ABSTRACT Observation shows that nebular emission, molecular gas, and young stars in giant galaxies are associated with rising X-ray bubbles inflated by radio jets launched from nuclear black holes. ...We propose a model where molecular clouds condense from low-entropy gas caught in the updraft of rising X-ray bubbles. The low-entropy gas becomes thermally unstable when it is lifted to an altitude where its cooling time is shorter than the time required to fall to its equilibrium location in the galaxy, i.e., . The infall speed of a cloud is bounded by the lesser of its free-fall and terminal speeds, so that the infall time here can exceed the free-fall time by a significant factor. This mechanism is motivated by Atacama Large Millimeter Array observations revealing molecular clouds lying in the wakes of rising X-ray bubbles with velocities well below their free-fall speeds. Our mechanism would provide cold gas needed to fuel a feedback loop while stabilizing the atmosphere on larger scales. The observed cooling time threshold of -the clear-cut signature of thermal instability and the onset of nebular emission and star formation-may result from the limited ability of radio bubbles to lift low-entropy gas to altitudes where thermal instabilities can ensue. Outflowing molecular clouds are unlikely to escape, but instead return to the central galaxy in a circulating flow. We contrast our mechanism to precipitation models where the minimum value of triggers thermal instability, which we find to be inconsistent with observation.
Abstract
The illumination pattern (or emissivity profile) of the accretion disc due to the reflection of X-rays in active galactic nucleus can be understood in terms of relativistic effects on the ...rays propagating from a source in a corona surrounding the central black hole, both on their trajectories and on the accretion disc itself. Theoretical emissivity profiles due to isotropic point sources as well as simple extended geometries are computed in general relativistic ray-tracing simulations performed on graphics processing units (GPUs). Such simulations assuming only general relativity naturally explain the accretion disc emissivity profiles determined from relativistically broadened emission lines which fall off steeply (with power-law indices of between 6 and 8) over the inner regions of the disc, then flattening off to almost a constant before tending to a constant power law of index 3 over the outer disc. Simulations for a variety of source locations, extents and geometries show how the emissivity profiles depend on these properties, and when combined with reverberation time lags allow the location and extent of the primary X-ray source to be constrained. Comparing the emissivity profile determined from the broadened iron K emission line in spectra of 1H 0707−495 obtained in 2008 January to theoretical emissivity profiles and applying constraints from reverberation lags suggest that there exists an extended region of primary X-ray emission located as low as 2r
g above the accretion disc, extending outwards to a radius of around 30r
g.
The X‐ray background can be used to constrain the accretion history of supermassive black holes (SMBHs) in active galactic nuclei (AGN), with the SMBH mass density related to the energy density due ...to accretion. A knowledge of the hard X‐ray bolometric correction, κ2‐10 keV, is a vital input into these studies, as it allows us to constrain the parameters of the accretion responsible for SMBH growth. Earlier studies assumed a constant bolometric correction for all AGN, and more recent work has suggested accounting for a dependence on AGN luminosity. Until recently, the variations in the disc emission in the ultraviolet (UV) have not been taken into account in this calculation; we show that such variations are important by construction of optical‐to‐X‐ray spectral energy distributions for 54 AGN. In particular, we use Far Ultraviolet Spectroscopic Explorer (FUSE) UV and X‐ray data from the literature to constrain the disc emission as well as possible. We find evidence for very significant spread in the bolometric corrections, with no simple dependence on luminosity being evident. Populations of AGN such as narrow‐line Seyfert 1 nuclei, radio‐loud and X‐ray‐weak AGN may have bolometric corrections which differ systematically from the rest of the AGN population. We identify other sources of uncertainty including intrinsic extinction in the optical–UV, X‐ray and UV variability and uncertainties in SMBH mass estimates. Our results suggest a more well‐defined relationship between the bolometric correction and Eddington ratio in AGN, with a transitional region at an Eddington ratio of ∼0.1, below which the bolometric correction is typically 15–25, and above which it is typically 40–70. We consider the potential‐implied parallels with the low/hard and high/soft states in Galactic black hole (GBH) accretion, and present bolometric corrections for the GBH binary GX 339−4 for comparison. Our findings reinforce previous studies proposing a multistate description of AGN accretion analogous to that for GBH binaries. Future calculations of the SMBH mass density may need to take into account the possible dependence of κ2‐10 keV on the Eddington ratio.
Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG-6-30-15 (ref. 1), broad iron K lines have been found in emission from several other Seyfert galaxies, from ...accreting stellar-mass black holes and even from accreting neutron stars. The iron K line is prominent in the reflection spectrum created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion of the line makes it sensitive to the strong gravity and spin of the black hole. The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.
We present broadband X-ray analyses of a sample of bright ultraluminous X-ray sources (ULX) with the goal of investigating the spectral similarity of this population to the known ULX pulsars, M82 ...X-2, NGC 7793 P13, and NGC 5907 ULX. We perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR data set of NGC 5907 ULX, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both M82 X-2 and NGC 7793 P13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. We then demonstrate that similar "hard" excesses are seen in all ULXs in the broadband sample. Finally, for ULXs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in M82 X-2, NGC 7793 P13, and NGC 5907 ULX. Based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ULX sample may be dominated by neutron star accretors. Compared to the known pulsar ULXs, our spectral fits for the remaining ULXs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. If these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations.
