Compiled by Bob Fosbury, Anna Pasquali and Rudi Albrecht, December 2000
The archive of the Vienna University Observatory was searched for material pertaining to early research on asteroids by Albrecht with A. Schnell and H.-M. Maitzen (Vienna).
Research at the ST-ECF on the physics and evolution of massive stars ranges from their main-sequence phase to their latest Wolf-Rayet (WR) stage. Rosa, with collaborators from Paris, Marseille and Potsdam, is making a census of OB stars in compact HII blobs in the Large Magellanic Cloud (Heydari-Malayeri et al. 1999a,b,c). HST photometry and spectroscopy are used to derive precise spectral types and constrain the age of the stellar population.
The subsequent evolution of OB stars off the main sequence is manifested by the Luminous Blue Variables (LBVs) which, as a result of outbursts, become considerably lower mass Wolf-Rayet (WR) stars. Such an outburst is believed to produce a circumstellar nebula such as the Homunculus embedding Eta Carinae (Schulte-Ladbeck et al. 1999). The Homunculus is highly bipolar and in order to derive its true 3D structure and its mass and dust content, Pasquali and Walsh are using polarimetric mapping of the nebula from optical to infrared wavelengths, both from HST and from the ground using adaptive optics.
Among the possible explanations of the bipolar morphology observed for several LBV nebulae are binarity and stellar rotation. Echelle spectra taken by Pasquali of a galactic LBV star, HD168625, show for the first time the detection of a binary system composed of a B supergiant and a G2 giant (Pasquali et al. 2000).
By forcing the stellar wind towards the equator, stellar rotation produces an equatorial wind enhancement. This generates a torus which directs the nebula ejection along the stellar polar axis. Characterised by a high gas density and low temperature, this torus is expected to contain molecules such as CO. Indeed, millimeter CO emission has been detected by Pasquali in the Galactic LBV AG Carinae and it has been used to constrain the mass-loss rate through the stellar equator and to revise the star's evolutionary status.
Thanks to the high mass loss induced by the LBV outburst, massive stars (M > 30 - 40 M_sun) terminate their evolution as Wolf-Rayet (WR) stars. The most massive one known in the Galaxy is LSS 4368, an oxygen-rich WR star. It is expected to erupt in a Eta Carinae-like fashion although much more powerfully. Rosa is monitoring this object using photometry and spectroscopy with the aim of detecting the eventual eruption and ejection of a shell. Maps of the environment in the nebular lines will be useful for distance determination should light echoes be observed following an outburst.
An HST view of N159 in the Large Magellanic Cloud. Stellar winds from hot newborn massive stars within the nebula sculpt ridges, arcs and filaments in the cloud, which is over 150 light-years across. A rare type of compact ionized 'blob' is resolved for the first time to be a butterfly-shaped nebula. A possible explanation of this bipolar shape is the outflow of gas from massive stars hidden in the central absorption zone. Such stars are so hot that their radiation pressure halts the infall of gas and directs it away from the stars in two opposite directions. A dense equatorial disk formed by matter still trying to fall onto the stars focuses the outstreaming matter into the bipolar directions. The red in this true-color image is from H-alpha and the yellow from [OIII].
Several aspects of the late evolutionary stages of intermediate mass stars (0.8 - 8 M_sun) have been studied by Kerber, Pirzkal and Walsh. The central stars of planetary nebulae (PNe) have been investigated with emphasis on the He-flash episode during which they undergo an envelope ejection for the second time. An example of this is offered by Sakuari's object which is being observed with ISO and the VLT (Kerber et al. 1999).
The photoionization state of a nebula can be used to trace its hydrodynamical interaction with the surrounding interstellar medium (Kerber et al. 2000), while chemical abundances are closely linked to the third dredge-up taking place in the central star (Pequignot et al. 2000).
