ING Scientific Highlights in 1994
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ING Scientific Highlights
in 1994*

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


[ 1993 Scientific Highlights | 1995 Scientific Highlights ]
[ SOLAR SYSTEM | STARS | GALAXIES | OBSERVATIONAL COSMOLOGY | OTHER ]
SOLAR SYSTEM


COMET SHOEMAKER-LEVY 9 

Impact L as seen from La PalmaIn common with many observatories all over the world, a major campaign was undertaken in July at the time of the impact on Jupiter of Comet Shoemaker-Levy 9. On the INT, the IDS, configured for low-resolution spectroscopy in the 3500 to 6600Å region, obtained a continuous sequence of spectra of impact L. Apart from recording the fireball, atomic line emission from sodium, magnesium, calcium and possibly iron was detected. The series of spectra showed a typical solar absorption line spectrum reflected from the surface of Jupiter, as well as the broad methane absorption band centered on 6190Å. Division by a spectrum obtained before the impact removed the solar spectrum, making it possible to see that sodium was present in emission even in the first spectrum taken 600 seconds after impact.

The JKT was employed in imaging the impacts through six narrow-band filters sampling the continuum and methane absorption bands. The sequence of events that emerges from the data is as follows. The fireball is first glimpsed 328 seconds after impact when it is still 2° beyond the limb, on the far side of Jupiter. As it comes to the horizon after 553 seconds it achieves maximum brightness but no emision lines are yet visible. By 600 seconds the fireball is fading and sodium is brightening, reaching its maximum after 848 seconds, after which it rapidly fades. In the meantime magnesium and calcium brighten and fade more slowly. A simple model is that the fireball is at first optically thick. As it expands and cools it becomes optically thin and the emission lines are seen.
 

More information

ING facilities involved: 

  • Isaac Newton Telescope, using IDS
  • Jacobus Kapteyn Telescope, using CCD imaging
Some references: 
  • Walton, N. et al, 1994, Spectrum, 3, 4 


ASTEROIDS 

Graphs of reflectance against wavelength for the asteroidsDifferent taxonomic groups in the asteroid belt are prevalent in certain ranges of heliocentric distance a. The majority of distant asteroids have surfaces of low albedo material whose colour varies from the grey C-type asteroids found at around a=3 au to the redder P- and D-types found progressively further out. Since cometary nuclei also exhibit low albedos, and cometary dust is neutral to red in colour, it has been suggested that there could be a link between asteroids and comets. A cometary nucleus that ceased to outgas could perhaps end up as an inert body like an ordinary asteroid.

With this in mind, researchers set out to investigate the reflectance properties of 19 D-type asteroids using the WHT Faint Object Spectrograph. Two asteroids, 102 Miriam and 914 Palisana, exhibited strong UV absorption, characterisitc of phyllosilicates, indicating that they are in fact more correctly classified as C-types. The colours of the remainder were quantified by the slopes of their reflectance spectra. A previously reported heliocentric colour gradient within the D-types was not confirmed, but is not ruled out by the new data. A reflectance-slope diameter correlation for D-types is confirmed in the sense that the smaller asteroids tend to be redder. It is possible that the redder objects are made of intrinsically weaker material and are less resistant to break-up by impacts. Although the D-type asteroids observed have very similar albedos (0.037 ± 0.007) to cometary nuclei, they possess a less diverse range of reflectance properties than cometary nuclei and are better defined as a group by their surfaces. Cometary nuclei cannot therefore be regarded as the spectral analogues of D-type asteroids in general, though the D-types are spectrally similar to some comets. If there is an evolutionary link, this could be explained if the surface compositions have altered over their lifetimes.
 

More information

ING facilities involved: 

  • William Herschel Telescope, using the FOS
Some references: 
  • Fitzsimmons, A. et al, 1994, "A spectroscopic survey of D-type asteroids", A&A, 282, 634 

 
STARS


SUPERNOVAE 

Image of supernova 1994D in NGC 4526Remarkably, after SN1993J, the period from March '94 to August '94 was to see the discovery of three further exceptional supernovae. The explosion of SN1994D in March 1994 was the closest thermonuclear (ie type Ia) event for 22 years. In April, SN1994I was discovered in M51, the brightest type Ib/c event for eight years, and in July, SN1994W appeared, one of the most peculiar type II supernovae ever seen. Following the success of the SN1993J archive, the observing programme and archive was extended to include the other three supernovae as well. 

The La Palma observations provided excellent pre- and post-maximal optical coverage of SN1994D, demonstrating that it was a classic type Ia supernova, believed to be caused by thermonuclear fusion of carbon and oxygen in a white dwarf in a binary system. The data included the first ever pre-maximum light coverage of a normal type Ia in the 8500 to 10000Å range. The earliest spectra provided a goldmine of information about the outer layers of the explosion, and possibly about the progenitor system as well. 

