ING Scientific Highlights in 1992
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ING Scientific Highlights
in 1992*

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

[ 1991 Scientific Highlights | 1993 Scientific Highlights



Mass constraints
Unequivocal evidence for a stellar-size black hole in the Galaxy has been sought for over two decades. CygX-1 is a strong candidate, but the massive companion star makes it difficult to set a lower limit of better than 3 solar masses to the mass of the compact object. The soft-X-ray transients have yielded potentially more important candidates because of the low mass of the companion (or secondary) star and its faintness after the X-ray outburst has subsided into quiescence. The 1975 transient A0620—00 is currently considered to be the strongest black-hole candidate, with a mass function of 2.9 solar masses, which represents the lower limit to the mass of the compact object. Although X-ray and optical light curves similar to those of A0620—00 are considered sufficient to label an object a 'black hole candidate', the only certain way of identifying the compact object is to study the radial velocity of the secondary star and measure the mass function.

In order to get a good coverage of the radial velocity curve of V404 Cyg, 73 spectra were obtained using the William Herschel Telescope and the Isaac Newton Telescope. Radial velocities were then derived by cross-correlating the spectra. These clearly demostrated a large-amplitude modulation with a period of 6.47 days and a large velocity amplitude of 210.6 km/s. From the mass function of the system, it's possible to set lower limits to the mass of the primary star. For a zero secondary star mass and a system inclination of 90 degrees, we get 6.26 solar masses which substantially exceeds the 3 solar masses maximum allowed mass of a neutron star. So astronomers concluded that the compact object had to be a black hole.

The most straightforward model for V404 Cyg is a binary system consisting of a black hole and a K0 III secondary. A main-sequence star is ruled out because its radius would only be 0.14 of its Roche-lobe radius, and it would thus be unable to transfer mass to the compact object. It must be close to the Roche-lobe radius to account for the X-ray outbursts. Alternatively, V404 Cyg may be a triple system, in which the G/K star observed in the spectra is a main sequence star orbiting a black hole/late-type-dwarf binary. The late-type dwarf must be less than 0.5 solar masses for its luminosity to be much lower than that of the G/K star, and thus not be detected in the spectra. This inner binary would then be similar to other X-ray transients, but the third star allows to place a limit on its total mass of 6.26 solar masses. Because the late-type star cannot be more than 0.5 solar masses, the compact object must still be a black hole, as discussed above.

More information

ING facilities involved:

  • William Herschel Telescope, using ISIS.
  • Isaac Newton Telescope, using IDS.
Pictures: Some references: 
  • Casares J., Charles P. A., Naylor T., 1992, "A 6.5-day periodicity in the recurrent nova V404 Cygni implying the presence of a black hole", Nature, 355, 614. 


Geometrical representation ofRaman scattering modelThe spectra of symbiotic stars simultaneously exhibit the signature of a cool giant and an ionised nebulosity. Symbiotic stars are unresolved binary systems consisting of a mass-losing red giant and a hot radiation source with an effective temperature of typically around 100,000K. The nebulosity is thought to be the ionised stellar wind of the red giant. The orbital periods of these binaries range from several hundred days to a few decades. The spectra of symbiotic stars often show two emission lines at 6825 and 7082Å which are much broader than other emission lines in the spectrum. These remained unidentified until researchers suggested that they could be due to Raman scattering of the OVI 1032 and 1038Å resonance lines by neutral hydrogen. Raman scattering is a well known physical process in the laboratory and it is observed in planetary atmospheres, but the emission features in symbiotic spectra would be the first example in any other astronomical object. Because of the geometry of symbiotic systems, and because in the Raman process there is an absence of diluting unscattered radiation, a high degree of polarisation was predicted in the 6825 and 7082Å lines.

During a WHT service night, ISIS spectra were obtained of six symbiotic stars and polarisation was confirmed to be clearly present in the 6825Å line in all the objects. Besides confirming the presence of Raman scattering, the observation revealed that there were structural variations in the degree and angle of polarisation across the line profile. This discovery now opens up the possibility of mapping the geometric structure of the neutral hydrogen scatter in three dimensions, because the Raman scattered emission lines contain velocity and vectorial polarisation information.

More information

ING facilities involved:

  • William Herschel Telescope, using ISIS
Some references: 
  • Schmid, H. and Schild, H., 1994, "Raman scattered emission lines in symbiotic stars: A spectropolarimetric survey", A&A, 281, 145
  • Schmid, H. and Schild, H., 1992, "Raman scattering, Spectropolarimetry and symbiotic stars", GEMINI Newsletter Royal Greenwich Obs., 37, 4



Model of ENLR in galaxyActive galaxies appear in a wide variety of guises, with observed phenomena covering vast ranges in luminosity and spatial scale. This diversity has hindered attempts to understand the nature of the fundamental energy source. In recent years, however, it has emerged that at least some of the radiation produced by active galactic nuclei (AGN) is emitted anisotropically. This has led to the development of "unified schemes" which seek to relate apparently differently classes of AGN. In one model an active nucleus is largely enclosed by a thick torus of obscuring material. The strong non-stellar continuum and broad emission lines characteristic of Seyfert 1 galaxies are seen when the object is oriented so that we have an unobstructed view of the nucleus but when the obscuring disk is edge-on, the nucleus is hidden and a Seyfert 2 galaxy is seen, with narrow emission lines and weak non-stellar continuum. 

