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ING Annual Report 1995/96
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Chapter 1Scientific Highlights
In the limited space available, it is impossible to make a comprehensive survey of the science being carried out by the ING telescopes. The following is therefore necessarily only a selection of highlights, intended to be representative of the scientific quality and range of research being undertaken.
THE DEEPEST GROUND-BASED COUNT OF GALAXIES
INT+Prime Focus, WHT+Cass Aux Camera
By combining a 26-h exposure taken with the prime focus CCD camera on the INT and an exposure taken with the CCD camera at the cassegrain auxiliary focus of the WHT astronomers have extended their determination of the form of the galaxy number-magnitude count relation on one CCD field to a blue magnitude limit of B=27.5 magnitudes. These data are deeper than any previously published B-band count.
True-colour image of faint blue galaxies at the edge of the observable Universe, formed from a 26-hour B-band and R-band exposure at INT and a 13-h exposure in B-band at WHT. Detailed analysis of the colours shows that the bulk of the faint blue galaxies lie at redshifts of about 2 and are probably in their first phase of star formation (courtesy of Tom Shanks). [GIF]
In recent years sensitive optical surveys have revealed a large population of "faint blue galaxies", which are believed to be young galaxies observed close to their time of formation. But there has been considerably uncertainty regarding the epochs at which these galaxies are observed, owing to the difficulties inherent in determining spectroscopic redshifts for very faint objects. Using the data from the long exposures taken at the ING telescopes and those from the HST Deep Field, a team of astronomers from the University of Durham, by modelling the numbers and colours of galaxies at the faintest detection limits, has come to the conclusion that the faint blue galaxies are likely to lie at high redshift (z»2).
It is remarkable that the galaxy number counts derived by the HST in the B-band is only one magnitude fainter than the ground-based counts from the WHT.
- N Metcalfe et al, 1995, "Galaxy number counts - III. Deep CCD observations to B=27.5 mag", MNRAS, 273, 257
- N Metcalfe et al, 1996, "Galaxy formation at high redshifts", Nature, 383, 236
- C Frenk, 1997, "How galaxies formed", 1996/1997 PPARC Annual Report, 22
FIRST DETECTION OF BROWN DWARFS
WHT+ISIS, INT+Prime Focus
For decades researchers have speculated about the existence of brown dwarfs - celestial objects which probably constitute a link between stars with lower masses and giant planets, such as Jupiter, whose mass is approximately one thousandth of the mass of the Sun. There is no reason to assume that these substellar objects cannot form randomly in space through a process similar to that of the stars; i.e. as a result of gravitational collapse and fragmentation of dust and gas clouds. However, despite many searches carried out, their existence had not yet been unequivocably proved.
A brown dwarf is a self-gravitating gaseous object composed mainly of hydrogen and helium, whose mass is too small to induce stable hydrogen fusion in its interior. All the theoretical surveys conducted agree that the limiting mass which separates stars from brown dwarfs is about 7 or 8% of the mass of the Sun. Incapable of generating nuclear energy, the gravitational contraction of a brown dwarf takes place unavoidably until the pressure of the degenerated electrons in its interior interrupts the whole process. The nearby star cluster of the Pleiades, a group of stars which formed about a hundred million years ago at a distance of approximately 400 light years (3780 billion kilometers) from the Sun, is considered to be one of the most suitable astronomical sources for the detection, and the subsequent study of brown dwarfs. At such early ages, these objects should be undergoing gravitational contraction, radiating much more energy than in later stages of their evolution. More massive brown dwarfs in the Pleiades should be detectable in sufficiently deep surveys.
