arXiv subjects

Thomas Quinn

Publications and source records attributed to Thomas Quinn.

At least 91 records · Page 5Linked to original sources

arXiv2005

Fate of Supermassive Black Holes and Evolution of the M(BH)-sigma Relation in Merging Galaxies: The Effect of Gaseous Dissipation

We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using high-resolution self-consistent gasdynamical simulations of binary equal- and unequal-mass mergers of disk galaxies. The galaxy models are consistent with the LCDM paradigm of structure formation and the simulations include the effects of radiative cooling and star formation. We find that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ~ 100 pc. Instead, the final SMBH separation in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the satellite galaxy can be entirely disrupted before the merger is completed leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, we show that gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Moreover, we demonstrate that merging disk galaxies constructed to obey the M(BH)-sigma relation, move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. Collisionless simulations reveal that remnants tend to move away from the mean relation highlighting the role of gas-poor mergers as a possible source of scatter. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation provides the necessary fuel to the SMBHs and allows the merger remnants to satisfy the relation.

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arXiv2005

On the survival and disruption of Earth mass CDM micro-haloes

Neutralino dark matter leads to the formation of numerous earth mass dark matter haloes at redshifts z\approx 60 (Diemand et al. 2005). These abundant CDM micro-haloes have cuspy density profiles that can easily withstand the Galactic tidal field at the solar radius. Zhao, Taylor, Silk & Hooper (astro-ph/0502049) concluded that ``...the majority of dark matter substructures with masses \sim 10^{-6}M_o will be tidally disrupted due to interactions with stars in the Galactic halo''. However these authors assumed a halo density of stars that is at least an order of magnitude higher than observed. We show that the appropriate application of the impulse approximation is to the regime of multiple encounters, not single disruptive events as adopted by Zhao et al., which leads to a survival time of several Hubble times. Therefore we do not expect the tidal heating by Galactic stars to affect the abundance of micro-haloes. Disk crossing will cause some mass loss but the central cores are likely to survive and could be detected as gamma-ray sources with proper motions of several arc minutes per year.

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arXiv2004

The Origin and Properties of Intracluster Stars in a Rich Cluster

We use a multi million particle N-body + SPH simulation to follow the formation of a rich galaxy cluster in a Lambda+CDM cosmology, with the goal of understanding the origin and properties of intracluster stars. The simulation includes gas cooling, star formation, the effects of a uniform UVB and feedback from supernovae. Halos that host galaxies as faint as M_R = -19.0 are resolved by this simulation, which includes 85% of the total galaxy luminosity in a rich cluster. We find that the accumulation of intracluster light (ICL) is an ongoing process, linked to infall and stripping events. The unbound star fraction increases with time and is 20% at z = 0, consistent with observations of galaxy clusters. The surface brightness profile of the cD shows an excess compared to a de Vaucouleur profile near 200 kpc, which is also consistent with observations. Both massive and small galaxies contribute substantially to the formation of the ICL, with stars stripped preferentially from the outer parts of their stellar distributions. Simulated observations of planetary nebulae (PNe) show significant substructure in velocity space. Despite this, individual intracluster PNe might be useful mass tracers if more than 5 fields at a range of radii have measured line-of-sight velocities, where an accurate mass calculation depends more on the number of fields than the number of PNe measured per field. However, the orbits of IC stars are more anisotropic than those of galaxies or dark matter, which leads to a systematic underestimate of cluster mass relative to that calculated with galaxies, if not accounted for in dynamical models. Overall, the properties of ICL formed in a hierarchical scenario are in good agreement with current observations. (Abridged)

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arXiv2004

The formation and habitability of terrestrial planets in the presence of hot jupiters

`Hot jupiters,' giant planets with orbits very close to their parent stars, are thought to form farther away and migrate inward via interactions with a massive gas disk. If a giant planet forms and migrates quickly, the planetesimal population has time to re-generate in the lifetime of the disk and terrestrial planets may form (Armitage 2003). We present results of simulations of terrestrial planet formation in the presence of hot jupiters, broadly defined as having orbital radii <= 0.5 AU. We show that terrestrial planets similar to those in the Solar System can form around stars with hot jupiters, and can have water contents equal to or higher than the Earth's. For small orbital radii of hot jupiters (e.g. 0.15, 0.25 AU) potentially habitable planets can form, but for semi-major axes of 0.5 AU or greater their formation is suppressed. We show that the presence of an outer giant planet such as Jupiter does not enhance the water content of the terrestrial planets, but rather decreases their formation and water delivery timescales. We speculate that asteroid belts may exist interior to the terrestrial planets in systems with hot jupiters.

