Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?

In cosmological structure formation models, massive non-linear objects in the process of formation, such as galaxy clusters, are surrounded by large-scale shocks at or around the expected virial radius. Direct observational evidence for such virial shocks is currently lacking, but we show here that their presence can be inferred from future, high resolution, high-sensitivity observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study the detectability of virial shocks in mock SZ maps, using simple models of cluster structure (gas density and temperature distributions) and noise (background and foreground galaxy clusters projected along the line of sight, as well as the cosmic microwave background anisotropies). We find that at an angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will stand out from the noise, and can be detected at high significance. More generally, our results imply that the projected SZ surface brightness profile in future, high-resolution experiments will provide sensitive constraints on the density profile of cluster gas.Comment: 15 pages, submitted to Ap

A Population of Short-Period Variable Quasars from PTF as Supermassive Black Hole Binary Candidates

Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be common in galactic nuclei, as a result of frequent galaxy mergers. Hydrodynamical simulations of circumbinary discs predict strong periodic modulation of the mass accretion rate on time-scales comparable to the orbital period of the binary. As a result, SMBHBs may be recognized by the periodic modulation of their brightness. We conducted a statistical search for periodic variability in a sample of 35,383 spectroscopically confirmed quasars in the photometric database of the Palomar Transient Factory (PTF). We analysed Lomb-Scargle periodograms and assessed the significance of our findings by modeling each individual quasar's variability as a damped random walk (DRW). We identified 50 quasars with significant periodicity beyond the DRW model, typically with short periods of a few hundred days. We find 33 of these to remain significant after a re-analysis of their periodograms including additional optical data from the intermediate-PTF and the Catalina Real-Time Transient Survey (CRTS). Assuming that the observed periods correspond to the redshifted orbital periods of SMBHBs, we conclude that our findings are consistent with a population of unequal-mass SMBHBs, with a typical mass ratio as low as q = M2/M1 ~ 0.01.Comment: MNRAS (accepted), new section 4.

Constraints from Gravitational Recoil on the Growth of Supermassive Black Holes at High Redshift

Recent studies have shown that during their coalescence, binary supermassive black holes (SMBHs) experience a gravitational recoil with velocities of 100 km/s < v(kick) < 600 km/s. These velocities exceed the escape velocity v(esc) from typical dark matter (DM) halos at high-redshift (z>6), and therefore put constraints on scenarios in which early SMBHs grow at the centers of DM halos. Here we quantify these constraints for the most distant known SMBHs, with inferred masses in excess of 10^9 M(sun), powering the bright quasars discovered in the Sloan Digital Sky Survey at z>6. We assume that these SMBHs grew via a combination of accretion and mergers between pre-existing seed BHs in individual progenitor halos, and that mergers between progenitors with v(esc) < v(kick) disrupt the BH growth process. Our results suggest that under these assumptions, the z=6 SMBHs had a phase during which gained mass significantly more rapidly than under an Eddington-limited exponential growth rate.Comment: submitted to ApJ Letters, 5 emulateapj pages with 1 figur

On the Cosmological Evolution of the Luminosity Function and the Accretion Rate of Quasars

We consider a class of models for the redshift evolution (between 0\lsim z \lsim 4) of the observed optical and X-ray quasar luminosity functions (LFs), with the following assumptions: (i) the mass-function of dark matter halos follows the Press-Schechter theory, (ii) the black hole (BH) mass scales linearly with the halo mass, (iii) quasars have a constant universal lifetime, and (iv) a thin accretion disk provides the optical luminosity of quasars, while the X-ray/optical flux ratio is calibrated from a sample of observed quasars. The mass accretion rate $\dot{M}$ onto quasar BHs is a free parameter of the models, that we constrain using the observed LFs. The accretion rate $\dot M$ inferred from either the optical or X-ray data under these assumptions generally decreases as a function of cosmic time from $z \simeq 4$ to $z \simeq 0$. We find that a comparable accretion rate is inferred from the X-ray and optical LF only if the X-ray/optical flux ratio decreases with BH mass. Near $z\simeq 0$, $\dot M$ drops to substantially sub-Eddington values at which advection-dominated accretion flows (ADAFs) exist. Such a decline of $\dot M$, possibly followed by a transition to radiatively inefficient ADAFs, could explain both the absence of bright quasars in the local universe and the faintness of accreting BHs at the centers of nearby galaxies. We argue that a decline of the accretion rate of the quasar population is indeed expected in cosmological structure formation models.Comment: Latex, 23 pages, 9 figures, accepted for publication in Ap

