427 research outputs found
Adaptation and enslavement in endosymbiont-host associations
The evolutionary persistence of symbiotic associations is a puzzle.
Adaptation should eliminate cooperative traits if it is possible to enjoy the
advantages of cooperation without reciprocating - a facet of cooperation known
in game theory as the Prisoner's Dilemma. Despite this barrier, symbioses are
widespread, and may have been necessary for the evolution of complex life. The
discovery of strategies such as tit-for-tat has been presented as a general
solution to the problem of cooperation. However, this only holds for
within-species cooperation, where a single strategy will come to dominate the
population. In a symbiotic association each species may have a different
strategy, and the theoretical analysis of the single species problem is no
guide to the outcome. We present basic analysis of two-species cooperation and
show that a species with a fast adaptation rate is enslaved by a slowly
evolving one. Paradoxically, the rapidly evolving species becomes highly
cooperative, whereas the slowly evolving one gives little in return. This helps
understand the occurrence of endosymbioses where the host benefits, but the
symbionts appear to gain little from the association.Comment: v2: Correction made to equations 5 & 6 v3: Revised version accepted
in Phys. Rev. E; New figure adde
Fast outflows in compact radio sources: evidence for AGN-induced feedback in the early stages of radio source evolution
We present intermediate resolution, wide wavelength coverage spectra for a
complete sample of 14 compact radio sources taken with the aim of investigating
the impact of the nuclear activity on the circumnuclear (ISM) in the early
stages of radio source evolution. We observe spatially extended line emission
(up to 20 kpc) in the majority of sources which is consistent with a quiescent
halo. In the nuclear apertures we observe broad, highly complex emission line
profiles. Multiple Gaussian modelling of the [O III]5007 line reveals 2-4
components which can have FWHM and blueshifts relative to the halo of up to
2000 km/s. We interpret these broad, blueshifted components as material in
outflow and discuss the kinematical evidence for jet-driven outflows.
Comparisons with samples in the literature show that compact radio sources
harbour more extreme nuclear kinematics than their extended counterparts, a
trend seen within our sample with larger velocities in the smaller sources. The
observed velocities are also likely to be influenced by source orientation with
respect to the observer's line of sight. Nine sources have associated HI
absorption. In common with the optical emission line gas, the HI profiles are
often highly complex with the majority of the detected components significantly
blueshifted, tracing outflows in the neutral gas. The sample has been tested
for stratification in the ISM (FWHM/ionisation potential/critical density) as
suggested by Holt et al. (2003) for PKS1345+12 but we find no significant
trends within the sample using a Spearman Rank analysis. This study supports
the idea that compact radio sources are young radio loud AGN observed during
the early stages of their evolution and currently shedding their natal cocoons
through extreme circumnuclear outflows.Comment: Accepted for publication in MNRAS, 24 pages, 7 figure
New X-ray Constraints on Starburst and Seyfert Activity in the Barred Spiral Galaxy NGC 1672
The nearby barred spiral galaxy NGC 1672 shows dramatic starburst activity
and may also host a Seyfert 2 nucleus. We present new X-ray observations that
set constraints on starburst and Seyfert activity in NGC 1672. Two ROSAT HRI
exposures, taken in 1992 and 1997, are used to investigate long-term
variability of the known X-ray sources and to search for new sources of X-ray
emission. We find large-amplitude (about 69%) variability from X-3, one of the
off-nuclear sources located near an end of the galactic bar. X-3 has a peak
observed 0.2-2.0 keV luminosity of about 2.5 x 10^{39} erg/s, and it is
probably a luminous X-ray binary or young supernova remnant. We do not observe
variability of the nuclear source X-1 or the strong off-nuclear source X-2. Our
analyses also reveal two new off-nuclear sources, one of which is associated
with a bright region along a spiral arm, and we find evidence for large-scale
diffuse X-ray emission throughout part of the disk of NGC 1672. Furthermore, we
use ASCA data taken in 1995 to constrain the hard X-ray properties of NGC 1672.
