arXiv subjects

John Dubinski

Publications and source records attributed to John Dubinski.

At least 37 records · Page 2Linked to original sources

arXiv2005

Equilibrium Disk-Bulge-Halo Models for the Milky Way and Andromeda Galaxies

We describe a new set of self-consistent, equilibrium disk galaxy models that incorporate an exponential disk, a Hernquist model bulge, an NFW halo and a central supermassive black hole. The models are derived from explicit distribution functions for each component and the large number of parameters permit detailed modeling of actual galaxies. We present techniques that use structural and kinematic data such as radial surface brightness profiles, rotation curves and bulge velocity dispersion profiles to find the best-fit models for the Milky Way and M31. Through N-body realizations of these models we explore their stability against the formation of bars. The models permit the study of a wide range of dynamical phenomenon with a high degree of realism.

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arXiv2004

Deep CCD Surface Photometry of Galaxy Clusters II: Searching for Intracluster Starlight in non-cD clusters

We report the search for intracluster light in four Abell Type II/III (non-cD) galaxy clusters: Abell 801, 1234, 1553, & 1914. We find on average that these clusters contain $\sim$ 10% of their detected stellar luminosity in a diffuse component. We show that for two of the clusters the intracluster light closely follows the galaxy distribution, but in the other two cases, there are noticeable differences between the spatial distribution of the galaxies and the intracluster light. We report the results of a search for intracluster tidal debris in each cluster, and note that Abell 1914 in particular has a number of strong tidal features likely due to its status as a recent cluster merger. One of the Abell 1914 features appears to be spatially coincident with an extension seen in weak lensing maps, implying the feature traces a large amount of mass. We compare these results to numerical simulations of hierarchically-formed galaxy clusters, and find good general agreement between the observed and simulated images, although we also find that our observations sample only the brightest features of the intracluster light. Together, these results suggest that intracluster light can be a valuable tool in determining the evolutionary state of galaxy clusters.

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arXiv2003

Optimal Weak Lensing Skewness Measurements

Weak lensing measurements are entering a precision era to statistically map the distribution of matter in the universe. The most common measurement has been of the variance of the projected surface density of matter, which corresponds to the induced correlation in alignments of background galaxies. This measurement of the fluctuations is insensitive to the total mass content, like using waves on the ocean to measure its depths. But when the depth is shallow as happens near a beach, waves become skewed. Similarly, a measurement of skewness in the projected matter distribution directly measures the total matter content of the universe. While skewness has already been convincingly detected, its constraint on cosmology is still weak. We address optimal analyses for the CFHT Legacy Survey in the presence of noise. We show that a compensated Gaussian filter with a width of 2.5 arc minutes optimizes the cosmological constraint, yielding $ΔΩ_m/Ω_m\sim 10%$. This is significantly better than other filters which have been considered in the literature. This can be further improved with tomography and other sophisticated analyses.

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arXiv2003

High Performance Commodity Networking in a 512-CPU Teraflop Beowulf Cluster for Computational Astrophysics

We describe a new 512-CPU Beowulf cluster with Teraflop performance dedicated to problems in computational astrophysics. The cluster incorporates a cubic network topology based on inexpensive commodity 24-port gigabit switches and point to point connections through the second gigabit port on each Linux server. This configuration has network performance competitive with more expensive cluster configurations and is scaleable to much larger systems using other network topologies. Networking represents only about 9% of our total system cost of USD$561K. The standard Top 500 HPL Linpack benchmark rating is 1.202 Teraflops on 512 CPUs so computing costs by this measure are $0.47/Megaflop. We also describe 4 different astrophysical applications using complex parallel algorithms for studying large-scale structure formation, galaxy dynamics, magnetohydrodynamic flows onto blackholes and planet formation currently running on the cluster and achieving high parallel performance. The MHD code achieved a sustained speed of 2.2 teraflops in single precision or 44% of the theoretical peak.

