The analysis of 108mAg, 166mHo and 94Nb in decommissioning waste

Contaminated waste consisting of various materials results from the decommissioning of nuclear power plants. Disposal of Decommissioning Waste requires as much measurement data for the radionuclides contained within the waste as possible. Data that are obtained is then used to create an inventory of the radionuclides in the various types of wastes. This research investigates a route to obtaining additional measurement data for such inventories. The importance of these inventories is to provide as much information as possible so that a reliable risk assessment can be performed on the waste samples and the proposed method of disposal. [Continues.

The M-sigma relation in different environments

Galaxies become red and dead when the central supermassive black hole (SMBH) becomes massive enough to drive an outflow beyond the virial radius of the halo. We show that this final SMBH mass is larger than the final SMBH mass in the bulge of a spiral galaxy by up to an order of magnitude. The M‒σ relations in the two galaxy types are almost parallel (M ∝ σ4 + β, with β < 1) but offset in normalization, with the extra SMBH mass supplied by the major merger transforming the galaxy into an elliptical, or by mass gain in a galaxy cluster. This agrees with recent findings that the SMBH in two brightest cluster Galaxies are ∼10 × the expected M‒σ mass. We show that these results do not strongly depend on the assumed profile of the dark matter halo, so analytic estimates found for an isothermal potential are approximately valid in all realistic cases. Our results imply that there are in practice actually three M‒σ relations, corresponding to spiral galaxies with evolved bulges, field elliptical galaxies and cluster centre elliptical galaxies. A fourth relation, corresponding to cluster spiral galaxies, is also possible, but such galaxies are expected to be rare. All these relations have the form MBH = Cnσ4, with only a slight difference in slope between field and cluster galaxies, but with slightly different coefficients Cn. Conflating data from galaxies of different types and fitting a single relation to them tends to produce a higher power of σ

The M-sigma relation between supermassive black holes and their host galaxies

Supermassive black holes (SMBHs) are found in the centres of most galaxies. Their masses, and hence their gravitational potentials, are negligible compared with those of the host galaxy. However, several strong correlations between SMBH masses and host galaxy properties have been observed, notably the M − σ relation connecting the SMBH mass to the characteristic velocity of stars in the galaxy. The existence of these correlations implies that the SMBH influences the evolution of its host galaxy. In this review, we present the most promising physical model of this influence, known as the Active galactic nucleus (AGN) wind feedback model. Winds launched from the accretion disc around the SMBH can drive powerful outflows, provided that the SMBH is massive enough - this condition establishes the M − σ relation. Outflows can have a profound influence on the evolution of the host galaxy, by compressing its gas and driving it out, affecting the star formation rate. We present the current status of the model and the observational evidence for it, as well as the directions of future research

Growing supermassive black holes by chaotic accretion

We consider the problem of growing the largest supermassive black holes from stellar–mass seeds at high redshift. Rapid growth without violating the Eddington limit requires that most mass is gained while the hole has a low spin and thus a low radiative accretion efficiency. If, as was formerly thought, the black–hole spin aligns very rapidly with the accretion flow, even a randomly–oriented sequence of accretion events would all spin up the hole and prevent rapid mass growth. However, using a recent result that the Bardeen–Petterson effect causes counteralignment of hole and disc spins under certain conditions, we show that holes can grow rapidly in mass if they acquire most of it in a sequence of randomly oriented accretion episodes whose angular momenta Jd are no larger than the hole’s angular momentum Jh. Ultimately the hole has total angular momentum comparable with the last accretion episode. This points to a picture in which the accretion is chaotic on a lengthscale of order the disc size, that is <~0.1 pc

The end of the black hole dark ages and the origin of warm absorbers

We consider how the radiation pressure of an accreting supermassive black hole (SMBH) affects the interstellar medium around it. Much of the gas originally surrounding the hole is swept into a shell with a characteristic radius somewhat larger than the black hole's radius of influence (∼1–100 pc). The shell has a mass directly comparable to the (M-σ) mass that the hole will eventually reach, and may have a complex topology. We suggest that outflows from the central SMBHs are halted by collisions with the shell, and that this is the origin of the warm absorber components frequently seen in active galactic nucleus (AGN) spectra. The shell may absorb and reradiate some of the black hole accretion luminosity at long wavelengths, implying both that the bolometric luminosities of some known AGN may have been underestimated, and that some accreting SMBH may have escaped detection entirely

