15 research outputs found
CHANG-ES XI: Circular Polarization in the Cores of Nearby Galaxies
We detect 5 galaxies in the CHANG-ES (Continuum Halos in Nearby Galaxies --
an EVLA Survey) sample that show circular polarization (CP) at L-band in our
high resolution data sets. Two of the galaxies (NGC~4388 and NGC~4845) show
strong Stokes \%, two (NGC~660 and NGC~3628) have
values of \%, and NGC~3079 is a marginal detection at \%. The two strongest galaxies also have the most luminous X-ray
cores and the strongest internal absorption in X-rays. We have expanded on our
previous Faraday conversion interpretation and analysis and provide analytical
expressions for the expected signal for a general case in which the cosmic
ray electron energy spectral index can take on any value. We provide examples
as to how such expressions could be used to estimate magnetic field strengths
and the lower energy cutoff for CR electrons. Four out of our detections are
{\it resolved}, showing unique structures, including a {\it jet} in NGC~4388
and a CP `conversion disk' in NGC~4845. The conversion disk is inclined to the
galactic disk but is perpendicular to a possible outflow direction. Such CP
structures have never before been seen in any galaxy to our knowledge. None of
the galaxy cores show linear polarization at L-band. Thus CP may provide a
unique probe of physical conditions deep into radio AGNs.Comment: 30 pages, 4 figures, accepted to MNRA
CHANG-ES IV: Radio continuum emission of 35 edge-on galaxies observed with the Karl G. Jansky Very Large Array in D-configuration, Data Release 1
We present the first part of the observations made for the Continuum Halos in
Nearby Galaxies, an EVLA Survey (CHANG-ES) project. The aim of the CHANG-ES
project is to study and characterize the nature of radio halos, their
prevalence as well as their magnetic fields, and the cosmic rays illuminating
these fields. This paper reports observations with the compact D configuration
of the Karl G. Jansky Very Large Array (VLA) for the sample of 35 nearby
edge-on galaxies of CHANG-ES. With the new wide bandwidth capabilities of the
VLA, an unprecedented sensitivity was achieved for all polarization products.
The beam resolution is an average of 9.6" and 36" with noise levels reaching
approximately 6 and 30 microJy per beam for C- and L-bands, respectively
(robust weighting). We present intensity maps in these two frequency bands (C
and L), with different weightings, as well as spectral index maps, polarization
maps, and new measurements of star formation rates (SFRs). The data products
described herein are available to the public in the CHANG-ES data release
available at www.queensu.ca/changes. We also present evidence of a trend among
galaxies with larger halos having higher SFR surface density, and we show, for
the first time, a radio continuum image of the median galaxy, taking advantage
of the collective signal-to-noise ratio of 30 of our galaxies. This image shows
clearly that a typical spiral galaxy is surrounded by a halo of magnetic fields
and cosmic rays.Comment: 70 pages, of which 35 pages present the data of each galax
Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- II: First Results on NGC 4631
We present the first results from the CHANG-ES survey, a new survey of 35
edge-on galaxies to search for both in-disk as well as extra-planar radio
continuum emission. The motivation and science case for the survey are
presented in a companion paper (Paper I). In this paper (Paper II), we outline
the observations and data reduction steps required for wide-band calibration
and mapping of EVLA data, including polarization, based on C-array test
observations of NGC 4631.
With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes
at 6 GHz for the test data) we have achieved best rms noise levels of 22 and
3.5 Jy beam at 1.5 GHz and 6 GHz, respectively. New disk-halo
features have been detected, among them two at 1.5 GHz that appear as loops in
projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be
formed from a single wide-band observation in a single array configuration.
This map represents tangent slopes to the intensities within the band centered
at 1.5 GHz, rather than fits across widely separated frequencies as has been
done in the past and is also the highest spatial resolution spectral index map
yet presented for this galaxy. The average spectral index in the disk is
indicating that the emission is
largely non-thermal, but a small global thermal contribution is sufficient to
explain a positive curvature term in the spectral index over the band. Two
specific star forming regions have spectral indices that are consistent with
thermal emission. Polarization results (uncorrected for internal Faraday
rotation) are consistent with previous observations and also reveal some new
features. On broad scales, we find strong support for the notion that magnetic
fields constrain the X-ray emitting hot gas.Comment: Accepted to the Astronomical Journal, Version 2 changes: Added
acknowledgement to NRA
Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey
We introduce a new survey to map the radio continuum halos of a sample of 35
edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The
survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky
Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of
array configurations (B, C, and D) in order to compile the most comprehensive
data set yet obtained for the study of radio halo properties. This is the first
survey of radio halos to include all polarization products.
