Black hole foraging: feedback drives feeding

We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers. Momentum-driven feedback from an accreting hole gives significant orbital energy but little angular momentum to the surrounding gas. Once central accretion drops, the feedback weakens and swept-up gas falls back towards the SMBH on near-parabolic orbits. These intersect near the black hole with partially opposed specific angular momenta, causing further infall and ultimately the formation of a small-scale accretion disk. The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) super-solar abundances of the matter accreting on to the SMBH; and (vi) a lower central dark-matter density, and hence annihilation signal, than adiabatic SMBH growth implies. We also suggest a simple sub-grid recipe for implementing this process in numerical simulations.Comment: accepted for publication in ApJ Letters, 5 pages, 1 figur

Verification of successive convexification algorithm

In this report, I describe a technique which allows a non-convex optimal control problem to be expressed and solved in a convex manner. I then verify the resulting solution to ensure its physical feasibility and its optimality. The original, non-convex problem is the fuel-optimal powered landing problem with aerodynamic drag. The non-convexities present in this problem include mass depletion dynamics, aerodynamic drag, and free final time. Through the use of lossless convexification and successive convexification, this problem can be formulated as a series of iteratively solved convex problems that requires only a guess of a final time of flight. The solution’s physical feasibility is verified through a nonlinear simulation built in Simulink, while its optimality is verified through the general nonlinear optimal control software GPOPS-II.Aerospace Engineerin

The Summer Employment Experiences and the Personal/ Social Behaviors of Youth Violence Prevention Employment Program Participants and Those of a Comparison Group

The summer job market for teens in both Massachusetts and the U.S. over the past five years has been quite depressed, with record low summer employment rates for the nation's teens being set in the past three years (2010-2012).1 The teen summer employment rate in Massachusetts fell from 67% in 1999 to only 36% in 2012, a decline of 31 percentage points (Chart 1). Black and Hispanic teens, especially those residing in low income families and from high poverty neighborhoods, have experienced the greatest difficulties in finding employment in the summer. Lack of job opportunities reduces teens' exposure to the world of work and their ability to acquire both basic employability skills (attendance, team work, communicating with other workers and customers) and occupational skills. Being jobless all summer also increases their risk of social isolation (staying at home), hanging out on the street, and exposure to or participation in urban violence and delinquent behavior

The origins of slaty cleavage

Imperial Users onl

Consistent Testing for Recurrent Genomic Aberrations

Genomic aberrations, such as somatic copy number alterations, are frequently observed in tumor tissue. Recurrent aberrations, occurring in the same region across multiple subjects, are of interest because they may highlight genes associated with tumor development or progression. A number of tools have been proposed to assess the statistical significance of recurrent DNA copy number aberrations, but their statistical properties have not been carefully studied. Cyclic shift testing, a permutation procedure using independent random shifts of genomic marker observations on the genome, has been proposed to identify recurrent aberrations, and is potentially useful for a wider variety of purposes, including identifying regions with methylation aberrations or overrepresented in disease association studies. For data following a countable-state Markov model, we prove the asymptotic validity of cyclic shift $p$-values under a fixed sample size regime as the number of observed markers tends to infinity. We illustrate cyclic shift testing for a variety of data types, producing biologically relevant findings for three publicly available datasets.Comment: 35 pages, 7 figure

Misaligned gas discs around eccentric black-hole binaries and implications for the final-parsec problem

We investigate the evolution of low mass (Md /Mb = 0.005) misaligned gaseous discs around eccentric supermassive black hole (SMBH) binaries. These are expected to form from randomly oriented accretion events onto a SMBH binary formed in a galaxy merger. When expanding the interaction terms between the binary and a circular ring to quadrupole order and averaging over the binary orbit, we expect four non-precessing disc orientations: aligned or counter-aligned with the binary, or polar orbits around the binary eccentricity vector with either sense of rotation. All other orientations precess around either of these, with the polar precession dominating for high eccentricity. These expectations are borne out by smoothed particle hydrodynamics simulations of initially misaligned viscous circumbinary discs, resulting in the formation of polar rings around highly eccentric binaries in contrast to the co-planar discs around circular binaries. Moreover, we observe disc tearing and violent interactions between differentially precessing rings in the disc significantly disrupting the disc structure and causing gas to fall onto the binary with little angular momentum. While accretion from a polar disc may not promote SMBH binary coalescence (solving the `final-parsec problem'), ejection of this infalling low-angular momentum material via gravitational slingshot is a possible mechanism to reduce the binary separation. Moreover, this process acts on dynamical rather than viscous time scales, and so is much faster.Comment: 12 pages, 9 figures, Accepted for publication in MNRA