The Beautiful Mess in Abell 2255 Botteon, A.; Brunetti, G.; van Weeren, R. J. ...
Astrophysical journal/The Astrophysical journal,
07/2020, Volume:
897, Issue:
1
Journal Article
Peer reviewed
Open access
We present LOFAR observations of one of the most spectacular objects in the radio sky: Abell 2255. This is a nearby (z = 0.0806) merging galaxy cluster hosting one of the first radio halos ever ...detected in the intracluster medium (ICM). The deep LOFAR images at 144 MHz of the central ∼10 Mpc2 region show a plethora of emission on different scales, from tens of kiloparsecs to above megaparsec sizes. In this work, we focus on the innermost region of the cluster. Among the numerous interesting features observed, we discover remarkable bright and filamentary structures embedded in the radio halo. We incorporate archival WSRT 1.2 GHz data to study the spectral properties of the diffuse synchrotron emission and find a very complex spectral index distribution in the halo spanning a wide range of values. We combine the radio data with Chandra observations to investigate the connection between the thermal and nonthermal components by quantitatively comparing the radio and X-ray surface brightness and the spectral index of the radio emission with the thermodynamical quantities of the ICM. Despite the multitude of structures observed in the radio halo, we find that the X-ray and radio emission are overall well correlated. The fact that the steepest spectrum emission is located in the cluster center and traces regions with high entropy possibly suggests the presence of seed particles injected by radio galaxies that are spread in the ICM by the turbulence generating the extended radio halo.
ABSTRACT
We determined Faraday rotation measures (RMs) towards 137 pulsars in the northern sky, using Low-Frequency Array (LOFAR) observations at 110–190 MHz. This low-frequency RM catalogue, the ...largest to date, improves the precision of existing RM measurements on average by a factor of 20 – due to the low frequency and wide bandwidth of the data, aided by the RM-synthesis method. We report RMs towards 25 pulsars for the first time. The RMs were corrected for ionospheric Faraday rotation to increase the accuracy of our catalogue to ≈0.1 rad m−2. The ionospheric RM correction is currently the largest contributor to the measurement uncertainty. In addition, we find that the Faraday dispersion functions towards pulsars are extremely Faraday thin – mostly less than 0.001 rad m−2. We use these new precise RM measurements (in combination with existing RMs, dispersion measures, and distance estimates) to estimate the scale height of the Galactic halo magnetic field: 2.0 ± 0.3 kpc for Galactic quadrants i and ii above and below the Galactic plane (we also evaluate the scale height for these regions individually). Overall, our initial low-frequency catalogue provides valuable information about the 3D structure of the Galactic magnetic field.
Context. Cosmic rays and magnetic fields play an important role for the formation and dynamics of gaseous halos of galaxies. Aims. Low-frequency radio continuum observations of edge-on galaxies are ...ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. Spectral information can be used to test models of CRE propagation. Free–free absorption by ionized gas at low frequencies allows us to investigate the properties of the warm ionized medium in the disk. Methods. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129–163 MHz with the LOw Frequency ARray (LOFAR) and at 13–18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1–2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency. Results. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free–free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 ± 2 μG. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 ± 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation. Conclusions. Free–free absorption is probably important at and below about 150 MHz in the disks of edge-on galaxies. To reliably separate the thermal and nonthermal emission components, to investigate spectral steepening due to CRE energy losses, and to measure magnetic field strengths in the disk and halo, wide frequency coverage and high spatial resolution are indispensable.
ABSTRACT
We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest resolution images of NGC 6251 at these ...frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs, but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of t ≳ 250 Myr and t ≳ 210 Myr, respectively. We present the first detection of polarization at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of $B \lt 0.2 \, (\Lambda _B / 10\, {\rm kpc})^{-0.5}\, \mu$G for a coherence scale of $\Lambda _B \lt 60\, {\rm kpc}$ and $B \lt 13\, \mu \textrm{G~for} \Lambda _B = 240$ kpc.
