Extended radio emission in BL Lac objects - I: the images

We have observed 28 sources selected from the 1Jy sample of BL Lac objects (Stickel et al. 1991) with the Very Large Array (VLA) in A, B and D configurations at 1.36, 1.66 and 4.85 GHz, and/or with the Westerbork Synthesis Radio Telescope (WSRT) at 1.40 GHz. In this paper we present high sensitivity images at arcsecond resolution of the 18 objects showing extended structure in our images, and of another source from the FIRST (Faint Images of the Radio Sky at Twenty-cm) survey (Becker et al. 1995). In general our high sensitivity images reveal an amount of extended emission larger than previously reported. In some objects the luminosity of the extended structure is comparable with that of FR~II radio sources. A future paper will be devoted to the interpretation of these results.Comment: 12 pages, 35 figures, to appear on A&A Supp. Ser., postscript file with figures included available at http://www.ira.noto.cnr.it/staff/carlo/ds1030.ps.g

Solution generating theorems for perfect fluid spheres

The first static spherically symmetric perfect fluid solution with constant density was found by Schwarzschild in 1918. Generically, perfect fluid spheres are interesting because they are first approximations to any attempt at building a realistic model for a general relativistic star. Over the past 90 years a confusing tangle of specific perfect fluid spheres has been discovered, with most of these examples seemingly independent from each other. To bring some order to this collection, we develop several new transformation theorems that map perfect fluid spheres into perfect fluid spheres. These transformation theorems sometimes lead to unexpected connections between previously known perfect fluid spheres, sometimes lead to new previously unknown perfect fluid spheres, and in general can be used to develop a systematic way of classifying the set of all perfect fluid spheres. In addition, we develop new ``solution generating'' theorems for the TOV, whereby any given solution can be ``deformed'' to a new solution. Because these TOV-based theorems work directly in terms of the pressure profile and density profile it is relatively easy to impose regularity conditions at the centre of the fluid sphere.Comment: 8 pages, no figures, to appear in the proceedings of the NEB XII Conference (Recent Developments in Gravity), 29 June - 2 July, 2006, Napflio, Greec

Quasars at z=6: the survival of the fittest

The Sloan Digital Sky survey detected luminous quasars at very high redshift, z>6. Follow-up observations indicated that at least some of these quasars are powered by supermassive black holes (SMBHs) with masses in excess of billion solar masses. SMBHs, therefore, seem to have already existed when the Universe was less than 1 Gyr old, and the bulk of galaxy formation still has to take place. We investigate in this paper to which extent accretion and dynamical processes influence the early growth of SMBHs. We assess the impact of (i) black hole mergers, (ii) the influence of the merging efficiency and (iii) the negative contribution due to dynamical effects which can kick black holes out of their host halos (gravitational recoil). We find that if accretion is always limited by the Eddington rate via a thin disc, the maximum radiative efficiency allowed to reproduce the LF at z=6 is of order 12%, when the adverse effect of the gravitational recoil is taken into consideration. Dynamical effects cannot be neglected in studies of high-redshift SMBHs. If black holes can accrete at super-critical rate during an early phase, reproducing the observed SMBH mass values is not an issue, even in the case that the recoil velocity is in the upper limits range, as the mass ratios of merging binaries are skewed towards low values, where the gravitational recoil effect is very mild. We propose that SMBH growth at early times is very selective, and efficient only for black holes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure

A Lemaitre-Tolman-Bondi cosmological wormhole

We present a new analytical solution of the Einstein field equations describing a wormhole shell of zero thickness joining two Lema{\i}tre-Tolman-Bondi universes, with no radial accretion. The material on the shell satisfies the energy conditions and, at late times, the shell becomes comoving with the dust-dominated cosmic substratum.Comment: 5 pages, latex, no figures, to appear in Phys. Rev.

Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case

We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is made of the material that has too much angular momentum to be accreted directly. The torus accretes, however, because of the transport of angular momentum due to the magnetorotational instability (MRI). Initially, accretion is dominated by the polar funnel, as in the hydrodynamic inviscid case, where material has zero or very low angular momentum. At the later phase of the evolution, the torus thickens towards the poles and develops a corona or an outflow or both. Consequently, the mass accretion through the funnel is stopped. The accretion of rotating gas through the torus is significantly reduced compared to the accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller than the accretion rate in the inviscid, weakly rotating case.Our results do not change if we switch on or off resistive heating. Overall our simulations are very similar to those presented by Stone, Pringle, Hawley and Balbus despite different initial and outer boundary conditions. Thus, we confirm that MRI is very robust and controls the nature of radiatively inefficient accretion flows.Comment: submitted in Ap

Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney

A recent criticism of cosmological methodology and achievements by Disney (2000) is assessed. Some historical and epistemological fallacies in the said article have been highlighted. It is shown that---both empirically and epistemologically---modern cosmology lies on sounder foundations than it is portrayed. A brief historical account demonstrates that this form of unsatisfaction with cosmology has had a long tradition, and rather meagre results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel. Grav., accepted for publicatio