While the nuclear source X-1 is the dominant soft X-ray source in NGC 1672, we
find that the bulk of the 2-10 keV and 5-10 keV emission is spatially
coincident with the off-nuclear source X-3, giving it an apparent 0.2-8 keV
luminosity of about 6 x 10^{39} erg/s. A power-law plus Raymond-Smith model
provides an acceptable fit to the full-band ASCA spectra. We do not find any
evidence for a luminous but absorbed nuclear X-ray source. If there is a
luminous Seyfert 2 nucleus in NGC 1672, it must be obscured by a
`Compton-thick' torus with a column density of > 2 x 10^{24} cm^{-2}.Comment: 19 pages, AJ, accepted, also available from
http://www.astro.psu.edu/users/niel/papers/papers.htm
Fecundity and the demographic strategies of coral morphologies
JM, AB and SC were supported by fellowships from the Australian Research Council (FT110100609, FT0990652 and DP0880544 respectively). MD was supported by the ERC (BioTIME 250189) and the Scottish Funding Council (MASTS - HR09011).Understanding species differences in demographic strategies is a fundamental goal of ecology. In scleractinian corals, colony morphology is tightly linked with many demographic traits, such as size-specific growth and morality. Here, we test how well morphology predicts the colony size-fecundity relationship in eight species of broadcast-spawning corals. Variation in colony fecundity is greater among morphologies than between species with a similar morphology, demonstrating that colony morphology can be used as a quantitative proxy for demographic strategies. Additionally, we examine the relationship between size-specific colony fecundity and mechanical vulnerability (i.e. vulnerability to colony dislodgment). Interestingly, the relationship between size-specific fecundity and mechanical vulnerability varied among morphologies. For tabular species, the most fecund colonies are the most mechanically vulnerable, while the opposite is true for massive species. For corymbose and digitate colonies, mechanical vulnerability remains relatively constant as fecundity increases. These results reveal strong differences in the demographic trade-offs among species of different morphologies. Using colony morphology as a quantitative proxy for demographic strategies can help predict coral community dynamics and responses to anthropogenic change.Publisher PDFPeer reviewe
The host galaxies of three radio-loud quasars: 3C 48, 3C 345, and B2 1425+267
Observations with the Wide-Field/Planetary Camera-2 of the Hubble Space
Telescope (HST) are presented for three radio-loud quasars: 3C 48 (z=0.367), B2
1425+267 (z=0.366), and 3C 345 (z=0.594). All three quasars have luminous (~4
L^*) galaxies as hosts, which are either elliptical (B2 1425+267 and 3C 345) or
interacting (3C 48), and all hosts are 0.5 - 1.0 mag bluer in (V-I) than other
galaxies with the same overall morphology at similar redshifts to the quasars.
The host of 3C 48 has many H II regions and a very extended tidal tail.
All nine of the radio-loud quasars studied here and in Bahcall et al. (1997)
either have bright elliptical hosts or occur in interacting systems. There is a
robust correlation between the radio emission of the quasar and the luminosity
of host galaxy; the radio-loud quasars reside in galaxies that are on average
about 1 mag brighter than hosts of the radio-quiet quasars.Comment: Accepted for publication in ApJ. 3 postscript and 3 jpeg figures.
