Monthly Notices of the Royal Astronomical Society

doi: 10.1111/j.1365-2966.2009.16359.xpmid: N/A

Brown, Shea; Farnsworth, Damon; Rudnick, Lawrence

doi: 10.1111/j.1365-2966.2009.15867.xpmid: N/A

We explore for the first time the method of cross-correlation of radio synchrotron emission and tracers of large-scale structure in order to detect the warm-hot intergalactic medium (WHIM). We performed a cross-correlation of a 34°× 34° area of Two-Micron All-Sky Survey (2MASS) galaxies for two redshift slices (0.03 < z < 0.04 and 0.06 < z < 0.07) with the corresponding region of the 1.4 GHz Bonn survey. For this analysis, we assumed that the synchrotron surface brightness is linearly proportional to surface density of galaxies. We also sampled the cross-correlation function (CCF) using 24 distant fields of the same size from the Bonn survey, to better assess the noise properties. Though we obtained a null result, we found that by adding a signal weighted by the 2MASS image with a filament (peak) surface brightness of 1 (7) and 7 (49) mK would produce a 3σ positive correlation for the 0.03 < z < 0.04 and 0.06 < z < 0.07 redshift slices, respectively. These detection thresholds correspond to minimum energy magnetic fields as low as 0.2 μG, close to some theoretical expectations for filament field values. This injected signal is also below the rms noise of the Bonn survey, and demonstrates the power of this technique and its utility for upcoming sensitive continuum surveys such as those planned with the Murchison Widefield Array.

Mignone, A.; Rossi, P.; Bodo, G.; Ferrari, A.; Massaglia, S.

doi: 10.1111/j.1365-2966.2009.15642.xpmid: N/A

Relativistic magnetized jets are key elements in active galactic nuclei and in other astrophysical environments. Their structure and evolution involve a complex non-linear physics that can be approached by numerical studies only. Still, owing to a number of challenging computational aspects, only a few numerical investigations have been undertaken so far. In this paper, we present high-resolution three-dimensional numerical simulations of relativistic magnetized jets carrying an initially toroidal magnetic field. The presence of a substantial toroidal component of the field is nowadays commonly invoked and held responsible for the process of jet acceleration and collimation. We find that the typical nose cone structures, commonly observed in axisymmetric two-dimensional simulations, are not produced in the three-dimensional case. Rather, the toroidal field gives rise to strong current-driven kink instabilities leading to jet wiggling. However, it appears to be able to maintain a highly relativistic spine along its full length. By comparing low- and high-resolution simulations, we emphasize the impact of resolution on the jet dynamical properties.

Horner, J.; Lykawka, P. S.

doi: 10.1111/j.1365-2966.2009.15702.xpmid: N/A

The fact that the Centaurs are the primary source of the short-period comets is well established. However, the origin of the Centaurs themselves is still under some debate, with a variety of different source reservoirs being proposed in the last decade. In this work, we suggest that the Neptune Trojans (together with the Jovian Trojans) could represent an additional significant source of Centaurs. Using dynamical simulations of the first Neptune Trojan discovered (2001 QR322), together with integrations following the evolution of clouds of theoretical Neptune Trojans obtained during simulations of planetary migration, we show that the Neptune Trojan population contains a great number of objects which are unstable on both Myr and Gyr time-scales. Using individual examples, we show how objects that leave the Neptunian Trojan cloud evolve on to orbits indistinguishable from those of the known Centaurs, before providing a range of estimates of the flux from this region to the Centaur population. With only moderate assumptions, it is shown that the Trojans can contribute a significant proportion of the Centaur population, and may even be the dominant source reservoir. This result is supported by past work on the colours of the Trojans and the Centaurs, but it will take future observations to determine the full scale of the contribution of the escaped Trojans to the Centaur population.

Navarro, Julio F.; Ludlow, Aaron; Springel, Volker; Wang, Jie; Vogelsberger, Mark; White, Simon D. M.; Jenkins, Adrian; Frenk, Carlos S.; Helmi, Amina

doi: 10.1111/j.1365-2966.2009.15878.xpmid: N/A

We study the mass, velocity dispersion and anisotropy profiles of Λ cold dark matter (ΛCDM) haloes using a suite of N-body simulations of unprecedented numerical resolution. The Aquarius Project follows the formation of six different galaxy-sized haloes simulated several times at varying numerical resolution, allowing numerical convergence to be assessed directly. The highest resolution simulation represents a single dark matter halo using 4.4 billion particles, of which 1.1 billion end up within the virial radius. Our analysis confirms a number of results claimed by earlier work, and clarifies a few issues where conflicting claims may be found in the recent literature. The mass profile of ΛCDM haloes deviates slightly but systematically from the form proposed by Navarro, Frenk & White. The spherically averaged density profile becomes progressively shallower inwards and, at the innermost resolved radius, the logarithmic slope is γ≡− d ln ρ/d ln r≲ 1. Asymptotic inner slopes as steep as the recently claimed ρ∝r−1.2 are clearly ruled out. The radial dependence of γ is well approximated by a power law, γ∝rα (the Einasto profile). The shape parameter, α, varies slightly but significantly from halo to halo, implying that the mass profiles of ΛCDM haloes are not strictly universal: different haloes cannot, in general, be rescaled to look identical. Departures from similarity are also seen in velocity dispersion profiles and correlate with those in density profiles so as to preserve a power-law form for the spherically averaged pseudo-phase-space density, ρ/σ3∝r−1.875. The index here is identical to that of Bertschinger's similarity solution for self-similar infall on to a point mass from an otherwise uniform Einstein–de Sitter universe. The origin of this striking behaviour is unclear, but its robustness suggests that it reflects a fundamental structural property of ΛCDM haloes. Our conclusions are reliable down to radii below 0.4 per cent of the virial radius, providing well-defined predictions for halo structure when baryonic effects are neglected, and thus an instructive theoretical template against which the modifications induced by the baryonic components of real galaxies can be judged.

