2,571 research outputs found
Sub-millimetre observations of hyperluminous infrared galaxies
We present sub-mm photometry for 11 Hyperluminous Infrared Galaxies (HLIRGs)
and use radiative transfer models for starbursts and AGN to investigate the IR
emission. In all sources both a starburst and AGN are required to explain the
IR emission. The mean starburst fraction is 35%, with a range spanning 80%
starburst dominated to 80% AGN dominated. In all cases the starburst dominates
at rest-frame wavelengths >50 microns, with star formation rates >500 solar
masses per year. The trend of increasing AGN fraction with increasing IR
luminosity seen in IRAS galaxies peaks in HLIRGs, and is not higher than the
fraction seen in bright ULIRGs. The AGN and starburst luminosities correlate,
suggesting that a common physical factor, plausibly the dust masses, governs
their luminosities. Our results suggest that the HLIRG population is comprised
both of ULIRG-like galaxy mergers, and of young galaxies going through their
maximal star formation periods whilst harbouring an AGN. The coeval AGN and
starburst activity in our sources implies that starburst and AGN activity, and
the peak starburst and AGN luminosities, can be coeval in active galaxies
generally. When extrapolated to high-z our sources have comparable sub-mm
fluxes to sub-mm survey sources. At least some sub-mm survey sources are
therefore likely to be comprised of similar galaxy populations to those found
in the HLIRG population. It is also plausible from these results that high-z
sub-mm sources harbour heavily obscured AGN. The differences in X-ray and
sub-mm properties between HLIRGs at z~1 and sub-mm sources at z~3 implies
evolution between the two epochs. Either the mean AGN obscuration level is
greater at z~3 than at z~1, or the fraction of IR-luminous sources at z~3 that
contain AGN is smaller than that at z~1.Comment: 15 pages. Accepted for publication in MNRA
557 GHz Observations of Water Vapor Outflows from VY CMa and W Hydrae
We report the first detection of thermal water vapor emission in the 557 GHz,
ground state transition of ortho-HO toward VY Canis
Majoris. In observations obtained with the Submillimeter Wave Astronomy
Satellite (SWAS), we measured a flux of Jy, in a spectrally resolved
line centered on a velocity km s with a full width half
maximum of km s, somewhat dependent on the assumed line shape.
We analyze the line shape in the context of three different radial outflow
models for which we provide analytical expressions. We also detected a weaker
557 GHz emission line from W Hydrae. We find that these and other HO
emission line strengths scale as suggested by Zubko and Elitzur (2000).Comment: Astrophysical Journal Letters, accepte
Starburst and AGN activity in ultraluminous infrared galaxies
(Abridged) We examine the power source of 41 local Ultraluminous Infrared
Galaxies using archival infrared and optical photometry. We fit the observed
Spectral Energy Distributions (SEDs) with starburst and AGN components; each
component being drawn from a family of templates. We find all of the sample
require a starburst, whereas only half require an AGN. In 90% of the sample the
starburst provides over half the IR emission, with a mean fractional luminosity
of 82%. When combined with other galaxy samples we find that starburst and AGN
luminosities correlate over 6 decades in IR luminosity suggesting that a common
factor governs both luminosities, plausibly the gas masses in the nuclear
regions. We find that the mid-IR 7.7 micron line-continuum ratio is no
indication of the starburst luminosity, or the fractional AGN luminosity, and
therefore that this ratio is not a reliable diagnostic of the power source in
ULIRGs. We propose that the scatter in the radio-IR correlation in ULIRGs is
due to a skewed starburst IMF and/or relic relativistic electrons from a
previous starburst, rather than contamination from an obscured AGN. We show
that most ULIRGs undergo multiple starbursts during their lifetime, and by
inference that mergers between more than two galaxies may be common amongst
ULIRGs. Our results support the evolutionary model for ULIRGs proposed by
Farrah et al 2001, where they can follow many different evolutionary paths of
starburst and AGN activity in transforming merging spiral galaxies into
elliptical galaxies, but that most do not go through an optical QSO phase. The
lower level of AGN activity in our local sample than in z~1 HLIRGs implies that
the two samples are distinct populations. We postulate that different galaxy
formation processes at high-z are responsible for this difference.Comment: 24 pages, 8 figures. Accepted for publication in MNRA
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization
One well-known way to constrain the hydrogen neutral fraction, x_H, of the
high-redshift intergalactic medium (IGM) is through the shape of the red
damping wing of the Lya absorption line. We examine this method's effectiveness
in light of recent models showing that the IGM neutral fraction is highly
inhomogeneous on large scales during reionization. Using both analytic models
and "semi-numeric" simulations, we show that the "picket-fence" absorption
typical in reionization models introduces both scatter and a systematic bias to
the measurement of x_H. In particular, we show that simple fits to the damping
wing tend to overestimate the true neutral fraction in a partially ionized
universe, with a fractional error of ~ 30% near the middle of reionization.