Original figures may be found in ftp://eku.sns.ias.edu/pub/sofia/RadioLoud
Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula
We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula
obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and
High-resolution Spitzer IRS spectra of selected positions within the nebula,
and a near-infrared ground-based image made in the light of [Fe II]1.644
micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0
micron, resembles the general morphology of visible H-alpha and near-IR [Fe II]
line emission, while the 3.6 and 4.5 micron images are dominated by continuum
synchrotron emission. The 24 micron and 70 micron images show enhanced emission
that may be due to line emission or the presence of a small amount of warm dust
in the nebula on the order of less than 1% of a solar mass. The ratio of the
3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power
law index ranging from approximately 0.3 to 0.8 across the nebula. Combining
this information with optical and X-ray synchrotron images, we derive a
broadband spectrum that reflects the superposition of the flatter spectrum jet
and torus with the steeper diffuse nebula, and suggestions of the expected
pileup of relativistic electrons just before the exponential cutoff in the
X-ray. The pulsar, and the associated equatorial toroid and polar jet
structures seen in Chandra and HST images (Hester et al. 2002) can be
identified in all of the IRAC images. We present the IR photometry of the
pulsar. The forbidden lines identified in the high resolution IR spectra are
all double due to Doppler shifts from the front and back of the expanding
nebula and give an expansion velocity of approximately 1264 km/s.Comment: 21 pages, 4 tables, 16 figure
The Nature of LINERs
We present -band (m) spectroscopy of a sample of nine
galaxies showing some degree of LINER activity (classical LINERs, weak-[O {\sc
i}] LINERs and transition objects), together with -band spectroscopy for
some of them. A careful subtraction of the stellar continuum allows us to
obtain reliable [Fe {\sc ii}]m/Pa line ratios. We conclude
that different types of LINERs (i.e., photoionized by a stellar continuum or by
an AGN) cannot be easily distinguished based solely on the [Fe {\sc ii}]m/Pa line ratio.
The emission line properties of many LINERs can be explained in terms of an
aging starburst. The optical line ratios of these LINERs are reproduced by a
model with a metal-rich H {\sc ii} region component photoionized with a single
stellar temperature K, plus a supernova remnant (SNR) component.
The [Fe {\sc ii}] line is predominantly excited by shocks produced by SNRs in
starbursts and starburst-dominated LINERs, while Pa tracks H {\sc ii}
regions ionized by massive young stars. The contribution from SNRs to the
overall emission line spectrum is constrained by the [Fe {\sc ii}]m/Pa line ratio. Although our models for aging starbursts are
constrained only by these infrared lines, they consistently explain the optical
spectra of the galaxies also.
The LINER-starburst connection is tested by predicting the time dependence of
the ratio of the ionizing luminosity () to the supernova rate
(SNr), /(SNr). We predict the relative number of starbursts to
starburst-dominated LINERs (aging starbursts) and show that it is in
approximate agreement with survey findings for nearby galaxies.Comment: Accepted in ApJ (19 pages, 8 figures, uses emulateapj.sty
Elusive Active Galactic Nuclei
A fraction of active galactic nuclei do not show the classical Seyfert-type
signatures in their optical spectra, i.e. they are optically "elusive". X-ray
observations are an optimal tool to identify this class of objects. We combine
new Chandra observations with archival X-ray data in order to obtain a first
estimate of the fraction of elusive AGN in local galaxies and to constrain
their nature. Our results suggest that elusive AGN have a local density
comparable to or even higher than optically classified Seyfert nuclei. Most
elusive AGN are heavily absorbed in the X-rays, with gas column densities
exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with
obscuration. It is likely that in elusive AGN, the nuclear UV source is
completely embedded and the ionizing photons cannot escape, which prevents the
formation of a classical Narrow Line Region. Elusive AGN may contribute
significantly to the 30 keV bump of the X-ray background.Comment: accepted for publication in MNRAS Letters, 6 pages, 3 figures, typos
and references correcte
Supercooled liquid water cloud observed, analysed, and modelled at the top of the planetary boundary layer above Dome C, Antarctica
Abstract. A comprehensive analysis of the water budget over the Dome C (Concordia,
Antarctica) station has been performed during the austral summer 2018â2019
as part of the Year of Polar Prediction (YOPP) international campaign. Thin
(âŒ100âm deep) supercooled liquid water (SLW) clouds have been
detected and analysed using remotely sensed observations at the station
(tropospheric depolarization lidar, the H2O Antarctica Microwave Stratospheric and Tropospheric
