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Abstract:

Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a common by-product of major galaxy mergers. We use simple disk models for the circumbinary gas and for the binary–disk interaction to follow the orbital decay of SMBHBs with a range of total masses (M) and mass ratios (q), through physically distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior to the GW-driven phase, the viscous decay is generically in the stalled "secondary-dominated" regime. SMBHBs spend a non-negligible fraction of a fiducial time of 107 yr at orbital periods between days ≲torb≲ yr, and we argue that they may be sufficiently common to be detectable, provided they are luminous during these stages. A dedicated optical or X-ray survey could identify coalescing SMBHBs statistically, as a population of periodically variable quasars, whose abundance obeys the scaling Nvar ∝ tαvar within a range of periods around tvar∼ tens of weeks. SMBHBs with M ≲ 107 M☉, with 0.5 ≲ α ≲ 1.5, would probe the physics of viscous orbital decay, whereas the detection of a population of higher-mass binaries, with α = 8/3, would confirm that their decay is driven by GWs. The lowest-mass SMBHBs (M ≲ 105–6 M☉) enter the GW-driven regime at short orbital periods, when they are already in the frequency band of the Laser Interferometric Space Antenna (LISA). While viscous processes are negligible in the last few years of coalescence, they could reduce the amplitude of any unresolved background due to near-stationary LISA sources. We discuss modest constraints on the SMBHB population already available from existing data, and the sensitivity and sky coverage requirements for a detection in future surveys. SMBHBs may also be identified from velocity shifts in their spectra; we discuss the expected abundance of SMBHBs as a function of their orbital velocity.

Abstract:

Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.

Abstract:

Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies. © 2009 Macmillan Publishers Limited. All rights reserved.

Abstract:

We describe a technique that finds orbits through the Galaxy that are consistent with measurements of a tidal stream, taking into account the extent that tidal streams do not precisely delineate orbits. We show that if accurate line-of-sight velocities are measured along a well defined stream, the technique recovers the underlying orbit through the Galaxy and predicts the distances and proper motions along the stream to high precision. As the error bars on the location and velocities of the stream grow, the technique is able to find more and more orbits that are consistent with the data and the uncertainties in the predicted distances and proper motions increase. With radial-velocity data along a stream ~40deg long and <0.3deg wide on the sky accurate to ~1 km/s the precisions of the distances and tangential velocities along the stream are 4 percent and 5 km/s, respectively. The technique can be used to diagnose the Galactic potential: if circular-speed curve is actually flat, both a Keplerian potential and Phi(r) proportional to r are readily excluded. Given the correct radial density profile for the dark halo, the halo's mass can be determined to a precision of 5 percent.