1,744,545 research outputs found
The impact of a young radio galaxy: clues from the cosmic ray electron population
Full text linkIn the framework of hierarchical structure formation, active galactic nuclei (AGN) feedback shapes the galaxy luminosity function. Low luminosity, galaxy-scale double radio sources are ideal targets to investigate the interplay between AGN feedback and star formation. We use Very Large Array and BIMA millimetre-wave array observations to study the radio continuum emission of NGC 3801 between 1.4 and 112.4 GHz. We find a prominent spectral break at ~10 GHz, where the spectrum steepens as expected from cosmic ray electron (CRe) ageing. Using the equipartition magnetic field and fitting JP models locally, we create a spatially resolved map of the spectral age of the CRe population. The spectral age of τint = 2.0 ± 0.2 Myr agrees within a factor of 2 with the dynamical age of the expanding X-ray emitting shells. The spectral age varies only little across the lobes, requiring an effective mixing process of the CRe such as a convective backflow of magnetized plasma. The jet termination points have a slightly younger CRe spectral age, hinting at in situ CRe re-acceleration. Our findings support the scenario where the supersonically expanding radio lobes heat the interstellar medium (ISM) of NGC 3801 via shock waves, and, as their energy is comparable to the energy of the ISM, are clearly able to influence the galaxy's further evolution
Galaxy and mass assembly (GAMA) : AUTOZ spectral redshift measurements, confidence and errors
Full text linkThe Galaxy And Mass Assembly (GAMA) survey has obtained spectra of over 230 000 targets using the Anglo-Australian Telescope. To homogenize the redshift measurements and improve the reliability, a fully automatic redshift code was developed (AUTOZ). The measurements were made using a cross-correlation method for both the absorption-and the emission-line spectra. Large deviations in the high-pass-filtered spectra are partially clipped in order to be robust against uncorrected artefacts and to reduce the weight given to single-line matches. A single figure of merit (FOM) was developed that puts all template matches on to a similar confidence scale. The redshift confidence as a function of the FOM was fitted with a tanh function using a maximum likelihood method applied to repeat observations of targets. The method could be adapted to provide robust automatic redshifts for other large galaxy redshift surveys. For the GAMA survey, there was a substantial improvement in the reliability of assigned redshifts and in the lowering of redshift uncertainties with a median velocity uncertainty of 33 kms-1.Peer reviewe
The inside-out view on neutron-star magnetospheres
No full textWe construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron star's interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area, which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly rotating systems such as magnetars. Nonetheless, we demonstrate that magnetospheres qualitatively resembling those expected for both magnetars and pulsars are possible within our framework. The ‘inside-out’ approach taken in this paper should be more generally applicable to rotating neutron stars, where the interior and exterior regions are again not independent but evolve together
Erratum: Galactic synchrotron emissivity measurements between 250° < l < 355° from the GLEAM survey with the MWA [MNRAS, 472, (2017), (828-834)] doi:10.1093/mnras/stx2129
No full text© 2017 The Authors. This is an erratum to the paper 'Galactic synchrotron emissivity measurements between 250° < l < 355° from the GLEAM survey with the MWA', published in MNRAS, 465, 3163 (2017)
Erratum:Filament Hunting: Integrated HI 21cm Emission From Filaments Inferred by Galaxy Surveys [MNRAS, (2019)] DOI: 10.1093/mnras/stx509
Full text linkThis is an erratum to the paper' Filament Hunting: Integrated HI 21cm Emission From Filaments Inferred by Galaxy Surveys' that was published in MNRAS, 468, 857. Due to the use of an incorrect equation to calculate the thermal noise of an observation, the noise was underestimated in the paper, affecting some of our conclusions.</p
A simple disc wind model for broad absorption line quasars
No full textApproximately 20 per cent of quasi-stellar objects (QSOs) exhibit broad, blue-shifted absorption lines in their ultraviolet spectra. Such features provide clear evidence for significant outflows from these systems, most likely in the form of accretion disc winds. These winds may represent the ‘quasar’ mode of feedback that is often invoked in galaxy formation/evolution models, and they are also key to unification scenarios for active galactic nuclei (AGN) and QSOs. To test these ideas, we construct a simple benchmark model of an equatorial, biconical accretion disc wind in a QSO and use a Monte Carlo ionization/radiative transfer code to calculate the ultraviolet spectra as a function of viewing angle. We find that for plausible outflow parameters, sightlines looking directly into the wind cone do produce broad, blue-shifted absorption features in the transitions typically seen in broad absorption line (BAL) QSOs. However, our benchmark model is intrinsically X-ray weak in order to prevent overionization of the outflow, and the wind does not yet produce collisionally excited line emission at the level observed in non-BAL QSOs. As a first step towards addressing these shortcomings, we discuss the sensitivity of our results to changes in the assumed X-ray luminosity and mass-loss rate, Ṁwind. In the context of our adopted geometry, Ṁwind ∼ Ṁacc is required in order to produce significant BAL features. The kinetic luminosity and momentum carried by such outflows would be sufficient to provide significant feedback