After only 0.3% of the cluster's area had been explored using IAC80 telescope at Teide Observatory, a faint object was detected, whose extremely red colour possibly indicated a very low surface temperature. Firstly, its motion in space was confirmed to coincide with that of the stars of the cluster and, later, a precise photometric characterization was achieved. Several high resolution spectra between 600 and 900 nm were obtained with the WHT. These spectra confirmed the discovery of one of the coldest quasi-stellar objects known in the Universe. The spectral lines of neutral potassium between 767 and 770 nm indicated that it was an object with high surface gravity, as was expected for a brown dwarf, and the presence of prominent bands of titanium oxide and, especially, vanadium oxide at 750 nm allowed to derive its spectral classification and an estimate of its effective surface temperature, which turned out to be some 2350 K. The spectrum allowed to infere a velocity measurement of this object in regard to the Sun, which happened to be very similar to that of the stars in the cluster. All the entire set of observations suggested that it was a member of the cluster and, therefore, that its age was the same as the cluster's: 100 million years approximately, with a margin of error below 30%. It was the first time that the age of a celestial object of this nature had been so accurately determined, overcoming one of the most important restrictions preventing the true substellar nature of brown dwarf candidates to be classified. From the cluster's distance it was possible to determine that the luminosity of Teide 1 (this is how the discoverers decided to call the object) was one thousandth of the solar luminosity. The comparison of its principal features (luminosity, temperature and age) with all the evolutionary models available in the scientific literature led to the conclusion that Teide 1 had to be a brown dwarf.
In 1996 the International Time Project "Observational Properties of Brown Dwarfs" detected new brown dwarfs in the Pleiades cluster. Several have masses similar to Teide 1 (55 Jupiter masses approximately) or higher, but various present slightly lower masses. They were all first detected using the INT. Subsequent confirmation involved spectra from the WHT and infrared photometry from UKIRT and WHT. The Keck telescope was then used to detect the element lithium in the spectra of brown dwarfs. Lithium is an important test for brown dwarfs because it is destroyed by nuclear reactions in stars of low mass but not in brown dwarfs.
- R Rebolo et al, 1995, "Discovery of a brown dwarf in the Pleiades star cluster", Nature, 377, 129
- "Brown Dwarfs in the cluster of the Pleiades", 1995 CCI Annual Report, 13
- "New Brown Dwarfs in the Pleiades", 1996 CCI Annual Report, 7
- R Jameson, 1997, "The search for brown dwarfs", 1996/1997 PPARC Annual Report, 28
- "Another Brown Dwarf discerned", S&T, 12/95, 10
ING OBSERVATIONS OF COMET HALE-BOPP
WHT+ISIS, INT+IDS, JKT+CCD imaging
Comet Hale-Bopp was discovered at a heliocentric distance of 7.2 AU in July 1995. What was significant about this discovery was both the large distance at which it was discovered, and that it was already at an integrated magnitude of ~10.5. To put this into context, at the same distance from the Sun Comet Halley was at V=22.8. This difference was mostly due to the fact that Hale-Bopp had generated an atmosphere, or coma, around itself, while Halley had not. At such large distances the optical coma of a comet is dominated by scatered sunlight from dust grains. These are released from the comet nucleus (generally 1–20 km in diameter) through sublimation of surface ices, at this distance primarily volatiles such as CO. Therefore the presence of so much dust implied an extremely active nucleus, with either a large fraction of its surface undergoing outgassing, or perhaps just a very large nucleus.
Subsequent spectrophotometry with the WHT a month after discovery revealed the presence of the CN molecular band, formed from the HCN being released from the nucleus and then being photo-dissociated via solar UV photons. Monte-Carlo modelling of these data revealed an outgassing rate for the parent HCN molecule of 6e25 mol./second. This confirmed the high activity of the nucleus, as Halley had an outgassing rate a factor of 10 lower when it was at 4.5 AU from the Sun. This meant that the discovery of Comet Hale-Bopp at an unusually large heliocentric distance provided an unprecedented opportunity to follow its evolution from beyond Jupiter into the inner Solar System. To take advantage of this, spectroscopic follow-up was carried out using variously the WHT with ISIS and the INT with the IDS. A spectrum of the comet was obtained on 3 September 1996. Even though the comet was still 3.2 AU from the Sun, where most comets show little activity, Hale-Bopp had a spectrum tremendously rich in molecular species.
