1,398 research outputs found
Properties of dusty tori in active galactic nuclei - II. Type 2 AGN
(abridged) This paper is the second part of a work investigating the
properties of dusty tori in AGN by means of multi-component SED fitting. It
focuses on low luminosity, low redshift (z < 0.25) AGN selected among emission
line galaxies as well as X-ray, radio and mid-infrared selected type 2 AGN
samples from the literature. The available multi-band photometry covers the
spectral range from the u-band up to 160 um. The observed SED of each object is
fit to a set of multi-component models comprising a stellar component, a high
optical depth torus and cold emission from a starburst (SB). The contribution
of the various components (stars, torus, SB) is reflected in the position of
the objects on the IRAC colour diagram. The comparison of type 1 (as derived
from Hatziminaoglou et al. 2008) and type 2 AGN properties is broadly
consistent with the Unified Scheme. The estimated ratio between type 2 and type
1 objects is about 2-2.5:1. The AGN accretion-to-infrared luminosity ratio is
an indicator of the obscuration of the AGN since it scales down with the
covering factor. We find evidence supporting the receding torus paradigm, with
the estimated fraction of obscured AGN decreasing with increasing optical
luminosity over four orders of magnitude. The average star formation rates are
of ~ 10, 40 and 115 Mo/yr for the low-z, type 2 and quasar samples,
respectively; but this might simply reflect observational biases, as our
quasars are one to two orders of magnitude more luminous than the type 2 AGN.
For the majority of objects with 70 and/or 160 um detections an SB component
was needed in order to reproduce the data points, implying that the
far-infrared emission in AGN arises mostly from star formation; moreover, the
SB-to-AGN luminosity ratio shows a slight trend with increasing luminosity.Comment: 20 pages, 23 figures, accepted for publication in MNRAS; full
versions of tables 1-4 and Figs. 5, 14 and 15 only available as online
materia
Scattering of Straight Cosmic Strings by Black Holes: Weak Field Approximation
The scattering of a straight, infinitely long string moving with velocity
by a black hole is considered. We analyze the weak-field case, where the impact
parameter () is large, and obtain exact solutions to the equations of
motion. As a result of scattering, the string is displaced in the direction
perpendicular to the velocity by an amount , where . The second
term dominates at low velocities . The late-time
solution is represented by a kink and anti-kink, propagating in opposite
directions at the speed of light, and leaving behind them the string in a new
``phase''. The solutions are applied to the problem of string capture, and are
compared to numerical results.Comment: 19 pages, 5 figure
ISOCAM observations in the Lockman Hole - II The 14.3 micron deep survey: data reduction, catalogue and source counts
We present a new analysis of the ISOCAM 14.3 micron deep survey in a 20x20
square arcmins area in the Lockman Hole. This survey is intermediate between
the ultra-deep surveys and the shallow surveys in the ELAIS fields. The data
have been analyzed with the method presented by Lari et al. (2001). We have
produced a catalogue of 283 sources detected above the 5-sigma threshold, with
fluxes in the interval 0.1-8 mJy. The catalogue is 90% complete at 1 mJy. The
positional accuracy, estimated from the cross-correlation of infrared and
optical sources, is around 1.5 arcsec. The search for the optical counterparts
of the sources in the survey is performed on a medium-deep r' band optical
image (5-sigma depth of r'=25), making use of the radio detections when
available. The photometry has been checked through simulations and by comparing
the data with those presented in a shallower and more extended ISOCAM survey in
the Lockman Hole, that we have presented in a companion paper. Only 15% of the
14.3 micron sources do not have an optical counterpart down to r'=25 mag. We
use the 6.7/14.3 micron colour as a star/galaxy separator, together with a
visual inspection of the optical image and an analysis of the observed Spectral
Energy Distribution of the ISOCAM sources. The stars in the sample turn out to
be only 6% of the sample. We discuss the 14.3 micron counts of extragalactic
sources, combining our catalogue with that obtained from the shallower ISOCAM
survey. The data in the two surveys are consistent, and our results fully
support the claims in previous works for the existence of an evolving
population of infrared galaxies, confirming the evident departure from
non-evolutionary model predictions.Comment: Version accepted by Astronomy and Astrophysics. Images have been
heavily degraded to fill the archive size. A full resolution version can be
downloaded at the following link
http://dipastro.pd.astro.it/giulia/Deep/1252.ps.g
Null Strings in Schwarzschild Spacetime
The null string equations of motion and constraints in the Schwarzschild
spacetime are given. The solutions are those of the null geodesics of General
Relativity appended by a null string constraint in which the "constants of
motion" depend on the world-sheet spatial coordinate. Because of the extended
nature of a string, the physical interpretation of the solutions is completely
different from the point particle case. In particular, a null string is
generally not propagating in a plane through the origin, although each of its
individual points is. Some special solutions are obtained and their physical
interpretation is given. Especially, the solution for a null string with a
constant radial coordinate moving vertically from the south pole to the
north pole around the photon sphere, is presented. A general discussion of
classical null/tensile strings as compared to massless/massive particles is
given. For instance, tensile circular solutions with a constant radial
coordinate do not exist at all. The results are discussed in relation to
the previous literature on the subject.Comment: 16 pages, REVTEX, no figure
Radio source stacking and the infrared / radio correlation at microJy flux densities
We investigate the infrared / radio correlation using the technique of source
stacking, in order to probe the average properties of radio sources that are
