ING Scientific Highlights in 1996
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ING Scientific Highlights
in 1996*

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


[ 1995 Scientific Highlights | 1997 Scientific Highlights ]



Comet Hale-Bopp in 1995Comet 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.

Comet Hale-Bopp in 1996Subsequent 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.

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.

More information

ING facilities involved: 

  • JKT+CCD imaging 
Pictures:  Some references: 
  • 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, 1996, "ING observations of Comet Hale-Bopp", Spectrum Newsletter, 12, 4


1997 UG25, one of the faintest Kuiper Belt objectsIn 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.

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.

More information

ING facilities involved: 

Pictures:  Some references: 
  • D Pollacco, 1996, IAU circular 6328
  • "A Dying Star's Last Gasp", S&T, 05/96, 11


Line-of-sight velocity distribution of NGC 7331A 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.

More information

ING facilities involved: 

  • INT+Prime Focus 
Pictures:  Some references: 
  • 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


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.

More information

ING facilities involved: 

Some references: 
  • 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


NGC 6822 Dwarf GalaxyA large old to intermediate-age stellar population has been found in a couple of dwarf irregular (dIr) galaxies in the Local Group. The blue colours of dIr galaxies, their large numbers of HII regions, Ha flux, blue stars and relatively large amount of gas have led them to be referred to as young objects and to be preferred targets for the study of young stellar populations and recent star formation. The presence and the age of a stellar population older than a few thousands of millions of years in dIrs had been questioned, even though an underlying population of red stars has been detected in the nearest of them (often called Baade's sheet). But the presence of Baade's sheet is not, by itself, a proof of old age since it may be produced by stars just a few thousands of millions of years old. 

Star FormationHistory of NGC 6822Following a new approach to the problem (based on the comparison of observedand model colour-magnitude [CM] diagrams), star formation histories (SFHs) for old, intermediate and young ages have been obtained on the INT for three dIr galaxies: NGC6822, Pegasus and LGS3. Model CM diagrams are built up for different input SFHs and an accurate simulation of observational effects is a majoringredient. Although the resolution worsens in the determination of the SFH as we go to older ages, this work gives a definite answer to the question of the age of these dIrs in the sense that not only dIrs they begin to form stars about 15,000 million years ago but also that they show SFHs enhanced in the first half of their lives. 

This result is a new step towards the understanding of the mechanisms of the
formation and evolution of galaxies. The dIrs seem now to be as old as the other
kind of dwarf galaxies, the dwarf ellipticals (dE), which show few or no traces of
recent star formation but which might have a stronger relationship with dIrs than is usually assumed.

More information

ING facilities involved: 

  • INT+Prime Focus
Pictures Some references: 
  • A. Aparicio, C. Gallart, G. Bertelli, 1997,  "The star formation history of the Pegasus dwarf irregular galaxy",  Astron. J.114,  669 
  • A. Aparicio, C. Gallart, G. Bertelli, 1997, "The stellar content and the star formation history of the Local Group dwarf galaxy LGS 3.",  Astron. J., 114,  680
  • C. Gallart, A. Aparicio, 1997, "Old populations and the star formation history of the Local Group dwarf irregular galaxy NGC 6822.", Bull. Am. Astron. Soc., 29,  824 
  • C. Gallart, A. Aparicio, G. Bertelli, C. Chiosi, 1996, "The Local Group dwarf irregular galaxy NGC 6822. III. The recent star formation history.", Astron. J., 112, 2596 
  • C. Gallart, A. Aparicio, J. M. Vílchez, 1996, "The Local Group dwarf irregular galaxy NGC 6822. I. The stellar content.", Astron. J., 112, 1928 
  • C. Gallart, A. Aparicio, G. Bertelli, C. Chiosi, 1996, "The Local Group dwarf irregular galaxy NGC 6822. II. The old and intermediate-age star formation history.", Astron. J.,  112, 1950



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.

More information

ING facilities involved:

  • WHT
  • INT
  • JKT
Some references: 
  • 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

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Last modified: 13 December 2010