A type Ic supernova such as 1994I is a core-collapse event characterised by absent or weak hydrogen lines without the strong helium lines of a type Ib. It may have been a progenitor in a close binary system which has lost both its hydrogen and helium rich envelopes before collapse. To improve understanding of these events, good spectral coverage is required, and it is hoped to continue monitoring into 1995. 

Features in the spectra of the very peculiar type II supernova SN1994W appear to have come from two regions: broad hydrogen and helium emission from the rapidly moving ejecta, and narrow P Cygni features from a slow moving dense circumstellar wind, ionised by the EUV flash and now scattering part of the subsequently emitted optical light.
 

More information

ING facilities involved: 

  • William Herschel Telescope
Some references: 
  • Meikle, W. et al, 1994, Spectrum, 4, 7 


SPECTRALLY RESOLVED ECLIPSE MAPS OF AN ACCRETION DISK 

Image of supernova 1994D in NGC 4526Accretion disks play an important role in many astrophysical environments, such as active galactic nuclei, protostellar systems, X-ray binaries and cataclysmic variables. The lack of spatially resolved information, however, has meant that theoretical models for accretion disks are in general poorly constrained by observations. By using the shape of the light curves from an an eclipsing cataclysmic variable, UX Ursae Majoris, astronomers have reconstructed the spectral energy distribution across the face of an accretion disk. The spectral resolution is sufficient to reveal both the radial dependence of absorption and emission line features within the disk, and the spectral details of the bright spot formed at the point where the accretion stream from the secondary star collides with the disk. Such detailed reconstructions of accretion-disk spectra will help to bridge the gap between observations and theoretical models.
 
More information

ING facilities involved: 

  • Isaac Newton Telescope using the FOS spectrograph
Pictures: Some references: 
  • Rutten, R.G.M. et al, 1992, "Reconstruction of the accretion disk in six cataclysmic variable stars ", A&A, 260, 213
  • Rutten, R.G.M. et al, 1993, "Spectrally resolved eclipse maps of the accretion disk in UX Ursae Majoris", Nature, 362, 518 
  • Rutten, R.G.M. et al, 1994, "Spectral eclipse mapping of the accretion disk in the nova-like variable UX Ursae Majoris", A&A, 283, 441 
                      
GALAXIES


ELLIPTICAL GALAXIES

Ellipticity profiles ofgalaxiesAn extensive campaign to obtain surface photometry of a complete magnitude-limited sample of 56 elliptical galaxies from the Revised Shapley-Ames catalogue was completed by researchers. CCD photometry in B, V, and I was obtained from the JKT (for the northern sample) and various telescopes at ESO (for the south). Radial profiles of surface brightness, B-V, V-I and ellipticity were determined, together with third and fourth-order Fourier coefficients that quantify the deviations of these profiles from perfect ellipses. These coefficients prove to be sensitive diagnostics of the presence of dust. Narrow-band interference-filter photometry was also obtained in H alpha + [N II] to map the amount and morphology of dust and ionised gas. Optical long-slit spectroscopy was performed on the WHT, the INT and at ESO to determine the ratio of [N II] to H alpha. Dust lanes or patches were detected in 23 of the galaxies: allowing for selection effects the true fraction of elliptical galaxies containing dust is estimated to be about 80%. Ionised gas was detected in 32 galaxies, with emission line intensity ratios typical of LINER nuclei. The amounts of dust and gas detected are small: about 104 or 105 solar masses of ionised gas. The dust and ionised gas show a wide variety of distributions which are consistent with the gas and dust being physically associated, including some cases of patchy or filamentary structure suggestive of a recent interaction event. 
 
More information

ING facilities involved:

  • William Herschel Telescope, using ISIS
  • Isaac Newton Telescope, using IDS
  • Jacobus Kapteyn Telescope, using prime focus CCD camera
Some references: 
  • Goudfrooij, P. et al, 1994, "Interstellar matter in Shapley-Ames elliptical galaxies. I. Multicolour CCD surface photometry", A&AS, 104, 179
  • Goudfrooij, P. et al, 1994, "Interstellar matter in Shapley-Ames elliptical galaxies. II. The distribution of dust and ionised gas", A&AS, 105, 341


GALACTIC MODELS 

Rotation velocity fitsThe presence of a massive black-hole has been invoked to explain the observed rotation velocities and velocity dispersions at the centres of a number of galaxies of various classes, including M31, M32, NGC 3115 and NGC 4594.

Researchers measured stellar line-of-sight velocity profiles of these galaxies from high resolution, high signal-to-noise, long-slit spectroscopy using both ISIS on the WHT and the Multiple Mirror Telescope on Mount Hopkins. In all objects, significant deviations of the velocity profiles from Gaussian were found which are not accounted for by projection or seeing effects. The strongest deviations are asymmetries, primarily along the major axis, which indicate an excess of stars at low rotation velocities.

These velocity profiles rule out certain existing dynamical models. A new model for M32 has been constructed taking proper account of the new data, consistent with the central luminosity 'spike' being completely non-thermal. The observed velocity distributions near the centre can be fitted equally well by radially anisotropic models with a central black hole of mass no more than about 5 x 109 solar masses.
 

