112 research outputs found
Source finding, parametrization and classification for the extragalactic Effelsberg-Bonn HI Survey
Context. Source extraction for large-scale HI surveys currently involves
large amounts of manual labor. For data volumes expected from future HI surveys
with upcoming facilities, this approach is not feasible any longer.
Aims. We describe the implementation of a fully automated source finding,
parametrization, and classification pipeline for the Effelsberg-Bonn HI Survey
(EBHIS). With future radio astronomical facilities in mind, we want to explore
the feasibility of a completely automated approach to source extraction for
large-scale HI surveys.
Methods. Source finding is implemented using wavelet denoising methods, which
previous studies show to be a powerful tool, especially in the presence of data
defects. For parametrization, we automate baseline fitting, mask optimization,
and other tasks based on well-established algorithms, currently used
interactively. For the classification of candidates, we implement an artificial
neural network which is trained on a candidate set comprised of false positives
from real data and simulated sources. Using simulated data, we perform a
thorough analysis of the algorithms implemented.
Results. We compare the results from our simulations to the parametrization
accuracy of the HI Parkes All-Sky Survey (HIPASS) survey. Even though HIPASS is
more sensitive than EBHIS in its current state, the parametrization accuracy
and classification reliability match or surpass the manual approach used for
HIPASS data.Comment: 13 Pages, 13 Figures, 1 Table, accepted for publication in A&
A dynamical transition from atomic to molecular intermediate-velocity clouds
Towards the high galactic latitude sky, the far-infrared (FIR) intensity is
tightly correlated to the total hydrogen column density which is made up of
atomic (HI) and molecular hydrogen (H. Above a certain column density
threshold, atomic hydrogen turns molecular. We analyse gas and dust properties
of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore
their transition from the atomic to the molecular phase. Driven by
observations, we investigate the physical processes that transform a purely
atomic IVC into a molecular one. Data from the Effelsberg-Bonn HI-Survey
(EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS.
Modified black-body emission spectra are fitted to deduce dust optical depths
and grain temperatures. We remove the contribution of atomic hydrogen to the
FIR intensity to estimate molecular hydrogen column densities. Two IVCs show
different FIR properties, despite their similarity in HI, such as narrow
spectral lines and large column densities. One FIR bright IVC is associated
with H, confirmed by CO emission; the other IVC
is FIR dim and shows no FIR excess, which indicates the absence of molecular
hydrogen. We propose that the FIR dim and bright IVCs probe the transition
between the atomic and molecular gas phase. Triggered by dynamical processes,
this transition happens during the descent of IVCs onto the galactic disk. The
most natural driver is ram pressure exerted onto the cloud by the increasing
halo density. Because of the enhanced pressure, the formation timescale of
H is reduced, allowing the formation of large amounts of H within a
few Myr.Comment: 13 pages, 14 figures, accepted for publication by A&
Far-infrared excess emission as a tracer of disk-halo interaction
Given the current and past star-formation in the Milky Way in combination
with the limited gas supply, the re-fuelling of the reservoir of cool gas is an
important aspect of Galactic astrophysics. The infall of \ion{H}{i} halo clouds
can, among other mechanisms, contribute to solving this problem. We study the
intermediate-velocity cloud IVC135+54 and its spatially associated
high-velocity counterpart to look for signs of a past or ongoing interaction.
Using the Effelsberg-Bonn \ion{H}{i} Survey data, we investigated the interplay
of gas at different velocities. In combination with far-infrared Planck and
IRIS data, we extended this study to interstellar dust and used the correlation
of the data sets to infer information on the dark gas. The velocity structure
indicates a strong compression and deceleration of the infalling high-velocity
cloud (HVC), associated with far-infrared excess emission in the
intermediate-velocity cloud. This excess emission traces molecular hydrogen,
confirming that IVC135+54 is one of the very few molecular halo clouds. The
high dust emissivity of IVC135+54 with respect to the local gas implies that it
consists of disk material and does not, unlike the HVC, have an extragalactic
origin. Based on the velocity structure of the HVC and the dust content of the
IVC, a physical connection between them appears to be the logical conclusion.
Since this is not compatible with the distance difference between the two
objects, we conclude that this particular HVC might be much closer to us than
complex C. Alternatively, the indicators for an interaction are misleading and
have another origin.Comment: 11 pages, 10 figures, accepted for publication in A&
Structure and Kinematics of the Nearby Dwarf Galaxy UGCA 105
Owing to their shallow stellar potential, dwarf galaxies possess thick gas
disks, which makes them good candidates for studies of the galactic vertical
kinematical structure. We present 21 cm line observations of the isolated
nearby dwarf irregular galaxy UGCA 105, taken with the Westerbork Synthesis
Radio Telescope (WSRT), and analyse the geometry of its neutral hydrogen (HI)
disk and its kinematics. The galaxy shows a fragmented HI distribution. It is
more extended than the optical disk, and hence allows one to determine its
kinematics out to very large galacto-centric distances. The HI kinematics and
morphology are well-ordered and symmetric for an irregular galaxy. The HI is
sufficiently extended to observe a substantial amount of differential rotation.
