|Home > Public Information > Scientific Highlights > 2001|
discoveries following from observations
GALAXY | EXTRAGALACTIC
DISCOVERY OF THE FIRST BLACK HOLE IN THE HALO OF OUR GALAXY
X-ray novae or soft
X-ray transients constitute a subset of low-mass
X-ray binaries (LMXBs) that consist
of a late-type secondary star and a neutron
star or black hole exhibiting bright optical
and X-ray outbursts that are recurrent on time
scales of decades. During their
outbursts, they resemble persistent LMXBs
in which the light of the secondary
star is overwhelmed by a luminous accretion
disk surrounding the compact object. After a
year or less in some objects, the system
returns to quiescence. The secondary star now
contributes a much larger fraction of
the total light, and its atmospheric absorption
lines become visible in optical spectra.
Thus, quiescent X-ray novae provide the ideal
opportunity to study the nature and dynamical
properties of the binary system. These
studies have demonstrated so far that the mass of
the compact object in 10 X-ray
novae exceeds the theoretical maximum
mass of a neutron star and thus
must evidently be a black hole.
A previously unknown
X-ray transient, XTE J1118+480, was
discovered by the Rossi X-Ray Timing Explorer all-sky
monitor on 2000 March 29. An optical counterpart
was then identified and confirmed spectroscopically. The shape of the light curve
and its temporal evolution resembled those of
superhumps observed during superoutbursts of
short-period cataclysmic variables and outbursts of
some other soft X-ray transients. The binary system
was found at a distance of about 6,000 light years in a direction pointing
62 degrees away from the Galactic plane.
From spectroscopic observations
carried out by an international team of astronomers using a
number of telescopes, including the WHT, and spanning a couple of months,
the mass of the compact object was determined to be at least 6 times
the mass of the Sun. This lower limit to its mass firmly implies that
it is a black hole, the first one firmly identified in the Galactic halo.
UZ For is a member of the
AM Herculis type cataclysmic variables (CVs), in which a strongly magnetic
white dwarf accretes material from a late-type companion that fills its
Roche lobe. As material passes through the inner Lagrange point of
the system towards the white dwarf, the magnetic field does not initially
dominate the motion of the material. Closer to the white dwarf surface,
beyond the stagnation region, the field threads and disrupts the flow,
channelling infalling material into a funnel which terminates in a shock
front at or near the magnetic pole(s). Shock-heated plasma cools via
bremsstrahlung, Compton cooling, and cyclotron emission as it settles
onto the white dwarf, with the accretion stream also contributing to
the optical and ultraviolet emission. Magnetic interaction between the
white dwarf and its companion keeps the white dwarf in rotational synchronism
with the M dwarf companion, and the system rotation then leads to the
coherent variability observed in these systems.
The orbital period of UZ
For is 126.5 min, of which the white dwarf is eclipsed for approximately
8 min. The simultaneous rapid intensity and spectral variations
which are characteristic of the eclipses of cataclysmic variables make
these objects ideal targets for study with advanced photon-counting
detectors which record the time of arrival and the energy of each incident
photon. Although such detectors have long been available for high-energy
studies (e.g. proportional counters or CCD detectors operated in X-ray
photon-counting mode), they are only now becoming available for optical
work, based on the new development of superconducting tunnel junction
A photon incident
on an individual STJ breaks a number of the Cooper pairs responsible
for the superconducting state. Since the energy gap between the ground
state and excited state is only a few meV, each individual photon
creates a large number of free electrons, in proportion to the photon
energy. The amount of charge thus produced is detected and measured,
giving an accurate estimate of the photon arrival time as well as a direct
measurement of its energy. Arrays of such devices provide imaging capabilities.