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arXiv2003

GOTPM: A Parallel Hybrid Particle-Mesh Treecode

We describe a parallel, cosmological N-body code based on a hybrid scheme using the particle-mesh (PM) and Barnes-Hut (BH) oct-tree algorithm. We call the algorithm GOTPM for Grid-of-Oct-Trees-Particle-Mesh. The code is parallelized using the Message Passing Interface (MPI) library and is optimized to run on Beowulf clusters as well as symmetric multi-processors. The gravitational potential is determined on a mesh using a standard PM method with particle forces determined through interpolation. The softened PM force is corrected for short range interactions using a grid of localized BH trees throughout the entire simulation volume in a completely analogous way to P$^3$M methods. This method makes no assumptions about the local density for short range force corrections and so is consistent with the results of the P$^3$M method in the limit that the treecode opening angle parameter, $θ\to 0$. (abridged)

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arXiv2003

Detection of dark matter Skewness in the VIRMOS-DESCART survey: Implications for Ω_0

Weak gravitational lensing provides a direct statistical measure of the dark matter distribution. The variance is easiest to measure, which constrains the degenerate product σ_8Ω^0.6. The degeneracy is broken by measuring the skewness arising from the fact that densities must remain positive, which is not possible when the initially symmetric perturbations become non-linear. Skewness measures the non-linear mass scale, which in combination with the variance measures Ωdirectly. We present the first detection of dark matter skewness from the Virmos-Decart survey. We have measured the full three point function, and its projections onto windowed skewness. We separate the lensing mode and the B mode. The lensing skewness is detected for a compensated Gaussian on scales of 5.37 arc minutes to be κ^3=1.06+/-0.06x10^-6. The B-modes are consistent with zero at this scale. The variance for the same window function is κ^2= 5.32+/-0.62+/-0.98x10^-5, resulting in S_3=375^{+342}_{-124}. Comparing to N-body simulations, we find Ω_0<0.5 at 90% confidence. The Canada-France-Hawaii-Telescope legacy survey and newer simulations should be able to improve significantly on the constraint.

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arXiv2001

Intrinsic Shapes of Molecular Cloud Cores

We conduct an analysis of the shapes of molecular cloud cores using recently compiled catalogs of observed axis ratios of individual cores mapped in ammonia or through optical selection. We apply both analytical and statistical techniques to deproject the observed axis ratios in order to determine the true distribution of cloud core shapes. We find that neither pure oblate nor pure prolate cores can account for the observed distribution of core shapes. Intrinsically triaxial cores produce distributions which agree with observations. The best-fit triaxial distribution contains cores which are more nearly oblate than prolate.

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arXiv1999

The Dynamical Evolution of Galaxies in Clusters

The evolution of galaxies is driven strongly by dynamical processes including internal instabilities, tidal interactions and mergers. The cluster environment is a useful laboratory for studying these effects. I present recent results on simulations of interacting populations of spiral and elliptical galaxies in the cosmological collapse of a cluster, showing the formation of the central, brightest cluster galaxies through merging, the effect of tidal interactions and merging on galaxy morphology over cosmic history, and the distribution and kinematics of the tidal debris field. (See http://www.cita.utoronto.ca/~dubinski/rutgers98 for extensive figures and animations.)

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arXiv1999

Constraining Dark Halo Potentials with Tidal Tails

We present an extensive parameter survey to study the influence of halo mass profiles on the development of tidal tails in interacting disk galaxies. We model the galaxies using a fixed exponential disk with a central bulge and vary the halo potential over a range of parameters using both the Hernquist and NFW mass distributions, probing the effect of the halo mass, extent and concentration. We examine the consistency of the results against both observational and theoretical constraints on halo profiles and comment on the failures and weaknesses of different models. Galaxies with rising or flat rotation curves dominated by the halo are inhibited from forming tidal tails unless the halo is abruptly cut off just beyond the disk edge. Conversely, models with declining rotation curves -- resulting either from compact, low mass halos, or from massive disk components in low concentration dark halos -- produce tidal tails very similar to those observed in well-studied interacting systems. As argued by Springel & White (1998), a unifying, quantitative relation for all cases is that the ratio of the escape velocity to the circular velocity at around the solar radius must be v_e/v_c<2.5 for tidal tails to be produced. The galaxy models which appear to fit most of the observational constraints are those which have disk-dominated rotation curves and low concentration halos. We discuss our results in a cosmological context using recent studies which link halo properties to cosmological models.

