- ID:
- ivo://CDS.VizieR/J/A+A/654/A5
- Title:
- Faraday cubes Fields A B and C
- Short Name:
- J/A+A/654/A5
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with straight depolarisation canals and tracers of the magnetized and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI). This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to the depolarisation canals. We use the Rolling Hough Transform to compare the orientation of the depolarisation canals with that of the filamentary structure seen in HI and, based on starlight and dust polarisation data, with that of the plane-of-the-sky magnetic field. Stellar parallaxes from Gaia complement the starlight polarisation with the corresponding distances. Faraday tomography of the three fields shows a rich network of diffuse polarised emission at Faraday depths between -10rad/m^2^ and +15rad/m^2^. A complex system of straight depolarisation canals resembles that of the 3C 196 field. The depolarisation canals align both with the HI filaments and with the magnetic field probed by dust. The observed alignment suggests that an ordered magnetic field organises the multiphase ISM over a large area (~20{deg}). In one field, two groups of stars at distances below and above 200pc, respectively, show distinct magnetic-field orientations. These are both comparable with the orientations of the depolarisation canals in the same field. We conclude that the depolarisation canals likely trace the same change of the magnetic field as probed by the stars, which corresponds to the edge of the Local Bubble.
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- ID:
- ivo://CDS.VizieR/J/ApJ/714/1170
- Title:
- Faraday rotation at high Galactic latitude
- Short Name:
- J/ApJ/714/1170
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of the vertical magnetic field of the Milky Way toward the Galactic poles, determined from observations of Faraday rotation toward more than 1000 polarized extragalactic radio sources at Galactic latitudes |b|>=77{deg}, using the Westerbork Radio Synthesis Telescope and the Australia Telescope Compact Array. We find median rotation measures (RMs) of 0.0+/-0.5rad/m^2^ and +6.3+/-0.7rad/m^2^ toward the north and south Galactic poles, respectively, demonstrating that there is no coherent vertical magnetic field in the Milky Way at the Sun's position. If this is a global property of the Milky Way's magnetism, then the lack of symmetry across the disk rules out pure dipole or quadrupole geometries for the Galactic magnetic field. The angular fluctuations in RM seen in our data show no preferred scale within the range ~0.1{deg} to ~25{deg}. The observed standard deviation in RM of ~9rad/m^2^ then implies an upper limit of ~1uG on the strength of the random magnetic field in the warm ionized medium at high Galactic latitudes.
- ID:
- ivo://CDS.VizieR/J/ApJ/795/63
- Title:
- Faraday rotation from magnesium II absorbers
- Short Name:
- J/ApJ/795/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Strong singly ionized magnesium (Mg II) absorption lines in quasar spectra typically serve as a proxy for intervening galaxies along the line of sight. Previous studies have found a correlation between the number of these Mg II absorbers and the Faraday rotation measure (RM) at ~5 GHz. We cross-match a sample of 35752 optically identified non-intrinsic Mg II absorption systems with 25649 polarized background radio sources for which we have measurements of both the spectral index and RM at 1.4 GHz. We use the spectral index to split the resulting sample of 599 sources into flat-spectrum and steep-spectrum subsamples. We find that our flat-spectrum sample shows significant (~3.5{sigma}) evidence for a correlation between Mg II absorption and RM at 1.4 GHz, while our steep-spectrum sample shows no such correlation. We argue that such an effect cannot be explained by either luminosity or other observational effects, by evolution in another confounding variable, by wavelength-dependent polarization structure in an active galactic nucleus, by the Galactic foreground, by cosmological expansion, or by partial coverage models. We conclude that our data are most consistent with intervenors directly contributing to the Faraday rotation along the line of sight, and that the intervening systems must therefore have coherent magnetic fields of substantial strength (B{bar}=1.8+/-0.4{mu}G). Nevertheless, the weak nature of the correlation will require future high-resolution and broadband radio observations in order to place it on a much firmer statistical footing.