As a test of these ideas, the consortium called the "Lovers of Active Galaxies" observed the circum-nuclear region of the Seyfert 1 galaxy NGC 4151, using both the WHT and INT as part of and International Time Programme, looking for evidence of effects on the surrounding interstellar medium of ionizing radiation emitted by the nucleus. Like many Seyferts, NGC 4151 contains an extra-nuclear emission line region (ENLR) which is elongated over several kiloparsecs. The LAG consortium find that the ENLR emission line spectrum is characteristic of photoionization by a AGN continuum source and that the ionizing radiation field is very probably anisotropic.. Furthermore, the ionising parameter decreases with distance from the nucleus, consistent with geometrical dilution of the radiation of a central source. It is thought that ionising radiation from the nucleus could be being emitted into a broad cone whose axis is inclinned at a large angle to the plane of the galactic disk (by about 70 degrees) and that the ENLR is that segment of the disk which is illuminated by the cone. An elongated nuclear radio source is obsserved at an angle to the ENLR: the ejection axis of the radio-emitting plasma could conincide with the axis of the cone. This geometry is consistent with the fact that NGC 4151 is a Seyfert 1, since our line of sight would fall within the cone, giving a direct view of the nucleus.

More information

ING facilities involved: 

  • Isaac Newton Telescope, using IDS
  • William Herschel Telescope, using ISIS
Some references: 
  • Robinson, A. et al, 1994, "The extended narrow line region of NGC 4151", A&A, 291, 351 


Image of shell in NGC 2300 after galaxy model subtractionShell galaxies are a comparatively recent discovery and are a phenomenon that is not yet fully understood. Shells may arise as a consequence of low-velocity galaxy mergers or alternatively be caused by density waves induced by weak galaxy interactions. Studies - at least in the northern hemisphere - are hampered by the lack of known shell galaxies. A northern hemisphere programme has therefore been started by researchers to identify and analyse them in larger numbers. Using the JKT CCD camera they obtained broad band R images for a sample of 14 bright ellipticals which had been selected on the basis of independant evidence for a merger or intercation, such as extensive dust or kinematically decoupled cores. A new shell-finding algorithm was employed which subtracts a sophisticated elliptical galaxy model and detects shells from inspection of residual isophotes. Two of the sample of 14 were previously known shell galaxies and new shells were discovered in eight others. Partly because the sample was biased towards positive detection and partly because of the sensitivity of the CCD technique, the proportion of shell galaxies detected is much higher than in previous surveys.
More information

ING facilities involved: 

  • Jacobus Kapteyn Telescope with CCD camera
Some references: 
  • Forbes, D. and Thomson, R., 1992, "Shells and isophotal distortions in elliptical galaxies", MNRAS, 254, 723 


Object spectraIn our Galaxy, the stars of the stellar halo are amongst the oldest objects we can observe, and their spatial distribution, kinematics and chemistry are excellent indicators of the timescale and nature of the early evolution of the Galaxy. Since the halo stars are influenced by the whole distribution of the mass of the Galaxy including it's halo of dark matter, they also provide constraints on both the extent and shape of this mass distribution.

One current theory of galaxy formation suggests that the necessary densities for the formation of halo stars occurred in dwarf galaxy sized objects which later merged. If this model is correct, the stellar debris of these initial clumps should now be scattered throughout the halo and groups of stars should exist on the same orbits as their parents dust clouds. Most theories also predict considerable later accretion of halo stars from disrupted satellite galaxies, the remnants of which would also be on similar orbits. The kinematic streams should be detectable. Researchers have developed a selection technique to obtain high probability samples of halo blue horizontal branch (BHB) stars, using both direct and objective prism UK Schmidt plates scanned by the APM. The technique reliably eliminates white dwarf stars, quasars, hot subdwarfs and faint galaxies. 44 candidates were selected in this way in two fields using ISIS on the WHT; they identified 25 stars with A-type spectra in one field and 11 in another, and determined their velocities and distances. The dispersion of velocities of four stars in one field at 30kpc from the Sun is very low, 12 km sec-1, and suggests that these stars are physically associated and may have had a commonn origin. The remaining kinematic data show the Galaxy to have a massive halo extending to at least 30kpc with a flat rotation curve.

