Structure Of Elliptical Galaxies Research Articles

Radio and optical orientations of galaxies

We investigate the correlations between optical and radio isophotal position angles for 14302 SDSS galaxies with $r$ magnitudes brighter than 18 and which have been associated with extended FIRST radio sources. We identify two separate populations of galaxies using the colour, concentration and their principal components. Surprisingly strong statistical alignments are found: late-type galaxies are overwhelmingly biased towards a position angle differences of $0^{\circ}$ and early-type galaxies to $90^{\circ}$. The late-type alignment can be easily understood in terms of the standard picture in which the radio emission is intimately related to areas of recent star-formation. In early-type galaxies the radio emission is expected to be driven by accretion on to a nuclear black hole. We argue that the observed correlation of the radio axis with the minor axis of the large-scale stellar distribution gives a fundamental insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled form the rest of the galaxy. Our results imply that the galaxies are oblate spheroids with their radio emission aligned with the minor axis. Remarkably the strength of the correlation of the radio major axis with the optical minor axis depends on radio loudness. Those objects with a low ratio of FIRST radio flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This may reflect different formation histories for the different objects and we suggest we may be seeing the different behaviour of rationally supported and non-rotationally supported ellipticals.

Shrinking of Cluster Ellipticals: A Tidal Stripping Explanation and Implications for the Intracluster Light

We look for evidence of tidal stripping in elliptical galaxies through the analysis of homogeneous CCD data corresponding to a sample of 228 elliptical galaxies belonging to 24 clusters of galaxies at 0.015 < z < 0.080. We investigate departures from the standard magnitude-isophotal size relation as a function of environmental (clustercentric distance, local galaxy density) and structural (cluster velocity dispersion, Bautz-Morgan type) properties. We find that, for any particular galaxy luminosity, the elliptical galaxies in the inner and denser regions of the clusters are about 5% smaller than those in the outer regions, which is in good agreement with the finding of Strom and Strom based on photographic photometry. The null hypothesis (i.e., galaxy sizes are independent of the clustercentric distance or density) is rejected at a significance level of better than 99.7%. The numerical models of Aguilar and White predict that tidal stripping can lead to changes in the whole structure of elliptical galaxies, producing shrinkage and brightening of the galaxy qualitatively consistent with our measurements and also with the findings of Trujillo and coworkers that more centrally concentrated elliptical galaxies populate denser regions. Our observational results can be interpreted as evidence for the stripping of stars from elliptical galaxies in the central/denser regions of clusters, contributing to the intracluster light observed in these structures.

The structure of elliptical galaxies in the Virgo cluster. Results from the INT Wide Field Survey

We report on a complete CCD imaging survey of 226 elliptical galaxies in the North-East quadrant of the Virgo cluster, representative of the properties of giant and dwarf elliptical galaxies in this cluster. We fit their radial light profiles with the Sersic model of light distribution. We confirm the result of Graham & Guzman ([CITE], AJ, 125, 2936) that the apparent dichotomy between E and dE galaxies in the luminosity– plane no longer appears when other structural parameters are considered and can be entirely attributed to the onset of “core” galaxies at mag. When “core” galaxies are not considered, E and dE form a unique family with n linearly increasing with the luminosity. For 90 galaxies we analyze the color indices, both in the nuclear and in the outer regions. Both indices are bluer toward fainter luminosities. We find also that the outer color gradients do not show any significant correlation with the luminosity. The scatter in all color indicators increases significantly toward lower luminosities, e.g. galaxies fainter than have a spread > 0.5 mag.

Frequency map analysis of the orbital structure in elliptical galaxies

We present an application of the frequency map analysis to an elliptical galaxy which is represented by a generalization of a double-power-law spherical mass model. The density distribution ofthismodel variesasr ¹g close tothecentre andasr ¹4 atlargeradii. Westudythe case with g ¼ 1, which is known as the ‘weak-cusp’ model and which represents well the density profile of the ‘core’ galaxies observed by the Hubble Space Telescope. The final objective of our work is to improve our understanding of the dynamics of elliptical galaxies in asimilarwaytoMerritt &Fridman,findingtheregionsofstochasticity,lookingforresonances that might play an important role in sustaining the triaxial morphology, and analysing the diffusion of orbits. To this end, we use the frequency map analysis of Laskar, which has been applied widely in the field of celestial mechanics butwhich is a relatively new technique in the area of galactic dynamics. Finally, we show some useful features of this method in understanding the global dynamical structure of the system.

X-Ray Observations and the Structure of Elliptical Galaxies

We compare optical and high-quality X-ray data for three bright elliptical galaxies in the Virgo Cluster, NGC 4472, 4649, and 4636. The distribution of total mass in NGC 4472 and 4649 determined from X-ray data is sensitive to the stellar mass over a considerable range in galactic radius extending to r ≈ re, the effective radius. The agreement of X-ray and optically determined stellar masses provides a unique verification of the stellar mass-to-light ratio, which is essentially constant over the range 0.1 r/re 1. However, for NGC 4636 the dark matter is important at all radii 0.35re. Evidently, the dark to stellar mass ratio varies in quite different ways in elliptical galaxies of comparable optical luminosity, implying that the radial structure of dark halos may not be universal. There is some evidence in NGC 4636 for additional support of the hot interstellar gas at r 0.35re; either a field B ~ 10-4 G or a small (mechanically unstable) central region of high gas temperature (T ~ 107 K) is required. The global temperature structure in the hot interstellar medium of many recently observed elliptical galaxies is very similar, reaching a maximum near 3re-4re. This feature, which may suggest a new structural scale in these galaxies, is inconsistent with current theoretical gasdynamical models.

