ORCID Profile
0000-0002-9610-5629
Current Organisations
University of Hertfordshire
,
University of Tasmania
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Publisher: Oxford University Press (OUP)
Date: 05-09-2012
Publisher: EDP Sciences
Date: 24-10-2016
Publisher: Oxford University Press (OUP)
Date: 30-07-2012
Publisher: Elsevier BV
Date: 02-2007
Publisher: EDP Sciences
Date: 06-2015
Publisher: Oxford University Press (OUP)
Date: 12-2009
Publisher: Oxford University Press (OUP)
Date: 12-02-2022
Abstract: We present a method for applying spatially resolved adiabatic and radiative loss processes to synthetic radio emission from hydrodynamic simulations of radio sources from active galactic nuclei (AGNs). Lagrangian tracer particles, each representing an ensemble of electrons, are injected into simulations and the position, grid pressure, and time since the last strong shock are recorded. These quantities are used to track the losses of the electron packet through the radio source in a manner similar to the Radio AGN in Semi-analytic Environments formalism, which uses global source properties to calculate the emissivity of each particle ex situ. Freedom in the choice of observing parameters, including redshift, is provided through the post-processing nature of this approach. We apply this framework to simulations of jets in different environments, including asymmetric ones. We find a strong dependence of radio source properties on frequency and redshift, in good agreement with observations and previous modelling work. There is a strong evolution of radio spectra with redshift due to the more prominent inverse-Compton losses at high redshift. Radio sources in denser environments have flatter spectral indices, suggesting that spectral index asymmetry may be a useful environment tracer. We simulate intermediate Mach number jets that disrupt before reaching the tip of the lobe, and find that these retain an edge-brightened Fanaroff–Riley Type II morphology, with the most prominent emission remaining near the tip of the lobes for all environments and redshifts we study.
Publisher: EDP Sciences
Date: 11-2013
Publisher: Cambridge University Press
Date: 28-10-2010
Publisher: Oxford University Press (OUP)
Date: 16-01-2021
Abstract: The ARCADE 2 balloon bolometer along with a number of other instruments have detected what appears to be a radio synchrotron background at frequencies below about 3 GHz. Neither extragalactic radio sources nor diffuse Galactic emission can currently account for this finding. We use the locally measured cosmic ray electron population, demodulated for effects of the Solar wind, and other observational constraints combined with a turbulent magnetic field model to predict the radio synchrotron emission for the Local Bubble. We find that the spectral index of the modelled radio emission is roughly consistent with the radio background. Our model can approximately reproduce the observed antenna temperatures for a mean magnetic field strength B between 3 and 5 nT. We argue that this would not violate observational constraints from pulsar measurements. However, the curvature in the predicted spectrum would mean that other, so far unknown sources would have to contribute below 100 MHz. Also, the magnetic energy density would then dominate over thermal and cosmic ray electron energy density, likely causing an inverse magnetic cascade with large variations of the radio emission in different sky directions as well as high polarization. We argue that this disagrees with several observations and thus that the magnetic field is probably much lower, quite possibly limited by equipartition with the energy density in relativistic or thermal particles (B = 0.2−0.6 nT). In the latter case, we predict a contribution of the Local Bubble to the unexplained radio background at most at the per cent level.
Publisher: American Astronomical Society
Date: 10-2014
Publisher: Oxford University Press (OUP)
Date: 06-03-2023
Abstract: Hotspots of radio galaxies are regions of shock-driven particle acceleration. Multiple hotspots have long been identified as potential indicators of jet movement or precession. Two frequent explanations describe a secondary hotspot as either the location of a prior jet termination point or a deflected backflow-driven shock: the so-called dentist’s drill and splatter-spot models. We created high-resolution simulations of precessing jets with a range of parameters. In addition to the existing mechanisms, our results show three additional mechanisms for multiple hotspot formation: (1) the splitting of a large terminal hotspot into passive and active components (2) jet stream splitting resulting in two active hotspots and (3) dynamic multiple hotspot complexes that form as a result of jet termination in a turbulent cocoon, linked here to rapid precession. We show that these distinct types of multiple hotspots are difficult to differentiate in synthetic radio maps, particularly hotspot complexes that can easily be mistaken for the jet itself. We discuss the implication for hypothesized binary supermassive black hole systems where jet precession is a key component of the morphology, and show a selection of potential precession candidates found using the Low-Frequency Array Two-metre Sky Survey Data Release 2.
