arXiv subjects

T. Quinn

Publications and source records attributed to T. Quinn.

48 records · Page 3Linked to original sources

arXiv2002

The effect of non--gravitational gas heating in groups and clusters of galaxies

We present a set of gas-dynamical simulations of galaxy groups and clusters aimed at exploring the effect of non-gravitational heating. We use GASOLINE, a parallel Tree+SPH code, to simulate the formation of four cosmic halos with temperature 0.5<T<8 keV. Non-gravitational heating is implemented in two different ways: (1) by imposing a minimum entropy floor at a given redshift, 1<z<5; (2) by gradually heating gas, proportionally to the SN rate expected from semi-analytical modeling of galaxy formation. Our main results are the following. (a) An extra heating energy of about 1 keV per gas particle is required to reproduce the observed Lx-T relation, independent of whether it is provided so as to create an entropy floor of 50-100 keV cm^2, or is modulated in redshift; our SN feedback recipe provides only 1/3 keV/part. (b) The M-T relation is almost unaffected by non-gravitational heating and follows the M T^{3/2} scaling, with a normalization ~40% higher than observed, independent of the heating scheme. The inclusion of cooling in a run of a small group has the effects of increasing T_ew by ~30%, possibly reconciling simulated and observed M-T relations, and of decreasing Lx by ~40%. In spite of the inclusion of SN feedback energy, almost 40% of the gas becomes cold, in excess of current observational estimates. (abridged)

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arXiv2002

Comparison of Asteroids Observed in the SDSS with a Catalog of Known Asteroids

We positionally correlate asteroids from existing catalogs with a sample of $\about$18,000 asteroids detected by the Sloan Digital Sky Survey (SDSS, Ivezić {\em et al.} 2001). We find 2641 unique matches, which represent the largest sample of asteroids with both accurate multi-color photometry and known orbital parameters. The matched objects are predominantly bright, and demonstrate that the SDSS photometric pipeline recovers \about90% of the known asteroids in the observed region. For the recovered asteroids we find a large offset (\about 0.4 mag) between Johnson V magnitudes derived from SDSS photometry and the predicted catalog-based visual magnitudes. This offset varies with the asteroid color from 0.34 mag for blue asteroids to 0.44 mag for red asteroids, and is probably caused by the use of unfiltered CCD observations in the majority of recent asteroid surveys. This systematic photometric error leads to an overestimate of the number of asteroids brighter than a given absolute magnitude limit by a factor of \about 1.7. The distribution of the matched asteroids in orbital parameter space indicates strong color segregation. We confirm that some families are dominated by a single asteroid type (e.g. the Koronis family by red asteroids and the Themis family by blue asteroids), while others appear to be a mixture of blue and red objects (e.g. the Nysa/Polana family). Asteroids with the bluest \iz colors, which can be associated with the Vesta family, show particularly striking localization in orbital parameter space.

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arXiv2001

Predicting the number, spatial distribution and merging history of dark matter haloes

We present a new algorithm (PINOCCHIO, PINpointing Orbit-Crossing Collapsed HIerarchical objects) to predict accurately the formation and evolution of individual dark matter haloes in a given realization of an initial linear density field. Compared with the halo population formed in a large (360^3 particles) collisionless simulation of a CDM universe, our method is able to predict to better than 10 per cent statistical quantities such as the mass function, two-point correlation function and progenitor mass function of the haloes. Masses of individual haloes are estimated accurately as well, with errors typically of order 30 per cent in the mass range well resolved by the numerical simulation. These results show that the hierarchical formation of dark matter haloes can be accurately predicted using local approximations to the dynamics when the correlations in the initial density field are properly taken into account. The approach allows one to automatically generate a large ensemble of accurate merging histories of haloes with complete knowledge of their spatial distribution. The construction of the full merger tree for a 256^3 realisation requires a few hours of CPU-time on a personal computer, orders of magnitude faster than the corresponding N-body simulation would take, and without needing any extensive post-processing. The technique can be efficiently used, for instance, for generating the input for galaxy formation modeling.

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arXiv2001

Pre-heating the ICM in high resolution simulations: the effect on the gas entropy

We present results from high-resolution Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of non-gravitational heating on the entropy of the ICM. We simulate three systems, having emission-weighted temperature T=0.6,1 and 3 keV, with spatial resolution better than 1% of the virial radius. We consider the effect of different prescriptions for non-gravitational ICM heating, such as SN energy feedback, as predicted by semi-analytical models of galaxy formation, and two different minimum entropy floors, S_fl=50 and 100 keV cm^2, imposed at z=3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree which depends on total injected energy and on cluster mass. We use observational results on the excess entropy in central regions of galaxy systems, to constrain the amount of extra-heating required. We find that setting the entropy floor S_fl=50 keV cm^2, which corresponds to an extra heating energy of about 1 keV per particle, is able to reproduce the observed excess of ICM entropy.

