719 research outputs found
Hypervelocity stars and the environment of Sgr A*
Hypervelocity stars (HVSs) are a natural consequence of the presence of a
massive nuclear black hole (Sgr A*) in the Galactic Center. Here we use the
Brown et al. sample of unbound and bound HVSs together with numerical
simulations of the propagation of HVSs in the Milky Way halo to constrain three
plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an
inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar
binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars
off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the
predicted radial and velocity distributions of HVSs with the limited-statistics
dataset currently available, and show that the IMBH model appears to produce a
spectrum of ejection velocities that is too flat. Future astrometric and deep
wide-field surveys of HVSs should shed unambiguous light on the stellar
ejection mechanism and probe the Milky Way potential on scales as large as 200
kpc.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
Intergalactic Photon Spectra from the Far IR to the UV Lyman Limit for and the Optical Depth of the Universe to High Energy Gamma-Rays
We calculate the intergalactic photon density as a function of both energy
and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit
cutoff at 13.6 eV in a Lambda-CDM universe with and
. Our galaxy evolution model gives results which are
consistent with Spitzer deep number counts and the spectral energy distribution
of the extragalactic background radiation. We use our photon density results to
extend previous work on the absorption of high energy gamma-rays in
intergalactic space owing to interactions with low energy photons and the 2.7 K
cosmic background radiation. We calculate the optical depth of the universe,
tau, for gamma-rays having energies from 4 GeV to 100 TeV emitted by sources at
redshifts from ~0 to 5. We also give an analytic fit with numerical
coefficients for approximating . As an example of the
application of our results, we calculate the absorbed spectrum of the blazar
PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the
H.E.S.S. air Cherenkov gamma-ray telescope array.Comment: final version to be published in Ap
Growing massive black holes through super-critical accretion of stellar-mass seeds
The rapid assembly of the massive black holes that power the luminous quasars
observed at remains a puzzle. Various direct collapse models have
been proposed to head-start black hole growth from initial seeds with masses
, which can then reach a billion solar mass while
accreting at the Eddington limit. Here we propose an alternative scenario based
on radiatively inefficient super-critical accretion of stellar-mass holes
embedded in the gaseous circum-nuclear discs (CNDs) expected to exist in the
cores of high redshift galaxies. Our sub-pc resolution hydrodynamical
simulations show that stellar-mass holes orbiting within the central 100 pc of
the CND bind to very high density gas clumps that arise from the fragmentation
of the surrounding gas. Owing to the large reservoir of dense cold gas
available, a stellar-mass black hole allowed to grow at super-Eddington rates
according to the "slim disc" solution can increase its mass by 3 orders of
magnitudes within a few million years. These findings are supported by
simulations run with two different hydro codes, RAMSES based on the Adaptive
Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type
method, and with similar, but not identical, sub-grid recipes for star
formation, supernova feedback, black hole accretion and feedback. The low
radiative efficiency of super-critical accretion flows are instrumental to the
rapid mass growth of our black holes, as they imply modest radiative heating of
the surrounding nuclear environment.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
The Stellar Mass Evolution of Galaxies in the NICMOS Ultra Deep Field
We measure the build-up of the stellar mass of galaxies from z=6 to z=1.
Using 15 band multicolor imaging data in the NICMOS Ultra Deep Field we derive
photometric redshifts and masses for 796 galaxies down to H(AB)=26.5. The
derived evolution of the global stellar mass density of galaxies is consistent
with previous star formation rate density measurements over the observed range
of redshifts. Beyond the observed range, maintaining consistency between the
global stellar mass and the observed star formation rate suggests the epoch of
galaxy formation was z=16.Comment: 18 pages, 3 figures, submitted to AJ. Data at:
http://orca.phys.uvic.ca/~gwyn/MMM/nicmos.htm
Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies
We compute the expected gravitational wave signal from coalescing massive
black hole (MBH) binaries at the center of galaxies in a hierarchical structure
formation scenario in which seed holes of intermediate mass form far up in the
dark halo merger tree. The merger history of DM halos and MBHs is followed from
z=20 to the present in a LCDM cosmology. MBHs get incorporated through halo
mergers into larger and larger structures, sink to the center owing to
dynamical friction against the DM background, accrete cold material in the
merger remnant, and form MBH binary systems. Stellar dynamical interactions
cause the hardening of the binary at large separations, while gravitational
wave emission takes over at small radii and leads to the final coalescence of
the pair. The integrated emission from inspiraling MBH binaries results in a
gravitational wave background (GWB). The characteristic strain spectrum has the
standard h_c(f)\propto f^{-2/3} behavior only in the range 1E-9<f<1E-6 Hz. At
lower frequencies the orbital decay of MBH binaries is driven by the ejection
of background stars, and h_c(f) \propto f. At higher frequencies, f>1E-6 Hz,
the strain amplitude is shaped by the convolution of last stable circular orbit
emission. We discuss the observability of inspiraling MBH binaries by the
planned LISA. Over a 3-year observing period LISA should resolve this GWB into
discrete sources, detecting ~60 (~250) individual events above a S/N=5 (S/N=1)
confidence level. (Abridged)Comment: 11 pages, 8 figues. Revised version accepted to be published in ApJ
Discussion on number counts corrected and expande
Connecting Galaxy Evolution, Star Formation and the X-ray Background
As a result of deep hard X-ray observations by Chandra and XMM-Newton a
significant fraction of the cosmic X-ray background (CXRB) has been resolved
into individual sources. These objects are almost all active galactic nuclei
(AGN) and optical followup observations find that they are mostly obscured Type
2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs
s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the
peak in the cosmic star-formation rate, this paper proposes that the obscuring
material required for AGN unification is regulated by star-formation within the
host galaxy. We test this idea by computing CXRB synthesis models with a ratio
of Type 2/Type 1 AGN that is a function of both z and 2-10 keV X-ray
luminosity, L_X. The evolutionary models are constrained by parameterizing the
observed Type 1 AGN fractions from the recent work by Barger et al. The
parameterization which simultaneously best accounts for Barger's data, the CXRB
spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio
of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models
with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN
fractions. This particular evolution predicts a Type 2/Type 1 ratio of 1-2 for
log L_X > 44, and thus the deep X-ray surveys are missing about half the
obscured AGN with these luminosities. These objects are likely to be Compton
thick. Overall, these calculations show that the current data strongly supports
a change to the AGN unification scenario where the obscuration is connected
with star formation in the host galaxy rather than a molecular torus alone. The
evolution of the obscuration implies a close relationship between star
formation and AGN fueling, most likely due to minor mergers or interactions.Comment: 36 pages, 8 figures, ApJ in press. Minor changes to match published
versio
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