1,037 research outputs found
Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
Context. The combined use of carbon radio recombination lines (CRRLs) and the
158 m-[CII] line is a powerful tool for the study of the energetics and
physical conditions (e.g., temperature and density) of photodissociation
regions (PDRs). However, there are few observational studies that exploit this
synergy. Aims. Here we explore the relation between CRRLs and the 158
m-[CII] line in light of new observations and models. Methods. We present
new and existing observations of CRRLs in the frequency range 0.15--230 GHz
with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion~A (M42). We
complement these observations with SOFIA observations of the 158 m-[CII]
line. We studied two PDRs: the foreground atomic gas, known as the Veil, and
the dense PDR between the HII region and the background molecular cloud.
Results. In the Veil we are able to determine the gas temperature and electron
density, which we use to measure the ionization parameter and the photoelectric
heating efficiency. In the dense PDR, we are able to identify a layered PDR
structure at the surface of the molecular cloud to the south of the Trapezium
cluster. There we find that the radio lines trace the colder portion of the
ionized carbon layer, the C/C/CO interface. By modeling the emission of
the ~m-[CII] line and CRRLs as arising from a PDR we derive a thermal
pressure K cm and a radiation field
close to the Trapezium. Conclusions. This work provides
additional observational support for the use of CRRLs and the 158 m-[CII]
line as complementary tools to study dense and diffuse PDRs, and highlights the
usefulness of CRRLs as probes of the C/C/CO interface.Comment: 18 pages, 16 figures, accepted for publication in A&
The discovery of diffuse steep spectrum sources in Abell 2256
Context: Hierarchical galaxy formation models indicate that during their
lifetime galaxy clusters undergo several mergers. Here we report on the
discovery of three diffuse radio sources in the periphery of Abell 2256, using
the GMRT.
Aims: The aim of the observations was to search for diffuse ultra-steep
spectrum radio sources within the galaxy cluster Abell 2256.
Methods: We have carried out GMRT 325 MHz radio continuum observations of
Abell 2256. V, R and I band images of the cluster were taken with the 4.2m WHT.
Results: We have discovered three diffuse elongated radio sources located
about 1 Mpc from the cluster center. Two are located to the west of the cluster
center, and one to the southeast. The sources have a measured physical extent
of 170, 140 and 240 kpc, respectively. The two western sources are also visible
in deep low-resolution 115-165 MHz WSRT images, although they are blended into
a single source. For the combined emission of the blended source we find an
extreme spectral index of -2.05\pm 0.14 between 140 and 351 MHz. The extremely
steep spectral index suggests these two sources are most likely the result of
adiabatic compression of fossil radio plasma due to merger shocks.
Conclusions: The discovery of the steep spectrum sources implies the
existence of a population of faint diffuse radio sources in (merging) clusters
with such steep spectra that they have gone unnoticed in higher frequency
(\gtrsim 1 GHz) observations. An exciting possibility therefore is that such
sources will determine the general appearance of clusters in low-frequency high
resolution radio maps as will be produced by for example LOFAR or LWA.Comment: 5 pages, 2 figures, accepted for publication in A&A on October 16,
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Jet-driven outflows of ionized gas in the nearby radio galaxy 3C 293
Fast outflows of gas, driven by the interaction between the radio jets and interstellar medium
(ISM) of the host galaxy, are being observed in an increasing number of galaxies. One such
example is the nearby radio galaxy 3C 293. In this paper we present integral field unit
observations taken with OASIS on the William Herschel Telescope, enabling us to map the
spatial extent of the ionized gas outflows across the central regions of the galaxy. The jet-driven
outflow in 3C 293 is detected along the inner radio lobes with a mass outflow rate ranging
from ∼0.05 to 0.17 M yr−1 (in ionized gas) and corresponding kinetic power of ∼0.5–
3.5 × 1040 erg s−1. Investigating the kinematics of the gas surrounding the radio jets (i.e. not
directly associated with the outflow), we find linewidths broader than 300 km s−1 up to 5 kpc in
the radial direction from the nucleus (corresponding to 3.5 kpc in the direction perpendicular to
the radio axis at maximum extent). Along the axis of the radio jet linewidths >400 km s−1 are
detected out to 7 kpc from the nucleus and linewidths of >500 km s−1 at a distance of 12 kpc