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arXiv2004

Gravitational instability in binary protoplanetary disks; new constraints on giant planet formation

We use high resolution 3D SPH simulations to study the evolution of self-gravitating binary protoplanetary disks. Heating by shocks and cooling are included. We consider different orbital separations and masses of the disks and central stars. Isolated massive disks ($M \sim 0.1 M_{ødot}$) fragment into protoplanets as a result of gravitational instability for cooling times comparable to the orbital time. Fragmentation does not occur in binary systems with a separation of about 60 AU. This is because efficient heating owing to strong tidally induced spiral shocks damps any overdensity. The resulting temperatures, above 200 K, would vaporize water ice in the outer disk, posing a problem even for the other model of giant planet formation, core-accretion. Light disks ($M \sim 0.01 M_{\odot}$) do not fragment but remain cold because their low self-gravity inhibits strong shocks. Core accretion would not be hampered in the latter. At separations of about 120 AU the efficiency of fragmentation by disk instability rises and approaches that in isolated systems. If disk instability is the main formation mechanism for giant planets, on going surveys targeting binary systems should find considerably fewer planets in systems with separations below 100 AU.

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arXiv2004

A Strategy for Finding Near Earth Objects with the SDSS Telescope

We present a detailed observational strategy for finding Near Earth Objects (NEOs) with the Sloan Digital Sky Survey (SDSS) telescope. We investigate strategies in normal, unbinned mode as well as binning the CCDs 2x2 or 3x3, which affects the sky coverage rate and the limiting apparent magnitude. We present results from 1 month, 3 year and 10 year simulations of such surveys. For each cadence and binning mode, we evaluate the possibility of achieving the Spaceguard goal of detecting 90% of 1 km NEOs (absolute magnitude H <= 18 for an albedo of 0.1). We find that an unbinned survey is most effective at detecting H <= 20 NEOs in our sample. However, a 3x3 binned survey reaches the Spaceguard Goal after only seven years of operation. As the proposed large survey telescopes (PanStarss; LSST) are at least 5-10 years from operation, an SDSS NEO survey could make a significant contribution to the detection and photometric characterization of the NEO population.

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arXiv2004

The (In)Stability of Planetary Systems

We present results of numerical simulations which examine the dynamical stability of known planetary systems, a star with two or more planets. First we vary the initial conditions of each system based on observational data. We then determine regions of phase space which produce stable planetary configurations. For each system we perform 1000 ~1 million year integrations. We examine upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system (SS). We find that the resonant systems, 2 planets in a first order mean motion resonance, (HD82943 and GJ876) have very narrow zones of stability. The interacting systems, not in first order resonance, but able to perturb each other (upsilon And, 47UMa, and SS) have broad regions of stability. The separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443 and HD168443) are fully stable. Furthermore we find that the best fits to the interacting and resonant systems place them very close to unstable regions. The boundary in phase space between stability and instability depends strongly on the eccentricities, and (if applicable) the proximity of the system to perfect resonance. In addition to million year integrations, we also examined stability on ~100 million year timescales. For each system we ran ~10 long term simulations, and find that the Keplerian fits to these systems all contain configurations which may be regular on this timescale.