The image on the left was obtained on 25 August 1995 when the comet was 6.9 AU from the Sun and 6.3 AU from the Earth. A large number of stars are visible, as at this time the comet was in the direction of the constellation of Sagittarius [GIF]. On the right, dust jets observed in Comet Hale-Bopp with the JKT on 27 August 1996. The image spans 84 arcseconds, or roughly 170,000 km at the comet. Six jets can be seen emanating from the nucleus (courtesy of Alan Fitzsimmons) [JPEG|TIFF].
While the gradual brightening of the comet is clear, any short-term variability in the dust production, and hence outgassing, rate is difficult to obtain from these observations. Therefore in August 1996 CCD imaging of Hale-Bopp was obtained with the JKT over 13 nights, with the primary goal being an investigation into the short-term (hours–days) variability of the comet. By fitting the comet images with a modelled isophote distribution and subtracted it to reveal more clearly the underlying structure, a similar process to that used in the study of shell galaxies, it is possible to study the morphology of the coma. On 27 August 1996 comet Hale-Bopp was imaged with an R-band filter in seeing of 0.6 arcseconds using the JKT. Six well defined jets were seen emanating from the nucleus. These were due to the outgassing from the nucleus being confined to several localised hotspots, where the insulating mantle was thin or non-existent thereby allowing heating of the nuclear ices.
- A Fitzsimmons and I M Cartwright, 1996, "Optical spectroscopy of comet C/1995 O1 Hale-Bopp", MNRAS, 278, L37
- A Fitzsimmons et al, 1996, IAU circular 6361
- A Fitzsimmons et al, "ING observations of Comet Hale-Bopp", Spectrum Newsletter, 12, 4
DISCOVERY OF A NEW TYPE OF GALAXY: ONE IN WHICH THE BULGE ROTATES RETROGRADE TO THE DISK
WHT+ISIS, INT+Prime Focus
A team of astronomers found that the bulge of the large, nearby Sb galaxy NGC 7331 rotates retrograde to its disk. Analysis of spectra in the region of the near-IR Ca II triplet along the major axis shows that, in the radial range between 5
and 20 arcseconds, the line-of-sight velocity distribution of the absorption lines has two distinct peaks and can be decomposed into a fast-rotating component and a slower rotating, retrograde component. The radial surface brightness profile of the counterrotating component follows that of the bulge, obtained from a two-dimensional bulge-disk decomposition of a near-infrared K-band image, while the fast-rotating component follows the disk. At the radius at which the disk starts to dominate, the isophotes change from being considerably boxy to being very disky.
Although a number of spiral galaxies have been found that contain cold, counterrotating disks, this is the first galaxy known to have a boxy, probably triaxial, fairly warm, counterrotating component, which is dominating in the central regions. If it is a bar seen end-on, this bar has to be thicker than the disk. NGC 7331, even though it is a fairly early-type spiral, does not have a conventional, corotating bulge. The fact that the inner component is retrograde makes the astronomers believe that it was formed from infalling material in either stellar or gaseous form. Another possibility discused by the discoverers is that the structure has been there since the formation of the galaxy. In this case, it will be a challenge to explain the large change in orientation of the angular momentum when going outward radially.
Left: Gray-scale plot of the stellar line-of-sight velocity distribution along the major axis of NGC 7331, where for representation purposes, the data in the spatial direction have been smoothed with a gaussian of FWHM 4 arcseconds. LOSVD stands for Line-Of-Sight Velocity Distribution (courtesy of Francisco Prada).[GIF]
- F Prada et al, 1996, "A counterrotating bulge in the Sb galaxy NGC 7331", ApJ, 463, L9
- C M Gutiérrez et al, 1996, "Un bulbo retrógado en la galaxia cercana NGC 7331", IAC Noticias, 1/1996, 4
A GRAVITATIONALLY LENSED Z=2.515 STAR-FORMING GALAXY
The origin and evolution of galaxies is one of the holy grails of modern astronomy. It is interesting that despite a huge effort over the last few decades, the nature of galaxy evolution is still much less well understood than that of the stars from which the galaxies themselves are largely made. In order to study how galaxies change with time, the astronomer must isolate populations at different look-back times and compare them with the well-studied objects we see around us today. The major problem of this work is that the farther away you look, the fainter the sources become, and consequently isolating such a population from bright, close-by objects becomes very difficult.