A 6×6 array of 25×25
μm2 tantalum STJ device built at ESA was incorporated
into a cryogenic camera operated at the Nasmyth focus of the 4.2-m
William Herschel Telescope. The projected pixel size of 0.6×0.6 arcsec2 results in an array covering a sky area of
4×4 arcsec2. This camera, 'S-Cam2', is
a development of the system first applied to observations of the Crab
pulsar in 1999. Several modifications, including a new detector array,
and improved detector stability and uniformity, result in an improved
wavelength resolution of Δλ=30, 60, and 100 nm at λ=350, 500, and 650
For each individual detected
photon, the arrival time, x, y array element (or
pixel) , co-ordinateand energy channel are recorded. Photon arrival times
are recorded with an accuracy of about ±5 μs with respect to
GPS timing signals, which is specified to remain within 1 μs of UTC.
The characteristics of
STJ arrays are ideally suited to the observation of CVs. The high time
resolution, high efficiency, large dynamic range, and modest energy resolution
afforded by the S-Cam2 system allow a direct probing of the energy dependence
of the intensity variations across the eclipse, and investigation of the
details of the ingress and egress light curves, whose structure provides
important diagnostics of the emission mechanism.
Astronomers obtained data
for three eclipses of UZ For. They attributed two sharp changes in brightness
to the eclipse of two small accretion regions and localize them on the
surface of the white dwarf primary. The first of these is in the lower
hemisphere at the location seen by others in the optical, and in the EUV
and X-rays. The second is in the upper hemisphere, near the rotation axis,
and there is no evidence for any emission from this region in X-rays. The
diameter of the accretion spots is less than about 100 km.
The central velocity
dispersions of many Local Group dwarf spheroidal (dSph) galaxies are significantly
larger than expected for self-gravitating systems. Assuming virial equilibrium,
the implied mass-to-light (M/L) ratios reach as high
250, making the dSph galaxies among the most dark matter- dominated
systems in the universe. Given the apparent absence of dark matter in globular
clusters, dSph galaxies are also the smallest dark matter- dominated stellar
systems in the universe. As such, they have emerged as crucial testing
grounds for competing theories of dark matter.
Despite their importance,dynamical
models of dSph galaxies to date have been very simple. Most
analyses have relied on the use of single-mass
isotropic King models, with their associated
assumptions that mass follows light and that
the stellar velocity distribution is isotropic.
Hitherto, the validity of such assumptions
has remained unchallenged because of the
small size of the data sets. When only
small numbers of radial velocities are available,
there is a well-known degeneracy between mass
and velocity anisotropy. An increase in the
line-of-sight velocity dispersion at large
radii may by due to either (1) the
presence of large amounts of mass at
large radii or (2) tangential anisotropy
in the velocity distribution. This degeneracy
could be broken by means of improved modelling and a larger data set
with many more stars in the outer parts.
Observations were conducted
from 2000 June 23 to 26 at the William Herschel Telescope using the AF2/WYFFOS
multifiber positioner and spectrograph. A total of 284 stars were observed,
spanning the magnitude range of V
17.0-19.8. Of these, 159 were Draco members (extending to 25')
with spectra of sufficient quality to be included in the dynamical analyses.
The median velocity uncertainty for these 159 stars was 1.9 km s-1.
These are the first observations to probe the outermost regions of a strongly
dark matter-dominated dSph galaxy.
From subsequent analysis,
astronomers found that the velocity dispersion profile is flat or slowly
rising at large radii, which provides the first c lear signature of an
extended dark matter halo in any dSph galaxy.
Further studies of this
cocoon, whose composition remains a mystery, promise to illuminate the
early history of our own Galaxy, which presumably built up from such
dark-matter quanta. This result also fits with the bottom-up view of
galaxy formation, in which the gravitational fields of big galaxies
shred smaller ones and assimilate their stars, gas, and dark matter.
The Wide Angle Survey,
one of the ING Wide Field Survey programmes, brings together a
diverse range of scientific topics, merging the observational programme
to increase scientific effectiveness.