This bias is generic to any inhomogeneous model. However, the bias is reduced
and can even underestimate x_H if the observational sample only probes a subset
of the entire halo population, such as quasars with large HII regions. We also
find that the damping wing absorption profile is generally steeper than one
would naively expect in a homogeneously ionized universe. The profile steepens
and the sightline-to-sightline scatter increases as reionization progresses. Of
course, the bias and scatter also depend on x_H and so can, at least in
principle, be used to constrain it. Damping wing constraints must therefore be
interpreted by comparison to theoretical models of inhomogeneous reionization.Comment: 11 pages, 10 figures; submitted to MNRA
Pacer cell response to periodic Zeitgebers
Almost all organisms show some kind of time periodicity in their behavior.
Especially in mammals the neurons of the suprachiasmatic nucleus form a
biological clock regulating the activity-inactivity cycle of the animal. This
clock is stimulated by the natural 24-hour light-dark cycle. In our model of
this system we consider each neuron as a so called phase oscillator, coupled to
other neurons for which the light-dark cycle is a Zeitgeber. To simplify the
model we first take an externally stimulated single phase oscillator. The first
part of the phase interval is called the active state and the remaining part is
the inactive state. Without external stimulus the oscillator oscillates with
its intrinsic period. An external stimulus, be it from activity of neighboring
cells or the periodic daylight cycle, acts twofold, it may delay the change
form active to inactive and it may advance the return to the active state. The
amount of delay and advance depends on the strength of the stimulus. We use a
circle map as a mathematical model for this system. This map depends on several
parameters, among which the intrinsic period and phase delay and advance. In
parameter space we find Arnol'd tongues where the system is in resonance with
the Zeitgeber. Thus already in this simplified system we find entrainment and
synchronization. Also some other phenomena from biological experiments and
observations can be related to the dynamical behavior of the circle map
Squelched Galaxies and Dark Halos
There is accumulating evidence that the faint end of the galaxy luminosity
function might be very different in different locations. The luminosity
function might be rising in rich clusters and flat or declining in regions of
low density. If galaxies form according to the model of hierarchical clustering
then there should be many small halos compared to the number of big halos. If
this theory is valid then there must be a mechanism that eliminates at least
the visible component of galaxies in low density regions. A plausible mechanism
is photoionization of the intergalactic medium at a time before the epoch that
most dwarf galaxies form in low density regions but after the epoch of
formation for similar systems that ultimately end up in rich clusters. The
dynamical timescales are found to accommodate this hypothesis in a flat
universe with Omega_m < 0.4.
If small halos exist but simply cannot be located because they have never
become the sites of significant star formation, they still might have dynamical
manifestations. These manifestations are hard to identify in normal groups of
galaxies because small halos do not make a significant contribution to the
global mass budget. However, it could be entertained that there are clusters of
halos where there are only small systems, clusters that are at the low mass end
of the hierarchical tree. There may be places where only a few small galaxies
managed to form, enough for us to identify and use as test probes of the
potential. It turns out that such environments might be common. Four probable
groups of dwarfs are identified within 5 Mpc and the assumption they are
gravitationally bound suggests M/L_B ~ 300 - 1200 M_sun/L_sun, 6 +/- factor 2
times higher than typical values for groups with luminous galaxies.Comment: Accepted ApJ 569, (April 20), 2002, 12 pages, 6 figures, 1 tabl
Phenotypic heterogeneity of peripheral monocytes in healthy dogs
Monocytes are key cells of the innate immune system. Their phenotypic and functional roles have been investigated in humans, mice and other animals, such as the rat, pig and cow. To date, detailed phenotypic analysis of monocytes has not been undertaken in dogs. Two important surface markers in human monocytes are CD14 and MHC class II (MHC II). By flow cytometry, we demonstrated that canine monocytes can be subdivided into three separate populations: CD14posMHC IIneg, CD14posMHC IIpos and CD14negMHC IIpos. Both light and transmission electron microscopy confirmed the monocytic identity of all three populations. The CD14posMHC IIneg population could be distinguished on an ultrastructural level by their smaller size, the presence of more numerous, larger granules, and more pseudopodia than both of the other populations
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