1305.2195
Following the flow: tracer particles in astrophysical fluid simulations
Genel, Vogelsberger, ... Springel, Hernquist
Present two independent numerical schemes for passive tracer particles in the hydrodynamical moving-mesh code Arepo, and compare their performance for various problems, from simple tests to cosmological simulations. Purpose of tracer particles: allow the flow to be followed in a Lagrangian way, reliably tracing the evolution of the fluid; subsequently measure local instantaneous fluid property, thereby recording the thermodynamical hisotry of individual fluid parcels. "Velocity field tracers": advected according to the local velocity field of the fluid; find that such tracers do not in general follow the mass flow correctly (particularly complex flows). Discuss a novel implementation of "Monte Carlo tracers" which are moved along with fluid cells, and are exchanged probabilistically between them following the mass flux. This method reproduces the mass distribution of the fuild correctly by construction, but is more diffusive than the fluid itself. Show that this novel approach is more reliable and is appropriate for following hydrodynamical flows in mesh-based codes. The MC tracers can also naturally be transferred between fluid cells and other types of particles, such as stellar particles, so that the mass flow in cosmological simulations can be followed in its entirety. Demonstrate a possible use of the tracer particles by studying the thermodynamical history of the halo atmosphere in the Santa Barbara Cluster [?].
1305.2199
The most luminous quasars do not live in the most massive dark matter haloes at any redshift
Fanidakis, Maccio, Baugh, Lacey, Frenk
In contrast to expectation, in a galaxy formation model which includes AGN feedback, quasars are predicted to live in average DM halo environments with typical masses of a few times 1e12 Msun. This fundamental prediction arises from the fact that quasar activity (BH accretion with luminosity greater than 1e46 erg/sec) is inhibited in DM haloes where AGN feedback operates. The galaxy hosts of quasars in simulations are identified with over massive (in gas and stars) spheroidal galaxies, in which BH accretion is triggered via a galaxy merger or secular processes. Further show that the z=0 descendants of high redshift (z~6) QSOs span a wide range of morphologies, galaxy and halo masses. The z~6 BHs typically grow only by a modest factor by the present day. High z QSOs never inhabit the largest DM haloes at that time and their descendants are very seldom found in the most massive haloes at z=0. [Is this compatible with observations?] Also show that observationally it is very likely to find an enhancement in the abundance of galaxies around quasars at z~5. However, these enhancements are considerably weaker compared to the overdensities expected at the extreme peaks of the DM distribution. Thus, it is very unlikely that a quasar detected in the z>5 Universe pinpoints the location of the progenitors of superclusters in the local Universe.
1305.2200
Constraints on black hole fuelling modes from the clustering of X-ray AGN
Fanidakis, et al
Clustering analysis of X-ray selected AGN by compiling X-ray samples from the literature and re-estimating the DM halo masses of AGN in a uniform manner. Find: moderate luminosity AGN (1e42-44 erg/sec) in the z=0-1.3 Universe are typically found in DM haloes with mases of 1e13 Msun. Compare to theoretical predictions of the coupled galaxy and BH formation model GALFORM. Find good agreement when calculation includes the hot-halo mode of accretion onto the central BH. This type of accretion, which is additional to the common cold accretion during disk instabilities and galaxy mergers, is tightly coupled to the AGN feedback in the model. The hot-halo mode becomes prominent in DM haloes with masses greater than 1e12.5 Msun, where AGN feedback typically operates, giving rise to a distinct class of moderate luminosity AGN that inhabit rich clusters and superclusters. Cold gas fueling of the BH cannot produce the observationally inferred DM halo masses of X-ray AGN. Switching off AGN feedback in the model results in a large population of luminous quasars (>1e44 erg/sec) in DM haloes with masses up to 1d14 Msun, which is inconsistent with the observed clustering of quasars. The abundance of hot-halo AGN decreases significantly in the z~3-4 universe. At such high redshifts, the cold accretion mode is solely responsible for shaping the environment of moderate luminosity AGN. Analysis supports two accretion modes (cold and hot) for the fueling of supermassive BHs and strongly underlines the importance of AGN feedback in cosmological models both of galaxy formation and BH growth.
1305.2194
Galileon forces in the solar system
Andrews, Chu, Trodden
Obtain an analytic understanding of realistic astrophysical dynamics in the presence of a Vainshtein screened fifth force arising from IR modifications of GR. Attempt to solve (general flat spacetime galileon model) the scalar force law between well separated bodies located well within the Vainshtein radius of the Sun. Derive the exact static Green's function of the galileon wave equation linearized about the background field generated by the Sun, for the minimal cubic and maximally quartic galileon theories, and then introduce a method to compute the general leading order force law perturbatively away from these limits. Also show that the same NLs which produce the V screening effect present obstacles to an analytic calculation of the galileon forces between closely bound systems within the SS, such as that of the Earth and Moon. Within the test mass approximation, deduce that a large enough quartic gaileon interaction would suppress the effect on planetary perihelion precession below the level detectable by even the next-generation experiments.
1305.2198
Oscillations and stability of polytropic filaments
Breysse, Kamionkowski, Benson
Study the oscillations and stability of self-gravitating cylindrically symmetric fluid systems and collisionless systems. Study small perturbations to the equilibrium system and find the normal modes, using methods similar to those used in astroseismology. Find that there is a single sequence of purely radial modes that become unstable if the adiabatic exponent is less than 1. Nonradial modes can be divided into p modes, which are stable and pressure-driven, and g modes, which are gravity driven. The g modes become unstable if the adiabatic exponent is greater than the polytrope index [?]. These modes are analogous to the modes of a spherical star, but their behavior is somewhat different because a cylindrical geometry has less symmetry than a spherical geometry. This implies that perturbations are classified by a radial quantum number, an azimuthal quantum number, and wavelength in the z direction, which can become arbitrarily large. Find that decreasing this wavelength increases the frequency of stable modes and increases the growth rate of unstable modes. Use variational arguments to demonstrate that filaments of collisionless matter with ergodic distribution [?] functions are stable to purely radial perturbations, and that filaments with ergodic power-law distribution functions are stable to all perturbations.
1305.2204
The redshift and mass dependence on the formation of the Hubble sequence at z>1 from CANDELS/UDS
Mortlock, ... et al
Sample of 1188 massive galaxies with M*>1e10 Msun between 1<z<3 within UDS of CANDELS field; study structures and morphologies. Using this sample, determine how galaxy structure and morphology evolve with time. Visually classify sample into disks, ellipticals and peculiar systems and correct for z effects on the classifications through simulations. Find evolution in the fractions of galaxies at a given visual classification as a function of z. The peculiar population is dominant at z>2 with a substantial spheroid population, and a negligible disk population. [spheroid is elliptical?] Compute the transition redshift, z_trans, where the combined fraction of spheroids and disks is equal to that of peculiar galaxies, at z_trans=1.86 pm 0.62 for galaxies in the sample's stellar mass range. Find that this changes as a function of stellar mass, with Hubble-type systems becoming dominant at higher redshifts for higher mass galaxies (z_trans=2.2 pm 0.82), rather than for the lower mass galaxies (z_trans=1.73 pm 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. Compre visual classifications with Sersic index, the concentration, asymmetry and clumpiness (CAS) parameters, SFR and rest frame U-B color. Find links between the color of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, discuss the negligible z>2 disk fraction based on visual morphologies and speculate that this is an effect of forming disks appearing peculiar through processes such as violent disk instabilities or mergers. Conclude that to properly define high z morphology, a new and more exact classification scheme is needed.
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