, 357
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The IRAS Faint Source catalog galaxy
survey was a collaboration between several universities. Redshifts were
obtained for 1400 galaxies using the Faint Object Specrographs on the INT
and WHT. On the very last night of the campaign, an emission-line galaxy
was discovered with an unusual spectrum: lines of high excitation but weak
Lyman alpha emission. A follow-up spectrum on the WHT revealed a very high
redshift of z=2.286, making it one of the most distant galaxies known.
Nearly all the emission of this source, named 10214+4724, is in the infrared
and its total luminosity (at all wavelengths) is higher than any other
known object. The power source could be a radio-quiet quasar embeded in
a very dusty galaxy; the alternative explanation is a spectacular starburst,
a billion stars all forming at once within a vast dust cloud, in which
case the object would be a galaxy in the process of formation. Observations
revealed that the integrated polarization of the galaxy is 16±2%.
This implies that at least this fraction, and probably most, of the light
from 10214+4724 has been scattered. Regardless of the precise nature of
the galaxy, the illumination must therefore be hightly anisotropic,and
the similarity with objects like 3C368 tends to favour the hidden quasar
interpretation.
More
information
ING facilities involved:
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Isaac Newton Telescope,
using FOS-1
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William Hersschel Telescope,
using FOS-2
Some references:
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Rowan-Robinson, M.,
1991, "Most luminous object record broken again", JBAA, 101,
255
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OBSERVATIONAL
COSMOLOGY
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Counting galaxies as a function of
apparent magnitude is one of the classical cosmological tests and an important
probe of both the geometry and the evolutionary history of the Universe.
Previously, accurate counts had been possible from Schmidt photograghs
to about B=24mag. Now CCD detectors offer a factor of 10-20 improvement
in sensitivity, reaching far fainter magnitude limits. Important constraints
can now be placed on allowable combinations of q0
and evolution. Already the sheer number of galaxies seen is uncomfortably
large for a high q0 Universe.
Of particular interest is the
possible existence of a turn-over in the differential number count at B=27mag
which, if real, could indicate a redshift cut-off. This could be to do
to galaxies having strong Lyman limit systems and hence being invisible
in the B band above z=4; alternatively a low redshift of galaxy formation
could be the cause.
Researchers used the INT to obtain
a series of CCD exposures totalling 24 hours on one field; stacking these
has yielded the deepest B band image ever taken. About 3000 objects were
detected in an area 3.5 x 5.5 arcminutes with a magnitude limit of B=28mag.
The turn-over at B=27mag is as yet unconfirmed, though detailed corrections
for coincidence (merging of faint galaxy images) are not yet complete.
More
information
ING facilities involved:
-
Isaac Newton Telescope
using the prime focus CCD camera
Some references:
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Metcalfe, N. et al,
1991 "Galaxy number counts - II CCD observations to B=25 mag", MNRAS,
249,
498
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Metcalfe, N. et al,
1991, "Ultra-deep INT CCD imaging of the faintest galaxies", GEMINI Newsletter
Royal Greenwich Obs., 34, 12
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Metcalfe, N. et al.,1995,
"Galaxy number counts - III Deep CCD observations to B=27.5mag",MNRAS,
273,
257
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Researchers have discovered a quasar
with a redshift of z=4.7 as part of the high redshift quasar survey in
which UK Schmidt Telescope plates are scanned in the Automatic Plate Measuring
Machine (APM) to select objects of particular B-R, R-I colour. Spectroscopic
confirmation was obtained with the Faint Object Spectrograph on the INT.
Over half of the highest redshift
quasars have been discovered as a result of the UKST-APM-INT combination,
including no less than four of the top six. Although it just fails to take
the record for the highest known redshift (recently established at 4.89),
the I magnitude of this object is 17.5, making it the brightest known object
in the Universe at optical wavelengths. The particular importance of this
discovery is that it is sufficiently bright for relatively high dispersion
spectroscopy. Apart from the intrinsic interest in the object itself, detailed
study of the intervening gas clouds and galaxies is therefore posssible.