We present the drastic transformation of the X-ray properties of the active galactic nucleus (AGN) 1ES 1927+654, following a changing-look event. After the optical/ultraviolet outburst the power-law ...component, produced in the X-ray corona, disappeared, and the spectrum of 1ES 1927+65 instead became dominated by a blackbody component (kT ∼ 80-120 eV). This implies that the X-ray corona, ubiquitously found in AGNs, was destroyed in the event. Our dense ∼450 days long X-ray monitoring shows that the source is extremely variable in the X-ray band. On long timescales the source varies up to ∼4 dex in ∼100 days, while on short timescales up to ∼2 dex in ∼8 hr. The luminosity of the source is found to first show a strong dip down to , and then a constant increase in luminosity to levels exceeding the pre-outburst level 300 days after the optical event detection, rising up asymptotically to . As the X-ray luminosity of the source increases, the X-ray corona is recreated, and a very steep power-law component (Γ 3) reappears, and dominates the emission for 0.3-2 keV luminosities , ∼300 days after the beginning of the event. We discuss possible origins of this event, and speculate that our observations could be explained by the interaction between the accretion flow and debris from a tidally disrupted star. Our results show that changing-look events can be associated with dramatic and rapid transformations of the innermost regions of accreting supermassive black holes.
Galaxy-scale outflows, which are thought to provide the link connecting the central black hole to its host galaxy, are now starting to be observed. However, the physical origin of the mechanism ...driving the observed outflows, whether due to energy-driving or radiation-driving, is still debated; and in some cases, it is not clear whether the central source is an active galactic nucleus (AGN) or a nuclear starburst. Here, we study the role of radiation pressure on dust in driving galactic-scale AGN outflows, and analyse the dynamics of the outflowing shell as a function of the underlying physical parameters. We show that high-velocity outflows (≳1000 km s−1) with large momentum flux (≳10 L/c) can be obtained, by taking into account the effects of radiation trapping. In particular, the high observed values of the momentum boosts can be reproduced, provided that the shell is initially optically thick to the reprocessed infrared radiation. Alternatively, the inferred measurements of the momentum flux may be significantly biased by AGN variability. In this context, the observations of powerful outflows on kiloparsec scales, with no or weak signs of ongoing nuclear activity at the present time, could be re-interpreted as relics of past AGN episodes.
We present a multiwavelength morphological analysis of star-forming clouds and filaments in the central (≲50 kpc) regions of 16 low-redshift (z < 0.3) cool core brightest cluster galaxies. New Hubble ...Space Telescope imaging of far-ultraviolet continuum emission from young (≲10 Myr), massive (≳5 M⊙) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Lyα, narrow-band Hα, broad-band optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot (∼107–8 K) and warm ionized (∼104 K) gas phases, as well as the old stellar population and radio-bright active galactic nucleus (AGN) outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend towards and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed in situ by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. The morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star-forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to-free-fall time ratio is t
cool/t
ff ∼ 10. This condition is roughly met at the maximal projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.
We present an Integral Field Unit survey of 73 galaxy clusters and groups with the VIsible Multi Object Spectrograph on the Very Large Telescope. We exploit the data to determine the H α gas dynamics ...on kpc scales to study the feedback processes occurring within the dense cluster cores. We determine the kinematic state of the ionized gas and show that the majority of systems (∼2/3) have relatively ordered velocity fields on kpc scales that are similar to the kinematics of rotating discs and are decoupled from the stellar kinematics of the brightest cluster galaxy. The majority of the H α flux (>50 per cent) is typically associated with these ordered kinematics and most systems show relatively simple morphologies suggesting they have not been disturbed by a recent merger or interaction. Approximately 20 per cent of the sample (13/73) have disturbed morphologies which can typically be attributed to active galactic nuclei activity disrupting the gas. Only one system shows any evidence of an interaction with another cluster member. A spectral analysis of the gas suggests that the ionization of the gas within cluster cores is dominated by non-stellar processes, possibly originating from the intracluster medium itself.
We examine the radio properties of the brightest cluster galaxies (BCGs) in a large sample of X-ray selected galaxy clusters comprising the Brightest Cluster Sample (BCS), the extended BCS and ...ROSAT-ESO Flux Limited X-ray cluster catalogues. We have multifrequency radio observations of the BCG using a variety of data from the Australia Telescope Compact Array, Jansky Very Large Array and Very Long Baseline Array telescopes. The radio spectral energy distributions of these objects are decomposed into a component attributed to on-going accretion by the active galactic nuclei (AGN) that we refer to as ‘the core’, and a more diffuse, ageing component we refer to as the ‘non-core’. These BCGs are matched to previous studies to determine whether they exhibit emission lines (principally Hα), indicative of the presence of a strong cooling cluster core. We consider how the radio properties of the BCGs vary with cluster environmental factors. Line emitting BCGs are shown to generally host more powerful radio sources, exhibiting the presence of a strong, distinguishable core component in about 60 per cent of cases. This core component more strongly correlates with the BCG's O iii 5007 Å line emission. For BCGs in line emitting clusters, the X-ray cavity power correlates with both the extended and core radio emission, suggestive of steady fuelling of the AGN over bubble-rise time-scales in these clusters.
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