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
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.
Metadata only
The 2mass Cmd Of The Milky Way: Inferring Structure And History
(2002) Cole, AA; Weinberg, MD
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.
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.
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
The Cosmological Significance Of High-velocity Cloud Complex H
(2006) Simon, JD; Blitz, L; Cole, AA; Weinberg, MD; Cohen, M
We have used new and archival infrared and radio observations to search for a dwarf galaxy associated with the high-velocity cloud (HVC) known as `complex H.' Complex H is a large (Ω gtrsim 400 deg2) and probably nearby (d = 27 kpc) HVC whose location in the Galactic plane has hampered previous investigations of its stellar content. The H I mass of the cloud is 2.0 × 107(d/27 kpc)2 Msun, making complex H one of the most massive HVCs if its distance is more than ~20 kpc. Virtually all similar H I clouds in other galaxy groups are associated with low surface brightness dwarf galaxies. We selected mid-infrared sources observed by the MSX satellite in the direction of complex H that appeared likely to be star-forming regions and observed them at the wavelength of the CO J = 1 → 0 rotational transition in order to determine their velocities. Of the 60 observed sources, 59 show emission at Milky Way velocities, and we detected no emission at velocities consistent with that of complex H. We use these observations to set an upper limit on the ongoing star formation rate in the HVC of lesssim5 × 10-4 Msun yr-1. We also searched the 2MASS database for evidence of any dwarf-galaxy-like stellar population in the direction of the HVC and found no trace of a distant red giant population, with an upper limit on the stellar mass of ~106 Msun. Given the lack of evidence for either current star formation or an evolved population, we conclude that complex H cannot be a dwarf galaxy with properties similar to those of known dwarfs. Complex H is therefore one of the most massive known H I clouds that does not contain any stars. If complex H is self-gravitating, then this object is one of the few known dark galaxy candidates. These findings may offer observational support for the idea that the cold dark matter substructure problem is related to the difficulty of forming stars in low-mass dark matter halos; alternatively, complex H could be an example of a cold accretion flow onto the Milky Way.
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.
Open Access
Dark Matter Halo Response To The Disc Growth
(2006) Choi, JH; Lu, Y; Mo, HJ; Weinberg, MD
We consider the sensitivity of the circular-orbit adiabatic contraction approximation to the baryon condensation rate and the orbital structure of dark matter haloes in the Λ cold dark matter (ΛCDM) paradigm. Using one-dimensional hydrodynamic simulations including the dark matter halo mass accretion history and gas cooling, we demonstrate that the adiabatic approximation is approximately valid even though haloes and discs may assemble simultaneously. We further demonstrate the validity of the simple approximation for ΛCDM haloes with isotropic velocity distributions using three-dimensional N-body simulations. This result is easily understood: an isotropic velocity distribution in a cuspy halo requires more circular orbits than radial orbits. Conversely, the approximation is poor in the extreme case of a radial orbit halo. It overestimates the response of a core dark matter halo, where radial orbit fraction is larger. Because no astronomically relevant models are dominated by low angular momentum orbits in the vicinity of the disc and the growth time-scale is never shorter than a dynamical time, we conclude that the adiabatic contraction approximation is useful in modelling the response of dark matter haloes to the growth of a disc.
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
Bar-induced Evolution Of Dark Matter Cusps
(2005) Holley-Bockelmann, K; Weinberg, M; Katz, N
The evolution of a stellar bar transforms not only the galactic disc, but also the host dark matter halo. We present high-resolution, fully self-consistent N-body simulations that clearly demonstrate that dark matter halo central density cusps flatten as the bar torques the halo. This effect is independent of the bar formation mode and occurs even for rather short bars. The halo and bar evolution is mediated by resonant interactions between orbits in the halo and the bar pattern speed, as predicted by linear Hamiltonian perturbation theory. The bar lengthens and slows as it loses angular momentum, a process that occurs even in rather warm discs. We demonstrate that the bar and halo response can be critically underestimated for experiments that are unable to resolve the relevant resonant dynamics; this occurs when the phase space in the resonant region is undersampled or plagued by noise.