4,945 research outputs found
The outer filament of Centaurus A as seen by MUSE
We investigate signatures of a jet-interstellar medium (ISM) interaction
using optical integral-field observations of the so-called outer filament near
Centaurus A, expanding on previous results obtained on a more limited area.
Using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT during science
verification, we observed a significant fraction of the brighter emitting gas
across the outer filament. The ionized gas shows complex morphology with
compact blobs, arc-like structures and diffuse emission. Based on the
kinematics, we identified three main components. The more collimated component
is oriented along the direction of the radio jet. The other two components
exhibit diffuse morphology together with arc-like structures also oriented
along the radio jet direction. Furthermore, the ionization level of the gas is
found to decrease from the more collimated component to the more diffuse
components. The morphology and velocities of the more collimated component
confirm our earlier results that the outer filament and the nearby HI cloud are
likely partially shaped by the lateral expansion of the jet. The arc-like
structures embedded within the two remaining components are the clearest
evidence of a smooth jet-ISM interaction along the jet direction. This suggests
that, although poorly collimated, the radio jet is still active and has an
impact on the surrounding gas. This result indicates that the effect on the ISM
of even low-power radio jets should be considered when studying the influence
Active Galactic Nuclei can have on their host galaxy.Comment: 5 pages, 3 figures, Accepted for publication by A&
Emission lines and optical continuum in low-luminosity radio galaxies
We present spectroscopic observations of a complete sub-sample of 13
low-luminosity radio galaxies selected from the 2Jy sample. The underlying
continuum in these sources is carefully modelled in order to make a much-needed
comparison between the emission line and continuum properties of FRIs with
those of other classes of radio sources. We find that 5 galaxies in the sample
show a measurable UV excess: 2 of the these sources are BL Lacs and in the
remaining 3 galaxies we argue that the most likely contributor to the UV excess
is a young stellar component. Excluding the BL Lacs, we therefore find that
\~30% of the sample show evidence for young stars, which is similar to the
results obtained for higher luminosity samples. We compare our results with
far-infrared measurements in order to investigate the far-infrared-starburst
link. The nature of the optical-radio correlations is investigated in light of
this new available data and, in contrast to previous studies, we find that the
FRI sources follow the correlations with a similar slope to that found for the
FRIIs. Finally, we compare the luminosity of the emission lines in the FRI and
BL Lac sources and find a significant difference in the [OIII] line
luminosities of the two groups. Our results are discussed in the context of the
unified schemes.Comment: 18 pages, 31 figures, MNRAS in press, (all enquiries to Clive
Tadhunter ([email protected])
The unfriendly ISM in the radio galaxy 4C12.50 (PKS 1345+12)
The radio source 4C12.50 has often been suggested to be a prime candidate for
the link between ultraluminous infrared galaxies and young radio galaxies. A
VLBI study of the neutral hydrogen in the nuclear regions of this object shows
that most of the gas detected close to the systemic velocity is associated with
an off-nuclear cloud (~50 to 100 pc from the radio core) with a column density
of ~10^22 T_spin/100 K) cm^(-2) and an HI mass of a few times 10^5 to 10^6
M_sun. We consider a number of possibilities to explain the results. In
particular, we discus the possibility that this cloud indicates the presence of
a rich and clumpy interstellar medium in the centre, likely left over from the
merger that triggered the activity and that this medium influences the growth
of the radio source. The location of the cloud -- at the edge of the northern
radio jet/lobe -- suggests that the radio jet might be interacting with a gas
cloud. This interaction could be responsible for bending the young radio jet.
The velocity profile of the gas is relatively broad (~150$ km/s) and we
interpret this as kinematical evidence for interaction of the radio plasma with
the cloud. We also consider the model where the cloud is part of a broader
circumnuclear structure. Only a limited region of this structure would have
sufficient background radio brightness and large enough column depth in neutral
gas to obtain detectable HI absorption against the counterjet. The VLBI study
of the neutral hydrogen in 4C12.50 suggests that HI detected near the systemic
velocity (as it is often the case in radio galaxies) may not necessarily be
connected with a circumnuclear disk or torus (as is very often assumed) but
instead could be a tracer of the large-scale medium that surrounds the active
nucleus and that may influence the growth of the young radio source.Comment: 7 pages, 2 figures. Accepted for publication in A&
The Lockman Hole Project: new constraints on the sub-mJy source counts from a wide-area 1.4 GHz mosaic
This paper is part of a series discussing the results obtained in the
framework of a wide international collaboration - the Lockman Hole Project -
aimed at improving the extensive multiband coverage available in the Lockman
Hole region, through novel deep, wide-area, multifrequency (60, 150, 350 MHz,
and 1.4 GHz) radio surveys. This multifrequency, multi-band information will be
exploited to get a comprehensive view of star formation and active galactic
nucleus activities in the high-redshift Universe from a radio perspective. In
this paper, we present novel 1.4 GHz mosaic observations obtained with the
Westerbork Synthesis Radio Telescope. With an area coverage of 6.6 deg2, this
is the largest survey reaching an rms noise of 11 uJy/beam. In this paper, we
present the source catalogue (~6000 sources with flux densities S>55 uJy
(5sigma), and we discuss the 1.4 GHz source counts derived from it. Our source
counts provide very robust statistics in the flux range 0.1<S<1 mJy, and are in
excellent agreement with other robust determinations obtained at lower and
