P. D. Lynam |
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Observational Cosmology with Clusters of GalaxiesContents
IntroductionThe study of the formation, evolution and properties of the Universe and the structure within it is one of the most exciting facets of current astrophysical research. Clusters of galaxies occupy a prominent position in the hierarchy of cosmic structure since they contain the bulk of the observable matter in the Universe. These important cosmological probes punctuate the underlying large-scale matter distribution. As such, they are readily identifiable tools with which to investigate the behaviour of the Universe on intermediate and large scales, complementing studies of the Cosmic Background Radiation (CBR) and supernovae. These unique environments, harbouring hundreds to thousands of co-eval, frequently interacting galaxies, immersed in a high-temperature, x-ray emitting plasma are the ideal laboratories within which to isolate environmental and evolutionary effects in the galaxy population. Clusters provide the opportunity to examine the relationship between selected environments at measured epochs and thus enable us to witness successive phases in the development of the Universe. By comparing these observations with cosmological models, we achieve a greater understanding of our place within the Universe and can attempt to predict its fate with much greater confidence. Through our efforts to address these issues, cosmology has remained one of the most attractive subjects to science students and the layperson alike.The Importance of X-ray InformationMulti-wavelength studies of clusters of galaxies, pursuing several science goals are being undertaken. These use samples derived from the all-sky survey performed at x-ray wavelengths by the German Roentgen Satellite (ROSAT) which remains the premier survey of its type. The primary source catalogues used are the REFLEX I (ROSAT-ESO Flux Limited X-ray) cluster catalogue --- the most comprehensive unbiased cluster sample to date --- and its Northern celestial hemisphere counterpart: NORAS (Northern ROSAT All-Sky) cluster survey. The advantages of x-ray selection should be emphasized: The x-ray emitting cluster gas traces the underlying gravitational potential of the cluster. Therefore, x-ray properties provide an unbiased reflection of the physical nature of the cluster and thus circumvent the numerous flaws known to plague catalogues compiled from inspections of localized overdensities in the projected 2-dimensional optical galaxy distribution.Global Variations in Galaxy ContentIn an effort to conduct the first statistically meaningful investigation of the properties of clusters in both the x-ray and optical regime, deep, wide-field, multi-colour imaging of equal mass clusters in a series of redshift intervals is being performed. Massive clusters are expected to evolve at the highest rate and therefore to show the highest contrast in their galaxy content between different epochs. Thus, we hope to observe recent evolutionary phases more clearly. Furthermore, the same clusters are studied in detail simultaneously using the latest x-ray observatories, with the ultimate aim that cluster physics is understood well enough to predict optical characteristics from x-ray observations, and vice-versa. This project exploits a new generation of detectors, which enable nearby clusters to be simultaneously studied for the first time at both x-ray and optical wavelengths, across the whole of their extent on the sky. Limited by the field of view of conventional detectors, previous disparate investigations have typically sampled fewer objects and their stability has been susceptible to the inclusion of a few extremely unusual constituents. It is intended to use characteristics of the cluster colour-magnitude diagram to diagnose evolutionary traits in each cluster, and to determine bulk evolutionary trends in each redshift interval. We can then search for correlations between these optical indicators and the x-ray properties of the clusters, for example; x-ray temperature.Reliability of Brightest Cluster Galaxies as Distance IndicatorsHigh density environments such as clusters of galaxies host a unique class of galaxy: the Giant Ellipticals. These, the largest known galaxies, dominate the optical appearance of massive clusters and are frequently referred to as Brightest Cluster Galaxies (BCGs). Furthermore, giant ellipticals were thought to exhibit remarkable uniformity in their intrinsic luminosity. Hence, these galaxies are easily identifiable at large distances and have traditionally been used as cosmological distance indicators. However, relatively few studies of giant ellipticals have been performed and the apparent photometric homogeneity has often been assumed rather than demonstrated. Today, there are indications that the reliability of BCGs as distance indicators may have been overstated: comparisons between x-ray and optically selected BCG samples suggest selection effects introduce artificial correlations which have been misinterpreted as physical characteristics of these galaxies and erroneously used to give an impression of improved