What's in a Fermi Bubble: A Quasar Episode in the Galactic Center

Fermi bubbles, the recently observed giant (∼ 10 kpc high) gamma-ray emitting lobes on either side of our Galaxy (Su et al. 2010), appear morphologically connected to the Galactic center, and thus offer a chance to test several models of supermassive black hole (SMBH) evolution, feedback and relation with their host galaxies. We use a physical feedback model (King 2003, 2010) and novel numerical techniques (Nayakshin et al. 2009) to simulate a short burst of activity in Sgr A∗ , the central SMBH of the Milky Way, ∼ 6 Myr ago, temporally coincident with a star formation event in the central parsec. We are able to reproduce the bubble morphology and energetics both analytically (Zubovas et al. 2011) and numerically (Zubovas & Nayakshin, in prep). These results provide strong support to the model, which was also used to simulate more extreme environments (Nayakshin & Power 2010)

The observed m-σ relations imply that super-massive black holes grow by cold chaotic accretion

We argue that current observations of M-σ relations for galaxies can be used to constrain theories of super-massive black holes (SMBHs) feeding. In particular, assuming that SMBH mass is limited only by the feedback on the gas that feeds it, we show that SMBHs fed via a planar galaxy-scale gas flow, such as a disk or a bar, should be much more massive than their counterparts fed by quasi-spherical inflows. This follows from the relative inefficiency of active galactic nucleus feedback on a flattened inflow. We find that even under the most optimistic conditions for SMBH feedback on flattened inflows, the mass at which the SMBH expels the gas disk and terminates its own growth is a factor of several higher than the one established for quasi-spherical inflows. Any beaming of feedback away from the disk and any disk self-shadowing strengthen this result further. Contrary to this theoretical expectation, recent observations have shown that SMBHs in pseudobulge galaxies (which are associated with barred galaxies) are typically under- rather than overmassive when compared with their classical bulge counterparts at a fixed value of σ. We conclude from this that SMBHs are not fed by large (100 pc to many kpc) scale gas disks or bars, most likely because such planar flows are turned into stars too efficiently to allow any SMBH growth. Based on this and other related observational evidence, we argue that most SMBHs grow by chaotic accretion of gas clouds with a small and nearly randomly distributed direction of angular momentum

Misaligned accretion on to supermassive black hole binaries

We present the results of high-resolution numerical simulations of gas clouds falling on to binary supermassive black holes to form circumbinary accretion discs, with both prograde and retrograde cloud orbits. We explore a range of clouds masses and cooling rates. We find that for low-mass discs that cool fast enough to fragment, prograde discs are significantly shorter lived than similar discs orbiting retrograde with respect to the binary. For fragmenting discs of all masses, we also find that prograde discs fragment across a narrower radial region. If the cooling is slow enough that the disc enters a self-regulating gravitoturbulent regime, we find that alignment between the disc and binary planes occurs on a time-scale primarily dictated by the disc thickness. We estimate realistic cooling times for such discs, and find that in the majority of cases we expect fragmentation to occur. The longer lifetime of low-mass fragmenting retrograde discs allows them to drive significant binary evolution, and may provide a mechanism for solving the ‘last parsec problem’

Accretion disc viscosity: what do warped discs tell us?

Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling the flow of mass and angular momentum in the radial direction. In a previous paper, we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper, we discuss the observational evidence for the magnitude of the dissipative effects which act in non-planar discs, both to transfer and to eliminate the non-planarity. Estimates based on the model by Ogilvie, which assumes a small-scale, isotropic viscosity, give alignment time-scales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the time-scales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate

Instability of warped discs

Accretion discs are generally warped. If a warp in a disc is too large, the disc can ‘break’ apart into two or more distinct planes, with only tenuous connections between them. Further, if an initially planar disc is subject to a strong differential precession, then it can be torn apart into discrete annuli that precess effectively independently. In previous investigations, torque-balance formulae have been used to predict where and when the disc breaks into distinct parts. In this work, focusing on discs with Keplerian rotation and where the shearing motions driving the radial communication of the warp are damped locally by turbulence (the ‘diffusive’ regime), we investigate the stability of warped discs to determine the precise criterion for an isolated warped disc to break. We find and solve the dispersion relation, which, in general, yields three roots. We provide a comprehensive analysis of this viscous-warp instability and the emergent growth rates and their dependence on disc parameters. The physics of the instability can be understood as a combination of (1) a term that would generally encapsulate the classical Lightman–Eardley instability in planar discs (given by ∂(νΣ)/∂Σ < 0) but is here modified by the warp to include ∂(ν1|ψ|)/∂|ψ| < 0, and (2) a similar condition acting on the diffusion of the warp amplitude given in simplified form by ∂(ν2|ψ|)/∂|ψ| < 0. We discuss our findings in the context of discs with an imposed precession, and comment on the implications for different astrophysical systems