Publisher: Cambridge University Press (CUP)
Date: 08-2009
DOI: 10.1017/S1743921310006526
Abstract: Recently, high-resolution observations made with the help of the near-infrared adaptive optics integral field spectrograph SINFONI at the VLT proved the existence of massive and young nuclear star clusters in the centers of a s le of Seyfert galaxies. With the help of high-resolution hydrodynamical simulations with the pluto code, we follow the evolution of such clusters, especially focusing on mass and energy feedback from young stars. This leads to a filamentary inflow of gas on large scales (tens of parsecs), whereas a turbulent and very dense disk builds up on the parsec scale. Here we concentrate on the long-term evolution of the nuclear disk in NGC 1068 with the help of an effective viscous disk model, using the mass input from the large-scale simulations and accounting for star formation in the disk. This two-stage modeling enables us to connect the tens-of-parsecs scale region (observable with SINFONI) with the parsec-scale environment (MIDI observations). At the current age of the nuclear star cluster, our simulations predict disk sizes of the order 0.8 to 0.9 pc, gas masses of order 10 6 M ⊙ , and mass transfer rates through the inner boundary of order 0.025 M ⊙ yr −1 , in good agreement with values derived from observations.
Publisher: EDP Sciences
Date: 28-01-2015
Publisher: EDP Sciences
Date: 03-2018
DOI: 10.1051/0004-6361/201731815
Abstract: Context. The space based γ -ray observatory INTEGRAL of the European Space Agency (ESA) includes the spectrometer instrument “SPI”. This is a coded mask telescope featuring a 19-element Germanium detector array for high-resolution γ -ray spectroscopy, encapsulated in a scintillation detector assembly that provides a veto for background from charged particles. In space, cosmic rays irradiate spacecraft and instruments, which, in spite of the vetoing detectors, results in a large instrumental background from activation of those materials, and leads to deterioration of the charge collection properties of the Ge detectors. Aim. We aim to determine the measurement characteristics of our detectors and their evolution with time, that is, their spectral response and instrumental background. These incur systematic variations in the SPI signal from celestial photons, hence their determination from a broad empirical database enables a reduction of underlying systematics in data analysis. For this, we explore compromises balancing temporal and spectral resolution within statistical limitations. Our goal is to enable modelling of background applicable to spectroscopic studies of the sky, accounting separately for changes of the spectral response and of instrumental background. Methods. We use 13.5 years of INTEGRAL/SPI data, which consist of spectra for each detector and for each pointing of the satellite. Spectral fits to each such spectrum, with independent but coherent treatment of continuum and line backgrounds, provides us with details about separated background components. From the strongest background lines, we first determine how the spectral response changes with time. Applying symmetry and long-term stability tests, we eliminate degeneracies and reduce statistical fluctuations of background parameters, with the aim of providing a self-consistent description of the spectral response for each in idual detector. Accounting for this, we then determine how the instrumental background components change in intensities and other characteristics, most-importantly their relative distribution among detectors. Results. Spectral resolution of Ge detectors in space degrades with time, up to 15% within half a year, consistently for all detectors, and across the SPI energy range. Semi-annual annealing operations recover these losses, yet there is a small long-term degradation. The intensity of instrumental background varies anti-correlated to solar activity, in general. There are significant differences among different lines and with respect to continuum. Background lines are found to have a characteristic, well-defined and long-term consistent intensity ratio among detectors. We use this to categorise lines in groups of similar behaviour. The dataset of spectral-response and background parameters as fitted across the INTEGRAL mission allows studies of SPI spectral response and background behaviour in a broad perspective, and efficiently supports precision modelling of instrumental background.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 05-09-2014
Abstract: Astronomers commonly use type-Ia supernovae as standard distance measurement tools, though the physics of these bright sources are not fully understood. One product of the thermonuclear explosion is 56 Ni, which probably lies at the heart of the supernova cloud. Diehl et al. detected the gamma-ray emission from 56 Ni in SN2014J much earlier than expected, only about 20 days after the initial explosion. This early exposure suggests both an asymmetric event and the production of 56 Ni farther out in the ejecta than predicted. These sources will help astronomers measure distances far beyond where supernovae have been studied. Science , this issue p. 1162
Publisher: EDP Sciences
Date: 31-08-2012
Publisher: Oxford University Press (OUP)
Date: 30-04-2020
Abstract: Feedback from radio jets associated with active galactic nuclei (AGNs) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant, and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGNs in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant, and restarted radio source populations. Consistent with other recent work, our models predict that remnant radio lobes fade quickly. Any high (& per cent) observed fraction of remnant and restarted sources therefore requires a dominant population of short-lived jets. We speculate that this could plausibly be provided by feedback-regulated accretion.
