1,645 research outputs found
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
On the evolutionary behaviour of BL Lac objects
We present a new well defined sample of BL Lac objects selected from the
ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming
a flux limited sample down to f_X = 8 x 10^{-13} cgs, redshifts are known for
33 objects (and 31 of the complete sample). X-ray spectral properties were
determined for each object individually with the RASS data. The luminosity
function of RASS selected BL Lac objects is compatible with results provided by
objects selected with the Einstein observatory, but the RASS selected sample
contains objects with luminosities at least tenfold higher. Our analysis
confirms the negative evolution for X-ray selected BL Lac objects found in a
sample by the Einstein observatory, the parameterization provides similar
results. A subdivision of the sample into halves according to the X-ray to
optical flux ratio yielded unexpected results. The extremely X-ray dominated
objects have higher redshifts and X-ray luminosities and only this subgroup
shows clear signs of strong negative evolution. The evolutionary behaviour of
objects with an intermediate spectral energy distribution between X-ray and
radio dominated is compatible with no evolution at all. Consequences for
unified schemes of X-ray and radio selected BL Lac objects are discussed.We
suggest that the intermediate BL Lac objects are the basic BL Lac population.
The distinction between the two subgroups can be explained if extreme X-ray
dominated BL Lac objects are observed in a state of enhanced X-ray activity.Comment: 14 pages incl. 8 figures, accepted by A&
Multi-source self-calibration: Unveiling the microJy population of compact radio sources
Context. Very Long Baseline Interferometry (VLBI) data are extremely
sensitive to the phase stability of the VLBI array. This is especially
important when we reach {\mu}Jy r.m.s. sensitivities. Calibration using
standard phase referencing techniques is often used to improve the phase
stability of VLBI data but the results are often not optimal. This is evident
in blank fields that do not have in-beam calibrators. Aims. We present a
calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be
used after standard phase referencing on wide-field VLBI observations. This is
tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field-North and
the Hubble Flanking Fields. Methods. MSSC uses multiple target sources detected
in the field via standard phase referencing techniques and modifies the
visibili- ties so that each data set approximates to a point source. These are
combined to increase the signal to noise and permit self-calibration. In
principle, this should allow residual phase changes caused by the troposphere
and ionosphere to be corrected. By means of faceting, the technique can also be
used for direction dependent calibration. Results. Phase corrections, derived
using MSSC, were applied to a wide-field VLBI data set of the HDF-N comprising
of 699 phase centres. MSSC was found to perform considerably better than
standard phase referencing and single source self-calibration. All detected
sources exhibited dramatic improvements in dynamic range. Using MSSC, one
source reached the detection threshold taking the total detected sources to
twenty. 60% of these sources can now be imaged with uniform weighting compared
to just 45% with standard phase referencing. The Parseltongue code which
implements MSSC has been released and made publicly available to the
astronomical community (https://github.com/jradcliffe5/multi_self_cal).Comment: 7 pages, 4 figures, accepted to A&
Ions in Fluctuating Channels: Transistors Alive
Ion channels are proteins with a hole down the middle embedded in cell
membranes. Membranes form insulating structures and the channels through them
allow and control the movement of charged particles, spherical ions, mostly
Na+, K+, Ca++, and Cl-. Membranes contain hundreds or thousands of types of
channels, fluctuating between open conducting, and closed insulating states.
Channels control an enormous range of biological function by opening and
closing in response to specific stimuli using mechanisms that are not yet
understood in physical language. Open channels conduct current of charged
particles following laws of Brownian movement of charged spheres rather like
the laws of electrodiffusion of quasi-particles in semiconductors. Open
channels select between similar ions using a combination of electrostatic and
'crowded charge' (Lennard-Jones) forces. The specific location of atoms and the
exact atomic structure of the channel protein seems much less important than
certain properties of the structure, namely the volume accessible to ions and
the effective density of fixed and polarization charge. There is no sign of
other chemical effects like delocalization of electron orbitals between ions
and the channel protein. Channels play a role in biology as important as
transistors in computers, and they use rather similar physics to perform part
of that role. Understanding their fluctuations awaits physical insight into the
source of the variance and mathematical analysis of the coupling of the
fluctuations to the other components and forces of the system.Comment: Revised version of earlier submission, as invited, refereed, and
published by journa
Reconstructing the molecular life history of gliomas.