from the nucleus, indicating that the disturbed kinematics clearly extend well beyond the high
surface brightness radio structures of the jets. This is suggestive of the cocoon structure seen
in simulations of jet–ISM interaction and implies that the radio jets are capable of disturbing
the gas throughout the central regions of the host galaxy in all directions
ALMA observations of AGN fuelling:The case of PKS B1718-649
We present ALMA observations of the (CO)-C-12 (2-1) line of the newly born (t(radio) similar to 10(2) years) active galactic nucleus (AGN), PKS B1718-649. These observations reveal that the carbon monoxide in the innermost 15 kpc of the galaxy is distributed in a complex warped disk. In the outer parts of this disk, the CO gas follows the rotation of the dust lane and of the stellar body of the galaxy hosting the radio source. In the innermost kiloparsec, the gas abruptly changes orientation and forms a circumnuclear disk (r less than or similar to 700 pc) with its major axis perpendicular to that of the outer disk. Against the compact radio emission of PKS B1718-649 (r similar to 2 pc), we detect an absorption line at red-shifted velocities with respect to the systemic velocity (Delta v = +365 +/- 22 km s(-1)). This absorbing CO gas could trace molecular clouds falling onto the central super-massive black hole. A comparison with the near-infrared H-2 1-0 S(1) observations shows that the clouds must be close to the black hole (r less than or similar to 75 pc). The physical conditions of these clouds are different from the gas at larger radii, and are in good agreement with the predictions for the conditions of the gas when cold chaotic accretion triggers an active galactic nucleus. These observations on the centre of PKS B1718-649 provide one of the best indications that a population of cold clouds is falling towards a radio AGN, likely fuelling its activity
A deep study of A399-401: An application for wide-field facet calibration
We examine the particle acceleration mechanism in the Mpc-scale bridge
between Abell 399 and Abell 401 and assess in particular if the synchrotron
emission originates from first-order or second-order Fermi re-acceleration. We
use deep (~40 hours) LOw-Frequency ARray (LOFAR) observations from Abell 399
and Abell 401 and apply improved direction-dependent calibration to produce
deep radio images at three different resolutions at 144 MHz. With a
point-to-point analysis we find in the bridge trends between the radio emission
from our new maps and X-ray emission from an XMM Newton observation. By
analyzing our observations and results, we argue that second-order Fermi
re-acceleration is currently the most favoured process to explain the emission
from the radio bridge, where past AGN activity may be responsible for the
supply of fossil plasma needed for in-situ re-acceleration. The radio halos
from Abell 401 and Abell 399 are also consistent with a second-order Fermi
re-acceleration model
Searching for the largest bound atoms in space
(abridged) Radio recombination lines (RRLs) at frequencies < 250 MHz
trace the cold, diffuse phase of the ISM. Next generation low frequency
interferometers, such as LOFAR, MWA and the future SKA, with unprecedented
sensitivity, resolution, and large fractional bandwidths, are enabling the
exploration of the extragalactic RRL universe. We observed the radio quasar 3C
190 (z~1.2) with the LOFAR HBA. In reducing this data for spectroscopic
analysis, we have placed special emphasis on bandpass calibration. We devised
cross-correlation techniques to significantly identify the presence of RRLs in
a low frequency spectrum. We demonstrate the utility of this method by applying
it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new
observations of 3C 190. RRLs have been detected in the foreground of 3C 190 at
z = 1.12355 (assuming a carbon origin), owing to the first detection of RRLs
outside of the local universe (first reported in Emig et al. 2019). Towards the
Galactic supernova remnant Cas A, we uncover three new detections: (1)
C-transitions (n = 5) for the first time at low radio
frequencies, (2) H-transitions at 64 MHz with a FWHM of 3.1 km/s, the
most narrow and one of the lowest frequency detections of hydrogen to date, and
(3) C at v = 0 km/s in the frequency range 55-78 MHz for the
first time. Additionally we recover C, C, C, and
C from the -47 km/s and -38 km/s components. In the nearby starburst
galaxy, M 82, we do not find a significant feature. Our current searches for
RRLs in LOFAR observations are limited to narrow (< 100 km/s) features, owing
to the relatively small number of channels available for continuum estimation.
Future strategies making use of larger contiguous frequency coverage would aid
calibration to deeper sensitivities and broader features.Comment: 21 pages, 21 figures, accepted in A&
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