Publisher: Oxford University Press (OUP)
Date: 07-09-2023
Publisher: Oxford University Press (OUP)
Date: 03-10-2019
Abstract: Emission from the radioactive trace element 26Al has been observed throughout the Milky Way with the COMPTEL and INTEGRAL satellites. In particular, the Doppler shifts measured with INTEGRAL connect 26Al with superbubbles, which may guide 26Al flows off spiral arms in the direction of Galactic rotation. In order to test this paradigm, we have performed galaxy-scale simulations of superbubbles with 26Al injection in a Milky Way-type galaxy. We produce all-sky synthetic γ-ray emission maps of the simulated galaxies. We find that the 1809 keV emission from the radioactive decay of 26Al is highly variable with time and the observer’s position. This allows us to estimate an additional systematic variability of 0.2 dex for a star formation rate derived from 26Al for different times and measurement locations in Milky Way-type galaxies. High-latitude morphological features indicate nearby emission with correspondingly high-integrated γ-ray intensities. We demonstrate that the 26Al scale height from our simulated galaxies depends on the assumed halo gas density. We present the first synthetic 1809 keV longitude-velocity diagrams from 3D hydrodynamic simulations. The line-of-sight velocities for 26Al can be significantly different from the line-of-sight velocities associated with the cold gas. Over time, 26Al velocities consistent with the INTEGRAL observations, within uncertainties, appear at any given longitude, broadly supporting previous suggestions that 26Al injected into expanding superbubbles by massive stars may be responsible for the high velocities found in the INTEGRAL observations. We discuss the effect of systematically varying the location of the superbubbles relative to the spiral arms.
Publisher: EDP Sciences
Date: 07-2019
DOI: 10.1051/0004-6361/201935762
Abstract: Context. Gaseous halos play a key role in understanding inflow, feedback, and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains, and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large s les of galaxies. The LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS) has identified a galaxy stellar mass scale, 10 11 M ⊙ , above which the radio luminosities increase disproportionately. Aims. We investigate if radio luminosities of galaxies, especially the marked rise at galaxy masses around 10 11 M ⊙ , can be explained with standard assumptions regarding jet powers, scaling between black hole mass and galaxy mass, and gaseous halos. Methods. Based on observational data and theoretical constraints, we developed models for the radio luminosity of radio AGN in halos under infall, galactic wind, and hydrostatic conditions. We compared these models to LoTSS data for a large s le of galaxies in the mass range between 10 8.5 M ⊙ and 10 12 M ⊙ . Results. Under the assumption that the same characteristic upper limit to jet powers known from high galaxy masses holds at all masses, we find the maximum radio luminosities for the hydrostatic gas halos to lie close to the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at 10 11 M ⊙ is matched in our model and is related to a significant change in halo gas density around this galaxy mass, which is a consequence of lower cooling rates at a higher virial temperature. Wind and infall models overpredict the radio luminosities for small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. Conclusions. Radio AGN could have the same characteristic Eddington-scaled upper limit to jet powers in galaxies of all masses in the s le if the galaxies have hydrostatic gas halos in phases when radio AGN are active. We find no evidence of a change of the type of galaxy halo with the galaxy mass. Galactic winds and quasi-spherical cosmological inflow phases cannot frequently occur at the same time as powerful jet episodes unless the jet properties in these phases are significantly different from what we assumed in our model.
Publisher: EDP Sciences
Date: 09-2012
Publisher: EDP Sciences
Date: 28-01-2016
Publisher: EDP Sciences
Date: 10-2012
Publisher: EDP Sciences
Date: 07-2015
Publisher: Oxford University Press (OUP)
Date: 15-11-2017
Publisher: Oxford University Press (OUP)
Date: 09-08-2018
Publisher: EDP Sciences
Date: 17-11-2015
Publisher: Oxford University Press (OUP)
Date: 26-10-2019
Abstract: We examine the remnant phase of radio galaxies using three-dimensional hydrodynamical simulations of relativistic jets propagating through cluster environments. By switching the jets off once the lobes have reached a certain length we can study how the energy distribution between the lobes and shocked intracluster medium compares to that of an active source, as well as calculate synchrotron emission properties of the remnant sources. We see that as a result of disturbed cluster gas beginning to settle back into the initial cluster potential, streams of dense gas are pushed along the jet axis behind the remnant lobes, causing them to rise out of the cluster faster than they would due to buoyancy. This leads to increased adiabatic losses and a rapid dimming. The rapid decay of total flux density and surface brightness may explain the small number of remnant sources found in s les with a high flux density limit and may cause analytic models to overestimate the remnant fraction expected in sensitive surveys such as those now being carried out with LOFAR.
Publisher: Oxford University Press (OUP)
Date: 18-05-2011
Publisher: Oxford University Press (OUP)
Date: 21-12-2009
Publisher: Springer Science and Business Media LLC
Date: 29-02-2016
DOI: 10.1038/NATURE16978
Abstract: Microquasars are stellar-mass black holes accreting matter from a companion star and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 10(6) to 10(10) solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron-positron plasma. Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron-positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.