too faint to be detected individually. We compare the two methods used in the
literature to stack sources, and demonstrate that the creation of stacked
images leads to a loss of information. We stack infrared sources in the Spitzer
extragalactic First Look Survey (xFLS) field, and the three northern Spitzer
Wide-area Infrared Extragalactic survey (SWIRE) fields, using radio surveys
created at 610 MHz and 1.4 GHz, and find a variation in the absolute strength
of the correlation between the xFLS and SWIRE regions, but no evidence for
significant evolution in the correlation over the 24-um flux density range 150
uJy - 2 mJy. We carry out the first radio source stacking experiment using
70-um-selected galaxies, and find no evidence for significant evolution over
the 70-um flux density range 10 mJy - 100 mJy.Comment: 11 pages, 12 figures. Accepted for publication in MNRA
Information design for bowel cancer detection: The impact of using information visualisation to help patients prepare for colonoscopy screening
Colorectal cancer is one of the most commonly occurring cancers in the world, and colonoscopy is the most sensitive procedure to detect it. Colonoscopy success depends on the quality of bowel preparation, yet the way information is designed and communicated to patients does not meet their needs. By considering how information is conveyed through three different outputs (booklet, motion graphics and app), this study investigates the advantages of using visualised information when communicating bowel preparation instructions for colonoscopy screening. A user-centered multiple-methods approach was followed and results show how user performance benefits from the use of information visualisation. A set of guidelines is given to inform the development of bowel preparation instructions and other similar health related communications
Bathymetric Artifacts in Sea Beam Data: How to Recognize Them and What Causes Them
Sea Beam multibeam bathymetric data have greatly advanced understanding of the deep seafloor. However, several types of bathymetric artifacts have been identified in Sea Beam\u27s contoured output. Surveys with many overlapping swaths and digital recording on magnetic tape of Sea Beam\u27s 16 acoustic returns made it possible to evaluate actual system performance. The artifacts are not due to the contouring algorithm used. Rather, they result from errors in echo detection and processing. These errors are due to internal factors such as side lobe interference, bottom-tracking gate malfunctions, or external interference from other sound sources (e.g., 3.5 kHz echo sounders or seismic sound sources). Although many artifacts are obviously spurious and would be disregarded, some (particularly the omega effects described in this paper) are more subtle and could mislead the unwary observer. Artifacts observed could be mistaken for volcanic constructs, abyssal hill trends, hydrothermal mounds, slump blocks, or channels and could seriously affect volcanic, tectonic, or sedimentological interpretations. Misinterpretation of these artifacts may result in positioning errors when seafloor bathymetry is used to navigate the ship. Considering these possible geological misinterpretations, a clear understanding of the Sea Beam system\u27s capabilities and limitations is deemed essential
Strings in Cosmological and Black Hole Backgrounds: Ring Solutions
The string equations of motion and constraints are solved for a ring shaped
Ansatz in cosmological and black hole spacetimes. In FRW universes with
arbitrary power behavior [R(X^0) = a\;|X^0|^{\a}\, ], the asymptotic form of
the solution is found for both and and we plot the
numerical solution for all times. Right after the big bang (), the
string energy decreasess as and the string size grows as for . Very
soon [ ] , the ring reaches its oscillatory regime with frequency
equal to the winding and constant size and energy. This picture holds for all
values of \a including string vacua (for which, asymptotically, \a = 1).
In addition, an exact non-oscillatory ring solution is found. For black hole
spacetimes (Schwarzschild, Reissner-Nordstr\oo m and stringy), we solve for
ring strings moving towards the center. Depending on their initial conditions
(essentially the oscillation phase), they are are absorbed or not by
Schwarzschild black holes. The phenomenon of particle transmutation is
explicitly observed (for rings not swallowed by the hole). An effective horizon
is noticed for the rings. Exact and explicit ring solutions inside the
horizon(s) are found. They may be interpreted as strings propagating between
the different universes described by the full black hole manifold.Comment: Paris preprint PAR-LPTHE-93/43. Uses phyzzx. Includes figures. Text
and figures compressed using uufile
Far-Infrared Properties of Spitzer-selected Luminous Starbursts
We present SHARC-2 350 micron data on 20 luminous z ~ 2 starbursts with
S(1.2mm) > 2 mJy from the Spitzer-selected samples of Lonsdale et al. and
Fiolet et al. All the sources were detected, with S(350um) > 25 mJy for 18 of
them. With the data, we determine precise dust temperatures and luminosities
for these galaxies using both single-temperature fits and models with power-law
mass--temperature distributions. We derive appropriate formulae to use when
optical depths are non-negligible. Our models provide an excellent fit to the
6um--2mm measurements of local starbursts. We find characteristic
single-component temperatures T1 ~ 35.5+-2.2 K and integrated infrared (IR)
luminosities around 10^(12.9+-0.1) Lsun for the SWIRE-selected sources.
Molecular gas masses are estimated at 4 x 10^(10) Msun, assuming
kappa(850um)=0.15 m^2/kg and a submillimeter-selected galaxy (SMG)-like
gas-to-dust mass ratio. The best-fit models imply >~2 kpc emission scales. We
also note a tight correlation between rest-frame 1.4 GHz radio and IR
luminosities confirming star formation as the predominant power source. The
far-IR properties of our sample are indistinguishable from the purely
submillimeter-selected populations from current surveys. We therefore conclude
that our original selection criteria, based on mid-IR colors and 24 um flux
densities, provides an effective means for the study of SMGs at z ~ 1.5--2.5.Comment: 13 pages, 4 figures, edited to match published version in ApJ 717,
29-39 (2010
- …