The nebular emission lines give an estimate of the reddening, both intrinsic and interstellar, to the object which can be compared with the extinction of the surrounding stellar field determined by multiband photometry. This can give an accurate estimate of the distance to the PNe.
Monitoring of the changing morphology of the circumstellar shell around the R CrB star UW Cen has been carried out by Kerber. Clouds of dust forming close to the photosphere of the central star lead to a variable illumination of the shell (the lighthouse effect), reflecting the changes in the dusty stellar environment (Clayton et al. 1999).
A number of Proto-Planetary Nebulae are observed to display an elliptical/bipolar morphology very similar to that found for their more evolved descendants, the PNe. Although detected with infrared imaging, their asymmetry can be examined quantitatively using polarimetry. Walsh is analysing the development of the bipolar morphology among Proto-Planetary Nebulae using polarimetric mapping in the K' band with SOFI on the ESO NTT (Ageorges & Walsh 2000).
ISOCAM photometry of Sakurai's object (V 4334 Sgr) at different epochs. Diamonds are from Feb 1997, squares from Sept and Oct 1997 and triangles from Feb 1998. UBVRi and JHK data are by Kerber et al. (1999) and from the literature.
Figure: Eta CarAn H band linear polarization map of the Homunculus nebula surrounding Eta Carina taken with the ESO ADONIS instrument. The plotted range is 0 to 37% and the central core, which was saturated in the original images, is set to zero polarization.
Deuterium is believed to be entirely produced in the Big Bang and is destroyed by nuclear processing in stars. Measurements at high redshift of absorption lines to quasars provide the D/H ratio in near-primordial clouds, while for the Milky Way, D/H is measured from HST spectra of Lyman series absorption lines. Walsh, working with collaborators at the IAP and at Meudon in Paris, have observed deuterium for the first time in the optical region in the Balmer series, in emission. Weak lines at a constant spectral offset from the bright H-alpha and H-beta lines have been detected in the Orion Nebula and four other HII regions, including one in the Small Magellanic Cloud. Deuterium emission is produced through fluoresence excitation on the surfaces of cold neutral clouds in the HII region. Modelling of the complex physics is underway in order to determine the D/H ratio from the deuterium emission lines.
During the past two decades, the diffuse ionized gas (DIG) located outside classical HII regions has been recognized as a major component of the ISM. Called extraplanar or halo DIG, it is seen best in deep H-alpha imaging of edge-on galaxies. The counterpart in our Milky Way galaxy is known as the 'Reynolds layer'. Early attempts to understand its ionization identified as the culprit the Lyman continum escaping from classical HII regions in the galactic disks. The unexpected line ratios observed in the weak emission lines of non-hydorgen ions, in particular the unexpectedly low HeI emission and strange ratios of [OIII], [OI], [SII] and [NII] have recently have stimulated attempts to include non-photoionizing energy sources (e.g., shocks) into the interpretation with only modest success.
However, one of the key ingredients in the photoionization models used so far is obviously erroneus, namely the geometry of the problem. Predictions are derived from models based on the classical spherically symetric, isotropic and homogeneous model. In fact, a more appropriate model would to assume that a volume of ISM sees a faint stellar and nebular Lyman continuum from half of the solid angle with almost no nebular radiation from the other half.
In order to explore the main effects of such a geometry, Rosa and R. Tuellmann (ESO) are modifying a Monte Carlo photoionization code to implement galactic disk/halo conditions. The original 1D, spherically symmetric code has now been extended to treat explicitly the required 3D geometry. First experiments have shown that the new geometry has a major impact, because the initial guesses required for the iterative process of the program differ dramatically from those of classical stroemgren spheres.
Using parallel data from NICMOS on HST, Pasquali, Brigas (Cagliari) and De Marchi (STScI) have examined near-IR images of Galactic globular clusters to derive the mass function at low masses (Pasquali et al. 2000). These results extend to J~25 and H~24 and show that the mass function does not depend on the radial distance from the centre of the cluster and so probably represents the IMF without distortion by tidal stripping or evaporation.