higher flux densities. A clear excess is found with respect to the counts
predicted by the semi-empirical radio sky simulations developed in the
framework of the Square Kilometre Array Simulated Skies project. A preliminary
analysis of the identified (and classified) sources suggests this excess is to
be ascribed to star-forming galaxies, which seem to show a steeper evolution
than predicted.Comment: accepted for publication on MNRAS. New version that corrects latex
errors and contain the correct version of figure 1
Dose-Effects Models for Space Radiobiology: An Overview on Dose-Effect Relationships
Space radiobiology is an interdisciplinary science that examines the biological effects of ionizing radiation on humans involved in aerospace missions. The dose-effect models are one of the relevant topics of space radiobiology. Their knowledge is crucial for optimizing radioprotection strategies (e.g., spaceship and lunar space station-shielding and lunar/Mars village design), the risk assessment of the health hazard related to human space exploration, and reducing damages induced to astronauts from galactic cosmic radiation. Dose-effect relationships describe the observed damages to normal tissues or cancer induction during and after space flights. They are developed for the various dose ranges and radiation qualities characterizing the actual and the forecast space missions [International Space Station (ISS) and solar system exploration]. Based on a Pubmed search including 53 papers reporting the collected dose-effect relationships after space missions or in ground simulations, 7 significant dose-effect relationships (e.g., eye flashes, cataract, central nervous systems, cardiovascular disease, cancer, chromosomal aberrations, and biomarkers) have been identified. For each considered effect, the absorbed dose thresholds and the uncertainties/limitations of the developed relationships are summarized and discussed. The current knowledge on this topic can benefit from further in vitro and in vivo radiobiological studies, an accurate characterization of the quality of space radiation, and the numerous experimental dose-effects data derived from the experience in the clinical use of ionizing radiation for diagnostic or treatments with doses similar to those foreseen for the future space missions. The growing number of pooled studies could improve the prediction ability of dose-effect relationships for space exposure and reduce their uncertainty level. Novel research in the field is of paramount importance to reduce damages to astronauts from cosmic radiation before Beyond Low Earth Orbit exploration in the next future. The study aims at providing an overview of the published dose-effect relationships and illustrates novel perspectives to inspire future research
LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
We present the first LOFAR observations of the radio jet in the quasar
4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved
resolution is very well matched to that of archival Jansky Very Large Array
(JVLA) observations at higher radio frequencies as well as the archival X-ray
images obtained with {\it Chandra}. We found that, for several knots along the
jet, the radio flux densities measured at hundreds of MHz lie well below the
values estimated by extrapolating the GHz spectra. This clearly indicates the
presence of spectral curvature. Radio spectral curvature has been already
observed in different source classes and/or extended radio structures and it
has been often interpreted as due to intrinsic processes, as a curved particle
energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich}
effect, free-free or synchrotron self absorption to name a few). Here we
discuss our results according to the scenario where particles undergo
stochastic acceleration mechanisms also in quasar jet knots.Comment: 13 pages, 4 tables, 4 figures, pre-proof version, published on the
Astrophysical Journal (Harris, et al. 2019 ApJ, 873, 21
A Deep WSRT 1.4 GHz Radio Survey of the Spitzer Space Telescope FLSv Region
The First Look Survey (FLS) is the first scientific product to emerge from
the Spitzer Space Telescope. A small region of this field (the verification
strip) has been imaged very deeply, permitting the detection of cosmologically
distant sources. We present Westerbork Synthesis Radio Telescope (WSRT)
observations of this region, encompassing a ~1 sq. deg field, centred on the
verification strip (J2000 RA=17:17:00.00, DEC=59:45:00.000). The radio images
reach a noise level of ~ 8.5 microJy/beam - the deepest WSRT image made to
date. We summarise here the first results from the project, and present the
final mosaic image, together with a list of detected sources. The effect of
source confusion on the position, size and flux density of the faintest sources
in the source catalogue are also addressed. The results of a serendipitous
search for HI emission in the field are also presented. Using a subset of the
data, we clearly detect HI emission associated with four galaxies in the
central region of the FLSv. These are identified with nearby, massive galaxies.Comment: 9 pages, 6 figures (fig.3 in a separate gif file). Accepted for
publication in A&A. The full paper and the related material can be downloaded
from http://www.astron.nl/wsrt/WSRTsurveys/WFLS
Large-scale HI in nearby radio galaxies: segregation in neutral gas content with radio source size
We present results of a study of neutral hydrogen (HI) in a complete sample
of nearby non-cluster radio galaxies. We find that radio galaxies with large
amounts of extended HI (M_HI >= 10^9 M_solar) all have a compact radio source.
The host galaxies of the more extended radio sources, all of Fanaroff & Riley
type-I, do not contain these amounts of HI. We discuss several possible
explanations for this segregation. The large-scale HI is mainly distributed in
disk- and ring-like structures with sizes up to 190 kpc and masses up to 2 x
10^10 M_solar. The formation of these structures could be related to past
merger events, although in some cases it may also be consistent with a
cold-accretion scenario.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letters. A
version with full resolution figures can be found at
http://www.astro.rug.nl/~emonts/emonts_HIletter_jan07.pd
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