More information

ING facilities involved: 

  • William Herschel Telescope, using ISIS
Some references: 
  • Van der Marel, R., 1994, "Velocity profiles of galaxies with claimed Black Holes - Part One - Observations of M31, M32, NGC3115 and NGC4594E", MNRAS, 268, 521 
  • Van der Marel, R., 1994, "Velocity profiles of galaxies with claimed Black Holes - Part Three - Observations and models for M87", MNRAS, 270, 271 

 
OBSERVATIONAL COSMOLOGY


HIGH Z RADIO GALAXIES

Image of 8C1435+635 with radio contours superimposedIn principle, radio galaxies are sufficiently luminous that they can be observed out to a redshift of z=5, and the properties of objects at such high redshifts are of great astrophysical interest. Unfortunately the geometry of the Universe, the luminosity function and a probably drop in the numbers of radio sources past z=2 all conspire to make z=3 objects very rare. 

A search for high redshift radio galaxies was conducted by a consortium of researchers, who selected objects at a frequency of 38 MHz with unusual radio spectra: some had steep, straight spectra and some had highly curved spectra that flattened or turned over at low frequency. VLA maps of the selected candidates provided positions that were sufficiently accurate for optical spectroscopy without further imaging, and follow-up spectra were obtained with ISIS on the WHT. One candidate, 8C1435+635, showed a resolved, spatially extended emission line centered on 639.0 nm with several components with velocities between -1500 and +700 kms-1. A further line object was detected at 820.5nm. Identifying these with lyman alpha and C IV yields a (Ly alpha) redshift of z=4.255, the most distant galaxy known. The radio luminosity, even allowing for possible gravitational amplification, places it at the very top of the radio luminosity function, suggesting that the evolution of radio-loud AGN at high redshift may be similar to the radio-quiet quasars.
 

More information

ING facilities involved: 

  • William Herschel Telescope, using ISIS
Some references: 
  • Lacy, M. et al, 1994, "8C 1435+635: a radio galaxy at z=4.25", MNRAS, 271, 504 


DWARF GALAXY POPULATIONS

A grayscale image of a portion of cluster Abell 963The surface density of faint galaxies significantly exceeds what would be expected if the observed properties of local field galaxies were simply extrapolated to large redshift under the assumption of no evolution. The deepest optical counts are inconsistent with no evolution or even modest evolution irrespective of cosmological model, and show that galaxy colours are bluer at higher apparent magnitudes. Most models devised to account for the excess galaxies seen in the number-magnitude counts invoke a large population of low-luminosity galaxies at moderate redshifts.

Astronomers investigated the faint end of the luminosity function directly at moderate z (=0.2) using deep R-band observations of the Abell distant cluster A963 made with the Hitch-hiker parallel CCD camera on the WHT. By subtracting a mean-field component from the observed galaxy counts in the cluster area, they deduced the existence of a large drawf population in the cluster, giving rise to a significant excess over the mean down to the sample limit at mR=24.5. The implied dwarf galaxy luminosity function has a steep slope, alpha=-1.8.
 

More information

ING facilities involved: 

  • William Herschel Telescope, using Hitch-hiker CCD camera
Some references: 
  • Driver, S. et al, 1994, "Dwarf galaxies at z=0.2 - Photometry of the cluster Abell 963", MNRAS, 268, 393 


OTHER HIGHLIGHTS 

Two dwarf spheroidal galaxies were studied using ISIS on the WHT. One of these had evidence for rotation around the major axis. Further observations will be needed to show that these galaxies are being tidally disrupted, or that there is a large dark matter component within the galaxy. (Hargreaves et al, 1994, MNRAS, 269, 957 and 271, 693).

The Crab nebula, the remnant of the supernova of 1054 and thought to have been a type II supernova, has been shown to have a diffuse halo out to about 2 arcminutes beyond the edge of the nebula. The mass of this halo has proven to be about 4 solar masses, and when added to the 4 solar masses already visible gives a progenitor mass of 8 solar masses. Theories about Type II supernovae say that the progenitors must have at least a mass of 8 solar masses to explode, and the Crab is now in agreement with this. (Murdin, 1994, MNRAS, 269, 89).

The first determination of a distance to a High Velocity Cloud (HVC) has been made using WHT and JKT spectroscopy. These techniques have been used by other researchers to study a number of halo and disk HVC's using the INT. (Little et al, 1994, ApJ, 427, 267; Shaw et al, 1994, Spectrum, No.3, 8; Centurión et al, 1994, Aston Astrophys, 292, 261.

New white dwarfs have been found using the INT and JKT doing optical follow-ups on ROSAT and EUV surveys. One of these objects was the hottest hydrogen-rich DA white dwarf detected. (Barstow et al, 1994, MNRAS, 271, 175). Also found in the same follow-ups was a new AM Her system, which has the shortest period known for this type of object. (Osborne et al, 1994, MNRAS, 270, 650).
 



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Last modified: 13 December 2010