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arXiv1998

The Origin of the Brightest Cluster Galaxies

Most clusters and groups of galaxies contain a giant elliptical galaxy in their centres which far outshines and outweighs normal ellipticals. The origin of these brightest cluster galaxies is intimately related to the collapse and formation of the cluster. Using an N-body simulation of a cluster of galaxies in a hierarchical cosmological model, we show that galaxy merging naturally produces a massive, central galaxy with surface brightness and velocity dispersion profiles similar to observed BCG's. To enhance the resolution of the simulation, 100 dark halos at $z=2$ are replaced with self-consistent disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000 particles for the 20 largest galaxies and 10000 particles for the remaining ones. This technique allows us to analyze the stellar and dark matter components independently. The central galaxy forms through the merger of several massive galaxies along a filament early in the cluster's history. Galactic cannibalism of smaller galaxies through dynamical friction over a Hubble time only accounts for a small fraction of the accreted mass. The galaxy is a flattened, triaxial object whose long axis aligns with the primordial filament and the long axis of the cluster galaxy distribution agreeing with observed trends for galaxy-cluster alignment.

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arXiv1997

Constraining dark halos with tidal tails

Massive and extended dark halos can inhibit the formation of long tidal tails in galaxy collisions. We examine this effect using an extensive survey of simulations with different dark halo potentials to constrain halo properties of interacting galaxies. These constraints are compared to other observational limits and theoretical predictions of halo structure. The dark halos predicted by $Ω=1$ cosmological models like CDM are too massive and extended to produce the long tidal tails seen in nearby galaxy collisions. There is also a conflict with the halo potentials inferred from satellite kinematics; such halos would likewise inhibit tail formation in galaxy collisions.

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arXiv1997

Searching for Machos (and other Dark Matter Candidates) in a Simulated Galaxy

We conduct gravitational microlensing experiments in a galaxy taken from a cosmological N-body simulation. Hypothetical observers measure the optical depth and event rate toward hypothetical LMCs and compare their results with model predictions. Since we control the accuracy and sophistication of the model, we can determine how good it has to be for statistical errors to dominate over systematic ones. Several thousand independent microlensing experiments are performed. When the ``best-fit'' triaxial model for the mass distribution of the halo is used, the agreement between the measured and predicted optical depths is quite good: by and large the discrepancies are consistent with statistical fluctuations. If, on the other hand, a spherical model is used, systematic errors dominate. Even with our ``best-fit'' model, there are a few rare experiments where the deviation between the measured and predicted optical depths cannot be understood in terms of statistical fluctuations. In these experiments there is typically a clump of particles crossing the line of sight to the hypothetical LMC. These clumps can be either gravitationally bound systems or transient phenomena in a galaxy that is still undergoing phase mixing. Substructure of this type, if present in the Galactic distribution of Machos, can lead to large systematic errors in the analysis of microlensing experiments. We also describe how hypothetical WIMP and axion detection experiments might be conducted in a simulated N-body galaxy.

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arXiv1997

Tidal Tales Two: The Effect of Dark Matter Halos on Tidal Tail Morphology and Kinematics

We examine the effect of different dark matter halo potentials on the morphology and kinematics of tidals tails in a merger model of NGC 7252. We find that models of merging galaxies with low halo masses of Mh = 4-8 M{disk+bulge} (Mdb) can fit the observed morphology and kinematics of the NGC 7252 tails while galaxies with high mass halos (Mh = 16-32 Mdb) fail in this respect. In high mass models, the deep potential only allows weakly bound disk material (stars or gas) at R > 5 disk scale lengths to be ejected in tidal tails which tend to fall back onto the parent galaxies before the final merger. Galaxies with massive, low density halos are somewhat more successful at ejecting tidal debris during mergers, but still have difficulties recreating the thin, gas-rich tails observed in NGC 7252. Our models suggest upper limits for the dark halo masses in the NGC 7252 progenitor galaxies of roughly Mh <= 10 Mdb. We note, however, that our calculations have focused on the rather idealized case of the isolated merging of galaxies with distinct dark matter halos; calculations which employ more realistic (cosmological) initial conditions are needed to fully explore the use of tidal tails in constraining dark matter in galaxies.