photometric reliability in what are contaminated samples. Furthermore, there are indications that the structure of these objects may be affected by their proximity to the dynamical center of the cluster. Clarifying these issues has far-reaching repercussions for modern cosmology since it represents a significant revision of one of the steps in the cosmological distance ladder and directly affects measurements of the current rate of expansion of the Universe employing giant ellipticals.The Distribution of Brightest Cluster GalaxiesAs long as it is free of selection biases, the distribution of a class of object contains information about the objects' past. For example, a smooth, symmetrical distribution indicates a common history for all the objects. A more complicated distribution results from differences between members of a population which arose at some time in the past. Distribution studies can therefore provide clues to the formation and evolution of objects. Formation scenarios for the apparently special BCGs must address the question: do they share the same evolutionary history as their less luminous cluster co-habitees, or have they formed as a result of processes unique to their privileged location within the cluster? For individual clusters, there is observational evidence to support and contradict both the idea that BCGs formed in a monolithic fashion with their host cluster, and that BCGs are built-up by successive galaxy mergers. BCG studies have seen a renaissance in recent years and as large and unbiased samples become available. Formerly believed to be smooth and homogeneous, recent tantalizing infra-red analyses are suggestive of a bimodal BCG distribution in which the two modes are moderated by cluster mass: In massive clusters (those with x-ray luminosities greater than or equal to 2 x 1044 erg s-1) we interpret a uniform distribution of bright BCGs as evidence for passive evolution from a single burst at redshift, z ~ 1 to the present day; while giant ellipticals in low-mass (x-ray luminosities less than 2 x 1044 erg s-1) clusters show a wide range of evolutionary history --- including significant mass growth (presumably, through mergers) by up to a factor of 4 --- over the same cosmic time interval. There are several ways to proceed and several dichotomies to investigate: Comparisons of x-ray coincident BCGs versus alternative giant cluster ellipticals; presence versus absence of amorphous, low surface-brightness stellar envelopes identifying the cD galaxies; presence versus absence of colour gradients between giant ellipticals isolated via different selection techniques; the frequency of nuclear multiplicity within x-ray centroid coincident galaxies compared with other galaxies in the cluster; the correlation between `cuspy'/flat nuclear profiles and optical/x-ray selection. Programs pursuing comparative investigations of (optical) colour gradients, cD envelopes and (infra-red) core profiles, based on x-ray selected samples are currently acquiring data.With studies of clusters of galaxies, the populations within them and with contributions provided by the investigations and methods outlined above, we approach a consistent interpretation of observations, past and present. The study of these objects shall, in the next few years, drive and enhance our understanding of structure formation in the Universe. Minor Bodies in the Solar SystemMinor Bodies in the Solar SystemAsteroids, comets and meteoroids remain important subjects of astrophysical research. In particular, our knowledge of the mass distribution of these objects is limited. On-going search projects are continually discovering new objects. Therefore, estimates of the number, location and mass of minor bodies in the Solar System are frequently revised, and subject to debate. Of topical interest is the potential danger to civilization posed by the population of Near Earth Objects (NEOs). If we have a knowledge of the population of NEOs then the likelihood and severity of an impact between Earth and potentially hazardous asteroids can be computed.Additionally, missions such as Giotto, NEAR, Rosetta and OSIRIS-REx illustrate that rendez-vous and close approach missions are an important technique in our efforts to understand the nature of minor bodies. Residing in the outermost regions of the Solar System since its birth, these objects are believed to be chemically and thermally unchanged. Their contents, consisting of amalgamations of dust grains in a delicate frosted matrix, reflect the conditions that prevailed in the gas and dust cloud from which the Sun and planets formed. Analyzing specimens of these objects constrains Solar System formation scenarios and provides a data-point with possibly the maximum weighting in setting the standard abundance distribution - a key input to cosmological models. A Potential ThreatOccasionally, these objects approach the warmer, inner Solar System along highly elliptical orbits. The increased Solar radiation causes surface ices to sublime, liberating the dust grains. Over time, clouds of these particles spread out both leading and following the parent body. Eventually a complete torus of meteoroids, enclosing