Publisher: Oxford University Press (OUP)
Date: 21-07-2014
Publisher: Oxford University Press (OUP)
Date: 11-09-2022
Abstract: A strong nuclear kilomaser, W1, has been found in the nearby galaxy NGC 253, associated with a forming super star cluster. Kilomasers could arise from the accretion disc around supermassive stars (& M⊙), hypothetical objects that have been proposed as polluters responsible for the chemical peculiarities in globular clusters. The supermassive stars would form via runaway collisions, simultaneously with the cluster. Their discs are perturbed by stellar flybys, inspiralling and colliding stars. This raises the question if an accretion disc would at all be able to survive in such a dynamic environment and mase water lines. We investigated what the predicted maser spectrum of such a disc would look like using 2D hydrodynamic simulations and compared this to the W1 kilomaser. We derived model maser spectra from the simulations by using a general maser model for appropriate disc temperatures. All our model discs survived. The model maser spectra for the most destructive case for the simulations of M⋆ = 1000 M⊙ are a reasonable match with the W1 kilomaser spectrum in terms of scaling, flux values and some of the signal trends. Details in the spectrum suggest that a star of a few 1000 M⊙ might fit even better, with 10 000 M⊙ clearly giving too large velocities. Our investigations thus support the hypothesis that kilomasers could pinpoint supermassive stars.
Publisher: EDP Sciences
Date: 07-2019
DOI: 10.1051/0004-6361/201833856
Abstract: Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ -ray line signal in the sky. For the past 50 yr, this 511 keV emission – predominantly from the galactic bulge region and from a low surface-brightness disk – has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (≲10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this “positron puzzle” by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derived spectra for in idual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determined Doppler-broadening from γ -ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We found line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2–3 km s −1 deg −1 ), and measurements of radioactive 26 Al (7.5–9.5 km s −1 deg −1 ). The velocity gradient in the inner ±30° is determined to be 4 ± 6 km s −1 deg −1 . The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave the question open of whether positron annihilation is associated with the high velocities seen in 26 Al or rather with ordinarily rotating components of the Milky Way’s interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation and constrain bulk Doppler-broadening contributions to ≲1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.
Publisher: EDP Sciences
Date: 10-2014
Publisher: Oxford University Press (OUP)
Date: 21-04-2010
Publisher: Oxford University Press (OUP)
Date: 09-10-2017
Publisher: Oxford University Press (OUP)
Date: 22-11-2014
Publisher: Oxford University Press (OUP)
Date: 26-04-2018
Publisher: Oxford University Press (OUP)
Date: 04-2007
Publisher: Cambridge University Press (CUP)
Date: 09-2010
DOI: 10.1017/S1743921310015899
Abstract: 3D-MHD numerical simulations of bipolar, hypersonic, weakly magnetized jets and synthetic synchrotron observations are presented to study the structure and evolution of magnetic fields in FR II radio sources. The magnetic field setup in the jet is initially random. The power of the jets as well as the observational viewing angle are investigated. We find that synthetic polarization maps agree with observations and show that magnetic fields inside the sources are shaped by the jets' backflow. Polarimetry statistics correlates with time, the viewing angle and the jet-to-ambient density contrast. The magnetic structure inside thin elongated sources is more uniform than for ones with fatter cocoons. Jets increase the magnetic energy in cocoons, in proportion to the jet velocity. Both, filaments in synthetic emission maps and 3D magnetic power spectra suggest that turbulence develops in evolved sources.
Publisher: Oxford University Press (OUP)
Date: 04-11-2010
Publisher: Oxford University Press (OUP)
Date: 26-09-2023
Publisher: EDP Sciences
Date: 06-2005
Publisher: EDP Sciences
Date: 02-2023
DOI: 10.1051/0004-6361/202346008
Abstract: Context. The existence of magnetic fields in the circumgalactic medium (CGM) is largely unconstrained. Their detection is important as magnetic fields can have a significant impact on the evolution of the CGM, and, in turn, the fields can serve as tracers for dynamical processes in the CGM. Aims. Using the Faraday rotation of polarised background sources, we aim to detect a possible excess of the rotation measure in the surrounding area of nearby galaxies. Methods. We used 2461 residual rotation measures (RRMs) observed with the LOw Frequency ARray (LOFAR), where the foreground contribution from the Milky Way is subtracted. The RRMs were then studied around a subset of 183 nearby galaxies that was selected by apparent B -band magnitude. Results. We find that, in general, the RRMs show no significant excess for small impact parameters (i.e., the perpendicular distance to the line of sight). However, if we only consider galaxies at higher inclination angles and sightlines that pass close to the minor axis of the galaxies, we find significant excess at impact parameters of less than 100 kpc. The excess in |RRM| is 3.7 rad m −2 with an uncertainty between ±0.9 rad m −2 and ±1.3 rad m −2 depending on the statistical properties of the background (2.8 σ –4.1 σ ). With electron densities of ∼10 −4 cm −3 , this suggests magnetic field strengths of a few tenths of a microgauss. Conclusions. Our results suggest a slow decrease in the magnetic field strength with distance from the galactic disc, as expected if the CGM is magnetised by galactic winds and outflows.