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arXiv2001

Solar System Objects Observed in the SDSS Commissioning Data

We discuss measurements of the properties of about 10,000 asteroids detected in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 < r < 21.5, with a baseline of 5 minutes. Extensive tests show that the sample is at least 98% complete, with the contamination rate of less than 3%. We find that the size distribution of asteroids resembles a broken power-law, independent of the heliocentric distance: D^{-2.3} for 0.4 km < D < 5 km, and D^{-4} for 5 km < D < 40 km. As a consequence of this break, the number of asteroids with r < 21.5 is ten times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids (r < 18). The observed counts imply that there are about 530,000 objects with D>1 km in the asteroid belt, or about four times less than previous estimates. The distribution of main belt asteroids in the 4-dimensional SDSS color space is bimodal, and the two groups can be associated with S (rocky) and C (carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric distribution of asteroids and suggests the existence of two distinct belts: the inner rocky belt, about 1 AU wide (FWHM) and centered at R~2.8 AU, and the outer carbonaceous belt, about 0.5 AU wide and centered at R~3.2 AU. The colors of Hungarias, Mars crossers, and near-Earth objects are more similar to the C-type than to S-type asteroids, suggesting that they originate in the outer belt. (abridged).

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arXiv2000

The descendents of Lyman Break Galaxies in galaxy clusters: spatial distribution and orbital properties

We combine semi-analytical methods with a ultra-high resolution simulation of a galaxy cluster (of mass 2.3 10^14h-1Msolar, and 4 10^6 particles within its virial radius) formed in a standard CDM universe to study the spatial distribution and orbital properties of the present-day descendents of Lyman Break Galaxies (LBGs). At the present time only five (out of 12) of halos containing LBGs survive as separate entities inside the cluster virial radius. Their circular velocities are in the range 200 - 550 km/sec. Seven halos merged together to form the central object at the very center of the cluster. Using semi-analytical modeling of galaxy evolution we show that descendents of halos containing LBGs now host giant elliptical galaxies. Galaxy orbits are radial, with a pericenter to apocenter ratio of about 1:5. The orbital eccentricities of LBGs descendents are statistically indistinguishable from those of the average galaxy population inside the cluster, suggesting that the orbits of these galaxies are not significantly affected by dynamical friction decay after the formation of the cluster's main body. In this cluster, possibly due to its early formation time, the descendents of LBGs are contained within the central 60% of the cluster virial radius and have an orbital velocity dispersion lower than the global galaxy population, originating a mild luminosity segregation for the brightest cluster members. Mass estimates based only on LBGs descendents (especially including the central cD) reflect this bias in space and velocity and underestimate the total mass of this well virialized cluster by up to a factor of two compared to estimates using at least 20 cluster members.

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arXiv2000

The Effects of Gas Dynamics, Cooling, Star Formation, and Numerical Resolution in Simulations of Cluster Formation

We present the analysis of a suite of simulations of a Virgo mass galaxy cluster. Undertaken within the framework of standard cold dark matter cosmology, these simulations were performed at differing resolutions and with increasingly complex physical processes, with the goal of identifying the effects of each on the evolution of the cluster. We focus on the cluster at the present epoch and examine properties including the radial distributions of density, temperature, entropy and velocity. We also map `observable' projected properties such as the surface mass density, X-ray surface brightness and SZ signature. We identify significant differences between the simulations, which highlights the need for caution when comparing numerical simulations to observations of galaxy clusters. While resolution affects the inner density profile in dark matter simulations, the addition of a gaseous component, especially one that cools and forms stars, affects the entire cluster. We conclude that both resolution and included physical processes play an important role in simulating the formation and evolution of galaxy clusters. Therefore, physical inferences drawn from simulations that do not include a gaseous component that can cool and form stars present a poor representation of reality. (Abridged)

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arXiv1999

Direct N-body Simulations of Rubble Pile Collisions

There is increasing evidence that many km-sized bodies in the Solar System are piles of rubble bound together by gravity. We present results from a project to map the parameter space of collisions between km-sized spherical rubble piles. The results will assist in parameterization of collision outcomes for Solar System formation models and give insight into fragmentation scaling laws. We use a direct numerical method to evolve the positions and velocities of the rubble pile particles under the constraints of gravity and physical collisions. We test the dependence of the collision outcomes on impact parameter and speed, impactor spin, mass ratio, and coefficient of restitution. Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as the primordial disk during early planet formation) so that the maximum strain on the component material does not exceed the crushing strength. We compare our results with analytic estimates and hydrocode simulations. Off-axis collisions can result in fast-spinning elongated remnants or contact binaries while fast collisions result in smaller fragments overall. Clumping of debris escaping from the remnant can occur, leading to the formation of smaller rubble piles. In the cases we tested, less than 2% of the system mass ends up orbiting the remnant. Initial spin can reduce or enhance collision outcomes, depending on the relative orientation of the spin and orbital angular momenta. We derive a relationship between impact speed and angle for critical dispersal of mass in the system. We find that our rubble piles are relatively easy to disperse, even at low impact speed, suggesting that greater dissipation is required if rubble piles are the true progenitors of protoplanets.