As part of the Virgo
survey component some 25 square degrees of Virgo were obtained in the
B photometric band, and the pipeline processed object catalogues were
analysed. More than 500 Low Surface Brightness galaxies Btot<21 were discovered
by comparing the light profiles of the millions of objects in the
data frames with those of previously known template LSB galaxies.
Using this data astronomers
at Cambridge discovered a new nearby dwarf galaxy in the constellation of Cepheus. This LSB dwarf galaxy is a typical
example of previously unknown nearby galaxies, and it had been previously
overlooked because of its low surface brightness relative to the night
function in Virgo, when combined with the much flatter function found
in the field, will enable the efficiency of low mass galaxy formation
in differing environments to be investigated. First results are indicating
a strong environmental dependence, which would need to be taken
into consideration by Cold Dark Matter theories.
A GIANT STREAM OF METAL-RICH STARS IN THE HALO OF THE GALAXY M31
framework of hierarchical structure formation, large spiral galaxies
like the Milky Way or Andromeda arose from the merger of many small
galaxies and protogalaxies. Later in their evolution, spiral galaxies
become the dominant component in such mergers, cannibalizing smaller
systems that fall within their sphere of influence. The complete destruction
of the victim is usually progressive, and may take several orbits.
However, the stellar debris from the destroyed dwarf galaxy follows
a similar orbital trajectory to the progenitor, which is likely to
have started life far away from the place of its final demise, and
so the tidally disrupted matter tends to be deposited over a broad range
in distance from the larger galaxy. Over time, with the accumulation
of many such mergers, large galaxies develop an extensive stellar and
dark-matter 'halo', the latter being by far the most massive component
of the galaxy. Meanwhile, part of the (dissipative) gas component of
the smaller galaxies feeds the growth of the disk of the larger galaxy.
This is seen in numerical simulations of galaxy formation, which result
in galactic haloes comprising clumps of dark matter. If this prediction
is correct, then haloes should possess significant substructure—in contrast
to previous suggestions, which predict the dark and luminous components
of haloes to be distributed smoothly.
Andromeda or M31 galaxy
is our Galaxy's "big sister", twice as large but otherwise very similar.
It is the nearest large galaxy, lying only 2.2 million light-years away.
Astronomers have known for some years that our own Galaxy is a cannibal.
Its outer parts are threaded with tell-tale streams of stars from small
galaxies it has engulfed.
The first sensitive panoramic
wide field imaging
survey of M31 using the Wide Field Camera on the Isaac Newton Telescope
has unambiguously revealed the presence of a giant stellar stream
within M31's halo. The source of the stream is likely to be either,
or both, of the peculiar dwarf galaxies M32 and NGC205, close companions
of M31, which may have lost a substantial amount of stars, gas and dust
due to their tidal interactions with the massive host galaxy. The broad
agreement of the metallicity distribution of the stream stars with
these two dwarf satellites together with their alignment, physical
proximity, and distorted morphological appearance, point to a common
origin. The well-known disparity in properties between the Milky
Way and M31 stellar haloes would be understandable if the majority
of M31's stellar halo arose as relatively recent tidal debris from prolonged
bouts of aggressive tidal interaction with its two nearest neighbour
satellites. Together with recent observations of tidal debris in
the Milky Way halo, these results clearly demonstrate that the epoch
of galaxy building still continues, and that substructure in the form
of huge, recently-deposited tidal streams, could be a generic feature
of large galaxy haloes.
The new survey was possible only because the digital detector arrays such as the Wide Field Camera now cover fairly large areas of sky. Even so, more than fifty long exposures had to be pieced together to give a panorama of the halo on one side of Andromeda.
