Weinberg, Martin

Job Title

Professor, Department of Astronomy

Last Name

Weinberg

First Name

Martin

Discipline

Astrophysics and Astronomy

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Now showing 1 - 10 of 69

Open Access
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.
Open Access
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.
Open Access
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.
Metadata only
The 2mass Cmd Of The Milky Way: Inferring Structure And History
(2002) Cole, AA; Weinberg, MD
Open Access
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.
Metadata only
The Shape Of The Disk: Clues From The Kinematics Of Disk Stars
(1996) Weinberg, MD; Schechter, P; Binney, J; deZeeuw, T
Metadata only
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
Open Access
The Morphological Diversities Among Star-forming Galaxies At High Redshifts In The Great Observatories Origins Deep Survey
(2006) Ravindranath, S; Giavalisco, M; Ferguson, HC; Conselice, C; Katz, N; Weinberg, M; Lotz, J; Dickinson, M; Fall, SM; Mobasher, B; Papovich, C
We used the deep, multiwavelength images obtained by the Great Observatories Origins Deep Survey (GOODS) to identify ~4700 Lyman break galaxies (LBGs) at z > 2.5, and 292 starburst galaxies at z ~ 1.2. We present the results from morphological analysis based on light profile shape and ellipticity for ~1333 of the most luminous LBGs. About 40% of LBGs at z ~ 3 have exponential profiles, ~30% of the galaxies have steep (r1/4-like) profiles, and ~30% of LBGs have multiple cores of disturbed morphologies suggestive of close pairs or mergers. The fraction of spheriod-like LBGs decrease by about 15% from z ~ 5 to 3. A comparison of LBGs with the starburst galaxies at z ~ 1.2 shows that disklike and merger morphologies are dominant, but the fraction of spheroid-like profiles is about 20% higher among LBGs. The ellipticity distribution for LBGs exhibits a pronounced skew toward high ellipticities (epsilon > 0.5), which cannot be explained by morphologies similar to the local disks and spheroids viewed at random orientations. The peak of the distribution evolves toward lower epsilon, from 0.7 at z = 4 to ~0.5 at z = 3. The ellipticity distribution for the z ~ 1.2 galaxies is relatively flat, similar to that seen for present-day galaxies. The dominance of elongated morphologies suggests that in a significant fraction of LBGs we may be witnessing star formation in clumps along gas-rich filaments, or the earliest gas-rich bars that encompass essentially the entire visible galaxy.
Open Access
Noise-driven Evolution In Stellar Systems - Ii. A Universal Halo Profile
(2001) Weinberg, MD
Disc instabilities such as arm and bar formation, minor mergers and tidal encounters drive a galaxy from equilibrium. Using the theory that describes the evolution of a galaxy halo as a result of stochastic fluctuations developed in the companion paper to this one, we show that this sort of noise evolves a halo toward a standard profile, independent of its initial profile and concentration. This process can substantially redistribute the mass in dark-matter haloes in the 10 Gyr since formation. Three different noise processes are studied: (i) a bombardment by blobs of mass that are small compared to the halo mass (‘shrapnel’); (ii) orbital evolution of substructure by dynamical friction (‘satellites’) and (iii) noise caused by the orbit of blobs in the halo (‘black holes’). The power spectrum in the shrapnel and satellite cases is continuous and produces the universal form by exciting the same discrete modes independent of the noise source. These modes dominate the evolution of the mass profile. The power spectrum for black holes is discrete and has a different form with a much slower rate of evolution. The predicted convergence in evolution may help explain the similarity of galaxy properties in widely differing environments.
Open Access
Adiabatic Invariants In Stellar Dynamics .1. Basic Concepts
(1994) Weinberg, MD
The adiabatic criterion, widely used in astronomical dynamics, is based on the harmonic oscillator. It asserts that the change in action under a slow varying perturbation is exponentially small. Recent mathematical results precisely define the conditions for invariance show that this model does not apply in general. In particular, a slowly varying perturbation may cause significant evolution stellar dynamical systems even if its time scale is longer than any internal orbital time scale. This additional 'heating' may have serious implications for the evolution of star clusters and dwarf galaxies which are subject to long-term environmental forces. the mathematical developments leading to these results are reviewed, and the conditions for applicability to and further implications for stellar systems are discussed. Companion papers present a computational method for a general time-dependent disturbance and detailed example.