Curran, S. J.; Tzanavaris, P.; Darling, J. K.; Whiting, M. T.; Webb, J. K.; Bignell, C.; Athreya, R.; Murphy, M. T.

doi: 10.1111/j.1365-2966.2009.15879.xpmid: N/A

We present the results of three separate searches for H i 21-cm absorption in a total of 12 damped Lyman α absorption systems (DLAs) and sub-DLAs over the redshift range zabs= 0.86–3.37. We find no absorption in the five systems for which we obtain reasonable sensitivities and add the results to those of other recent surveys in order to investigate factors which could have an effect on the detection rate. We provide evidence that the mix of spin temperature/covering factor ratios seen at low redshift may also exist at high redshift, with a correlation between the 21-cm line strength and the total neutral hydrogen column density, indicating a roughly constant spin temperature/covering factor ratio for all of the DLAs searched. Also, by considering the geometry of a flat expanding Universe together with the projected sizes of the background radio emission regions, we find, for the detections, that the 21-cm line strength is correlated with the size of the absorber. For the non-detections, it is apparent that larger absorbers (covering factors) are required in order to exhibit 21-cm absorption, particularly if these DLAs do not arise in spiral galaxies. We also suggest that the recent zabs= 2.3 detection towards TXS 0311+430 arises in a spiral galaxy, but on the basis of a large absorption cross-section and high metallicity, rather than a low spin temperature.

Margutti, R.; Genet, F.; Granot, J.; Duran, R. Barniol; Guidorzi, C.; Chincarini, G.; Mao, J.; Schady, P.; Sakamoto, T.; Miller, A. A.; Olofsson, G.; Bloom, J. S.; Evans, P. A.; Fynbo, J. P. U.; Malesani, D.; Moretti, A.; Pasotti, F.;

Sanderson, Alastair J. R.; Ponman, Trevor J.

doi: 10.1111/j.1365-2966.2009.15888.xpmid: N/A

We present a parametric analysis of the intracluster medium and gravitating mass distribution of a statistical sample of 20 galaxy clusters using the phenomenological cluster model of Ascasibar and Diego. We describe an effective scheme for the estimation of errors on model parameters and derived quantities using bootstrap resampling. We find that the model provides a good description of the data in all cases and we quantify the mean fractional intrinsic scatter about the best-fitting density and temperature profiles, finding this to have median values across the sample of 2 and 5 per cent, respectively. In addition, we demonstrate good agreement between r500 determined directly from the model and that estimated from a core-excluded global spectrum. We compare cool core and non-cool core clusters in terms of the logarithmic slopes of their gas density and temperature profiles and the distribution of model parameters and conclude that the two categories are clearly separable. In particular, we confirm the effectiveness of the logarithmic gradient of the gas density profile measured at 0.04r500 in differentiating between the two types of cluster.

Barnes, Peter J.; Yonekura, Yoshinori; Ryder, Stuart D.; Hopkins, Andrew M.; Miyamoto, Yosuke; Furukawa, Naoko; Fukui, Yasuo

doi: 10.1111/j.1365-2966.2009.15890.xpmid: N/A

We report Mopra Australia Telescope National Facility (ATNF), Anglo-Australian Telescope and Atacama Submillimeter Telescope Experiment observations of a molecular clump in Carina, BYF73 = G286.21+0.17, which give evidence of large-scale gravitational infall in the dense gas. From the millimetre and far-infrared data, the clump has a mass of ∼2 × 104 M⊙, luminosity of ∼2–3 × 104 L⊙ and diameter of ∼0.9 pc. From radiative transfer modelling, we derive a mass infall rate of ∼3.4 × 10−2 M⊙ yr−1. If confirmed, this rate for gravitational infall in a molecular core or clump may be the highest yet seen. The near-infrared K-band imaging shows an adjacent compact H ii region and IR cluster surrounded by a shell-like photodissociation region showing H2 emission. At the molecular infall peak, the K imaging also reveals a deeply embedded group of stars with associated H2 emission. The combination of these features is very unusual, and we suggest that they indicate the ongoing formation of a massive star cluster. We discuss the implications of these data for competing theories of massive star formation.

An, T.; Hong, X. Y.; Hardcastle, M. J.; Worrall, D. M.; Venturi, T.; Pearson, T. J.; Shen, Z.-Q.; Zhao, W.; Feng, W. X.

doi: 10.1111/j.1365-2966.2009.15899.xpmid: N/A

We present results on the compact steep-spectrum quasar 3C 48 from observations with the Very Long Baseline Array (VLBA), the Multi-Element Radio Linked Interferometer Network (MERLIN) and the European Very long baseline interferometry (VLBI) Network (EVN) at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz, respectively. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C ∼ 0.25 arcsec north of the core, where the jet starts to bend to the north-east. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet–interstellar-medium interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 yr ago suggests a significant northward motion for A2 with an apparent transverse velocity βapp= 3.7 ± 0.4c. The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of ≳0.96c if the jet is viewed at an inclination angle less than 20°. A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin–Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knots.