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arXiv1997

Parallel TreeSPH

We describe PTreeSPH, a gravity treecode combined with an SPH hydrodynamics code designed for massively parallel supercomputers having distributed memory. Our computational algorithm is based on the popular TreeSPH code of Hernquist & Katz (1989). PTreeSPH utilizes a domain decomposition procedure and a synchronous hypercube communication paradigm to build self-contained subvolumes of the simulation on each processor at every timestep. Computations then proceed in a manner analogous to a serial code. We use the Message Passing Interface (MPI) communications package, making our code easily portable to a variety of parallel systems. PTreeSPH uses individual smoothing lengths and timesteps, with a communication algorithm designed to minimize exchange of information while still providing all information required to accurately perform SPH computations. We have additionally incorporated cosmology, periodic boundary conditions with forces calculated using a quadrupole Ewald summation method, and radiative cooling and heating from a parameterized ionizing background following Katz, Weinberg & Hernquist (1996). The addition of other physical processes, such as star formation, is straightforward. A cosmological simulation from z=49 to z=2 with 64^3 gas particles and 64^3 dark matter particles requires ~6000 node-hours on a Cray T3D, with a communications overhead of ~10% and is load balanced to a ~90% level. When used on the new Cray T3E, this code will be capable of performing cosmological hydrodynamical simulations down to z=0 with ~2x10^6 particles, or to z=2 with ~10^7 particles, in a reasonable amount of time. Even larger simulations will be practical in situations where the matter is not highly clustered or when periodic boundaries are not required.

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arXiv1996

A Parallel Tree Code

We describe a new implementation of a parallel N-body tree code. The code is load-balanced using the method of orthogonal recursive bisection to subdivide the N-body system into independent rectangular volumes each of which is mapped to a processor on a parallel computer. On the Cray T3D, the load balance in the range of 70-90\% depending on the problem size and number of processors. The code can handle simulations with $>$ 10 million particles roughly a factor of 10 greater than allowed in vectorized tree codes.

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arXiv1995

Using Tidal Tails to Probe Dark Matter Halos

We use simulations of merging galaxies to explore the sensitivity of the morphology of tidal tails to variations of the halo mass distributions in the parent galaxies. Our goal is to constrain the mass of dark halos in well-known merging pairs. We concentrate on prograde encounters between equal mass galaxies which represent the best cases for creating tidal tails, but also look at systems with different relative orientations, orbital energies and mass ratios. As the mass and extent of the dark halo increase in the model galaxies, the resulting tidal tails become shorter and less massive, even under the most favorable conditions for producing these features. Our simulations imply that the observed merging galaxies with long tidal tails ($\sim 50-100$ kpc) such as NGC 4038/39 (the Antennae) and NGC 7252 probably have halo:disk+bulge mass ratios less than 10:1. These results conflict with the favored values of the dark halo mass of the Milky Way derived from satellite kinematics and the timing argument which give a halo:disk+bulge mass ratio of $\sim 30:1$. However, the lower bound of the estimated dark halo mass in the Milky Way (mass ratio $\sim 10:1$) is still consistent with the inferred tidal tail galaxy masses. Our results also conflict with the expectations of $Ω=1$ cosmologies such as CDM which predict much more massive and extended dark halos.

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arXiv1995

Nearly Self-Consistent Disk-Bulge-Halo Models for Galaxies

We describe methods for setting up self-consistent disk-bulge-halo galaxy models. The bulge and halo distribution functions (\df) are functions of $E$ and $L_z$ only. The halo's flattening and rotation can be specified. The disk \df\ is a function of $E$ and $L_z$ and a third ``integral'', $E_z$, the vertical energy, which is approximately conserved in a warm disk with vertical extent. The models also have finite extent making them suitable for N-body simulation. A simulation of a sample model shows that in practice the models are very close to equilibrium making them ideal for experiments on instabilities in galactic disks. We also present a sequence of models closely resembling the Milky Way mass distribution with 5 exponential scale radii and varying halo mass and radial extent.

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arXiv1995

TURBULENCE IN MOLECULAR CLOUDS

We generate random Gaussian turbulent velocity fields with a Kolmogorov spectrum and use these to obtain synthetic line-of-sight velocity profiles. The profiles are found to be similar to line profiles observed in molecular clouds. We suggest methods for analysing measured line profiles to test whether they might arise from Gaussian Kolmogorov turbulence.

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