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arXiv2004

Disk-Planet Interaction: Triggered Formation and Migration

We present three-dimensional SPH calculations of giant planets embedded in gaseous disks. Our findings are collected into a map of parameter space, exhibiting four distinct regions: Type I migration, gap formation, triggered formation of more planets, and wholly unstable disks. For Type I migration of the planet due to secular interactions with the disk material, the migration rate depends linearly on the disk mass, and is independent of the initial planet mass. For more massive disks, the planet can disturb the disk strongly enough to trigger the collapse of gas into additional giant planets. When additional planets form, their interaction as point masses dominates the subsequent behavior of the system. This mechanism allows for the rapid formation of Jupiter-mass and higher planets. Migration due to interaction with the disk can significantly change the orbits of giant planets in gas disks.

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arXiv2004

Simulations of Gaseous Disc-Embedded Planet Interaction

We present three-dimensional self-gravitating smoothed-particle hydrodynamics (SPH) simulations of an isothermal gaseous disc interacting with an embedded planet. Discs of varying stability are simulated with planets ranging from 10 Earth-masses to 2 Jupiter-masses. The SPH technique provides the large dynamic range needed to accurately capture the large scale behavior of the disc as well as the small scale interaction of the planet with surrounding material. Most runs used 10^5 gas particles, giving us the spatial resolution required to observe the formation of planets. We find four regions in parameter space: low-mass planets undergo Type I migration; higher-mass planets can form a gap; the gravitational instability mode of planet formation in marginally stable discs can be triggered by embedded planets; discs that are completely unstable can fragment to form many planets. The disc stability is the most important factor in determing which interaction a system will exhibit. For the stable disc cases, our migration and accretion time-scales are shorter and scale differently than previously suggested.

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arXiv2003

Giant planet formation from disk instability; cooling and heating

We present the results of high resolution SPH simulations of the evolution of gravitationally unstable protoplanetary disks. We report on calculations in which the disk is evolved using a locally isothermal or adiabatic equation of state (with shock heating), and also on new simulations in which cooling and heating by radiation are explicitly modeled. We find that disks with a minimum Toomre parameter $< 1.4$ fragment into several gravitationally bound protoplanets with masses from below to a few Jupiter masses. This is confirmed also in runs where the disk is given a quiet start, growing gradually in mass over several orbital times. A cooling time comparable to the orbital time is needed to achieve fragmentation, for disk masses in the range $0.08-0.1 M_{\odot}$. After about 30 orbital times, merging between the bound condensations always leads to 2-3 protoplanets on quite eccentric orbits.

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arXiv2003

The evolution of gravitationally unstable protoplanetary disks: fragmentation and possible giant planet formation

We carry out a large set of very high resolution, three dimensional smoothed particle hydrodynamics (SPH) simulations describing the evolution of gravitationally unstable gaseous protoplanetary disks. We consider a broad range of initial disk parameters. Disk masses out to 20 AU range from 0.075 to 0.125 $M_{\odot}$, roughly consistent with the high-end of the mass distribution inferred for disks around T Tauri stars. The initial disks span minimum $Q$ parameters between 0.8 and 2, with most models being around $\sim 1.4$. The disks are evolved assuming either a locally isothermal equation of state or an adiabatic equation of state with varying $γ$. When overdensities above a specific threshold appear as a result of gravitational instability in a locally isothermal calculation, the equation of state is switched to adiabatic to account for the increased optical depth. We show that when a disk has a minimum $Q$ parameter less than 1.4 strong trailing spiral instabilities, typically three or four armed modes, form and grow until fragmentation occurs along the arms after about 5 mean disk orbital times. The resulting clumps contract quickly to densities several orders of magnitude higher than the initial disk density, and the densest of them survive even under adiabatic conditions. These clumps are stable to tidal disruption and merge quickly, leaving 2-3 protoplanets on fairly eccentric orbits (the mean eccentricity being around 0.2) with masses between 0.7 and more than $7 M_{Jup}$, well in agreement with those of detected extrasolar planets.after $\sim 10^3$ years. Fragmentation is not strongly dependent on whether the disk starts from a marginally unstable state or gradually achieves it, as shown by a test where the disk gradually achieves the critical $Q$ by growing in mass.

