ING 2001 Scientific Highlights
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Scientific Highlights

Astronomical discoveries following from observations
carried out with the ING telescopes

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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.

Most compact objects are found close to the Galactic plane, which makes this discovery particularly interesting. It opens questions as to how this object was formed. The black hole could either have formed where it currently is found, or it could have been kicked out of the galactic plane during a violent stage of its evolution.

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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 (STJ) devices.

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 Aproximate0.6×0.6 arcsec2 results in an array covering a sky area of aproximate 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 nm respectively.
0.6×0.6 arcsec

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.

UZ For Eclipse 1
Sky-subtracted and flat-fielded total count s-1 (top), count s -1 in three bands, 340-490 nm (labelled 'B' for ease of reference), 490-580 nm ('V') and 580-700 nm ('R'), and two colour ratios constructed from the three energy-resolved light curves for eclipse 1. Lower values of these ratios imply a redder colour. Data are displayed in bins of 2 s for the intensities, and 5 s for the colour ratios, with error bars for the ratios corresponding to photon statistical errors.  [ GIF | TIFF ]

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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 as 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.

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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 sky.

Most Local Group galaxies are satellites of the Milky Way and Andromeda systems leaving only a few outliers to use as probes of the dynamical evolution of the Local Group and for characterizing the unperturbed evolution of nearby dwarf galaxies. 

The luminosity 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.

Cepheus Galaxy
Colour composite of Cepheus galaxy created from 1200 second exposures in g', r' and i'-band images taken in sub-arcsec conditions using the Wide Field Camera on the INT  [ JPEG | TIFF ]

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Within the 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.

Surface density of RGB stars over the southeastern halo of M31
Surface density of RGB stars over the southeastern halo of M31. The over-density of stars is seen as a stream extending out of M31 close to, but distinct from, the minor axis. [ GIF | TIFF ]

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.

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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.

UGC 10214
From observations carried out as part of the ING Wide-Field Survey astronomers have been able to identify one place where a dark galaxy may exist. They noticed that a galaxy called UGC 10214, shown above, has a stream of material flowing out of it, as if it is interacting with another galaxy. In this case, the stream of material is apparently flowing towards nothing. [ JPEG | TIFF ]

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A prediction 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 halo. 

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.

Color-magintude diagrams
Color-magnitude diagrams of the (panel a) control and (panels b and c) target fields. Panel a the distribution of the foreground Milky Way stars. The overdense strip at ( provides B-R) &sime; 0.8, 22 ≤ "V" ≤ 23.5 in panel b CMD is interpreted as produced by a stellar system at a distance of 51 ± 12 kpc from us, which could make it part of the Sagittarius northern stream or, alternatively, could be the trace of a hitherto unknown tidally disrupted dwarf galaxy. Squares represent variable star candidates. Panel c shows the CMD of the target field with an old, low-metallicity (age: 12 Gyr; metallicity: 1/20 solar) isochrone from the Padua library superposed. The isochrone MS shape shows good agreement with the hypothetical target field MS. Also, the variable star candidates (squares) fall in the predicted region of the horizontal branch. [ JPEG | TIFF ]

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 be rejected.

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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.

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Last modified: 24 November 2011