131 research outputs found
The Far-infrared Continuum of Quasars
ISO provides a key new far-infrared window through which to observe the
multi-wavelength spectral energy distributions (SEDs) of quasars and active
galactic nuclei (AGN). It allows us, for the first time, to observe a
substantial fraction of the quasar population in the far-IR, and to obtain
simultaneous, multi-wavelength observations from 5--200 microns. With these
data we can study the behavior of the IR continuum in comparison with
expectations from competing thermal and non-thermal models. A key to
determining which mechanism dominates, is the measurement of the peak
wavelength of the emission and the shape of the far-IR--mm turnover. Turnovers
which are steeper than frequency^2.5 indicate thermal dust emission in the
far-IR.
Preliminary results from our ISO data show broad, fairly smooth, IR continuum
emission with far-IR turnovers generally too steep to be explained by
non-thermal synchrotron emission. Assuming thermal emission throughout leads to
a wide inferred temperature range of 50-1000 K. The hotter material, often
called the AGN component, probably originates in dust close to and heated by
the central source, e.g. the ubiquitous molecular torus. The cooler emission is
too strong to be due purely to cool, host galaxy dust, and so indicates either
the presence of a starburst in addition to the AGN or AGN-heated dust covering
a wider range of temperatures than present in the standard, optically thick
torus models.Comment: 4 pages, to be published in the proceedings of "The Universe as Seen
by ISO," ed. M. Kessler. This and related papers can be found at
http://hea-www.harvard.edu/~ehooper/ISOkp/ISOkp.htm
Infrared Properties of High Redshift and X-ray Selected AGN Samples
The NASA/ISO Key Project on active galactic nuclei (AGN) seeks to better
understand the broad-band spectral energy distributions (SEDs) of these sources
from radio to X-rays, with particular emphasis on infrared properties. The ISO
sample includes a wide variety of AGN types and spans a large redshift range.
Two subsamples are considered herein: 8 high-redshift (1 < z < 4.7) quasars;
and 22 hard X-ray selected sources.
The X-ray selected AGN show a wide range of IR continuum shapes, extending to
cooler colors than the optical/radio sample of Elvis et al. (1994). Where a
far-IR turnover is clearly observed, the slopes are < 2.5 in all but one case
so that non-thermal emission remains a possibility. The highest redshift
quasars show extremely strong, hot IR continua requiring ~ 100 solar masses of
500 - 1000 Kelvin dust with ~ 100 times weaker optical emission. Possible
explanations for these unusual properties include: reflection of the optical
light from material above/below a torus; strong obscuration of the optical
continuum; or an intrinsic deficit of optical emission.Comment: 8 pages, 3 figures (2 color), to be published in the Springer Lecture
Notes of Physics Series as part of the proceedings for "ISO Surveys of a
Dusty Universe," a workshop held at Ringberg Castle, Germany, November 8 -
12, 1999. Requires latex style files for this series: cl2emult.cls,
cropmark.sty, lnp.sty, sprmindx.sty, subeqnar.sty (included with submission
Comparison of four dietary pattern indices in Australian baby boomers: Findings from the Busselton Healthy Ageing Study
The assessment of dietary patterns comprehensively represents the totality of the diet, an important risk factor for many chronic diseases. This study aimed to characterise and compare four dietary pattern indices in middle-aged Australian adults. In 3458 participants (55 % female) from the Busselton Healthy Ageing Study (Phase Two), a validated food frequency questionnaire was used to capture dietary data between 2016 and 2022. Four dietary patterns [Australian Dietary Guideline Index 2013 (DGI-2013); the Mediterranean Diet Index (MedDiet); the Literature-based Mediterranean Diet Index (Lit-MedDiet); and the EAT-Lancet Index], were calculated and compared by measuring total and sub-component scores, and concordance (Ïc). Cross-sectional associations between the dietary indices and demographic, lifestyle, and medical conditions were modelled with linear regression and restricted cubic splines. Participants had the highest standardised scores for the DGI-2013 followed by the EAT-Lancet Index and the MedDiet, with the lowest standardised scores observed for the Lit-MedDiet. The DGI-2013 had the lowest agreement with the other scores (Ïc †0.47). These findings indicate that the diets included in this Australian cohort align more closely with the Australian Dietary Guidelines than with the other international dietary patterns, likely due to the wide variation of individual food group weightings in the construction of these indices
The Formation of the First Stars I. Mass Infall Rates, Accretion Disk Structure and Protostellar Evolution
We present a theoretical model for primordial star formation. First we
describe the structure of the initial gas cores as virialized,
quasi-hydrostatic objects in accord with recent high resolution numerical
