arXiv subjects

Thomas Quinn

Publications and source records attributed to Thomas Quinn.

At least 73 records · Page 4Linked to original sources

arXiv2008

The ACS Nearby Galaxy Survey Treasury I. The Star Formation History of the M81 Outer Disk

The ACS Nearby Galaxy Survey Treasury (ANGST) is a large Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) treasury program to obtain resolved stellar photometry for a volume-limited sample of galaxies out to 4 Mpc. As part of this program, we have obtained deep ACS imaging of a field in the outer disk of the large spiral galaxy M81. The field contains the outskirts of a spiral arm as well as an area containing no current star formation. Our imaging results in a color-magnitude diagram (CMD) reaching to F814W = 28.8 and F606W = 29.5, one magnitude fainter than the red clump. Through detailed modeling of the full CMD, we quantify the age and metallicity distribution of the stellar populations contained in the field. The mean metallicity in the field is -1<[M/H]<0 and only a small fraction of stars have ages <~1 Gyr. The results show that most of the stars in this outer disk field were formed by z~1 and that the arm structure at this radius has a lifetime of >~100 Myr. We discuss the measured evolution of the M81 disk in the context of surveys of high-redshift disk galaxies and deep stellar photometry of other nearby galaxies. All of these indicate that massive spiral disks are mostly formed by z~1 and that they have experienced rapid metal enrichment.

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arXiv2008

The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks Versus Substructures

Cold dark matter cosmogony predicts triaxial dark matter halos, whereas observations find quite round halos. This is most likely due to the condensation of baryons leading to rounder halos. We examine the halo phase space distribution basis for such shape changes. Triaxial halos are supported by box orbits, which pass arbitrarily close to the density center. The decrease in triaxiality caused by baryons is thought to be due to the scattering of these orbits. We test this hypothesis with simulations of disks grown inside triaxial halos. After the disks are grown we check whether the phase space structure has changed by evaporating the disks and comparing the initial and final states. While the halos are substantially rounder when the disk is at full mass, their final shape after the disk is evaporated is not much different from the initial. Likewise, the halo becomes (more) radially anisotropic when the disk is grown, but the final anisotropy is consistent with the initial. Only if the baryons are unreasonably compact or massive does the halo change irreversibly. We show that the character of individual orbits is not generally changed by the growing mass. Thus the central condensation of baryons does not destroy enough box orbits to cause the shape change. Rather, box orbits merely become rounder along with the global potential. However, if angular momentum is transferred to the halo, either via satellites or via bars, a large irreversible change in the halo distribution occurs. The ability of satellites to alter the phase space distribution of the halo is of particular concern to galaxy formation simulations since halo triaxiality can profoundly influence the evolution of disks.

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arXiv2008

The Formation of the Oort Cloud in Open Cluster Environments

We study the influence of an open cluster environment on the formation and current structure of the Oort cloud. To do this, we have run 19 different simulations of the formation of the Oort Cloud for 4.5 Gyrs. In each simulation, the solar system spends its first 100 Myrs in a different open cluster environment before transitioning to its current field environment. We find that, compared to forming in the field environment, the inner Oort Cloud is preferentially loaded with comets while the Sun resides in the open cluster and that most of this material remains locked in the interior of the cloud for the next 4.4 Gyrs. In addition, the outer Oort Cloud trapping efficiencies we observe in our simulations are lower than previous formation models by about a factor of 2, possibly implying an even more massive early planetesimal disk. Furthermore, some of our simulations reproduce the orbits of observed extended scattered disk objects, which may serve as an observational constraint on the Sun's early environment. Depending on the particular open cluster environment, the properties of the inner Oort Cloud and extended scattered disk can vary widely. On the other hand, the outer portions of the Oort Cloud in each of our simulations are all similar.

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arXiv2008

The Formation of Polar Disk Galaxies

Polar Ring Galaxies, such as NGC4650A, are a class of galaxy which have two kinematically distinct components that are inclined by almost 90 degrees to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain, and the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that Polar Ring Galaxies are simply an extreme example of the angular moment misalignment that occurs during the hierarchical structure formation characteristic of Cold Dark Matter cosmology. In our model, Polar Ring Galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.