In this first paper, we outline the scientific motivation of the survey, the
specific science goals, and the expected improvements in noise levels and
spatial coverage from the survey. Our goals include investigating the physical
conditions and origin of halos, characterizing cosmic ray transport and wind
speed, measuring Faraday rotation and mapping the magnetic field, probing the
in-disk and extraplanar far-infrared - radio continuum relation, and
reconciling non-thermal radio emission with high-energy gamma-ray models. The
sample size allows us to search for correlations between radio halos and other
properties, including environment, star formation rate, and the presence of
AGNs. In a companion paper (Paper II) we outline the data reduction steps and
present the first results of the survey for the galaxy, NGC 4631.Comment: 17 pages, 1 figure, accepted to the Astronomical Journal, Version 2
changes: added acknowledgement to NRA
Magnetic fields in late-type edge-on galaxies
Viele Beobachtungen von Galaxien zeigen großskalige reguläre Magnetfelder. Der Ursprung und die Verstärkungsmechanismen kosmischer Magnetfelder sind dabei Gegenstand aktueller Forschung. Dabei sind die Details der Verstärkung jener Magnetfelder noch nicht vollständig verstanden. In dieser Arbeit wurden vier weitere Galaxien in Hinsicht auf deren Magnetfelder untersucht und es wurden X-förmige Magnetfelder in zwei der Galaxien gefunden. Weiterhin wurde die Depolarisation der polarisierten Strahlung näher untersucht. Während instrumentelle Depolarisation durch die hohe Datenqualität minimiert werden kann, ist die Depolarisation mittels interner Faraday Dispersion nicht zu vermeiden. Ein auf diesem Effekt basiertes Modell kann die fehlende polarisierte Emission in drei der Galaxien erfolgreich vorhersagen. In der vierten Galaxie stimmt das Modell nicht mit der Beobachtung überein, was zu dem Schluss führt, dass diese Galaxie kein großskaliges reguläres Magnetfeld besitzt
Jülich LOFAR Long Term Archive
The LOFAR radio telescope produces yearly huge amounts of data comparable to the data produced by other experiments like the ones conducted at the Large Hadron Collider, and has to deal as well with the challenges big data brings in terms of storage, compute and network. Regarding the storage challenge a tiered heterogeneous structure is needed: a smart solution for being able to store the vast amount of data and at the same time allow efficient access and (re)use of it. The data collected by the radio telescope is first sent to a central processor, and distributed later to the three sites in Europe which form the Long Term Archive: SURF in the Netherlands, the Jülich Supercomputing Centre in Germany and PSNC in Poland. The poster describes the Jülich LTA which consists of a distributed storage system based on the dCache software. At the moment the LOFAR LTA contains around 55 petabytes of data. About one third of it is hosted in Jülich, with a yearly growth of 2 petabytes
New constraints on galactic CRE transport from radio continuum observations
International audienceThe importance of cosmic rays and magnetic fields for the physics of the interstellar medium and the evolution of galaxies is demonstrated by a number of recent numerical simulations. We present new observations of the cosmic ray electron transport as well as the magnetic field strength and structure in galactic halos from radio-continuum polarization studies of edge-on disk galaxies obtained by the CHANG-ES (Continuum Halos in Nearby Galaxies - an EVLA Survey) project.The observations are discussed in the context of transport models for Cosmic Ray Eelctrons (CREs) from the star-forming disks into the halo. The observations also allow to derive the magentic field structure in galactic halos. In addition, it is demonstrated how recent low frequency observations of edge-on galaxies with LOFAR help to further constrain the problem
Systematic search for tidal features around nearby galaxies
Context. In hierarchical models of galaxy formation, stellar tidal streams are expected around most, if not all, galaxies. Although these features may provide useful diagnostics of the Λ CDM model, their observational properties remain poorly constrained because they are challenging to detect and interpret and have been studied in detail for only a sparse sampling of galaxy population. More quantitative, systematic approaches are required. We advocate statistical analysis of the counts and properties of such features in archival wide-field imaging surveys for a direct comparison against results from numerical simulations. Aims. We aim to study systematically the frequency of occurrence and other observational properties of tidal features around nearby galaxies. The sample we construct will act as a foundational dataset for statistical comparison with cosmological models of galaxy formation. Methods. Our approach is based on a visual classification of diffuse features around a volume-limited sample of nearby galaxies, using a post-processing of Sloan Digital Syk Survey (SDSS) imaging optimized for the detection of stellar structure with low surface brightness. Results. At a limiting surface brightness of 28 mag arcsec−2, 14% of the galaxies in our sample exhibit evidence of diffuse features likely to have arisen from minor merging events. Our technique recovers all previously known streams in our sample and yields a number of new candidates. Consistent with previous studies, coherent arc-like features and shells are the most common type of tidal structures found in this study. We conclude that although some detections are ambiguous and could be corroborated or refuted with deeper imaging, our technique provides a reliable foundation for the statistical analysis of diffuse circumgalactic features in wide-area imaging surveys, and for the identification of targets for follow-up studies
CHANG-ES: XVIII—The CHANG-ES Survey and Selected Results
International audienceThe CHANGES (Continuum Halos in Nearby Galaxies) survey of 35 nearby edge-on galaxies is revealing new and sometimes unexpected and startling results in their radio continuum emission. The observations were in wide bandwidths centred at 1.6 and 6.0 GHz. Unique to this survey is full polarization data showing magnetic field structures in unprecedented detail, resolution and sensitivity for such a large sample. A wide range of new results are reported here, some never before seen in any galaxy. We see circular polarization and variability in active galactic nuclei (AGNs), in-disk discrete features, disk-halo structures sometimes only seen in polarization, and broad-scale halos with reversing magnetic fields, among others. This paper summarizes some of the CHANGES results seen thus far