Shapes Of Elliptical Galaxies Research Articles

Variation in the intrinsic shapes of elliptical galaxies

We present a method to determine the variation in the intrinsic shapes of the light distribution of elliptical galaxies by combining the profiles of photometric data from the literature with triaxial models. The inferred shape variation is given by a Bayesian probability distribution, assuming a uniform prior. The likelihood of obtaining the data is calculated by using ensembles of triaxial models. We apply the method to infer the shape variation of a galaxy, using the ellipticities and the difference in the position angles at two suitably chosen points from the profiles of the photometric data. We find that the parameters that are best constrained are the short to long axial ratios at small and at large radii, and the absolute value of the triaxiality difference between these radii. We compare our results with the available previous results. We apply the methodology to a sample of 10 galaxies.

Dissipationless collapse of a set ofNmassive particles

The formation of self-gravitating systems is studied by simulating the collapse of a set of N particles which are generated from several distribution functions. We first establish that the results of such simulations depend on N for small values of N. We complete a previous work by Aguilar and Merritt concerning the morphological segregation between spherical and elliptical equilibria. We find and interpret two new segregations: one concerns the equilibrium core size and the other the equilibrium temperature. All these features are used to explain some of the global properties of self-gravitating objects: origin of globular clusters and central black hole or shape of elliptical galaxies.

Are there sextuplet and octuplet image systems?

We study gravitational lensing by the family of scale-free galaxies with flat rotation curves. The models are defined by a shape function, which prescribes the radius of the isophote as a function of the position angle from the major axis. The critical curves are analytic, while the caustic network is reducible to a simple quadrature. The cusps are always located at the turning points of the shape function. We show that the models with exactly elliptic isophotes never admit butterfly or swallowtail cusps and so have at most four (or five) images. Higher order imaging is brought about by deviations of the isophotes from pure ellipses, such as pointedness caused by embedded discs or boxiness caused by recent merging. The criteria for the onset of sextuple and octuple imaging can be calculated analytically in terms of the ellipticity e and the fourth-order Fourier coefficients (a4 and b4) used by observers to parametrize the isophote shapes. The six or eight images are arranged roughly in a circle, which appears as an incomplete Einstein ring if inadequately resolved. Using data on the shapes of elliptical galaxies and merger remnants, we estimate that ∼1 per cent of all multiply imaged quasars may be sextuplet systems or higher. Forthcoming satellites such as the Global Astrometric Interferometer for Astrophysics (GAIA) will provide data sets of ∼4000 multiply imaged systems, and so ∼40 will show sextuple imaging or higher.

Uncovering the Intrinsic Shapes of Elliptical Galaxies. IV. Tests on Simulated Merger Remnants

We test the methods developed in previous papers for inferring the intrinsic shapes of elliptical galaxies, using simulated objects from N-body experiments. The shapes of individual objects are correctly reproduced to within the statistical errors; close inspection of the results indicates a small systematic bias in the sense of underestimating both the triaxiality and short-to-long axis ratio. We also test the estimation of parent shape distributions, using samples of independent, randomly oriented objects. The best results are statistically accurate, but on average the parent distributions are again slightly biased in the same sense. Using the posterior probability densities for the individual objects, we estimate the magnitude of the bias to be 0.1 in both shape parameters. The results support the continued use of these methods on real systems.

Chaos and the Shapes of Elliptical Galaxies

Hubble Space Telescope observations reveal that the density of stars in most elliptical galaxies rises toward the center in a power-law cusp. Many of these galaxies also contain central dark objects, possibly supermassive black holes. The gravitational force from a steep cusp or black hole will destroy most of the box orbits that constitute the “backbone” of a triaxial stellar system. Detailed modeling demonstrates that the resulting chaos can preclude a self-consistent, strongly triaxial equilibrium. Most elliptical galaxies may therefore be nearly axisymmetric, either oblate or prolate.

Intrinsic shapes of discy and boxy ellipticals

Statistical test for intrinsic shapes of elliptical galaxies have given so far inconclusive and sometime contradictory results. These failures have been often charged to the fact that classical tests consider only the two asymmetric shapes (oblate versus prolate), while ellipticals are truly triaxial bodies. On the other and, recent analyses indicate that the class of elliptical galaxies could be a mixture of two families, a family of fast-rotating, disc-like ellipticals (disky) and a family of slow-rotating, box-like ellipticals (boxy). The tests for intrinsic shapes of elliptical galaxies are reviewed using data of better quality (CCD) with respect to previous applications. Two samples of disky and boxy ellipticals are analyzed.

Automated detection of clusters of galaxies

view Abstract Citations (19) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Automated detection of clusters of galaxies. Dodd, R. J. ; MacGillivray, H. T. Abstract The authors describe a method for the automated detection and parametrization of rich clusters of galaxies in deep wide-angle surveys obtained by purely objective means from machine-produced data. The technique is applied to one field of the ESO/SERC Southern Sky survey and also to a variety of synthetic galaxy fields produced from Monte Carlo simulations. The results indicate that clusters of galaxies are generally highly flattened, the distribution of cluster shapes being more elongated than the shapes of elliptical galaxies. The potential of the method for investigation of the geometric properties of clusters by objective means in a much larger sample over a wide area of sky is discussed. Publication: The Astronomical Journal Pub Date: October 1986 DOI: 10.1086/114206 Bibcode: 1986AJ.....92..706D Keywords: Astronomical Photography; Automatic Control; Cluster Analysis; Galactic Clusters; Computational Astrophysics; Ellipticity; Mass Distribution; Monte Carlo Method; Photographic Plates; Sky Surveys (Astronomy); Astronomy full text sources ADS | Related Materials (1) Erratum: 1987AJ.....93.1287D

On the shape of elliptical galaxies

We propose a test for the existence of prolate elliptical galaxies based on the properties of the photometric parameters introduced in the Cordoba surface photometry.

Tests for the intrinsic shape of elliptical galaxies

Tests for the intrinsic shape of elliptical galaxies

Concluding remarks

At an I.A .U. Symposium in 1977, Y. B. Zel’dovich said, ‘It will only be a few years before the origin and evolution of galaxies is understood.’ It is hard to guess whether the talks at this Discussion Meeting would have reinforced or moderated his optimism, but I have no doubt that Professor Zel’dovich would have been impressed by the variety of new observations and new ideas presented here. There has certainly been some progress towards understanding galaxies and their origins. A few years ago we were worse than ignorant about some crucial points: we ‘knew’ some things that were not so. Let me mention two such things in particular. It used to be tacitly accepted that elliptical galaxies owed their shape to rotational flattening. But the observations now show that the rotation rates are too slow. The internal dynamics are complex: elliptical galaxies may be triaxial — bars spinning end-over-end — and the random component of the stellar velocities may be anisotropic. According to Dr Binney (who was persuasively advocating such ideas even before the rotation data forced the problem on everyone’s attention) the shape of elliptical galaxies offers clues to the way in which they originated in an asymmetrical collapse. A second tacit assumption was that the ‘luminous’ content of galaxies dominated their dynamics - that the total gravitating mass is distributed in the same way as the stars we see. But, as Dr Gunn and others have told us, the luminous content of galaxies may be swamped by ten times as much dark matter: this material forms a halo around massive isolated galaxies, and provides the virial mass in rich clusters.