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arXiv2007

Rapid Formation of Supermassive Black Hole Binaries in Galaxy Mergers with Gas

Supermassive black holes (SMBHs) are a ubiquitous component of the nuclei of galaxies. It is normally assumed that, following the merger of two massive galaxies, a SMBH binary will form, shrink due to stellar or gas dynamical processes and ultimately coalesce by emitting a burst of gravitational waves. However, so far it has not been possible to show how two SMBHs bind during a galaxy merger with gas due to the difficulty of modeling a wide range of spatial scales. Here we report hydrodynamical simulations that track the formation of a SMBH binary down to scales of a few light years following the collision between two spiral galaxies. A massive, turbulent nuclear gaseous disk arises as a result of the galaxy merger. The black holes form an eccentric binary in the disk in less than a million years as a result of the gravitational drag from the gas rather than from the stars.

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arXiv2007

The age dependence of galaxy clustering

We construct mock galaxy catalogues to analyse clustering properties of a Lambda cold dark matter (LCDM) universe within a cosmological dark matter simulation of sufficient resolution to resolve structure down to the scale of dwarfs. We show that there is a strong age-clustering correlation for objects likely to host luminous galaxies, which includes the satellite halo (subhalo) population. Older mock galaxies are significantly more clustered in our catalog, which consists of satellite haloes as well as the central peaks of discrete haloes, selected solely by peak circular velocity. This age dependence is caused mainly by the age-clustering relation for discrete haloes, recently found by Gao et al., acting mostly on field members, combined with the tendency for older mock galaxies to lie within groups and clusters, where galaxy clustering is enhanced. Our results suggest that the clustering age dependence is manifested in real galaxies. At small scales (less than ~5 Mpc/h), the very simple assumption that galaxy colour depends solely on halo age is inconsistent with the strength of the observed clustering colour trends, where red galaxies become increasingly more clustered than blue galaxies toward smaller scales, suggesting that luminosity weighted galaxy ages do not closely trace the assembly epoch of their dark matter hosts. The age dependence is present but is much weaker for satellite haloes lying within groups and clusters than for the global population.

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arXiv2007

The impact of mergers on relaxed X-ray clusters - II. Effects on global X-ray and SZ properties and their scaling relations

We use the suite of simulations presented in Poole et al (2006) to examine global X-ray and Sunyaev-Zel'dovich (SZ) observables for systems of merging relaxed X-ray clusters. In all of the scaling relations generated from the properties we have studied, we observe a generic evolution (in good qualitative agreement with previous authors): a rapid transient roughly along the mass scaling relations, a subsequent slow drift across the scatter until virialization, followed by a slow evolution along and up the mass scaling relations as cooling recovers in the cluster cores. However, this drift is not sufficient to account for the observed scatter in the scaling relations. We also study the effects of mergers on several theoretical temperature measures of the intracluster medium: emission weighted measures (T_ew), the spectroscopic-like measure proposed by Mazzotta et al (2004; T_sl) and plasma model fits to the integrated spectrum of the system (T_spec). We find that T_sl tracks T_spec for the entire duration of our mergers, illustrating that it remains a good tool for observational comparison even for highly disturbed systems. Furthermore, the transient temperature increases produced during first and second pericentric passage are 15-40% larger for T_ew than for T_sl or T_spec. This suggests that the effects of transient temperature increases on sigma_8 and Omega_M derived by Randall et al (2002) are over estimated. Lastly, we examine the X-ray SZ proxy proposed by Kravtsov et al (2006) and find that the tight mass scaling relation they predict remains secure through the entire duration of a merger event, independent of projection effects.

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arXiv2007

High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability

The water content and habitability of terrestrial planets are determined during their final assembly, from perhaps a hundred 1000-km "planetary embryos" and a swarm of billions of 1-10 km "planetesimals." During this process, we assume that water-rich material is accreted by terrestrial planets via impacts of water-rich bodies that originate in the outer asteroid region. We present analysis of water delivery and planetary habitability in five high-resolution simulations containing about ten times more particles than in previous simulations (Raymond et al 2006a, Icarus, 183, 265-282). These simulations formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five planets in the habitable zone. Every planet from each simulation accreted at least the Earth's current water budget; most accreted several times that amount (assuming no impact depletion). Each planet accreted at least five water-rich embryos and planetesimals from past 2.5 AU; most accreted 10-20 water-rich bodies. We present a new model for water delivery to terrestrial planets in dynamically calm systems, with low-eccentricity or low-mass giant planets -- such systems may be very common in the Galaxy. We suggest that water is accreted in comparable amounts from a few planetary embryos in a "hit or miss" way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects such as planetary mass and location, and giant planet properties.