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arXiv2003

Evolution of the Mass Function of Dark Matter Haloes

We use a high resolution $Λ$CDM numerical simulation to calculate the mass function of dark matter haloes down to the scale of dwarf galaxies, back to a redshift of fifteen, in a 50 $h^{-1}$Mpc volume containing 80 million particles. Our low redshift results allow us to probe low $σ$ density fluctuations significantly beyond the range of previous cosmological simulations. The Sheth and Tormen mass function provides an excellent match to all of our data except for redshifts of ten and higher, where it overpredicts halo numbers increasingly with redshift, reaching roughly 50 percent for the $10^{10}-10^{11} \msun$ haloes sampled at redshift 15. Our results confirm previous findings that the simulated halo mass function can be described solely by the variance of the mass distribution, and thus has no explicit redshift dependence. We provide an empirical fit to our data that corrects for the overprediction of extremely rare objects by the Sheth and Tormen mass function. This overprediction has implications for studies that use the number densities of similarly rare objects as cosmological probes. For example, the number density of high redshift (z $\simeq$ 6) QSOs, which are thought to be hosted by haloes at 5$σ$ peaks in the fluctuation field, are likely to be overpredicted by at least a factor of 50%. We test the sensitivity of our results to force accuracy, starting redshift, and halo finding algorithm.

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arXiv2003

Gasoline: An adaptable implementation of TreeSPH

The key algorithms and features of the Gasoline code for parallel hydrodynamics with self-gravity are described. Gasoline is an extension of the efficient Pkdgrav parallel N-body code using smoothed particle hydrodynamics. Accuracy measurements, performance analysis and tests of the code are presented. Recent successful Gasoline applications are summarized. These cover a diverse set of areas in astrophysics including galaxy clusters, galaxy formation and gas-giant planets. Future directions for gasdynamical simulations in astrophysics and code development strategies for tackling cutting edge problems are discussed.

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arXiv2003

Simulations of unstable gaseous disks and the origin of giant planets

We study the evolution of cold, gravitationally unstable protoplanetary gaseous disks performing 3D SPH simulations with up to a million particles on large parallel machines. We show that self-gravitating protoplanets can form in disks with initial $Q_{min} \sim 1.4$ provided that disks are evolved with a locally isothermal equation of state until localized regions reach a sufficiently high density. Lower resolution simulations carried out for as long as a thousand years show that systems of giant protoplanets with masses and orbits comparable to those of the observed extrasolar planets arise naturally.

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arXiv2003

Formation of giant planets by fragmentation of protoplanetary disks

The evolution of gravitationally unstable protoplanetary gaseous disks has been studied with the use of three-dimensional smoothed particle hydrodynamics simulations with unprecedented resolution. We have considered disks with initial masses and temperature profiles consistent with those inferred for the protosolar nebula and for other protoplanetary disks. We show that long-lasting, self-gravitating protoplanets arise after a few disk orbital periods if cooling is efficient enough to maintain the temperature close to 50 K. The resulting bodies have masses and orbital eccentricities similar to those of detected extrasolar planets.

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arXiv2002

The Structural Evolution of Substructure

We investigate the evolution of substructure in cold dark matter halos using N-body simulations of tidal stripping of substructure halos (subhalos) within a static host potential. We find that halos modeled following the Navarro, Frenk & White (NFW) mass profile lose mass continuously due to tides from the massive host, leading to the total disruption of satellite halos with small tidal radii. The structure of stripped NFW halos depends mainly on the fraction of mass lost, and can be expressed in terms of a simple correction to the original NFW profile. We apply these results to substructure in the Milky Way, and conclude that the dark matter halos surrounding its dwarf spheroidal (dSph) satellites have circular velocity curves that peak well beyond the luminous radius at velocities significantly higher than expected from the stellar velocity dispersion. Our modeling suggests that the true tidal radii of dSphs lie well beyond the putative tidal cutoff observed in the surface brightness profile, suggesting that the latter are not really tidal in origin but rather features in the light profile of limited dynamical relevance. For Draco, in particular, our modeling implies that its tidal radius is much larger than derived by Irwin & Hatzidimitriou (1995), lending support to the interpretation of recent Sloan survey data by Odenkirchen et al. (2001). Similarly, our model suggests that Carina's halo has a peak circular velocity of ~55 km/s, which may help explain how this small galaxy has managed to retain enough gas to undergo several bursts of star formation. Our results imply a close correspondence between the most massive subhalos expected in a CDM universe and the known satellites of the Milky Way, and suggest that only subhalos with peak circular velocities below 35 km/s lack readily detectable luminous counterparts.

