The relation between accretion rate and jet power in X-ray luminous elliptical galaxies

Using Chandra X-ray observations of 9 nearby, X-ray luminous ellipticals with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the X-ray data and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, inferred from the energies and timescales required to inflate the cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates by a power law model. A significant fraction (2.2^{+1.0}_{-0.7} per cent, for P_jet=10^{43} erg/s) of the energy associated with the rest mass of material entering the accretion radius eventually emerges in the jets. The data also hint that this fraction may rise slightly with increasing jet power. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The tight correlation between P_Bondi and P_jet suggests that the Bondi formulae provide a reasonable description of the accretion process, despite the likely presence of magnetic pressure and angular momentum in the accreting gas, and that the accretion flows are approximately stable over timescales of a few million years. Our results show that the black hole `engines' at the hearts of large elliptical galaxies and groups can feed back sufficient energy to stem cooling and star formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.Comment: Accepted for publication in MNRAS. 10 pages, 4 figures. Includes an enhanced statistical analysis and some additional data. Conclusions unchange

Fe Emission And Ionized Excess Absorption in the Luminous Quasar 3C109 With XMM-Newton

We report results from an XMM-Newton observation of the broad-line radio galaxy 3C 109 (z=0.3056). Previous ASCA data revealed the presence of a broad iron line from the accretion disc with which the XMM-Newton spectrum is fully consistent. However, although improving the ASCA constraints on the line parameters, the quality of the data is not high enough to distinguish between an untruncated accretion disc extending down to small radii close to the black hole and a scenario in which the innermost 20-30 gravitational radii are missing. For this reason, our results are model-dependent and the hard data can be modeled equally well by considering an absorption scenario in which a large column of neutral gas partially covers the X-ray continuum source. However, the absorber would have to comprise hundreds/thousands very compact clouds close to the X-ray source, which seems rather extreme a requirement. The 2-10 keV intrinsic luminosity of 3C 109 is of the order of 2-3 x 10{sup 45} erg s{sup -1} regardless of the adopted model. A recent black hole mass estimate of {approx} 2 x 10{sup 8} M{sub {circle_dot}} implies that L{sub bol}/L{sub Edd} > 1. If partial covering is excluded, the observed reflection fraction (of the order of unity), steep photon index (1.86), and Fe line equivalent width (about 100 eV) all suggest to exclude that the X-ray continuum is strongly beamed indicating that the large Eddington ratio is associated with a radiatively efficient accretion process and making it unlikely that the innermost accretion disc is replaced by a thick radiatively inefficient medium such as in advection-dominated accretion models. We also confirm previous findings on the detection of low energy absorption in excess of the Galactic value, where we find excellent agreement with previous results obtained in X-rays and at other wavelengths (optical and infrared). The better quality of the XMM-Newton data enables us to attribute the excess absorption to slightly ionized gas in the line of sight, located at the redshift of 3C 109. The most likely interpretation for the excess absorption is that the line-of-sight is grazing the obscuring torus of unified models, which is consistent with the inclination inferred from the Fe line profile (about 40{sup o}) and with the hybrid radio-galaxy/quasar nature of 3C 109

A Bayesian analysis of regularised source inversions in gravitational lensing

Strong gravitational lens systems with extended sources are of special interest because they provide additional constraints on the models of the lens systems. To use a gravitational lens system for measuring the Hubble constant, one would need to determine the lens potential and the source intensity distribution simultaneously. A linear inversion method to reconstruct a pixellated source brightness distribution of a given lens potential model was introduced by Warren & Dye. In the inversion process, a regularisation on the source intensity is often needed to ensure a successful inversion with a faithful resulting source. In this paper, we use Bayesian analysis to determine the optimal regularisation constant (strength of regularisation) of a given form of regularisation and to objectively choose the optimal form of regularisation given a selection of regularisations. We consider and compare quantitatively three different forms of regularisation previously described in the literature for source inversions in gravitational lensing: zeroth-order, gradient and curvature. We use simulated data with the exact lens potential to demonstrate the method. We find that the preferred form of regularisation depends on the nature of the source distribution.Comment: 18 pages, 10 figures; Revisions based on referee's comments after initial submission to MNRA

A New View of the Dwarf Spheroidal Satellites of the Milky Way From VLT/FLAMES: Where are the Very Metal Poor Stars?

As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph): Sculptor, Sextans, Fornax and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early Universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] {approx} -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E ~ 270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462, in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 M_Sun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a modest natal 'kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first ever for an isolated stellar-mass BH using any technique

Magnetic Fields in the Center of the Perseus Cluster

We present Very Long Baseline Array (VLBA) observations of the nucleus of NGC1275, the central, dominant galaxy in the Perseus cluster of galaxies. These are the first observations to resolve the linearly polarized emission from 3C84, and from them we determine a Faraday rotation measure (RM) ranging from 6500 to 7500 rad m{sup -2} across the tip of the bright southern jet component. At 22 GHz some polarization is also detected from the central parsec of 3C84, indicating the presence of even more extreme RMs that depolarize the core at lower frequencies. The nature of the Faraday screen is most consistent with being produced by magnetic fields associated with the optical filaments of ionized gas in the Perseus Cluster

Machos in M31? Absence of Evidence but not Evidence of Absence

We present results of a microlensing survey toward the Andromeda Galaxy (M31) carried out during four observing seasons at the Isaac Newton Telescope (INT). This survey is part of the larger microlensing survey toward M31 performed by the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration. Using a fully automated search algorithm, we indentify 14 candidate microlensing events, three of which are reported here for the first time. Observations obtained at the Mayall telescope are combined with the INT data to produce composite light curves for these candidates. The results from the survey are compared with theoretical predictions for the number and distribution of events. These predictions are based on a Monte Carlo calculation of the detection efficiency and disk-bulge-halo models for M31. The models provide the full phase-space distribution functions (DFs) for the lens and source populations and are motivated by dynamical and observational considerations. They include differential extinction and span a wide range of parameter space characterized primarily by the mass-to-light ratios for the disk and bulge. For most models, the observed event rate is consistent with the rate predicted for self-lensing--a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence level. The event distribution does show a large near-far asymmetry hinting at a halo contribution to the microlensing signal. Two candidate events are located at particularly large projected radii on the far side of the disk. These events are difficult to explain by self lensing and only somewhat easier to explain by MACHO lensing. A possibility is that one of these is due to a lens in a giant stellar stream

The Low-Power Nucleus of PKS 1246-410 in the Centaurus Cluster

We present Chandra, Very Large Array (VLA), and Very Long Baseline Array (VLBA) observations of the nucleus of NGC 4696, a giant elliptical in the Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS 1246-410 in the Centaurus cluster is a nearby example of a radio galaxy in a dense cluster environment. In analyzing the new X-ray data we have found a compact X-ray feature coincident with the optical and radio core. While nuclear emission from the X-ray source is expected, its luminosity is low, < 10{sup 40} erg s{sup -1}. We estimate the Bondi accretion radius to be 30 pc and the accretion rate to be 0.01 M{sub {circle_dot}} y{sup -1} which under the canonical radiative efficiency of 10% would overproduce by 3.5 orders of magnitude the radiative luminosity. Much of this energy can be directed into the kinetic energy of the jet, which over time inflates the observed cavities seen in the thermal gas. The VLBA observations reveal a weak nucleus and a broad, one-sided jet extending over 25 parsecs in position angle -150 degrees. This jet is deflected on the kpc-scale to a more east-west orientation (position angle of -80 degrees)