The most obvious and systematic method is to conduct large spectroscopic surveys to determine redshifts for as many faint galaxies as possible. The disadvantage of this approach is that even at the faint limits achievable with 10m telescopes, only a tiny fraction of galaxies lies beyond about a redshift of 1. Thus a huge number of redshifts must be accumulated before even one distant source is located. What is needed is a method of selection which would only be sensitive to very distant galaxies. One of these methods is based on gravitational lensing by clusters of galaxies, in which the selection is purely geometrical.
Giant arcs in clusters were first recognised in the mid-1980s and the great potential of lensing as a cosmological tool was realised soon afterwards. The magnification and distorsion induced by the lensing depends solely on the position and distance of the source with respect to the lensing cluster. Thus low-luminosity sources may be magnified just as often as high luminosity ones by virtue of their alignment with the lens. The magnification allows the astronomers to obtain spectra and redshifts for objects otherwise too faint for such study with today's telescopes. Moreover, in addition to the boosting of the apparent magnitude, the lensing spatially magnifies the objects, whose components may then be studied individually. A second advantage of this technique is its ability to amplify sources over a wide redshift range (z>0.5).
Data from the HST enables the construction of very precise mass models for selected lensing clusters. A good example is the recent analysis of Abell 2218 (z=0.175), where the resolution of the HST allowed the construction of a detailed mass model constrained by as many as seven multiply-imaged sources. Based on these mass models, a number of the arclets were predicted to have redshifts z>1.
As part of a major effort to verify the lensing inversion method for Abell 2218, astronomers secured spectra for a large sample of faint arclets. For this purpose, the Low Dispersion Survey Spectrograph (LDSS-2) at the WHT was used. As a result, a redshift of z=2.515 for a refracted galaxy was obtained and this was the first confirmation of a redshift predicted by a cluster lensing model.
The source responsible for the lensed images appeared to be a blue galaxy whose on-going star formation rate of 7–11 solar masses per year is similar to that of similar sources found at higher redshift using the Lyman limit cutoff as a high-z locator. Its brightness was magnified almost 3 magnitudes thanks to the lensing process.
- T M D Ebbels et al, 1996, "Identification of a gravitationally lensed z=2.515 star-forming galaxy", MNRAS, 281, L75
- "The Universe through a gravitational lens", PPARC bulletin, 3, 20
- T Ebbels et al, 1996, “A gravitationally lensed z=2.515 star-forming galaxy”, Spectrum Newsletter, 9, 4
A DYING STAR'S LAST GASP: SAKURAI OBJECT
In February 1995 a Japanese amateur astronomer discovered a nova in the constellation of Sagittarius (now known as V4334 Sagittarii). Its pre-discovery light curve indicated that it was unusual in that it had apparently been evolving only very slowly compared to a normal nova. Spectroscopic observations post discovered with the WHT showed the star to have little resemblance to any previously observed nova and in fact looked more like a solar type object shrouded in dust and with some level of hydrogen deficiency. Further observations revealed the presence of a nebula shell some 45 arcseconds in diameter. Thanks to a PATT award the ING has been monitoring this event since discovery and has witnessed gross spectral changes as the star has cooled.
The discovery of a Planetary Nebula at the WHT is important in that it indicates we are dealing with an evolved star. Planetary Nebula occur when a star evolves from red supergiant to a white dwarf expelling material. During this evolution the star rapidly heats up in 10,000–20,000 years reaching a surface temperature of 100,000 K or more, and this causes the expelled material to become visible. When the star becomes a white dwarf nuclear reactions no longer occur and the star simply fades and cools.