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arXiv2002

The Galaxy Luminosity Function and Luminosity Density at Redshift z=0.1

Using a catalog of 147,986 galaxy redshifts and fluxes from the Sloan Digital Sky Survey (SDSS) we measure the galaxy luminosity density at z=0.1 in five optical bandpasses corresponding to the SDSS bandpasses shifted to match their restframe shape at z=0.1. We denote the bands {0.1}{u}, {0.1}{g}, {0.1}{r}, {0.1}{i}, {0.1}{z}, with λ_{eff} = [3216, 4240, 5595, 6792, 8111] Angstroms respectively. We use a maximum likelihood method which allows for a general form for the shape of the luminosity function, simple luminosity and number evolution, incorporates flux uncertainties, and accounts for the flux limits of the survey. We find luminosity densities at z=0.1 in absolute AB magnitudes in a Mpc^3 of [-14.10 \pm 0.15, -15.18 \pm 0.03, -15.90 \pm 0.03, -16.24 \pm 0.03, -16.56 \pm 0.02] in [{0.1}{u}, {0.1}{g}, {0.1}{r}, {0.1}{i}, {0.1}{z}], respectively, using Ω_0 =0.3, Ω_Λ=0.7, and h=1, and using Petrosian magnitudes. Similar results are obtained using Sersic model magnitudes, suggesting that flux from outside the Petrosian apertures is not a major correction. In the {0.1}{r} band, the best fit Schechter function to our results has ϕ_\ast = (1.49 \pm 0.04) \times 10^{-2} h^3 Mpc^{-3}, M_\ast - 5\log_{10} h = -20.44 \pm 0.01, and α= -1.05\pm 0.01. In solar luminosities, the luminosity density in {0.1}{r} is (1.84 \pm 0.04) h 10^8 L_{{0.1}{r},\odot} Mpc^{-3}. Our results are consistent with other estimates of the luminosity density, from the Two-degree Field Galaxy Redshift Survey and the Millenium Galaxy Catalog. They represent a substantial change (\sim 0.5 mag) from earlier SDSS luminosity density results based on commissioning data, almost entirely because of the inclusion of evolution in the luminosity function model.

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arXiv2001

Tidal debris of dwarf spheroidals as a probe of structure formation models

Recent observations suggest that Carina and other nearby dwarf spheroidal galaxies are surrounded by unbound stars tidally stripped by the Milky Way. We run high-resolution N-Body simulations of dwarf galaxies orbiting within the Milky Way halo to determine if such observations can be explained with dark matter potentials as those implied by current structure formation models. We show that tidal forces acting on dwarfs with constant density cores or with cuspy profiles having a low concentration parameter ($c < 5$) lead to flat outer stellar density profiles like that of Carina for a variety of orbital configurations. On the contrary, it is more difficult to remove stars from cuspy dark matter halos with concentrations as high as predicted by CDM models at the mass scale of dwarf galaxies ($c \simgt 10$) and the data can only be reproduced assuming nearly radial orbits. Our simulations show that Carina is losing mass at a fractional rate $< 0.1$ Gyr$^{-1}$ and its mass-to-light ratio could be inflated by at most a factor of 2 due to unbound stars projected along the line of sight. We follow the evolution of the tidal debris within a triaxial clumpy cold dark matter Milky Way halo which causes differential precession and small scale heating of the stellar streams. This renders their use as a dynamical tracer of the Galactic potential practically useless, but does provide a novel test of the nature of the dark matter. Models with warm dark matter (WDM) or fluid dark matter (FDM) produce dwarf halos with lower central densities than CDM and would be consistent with the observed tidal tails even for orbits with eccentricities as low as indicated by current data on nearby dwarf spheroidals. Galactic halos in FDM are smooth and spherical and would be favored by the detection of coherent streams.

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