Weinberg, Martin

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Professor, Department of Astronomy
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Weinberg
First Name
Martin
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Astrophysics and Astronomy
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Now showing 1 - 10 of 69
  • Publication
    The Dynamics Of Tidal Tails From Massive Satellites
    (2007) Choi, JH; Weinberg, MD; Katz, N
    We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations. We describe the essential dynamical mechanisms and morphological consequences of tail production in satellites with masses greater than 0.0001 of the host halo virial mass. We identify two important dynamical coconspirators: (1) the points where the attractive force of the host halo and satellite are balanced (X-points) do not occur at equal distances from the satellite centre or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails and (2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite's gravity leading to large offsets of the ejecta orbits from the satellite orbit. The effect of the satellite's self-gravity decreases only weakly with a decreasing ratio of satellite mass to host halo mass, proportional to (Ms/Mh)1/3, demonstrating the importance of these effects over a wide range of subhalo masses. Not only will the morphology of the leading and trailing tails for massive satellites be different, but the observed radial velocities of the tails will be displaced from that of the satellite orbit; both the displacement and the maximum radial velocity is proportional to satellite mass. If the tails are assumed to follow the progenitor satellite orbits, the tails from satellites with masses greater than 0.0001 of the host halo virial mass in a spherical halo will appear to indicate a flattened halo. Therefore, a constraint on the Milky Way halo shape using tidal streams requires mass-dependent modelling. Similarly, we compute the distribution of tail orbits both in Er–r−2 space and in E–Lz space, advocated for identifying satellite stream relics. The acceleration of ejecta by a massive satellite during escape spreads the velocity distribution and obscures the signature of a well-defined ‘moving group’ in phase space. Although these findings complicate the interpretation of stellar streams and moving groups, the intrinsic mass dependence provides additional leverage on both halo and progenitor satellite properties.
  • Publication
    Perturbations Of Spherical Stellar Systems During Flyby Encounters
    (2000) Vesperini, E; Weinberg, MD
    We study the internal response of a galaxy to an unbound encounter and present a survey of orbital parameters covering typical encounters in different galactic environments. Overall, we conclude that relatively weak encounters by low-mass interloping galaxies can cause observable distortions in the primaries. The resulting asymmetries may persist long after the interloper is evident. We focus our attention on the production of structure in dark halos and in cluster elliptical galaxies. Any distortion produced in a dark halo can distort the embedded stellar disk, possibly leading to the formation of lopsided and warped disks. We show that distant encounters with pericenters in the outer regions of a halo can excite strong and persistent features in the inner regions. Features excited in an elliptical galaxy are directly observable, and we predict that asymmetries in the morphologies of these systems can be produced by relatively small perturbers. For example, a flyby on an orbit with pericenter equal to the half-mass radius of the primary system and velocity of 200 km s-1 (a value typical for groups) can result in a significant dipole distortion for perturbers with mass as small as 5% of the primary's mass. We use these detailed results to predict the distribution of the A parameter defined by Abraham et al. (sensitive to lopsidedness) and the shift between the center of mass of the primary system and the position of the peak of density for a range of environments. We find that high-density, low velocity dispersion environments are more likely to host galaxies with significant asymmetries. Our distribution for the A parameter is in good agreement with the range spanned by the observed values for local galaxy clusters and for distant galaxies in the Medium Deep Survey and in the Hubble Deep Field. Assuming that primordial galaxies were located in dense environments with previrialized low velocity dispersions, our conclusions are consistent with the observational results showing a systematic trend for galaxies at larger redshifts to be more asymmetric than local galaxies. Finally, we propose a generalized asymmetry parameter A(r) which provides detailed information on the radial structure of the asymmetry produced by the mechanism explored in our work.