- ID:
- ivo://CDS.VizieR/J/ApJ/707/114
- Title:
- Faraday rotation in Cen A
- Short Name:
- J/ApJ/707/114
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of an Australia Telescope Compact Array 1.4GHz spectropolarimetric aperture synthesis survey of 34deg^2^ centered on Centaurus A-NGC 5128. A catalog of 1005 extragalactic compact radio sources in the field to a continuum flux density of 3mJy/beam is provided along with a table of Faraday rotation measures (RMs) and linear polarized intensities for the 28% of sources with high signal to noise in linear polarization. We use the ensemble of 281 background polarized sources as line-of-sight probes of the structure of the giant radio lobes of Centaurus A. This is the first time such a method has been applied to radio galaxy lobes and we explain how it differs from the conventional methods that are often complicated by depth and beam depolarization effects. Assuming a magnetic field strength in the lobes of 1.3B1uG, where B1=1 is implied by equipartition between magnetic fields and relativistic particles, the upper limit we derive on the maximum possible difference between the average RM of 121 sources behind Centaurus A and the average RM of the 160 sources along sightlines outside Centaurus A implies an upper limit on the volume-averaged thermal plasma density in the giant radio lobes of <ne><5x10^-5^B1^-1^cm^-3^. We use an RM structure function analysis and report the detection of a turbulent RM signal, with rms=17rad/m^2^ and scale size 0.3deg, associated with the southern giant lobe. We cannot verify whether this signal arises from turbulent structure throughout the lobe or only in a thin skin (or sheath) around the edge, although we favor the latter. The RM signal is modeled as possibly arising from a thin skin with a thermal plasma density equivalent to the Centaurus intragroup medium density and a coherent magnetic field that reverses its sign on a spatial scale of 20kpc. For a thermal density of n1 10^-3^cm^-3^, the skin magnetic field strength is 0.8n1^-1^uG.
- ID:
- ivo://CDS.VizieR/J/MNRAS/467/1776
- Title:
- Faraday rotation in Magellanic Bridge
- Short Name:
- J/MNRAS/467/1776
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an investigation into the magnetism of the Magellanic Bridge, carried out through the observation of Faraday rotation towards 167 polarized extragalactic radio sources spanning the continuous frequency range of 1.3-3.1GHz with the Australia Telescope Compact Array. Comparing measured Faraday depth values of sources 'on' and 'off' the Bridge, we find that the two populations are implicitly different. Assuming that this difference in populations is due to a coherent field in the Magellanic Bridge, the observed Faraday depths indicate a median line-of-sight coherent magnetic-field strength of B||~=0.3uG directed uniformly away from us. Motivated by the varying magnitude of Faraday depths of sources on the Bridge, we speculate that the coherent field observed in the Bridge is a consequence of the coherent magnetic fields from the Large and Small Magellanic Clouds being pulled into the tidal feature. This is the first observation of a coherent magnetic field spanning the entirety of the Magellanic Bridge and we argue that this is a direct probe of a 'pan-Magellanic' field.
- ID:
- ivo://CDS.VizieR/J/other/Galax/7.43
- Title:
- Faraday rotation of extended emission
- Short Name:
- J/other/Galax/7.
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resource, can be used as a probe of the large-scale Galactic magnetic field. We used polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near 1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. We calculated RMs from a linear fit to the polarisation angles as a function of wavelength squared in four frequency channels, for both the EG sources and the XE. Across the CGPS area, l=55 to l=193 degrees, b=-3 to b=5 degrees, the RMs of the XE closely track the RMs of the EG sources, with XE RMs about half the value of EG-source RMs. The exceptions are places where large local HII complexes heavily depolarise more distant emission. We conclude that there is valuable information in the XE RM dataset. The factor of 2 between the two types of RM values is close to that expected from a Burn slab model of the ISM. This result indicates that, at least in the outer Galaxy, the EG and XE sources are likely probing similar depths, and that the Faraday rotating medium and the synchrotron emitting medium have similar variation with galactocentric distance.
- ID:
- ivo://CDS.VizieR/J/ApJ/887/L7
- Title:
- Faraday rotation of radio sources from THOR
- Short Name:
- J/ApJ/887/L7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present first results for Faraday rotation of compact polarized sources (1-2GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39{deg}<l<52{deg}, we find rotation measures (RMs) in the range -310rad/m^2^<=RM<=+4219rad/m^2^, with the highest values concentrated within a degree of l=48{deg} at the Sagittarius arm tangent. Most of the high RMs arise in diffuse plasma, along lines of sight that do not intersect HII regions. For l>49{deg}, RM drops off rapidly, while at l<47{deg}, the mean RM is higher with a larger standard deviation than at l>49{deg}. We attribute the RM structure to the compressed diffuse warm ionized medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of fast radio bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys.