Radio Jet-Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018

As part of our study of the magnetic fields of AGN we have recently observed a large sample of blazars with the Very Long Baseline Array. Here we report the discovery of a striking two-component jet in the source 1055+018, consisting of an inner spine with a transverse magnetic field, and a fragmentary but distinct boundary layer with a longitudinal magnetic field. The polarization distribution in the spine strongly supports shocked-jet models while that in the boundary layer suggests interaction with the surrounding medium. This behavior suggests a new way to understand the differing polarization properties of strong- and weak-lined blazars.Comment: LaTex; 10 pages; 6 figures; reference fix; to appear in ApJL, 518, 1999 June 2

Dynamical Friction in a Gaseous Medium

Using time-dependent linear perturbation theory, we evaluate the dynamical friction force on a massive perturber M_p traveling at velocity V through a uniform gaseous medium of density rho_0 and sound speed c_s. This drag force acts in the direction -\hat V, and arises from the gravitational attraction between the perturber and its wake in the ambient medium. For supersonic motion (M=V/c_s>1), the enhanced-density wake is confined to the Mach cone trailing the perturber; for subsonic motion (M<1), the wake is confined to a sphere of radius c_s t centered a distance V t behind the perturber. Inside the wake, surfaces of constant density are hyperboloids or oblate spheroids for supersonic or subsonic perturbers, respectively, with the density maximal nearest the perturber. The dynamical drag force has the form F_df= - I 4\pi (G M_p)^2\rho_0/V^2. We evaluate I analytically; its limits are I\to M^3/3 for M>1. We compare our results to the Chandrasekhar formula for dynamical friction in a collisionless medium, noting that the gaseous drag is generally more efficient when M>1 but less efficient when M<1. To allow simple estimates of orbit evolution in a gaseous protogalaxy or proto-star cluster, we use our formulae to evaluate the decay times of a (supersonic) perturber on a near-circular orbit in an isothermal \rho\propto r^{-2} halo, and of a (subsonic) perturber on a near-circular orbit in a constant-density core. We also mention the relevance of our calculations to protoplanet migration in a circumstellar nebula.Comment: 17 pages, 5 postscript figures, to appear in ApJ 3/1/9

XMM-Newton Observation of an X-ray Trail Between the Spiral Galaxy NGC6872 and the Central Elliptical NGC6876 in the Pavo Group

We present XMM-Newton observations of a trail of enhanced X-rayemission extending along the 8'.7 X 4' region between the spiral NGC6872 and the dominant elliptical NGC6876 in the Pavo Group,the first known X-ray trail associated with a spiral galaxy in a poor galaxy group and, with projected length of 90 kpc, one of the longest X-ray trails observed in any system. The X-ray surface brightness in the trail region is roughly constant beyond ~20 kpc of NGC6876 in the direction of NGC6872. The trail is hotter (~ 1 keV) than the undisturbed Pavo IGM (~0.5 keV) and has low metal abundances (0.2 Zsolar). The 0.5-2 keV luminosity of the trail, measured using a 67 X 90 kpc rectangular region, is 6.6 X 10^{40} erg/s. We compare the properties of gas in the trail to the spectral properties of gas in the spiral NGC6872 and in the elliptical NGC6876 to constrain its origin. We suggest that the X-ray trail is either IGM gas gravitationally focused into a Bondi-Hoyle wake, a thermal mixture of ~64% Pavo IGM gas with ~36% galaxy gas that has been removed from the spiral NGC6872 by turbulent viscous stripping, or both, due to the spiral's supersonic motion at angle xi ~ 40 degrees with respect to the plane of the sky, past the Pavo group center (NGC6876) through the densest region of the Pavo IGM. Assuming xi = 40 degrees and a filling factor eta in a cylindrical volume with radius 33 kpc and projected length 90 kpc, the mean electron density and total hot gas mass in the trail is 9.5 X 10^{-4}*eta^{-1/2} cm^{-3} and 1.1 X 10^{10}*eta^{1/2} Msolar, respectively.Comment: typos corrected in Eq. 7 & 8, figures and discussion unchanged, 39 pages, 11 postscript figures, submitted to Ap

Three-Dimensional Magnetohydrodynamic Simulations of Spherical Accretion

We present three-dimensional numerical magnetohydrodynamic simulations of radiatively inefficient spherical accretion onto a black hole. The simulations are initialized with a Bondi flow, and with a weak, dynamically unimportant, large-scale magnetic field. The magnetic field is amplified as the gas flows in. When the magnetic pressure approaches equipartition with the gas pressure, the field begins to reconnect and the gas is heated up. The heated gas is buoyant and moves outward, causing line stretching of the frozen-in magnetic field. This leads to further reconnection, and more heating and buoyancy-induced motions, so that the flow makes a transition to a state of self-sustained convection. The radial structure of the flow changes dramatically from its initial Bondi profile, and the mass accretion rate onto the black hole decreases significantly. Motivated by the numerical results, we develop a simplified analytical model of a radiatively inefficient spherical flow in which convective transport of energy to large radii plays an important role. In this "convection-dominated Bondi flow" the accretion velocity is highly subsonic and the density varies with radius as ~R^{-1/2} rather than the standard Bondi scaling ~R^{-3/2}. We estimate that the mass accretion rate onto the black hole is significantly less than the Bondi accretion rate. Convection-dominated Bondi flows may be relevant for understanding many astrophysical phenomena, e.g. post-supernova fallback and radiatively inefficient accretion onto supermassive black holes, stellar-mass black holes and neutron stars.Comment: 23 pages, 6 figures, submitted to Ap