Ironically, one by product of the
APM high-redshift quasar search has been one of the nearest and faintest
stars yet discovered. The star, called BRI0021-0214, was selected on the
basis of its BJ-R, R-I colours, then confirmed as a low temperature
M dwarf by a spectrum obtained with the Faint Object Spectrograph on the
INT. Infrared obsservations indicate a bolometric luminosity of about 14mag
or 10-4 solar luminosities, and an effective temperature of
around 2250K. The star, which is about 10pc distant, is very near the hydrogen
burning limit of 0.075 solar masses. The discovery of such stars is important
for the determination of the stellar luminosity function at its faint end
and the contribution of low mass stars to the galactic mass density.
More
information
ING facilities involved:
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Isaac Newton Telescope,
using FOS-1
Some references:
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McMahon, R. and Irwin,
M., 1992, "APM surveys for high-redshift quasars", Digitised Optical Sky
Surveys. Proceedings of the Conference on "Digitised Optical Sky Surveys",
held in Edinburgh, Scotland, June 18-21, 417
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GRAVITATIONAL LENSING
During
the past few years, several giant luminous arcs and faint blue gravitationally
distorted images have been discovered in the centres of various distant
and rich clusters of galaxies. The redshift measurements of such structures
have proved that they are background galaxies gravitationally distorted
by the cluster core. At present, more than 10 different clusters of galaxies
show an arclike structure or distorted images, though few of them have
been confirmed spectroscopically. In the course of an observing run with
the INT, researchers discovered that the rich cluster of galaxies Abell
2390 contains a strange linear object with could be an arc. Spectra of
the structure were obtained with the WHT and the ESO 3.6m which show an
emission line along its whole length. The identification of this emission
line as [OIII] at a wavelength of 3727Å leads to a redshift of z=0.913
for the structure.
The double quasar Q0957+561AB is believed
to be a gravitationally lensed image of a single quasar at z=1.41. It is
seen as two images because of an intervening galaxy, a giant cD elliiptical
in a cluster of redshift z=0.36. Both quasar images vary in brightness
but there is a time delay of 415 days between the appearance of brightness
variations in the two images and this is attributed to the difference in
length of the light paths from the quasar. Simple models have been developed
to account for the properties of the lensing system. The lensing galaxy
is represented by a circularly symmetric smooth surface density profile
whose parameters are the core radius and the velocity dispersion. The model
shows that the time delay is a function of measurable parameters such as
distances and the velocity dispersion, but scales inversely with the Hubble
constant H0. Consequently, an estimate
of the value of H0 is possible. Researchers
using ISIS on the WHT to obtain spectra of the galaxy and quasar images,
and measured the line-of-sight velocity dispersion of the galaxy as 303±50
kms-1. Using the model, the value of H0
becomes 50±17 kms-1Mpc-1.
A key uncertainty in this calculation is the contribution of the cluster
to the lensing effect: this can be estimated from the cluster velocity
dispersion, for which further obsservations will be needed.
More
information
ING facilities involved:
-
Isaac Newton Telescope,
using prime focus CCD
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William Herschel Telescope,
using FOS
Some references:
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Pello, R. et al, 1991
"A straight gravitational image in Abell 2390 - a striking case of lenssing
by a cluster of galaxies", ApJ, 366, 405
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Leborgne, J.-F. et
al, 1991, "Photometric and spectroscopic observations of cluster of galaxies
Abell 2390", A&AS, 88, 133
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OTHER HIGHLIGHTS
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The discovery of an expanding
bubble of hot gas at the centre of the galaxy, along with evidence for
a blue object, gives weight to the theory that there is a massive accreting
black hole at the centre of the galaxy. These discoveries were made using
a Fabry-Perot spectrometer (FAST) on the WHT.
The best candidate yet for a
stellar size black hole was found using the WHT and INT, in an X-ray binary
system V404 Cyg.
A survey of all the northern
symbiotic stars (consisting of a cool continuum and absorption bands) was
carried out using the INT and WHT.
Researchers continue to use
the WHT with GHRIL to make maps of the surface of Betelgeuse (see Scientific
Highlights 1990) and have shown that the features are changing with time
and are thought to be due to large convection cells within the star.
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