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arXiv2006

The impact of mergers on relaxed X-ray clusters - I. Dynamical evolution and emergent transient structures

We report on the analysis of a suite of SPH simulations (incorporating cooling and star formation) of mergers involving idealised X-ray clusters whose initial conditions resemble relaxed clusters with cool compact cores observed by Chandra and XMM. The simulations sample the most interesting, theoretically plausible, range of impact parameters and progenitor mass ratios. We find that all mergers evolve via a common progression. We illustrate this progression in the projected gas density, X-ray surface brightness, SZ, temperature, and gas entropy maps. Several different classes of transient ``cold front''-like features can arise over the course of a merger. We find that all of these classes are present in Chandra and XMM observations of merging systems and propose a naming scheme for these features: ``comet-like'' tails, bridges, plumes, streams and edges. In none of the cases considered do the initial cool compact cores of the primary and the secondary get destroyed during the course of the mergers. We quantify the evolving morphology of our mergers using centroid variance, power ratios and offset between the X-ray and the projected mass maps. We find that the centroid variance best captures the dynamical state of the cluster. Placing the system at z=0.1, we find that all easily identified observable traces of the secondary disappear from a simulated 50 ks Chandra image following the second pericentric passage. The system, however, takes approximately 2 additional Gyrs to relax and virialize. Temperature fluctuations at the level of 20% can persist in the final systems well past the point of virialization, suggesting that that the existence of temperature fluctuations, in and of themselves, do not necessarily indicate a disturbed or unrelaxed system.

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arXiv2006

Is there Evidence for Flat Cores in the Halos of Dwarf Galaxies?: The Case of NGC 3109 and NGC 6822

Two well studied dwarf galaxies -- NGC 3109 and NGC 6822 -- present some of the strongest observational support for a flat core at the center of galactic dark matter (DM) halos. We use detailed cosmologically motivated numerical models to investigate the systematics and the accuracy of recovering parameters of the galaxies. Some of our models match the observed structure of the two galaxies remarkably well. Our analysis shows that the rotation curves of these two galaxies are instead quite compatible with their DM halos having steep cuspy density profiles. The rotation curves in our models are measured using standard observational techniques. The models reproduce the rotation curves of both galaxies, the disk surface brightness profiles as well as the profile of isophotal ellipticity and position angle. The models are centrally dominated by baryons; however, the dark matter component is globally dominant. The simulated disk mass is marginally consistent with a stellar mass-to-light ratio in agreement with the observed colors. We show that non-circular motions combined with gas pressure support and projection effects results in a large underestimation of the circular velocity in the central $\sim 1$ kpc region, creating the illusion of a constant density core. Although the systematic effects mentioned above are stronger in barred systems, they are also present in axisymetric disks. Our results strongly suggest that there is no contradiction between the observed rotation curves in dwarf galaxies and the cuspy central dark matter density profiles predicted by Cold Dark Matter models.

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arXiv2006

The Secular Evolution of Disk Structural Parameters

We present a comprehensive series of $N$-body as well as $N$-body + SPH simulations to study the secular evolution of the structure of disk galaxies. Our simulations are organized in a hierarchy of increasing complexity, ranging from rigid-halo collisionless simulations to fully live simulations with gas and star formation. Comparisons between the different types of simulations allow us to isolate the role of various physical mechanisms. We focus on the evolution of systems expected in a LCDM universe. Our goal is to examine which structural properties of disk galaxies may result from secular evolution rather than from direct hierarchical assembly. In the vertical direction, we find that various mechanisms can lead to heating. The strongest heating occurs during the vertical buckling instability of a bar. Among the consequences of this instability is the formation of peanut-shaped bulges which produce clear kinematic signatures when observed face-on. We find that bars are robust structures that are not destroyed by buckling. They can be destroyed instead by a central mass concentration but we find that this mass needs to be a large fraction of the total mass of the disk. We then study the evolution of stellar surface density profiles showing how angular momentum redistribution leads to increasing central densities and disk scale lengths and to profile breaks at large radii. The breaks in these simulations are in excellent agreement with observed breaks, even when the evolution is purely collisionless. Disk scale-lengths increase even when the total disk angular momentum is conserved; thus mapping halo angular momenta to scale-lengths is non-trivial. [Abridged]

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arXiv2006

Fragmentation of gravitationally unstable gaseous protoplanetary disks with radiative transfer