Publisher: Elsevier BV
Date: 10-2003
Publisher: Springer Science and Business Media LLC
Date: 2004
Publisher: Oxford University Press (OUP)
Date: 24-01-2013
DOI: 10.1093/MNRAS/STS564
Publisher: Oxford University Press (OUP)
Date: 24-11-2020
Abstract: Radioactive 26Al is an excellent tracer for metal ejection in the Milky Way, and can provide a direct constraint on the modelling of supernova feedback in galaxy evolution. Gamma-ray observations of the 26Al decay line have found high velocities and hence require a significant fraction of the Galactic 26Al in the hot component. At the same time, meteoritic data combined with simulation results suggest that a significant amount of 26Al makes its way into stars before decay. We investigated the distribution into hot and cold channels with a simulation of a Milky-Way-like galaxy with massive-star feedback in superbubbles and with ejecta traced by 26Al. About 30–40 per cent of the ejecta remain hot, with typical cooling times of the order Gyr. 26Al traces the footpoints of a chimney-fed outflow that mixes metals turbulently into the halo of the model galaxy on a scale of at least 50 kpc. The rest diffuses into cold gas ≲ 104 K, and may therefore be quickly available for star formation. We discuss the robustness of the result by comparison to a simulation with a different global flow pattern. The branching ratio into hot and cold components is comparable to that of longer term average results from chemical evolution modelling of galaxies, clusters, and the intracluster medium.
Publisher: Oxford University Press (OUP)
Date: 23-11-2017
Publisher: EDP Sciences
Date: 30-03-2023
DOI: 10.1051/0004-6361/202244457
Abstract: Diffuse γ -ray line emission traces freshly produced radioisotopes in the interstellar gas, providing a unique perspective on the entire Galactic cycle of matter from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium (ISM). We aim to construct a model of nucleosynthesis ejecta on a galactic scale that is specifically tailored to complement the physically most important and empirically accessible features of γ -ray measurements in the MeV range, in particular for decay γ rays such as 26 Al, 60 Fe, or 44 Ti. Based on properties of massive star groups, we developed a Population SYnthesis COde (PSYCO), which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate (SFR), density profiles, or stellar evolution models. As a result, we obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the population synthesis calculations of in idual massive star groups. Based on a variety of stellar evolution models, supernova (SN) explodabilities, and density distributions, we find that the measured 26 Al distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a SFR of 4–8 M ⊙ yr −1 and a spiral-arm-dominated density profile with a scale height of at least 700 pc. Our model requires that most massive stars indeed undergo a SN explosion. This corresponds to a SN rate in the Milky Way of 1.8–2.8 per century, with quasi-persistent 26 Al and 60 Fe masses of 1.2–2.4 M ⊙ and 1–6 M ⊙ , respectively. Comparing the simulated morphologies to SPI data suggests that a frequent merging of superbubbles may take place in the Galaxy, and that an unknown yet strong foreground emission at 1.8 MeV could be present.
Publisher: Cambridge University Press (CUP)
Date: 2021
DOI: 10.1017/PASA.2021.48
Abstract: The cosmic evolution of the chemical elements from the Big Bang to the present time is driven by nuclear fusion reactions inside stars and stellar explosions. A cycle of matter recurrently re-processes metal-enriched stellar ejecta into the next generation of stars. The study of cosmic nucleosynthesis and this matter cycle requires the understanding of the physics of nuclear reactions, of the conditions at which the nuclear reactions are activated inside the stars and stellar explosions, of the stellar ejection mechanisms through winds and explosions, and of the transport of the ejecta towards the next cycle, from hot plasma to cold, star-forming gas. Due to the long timescales of stellar evolution, and because of the infrequent occurrence of stellar explosions, observational studies are challenging, as they have biases in time and space as well as different sensitivities related to the various astronomical methods. Here, we describe in detail the astrophysical and nuclear-physical processes involved in creating two radioactive isotopes useful in such studies, $^{26}\\mathrm{Al}$ and $^{60}\\mathrm{Fe}$ . Due to their radioactive lifetime of the order of a million years, these isotopes are suitable to characterise simultaneously the processes of nuclear fusion reactions and of interstellar transport. We describe and discuss the nuclear reactions involved in the production and destruction of $^{26}\\mathrm{Al}$ and $^{60}\\mathrm{Fe}$ , the key characteristics of the stellar sites of their nucleosynthesis and their interstellar journey after ejection from the nucleosynthesis sites. This allows us to connect the theoretical astrophysical aspects to the variety of astronomical messengers presented here, from stardust and cosmic-ray composition measurements, through observation of $\\gamma$ rays produced by radioactivity, to material deposited in deep-sea ocean crusts and to the inferred composition of the first solids that have formed in the Solar System. We show that considering measurements of the isotopic ratio of $^{26}\\mathrm{Al}$ to $^{60}\\mathrm{Fe}$ eliminate some of the unknowns when interpreting astronomical results, and discuss the lessons learned from these two isotopes on cosmic chemical evolution. This review paper has emerged from an ISSI-BJ Team project in 2017–2019, bringing together nuclear physicists, astronomers, and astrophysicists in this inter-disciplinary discussion.