At the time of their clinical manifestation, the heterogeneous group of adult and pediatric gliomas carries a wide range of diverse somatic genomic alterations, ranging from somatic single-nucleotide variants to structural chromosomal rearrangements. Somatic abnormalities may have functional consequences, such as a decrease, increase or change in mRNA transcripts, and cells pay a penalty for maintaining them. These abnormalities, therefore, must provide cells with a competitive advantage to become engrained into the glioma genome. Here, we propose a model of gliomagenesis consisting of the following five consecutive phases that glioma cells have traversed prior to clinical manifestation: (I) initial growth; (II) oncogene-induced senescence; (III) stressed growth; (IV) replicative senescence/crisis; (V) immortal growth. We have integrated the findings from a large number of studies in biology and (neuro)oncology and relate somatic alterations and other results discussed in these papers to each of these five phases. Understanding the story that each glioma tells at presentation may ultimately facilitate the design of novel, more effective therapeutic approaches. Acta Neuropathol 2018 May; 135(5):649-670
Solving condensed-matter ground-state problems by semidefinite relaxations
We present a new generic approach to the condensed-matter ground-state
problem which is complementary to variational techniques and works directly in
the thermodynamic limit. Relaxing the ground-state problem, we obtain
semidefinite programs (SDP). These can be solved efficiently, yielding strict
lower bounds to the ground-state energy and approximations to the few-particle
Green's functions. As the method is applicable for all particle statistics, it
represents in particular a novel route for the study of strongly correlated
fermionic and frustrated spin systems in D>1 spatial dimensions. It is
demonstrated for the XXZ model and the Hubbard model of spinless fermions. The
results are compared against exact solutions, quantum Monte Carlo, and Anderson
bounds, showing the competitiveness of the SDP method.Comment: 8 pages, 3 figures; original title "Approaching condensed matter
ground states from below"; improved numerics, added references; published
version, including appendice
Occurrence and Global Properties of Narrow CIV lambda 1549 Absorption Lines in Moderate-Redshift Quasars
A statistical study is presented of (a) the frequency of narrow CIV lambda
1549 absorption lines in 1.5 <~ z <~ 3.6 radio-quiet and radio-loud quasars,
and of (b) the UV and radio properties of the absorbed quasars. The quasar
sample is unbiased with respect to absorption properties and the radio-quiet
and radio-loud subsamples are well matched in redshift and luminosity. A
similarly high incidence (>~50%) of narrow CIV absorbers is detected for the
radio-quiet and radio-loud quasars, and a constant ~25% of all the quasars,
irrespective of radio type display associated CIV absorbers stronger than a
rest equivalent width of 0.5A. Both radio-quiet and radio-loud quasars with
narrow absorption lines have systematically redder continua, especially
strongly absorbed objects. There is evidence of inclination dependent dust
reddening and absorption for the radio quasars. An additional key result is
that the most strongly absorbed radio quasars have the largest radio source
extent. This result is in stark contrast to a recent study of the low-frequency
selected Molonglo survey in which a connection between the strength of the
narrow absorbers and the (young) age of the radio source has been proposed. The
possible origin of these discrepant results is discussed and may be related to
the higher source luminosity for the quasars studied here.Comment: 28 pages, including 10 figures and 8 tables. To appear in ApJ, 599,
December 200
Nowhere to Hide: Radio-faint AGN in the GOODS-N field. I. Initial catalogue and radio properties
(Abridged) Conventional radio surveys of deep fields ordinarily have
arc-second scale resolutions often insufficient to reliably separate radio
emission in distant galaxies originating from star-formation and AGN-related
activity. Very long baseline interferometry (VLBI) can offer a solution by
identifying only the most compact radio emitting regions in galaxies at
cosmological distances where the high brightness temperatures (in excess of
K) can only be reliably attributed to AGN activity. We present the first
in a series of papers exploring the faint compact radio population using a new
wide-field VLBI survey of the GOODS-N field. The unparalleled sensitivity of
the European VLBI Network (EVN) will probe a luminosity range rarely seen in
deep wide-field VLBI observations, thus providing insights into the role of AGN
to radio luminosities of the order across cosmic
time. The newest VLBI techniques are used to completely cover an entire 7'.5
radius area to milliarcsecond resolutions, while bright radio sources ( mJy) are targeted up to 25 arcmin from the pointing centre. Multi-source
self-calibration, and a primary beam model for the EVN array are used to
correct for residual phase errors and primary beam attenuation respectively.
This paper presents the largest catalogue of VLBI detected sources in GOODS-N
comprising of 31 compact radio sources across a redshift range of 0.11-3.44,
almost three times more than previous VLBI surveys in this field. We provide a
machine-readable catalogue and introduce the radio properties of the detected
sources using complementary data from the e-MERLIN Galaxy Evolution survey
(eMERGE).Comment: 15 pages, 8 figures, accepted in A&A. Machine-readable table
available upon reques
Conceptualizing Place Borders as Narrative: Observations From Berlin-Wedding, a Neighbourhood in Transformation
Place is of central significance to urban planning processes that specifically target community involvement and co-ownership of development decisions. Consequently, the intriguing but often daunting task of understanding how a sense of place emerges, develops, and evolves has been a subject of interdisciplinary study that links the social sciences, humanities, and more recently, cognitive sciences. Since Kevin Lynch’s classic study of urban images and mental maps, borders within cities have either directly or indirectly featured as vital meaning-making elements of place identities. However, despite some remarkable precedents, analysis of political and socio-cultural borders has only begun to link place-making and bordering processes in ways that resonate with urban planning studies. In this article, we will suggest that borders emerge in the embodied creation of social space as a means to interpret the environment and stabilise ways of knowing the wider world. Building on our own previous research on participatory place-making initiatives in Berlin, we will indicate how border stories (i.e., the social communication of neighbourhood distinction, relationality, and transformation) represent vital knowledges of place. These knowledges reflect embodied experiences of place as well as contestations and tensions that characterise place development processes. Perhaps most importantly in terms of planning, the salience of urban borders lies in broadening understanding of how and why places function—or fail to function—as communities
PKS 1018-42: A Powerful Kinetically Dominated Quasar
We have identified PKS 1018-42 as a radio galaxy with extraordinarily
powerful jets, over twice as powerful as any 3CR source of equal or lesser
redshift except for one (3C196). It is perhaps the most intrinsically powerful
extragalactic radio source in the, still poorly explored, Southern Hemisphere.
PKS 1018-42 belongs to the class of FR II objects that are kinetically
dominated, the jet kinetic luminosity, (calculated at 151 MHz), is 3.4 times larger than the
total thermal luminosity (IR to X-ray) of the accretion flow, . It is the fourth most kinetically dominated
quasar that we could verify from existing radio data. From a review of the
literature, we find that kinetically dominated sources such as PKS 1018-42 are
rare, and list the 5 most kinetically dominated sources found from our review.
Our results for PKS 1018-42 are based on new observations from the Australia
Telescope Compact Array.Comment: To appear in ApJ Letter
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