Quasars and radio galaxies at redshifts higher than ~2 are believed to mark the sites of formation of what will become the most massive galaxies in the Universe. There is compelling evidence that these two classes of object show axial symmetry and are members of a common parent population, distinguished principally by their orientation with respect to the plane of the observer's sky. At a given radio power, the galaxies are optically fainter than the quasars by four or five magnitudes due to an opaque equatorial torus acting as a 'natural coronograph' which obscures our direct view of the active nucleus. This allows a clear, glare-free view of the quasar host galaxy.
Working with the optical spectropolarimeter (LRISp) on the Keck II 10m telescope and the infrared spectrograph (ISAAC) on the first ESO VLT 8m telescope, Fosbury (with collaborators from Arcetri, Caltech, ESO, Hertfordshire University and the Keck Observatory) have studied the spectra, from Lyman-alpha to H-alpha, of a sample of powerful radio galaxies with redshifts around 2.5 which corresponds to the epoch of maximum space density of quasars. These results (Vernet et. al. 2001) suggest a picture where the assembly of the massive elliptical galaxy is accompanied by a rapid chemical enrichment of the interstellar medium and the dispersal of large quantities of dust produced by the high star formation rate. This dust renders the young stars essentially invisible in the ultraviolet so that they reveal themselves almost exclusively in the far-infrared/sub-mm spectrum as cool re-radiation. Some of the dust serves to scatter a relatively constant fraction (a few %) of ultraviolet and optical radiation from the quasar which is observed as a linearly polarized, spatially extended continuum with superimposed broad quasar emission lines. The intense quasar radiation field ionizes much of the interstellar medium, producing strong, narrow emission lines which can be used to measure the physical conditions, chemical composition and kinematic state of the gas throughout the galaxy. These studies are allowing a close quantitative link to be made between the properties of galaxies from the ultraviolet to mm wavelengths and are very relevant to the future opportunities to be provided by FIRST, NGST and ALMA.
In order to aid the interpretation of these distant and consequently faint objects, a detailed study has been made by Fosbury with van Bemmel and Vernet (ESO) of the nearest powerful radio galaxy, Cygnus A, using similar observational techniques. The combination of multicolour imaging with HST and spatially resolved spectropolarimetry with Keck has allowed a detailed correspondence to be made between spectral continuua ‹ characterised by their polarization properties ‹ and spatial structures in the image. This results in a clear manifestation of the "ionization cones" and equatorial ring of young stars anticipated by the quasar/radio galaxy unification scheme. The very high s/n spectroscopy has allowed the identification and measurement of a rich harvest of emission lines, including some from highly ionized species such as S^{11+} and Ar^{10+}.
The FGS interferometers on HST offer the possibility to attain spatial resolution significantly better than that achievable by direct imaging. A pilot FGS project by Hook with E. Schreier (STScI) and G. Miley (Leiden) has studied the optical structure of several bright AGN and found them all to be point-like at the level of 20mas (Hook et al. 2000). The refurbished FGS1r offers significantly better performance and greater stability and cycle 9 HST time was awarded to continue this work which is expected to be able to detect structure on the scale of 5mas corresponding to sub-parsec distances away from the AGN broad-line region.
Colour images of the radio galaxy Cygnus A constructed from HST (WFPC 2) and Keck (LRIS) data. (a) is made from four broad bands corresponding approximately to B, V, R and I, and shows the young and old stellar populations with regions of obscuring dust. (b) includes the narrow-band emission from H-alpha (red) and [OIII] (green) emission lines in addition to broadband B; it shows more clearly the biconical structure illuminated by the hidden quasar as well as the equatorial ring of young, blue stars. The images are each 10" on a side.