Moreover, UGCA 105 shows strong signatures for the presence of a kinematically
anomalous gas component. Performing tilted-ring modelling by use of the
least-squares fitting routine TiRiFiC, we found that the HI disk of UGCA 105
has a moderately warped and diffuse outermost part. Probing a wide range of
parameter combinations, we succeeded in modelling the data cube as a disk with
a strong vertical gradient in rotation velocity (), as well as vertically increasing inwards motion
() within the radius of the stellar
disk. The inferred radial gas inflow amounts to , which is similar to the star formation rate of the galaxy. The
observed kinematics are hence compatible with direct or indirect accretion from
the intergalactic medium, an extreme backflow of material that has formerly
been expelled from the disk, or a combination of both.Comment: 15 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
SoFiA: a flexible source finder for 3D spectral line data
We introduce SoFiA, a flexible software application for the detection and
parameterization of sources in 3D spectral-line datasets. SoFiA combines for
the first time in a single piece of software a set of new source-finding and
parameterization algorithms developed on the way to future HI surveys with
ASKAP (WALLABY, DINGO) and APERTIF. It is designed to enable the general use of
these new algorithms by the community on a broad range of datasets. The key
advantages of SoFiA are the ability to: search for line emission on multiple
scales to detect 3D sources in a complete and reliable way, taking into account
noise level variations and the presence of artefacts in a data cube; estimate
the reliability of individual detections; look for signal in arbitrarily large
data cubes using a catalogue of 3D coordinates as a prior; provide a wide range
of source parameters and output products which facilitate further analysis by
the user. We highlight the modularity of SoFiA, which makes it a flexible
package allowing users to select and apply only the algorithms useful for their
data and science questions. This modularity makes it also possible to easily
expand SoFiA in order to include additional methods as they become available.
The full SoFiA distribution, including a dedicated graphical user interface, is
publicly available for download.Comment: MNRAS, accepted. SoFiA is registered at the Astrophysics Source Code
Library with ID ascl:1412.001. Download SoFiA at
https://github.com/SoFiA-Admin/SoFi
Compatibility studies between Radio Astronomy and three upcoming technologies
In recent years, the increased use of high-frequency technology in the millimetre and microwave range, including mobile phones, automobiles, and industrial equipment, has further reduced and threatened the spectrum assigned to the radio astronomy service (RAS). As a scientific and passive service, RAS requires protection from commercial services to observe the extremely faint celestial signals. As spectrum use for land, air, and space communications grows, protecting RAS operations from radio frequency interference is becoming more challenging.
This report examines the impact of advanced technologies on radio astronomy, specifically car radar at 77 GHz, 5G and Wi-Fi device deployments at 6.6 GHz. These technologies are evaluated for their potential impact on Italian radio telescopes: the Radio Observatories of Medicina and Noto and the Sardinia Radio Telescope (SRT). Of particular concern is the potential threat posed by car radars to future high-density prospects, as well as the historical importance of the 6.6 GHz frequency for radio astronomical observation of methanol emissions from stars
The nitrogen stock of the ice-rich yedoma domain
Recent studies on permafrost organic matter (OM) suggest that a portion of previously frozen carbon will enter the active carbon cycle as high latitudes warm. Less is known about the fate of other OM components, including nutrients such as nitrogen (N). The abundance and availability of N following permafrost thaw will regulate the ability of plants to offset carbon losses. Additionally, lateral N losses could alter aquatic food webs. There is growing evidence that some N is lost vertically as N2O, a greenhouse gas 300 times stronger than CO2 over 100 years. Despite broad recognition of its role regulating both carbon and non-carbon aspects of the permafrost climate feedback, estimates of permafrost N remain uncertain. To address this knowledge gap, we quantified N content for different stratigraphic units, including yedoma, Holocene cover deposits, refrozen thermokarst deposits, taberal sediments, and active layer soils. The resulting N estimates from this one permafrost region were similar in magnitude to previous estimates for the entire permafrost zone. We conclude that the permafrost N pool is much larger than currently appreciated and a substantial pool of permafrost N could be mobilized after thaw, with continental-scale consequences for biogeochemical budgets and global-scale consequences
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