the orbit of the parent object, is formed. Most periodic comets, some asteroids and at least one hybrid object have associated meteoroid streams. Those streams that intersect the orbit of Earth may cause meteor showers: when the Earth traverses a stream node, ground-based observers witness increased meteor activity as the enhanced meteoroid flux is frictionally heated in high-velocity collisions with the atmosphere. The presence of these micron-sized particles in the near-Earth environment represents a more immediate threat to humankind's activities than, for example, a collision with a NEO.As a result of meteoroid impacts, spacecraft surfaces can undergo erosive processes during repeated impacts on, or around, the same site on their structure. Individual impacts can generate plasmas, which subsequently drift across the surfaces of the spacecraft and interact with electrical connections, causing control anomalies (as is believed to be the case with the mid-term failure of the Olympus telecommunications satellite. The components most vulnerable to impact are solar panels: both forward impacts on the cell face and rear impacts on the substrate obliterate cell coatings reduce efficiency and lead to differential charging of spacecraft surfaces. Normally, such reductions in efficiency during operational lifetime are projected for in platform design. However, the experience of space platforms such as LDEF, HST, Olympus and the Mir space station during the 1993 passage of Earth through its node with Perseid meteoroid stream illustrate the the risk to spacecraft longevity from both the background meteoroid complex and an enhanced meteoroid flux during the traversal of a stream. The Leonid Meteoroid StreamThe Leonid meteoroid stream was anticipated to reach a peak level of activity following the return of its parent body (comet 55P/Tempel-Tuttle) and the densest part of the stream to the inner Solar System around the turn of the Millennium. It became incumbent on space platform operators to legislate for the dangers faced when the Leonid meteor shower reached its predicted peak in activity. It was expected that this spacecraft population would be subject to an enhanced meteoroid flux. The probability of any given spacecraft being hit by a meteoroid during the relatively short duration of an anticipated Leonid storm was estimated to be considerably higher than the probability of being hit by members of the normal sporadic background meteoroid population during a satellite's typical operational lifetime.As part of a broad consultancy to spacecraft operators (e.g. NASA, ESA, British Aerospace) a historical investigation of the Leonid meteoroid stream, endeavoring to obtain original observations of the Leonids and associated showers and storms in an effort to review and critically evaluate the available information, was conducted. Attempts were made to predict events around the turn of the millennium by constructing activity profiles of past Leonid apparitions from source observations, typically in the form of corrected Zenithal Hourly Rate (Z.H.R.) versus solar longitude diagrams. Then, by making justifiable assumptions, to estimate the level of activity during the next passage of the ortho-Leonid stream over the period 1997-2002. What became clear during this study was the difficulty encountered in using historical observations to predict future stream activity with any precision. The lack of a consistent system for observing meteor activity (particularly, having no reliable method of recording magnitude distributions) - even for contemporary observations - greatly reduces their usefulness. The resulting paucity of data enables only bulk results to be drawn from studies of this type and, as was revealed subsequently, only weakly constrains computational evolutionary modeling of meteoroid streams. Avoidance ManoeuvresDespite these difficulties, as a result of this and other studies, a conservative approach was adopted for a number of spacecraft - particularly those with larger surface areas - at specified, high-risk intervals throughout the period of enhanced Leonid activity: Projected (e.g. Space Shuttle, Envisat) launches were postponed, and on-station satellites (e.g. HST) were reoriented, reconfiguring their orbits to perform damage limitation manoeuvres by feathering their solar panels to the stream radiant.As the number of space-fairing operators, their launch capabilities, spacecraft size and spacecraft costs grow, the importance of understanding the threat of the meteoroidal complex is likely to increase correspondingly. The case of undertaking avoidance manoeuvres and no reported impact during the Leonids at the turn of the Millennium demonstrates that even comparatively crude analyses can be beneficial to space operations. Furthermore since meteoroids represent the low-mass end of the minor body mass spectrum, continued study of these relatively common objects can provide insights into the nature of their more massive relatives, the comets, asteroids and potentially hazardous NEOs. Publications
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| This document last updated (UTC): Tuesday 19 March 2024 |