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arXiv1998

Properties of Galaxy Clusters: Mass and Correlation Functions

We analyse parallel N-body simulations of three Cold Dark Matter (CDM) universes to study the abundance and clustering of galaxy clusters. The simulations cover a volume comparable to the forthcoming SDSS. We are able to make robust measurements of cluster properties to a redshift larger than unity. We extract halos using two independent, public domain group finders (FOF & HOP) and find consistent results. The correlation function of clusters is in very good agreement with a simple analytic prescription based upon a Lagrangian biasing scheme developed by Mo & White (1996) and the Press-Schechter (PS) formalism for the mass function. The R_0--D_c relation for the open CDM model is in good agreement with the results from the APM Cluster Survey. The SCDM universe shows a robust deviation in the shape and evolution of the mass function when compared with that predicted by the PS formalism. Critical models with a low sigma_8 normalization or small shape parameter Gamma show an excess of massive clusters compared with the PS prediction. When cluster normalized, the SCDM universe at z =1 contains 10 times more clusters with temperatures greater than 7keV, compared with the PS prediction. The agreement between the analytic and N-body mass functions of SCDM can be improved if the value of the delta_c (the extrapolated linear theory threshold for collapse) is revised to be $ delta_c(z) = 1.685[(0.7/sigma_8)(1+z)]^{-0.125}. Our best estimate for the amplitude of fluctuations inferred from the local cluster abundance for SCDM is sigma_{8} = 0.5 \pm 0.04. However, the discrepancy between the temperature function predicted in a critical density universe and that observed at z=0.33 (Henry et al. 1998) remains. (abridged)

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arXiv1998

The seeds of rich galaxy clusters in the Universe

The discovery of a population of young galaxies at an epoch when the universe was about one tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models this is the population of primeval galaxies that built themselves up to the size of present--day galaxies through the process of repeated mergers. But the recent detection of a large concentration of these primeval galaxies (Steidel et al. 1997) appears to be incompatible with hierarchical clustering models, which generally predict that clusters of this size are fully formed later in time. Here we use a combination of two powerful theoretical techniques --semi-analytic modelling and N-body simulations-- to show that such large concentrations should be quite common in a universe dominated by cold dark matter, and that they are the progenitors of the rich galaxy clusters seen today. We predict the clustering properties of primeval galaxies which should, when compared with data that will be collected in the near future, test our understanding of galaxy formation within the framework of a universe dominated by cold dark matter.

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arXiv1997

On the origin of early type galaxies and the evolution of the interaction rate in the field

Using cosmological N-body simulations of critical (SCDM) and open (Omega_0=0.3, OCDM) cold dark matter models we have identified dark matter halos which are associated with the progenitors of present day bright early type galaxies. By following their merging history, we show how early type galaxies that formed within massive halos at redshift around 3 are now preferentially residing in clusters and groups. On the other hand, those that formed through major merging events between redshift 1 and the present have not yet been accreted into larger, virialized structures. This result is in agreement with analytical predictions in hierarchical clustering models. CDM models are able to explain both the ancient and uniform population of ellipticals that dominates in clusters together with the more recent and heterogeneous population of field ellipticals. Predictions for the comoving number density of bright early-type galaxies are given, and are shown to be consistent with the observed luminosity function. We predict that the number density of interacting bright binary galaxies, from which the field population of ellipticals may have originated, is proportional to (1+z)**4.2+-0.28 and (1+z)**2.5+-0.42 in SCDM and OCDM respectively. This result is consistent with previous analytical estimates and is discussed together with recent observational constraints.

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arXiv1996

The Local Group as a test of cosmological models

The dynamics of the Local Group and its environment provide a unique challenge to cosmological models. The velocity field within 5h-1 Mpc of the Local Group (LG) is extremely ``cold''. The deviation from a pure Hubble flow, characterized by the observed radial peculiar velocity dispersion, is measured to be about 60km/s. We compare the local velocity field with similarly defined regions extracted from N-body simulations of Universes dominated by cold dark matter (CDM). This test is able to strongly discriminate between models that have different mean mass densities. We find that neither the Omega=1 (SCDM) nor Omega=0.3 (OCDM) cold dark matter models can produce a single candidate Local Group that is embedded in a region with such small peculiar velocities. For these models, we measure velocity dispersions between 500-700km/s and 150-300km/s respectively, more than twice the observed value. Although both CDM models fail to produce environments similar to those of our Local Group on a scale of a few Mpc, they can give rise to many binary systems that have similar orbital properties as the Milky Way--Andromeda system. The local, gravitationally induced bias of halos in the CDM ``Local Group'' environment, if defined within a sphere of 10 Mpc around each Local Group is about 1.5, independent of Omega. No biasing scheme could reconcile the measured velocity dispersions around Local Groups with the observed one. Identification of binary systems using a halo finder (named Skid (http://www-hpcc.astro.washington.edu/tools/DENMAX for a public version)) based on local density maxima instead of a simple linking algorithm, gives a much more complete sample. We show that a standard ``friend of friends'' algorithm would miss 40% of the LG candidates present in the simulations.

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