ING Scientific Highlights in 1995
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ING Scientific Highlights
in 1995*

*Astronomical discoveries following from observations carried out with the ING telescopes


[ 1994 Scientific Highlights | 1996 Scientific Highlights ]



ISIS spectrum of Teide 1For 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.

More information

ING facilities involved: 

  • INT+Prime Focus 
Pictures:  Some references: 
  • 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


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.

More information

ING facilities involved: 

Some references: 
  • 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 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.

More information

ING facilities involved: 

  • WHT+Cass Aux Camera
Some references: 
  • S Rawlings et al, 1995, "A radio galaxy at redshift 4.41", Nature, 383, 502
  • "Redshift records renewed", S&T, 01/97, 12



M100's CoreThe 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.

More information

ING facilities involved: 

Pictures:  Some references: 
  • 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



INT Image of Galaxy Dwingeloo 1The disk of the Milky Way contains gas and dust which obscures about 20% of the extragalactic sky, the so-called "Zone of Avoidance". Nearby galaxies hidden behind the zone of avoidance may have an important influence on the dynamics of the Local Group and its peculiar motion relative to the cosmic microwave background radiation. Such galaxies suffer extinction by gas and dust at optical wavelengths and confusion by stars in the infrared. However, 21 cm neutral atomic hydrogen (HI) emission, associated with late-type galaxies, may be observed if the velocity of the emission differs from that of the local gas. The Dwingeloo Obscured Galaxy Search is a long-term international collaboration of astronomers to search about 2000 degress2 of the northern galactic plane at 21 cm for new galaxies, using the Dwingeloo 25 m radio telescope. The first result of this search, in August 1994, was the detection of a new galaxy, Dwingeloo 1. Follow-up service imaging on UKIRT and the INT revealed it to be an SBb barred spiral. Dwingeloo 1 is rotating at about 130 km per second, has a mass about one-third that of the Milky Way, and lies about 3 kpc distant. Its angular and radial proximity to Maffei 1, Maffei 2 and IC342 suggest that it is a prominent member of this group of galaxies, a close neighbour of the Local Group.

More information

ING facilities involved: 

  • Isaac Newton Telescope using Prime Focus CCD
  • William Herschel Telescope using ISIS spectrograph
Pictures:  Some references: 
  • R Kraan-Korteweg et al, 1994, "Discovery of a nearby spiral galaxy behind the Milky Way", Nature, 372, 77 
  • P Henning, et al, 1995, "The Dwingeloo obscured galaxies survey", AAS, 187, 5305
  • P A Henning, et al, 1998, "Galaxies discovered behind the Milky Way by the Dwingeloo obscured galaxies survey", AJ, 115, 584 
  • A Loan, et al, 1996, "Optical observations of Dwingeloo 1, a nearby barred spiral galaxy behind the Milky Way", MNRAS, 280, 537
  • "A New Nearby Spiral Galaxy", 1995 CCI Annual Report, 33 
  • NASA Astronomy Picture of the Day



The WHT Deep FieldBy 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.

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.

More information

ING facilities involved:

  • INT+Prime Focus 
  • WHT+Cass Aux Camera
Pictures:  Some references: 
  • 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


X-ray-emitting galaxy cluster histogramThe 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.

More information

ING facilities involved: 

  • WHT
  • INT
  • JKT 
Pictures:  Some references: 
  • 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


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).

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Last modified: 13 December 2010