22,792 research outputs found
Major Galaxy Mergers and the Growth of Supermassive Black Holes in Quasars
Despite observed strong correlations between central supermassive black holes
(SMBHs) and star-formation in galactic nuclei, uncertainties exist in our
understanding of their coupling. We present observations of the ratio of
heavily-obscured to unobscured quasars as a function of cosmic epoch up to z~3,
and show that a simple physical model describing mergers of massive, gas-rich
galaxies matches these observations. In the context of this model, every
obscured and unobscured quasar represent two distinct phases that result from a
massive galaxy merger event. Much of the mass growth of the SMBH occurs during
the heavily-obscured phase. These observations provide additional evidence for
a causal link between gas-rich galaxy mergers, accretion onto the nuclear SMBH
and coeval star formation.Comment: Accepted for publication in Science. Published by Science Express on
March 25th. 17 pages, 5 figures, including supplemental online materia
Constraints on turbulent velocity broadening for a sample of clusters, groups and elliptical galaxies using XMM-Newton
Using the width of emission lines in XMM-Newton Reflection Grating
Spectrometer spectra, we place direct constraints on the turbulent velocities
of the X-ray emitting medium in the cores of 62 galaxy clusters, groups and
elliptical galaxies. We find five objects where we can place an upper limit on
the line-of-sight broadening of 500 km/s (90 per cent confidence level), using
a single thermal component model. Two other objects are lower than this limit
when two thermal components are used. Half of the objects examined have an
upper limit on the velocity broadening of less than 700 km/s. To look for
objects which have significant turbulent broadening, we use Chandra spectral
maps to compute the expected broadening caused by the spatial extent of the
source. Comparing these with our observed results, we find that Klemola 44 has
extra broadening at the level of 1500 km/s. RX J1347.5-1145 shows weak evidence
for turbulent velocities at 800 km/s. In addition we obtain limits on
turbulence for Zw3146, Abell 496, Abell 1795, Abell 2204 and HCG 62 of less
than 200 km/s. After subtraction of the spatial contribution and including a 50
km/s systematic uncertainty, we find at least 15 sources with less than 20 per
cent of the thermal energy density in turbulence.Comment: 17 pages, 17 figures, accepted by MNRAS. Includes minor edits to
proo
Searching for the missing iron in the core of the Centaurus cluster
We re-analyse a combined 198 ks Chandra observation of NGC4696, the brightest
galaxy of the Centaurus cluster. We extract temperature and metallicity
profiles from the data, and we confirm the presence of a sharp drop in iron
abundance, from ~1.8 Zsolar to ~0.4 Zsolar, within the central 5 kpc of the
cluster. We estimate that this abundance drop corresponds to a total "missing"
iron mass of 1.4e06 Msolar. We propose that part of this missing iron is locked
up in cool (~19 K) far-IR emitting dust, as found by Spitzer and Herschel
observations. This can occur if the iron injected by stellar mass loss in the
central region is in grains, which remain in that form as the injected dusty
cold gas mixes and joins the cold dusty filamentary nebula observed within the
same region. The bubbling feedback process observed in the cluster core then
drags filaments outward and dumps them at 10-20 kpc radius, where the
metallicity is high.Comment: 8 pages, accepted for publication in MNRA
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
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