Spectral variations in the Sakurai object during a period of 14 day during april/may 1995. The cooling of the star is accompanied by a remarkable strengthening in H alpha as well as the appearance of other features (courtesy of Don Pollacco). [GIF]
More recent work has shown that this may not be the end of the story, for some or even most stars. Just as the star reaches the white dwarf phase instabilities within its interior can cause an explosive event called a shell flash. In some objects this event can be so intense that material around the core of the star violently starts undergoing nuclear reactions. This can cause the star to go through a second supergiant phase and Planetary Nebula ejection before settling down to become a white dwarf. The time scale for this evolution is rapid taking anywhere from a few months to a few years to evolve from a white dwarf - red supergiant - hot Planetary Nebula central star. It is this evolution that Sakurai's object is currently undergoing.
During this century there is only one other object that is known to have undergone a shell flash of this magnitude: the central star of the old Planetary Nebula Abell 58 or V605 Aql. This object was first spotted as an unusually slow nova in 1918 and reaching about 10th magnitude in 1920. During its slow fade the light curve underwent rapid and large fluctuations similar to those seen in R Corona Borealis stars. The star was finally lost to observers around 1923 and was essentially forgotten about. In 1989 the star was recovered again as a very hot Wolf-Rayet star shrouded in dust and gas and having a brightness of around the 22nd magnitude and its ejected nebula contains virtually no hydrogen. HST imaging shows this new nebula to be 0.5 arcsec in diameter and containing very non-uniformly distributed material.
- D Pollacco, 1996, IAU circular 6328
- "A Dying Star's Last Gasp", S&T, 05/96, 11
A RADIO GALAXY AT REDSHIFT 4.41
WHT+ISIS, +Cass Aux Camera
The most distant astronomical objects observed are quasars at redshifts of z=4.9, corresponding to a time when the Universe was less than a billion years old. This leaves little time during which quasars and their host galaxies could form. In principle, the evolutionary state of the host galaxies can be probed by determining how many stars have formed, but this task is not straightforward because light from the quasar itself overwhelms any accompanying starlight. High-redshift radio galaxies – the likely progenitors of luminous elliptical galaxies – provide better targets for such studies, as optical emissions from their active nuclei are observed to be faint. The radio galaxy 6C0140+326, discovered in the optical following to observations by the WHT, shows no evidence for either a stellar continuum or an obscured quasar nucleus. The astronomers conclude that the galaxy associated with the radio source is neither fully formed nor obviously in the process of forming stars. This implies that at least some giant elliptical galaxies are still immature at z»4.5 and that if the intense bursts of star formation thought to produce the bulk of their stellar populations occur during the radio-bright phase, these star-forming regions are obscured by dust and gas.
6C 0140+326 has a redshift of 4.41, exceeding that of the previous record-setting radio galaxy, 8C 1435+635 at z=4.25, also discovered by the WHT.
- S Rawlings et al, 1995, "A radio galaxy at redshift 4.41", Nature, 383, 502
- "Redshift records renewed", S&T, 01/97, 12
THE WENSS SURVEY
WHT, INT, and JKT
The night-time CCI International Time Programme (ITP) observations for the period February 1995 to January 1996 were carried out by a consortium of astronomers following up various aspects of the Westerbork Northern Sky Survey (WENSS). This is a radio survey of the northern sky at the relatively low frequency of 327 MHz. Much of the work in the spring/summer concentrated on the mini-survey region, a 500 square degree area centred on the north ecliptic pole. The radio sources in the survey were split into several subgroups and a high success rate was achieved in following up each one: nearby galaxies, flat (quasars), peaked and ultra-steep (high-redshift galaxies) spectrum radio sources, and gravitational lenses.
The observations were carried out with CCI telescopes, among them, WHT, INT and JKT, both imaging and spectroscopy. These observations have improved the understanding of low-flux radio sources at both low and high redshift. The work at low redshift has allowed the construction of luminosity functions in the optical and in the radio, for nearby weak radio sources. It is clear from the work on flat-spectrum and ultra-steep spectrum radio sources that the WENSS survey allow the study and selection of objects to consistently higher redshifts than have generally been possible with higher flux radio surveys, and is therefore extremely well suited to the study of the high-redshift universe.
