213 research outputs found
Alien plant species that invade high elevations are generalists : support for the directional ecological filtering hypothesis
QUESTIONS : The richness of invasive alien plant species tends to decrease with increasing elevation. This pattern may be due to alien plant species requiring traits allowing survival at high elevations (the Abiotic Limitation Hypothesis; ALH). In contrast, the more recent Directional Ecological Filtering Hypothesis (DEFH) suggests that only species with broad environmental tolerances will successfully spread from lowlands (where most introductions occur) to high elevations. Here we test the support for the DEFH and the ALH along an elevational gradient by asking: First, are alien species that occur at higher elevations generalists? Second, do alien species occurring at higher elevations exhibit traits that distinguishes them from lowland alien species? LOCATION : Sani Pass, Maloti-Drakensberg Transfrontier Conservation Area, South Africa.METHODS : A nestedness analysis was conducted to test whether alien species were nested along the elevational gradient, and ANOVA and Chi2 tests (supplemented by resampling procedures) were used to determine if functional traits differed between high and low elevation alien species.RESULTS : Significant nestedness of the alien flora indicates that alien species occurring at high elevations are generalists, being widespread across the elevational gradient. Compared to low elevation aliens, plant height was lower and cold tolerance weaker for high elevation species.
CONCLUSION : We found support for the DEFH with the majority of high elevation aliens being widespread generalists. Overall only two of the 11 functional traits differed between high and low elevation alien species, with only one trait supporting the ALH: shorter plant stature found at higher elevations. Therefore, complementing nestedness analyses with trait data provides a more nuanced insight into the determinants of alien richness patterns along elevational gradients and highlights how the two contemporary hypotheses might not be mutually exclusive.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1654-11032018-03-31hb2017Plant Production and Soil Scienc
Genesis of the dusty Universe: modeling submillimetre source counts
We model the evolution of IR galaxies using a phenomenological approach to
match the observed source counts at different IR wavelengths. We introduce a
new algorithm for reproducing source counts based on direct integration of
probability distributions rather than Monte-Carlo sampling. We construct a
simple model for the evolution of the luminosity function and the colour
distribution of IR galaxies which utilizes a minimum number of free parameters.
Moreover we analyze how each of these parameters is constrained by
observational data. The model is based on pure luminosity evolution and adopts
the Dale & Helou SED templates. We find that the 850um source counts and their
redshift distribution depend strongly on the shape of the luminosity evolution
function, but only weakly on the details of the SEDs. We derive the best-fit
evolutionary model using the 850um counts and redshift distribution as
constraints. Moreover our best-fit shows a flattening of the faint end of the
luminosity function towards high redshifts and requires a colour evolution
which implies the typical dust temperatures of objects with the same
luminosities to decrease with redshift. We compare our best-fit model to
observed source counts at shorter and longer wavelengths which indicates our
model reproduces the 70um and 1100um source counts remarkably well, but
under-produces the counts at intermediate wavelengths. Analysis reveals that
the discrepancy arises at low redshifts, indicating that revision of the
adopted SED library towards lower dust temperatures (at a fixed infrared
luminosity) is required. This modification is equivalent to a population of
cold galaxies existing at low redshifts, as also indicated by recent Herschel
results, which are underrepresented in IRAS sample. We show that the modified
model successfully reproduces the source counts in a wide range of IR and submm
wavelengths.Comment: 21 pages, 11 figures, 2 tables. Accepted for publication in MNRAS.