Constraining the Lifetime of Quasars from their Spatial Clustering

The lifetime t_Q of the luminous phase of quasars is constrained by current observations to be between 10^6 and 10^8 years, but is otherwise unkown. We model the quasar luminosity function in detail in the optical and X-ray bands using the Press-Schechter formalism, and show that the expected clustering of quasars depends strongly on their assumed lifetime. We quantify this dependence, and find that existing measurements of the correlation length of quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show that future measurements of the power spectrum of quasars out to z=3, from the 2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and in principle allow a precise determination of t_Q. We estimate the systematic errors introduced by uncertainties in the modeling of the quasar-halo relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj

Witnessing the formation of a brightest cluster galaxy at z>2

We present deep observations taken with the HST Advanced Camera for Surveys of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting the powerful radio source MRC 1138-262 is associated with one of the most extensive merger systems known in the early universe. Our HST/ACS image shows many star-forming galaxies merging within a ~200 kpc region that emits both diffuse line emission and continuum in the rest-frame UV. Because this galaxy lives in an overdense environment, it represents a rare view of a brightest cluster galaxy in formation at z>2 which may serve as a testbed for predictions of massive cluster galaxy formation.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies", Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko

Gravitational-wave memory revisited: memory from the merger and recoil of binary black holes

Gravitational-wave memory refers to the permanent displacement of the test masses in an idealized (freely-falling) gravitational-wave interferometer. Inspiraling binaries produce a particularly interesting form of memory--the Christodoulou memory. Although it originates from nonlinear interactions at 2.5 post-Newtonian order, the Christodoulou memory affects the gravitational-wave amplitude at leading (Newtonian) order. Previous calculations have computed this non-oscillatory amplitude correction during the inspiral phase of binary coalescence. Using an "effective-one-body" description calibrated with the results of numerical relativity simulations, the evolution of the memory during the inspiral, merger, and ringdown phases, as well as the memory's final saturation value, are calculated. Using this model for the memory, the prospects for its detection are examined, particularly for supermassive black hole binary coalescences that LISA will detect with high signal-to-noise ratios. Coalescing binary black holes also experience center-of-mass recoil due to the anisotropic emission of gravitational radiation. These recoils can manifest themselves in the gravitational-wave signal in the form of a "linear" memory and a Doppler shift of the quasi-normal-mode frequencies. The prospects for observing these effects are also discussed.Comment: 6 pages, 2 figures; accepted to the proceedings of the 7th International LISA Symposium; v2: updated figures and signal-to-noise ratios, several minor changes to the tex

The Stellar Population of the M31 Spiral Arm Around OB Association A24

A study of the stellar population of the M31 spiral arm around OB association A24 was carried out based on the photometric data obtained from deep V and JHK imaging. The luminosity function was obtained for -7 <~ Mbol <~ -3.5 by applying the extinction correction corresponding to Av=1 and the bolometric correction BC(K) as an empirical function of (J-K)o. In comparing the observed color-luminosity diagrams with semitheoretical isochrones modified for the dust-shell effects, we found the young population of t <~ 30 Myr with supergiants of Mbol <~ -5, the bulk of the intermediate-age population of t ~ 0.2 - 2.5 Gyr with bright asymptotic giant branch (AGB) stars of -5 <~ Mbol <~ -4, and old populations of t ~> 3 Gyr with AGB and red giant branch (RGB) stars of Mbol ~> -4. The average star formation rate was estimated to be ~1.8x10^4 M_o/Myr and ~0.7x10^4 M_o/Myr per deprojected disk area of 1 kpc^2 from the number density of B0 V stars around Mv=-4.0 (age ~10 Myr) and the number density of bright AGB stars around Mbol = -4.3 (age ~1 Gyr), respectively. A study of the local variation in the V and the J and H luminosity functions revealed a kind of anticorrelation between the population of the young component and that of the intermediate-age component when subdomains of ~100 pc scales were concerned. This finding suggests that the disk domain around the A24 area experienced a series of star formation episodes alternatively among different subdomains with a timescale of a few spiral passage periods. Brief discussions are given about the interstellar extinction and about the lifetimes of bright AGB stars and the highly red objects (HROs) in the same area.Comment: 27 pages, 11 figures, accepted: ApJ, July 1, 199

The Refined Topological Vertex

We define a refined topological vertex which depends in addition on a parameter, which physically corresponds to extending the self-dual graviphoton field strength to a more general configuration. Using this refined topological vertex we compute, using geometric engineering, a two-parameter (equivariant) instanton expansion of gauge theories which reproduce the results of Nekrasov. The refined vertex is also expected to be related to Khovanov knot invariants.Comment: 70 Pages, 23 Figure