Alexov has worked with D. Silva (ESO) and M. Pierce (Indiana) to analyse deep, wide field, multicolour images of the z = 0.39 cluster, CL0024+1654 which exhibits the excess blue galaxy population, known as the "Butcher-Oemler" effect, seen commonly in clusters at redshifts of a few tenths. Such studies have previously concentrated on the core regions only and this new work goes deep enough to see that the well-known faint blue population of field galaxies extends into the cluster itself with a surface density which tracks the cluster population. This suggests that, at this epoch, the faint blue population of galaxies ‹ which has disappeared by redshift zero ‹ was present also in clusters.
Deep, morphologically-selected counts and photometric redshifts are being used by Bristow and collaborators (Phillipps et al. 2001) to place constraints on cosmological models, particularly on the density and the cosmological constant. The analysis of data from the Hubble Deep Fields for early-type galaxies favours an Einstein-de Sitter universe with simple passive evolution. Detailed image simulations carried out at the ST-ECF have proven the methodology and allow it to be applied with confidence to other fields.
Cristiani with Tae-Sun Kim & S. D'Odorico (ESO) have been using the newly commissioned high resolution spectrograph on the ESO VLT (UVES) to address the nature and evolution of the Ly-alpha absorbers seen against quasars.
The ultraviolet sensitivity of this new spectrograph, enabling observations down to the atmospheric cutoff, has allowed new measurements to be made at intermediate redshift (z ~ 2) which complement recent HST work on local quasars and existing groundbased results at higher redshifts. The evolution of the number of Ly-a absorbers per unit redshift is governed by two main factors: the Hubble expansion and the strength of the metagalactic UV background. At high z, the expansion and the evolution of the UV-background work together to produce a steep evolution of the number of lines with z. At low z, the UV background starts to decrease with time due to the reducing number of ionizing sources and this effect counteracts the Hubble expansion. As a result, the evolution of the number of lines becomes shallower. These new results are leading to significant revisions to the evolution models, suggesting that galaxies play a more important role in the provision of ionizing photons down to redshifts around 1 than was originally thought. From z ~ 1 until the present epoch, the rapidly decreasing global star formation rate produces a qualitative change in the behaviour of the quasar absorption lines.
Images of distant galaxies are distorted by the gravitational field of the Large Scale Structure in the Universe. As light passes regions of varying gravitational potential, images are distorted in a subtle way. This effect is called "Cosmic Shear" and is similar to the well-studied lensing of galaxies by foreground clusters. However, cosmic shear cannot be detected on individual galaxies and requires statistical methods for detection. The main effect is that distant galaxies at small angular separation tend to be aligned in a coherent pattern. Cosmic Shear measurements can be used to distinguish between cosmological models.
The gravitational distortion of high-redshift galaxies has been measured on scale of a few arcminutes to a few degrees using groundbased measurements. The power to discriminate between cosmological models, however, is largest for the smallest angular scales. HST has a unique advantage in measuring cosmic shear on the scale of one arcminute and less. Since any random pointing can be used, such a project can exploit parallel observations. As one of the largest European HST projects, a group based at the ST-ECF (Fosbury, Freudling, Micol, Miralles, Pirzkal) have been working with P. Schneider & H. Haemmerle (Bonn) and T. Erben (MPA, Garching), using the unfiltered CCD imaging mode on the HST imaging spectrograph (STIS) for that purpose. This mode has high sensitivity and a clean, stable point-spread-function, making it ideal for such measurements. A total of more than 100 pointings have been accumulated and processed so far. Preliminary analysis shows that, as expected, the amplitude of cosmic shear continues to rise at the smallest separations.
For a complete analysis of the shear measurements, the redshift distribution of the galaxies has to be known. The group is currently obtaining VLT optical and IR images for a subset of the STIS fields in order to estimate this distribution using photometric redshifts.