Radiometer (HAMSTRAD), net
surface radiation from the Baseline Surface Radiation Network (BSRN)), radiosondes, and satellite observations (CALIOP, Cloud-Aerosol LIdar with Orthogonal Polarization/CALIPSO, Cloud Aerosol Lidar and Infrared
Pathfinder Satellite Observations) combined with a specific
configuration of the numerical weather prediction model: ARPEGE-SH (Action
de Recherche Petite Echelle Grande Echelle â Southern Hemisphere). The
analysis shows that SLW clouds were present from November to March, with the
greatest frequency occurring in December and January when âŒ50â% of the days in summer time exhibited SLW clouds for at least 1âh. Two case studies are used to illustrate this phenomenon. On 24 December 2018, the atmospheric planetary boundary layer (PBL) evolved
following a typical diurnal variation, which is to say with a warm and dry
mixing layer at local noon thicker than the cold and dry stable layer at
local midnight. Our study showed that the SLW clouds were observed at Dome C
within the entrainment and the capping inversion zones at the top of the
PBL. ARPEGE-SH was not able to correctly estimate the ratio between liquid
and solid water inside the clouds with the liquid water path (LWP) strongly
underestimated by a factor of 1000 compared to observations. The lack of
simulated SLW in the model impacted the net surface radiation that was 20â30âWâmâ2 higher in the BSRN observations than in the ARPEGE-SH
calculations, mainly attributable to the BSRN longwave downward surface
radiation being 50âWâmâ2 greater than that of ARPEGE-SH. The second
case study took place on 20 December 2018, when a warm and wet episode
impacted the PBL with no clear diurnal cycle of the PBL top. SLW cloud
appearance within the entrainment and capping inversion zones coincided with
the warm and wet event. The amount of liquid water measured by HAMSTRAD was
âŒ20 times greater in this perturbed PBL than in the typical
PBL. Since ARPEGE-SH was not able to accurately reproduce these SLW clouds,
the discrepancy between the observed and calculated net surface radiation
was even greater than in the typical PBL case, reaching +50âWâmâ2,
mainly attributable to the downwelling longwave surface radiation from BSRN
being 100âWâmâ2 greater than that of ARPEGE-SH. The model was then run
with a new partition function favouring liquid water for temperatures below
â20 down to â40ââC. In this test mode, ARPEGE-SH has
been able to generate SLW clouds with modelled LWP and net surface radiation
consistent with observations during the typical case, whereas, during the
perturbed case, the modelled LWP was 10 times less than the observations and
the modelled net surface radiation remained lower than the observations by
âŒ50âWâmâ2. Accurately modelling the presence of SLW
clouds appears crucial to correctly simulate the surface energy budget over
the Antarctic Plateau
A comparative HST imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies: Paper I
We present the first results from a major HST WFPC2 imaging study aimed at
providing the first statistically meaningful comparison of the morphologies,
luminosities, scalelengths and colours of the host galaxies of radio-quiet
quasars, radio-loud quasars, and radio galaxies. We describe the design of this
study and present the images which have been obtained for the first half of our
33-source sample. We find that the hosts of all three classes of luminous AGN
are massive elliptical galaxies, with scalelengths ~=10 kpc, and R-K colours
consistent with mature stellar populations. Most importantly this is the the
first unambiguous evidence that, just like radio-loud quasars, essentially all
radio-quiet quasars brighter than M_R = -24 reside in massive ellipticals. This
result removes the possibility that radio `loudness' is directly linked to host
galaxy morphology, but is however in excellent accord with the
black-hole/spheroid mass correlation recently highlighted by Magorrian et al.
(1998). We apply the relations given by Magorrian et al. to infer the expected
Eddington luminosity of the putative black hole at the centre of each of the
spheroidal host galaxies we have uncovered. Comparison with the actual nuclear
R-band luminosities suggests that the black holes in most of these galaxies are
radiating at a few percent of the Eddington luminosity; the brightest host
galaxies in our low-z sample are capable of hosting quasars with M_R = -28,
comparable to the most luminous quasars at z = 3. Finally we discuss our
host-derived black-hole masses in the context of the
radio-luminosity:black-hole mass correlation recently uncovered for nearby
galaxies by Franceschini et al. (1998), and the resulting implications for the
physical origin of radio loudness.Comment: Submitted for publication in the Astrophysical Journal, 55 pages of
latex, plus 12 postscript figures (Figures 1a-1s (greyscales of images and
model fits, and Figures 2a-2g (luminosity profiles and model fits) can be
downloaded from http://www.roe.ac.uk/astronomy/html/rjm1.shtml
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