We report on the results of the first 3D SPH simulation of massive, gravitationally unstable protoplanetary disks with radiative transfer. We adopt a flux-limited diffusion scheme justified by the high opacity of most of the disk. The optically thin surface of the disk cools as a blackbody. The disks grow slowly in mass starting from a Toomre-stable initial condition to the point at which they become marginally unstable. We find that gravitationally bound clumps with masses close to a Jupiter mass can arise. Fragmentation appears to be driven by vertical convective-like motions capable of transporting the heat from the disk midplane to its surface on a timescale of only about 40 years at 10 AU. A larger or smaller cooling efficiency of the disk at the optically thin surface can promote or stifle fragmentation by affecting the vertical temperature profile, which determines whether convection can happen or not, and by regulating the accretion flow from optically thin regions towards overdense regions. We also find that the chances of fragmentation increase for a higher mean molecular weight $μ$ since compressional heating is reduced. Around a star with mass $1 M_{\odot}$ only disks with $μ\ge 2.4$, as expected for gas with a metallicity comparable to solar or higher, fragment. This suggests that disk instability, like core-accretion, should be more effective in forming gas giants at higher gas metallicities, consistent with the observed correlation between metallicity of the planet-hosting stars and frequency of planets

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arXiv2006

Gravitational Instabilities in Gaseous Protoplanetary Disks and Implications for Giant Planet Formation

Protoplanetary gas disks are likely to experience gravitational instabilites (GI's) during some phase of their evolution. Density perturbations in an unstable disk grow on a dynamic time scale into spiral arms that produce efficient outward transfer of angular momentum and inward transfer of mass through gravitational torques. In a cool disk with rapid enough cooling, the spiral arms in an unstable disk form self-gravitating clumps. Whether gas giant protoplanets can form by such a disk instability process is the primary question addressed by this review. We discuss the wide range of calculations undertaken by ourselves and others using various numerical techniques, and we report preliminary results from a large multi-code collaboration. Additional topics include -- triggering mechanisms for GI's, disk heating and cooling, orbital survival of dense clumps, interactions of solids with GI-driven waves and shocks, and hybrid scenarios where GI's facilitate core accretion. The review ends with a discussion of how well disk instability and core accretion fare in meeting observational constraints.

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arXiv2006

Multi-scale simulations of merging galaxies with supermassive black holes

We present the results of the first multi-scale N-Body+SPH simulations of merging galaxies containing central supermassive black holes (SMBHs) and having a spatial resolution of only a few parsecs. Strong gas inflows associated with equal-mass mergers produce non-axisymmetric nuclear disks with masses of order $10^9 M_{\odot}$, resolved by about $10^6$ SPH particles. Such disks have sizes of several hundred parsecs but most of their mass is concentrated within less than $50$ pc. We find that a close SMBH pair forms after the merger. The separation of the two SMBHs then shrinks further owing to dynamical friction against the predominantly gaseous background. The orbits of the SMBHs decay down to the minimum resolvable scale in a few million years for an ambient gas temperature and density typical of a region undergoing a starburst. These results suggest the initial conditions necessary for the eventual coalescence of the two holes arise naturally from the merging of two equal-mass galaxies whose structure and orbits are consistent with the predictions of the $Λ$CDM model. Our findings have important implications for planned gravitational wave detection experiments such as {\it LISA}.

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arXiv2005

High-resolution simulations of the final assembly of Earth-like planets 1: terrestrial accretion and dynamics

The final stage in the formation of terrestrial planets consists of the accumulation of ~1000-km ``planetary embryos'' and a swarm of billions of 1-10 km ``planetesimals.'' During this process, water-rich material is accreted by the terrestrial planets via impacts of water-rich bodies from beyond roughly 2.5 AU. We present results from five high-resolution dynamical simulations. These start from 1000-2000 embryos and planetesimals, roughly 5-10 times more particles than in previous simulations. Each simulation formed 2-4 terrestrial planets with masses between 0.4 and 2.6 Earth masses. The eccentricities of most planets were ~0.05, lower than in previous simulations, but still higher than for Venus, Earth and Mars. Each planet accreted at least the Earth's current water budget. We demonstrate several new aspects of the accretion process: 1) The feeding zones of terrestrial planets change in time, widening and moving outward. Even in the presence of Jupiter, water-rich material from beyond 2.5 AU is not accreted for several millions of years. 2) Even in the absence of secular resonances, the asteroid belt is cleared of >99% of its original mass by self-scattering of bodies into resonances with Jupiter. 3) If planetary embryos form relatively slowly, following the models of Kokubo & Ida, then the formation of embryos in the asteroid belt may have been stunted by the presence of Jupiter. 4) Self-interacting planetesimals feel dynamical friction from other small bodies, which has important effects on the eccentricity evolution and outcome of a simulation.

