2,117 research outputs found
FIRST-based survey of Compact Steep Spectrum sources I. MERLIN images of arc-second scale objects
Compact Steep Spectrum (CSS) sources are powerful extragalactic radio sources
with angular dimensions of the order of a few arcseconds or less. Such a
compactness is apparently linked to the youth of these objects. The majority of
CSSs investigated so far have been known since the early 1980s. This paper is
the first in a series where we report the results of an observational campaign
targeted on a completely new sample of CSSs which are significantly weaker than
those investigated before. The ultimate goal of that campaign is to find out
how ``weak'' CSSs compare to ``strong'', classical ones, especially with regard
to the morphologies. Here we present an analysis of morphological and physical
properties of five relatively large sources based on MERLIN observations at 1.6
and 5 GHz.Comment: 8 pages, 7 figures, A&A in pres
Convergence of bark investment according to fire and climate structures ecosystem vulnerability to future change
Fire regimes in savannas and forests are changing over much of the world. Anticipating the impact of these changes requires understanding how plants are adapted to fire. Here we test whether fire imposes a broad selective force on a key fire-tolerance trait, bark thickness, across 572 tree species distributed worldwide. We show that investment in thick bark is a pervasive adaptation in frequently burned areas across savannas and forests in both temperate and tropical regions where surface fires occur. Geographic variability in bark thickness is largely explained by annual burned area and precipitation seasonality. Combining environmental and species distribution data allowed us to assess the vulnerability to future climate and fire conditions: tropical rainforests are especially vulnerable, whereas seasonal forests and savannas are more robust. The strong link between fire and bark thickness provides an avenue for assessing the vulnerability of tree communities to fire and demands inclusion in global models
FIRST-based survey of Compact Steep Spectrum sources, II. MERLIN and VLA observations of Medium-sized Symmetric Objects
A new sample of candidate Compact Steep Spectrum (CSS) sources that are much
weaker than the CSS source prototypes has been selected from the VLA FIRST
catalogue. MERLIN `snapshot' observations of the sources at 5 GHz indicate that
six of them have an FR II-like morphology, but are not edge-brightened as is
normal for Medium-sized Symmetric Objects (MSOs) and FR IIs. Further
observations of these six sources with the VLA at 4.9 GHz and MERLIN at 1.7
GHz, as well as subsequent full-track observations with MERLIN at 5 GHz of what
appeared to be the two sources of greatest interest are presented. The results
are discussed with reference to the established evolutionary model of CSS
sources being young but in which not all of them evolve to become old objects
with extended radio structures. A lack of stable fuelling in some of them may
result in an early transition to a so-called coasting phase so that they fade
away instead of growing to become large-scale objects. It is possible that one
of the six sources (1542+323) could be labelled as a prematurely `dying' MSO or
a `fader'.Comment: 13 pages, matches the version printed in Astronomy & Astrophysic
Is the expansion of sugarcane over pasturelands a sustainable strategy for Brazil's bioenergy industry?
The authors gratefully thank the São Paulo Research Foundation (FAPESP) (grants # 2014/08632-9 # 2015/14122-6, # 2013/17581-6, # 2014/16612-8 and 2018/09845-7) for the scholarship granted while this research was carried out, and CNPq (grants # 402992/2013-0 and # 311661/2014-9) for the financial support of the present research. Anonymous reviewers are also thanked for their valuable criticisms and comments, which led to substantial improvements of this manuscript.Peer reviewedPostprintPostprin
The Cosmic Lens All-Sky Survey: I. Source selection and observations
The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative
program which has obtained high-resolution radio images of over 10000
flat-spectrum radio sources in order to create the largest and best studied
statistical sample of radio-loud gravitationally lensed systems. With this
survey, combined with detailed studies of the lenses found therein, constraints
can be placed on the expansion rate, matter density, and dark energy (e.g.
cosmological constant, quintessence) content of the Universe that are
complementary to and independent of those obtained through other methods. CLASS
is aimed at identifying lenses where multiple images are formed from compact
flat-spectrum radio sources, which should be easily identifiable in the radio
maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not
a factor, and the relative insensitivity of the instrument to environmental
conditions leads to nearly uniform sensitivity and resolution over the entire
survey. In four observing seasons from 1994-1999, CLASS has observed 13783
radio sources with the VLA at 8.4 GHz at 0.2 arcsecond resolution. When
combined with the JVAS survey, the CLASS sample contains over 16,000 images. A
complete sample of 11685 flat-spectrum sources was observed, selected from GB6
catalogue at 4.85 GHz and the NVSS at 1.4 GHz. So far, CLASS has found 16 new
gravitational lens systems, and the JVAS/CLASS survey contains a total of 22
lenses. (Abridged)Comment: 14 pages, 9 figures, accepted for publication in MNRA
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