- ID:
- ivo://CDS.VizieR/J/A+A/597/A98
- Title:
- Faraday tomography of foreground towards IC342
- Short Name:
- J/A+A/597/A98
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Magnetic fields pervade the interstellar medium (ISM), but are difficult to detect and characterize. The new generation of low-frequency radio telescopes, such as the Low Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides advancements in our capability of probing Galactic magnetism through low-frequency polarimetry. Maps of diffuse polarized radio emission and the associated Faraday rotation can be used to infer properties of, and trace structure in, the magnetic fields in the ISM. However, to date very little of the sky has been probed at high angular and Faraday depth resolution. We observed a 5 by 5 degree region centred on the nearby galaxy IC 342 (l=138.2, b=+10.6) using the LOFAR High Band Antennas in the frequency range 115-178MHz. We imaged this region at 4.5x3.8arcmin^2^ resolution and performed Faraday tomography to detect foreground Galactic polarized synchrotron emission separated by Faraday depth (different amounts of Faraday rotation). Our Faraday depth cube shows rich polarized structure, with up to 30K of polarized emission at 150MHz. We clearly detect two polarized features that extend over most of the field but are clearly separated in Faraday depth. Simulations of the behaviour of the depolarization of Faraday-thick structures at such low frequencies show that such structures would be too strongly depolarized to explain the observations. These structures are therefore rejected as the source of the observed polarized features. Only Faraday thin structures will not be strongly depolarized at low frequencies; producing such structures requires localized variations in the ratio of synchrotron emissivity to Faraday depth per unit distance. Such variations can arise from several physical phenomena, such as a transition between regions of ionized and (mostly) neutral gas. We conclude that the observed polarized emission is Faraday thin, and propose that the emission originates from two mostly neutral clouds in the local ISM. Using maps of the local ISM to estimate distances to these clouds, we have modelled the Faraday rotation for this line of sight and estimated that the strength of the line of sight component of magnetic field of the local ISM for this direction varies between -0.86 and +0.12uG (where positive is towards the Earth). We propose that this may be a useful method for mapping magnetic fields within the local ISM in all directions towards nearby neutral clouds.
- ID:
- ivo://CDS.VizieR/J/ApJ/405/94
- Title:
- Far-infrared emission from 5 galaxy clusters
- Short Name:
- J/ApJ/405/94
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have analyzed IRAS image data using a random position, multiple-aperture photometry method to study diffuse far-infrared emission for a sample of 56 clusters of galaxies at 60 and 100um. Five of the clusters in the sample exhibit excess far-infrared emission, but at low levels of significance. For A496 (Perseus) and MKW 1, this emission appears to be associated with a strong point source: in A2344, the observed excess is probably due to a foreground cirrus cloud. Two clusters, A262 and A2670, show evidence for extended diffuse emission. This emission is unlikely to be due to point sources associated with the clusters. In these two clusters, the diffuse emission shows evidence of being extended along the cluster's major axis. Far-infrared maps at 60 and 100um are presented for these five clusters. We find that contamination due to diffuse Galactic cirrus represents the major source of uncertainty in the measurement of far-infrared emission. The mean cluster fluxes for the sample as a whole are 44 and 107mJy for a 4' diameter aperture at 60 and 100um, respectively. The corresponding fluxes for a 10' diameter aperture are 138 and 253mJy at 60 and 100um. Photometry for the entire sample shows statistical evidence for excess emission at 60um associated with clusters in apertures 4' and 10' in diameter. This result is unlikely to be caused by random sampling errors. The flux distributions are analyzed for evidence that excess far-infrared radiation correlates with cluster environmental characteristics. Although we find evidence that poorer X-ray clusters are stronger far-infrared emitters, this result is tentative owing to the small sample sizes involved. The far-infrared luminosities, dust temperatures, and dust masses which these results imply are also presented. Finally, the origin and possible heating mechanism for dust in the intracluster medium is discussed.
- ID:
- ivo://CDS.VizieR/J/A+A/418/131
- Title:
- Far-infrared loops in the 2nd Galactic Quadrant
- Short Name:
- J/A+A/418/131
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of an investigation of the large structure of the diffuse interstellar medium in the 2nd Galactic Quadrant (90{deg}<=l<=180{deg}). 145 loops were identified on IRAS based on far-infrared maps. Our catalogue lists their basic physical properties. The distribution clearly suggests that there is an efficient process that can generate loop-like features at high galactic latitudes. Distances are provided for 30 loops. We also give an observational estimate on the volume filling factor of the hot gas in the Local Arm, 4.6%<=f_2nd_<6.4%.