More information

ING facilities involved:

  • William Herschel Telescope, using ISIS
Some references: 
  • Arnold, R and Gilmore, G., 1992, "Halo blue horizonntal branch stars - Spectroscopy in two fields", MNRAS, 257, 225



High dispersion blue spectrum of galaxyAccording to standard Big Bang nucleosynthesis theory the light elements - deuterium, helium 3, helium 4 and lithium 7 - owe their origin in whole or part to nuclear reactions that took place in the whole Universe about two minutes after the Big Bang. Indeed the theory has been remarkably successful in explaining and predicting the primordial abundances of these elements relative to hydrogen. Furthermore, these abundances can be used to estimate the mean baryonic (i.e. ordinary matter) density of the Universe today. The primordial helium mass fraction Y is also related to two other fundamental physics parameters: the number of light neutrino "flavours" -- the three known ones correspond to the three known leptons: the electron, the muon, and the tau meson - and the half life of the neutron. Consequently, an accurate determination of Y simultaneously provides an important test of standard Big Bang nucleosynthesis theory, and sets cosmological constraints on the values of these two parameters.

Researchers used the INT and the AAT to measure the primordial helium abundance from the emission lines of HII galaxies. Combined with selected data from the literature relating to extragalactic HII regions in general, they determined Y to be 0.228±0.005 or Y < 0.242 with 95 per cent confidence. This value places in turn an upper limit of 10.4 minutes on the half-life of the neutron and limits the number of neutrino species to < 3.2. These limits based on purely astronomical work, have been confirmed by experiments in the laboratory (10.25 minutes and 3.01±0.01 species). In the course of this work, HII galaxies with broad Wolf-Rayet emission features were excluded from the analysis since they often showed higher helium abundance, supporting a suggestion that there may be additional local sources of helium in the form of winds from Wolf-Rayet stars. 

More information

ING facilities involved: 

  • Isaac Newton Telescope, using IDS
Some references: 
  • Pagel,B.E. et al, 1992, "The primordial helium abundance from observations of extragalactic HII regions", MNRAS, 255, 325 



Image of supernovaafter 30 daysFor some time, astronomers have considered the idea of using Type Ia supernovae to determine the extragalactic distance scale. Supernovae of this type occur in the late stages of evolution of a binary system consisting of a white dwarf star orbiting a companion star. As the companion evolves to the red giant phase it overflows its Roche Lobe and matter accretes onto the surface of the white dwarf, gradually increasing the gravitational pressure. When the white dwarf mass reaches a critical value, the nuclear fuel ignites explosively. The subsequent growth and decay of light from the supernova is characteristic of the type, and the intrinisc luminosity of the explosion is thought to be independent of distance and therefore usable as a "standard candle". A systematic search for distant supernovae is being made on the INT as part of an international collaboration by a team of astronomers.

The light curves of newly discovered supernovae will be monitored over the period of visibility to determine their types, and in the case of type Ia discoveries their "standard candle" properties will be used to measure their distances. The distances and redshifts combined will provide a measure of the decleration parameter of the Universe to the critical density.

As part of this campaign, one type Ia supernova has already been discovered in a distant galaxy. The new supernova was detected at R=22mag on an INT CCD prime focus image and then confirmed in follow-up images over a period of weeks. A spectrum of the host galaxy was obtained with the WHT and the redshift measured to be z=0.457, making it the most distant supernova ever observed.

More information

ING facilities involved: 

  • Isaac Newton Telescope and prime focus CCD
  • William Herschel Telescope, using LDSS2
Some references: 
  • Perlmutter, S. et al, 1995, "A supernova at z=0.458 and implications for measuring the cosmological deceleration", ApJ, 440, 41



The highest redshift gravitationally lensed quasar yet has been discovered using the WHT. At a redshift of z=4.5, the two images of BRI0952-01 are just 0.95 arcseconds apart.

The WHT and INT were used in a survey of high latitude IRAS objects to look for Seyfert galaxies. The results confirmed a model whereby the 2 types of Seyfert galaxies are due to obscuration, coming from a single population.

ISIS on the WHT was used to demonstrate that the standard methods of obtaining velocity dispersions of galaxies could lead to wrong answers. More sophisticated models are therefore needed to fir the nuclei of galaxies better.

The WHT was used with adaptive optics to detect yellow supergiants, which act as standard candles, in NGC 4532. A value of 80 kms-1Mpc-1 for the Hubble constant was obtained, though lower values may be allowed.

INT images have been used to reconfirm the accepted fact that spiral disks are optically thin at optical wavelengths. There had been some new models put forward which had cast doubt on the previous results, but observations of NGC 450 and UGC 807 have shown that there is no absorbtion and that the disks are transparent.

Ultra-high resolution images of the core of M15 were obtained using the WHT and GHRIL, followed by computer post exposure sharpening (TRIFFID). It was used to examine an X-ray binary near the centre of M15. Only the HST has been able to take better images than this.

An extensive programme was carried out on the JKT to measure photometry of thousands of binary stars.

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Last modified: 15 March 2016