Publisher: EDP Sciences
Date: 12-2001
Publisher: Springer Science and Business Media LLC
Date: 06-2020
Publisher: Oxford University Press (OUP)
Date: 10-06-2022
Abstract: We report new observations of ‘Hanny’s Voorwerp’ (hereafter HV) taken from the second data release of the LOFAR Two-metre Sky Survey (LoTSS). HV is a highly-ionized region in the environs of the galaxy IC2497, first discovered by the Galaxy Zoo project. The new 150 MHz observations are considered in the context of existing multi frequency radio data and archival narrow-band imaging from the Hubble Space Telescope, centred on the [O iii] emission line. The combined sensitivity and spatial resolution of the LoTSS data – which far exceed what was previously available at radio frequencies – reveal clear evidence for large-scale extended emission emanating from the nucleus of IC2497. The radio jet appears to have punched a hole in the neutral gas halo, in a region co-located with HV. The new 150 MHz data, alongside newly-processed archival 1.64 GHz eVLA data, reveal that the extended emission has a steep spectrum, implying an age & yr. The jet supplying the extended 150 MHz structure must have ‘turned off’ long before the change in X-ray luminosity reported in recent works. In this picture, a combination of jet activity and the influence of the radiatively efficient active galactic nucleus are responsible for the unusual appearance of HV.
Publisher: Springer-Verlag
Date: 2005
DOI: 10.1007/11403913_8
Publisher: EDP Sciences
Date: 09-2014
Publisher: Oxford University Press (OUP)
Date: 08-2011
Publisher: Cambridge University Press (CUP)
Date: 08-2012
DOI: 10.1017/S1743921314005663
Abstract: We summarize the results presented in Krause et al . (2012a, K12) on the impact of supernova-driven shells and dark-remnant accretion on gas expulsion in globular cluster infancy.
Publisher: Oxford University Press (OUP)
Date: 21-09-2018
Publisher: EDP Sciences
Date: 16-02-2016
Publisher: Oxford University Press (OUP)
Date: 21-12-2012
Publisher: EDP Sciences
Date: 04-2002
Publisher: Oxford University Press (OUP)
Date: 21-01-2007
Publisher: EDP Sciences
Date: 24-01-2013
Publisher: EDP Sciences
Date: 02-2005
Publisher: Elsevier BV
Date: 06-2021
Publisher: Oxford University Press (OUP)
Date: 13-10-2021
Abstract: We have carried out relativistic three-dimensional simulations of high-power radio sources propagating i,nto asymmetric cluster environments. We offset the environment by 0 or 1 core radii (equal to 144 kpc), and incline the jets by 0, 15, or 45° away from the environment centre. The different environment encountered by each radio lobe provides a unique opportunity to study the effect of environment on otherwise identical jets. We find that the jets become unstable towards the end of the simulations, even with a Lorentz factor of 5 they nevertheless develop typical Fanaroff–Riley class II radio morphology. The jets propagating into denser environments have consistently shorter lobe lengths and brighter hotspots, while the axial ratio of the two lobes is similar. We reproduce the recently reported observational anticorrelation between lobe length asymmetry and environment asymmetry, corroborating the notion that observed large-scale radio lobe asymmetry can be driven by differences in the underlying environment.
Publisher: Oxford University Press (OUP)
Date: 09-02-2022
Abstract: Simulations are very useful for testing our theoretical understanding of star formation by varying the initial conditions or treatment of various physical mechanisms. However, large well-resolved simulations including complex physics are computationally costly and therefore are normally only performed once. This leads to a crisis in modelling, because star formation is a chaotic system, where a small variation in initial conditions can be magnified to a large change in results. We create a suite of cluster-scale simulations with 30 different random realizations of the turbulent velocity field applied to the same initial conditions of an isolated spherical molecular cloud. We test commonly used metrics of star formation activity and cloud structure and measure the variance cause by this random variation in initial conditions to quantify the error that should be applied to single-realization simulations. We find that after 5 Myr the number of stars varies by 58 per cent of the mean, the star formation efficiency by 60 per cent of the mean, and the shape of the column density PDF by 7 per cent of the mean. We provide a standard deviation at different times that should be applied as an error margin on all single realization simulations to enable robust statistical comparison.
