3,557 research outputs found
Renyi Entropies, the Analytic Bootstrap, and 3D Quantum Gravity at Higher Genus
We compute the contribution of the vacuum Virasoro representation to the
genus-two partition function of an arbitrary CFT with central charge .
This is the perturbative pure gravity partition function in three dimensions.
We employ a sewing construction, in which the partition function is expressed
as a sum of sphere four-point functions of Virasoro vacuum descendants. For
this purpose, we develop techniques to efficiently compute correlation
functions of holomorphic operators, which by crossing symmetry are determined
exactly by a finite number of OPE coefficients; this is an analytic
implementation of the conformal bootstrap. Expanding the results in ,
corresponding to the semiclassical bulk gravity expansion, we find
that---unlike at genus one---the result does not truncate at finite loop order.
Our results also allow us to extend earlier work on multiple-interval Renyi
entropies and on the partition function in the separating degeneration limit.Comment: 63 pages + ref
The nature of the absorbing torus in compact Radio Galaxies
We present BeppoSAX observations of the two radio galaxies PKS 1934-63 and S5
1946+708. Strong Iron Kalpha lines are detected in both objects indicating that
the two sources are absorbed by column densities higher than 10^24 cm^(-2).
Combining radio continuum, HI absorption and X-ray data we can constrain the
physical state and the dimensions of the absorber. We find that the putative
obscuring torus is molecular and located at a distance higher than 20 pc from
the center in S5 1946+70. In PKS 1934-63 no radio nucleus has been observed. If
this is due to free-free absorption a radius of only a few pc is inferred.
Since the Kalpha lines have been detected only at 90% confidence, we also
discuss the implications if they would be not as strong as found from our data.
From our results it appears that the combination of hard X-ray data and
spatially resolved HI absorption measurements provides a powerful diagnostic
for conditions in the absorbing ``torus''.Comment: 8 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
The Smith Cloud: HI associated with the Sgr dwarf?
The Smith high velocity cloud (V(LSR) = 98 kms) has been observed at two
locations in the emission lines [OIII]5007, [NII]6548 and H-alpha. Both the
[NII] and H-alpha profiles show bright cores due to the Reynolds layer, and red
wings with emission extending to V(LSR) = 130 kms. This is the first
simultaneous detection of two emission lines towards a high velocity cloud,
allowing us to form the ratio of these line profiles as a function of LSR
velocity. At both cloud positions, we see a clear distinction between emission
at the cloud velocity, and the Reynolds layer emission (V(LSR) = 0). The
[NII]/H-alpha ratio (=0.25) for the Reynolds layer is typical of the warm
ionised medium. At the cloud velocity, this ratio is enhanced by a factor of
3-4 compared to emission at rest with respect to the LSR. A moderately deep
upper limit at [OIII] (0.12R at 3-sigma) was derived from our data. If the
emission arises from dilute photoionisation from hot young stars, the highly
enhanced [NII]/H-alpha ratio, the [OIII] non-detection and weak H-alpha
emission (0.24-0.30R) suggest that the Smith Cloud is 26+/-4 kpc from the Sun,
at a Galactocentric radius of 20+/-4 kpc. This value assumes that the emission
arises from an optically thick slab, with a covering fraction of unity as seen
by the ionizing photons, whose orientation is either (a) parallel to the
Galactic disk, or (b) such as to maximize the received flux from the disk. The
estimated mass and size of the cloud are 4x10^6 Msun and 6 kpc. We discuss a
possible association with the much larger Sgr dwarf, at a galactocentric radius
of 16+/-2 kpc, which lies within 35 degrees (~12 kpc) of the Smith Cloud.Comment: 18 pages, 14 figures, mn.sty. Our first application of a new method
for establishing distances to high velocity clouds. This version matches
paper to appear in MNRAS, 299, 611-624 (Sept. 11 issue
A Decade Of Starspot Activity On The Eclipsing Short-Period RS Canum Venaticorum Star WY Cancri: 1988-1997
We present optical photometry of the short-period eclipsing RS CVn system WY Cancri for the years 1988â1997. For each light curve, we model the distortion waves in order to study the behavior of starspots in this system. After removing the spot effects f
Topologically Massive Gravity and Ricci-Cotton Flow
We consider Topologically Massive Gravity (TMG), which is three dimensional
general relativity with a cosmological constant and a gravitational
Chern-Simons term. When the cosmological constant is negative the theory has
two potential vacuum solutions: Anti-de Sitter space and Warped Anti-de Sitter
space. The theory also contains a massive graviton state which renders these