studies. The accretion rate can then be related to characteristic densities and
temperatures that are set by the cooling properties of molecular hydrogen. We
allow for rotation of the gas core, assuming angular momentum conservation
inside the sonic point of the flow. In the typical case, most mass then reaches
the star via an accretion disk. The structure of the inner region of this disk
is described with the standard theory of viscous disks, but with allowance for
the substantial energies absorbed in ionizing and dissociating the gas. The
size of the protostar and its luminosity depend upon the accretion rate, the
energetics of the accreting gas, and the ability of the radiation to escape
from the stellar accretion shock. We combine these models for the infall rate,
inner disk structure, and protostellar evolution to predict the radiation field
that is the basis for radiative feedback processes acting against infall (Paper
II). For realistic initial angular momenta, the photosphere of the protostar is
much smaller and hotter than in the spherical case, leading to stronger
radiative feedback at earlier stages in the evolution. In particular, once the
star is older than its Kelvin-Helmholtz time, contraction towards the main
sequence causes a rapid increase in ionizing and far-ultraviolet luminosity at
masses ~30Msun in the fiducial case. Since the cores out of which the first
stars formed were much more massive than 30Msun and since feedback is
dynamically unimportant at lower masses, we conclude that the first stars
should have had masses >~30Msun.Comment: 20 pages, Accepted to ApJ, some re-arrangement of text for improved
clarit
Chandra Observations of the Dwarf Nova WX Hyi in Quiescence
We report Chandra observations of the dwarf nova WX Hyi in quiescence. The
X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S
and Fe. The various ionization states implied by the lines suggest that the
emission is produced within a flow spanning a wide temperature range, from T ~
10^6 K to T >~ 10^8 K. Line diagnostics indicate that most of the radiation
originates from a very dense region, with n ~ 10^{13}-10^{14} cm^{-3}. The
Chandra data allow the first tests of specific models proposed in the
literature for the X-ray emission in quiescent dwarf novae. We have computed
the spectra for a set of models ranging from hot boundary layers, to hot
settling flows solutions, to X-ray emitting coronae. WX Hyi differs from other
dwarf novae observed at minimum in having much stronger low temperature lines,
which prove difficult to fit with existing models, and possibly a very strong,
broad O VII line, perhaps produced in a wind moving at a few x 10^3 km/s. The
accretion rate inferred from the X-rays is lower than the value inferred from
the UV. The presence of high-velocity mass ejection could account for this
discrepancy while at the same time explaining the presence of the broad O VII
line. If this interpretation is correct, it would provide the first detection
of a wind from a dwarf nova in quiescence.Comment: accepted to ApJ; 19 pages, 3 figures, 1 tabl
Galaxy Clusters in the IRAC Dark Field. II. Mid-Infrared Sources
We present infrared (IR) luminosities, star formation rates (SFR), colors, morphologies, locations, and active galactic nuclei (AGNs) properties of 24 ÎŒm detected sources in photometrically detected high-redshift clusters in order to understand the impact of environment on star formation (SF) and AGN evolution in cluster galaxies. We use three newly identified z = 1 clusters selected from the IRAC dark field; the deepest ever mid-IR survey with accompanying, 14 band multiwavelength data including deep Hubble Space Telescope imaging and deep wide-area Spitzer MIPS 24 ÎŒm imaging. We find 90 cluster members with MIPS detections within two virial radii of the cluster centers, of which 17 appear to have spectral energy distributions dominated by AGNs and the rest dominated by SF. We find that 43% of the star-forming sample have IR luminosities L_(IR) > 10^(11) L_â(luminous IR galaxies). The majority of sources (81%) are spirals or irregulars. A large fraction (at least 25%) show obvious signs of interactions. The MIPS-detected member galaxies have varied spatial distributions as compared to the MIPS-undetected members with one of the three clusters showing SF galaxies being preferentially located on the cluster outskirts, while the other two clusters show no such trend. Both the AGN fraction and the summed SFR of cluster galaxies increase from redshift zero to one, at a rate that is a few times faster in clusters than over the same redshift range in the field. Cluster environment does have an effect on the evolution of both AGN fraction and SFR from redshift one to the present, but does not affect the IR luminosities or morphologies of the MIPS sample. SF happens in the same way regardless of environment making MIPS sources look the same in the cluster and field, however the cluster environment does encourage a more rapid evolution with time as compared to the field
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