137 research outputs found

    Modelling Galaxies with f(E,Lz); a Black Hole in M32

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    A technique for the construction of axisymmetric distribution functions for individual galaxies is presented. It starts from the observed surface bright- ness distribution, which is deprojected to gain the axisymmetric luminosity density, from which follows the stars' gravitational potential. After adding dark mass components, such as a central black hole, the two-integral distribu- tion function (2I-DF) f(E,Lz), which depends only on the classical integrals of motion in an axisymmetric potential, is constructed using the Richardson- Lucy algorithm. This algorithm proved to be very efficient in finding f(E,Lz) provided the integral equation to be solved has been properly modified. Once the 2I-\df\ is constructed, its kinematics can be computed and compared with those observed. Many discrepancies may be remedied by altering the assumed inclination angle, mass-to-light ratio, dark components, and odd part of the 2I-DF. Remaining discrepancies may indicate, that the distribution function depends on the non-classical third integral, or is non-axisymmetric. The method has been applied to the nearby elliptical galaxy M32. A 2I-DF with ~55 degrees inclination and a central black hole (or other compact dark mass inside ~1pc) of 1.6-2*10^6 Msun fits the high-spatial-resolution kinema- tic data of van der Marel et al. remarkably well. 2I-DFs with a significantly less or more massive central dark mass or with edge-on inclination can be ruled out for M32. Predictions are made for HST-observations: spectroscopy using its smallest square aperture of 0.09"*0.09" should yield a non-gaussian central velocity profile with broad wings, true and gaussian-fit velocity dispersion of 150-170km/s and 120-130km/s, respectively.Comment: 14 pages, 9 figures, uuencoded compressed ps file (468k), Ref: OUTP-94-04

    A Very Fast and Momentum-Conserving Tree Code

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    The tree code for the approximate evaluation of gravitational forces is extended and substantially accelerated by including mutual cell-cell interactions. These are computed by a Taylor series in Cartesian coordinates and in a completely symmetric fashion, such that Newton's third law is satisfied by construction and hence momentum exactly conserved. The computational effort is further reduced by exploiting the mutual symmetry of the interactions. For typical astrophysical problems with N=10^5 and at the same level of accuracy, the new code is about four times faster than the tree code. For large N, the computational costs are found to scale almost linearly with N, which can also be supported by a theoretical argument, and the advantage over the tree code increases with ever larger N.Comment: revised version (accepted by ApJ Letters), 5 pages LaTeX, 3 figure

    A fast multipole method for stellar dynamics

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    The approximate computation of all gravitational forces between NN interacting particles via the fast multipole method (FMM) can be made as accurate as direct summation, but requires less than O(N)\mathcal{O}(N) operations. FMM groups particles into spatially bounded cells and uses cell-cell interactions to approximate the force at any position within the sink cell by a Taylor expansion obtained from the multipole expansion of the source cell. By employing a novel estimate for the errors incurred in this process, I minimise the computational effort required for a given accuracy and obtain a well-behaved distribution of force errors. For relative force errors of ∌10−7\sim10^{-7}, the computational costs exhibit an empirical scaling of ∝N0.87\propto N^{0.87}. My implementation (running on a 16 core node) out-performs a GPU-based direct summation with comparable force errors for N≳105N\gtrsim10^5.Comment: 21 pages, 15 figures, accepted for publication in Journal for Computational Astrophysics and Cosmolog

    Black hole foraging: feedback drives feeding

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    We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers. Momentum-driven feedback from an accreting hole gives significant orbital energy but little angular momentum to the surrounding gas. Once central accretion drops, the feedback weakens and swept-up gas falls back towards the SMBH on near-parabolic orbits. These intersect near the black hole with partially opposed specific angular momenta, causing further infall and ultimately the formation of a small-scale accretion disk. The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) super-solar abundances of the matter accreting on to the SMBH; and (vi) a lower central dark-matter density, and hence annihilation signal, than adiabatic SMBH growth implies. We also suggest a simple sub-grid recipe for implementing this process in numerical simulations.Comment: accepted for publication in ApJ Letters, 5 pages, 1 figur

    Dynamical Models for the Milky Way

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    The only way to map the Galaxy's gravitational potential Ί(x)\Phi({\bf x}) and the distribution of matter that produces it is by modelling the dynamics of stars and gas. Observations of the kinematics of gas provide key information about gradients of Ί\Phi within the plane, but little information about the structure of Ί\Phi out of the plane. Traditional Galaxy models {\em assume}, for each of the Galaxy's components, arbitrary flattenings, which together with the components' relative masses yield the model's equipotentials. However, the Galaxy's isopotential surfaces should be {\em determined\/} directly from the motions of stars that move far from the plane. Moreover, from the kinematics of samples of such stars that have well defined selection criteria, one should be able not only to map Ί\Phi at all positions, but to determine the distribution function fi(x,v)f_i({\bf x},{\bf v}) of each stellar population ii studied. These distribution functions will contain a wealth of information relevant to the formation and evolution of the Galaxy. An approach to fitting a wide class of dynamical models to the very heterogeneous body of available data is described and illustrated.Comment: 10 pages, LaTeX, style file and 4 figures included. Invited talk presented at the meeting ``Formation of the Galactic Halo ... Inside and Out'', Tucson, October 9-11. Full .ps file available at ftp://ftp.physics.ox.ac.uk/pub/local/users/dehnen/MilkyWayModels.ps.g

    Approximating Stellar Orbits: Improving on Epicycle Theory

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    Already slightly eccentric orbits, such as those occupied by many old stars in the Galactic disk, are not well approximated by Lindblad's epicycle theory. Here, alternative approximations for flat orbits in axisymmetric stellar systems are derived and compared to results from numeric integrations. All of these approximations are more accurate than Lindblad's classical theory. I also present approximate, but canonical, maps from ordinary phase-space coordinates to a set of action-angle variables. Unfortunately, the most accurate orbit approximation leads to non-analytical R(t). However, from this approximation simple and yet very accurate estimates can be derived for the peri- and apo-centers, frequencies, and actions integrals of galactic orbits, even for high eccentricities. Moreover, further approximating this approximation allows for an analytical R(t) and still an accurate approximation to galactic orbits, even with high eccentricities.Comment: accepted for publication in AJ; 12 pages LaTeX, 9 figures (coloured only here, not in AJ) uses aas2pp4.st

    The outer rotation curve of the Milky Way

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    A straightforward determination of the circular-speed curve vc(R) of the Milky Way suggests that near the Sun, vc starts to rise approximately linearly with R. If this result were correct, the Galactic mass density would have to be independent of radius at R ~> R0. We show that the apparent linear rise in v_c arises naturally if the true circular-speed curve is about constant or gently falling at R0 1.25 R0 are actually concentrated into a ring of radius ~1.6 R0.Comment: 3 pages, LaTeX, uses mn.sty, 5 .ps figures, submitted to MNRA

    Mass models of the Milky Way

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    A parameterized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scale length R_d* of the stellar disk to R0. The disk and bulge dominate v_c(R) at R<R0 only for R_d*/R0< 0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R << R0 to affect profoundly the structure of the best-fitting model at R >> R0. For example, changing the disk slightly from an exponential surface-density profile significantly changes the form of v_c(R) at R >> R0, where the disk makes a negligible contribution to v_c. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct halos rather than outwards-increasing mass-to-light ratios. The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z > R and at R > R0 are especially important.Comment: 10 pages, LaTeX, mn.sty, 5 .ps figures, accepted by MNRAS major revision involving new cepheid & hipparcos dat
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