Figure: STISA striking example of one of the longer pointings from the STIS parallel imaging program dedicated to the measurement of cosmic shear (HST program 8562, PI: Schneider). The image consists of 54 sub-exposures with a total integration time of 20,574s, carefully combined using a cross-correlation procedure developed by Pirzkal. The field of view is about 50 arcsec. The faintest galaxies have a magnitude V ~ 30 with half-light radii of about 150mas. Cosmic shear is the statistical distortion (by about 3%) of the images of distant galaxies introduced by the large-scale distribution of matter between them and us. The careful analysis of the second order moment of the galaxies in several hundreds of those images along random lines of sight will allow us to quantify accurately this effect on the scale of 1 arcminute.
Compiled by Rudi Albrecht, December 2000
In dense fields such as globular clusters the new generation of very large telescopes such as the VLT can rapidly collect enough photons for high quality photometry but usually at a relatively low resolution which causes photometric measurements to be affected by the effects of crowding. A method called GIRA has been developed at the ST-ECF which allows astrometric information from space-based cameras such as HST/WFPC2 to be used to obtain improved photometry in these cases. GIRA is a two-channel photometric code based on the Richardson-Lucy image restoration method. It has been tested on WFPC2/VLT-FORS images of globular clusters and further VLT time has been awarded for a more detailed search for variables in the core of NGC 288 based on GIRA photometry (Hook and Pirzkal with L. Lucy (IC London).
A slitless spectroscopy simulator (SLIM) was designed and implemented in Python. This project aimed at generating ACS two-dimensional, photometrically calibrated simulations of the three ACS cameras (WFC, HRC, SBC) with either the ACS grism or the ACS prisms. This software has been a test bed to determine the usefulness of Python for such a project. Results produced by SLIM have been routinely used to plan for the upcoming ACS SMOV and will be used in the future to test the ECF ACS grism extraction pipeline. The simulator has been made available to interested parties. (Pirzkal, and members of ECF/ACS team).
The focus was on shared development and use of Java components with various other sites (STScI, CADC, ISO IDC, NASA ADS). Java components and their interfaces were defined and discussed in the framework of the JSky initiative. As a result a number of Java components are now utilized in various configurations at these sites as part of their archive interface (Dolensky with other ECF and ESO staff).
Procedures were developed to identify, in the HST Archive, WFPC frames suitable for combination and to combine them in the best possible manner. Astronomers are presented with observations which are already grouped together, and whose offsets have been pre-computed; they are also offered cleaner (cosmic ray free) and deeper (co-added) frames and a higher resolution ('imcombined') product (Micol).
An experimental pipeline was developed to automatically extract scientific parameters from WFPC 2 frames and to build a so-called characterisation data base, as a means of statistically describing the scientific contents of the WFPC 2 archive: object detection, characterisation, as well as astrometric and photometric information. Together with a powerful user interface this will allow sophisticated archive queries, and, on a longer time scale, to apply more specialised scientific pipelines on selected types of data. (Micol, Pirenne, Benvenuti, Albrecht, together with D. Schade, D. Durand and S. Gaudet, CADC).
The ASTROVIRTEL project is aimed at enhancing the scientific return of the ESO/ST-ECF archive by exploiting it as a Virtual Observatory. The aim is the development of new data mining tools and the addition of new data sources to the archive. Five different research groups with a wide spectrum of science requirements are being suported (Benvenuti, Pierfederici, Pirenne, Micol, Albrecht, Dolensky).
Studies were started to determine the next archive technology to use in the era of large survey telescopes. The aim is to identify the cost-effective implementation of a Petabyte archive (Pirenne with A. Wicenec, ESO)
Concepts for high performance graphical user interfaces to archives are being developed, which utilize modern multi-media technologies to allow users to optimally browse the data archive (Pirenne).
Neural networks (in particular Kohonen self-organizing maps) are being used to characterize the contents of large data bases containig scientific literature. The goal is to allow users to search the data base for information on specific topics, and to find, in this data base, unexpected relations which might lead to new knowledge (Albrecht with D. Merkl, University of Vienna).