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arXiv2005

Dark Matter Subhaloes in Numerical Simulations

We use cosmological LCDM numerical simulations to model the evolution of the substructure population in sixteen dark matter haloes with resolutions of up to seven million particles within the virial radius. The combined substructure circular velocity distribution function (VDF) for hosts of 10^11 to 10^14 Msun at redshifts from zero to two or higher has a self-similar shape, is independent of host halo mass and redshift, and follows the relation: dn/dv=(1/8)(v_cmax/v_cmax,host)^-4. Halo to halo variance in the VDF is a factor of roughly two to four. At high redshifts, we find preliminary evidence for fewer large substructure haloes (subhaloes). Specific angular momenta are significantly lower for subhaloes nearer the host halo centre where tidal stripping is more effective. The radial distribution of subhaloes is marginally consistent with the mass profile for r >~ 0.3r_vir, where the possibility of artificial numerical disruption of subhaloes can be most reliably excluded by our convergence study, although a subhalo distribution that is shallower than the mass profile is favoured. Subhalo masses but not circular velocities decrease toward the host centre. Subhalo velocity dispersions hint at a positive velocity bias at small radii. There is a weak bias toward more circular orbits at lower redshift, especially at small radii. We additionally model a cluster in several power law cosmologies of P ~ k^n, and demonstrate that a steeper spectral index, n, results in significantly less substructure.

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arXiv2005

Terrestrial Planet Formation in Disks with Varying Surface Density Profiles

The ``minimum-mass solar nebula'' (MMSN) model estimates the surface density distribution of the protoplanetary disk by assuming the planets to have formed in situ. However, significant radial migration of the giant planets likely occurred in the Solar system, implying a distortion in the values derived by the MMSN method. The true density profiles of protoplanetary disks is therefore uncertain. Here we present results of simulations of late-stage terrestrial accretion, each starting from a disk of planetary embryos. We assume a power-law surface density profile that varies with heliocentric distance r as r^-alpha, and vary alpha between 1/2 and 5/2 (alpha = 3/2 for the MMSN model). We find that for steeper profiles (higher values of alpha), the terrestrial planets (i) are more numerous, (ii) form more quickly, (iii) form closer to the star, (iv) are more massive, (v) have higher iron contents, and (vi) have lower water contents. However, the possibility of forming potentially habitable planets does not appear to vary strongly with alpha.

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arXiv2005

Evolution of the Density Profiles of Dark Matter Haloes

We use numerical simulations in a Lambda CDM cosmology to model density profiles in a set of 16 dark matter haloes with resolutions of up to 7 million particles within the virial radius. These simulations allow us to follow robustly the formation and evolution of the central cusp over a large mass range of 10^11 to 10^14 M_sun, down to approximately 0.5% of the virial radius, and from redshift 5 to the present. The cusp of the density profile is set at redshifts of 2 or greater and remains remarkably stable to the present time, when considered in non-comoving coordinates. We fit our haloes to a 2 parameter profile where the steepness of the asymptotic cusp is given by gamma, and its radial extent is described by the concentration, c_gamma. In our simulations, we find gamma = 1.4 - 0.08Log(M/M_*) for haloes of 0.01M_* to 1000M_*, with a large scatter of gamma ~ +/-0.3$; and c_gamma = 8*M/M_*^{-0.15}, with a large M/M_* dependent scatter roughly equal to +/- c_gamma. Our redshift zero haloes have inner slope parameters ranging approximately from r^{-1} to r^{-1.5}, with a median of roughly r^{-1.3}. This 2 parameter profile fit works well for all our halo types, whether or not they show evidence of a steep asymptotic cusp. We also model a cluster in power law cosmologies of P ~ k^n (n=0,-1,-2,-2.7). We find larger concentration radii and shallower cusps for steeper n. The minimum resolved radius is well described by the mean interparticle separation. The trend of steeper and more concentrated cusps for smaller $M/M_*$ haloes clearly shows that dwarf sized Lambda CDM haloes have, on average, significantly steeper density profiles within the inner few percent of the virial radius than inferred from recent observations. Code to reproduce this profile can be downloaded from http://www.icc.dur.ac.uk/~reed/profile.html

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