COMPLETELY DARK GALAXIES
The universe could be harbouring numerous galaxies that have no stars at all and are made entirely of dark matter. Astronomers may ultimately discover that completely dark galaxies outnumber the familiar kind populated by shining stars and gas, perhaps by as many as 100 to 1. There is already a considerable amount of evidence that bright galaxies contain large amounts of dark matter, often ten times more than the mass of all their stars put together. There must be extra mass that we do not see to account for the observed movements of the stars under the influence of the gravity of the whole galaxy. In some galaxies we see so few stars they are incapable of holding themselves together as a galaxy. They would have long since scattered through space without the gravity of unseen matter to keep them together. But the question is: How do we look for these largely or even completely dark galaxies?
It's a difficult challenge, and the best technique will depend on the nature of the dark matter, which is still unknown. If the dark matter is composed entirely of fundamental particles, dark galaxies may act as gravitational lenses, distorting the appearance of; distant galaxies that happen to lie behind them. If the dark matter includes some brown dwarfs their infrared radiation may be detectable. The same will be true if the galaxies contain any dead stars, such as white dwarfs or black holes. If they are nearby, it might be possible to detect these stellar remnants acting as gravitational lenses on the light of individual stars in other galaxies beyond them. Several lensing events in a small area of sky would suggest the presence of a dark galaxy.
On place where a dark galaxy may exist has been identified using images taken with the INT Wide Field Camera. A galaxy called UGC 10214 has a stream of material flowing out of it, as if it is interacting with another galaxy. But in this case, there is no other galaxy or source of visible light present, hence the companion galaxy may be completely dark.
of standard cosmology is that dwarf protogalaxies
are the first to born as individual systems
in the universe. Afterward, many
of these merge to form larger galaxies such
as the Milky Way. The
way in which this process takes place
has consequences for the present-day structure
of the Milky Way. The significant issues are
how the merging efficiency compares with
the star formation efficiency in the protogalactic
fragments and how the fragment merging
and disruption compare with the age
of the Milky Way. If fragments are able
to form stars before merging, they will
collapse nondissipatively. If disruption
was not complete, Galactic precursors should
be visible today as dwarf galaxy satellites
or as stellar streams within the Galactic
The Sagittarius dwarf
galaxy, the closest Milky Way satellite
in an advanced state of tidal disruption,
provides a "living" test for tidal interaction
models and for galaxy formation theories. It was
soon apparent that its extent was larger than
at first assumed, and dynamical models
predict that the stream associated with the
galaxy should envelop the whole Milky Way in
an almost polar orbit.
Using the Wide Field
Camera on the Isaac Newton Telescope, astronomers detected a
very low density stellar system at 50 ±
10 kpc from the Galactic centre that could
be related to a merger process.
The found system is 60°
north and 46±12 kpc away from the centre
of the Sagittarius dwarf galaxy. If it
is really associated with this galaxy, it would
confirm predictions of dynamical interaction models indicating
that tidal debris from Sagittarius
could extend along a stream completely enveloping the
Milky Way in a polar orbit. However, the possibility that
it corresponds to a hitherto unknown
galaxy, also probably tidally stripped, cannot
DISTANT GALAXIES ARE IN THE RED
The panoramic IR camera, CIRSI has been used to carry out a large-scale survey of distant galaxies in the prime focus of the INT. The main goal of the project was to study the Universe when it was 7 billion years old, or around half its current age.
The recently completed infrared sky survey has detected over 50,000 galaxies in a patch of sky covering roughly the area of a full Moon. Although only one fifth of the data has been analysed so far, already three times as many very red galaxies have been found as was expected.
One possibility is that these galaxies have more old stars in them than expected. Old stars tend to be large and relatively cool -hence the red colour. A second possibility is that the galaxies are very dusty, where scattering by dust particles causes objects to appear red.
A second significant result is the discovery that these red galaxies seem to clump together much more than galaxies in the nearby Universe. One possible explanation is that these red galaxies are merging with each other to form single more massive galaxies.
This merging process would explain why the astronomers are seeing more galaxies in the past than expected. If galaxies merge, their total number will decrease to the present-day value.
|Top | Back|