The dust budget crisis in high-redshift submillimetre galaxies
Full text linkKR acknowledges support from the European Research Council Starting Grant (P.I. V. Wild). HLG acknowledges support from the Science, Technology and Facilities Council.We apply a chemical evolution model to investigate the sources and evolution of dust in a sample of 26 high-redshift (z > 1) submillimetre galaxies (SMGs) from the literature, with complete photometry from ultraviolet to the submillimetre. We show that dust produced only by low-intermediate-mass stars falls a factor 240 short of the observed dust masses of SMGs, the well-known 'dust-budget crisis'. Adding an extra source of dust from supernovae can account for the dust mass in 19 per cent of the SMG sample. Even after accounting for dust produced by supernovae the remaining deficit in the dust mass budget provides support for higher supernova yields, substantial grain growth in the interstellar medium or a top-heavy IMF. Including efficient destruction of dust by supernova shocks increases the tension between our model and observed SMG dust masses. The models which best reproduce the physical properties of SMGs have a rapid build-up of dust from both stellar and interstellar sources and minimal dust destruction. Alternatively, invoking a top-heavy IMF or significant changes in the dust grain properties can solve the dust budget crisis only if dust is produced by both low-mass stars and supernovae and is not efficiently destroyed by supernova shocks.Peer reviewe
Cataclysmic variables from a ROSAT/2MASS selection. I, Four new intermediate polars
Full text linkWe report the first results from a new search for cataclysmic variables (CVs) using a combined X-ray (ROSAT)/infrared (2MASS) target selection that discriminates against background active galactic nuclei. Identification spectra were obtained at the Isaac Newton Telescope for a total of 174 targets, leading to the discovery of 12 new CVs. Initially devised to find short-period low-mass-transfer CVs, this selection scheme has been very successful in identifying new intermediate polars. Photometric and spectroscopic follow-up observations identify four of the new CVs as intermediate polars: 1RXS J063631.9+353537 P(orb)similar or equal to 201 min, P-spin= 1008.3408 s or 930.5829 s), 1RXS J070407.9+262501 (P(orb)similar or equal to 250 min, P-spin= 480.708 s) 1RXS J173021.5-055933 (P-orb= 925.27 min, P-spin= 128.0 s), and 1RXS J180340.0+401214 (P-orb= 160.21 min, P-spin= 1520.51 s). RX J1730, also a moderately bright hard X-ray source in the INTEGRAL/IBIS Galactic plane survey, resembles the enigmatic AE Aqr. It is likely that its white dwarf is not rotating at the spin equilibrium period, and the system may represent a short-lived phase in CV evolution
A scaling-relation for disc galaxies: circular-velocity gradient versus central surface brightness
Full text linkFor disc galaxies, a close relation exists between the distribution of light and the
shape of the rotation curve. We quantify this relation by measuring the inner circular-
velocity gradient dRV (0) for spiral and irregular galaxies with high-quality rotation
curves. We find that dRV (0) correlates with the central surface brightness μ0 over
more than two orders of magnitude in dRV (0) and four orders of magnitudes in μ0.
This is a scaling-relation for disc galaxies. It shows that the central stellar density of a
galaxy closely relates to the inner shape of the potential well, also for low-luminosity
and low-surface-brightness galaxies that are expected to be dominated by dark matter.
Key words: dark matter – galaxies: kinematics and dynamics – galaxies: structure
Early evolution of embedded clusters
Full text linkThis research was supported by the DFG cluster of excellence ‘Origin and Structure of the Universe’ (JED, BE).We examine the combined effects of winds and photoionizing radiation from O-type stars on embedded stellar clusters formed in model turbulent molecular clouds covering a range of masses and radii. We find that feedback is able to increase the quantities of dense gas present, but decreases the rate and efficiency of the conversion of gas to stars relative to control simulations in which feedback is absent. Star formation in these calculations often proceeds at a rate substantially slower than the freefall rate in the dense gas. This decoupling is due to the weakening of, and expulsion of gas from, the deepest parts of the clouds' potential wells where most of the star formation occurs in the control simulations. This results in large fractions of the stellar populations in the feedback simulation becoming dissociated from dense gas. However, where star formation does occur in both control and feedback simulations, it does so in dense gas, so the correlation between star formation activity and dense gas is preserved. The overall dynamical effects of feedback on the clusters are minimal, with only small fraction of stars becoming unbound, despite large quantities of gas being expelled from some clouds. This owes to the settling of the stars into virialized and stellar-dominated configurations before the onset of feedback. By contrast, the effects of feedback on the observable properties of the clusters - their U-, B- and V-band magnitudes - are strong and sudden. The time-scales on which the clusters become visible and unobscured are short compared with the time-scales which the clouds are actually destroyed.Peer reviewe
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