During the survey, a good candidate for a giant radio galaxy was found: Mrk 1498 (B1626+5153). These kinds of extragalactic radio sources with dimensions greater than 1.5 Mpc are rare in the cosmos, but provide in principle a good laboratory for studying both the physics of the radio galaxy phenomenon and the nature of the intergalactic medium. It is uncertain whether these sources attain such large sizes because the ratio of jet power to the density of the surrounding medium is unusually large, or because the sources are simply much older than the average radio source of the type and so have had time to expand to unusually large dimensions.
Mrk 1498 is a classical double source which has a maximum dimension of at least 1.6 Mpc, a flux density at 325 MHz of 1.9 Jy and spectral index of –0.66. Optical spectra with the WHT show a narrow line emission spectrum typical of many radio galaxies and yield a redshift of z=0.056. The H-alpha line clearly has a broad line component, making Mrk 1498 the third known giant radio galaxy exhibiting broad permitted lines.
Most available evidence supports the view that the main differences among radio galaxies and radio quasars may be understood as an orientation effect. At some orientations one can see the central source directly, including the broad permitted lines, while at others the center is hidden and only the larger scale narrow emission line gas and large scale radio emission is visible. Of the dozen or so giant radio sources known, three, including Mrk 1498, show broad optical permitted lines, broadly consistent with the predictions of this orientation unification model.
- H J A Röttgering et al, 1996, "WN 1626+5153: a giant radio galaxy from the WENSS survey", MNRAS, 282, 1033
- A P Schoenmakers et al, "Giant Radio Galaxies from the WENSS", 1995/1996 Annual Report of the Utrecht Astronomical Institute, 19
- "WENNS", 1996 CCI Annual Report, 12
- "Giant Radio Galaxies", 1995 NFRA Annual Report, 35
GALAXY'S HEART IS HEAVY
An extensive new study of the Galactic center stellar cluster was carried out thanks to observations with the WHT and other ground-based telescopes. One of the conclusions of such study is that the central parsec is powered by a cluster of about two dozen luminous and helium-rich blue supergiants/Wolf Rayet stars (Teff=20,000–30,000 K) with ZAMS masses up to 100 solar masses approximately. The most likely scenario for the formation of the massive stars is a small star formation burst between 3e+6 and 7e+6 years ago. In this scenario the Galactic center is presently in a short-lived, post-main-sequence "wind phase". In addition, there is evidence for another star formation event about 108 years ago, as well as for recently formed massive stars that may have been transported into the central core along with orbiting gas streamers. The radial velocity dispersion of 35 early- and late-type stars with distances of 1–12 arcseconds from Sgr A*, a luminous radio-source near the Galactic center, is 154±19 km/s. These new results strongly favor the existence of a central dark mass of 3e+6 solar masses approximately (with density>=108.5 solar masses/pc3, and M/L>=10 solar masses/solar luminosities) within 0.14 pc of the dynamic center.
- A Krabbe et al, 1995, "The Nuclear Cluster of the Milky Way: star formation and velocity dispersion in the central 0.5 parsec", ApJ, 447, L95
- A Krabbe et al, 1993, "FAST: a near-infrared imaging Fabry-Perot spectrometer", PASP, 105, 1472
- "Our Galaxy's Heavy Heart", S&T, 02/96, 14
THE CURIOUS M100'S CORE
The inner region of the barred spiral NGC 4321 (M100) shows remarkably different morphology in the optical and the near-infrared. Whereas in the optical it is dominated by two spiral arms lying in an ovally shaped region of enhanced star formation, a K-band image reveals an inner bar aligned with the 5 kpc stellar bar and a pair of leading arms emerging from its ends. Neither feature is observed directly in the optical.