  • Publication
    A Rigorous Reanalysis Of The Iras Variable Population: Scale Lengths, Asymmetries, And Microlensing
    (1997) Nikolaev, S; Weinberg, MD
    Previous work reported a bar signature in color-selected IRAS variable stars. Here, we estimate the source density of these variables while consistently accounting for spatial incompleteness in the data using a likelihood approach. The existence of the bar is confirmed with a shoulder at a approx 3 kpc, an axis ratio of a:b = 2.2-2.7, and a position angle of 19° ± 1°. The ratio of nonaxisymmetric to axisymmetric components gives a similar estimate for the bar size of a = 3.3 ± 0.1 kpc and a position angle of phi0 = 24° ± 2°. We estimate a scale length of 4.00 ± 0.55 kpc for the IRAS variable population, suggesting that these stars represent the old disk population. We use this density reconstruction to estimate the optical depth to microlensing for the large-scale bar in the Galactic disk. We find an enhancement over an equivalent axisymmetric disk by up to 40%, but this still too small to account for the MACHO result. In addition, we note a possibility for a significant asymmetry at positive and negative longitudes with optical depths comparable to those in Baade's window. Independent of our reconstruction, an infrared microlensing survey may be a sensitive tool for detecting or constraining structural asymmetries. More generally, this is a pilot study for Bayesian star count analyses. The Bayesian approach allows the assessment of prior probabilities to the unknown parameters of the model; the resulting likelihood function is straightforwardly modified to incorporate all available data.
  • Publication
    Noise-driven Evolution In Stellar Systems - I. Theory
    (2001) Weinberg, MD
    We present a theory for describing the evolution of a galaxy caused by stochastic events such as weak mergers, transient spiral structure, orbiting blobs, etc. This noise excites large-scale patterns that drive the evolution of the galactic density profile. In a dark matter halo, the repeated stochastic perturbations preferentially ring the lowest-order modes with only a very weak dependence on the details of their source. The subsequent redistribution of halo mass is determined only by the mechanics of these modes. The halo profile then evolves toward a universal asymptotic form for a wide variety of noise sources. Such a convergence may help explain the similarity of normal galaxy morphology in diverse environments. A variety of other applications are discussed.
  • Publication
    The Shape Of The Disk: Clues From The Kinematics Of Disk Stars
    (1996) Weinberg, MD; Schechter, P; Binney, J; deZeeuw, T
  • Publication
    The 2-micron All-sky Survey - Survey Rationale And Initial Testing
    (1994) KLEINMANN, SG; LYSAGHT, MG; PUGHE, WL; SCHNEIDER, SE; SKRUTSKIE, MF; Weinberg, MD; PRICE, SD; MATTHEWS, K; SOIFER, BT; HUCHRA, JP; BEICHMAN, CA; CHESTER, TJ; JARRETT, T; KOPAN, GL; LONSDALE, CJ; ELIAS, J; LIEBERT, JW; SEITZER, P
  • Publication
    Fluctuations In Finite-n Equilibrium Stellar Systems
    (1998) Weinberg, MD
    Gravitational amplification of Poisson noise in stellar systems is important on large scales. For example, it increases the dipole noise power by roughly a factor of 6 and the quadrupole noise by 50 per cent for a King model profile. The dipole noise is amplified by a factor of 15 for the core-free Hernquist model. The predictions are computed by summing over the wakes caused by each star in the system — the dressed-particle formalism of Rostoker & Rosenbluth — and are demonstrated by N-body simulation. This result implies that a collisionless N-body simulation is impossible; the fluctuation noise which causes relaxation is an intrinic part of self-gravity. In other words, eliminating two-body scattering at interparticle scales does not eliminate relaxation altogether. Applied to dark matter haloes of disc galaxies, particle numbers of at least 106 will be necessary to suppress this noise at a level that does not dominate or significantly affect the disc response. Conversely, haloes are most likely far from phase-mixed equilibrium and the resulting noise spectrum may seed or excite observed structure such as warps, spiral arms and bars. For example, discreteness noise in the halo, similar to that caused by a population of 106-M⊙ black holes, can produce observable warping and possibly excite or seed other disc structure.
  • Publication
    Beyond The Best-fit Parameter: New Insight On Galaxy Structure Decomposition From Galphat
    (2009) Yoon, Ilsang; Weinberg, M; Katz, N
    We introduce a novel image decomposition package, Galphat, that provides robust estimates of galaxy surface brightness profiles using Bayesian Markov Chain Monte Carlo. The Galphat-determined posterior distribution of parameters enables us to assign rigorous statistical confidence intervals to maximum a posteriori estimates and to test complex galaxy formation and evolution hypotheses. We describe the Galphat algorithm, assess its performance using test image data, and demonstrate that it has sufficient speed for production analysis of a large galaxy sample. Finally we briefly introduce our ongoing science program to study the distribution of galaxy structural properties in the local universe using Galphat.