Models of Galaxies with Central Black Holes: Simulation Methods

We present a method for simulating numerically the effect of the adiabatic growth of black holes on the structure of elliptical galaxies. Using a parallel self--consistent field code, we add black holes to N--body realizations of model distribution functions for spherical galaxies, using a continuous mass--spectrum. The variable particle mass, combined with a simple multiple timestep integration scheme, makes it possible to evolve the models for many dynamical times with $N \sim 10^6-10^8$, allowing high spatial and mass resolution. This paper discusses verification of the code using analytic models for spherical galaxies, comparing our numerical results of the effect of central black holes on the structure of the galaxies with previously published models. The intrinsic and projected properties of the final particle distribution, including higher order moments of the velocity distribution, permit comparison with observed characteristics of real galaxies, and constrain the masses of any central black holes present in those galaxies. Our technique is promising and is easily extended to axisymmetric and triaxial galaxies.

Photometric and kinematic properties of disky elliptical galaxies

New insights in the structure of elliptical galaxies have been obtained indicating the presence of faint stellar disks in some of these systems. Clear correlations between the isophotal shapes, the kinematic, radio and X-ray properties of disky E's were found, suggesting that these galaxies form together with SO's a continuous transition of D/B ratio in the Hubble sequence (Bender et al. 1989). The isophotal shapes of these galaxies have been quantified by the fourth cosine coefficient a4 of the Fourier expansion of the deviations from perfect ellipses, which yields positive values when the isophotes are pointed along the major axis (Carter 1987, Lauer 1985, Jedrzewski 1987, Bender and Möllenhoff 1987).

Correlations between line strengths and fine structure in elliptical galaxies

Les correlations entre les forces des raies, Hβ, CN et (Mg I + MgH), et la structure fine sont analysees pour 36 galaxies elliptiques geantes. Les resultats sont utilises pour estimer les contributions des variations de l'âge et de la metallicite sur la dispersion des forces de raie.

The dynamics of shell formation

The paper investigates the dynamics of shell formation, addressing issues such as general requirements for shell formation systems, expected sharpness of shells, longevity of shell systems, and comparison of the competing models for the origin of the shell-forming stars. Shell formation is studied first in a one-dimensional potential, which gives new insights into the formation of shear shells, and then in an integrable three dimensional potential, which can cause an initially localized particle distribution to produce shells. The sharpness of shells is governed by the extent of the initial particle distribution in the angular dimensions. Approximate analytical expressions for shell thickness and spacing are derived in terms of the three frequencies which govern particle motions in the potential. The expressions apply to the limit when the initial particle distribution has become well sheared in phase space. Tightly wrapped shells make the invariant surfaces of a galaxy's potential visible and will provide valuable information concerning the structure of elliptical galaxies. 42 refs.

Surface Photometry and the Structure of Elliptical Galaxies

This paper reviews surface photometry of bulges and elliptical galaxies. Work prior to 1982 is discussed by Kormendy (1982a; hereafter K82). Since then, the subject has gone through a revolution. CCD detectors have come into common use, providing photometry accurate enough to measure new classes of subtle properties of ellipticals. Together with improvements in seeing, CCDs have allowed the resolution and study of galaxy cores and nuclei (Section 2). Newly discovered structural details, such as dust, shells, and dynamical subsystems, show the importance of accretion events in galaxy evolution (Sections 3-6). Better measurements of parameter scaling laws have led to improved constraints on galaxy formation (Section 8). Finally, CCDs provide accurate measurements of departures from elliptical isophotes (Section 9) and color gradients (Section 10). These observations are currently producing a quantum jump in our understanding of elliptical galaxies. Some of the present subjects are discussed in more detail in recent reviews by Kormendy (1987a; hereafter K87), Okamura (1988), and Nieto (1988). Techniques are discussed by de Vaucouleurs (1979, 1984), Nieto (1982), Capaccioli & de Vaucouleurs (1983), Capaccioli (1985, 1987, 1988a,b), Okamura (1988), and Djorgovski & Dickinson (1989). Compilations of photometry references for individual galaxies are found in Davoust & Pence (1982) and Pence & Davoust (1985). Unless otherwise noted, we assume that the Hubble constant is H_0 = 50 km s^(-1) Mpc^(-1).

Hierarchical merging and the structure of elliptical galaxies

N-body simulations are employed to investigate relationships between the global properties of collisionless, self-gravitating systems (e.g., galaxies) that result from hierarchical merging. Each galaxy of mass M is assumed to form by the merging of two identical progenitors of mass 1/2 M from an interpenetrating parabolic encounter. The remnant galaxies depart strongly from homology with their progenitors at all stages in the merging hierarchy. A power-law relation between galaxy mass and central velocity dispersion results, with exponent similar to the observed luminosity-velocity dispersion relation for ellipticals. Since this relation appears to be entirely scale-free, we inspect observational data on other stellar systems supported primarily by random motions (globular clusters and dwarf elliptical galaxies). The possibily that hierarchical merging has operated over many decades in mass (10/sup 5/< or approx. =M/M/sub sun/ < or approx. =10/sup 12/) is discussed.