NGC 4321 is a nuclear starburst induced and maintained by a global bar-driven density wave. The location of the starburst in the circumnuclear "ring" is related to the slowing down of the radial gas inflow in the presence of inner Lindblad resonances. Understanding the details of such radial flows in barred galaxies may well shed light on the origin and fueling of active galactic nuclei.
H-alpha continuum subtracted image of the central region of M100, as obtained using the TAURUS instrument on the WHT, with sub-arcsecond resolution. The H-alpha emission shows where massive stars are presently1y forming, represented by white in this false colour image. Note that the spiral arms visible in this image connect directly to the spiral arms in the disc of the galaxy (courtesy of J. H. Knapen). [JPEG|TIFF]
- J H Knapen et al, 1995, "The striking near-infrared morphology of the inner region in M100", ApJ, 443, L73
- "M100's Curious Core", S&T, 10/95, 13
DEFICIT OF DISTANT X-RAY-EMITTING GALAXY CLUSTERS AND IMPLICATIONS FOR CLUSTER EVOLUTION
WHT, INT, and JKT
The ROSAT International X-ray Optical Survey (RIXOS) was aimed at the optical identification of a complete sample of ~400 serendipitous X-ray sources found in 81 northern ROSAT fields, achieved using an International Time award on the Canarian Telescopes. Fields at high Galactic latitude (b>+28°) were selected with exposure times longer than 8000 seconds achieving a limiting flux optimized for wide-area optical follow-up. In total, 385 X-ray sources were catalogued over 20.4 deg2 to a limiting flux of fX>=3e+10–14 erg/s/cm2 in the 0.5–2.0 keV energy band.
An overview of the various stages of data preparation and acquisition for RIXOS included: source searching and positional calibration of the X-ray images, the construction of finding charts around each of the sources using digitised sky-survey plates, a search for previously-known catalogued sources from on-line services, deep imaging of the optically empty fields using the Nordic Optical Telescope and the INT, spectroscopic observation of the brighter sources with the INT and of the fainter ones with the WHT, and, finally, multicolour imaging photometry of extended or interesting objects using the JKT. The results of the RIXOS survey provided a sample which is complete over 15 deg2 of sky, including 319 X-ray sources of which the largest population is of Active Galactic Nuclei (AGN), followed by stars, clusters of galaxies, Emission Line Galaxies (ELG), and finally, just one "normal" galaxy.
The most significant scientific result from the survey was the deficit of distant X-ray-emitting galaxy clusters found. Clusters of galaxies are the largest gravitationally bound systems in the Universe and therefore provide important constraints on the formation and evolution of large-scale structure. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution, and the intracluster gas is compressed and shock-heated by the gravitational collapse. If the gas is in hydrostatic equilibrium the resulting X-ray emission is related in a simple way to the evolving gravitational potential. But if processes such as radiative cooling or pre-collapse heating of the gas are also important, the X-ray evolution will be strongly influenced by the thermal history of the gas. In the RIXOS project very few distant clusters were identified, and their redshift distribution seems to be inconsistent with simple models based on the evolution of the gravitational potential. These results thus suggest that radiative cooling or non-gravitational heating of intracluster gas must be important in the evolution of clusters.
- F J Castander et al, 1995, "Deficit in distant X-ray-emitting galaxy clusters and implications for cluster evolution", Nature, 377, 39
- "The ROSAT International X-ray/Optical Survey (RIXOS)", 1995 CCI Annual Report, 6
AND FINALLY SOME CURIOSITIES
The maximum redshift for quasar fuzz seen from Earth has grown to z=2.3 thanks to observations obtained with the WHT (I Aretxaga et al, 1995, MNRAS, 275, L27). WHT also discovered the most distant giant double radio source: 4C 39.24 at z=1.887 (J D B Law-Green et al, 1995, MNRAS, 277, 995), and showed that active galaxies with large double radio lobes are not enormously less common at redshifts above unity than they are closer (G Cotter et al, 1996, MNRAS, 281,1081).
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