Aims. Radio synchrotron polarization maps of the Galaxy can be used to infer the properties of interstellar turbulence in the diffuse magneto-ionic medium (MIM). In this paper, we investigate the ...normalized spatial gradient of linearly polarized synchrotron emission (|∇P|/|P|) as a tracer of turbulence, the relationship of the gradient to the sonic Mach number of the MIM, and changes in morphology of the gradient as a function of Galactic position in the southern sky. Methods. We used data from the S-band Polarization All Sky Survey (S-PASS) to image the normalized spatial gradient of the linearly polarized synchrotron emission (|∇P|/|P|) in the entire southern sky at 2.3 GHz. The spatial gradient of linear polarization reveals rapid changes in the density and magnetic fluctuations in the MIM due to magnetic turbulence as a function of Galactic position. We made comparisons of these data to ideal MHD numerical simulations. To constrain the sonic Mach number (Ms), we applied a high-order moments analysis to the observations and to the simulated diffuse, isothermal ISM with ideal magneto-hydrodynamic turbulence. Results. We find that polarization gradient maps reveal elongated structures, which we associate with turbulence in the MIM. Our analysis indicates that turbulent MIM is in a generally transonic regime. This result for the turbulent regime is more general than the ones deduced by the analysis of electron density variation data, because it is based on the stochastic imprints of the Faraday rotation effect, which is also sensitive to the magnetic field fluctuations. Filamentary structures are seen with typical widths down to the angular resolution, and the observed morphologies closely match numerical simulations and, in some cases, Hα contours. The |∇P|/|P| intensity is found to be approximately log-normal distributed. No systematic variations in the sonic Mach number are observed as a function of Galactic coordinates, which is consistent with turbulence in the WIM, as inferred by the analysis of Hα data. We conclude that the sonic Mach number of the diffuse MIM appears to be spatially uniform towards the Galactic plane and the Sagittarius-Carina arm, but local variations induced by nearby extended objects are also found.
Context.
Giant radio galaxies (GRGs) are physically large radio sources that extend well beyond their host galaxy environment. Their polarization properties are affected by the poorly constrained ...magnetic field that permeates the intergalactic medium on megaparsec scales. A low frequency (< 200 MHz) polarization study of this class of radio sources is now possible with LOFAR.
Aims.
Here we investigate the polarization properties and Faraday rotation measure (RM) of a catalog of GRGs detected in the LOFAR Two-meter Sky Survey. This is the first low frequency polarization study of a large sample of radio galaxies that were selected on their physical size. We explore the magneto-ionic properties of their under-dense environment and probe intergalactic magnetic fields using the Faraday rotation properties of their radio lobes. LOFAR is a key instrument for this kind of analysis because it can probe small amounts of Faraday dispersion (< 1 rad m
−2
), which are associated with weak magnetic fields and low thermal gas densities.
Methods.
We used RM synthesis in the 120−168 MHz band to search for polarized emission and to derive the RM and fractional polarization of each detected source component. We study the depolarization between 1.4 GHz and 144 MHz using images from the NRAO VLA Sky Survey. We investigate the correlation of the detection rate, the RM difference between the lobes, and the depolarization with different parameters as follows: the angular and linear size of the sources and the projected distance from the closest foreground galaxy cluster. In our sample, we also included
3C 236
, which is one of the largest radio galaxies known.
Results.
From a sample of 240 GRGs, we detected 37 sources in polarization, all of which have a total flux density above 56 mJy. We detected significant RM differences between the lobes, which would be inaccessible at gigahertz frequencies, with a median value of ∼1 rad m
−2
. The fractional polarization of the detected GRGs at 1.4 GHz and 144 MHz is consistent with a small amount of Faraday depolarization (a Faraday dispersion < 0.3 rad m
−2
). Our analysis shows that the lobes are expanding into a low-density (< 10
−5
cm
−3
) local environment that is permeated by weak magnetic fields (< 0.1
μ
G) with fluctuations on scales of 3−25 kpc. The presence of foreground galaxy clusters appears to influence the polarization detection rate up to 2
R
500
. In general, this work demonstrates the ability of LOFAR to quantify the rarefied environments in which these GRGs exist and highlights them as an excellent statistical sample to use as high precision probes of magnetic fields in the intergalactic medium and the Milky Way.