Publisher: Oxford University Press (OUP)
Date: 07-2013
Publisher: Oxford University Press (OUP)
Date: 20-01-2020
Abstract: A search of the first Data Release of the VISTA Variables in the Via Lactea (VVV) Survey discovered the exceptionally red transient VVV-WIT-01 (H − Ks = 5.2). It peaked before March 2010, then faded by ∼9.5 mag over the following 2 yr. The 1.6–22 μm spectral energy distribution in March 2010 was well fit by a highly obscured blackbody with T ∼ 1000 K and $A_{K_s} \\sim 6.6$ mag. The source is projected against the Infrared Dark Cloud (IRDC) SDC G331.062−0.294. The chance projection probability is small for any single event (p ≈ 0.01–0.02), which suggests a physical association, e.g. a collision between low mass protostars. However, blackbody emission at T ∼ 1000 K is common in classical novae (especially CO novae) at the infrared peak in the light curve due to condensation of dust ∼30–60 d after the explosion. Radio follow-up with the Australia Telescope Compact Array detected a fading continuum source with properties consistent with a classical nova but probably inconsistent with colliding protostars. Considering all VVV transients that could have been projected against a catalogued IRDC raises the probability of a chance association to p = 0.13–0.24. After weighing several options, it appears likely that VVV-WIT-01 was a classical nova event located behind an IRDC.
Publisher: Oxford University Press (OUP)
Date: 07-01-2022
Abstract: Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation c aign data set to obtain γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25–8000 keV, and we derive a flux limit of ${\\lesssim}10^{-8}\\, \\mathrm{erg\\, cm^{-2}\\, s^{-1}}$. The previously reported 511 keV line is not seen, and we find a flux limit of ${\\lesssim}1.7 \\times 10^{-4}\\, \\mathrm{ph\\, cm^{-2}\\, s^{-1}}$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e+s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses ${\\lesssim}8 \\times 10^{15}\\, \\mathrm{g}$. Our limits on the velocity-averaged DM annihilation cross section into e+e− are $\\langle \\sigma v \\rangle \\lesssim 5 \\times 10^{-28} \\left(m_{\\rm DM} / \\mathrm{MeV} \\right)^{2.5}\\, \\mathrm{cm^3\\, s^{-1}}$. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies.
Publisher: Oxford University Press (OUP)
Date: 17-12-2015
Publisher: EDP Sciences
Date: 09-2015
Publisher: EDP Sciences
Date: 11-2018
DOI: 10.1051/0004-6361/201732416
Abstract: Context. Feedback by massive stars shapes the interstellar medium and is thought to influence subsequent star formation. The details of this process are under debate. Aims. We exploited observational constraints on stars, gas, and nucleosynthesis ashes for the closest region with recent massive-star formation, Scorpius–Centaurus OB2, and combined them with three-dimensional (3D) hydrodynamical simulations in order to address the physics and history of the Scorpius–Centaurus superbubble. Methods. We used published cold gas observations of continuum and molecular lines from Planck , Herschel , and APEX. We analysed the Galactic All Sky Survey (GASS) to investigate shell structures in atomic hydrogen, and used Hipparcos and Gaia data in combination with interstellar absorption against stars to obtain new constraints for the distance to the Hi features. Hot gas is traced in soft X-rays via the ROSAT all sky survey. Nucleosynthesis ejecta from massive stars were traced with new INTEGRAL spectrometer observations via 26 Al radioactivity. We also performed 3D hydrodynamical simulations for the Sco–Cen superbubble. Results. Soft X-rays and a now more significant detection of 26 Al confirm recent (≈1 Myr ago) input of mass, energy, and nucleosynthesis ejecta, likely from a supernova in the Upper Scorpius (USco) subgroup. We confirm a large supershell around the entire OB association and perform a 3D hydrodynamics simulation with a conservative massive star population that reproduces the morphology of the superbubble. High-resolution GASS observations reveal a nested, filamentary supershell. The filaments are possibly related to the Vishniac clumping instability, but molecular gas (Lupus I) is only present where the shell coincides with the connecting line between the subgroups of the OB association, suggesting a connection to the cloud, probably an elongated sheet, out of which the OB association formed. Stars have formed sequentially in the subgroups of the OB association and currently form in Lupus I. To investigate the impact of massive star feedback on extended clouds, we simulate the interaction of a turbulent cloud with the hot, pressurised gas in a superbubble. The hot gas fills the tenuous regions of the cloud and compresses the denser parts. Stars formed in these dense clumps would have distinct spatial and kinematic distributions. Conclusions. The combined results from observations and simulations are consistent with a scenario where dense gas was initially distributed in a band elongated in the direction now occupied by the OB association. Superbubbles powered by massive stars would then repeatedly break out of the elongated parent cloud, and surround and squash the denser parts of the gas sheet and thus induce more star formation. The expected spatial and kinematic distribution of stars is consistent with observations of Sco–Cen. The scenario might apply to many similar regions in the Galaxy and also to active galactic nucleus (AGN)-related superbubbles.
