8,977 research outputs found
Deepest Near-IR Surface Photometry of Galaxies in the Local Sphere of Influence
We present near-IR, deep (4 mag deeper than 2MASS) imaging of 56 Local Volume
galaxies. Global parameters such as total magnitudes and stellar masses have
been derived and the new near-IR data combined with existing 21cm and optical
B-band data. We present multiwavelength relations such as the HI mass-to-light
ratio and investigate the maximum total baryonic mass a galaxy can have.Comment: 4 pages, 3 figures, To be published in the proceedings of "Galaxies
in the Local Volume", ed. B. Koribalski, H. Jerje
Observational Prospects for Afterglows of Short Duration Gamma-ray Bursts
If the efficiency for producing -rays is the same in short duration
(\siml 2 s) Gamma-Ray Bursts (GRBs) as in long duration GRBs, then the
average kinetic energy of short GRBs must be times less than that of
long GRBs. Assuming further that the relativistic shocks in short and long
duration GRBs have similar parameters, we show that the afterglows of short
GRBs will be on average 10--40 times dimmer than those of long GRBs. We find
that the afterglow of a typical short GRB will be below the detection limit
(\siml 10 \microJy) of searches at radio frequencies. The afterglow would be
difficult to observe also in the optical, where we predict R \simg 23 a few
hours after the burst. The radio and optical afterglow would be even fainter if
short GRBs occur in a low-density medium, as expected in NS-NS and NS-BH merger
models. The best prospects for detecting short-GRB afterglows are with early
(\siml 1 day) observations in X-rays.Comment: 5 pages, 2 figures, submitted to ApJ lette
Temporal Variability of the X-ray Emission of the Crab Nebula Torus
We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years
1991 to 1997 and have found significant changes in the emission structure of
the X-ray torus surrounding the pulsar. Certain regions increase in brightness
by about 20% over the six years, while others show decreases in surface
brightness. The origin of these changes is unclear, but a possible explanation
is that the bulk velocity of the synchrotron radiating electrons has decreased
on the order of 20% as well.Comment: 15 pages plus 6 figures, figure 1 and figure 6 are in color, to
appear in The Astrophysical Journal, Jan 1, 1999, Vol. 510, #
Online social network size is reflected in human brain structure
The increasing ubiquity of web-based social networking services is a striking feature of modern human society. The degree to which individuals participate in these networks varies substantially for reasons that are unclear. Here, we show a biological basis for such variability by demonstrating that quantitative variation in the number of friends an individual declares on a web-based social networking service reliably predicted grey matter density in the right superior temporal sulcus, left middle temporal gyrus and entorhinal cortex. Such regions have been previously implicated in social perception and associative memory, respectively. We further show that variability in the size of such online friendship networks was significantly correlated with the size of more intimate real-world social groups. However, the brain regions we identified were specifically associated with online social network size, whereas the grey matter density of the amygdala was correlated both with online and real-world social network sizes. Taken together, our findings demonstrate that the size of an individual's online social network is closely linked to focal brain structure implicated in social cognition
Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant
We present measurements of bi-directional motions in the jets of the radio
galaxy 1946+708 at z=0.101. This is a Compact Symmetric Object with striking
S-symmetry. Sensitive 15 GHz observations reveal a compact component at the
center of symmetry with a strongly inverted spectrum, that we identify as the
core. From five 4.9 GHz observations spread over 4 years we have determined the
velocities of four compact jet components. If simple kinematic models can be
applied then the inclination of the source and the bulk jet velocity can be
directly determined for any assumed value of the Hubble constant. Conversely,
the measurements already place constraints on the Hubble constant, and we show
how further observations of 1946+708 can yield an increasingly accurate
determination of H_0.Comment: in press at ApJ Letters, 12 page LaTex document includes 5 postscript
figure
Estimating the ground truth from multiple individual segmentations incorporating prior pattern analysis with application to skin lesion segmentation
Critical review of the impacts of grazing intensity on soil organic carbon storage and other soil quality indicators in extensively managed grasslands
Acknowledgements This work contributes to the N-Circle project (grant number BB/N013484/1), and CINAg (BB/N013468/1) Virtual Joint Centres on Agricultural Nitrogen (funded by the Newton Fund via UK BBSRC/NERC), U-GRASS (grant number NE/M016900/1), the Belmont Forum/FACCE-JPI DEVIL project (grant number NE/M021327/1), Soils-R-GGREAT (grant number NE/P019455/1), ADVENT (grant number NE/M019713/1), Sêr Cymru LCEE-NRN project, Climate-Smart Grass and the Scottish Government’s Strategic Research Programme.Peer reviewedPublisher PD
Polarization in the prompt emission of gamma-ray bursts and their afterglows
Synchrotron is considered the dominant emission mechanism in the production
of gamma-ray burst photons in the prompt as well as in the afterglow phase.
Polarization is a characteristic feature of synchrotron and its study can
reveal a wealth of information on the properties of the magnetic field and of
the energy distribution in gamma-ray burst jets. In this paper I will review
the theory and observations of gamma-ray bursts polarization. While the theory
is well established, observations have prove difficult to perform, due to the
weakness of the signal. The discriminating power of polarization observations,
however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of
Physics focus issue on Gamma Ray Burst
Cosmic Renaissance: The First Sources of Light
I review recent progress in understanding the formation of the first stars
and quasars. The initial conditions for their emergence are given by the now
firmly established model of cosmological structure formation. Numerical
simulations of the collapse and fragmentation of primordial gas indicate that
the first stars formed at redshifts z ~ 20 - 30, and that they were
predominantly very massive, with M_* > 100 M_sun. Important uncertainties,
however, remain. Paramount among them is the accretion process, which builds up
the final stellar mass by incorporating part of the diffuse, dust-free envelope
into the central protostellar core. The first quasars, on the other hand, are
predicted to have formed later on, at z ~ 10, in more massive dark matter
halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.Comment: 16 pages, 7 figures, invited review, to appear in PASP, Feb. 200
Anthropic reasoning in multiverse cosmology and string theory
Anthropic arguments in multiverse cosmology and string theory rely on the
weak anthropic principle (WAP). We show that the principle, though ultimately a
tautology, is nevertheless ambiguous. It can be reformulated in one of two
unambiguous ways, which we refer to as WAP_1 and WAP_2. We show that WAP_2, the
version most commonly used in anthropic reasoning, makes no physical
predictions unless supplemented by a further assumption of "typicality", and we
argue that this assumption is both misguided and unjustified. WAP_1, however,
requires no such supplementation; it directly implies that any theory that
assigns a non-zero probability to our universe predicts that we will observe
our universe with probability one. We argue, therefore, that WAP_1 is
preferable, and note that it has the benefit of avoiding the inductive
overreach characteristic of much anthropic reasoning.Comment: 7 pages. Expanded discussion of selection effects and some minor
clarifications, as publishe
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