solutions unstable for certain values of the parameters and boundary
conditions. We study the decay of these solutions due to the condensation of
the massive graviton mode using Ricci-Cotton flow, which is the appropriate
generalization of Ricci flow to TMG. When the Chern-Simons coupling is small
the AdS solution flows to warped AdS by the condensation of the massive
graviton mode. When the coupling is large the situation is reversed, and warped
AdS flows to AdS. Minisuperspace models are constructed where these flows are
studied explicitly
The Bolocam 1.1 mm Lockman Hole Galaxy Survey: SHARC II 350 micron Photometry and Implications for Spectral Models, Dust Temperatures, and Redshift Estimation
We present 350 micron photometry of all 17 galaxy candidates in the Lockman
Hole detected in a 1.1 mm Bolocam survey. Several of the galaxies were
previously detected at 850 microns, at 1.2 mm, in the infrared by Spitzer, and
in the radio. Nine of the Bolocam galaxy candidates were detected at 350
microns and two new candidates were serendipitously detected at 350 microns
(bringing the total in the literature detected in this way to three). Five of
the galaxies have published spectroscopic redshifts, enabling investigation of
the implied temperature ranges and a comparison of photometric redshift
techniques.
Lambda = 350 microns lies near the spectral energy distribution peak for z =
2.5 thermally emitting galaxies. Thus, luminosities can be measured without
extrapolating to the peak from detection wavelengths of lambda > 850 microns.
Characteristically, the galaxy luminosities lie in the range 1.0 - 1.2 x 10^13
L_solar, with dust temperatures in the range of 40 K to 70 K, depending on the
choice of spectral index and wavelength of unit optical depth. The implied dust
masses are 3 - 5 x 10^8 M_solar. We find that the far-infrared to radio
relation for star-forming ULIRGs systematically overpredicts the radio
luminosities and overestimates redshifts on the order of Delta z ~ 1, whereas
redshifts based on either on submillimeter data alone or the 1.6 micron stellar
bump and PAH features are more accurate.Comment: In Press (to appear in Astrophysical Journal, ApJ 20 May 2006 v643 1)
47 pages, 10 figures, 4 table
Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation
Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in
the radial density profile at some distance from the center. Using 22 galaxies
with known HI distributions as an example, we discuss the question of whether
this effect can be associated exclusively with external ionizing radiation, as
is commonly assumed. We show that before the surface density reaches
(the same for
galaxies of different types), it is hard to expect the gas to be fully ionized
by background radiation. For two of 13 galaxies with a sharp drop in the HI
profile, the "steepening" can actually be caused by ionization. At the same
time, for the remaining galaxies, the observed cutoff in the radial HI profile
is closer to the center than if it was a consequence of ionization by
background radiation and, therefore, it should be caused by other factors.Comment: 15 pages, 6 figure
Exploring the views of students on the use of Facebook in university teaching and learning
Facebook use among students is almost ubiquitous; however, its use for formal academic purposes remains contested. Through an online survey monitoring student use of module Facebook pages and focus groups, this study explores studentsâ current academic uses of Facebook and their
views on using Facebook within university modules. Students reported using Facebook for academic purposes, notably peerâpeer communication around group work and assessment â a use not always conceptualised by students as learning. Focus groups revealed that students are not ready or equipped for the collaborative style of learning envisaged by the tutor and see Facebook as their personal domain, within which they
will discuss academic topics where they see a strong relevance and purpose, notably in connection with assessment. Students use Facebook for their own mutually deïŹned purposes and a change in student mind- and skill-sets is required to appropriate the collaborative learning beneïŹts of Facebook in formal educational contexts
Unusual Higgs or Supersymmetry from Natural Electroweak Symmetry Breaking
This review provides an elementary discussion of electroweak symmetry
breaking in the minimal and the next-to-minimal supersymmetric models with the
focus on the fine-tuning problem -- the tension between natural electroweak
symmetry breaking and the direct search limit on the Higgs boson mass. Two
generic solutions of the fine-tuning problem are discussed in detail: models
with unusual Higgs decays; and models with unusual pattern of soft
supersymmetry breaking parameters.Comment: 23 pages, 6 figures; invited review by MPL
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