Supplementary information could be found at
http://www.strw.leidenuniv.nl/genesis
CO line emission in the halo of a radio galaxy at z=2.6
We report the detection of luminous CO(3-2) line emission in the halo of the
z=2.6 radio galaxy (HzRG) TXS0828+193, which has no detected counterpart at
optical to mid-infrared wavelengths implying a stellar mass < few x10^9 M_sun
and relatively low star-formation rates. With the IRAM PdBI we find two CO
emission line components at the same position at ~80 kpc distance from the HzRG
along the axis of the radio jet, with different blueshifts of few 100 km s^-1
relative to the HzRG and a total luminosity of ~2x10^10 K km s^-1 pc^2 detected
at 8 sigma significance. HzRGs have significant galaxy overdensities and
extended halos of metal-enriched gas often with embedded clouds or filaments of
denser material, and likely trace very massive dark-matter halos. The CO
emission may be associated with a gas-rich, low-mass satellite galaxy with
little on-going star formation, in contrast to all previous CO detections of
galaxies at similar redshifts. Alternatively, the CO may be related to a gas
cloud or filament and perhaps jet-induced gas cooling in the outer halo,
somewhat in analogy with extended CO emission found in low-redshift galaxy
clusters.Comment: MNRAS Letters, accepte
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
The CI lines as tracers of molecular gas, and their prospects at high redshifts
We examine the fine structure lines (492 GHz) and
(809 GHz) of neutral atomic carbon as bulk molecular
gas mass tracers and find that they can be good and on many occasions better
than CO transitions, especially at high redshifts. The notion of CI
emission as an H gas mass tracer challenges the long-held view of its
distribution over only a relatively narrow layer in the CII/CI/CO transition
zone in FUV-illuminated molecular clouds. Past observations have indeed
consistently pointed towards a more extended CI distribution but it was only
recently, with the advent of large scale imaging of its transition, that its surprising ubiquity in molecular clouds has
been fully revealed. In the present work we show that under {\it typical} ISM
conditions such an ubiquity is inevitable because of well known dynamic and
non-equilibrium chemistry processes maintaining a significant [C]/[CO]
abundance throughout Giant Molecular Clouds during their lifetime. These
processes are more intense in star-forming environments where a larger ambient
cosmic ray flux will also play an important role in boosting [C]/[CO].
The resulting CI lines can be bright and effective H mass tracers
especially for diffuse () gas while in UV-intense
and/or metal-poor environments their H-tracing capability diminishes
because of large scale CII production but nevertheless remains superior to that
of CO. The best place to take full advantage of CI's capacity to trace
H is not in the low- Universe, where large atmospheric absorption at 492
and 809 GHz precludes routine observations, but at high redshifts (\rm z\ga
1).Comment: Accepted for publication at the Monthly Notices of the Royal
Astronomical Society (29 pages, 5 figures
Millimeter imaging of submillimeter galaxies in the COSMOS field: Redshift distribution
We present new IRAM PdBI 1.3mm continuum observations at ~1.5" resolution of
28 SMGs previously discovered with the 870um bolometer LABOCA at APEX within
the central 0.7deg2 of the COSMOS field. 19 out of the 28 LABOCA sources were
detected with the PdBI at a >~3sigma level of ~1.4mJy/b. A combined analysis of
this new sample with existing interferometrically identified SMGs in the COSMOS
field yields the following results: 1) >~15%, and possibly up to ~40% of
single-dish detected SMGs consist of multiple sources, 2) statistical
identifications of multi-wavelength counterparts to the single-dish SMGs yield
that only ~50% of these single-dish SMGs have real radio or IR counterparts, 3)
~18% of interferometric SMGs have only radio or even no multi-wavelength
counterpart at all, and 4) ~50-70% of z>~3 SMGs have no radio counterparts down
to an rms of 7-12uJy at 1.4GHz. Using the exact interferometric positions to
identify proper multi-wavelength counterparts allows us to determine accurate
photometric redshifts for these sources. The redshift distributions of the
combined and the individual 1.1mm and 870um selected samples have a higher mean
and broader width than the redshift distributions derived in previous studies.