The polarization properties of radio sources at very low frequencies (<200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency ...Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region (10h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing.
Context. Mixed-morphology supernova remnants (MM SNRs) are a mysterious class of objects that display thermal X-ray emission within their radio shell. They are an older class of SNRs, and as such are ...profoundly affected by the environment into which they evolve. VRO 42.05.01 is a MM SNR of puzzling morphology in the direction of the Galactic anticentre. Aims. Low-frequency radio observations of supernova remnants are sensitive to synchrotron electrons accelerated in the shock front. We aim to compare the low-frequency emission to higher frequency observations to understand the environmental and shock acceleration conditions that have given rise to the observed properties of this source. Methods. We present a LOFAR High Band Antenna map centred at 143 MHz of the region of the Galactic plane centred at l = 166 ° , b = 3.5° at 143 MHz, with a resolution of 148″ and an rms noise of 4.4 mJy bm−1. Our map is sensitive to scales as large as 6°. We compared the LOw Frequency ARay (LOFAR) observations to archival higher frequency radio, infrared, and optical data to study the emission properties of the source in different spectral regimes. We did this both for the SNR and for OA 184, an H II region within our field of view. Results. We find that the radio spectral index of VRO 42.05.01 increases at low radio frequencies; i.e. the LOFAR flux is higher than expected from the measured spectral index value at higher radio frequencies. This observed curvature in the low-frequency end of the radio spectrum occurs primarily in the brightest regions of the source, while the fainter regions present a roughly constant power-law behaviour between 143 MHz and 2695 MHz. We favour an explanation for this steepening whereby radiative shocks have high compression ratios and electrons of different energies probe different length scales across the shocks, therefore sampling regions of different compression ratios.
Context. Cassiopeia A is one of the best-studied supernova remnants. Its bright radio and X-ray emission is due to shocked ejecta. Cas A is rather unique in that the unshocked ejecta can also be ...studied: through emission in the infrared, the radio-active decay of 44Ti, and the low-frequency free-free absorption caused by cold ionised gas, which is the topic of this paper. Aims. Free-free absorption processes are affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies can constrain a combination of these properties. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30–77 MHz and Very Large Array (VLA) L-band observations at 1–2 GHz to fit for internal absorption as parametrised by the emission measure. We simultaneously fit multiple UV-matched images with a common resolution of 17″ (this corresponds to 0.25 pc for a source at the distance of Cas A). The ample frequency coverage allows us separate the relative contributions from the absorbing gas, the unabsorbed front of the shell, and the absorbed back of the shell to the emission spectrum. We explored the effects that a temperature lower than the ~100–500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material, such as its mass and density. We also compiled integrated radio flux density measurements, fit for the absorption processes that occur in the radio band, and considered their effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 ± 0.48 M⊙ for an assumed gas temperatureof T = 100 K. This estimate is reduced for colder gas temperatures and, most significantly, if the ejecta are clumped. We measure the reverse shock to have a radius of 114″± 6″ and be centred at 23:23:26, +58:48:54 (J2000). We also find that a decrease in the amount of mass in the unshocked ejecta (as more and more material meets the reverse shock and heats up) cannot account for the observed low-frequency behaviour of the secular decline rate. Conclusions. To reconcile our low-frequency absorption measurements with models that reproduce much of the observed behaviour in Cas A and predict little mass in the unshocked ejecta, the ejecta need to be very clumped or the temperature in the cold gas needs to be low (~10 K). Both of these options are plausible and can together contribute to the high absorption value that we find.
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