ING Scientific Highlights in 1991
ING Banner
Home > Public Information > Scientific Highlights > 1991

ING Scientific Highlights
in 1991*

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


[ 1990 Scientific Highlights | 1992 Scientific Highlights ]


Image of BL Lac object MKN501BL 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:

  • William Herschel Telescope with TAURUS
Some references:
  • Abraham, R.G. et al., 1991, "Optical imaging of BL Lac host galaxies" MNRAS,252, 482



Photograph of NGC 891 with slit positions markedResearchers 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.
More information

ING facilities involved: 

  • Isaac Newton Telescope, using IDS
Some references: 
  • Bottema, R. et al, 1991 "The stellar velocity dispersion of the edge-on spiral galaxy NGC 891", A&A, 247, 357 


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: 

  • Isaac Newton Telescope, using FOS-1
  • William Hersschel Telescope, using FOS-2
Some references: 
  • Rowan-Robinson, M., 1991, "Most luminous object record broken again", JBAA, 101, 255 



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: 
  • Metcalfe, N. et al, 1991 "Galaxy number counts - II CCD observations to B=25 mag", MNRAS, 249, 498 
  • Metcalfe, N. et al, 1991, "Ultra-deep INT CCD imaging of the faintest galaxies", GEMINI Newsletter Royal Greenwich Obs., 34, 12 
  • Metcalfe, N. et al.,1995, "Galaxy number counts - III Deep CCD observations to B=27.5mag",MNRAS, 273, 257 



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: 

  • Isaac Newton Telescope, using FOS-1
Some references: 
  • 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



Image of the arc in B-bandDuring 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
  • William Herschel Telescope, using FOS
Some references: 
  • 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 
  • Leborgne, J.-F. et al, 1991, "Photometric and spectroscopic observations of cluster of galaxies Abell 2390", A&AS, 88, 133 


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. 

Top | Back

Contact:  (Public Relations Officer)
Last modified: 13 December 2010