Our sample supports the previous tentative trend that on average brighter
and/or mm-selected SMGs are located at higher redshifts. There is a tentative
offset between the mean redshift for the 1.1mm (=3.1+/-0.4) and 870um
(=2.6+/-0.4) selected samples, with the 1.1mm sources lying on average at
higher redshifts. Based on our nearly complete sample of AzTEC 1.1mm SMGs
within a uniform 0.15deg2 area we infer a higher surface density of z>~4 SMGs
than predicted by current cosmological models. In summary, our findings imply
that (sub-)millimeter interferometric identifications are crucial to build
statistically complete and unbiased samples of SMGs.Comment: 35 pages, 18 figures, 10 tables; accepted for publication in A&
Evidence for powerful AGN winds at high redshift: Dynamics of galactic outflows in radio galaxies during the "Quasar Era"
AGN feedback now appears as an attractive mechanism to resolve some of the
outstanding problems with the "standard" cosmological models, in particular
those related to massive galaxies. To directly constrain how this may influence
the formation of massive galaxies near the peak in the redshift distribution of
powerful quasars, z~2, we present an analysis of the emission-line kinematics
of 3 powerful radio galaxies at z~2-3 (HzRGs) based on rest-frame optical
integral-field spectroscopy obtained with SINFONI on the VLT. HzRGs are among
the most massive galaxies, so AGN feedback may have a particularly clear
signature. We find evidence for bipolar outflows in all HzRGs, with kinetic
energies that are equivalent to 0.2% of the rest-mass of the supermassive black
hole. Velocity offsets in the outflows are ~800-1000 km s^-1 between the
blueshifted and redshifted line emission, FWHMs ~1000 km s^-1 suggest strong
turbulence. Ionized gas masses estimated from the Ha luminosity are of order
10^10 M_s, similar to the molecular gas content of HzRGs, underlining that
these outflows may indicate a significant phase in the evolution of the host
galaxy. The total energy release of ~10^60 erg during a dynamical time of ~10^7
yrs corresponds to about the binding energy of a massive galaxy. Geometry,
timescales and energy injection rates of order 10% of the kinetic energy flux
of the jet suggest that the outflows are most likely driven by the radio
source. The global energy density release of ~10^57 erg s^-1 Mpc^-3 may also
influence the subsequent evolution of the HzRG by enhancing the entropy and
pressure in the surrounding halo and facilitating ram-pressure stripping of gas
in satellite galaxies that may contribute to the subsequent mass assembly of
the HzRG through low-dissipation "dry" mergers.Comment: A&A in press, minor edits & typo in table captions 2-
Deep observations of CO line emission from star-forming galaxies in a cluster candidate at z=1.5
We report results from a deep Jansky Very Large Array (JVLA) search for CO
1-0 line emission from galaxies in a candidate galaxy cluster at z~1.55 in the
COSMOS field. We target 4 galaxies with optical spectroscopic redshifts in the
range z=1.47-1.59. Two of these 4 galaxies, ID51613 and ID51813, are nominally
detected in CO line emission at the 3-4 sigma level. We find CO luminosities of
2.4x10^10 K km/s pc^2 and 1.3x10^10 K km/s pc^2, respectively. Taking advantage
from the clustering and 2-GHz bandwidth of the JVLA, we perform a search for
emission lines in the proximity of optical sources within the field of view of
our observations. We limit our search to galaxies with K<23.5 (AB) and
z_phot=1.2-1.8. We find 2 bright optical galaxies to be associated with
significant emission line peaks (>4 sigma) in the data cube, which we identify
with the CO line emission. To test the reliability of the line peaks found, we
performed a parallel search for line peaks using a Bayesian inference method.
Monte Carlo simulations show that such associations are statistically
significant, with probabilities of chance association of 3.5% and 10.7% for ID
51207 and ID 51380, respectively. Modeling of their optical/IR SEDs indicates
that the CO detected galaxies and candidates have stellar masses and SFRs in
the range (0.3-1.1)x10^11 M_sun and 60-160 M_sun/yr, with SFEs comparable to
that found in other star-forming galaxies at similar redshifts. By comparing
the space density of CO emitters derived from our observations with the space
density derived from previous CO detections at z~1.5, and with semi-analytic
predictions for the CO luminosity function, we suggest that the latter tend to
underestimate the number of CO galaxies detected at high-redshift. Finally, we
argue about the benefits of future blind CO searches in clustered fields with
upcoming submm/radio facilities.Comment: Accepted for publication in MNRAS. Abstract has been slightly
shortened compared to original pdf versio
Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers
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