Publisher: Oxford University Press (OUP)
Date: 02-10-2020
Abstract: The prospect of relativistic jets exhibiting complex morphologies as a consequence of geodetic precession has long been hypothesized. We have carried out a 3D hydrodynamics simulation study varying the precession cone angle, jet injection speed, and number of turns per simulation time. Using proxies for the radio emission we project the sources with different inclinations to the line of sight to the observer. We find that a number of different precession combinations result in characteristic ‘X’ shaped sources which are frequently observed in radio data, and some precessing jet morphologies may mimic the morphological signatures of restarting radio sources. We look at jets ranging in scale from tens to hundreds of kiloparsecs and develop tools for identifying known precession indicators of point symmetry, curvature, and jet misalignment from the lobe axis and show that, based on our simulation s le of precessing and non-precessing jets, a radio source that displays any of these indicators has a 98 per cent chance of being a precessing source.
Publisher: Oxford University Press (OUP)
Date: 14-02-2020
Abstract: Jet precession can reveal the presence of binary systems of supermassive black holes. The ability to accurately measure the parameters of jet precession from radio-loud active galactic nuclei is important for constraining the binary supermassive black hole population, which is expected as a result of hierarchical galaxy evolution. The age, morphology, and orientation along the line of sight of a given source often result in uncertainties regarding the jet path. This paper presents a new approach for efficient determination of precession parameters using a two-dimensional Markov chain Monte Carlo curve-fitting algorithm that provides us a full posterior probability distribution on the fitted parameters. Applying the method to Cygnus A, we find evidence for previous suggestions that the source is precessing. Interpreting in the context of binary black holes leads to a constraint of parsec scale and likely sub-parsec orbital separation for the putative supermassive binary.
Publisher: Cambridge University Press (CUP)
Date: 08-2015
DOI: 10.1017/S1743921315010649
Abstract: The origin of Galactic halo stars and the contribution of globular clusters (GC) to this stellar population have long been (and still are) debated. The discovery of multiple stellar populations with peculiar chemical properties in GCs both in the Milky Way and in Local Group galaxies recently brought a renewal on these questions. Indeed most of the scenarios that compete to reproduce the present-day GC characteristics call for fast expulsion of both gas and low-mass stars from these clusters in their early infancy. In this framework, the initial masses of GCs could have been 8 to 25 times higher than their present-day stellar mass, and they could have contributed to 5 to 20 % of the low-mass stars in the Galactic halo. Here we revisit these conclusions, which are in tension with observations of dwarf galaxies and of young massive star clusters in the Local Group. We come back in particular on the paradigm of gas expulsion from massive star clusters, and propose an alternative interpretation of the GC abundance properties. We conclude by proposing a major revision of the current concepts regarding the role massive star clusters play in the assembly of galactic haloes.
Publisher: EDP Sciences
Date: 06-2014
Publisher: EDP Sciences
Date: 07-2014
Publisher: Oxford University Press (OUP)
Date: 26-10-2010
Publisher: Oxford University Press (OUP)
Date: 14-11-2011
Publisher: EDP Sciences
Date: 24-10-2016
Publisher: EDP Sciences
Date: 2010
DOI: 10.1051/EAS/1044012
Publisher: EDP Sciences
Date: 04-2013
Publisher: EDP Sciences
Date: 12-2019
DOI: 10.1051/0004-6361/201935911
Abstract: Context. The diffuse gamma-ray emission of 26 Al at 1.8 MeV reflects ongoing nucleosynthesis in the Milky Way and traces massive-star feedback in the interstellar medium due to its 1 Myr radioactive lifetime. The morphology and dynamics of the interstellar medium are investigated in astrophysics through 3D hydrodynamic simulations in fine detail as there are few suitable astronomical probes available. Aims. We aim to compare a galactic-scale hydrodynamic simulation of the Galaxy’s interstellar medium, including feedback and nucleosynthesis, with gamma-ray data on 26 Al emission in the Milky Way, extracting constraints that are only weakly dependent on the particular realisation of the simulation or Galaxy structure. Methods. Due to constraints and biases in both the simulations and the gamma-ray observations, such comparisons are not straightforward. For a direct comparison, we performed maximum likelihood fits of both simulated sky maps and observation-based maximum entropy maps to measurements using INTEGRAL/SPI. In order to study general morphological properties, we compare the scale heights of 26 Al emission produced by the simulation to INTEGRAL/SPI measurements. Results. The direct comparison shows that the simulation describes the observed inner Galaxy well, however it differs significantly from the observed full-sky emission morphology. Comparing the scale height distribution, we see similarities for small-scale height features and a mismatch at larger-scale heights. We attribute this to prominent foreground emission sites which are not captured by the simulation.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Martin Krause.