Multifrequency radio-continuum observations of NGC 1569: evidence for a convective wind (original) (raw)
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The radio continuum halo of NGC 5775
Astronomy and Astrophysics
Sensitive, scaled-array VLA radio-continuum observations of the edge-on galaxy NGC 5775, at 6 and 20 cm, show evidence of a radio-continuum halo that is detected up to 10-15 kpc above and below the plane. In addition, the presence of a radio continuum bridge connecting NGC 5775 to neighbouring NGC 5774 is confirmed, lending further support to reports that these galaxies form an interacting pair. Detailed analysis of the radio continuum halo of NGC 5775 indicates that it is characterized by a relatively steep spectrum with a spectral index of ≈ 1.0. The spectral index of the disk is significantly flatter, averaging ≈ 0.6. To first order, there is no evidence for large-scale spectral changes as a function of distance from the mid-plane in either component. The superposition of the disk and halo components, with distinctly different spectral indices, accounts for the apparent spectral steepening of the observed (total) emission with distance from the mid-plane. The spectral results presented here are consistent with the findings for NGC 3556 (Bloemen et al. 1993), which could be studied with a 2-3 times better linear resolution. As with NGC 3556 the relatively flat spectrum of the disk of NGC 5775 appears to be the result of an ensemble of discrete sources, embedded in diffuse steep-spectrum emission as found in the halo. It appears that the gradual spectral steepening away from the mid-plane found in previous studies of edge-on galaxies may be attributed to insufficient angular resolution, supplemented by the missing-flux problem. In contrast to the absence of global spectral changes in the disk and halo there are localized features of flatter spectral index that appear to connect the disk to the halo. These "tentacles" of flatter spectral index are roughly vertical and may represent chimneys of localized convection.
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
The Astrophysical Journal, 2004
We present new multi-frequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major H ii regions is detected; the individual spectral indices are thermal for the youngest regions (showing largest Hα emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, though we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and which resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages of the H ii regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ∼ 50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical H ii galaxies with less recent star formation and more evolved stellar clusters.
A low-frequency radio halo associated with a cluster of galaxies
Nature, 2008
Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.
A radio study of the superwind galaxy NGC 1482
Monthly Notices of the Royal Astronomical …, 2004
We present multifrequency radio continuum as well as H i observations of the superwind galaxy NGC1482, with both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). This galaxy has a remarkable hourglass-shaped optical emission line outflow as well as bi-polar soft X-ray bubbles on opposite sides of the galactic disk. The low-frequency, lower-resolution radio observations show a smooth structure. From the non-thermal emission, we estimate the available energy in supernovae, and examine whether this would be adequate to drive the observed superwind outflow. The high-frequency, high-resolution radio images of the central starburst region located at the base of the superwind bi-cone shows one prominent peak and more extended emission with substructure. This image has been compared with the infrared, optical red-continuum, Hα, and, soft and hard X-ray images from Chandra to understand the nature and relationship of the various features seen at different wavelengths. The peak of infrared emission is the only feature which is coincident with the prominent radio peak, and possibly defines the centre of the galaxy.
CHANG-ES XXIII: influence of a galactic wind in NGC 5775
Monthly Notices of the Royal Astronomical Society, 2021
We present new radio continuum images of the edge-on starburst galaxy NGC 5775, from LOFAR (140 MHz) and the Karl G. Jansky Very Large Array CHANG-ES survey (1500 MHz). We trace the non-thermal radio halo up to 13 kpc from the disc, measuring the non-thermal spectral index and estimating the total equipartition magnetic field strength (≈13 mu\mumuG in the disc and ≈7 mu\mumuG above the plane). The radio halo has a similar extent at both frequencies, displays evidence for localized cosmic ray streaming coinciding with prominent H α filaments and vertical extensions of the regular magnetic field, and exhibits a boxy morphology especially at 140 MHz. In order to understand the nature of the disc–halo flow, we extend our previous model of cosmic ray propagation by implementing an iso-thermal wind with a tunable ‘flux tube’ (approximately hyperboloidal) geometry. This updated model is successful in matching the vertical distribution of non-thermal radio emission, and the vertical steepening ...
Extended diffuse radio emission in galaxy clusters
2019
Galaxy clusters are the most massive gravitationally bound systems in the universe which grow through mergers with other clusters, galaxy groups, and accretion of gas. The mergers generate shocks and turbulence in the intra-cluster medium (ICM). Along the shock edges, particles are accelerated to relativistic energies through multiple crossings at the shock front. The extended diffuse emission sources formed as a result are commonly classified as radio halos and radio relics. The precise nature and origin of the relativistic particles is nevertheless not well understood. We study the nature of the radio relic candidate in Abell 115. We obtain it's spectral properties using radio continuum observations at 150\,MHz with LOFAR, 610\,MHz with the GMRT, and 1.5\,GHz with the VLA. We confirm it's nature as a radio relic. Polarization analyses suggest that part of the large scale synchrotron source is embedded in a region of high ICM density. We show that the galaxy cluster A1430 h...
Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523
Monthly Notices of the Royal Astronomical Society, 2022
The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multiwavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (∼1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120–168 MHz and the Jansky Very Large Array at 1–2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a t...
Continuum Halos in Nearby Galaxies: An Evla Survey (Chang-Es). I. Introduction to the Survey
The Astronomical Journal, 2012
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) 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 active galactic nuclei. 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.
Monthly Notices of the Royal Astronomical Society: Letters
The advent of sensitive low-frequency radio observations has revealed a number of diffuse radio objects with peculiar properties that are challenging our understanding of the physics of the intracluster medium. Here, we report the discovery of a steep-spectrum radio halo surrounding the central brightest cluster galaxy (BCG) in the galaxy cluster SPT-CL J2031−4037. This cluster is morphologically disturbed yet has a weak cool core, an example of a cool-core/non-cool-core transition system, which harbours a radio halo ∼0.7 Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to that in the 325 MHz observation, and the spectral index of the part of the halo visible at the 325 MHz to 1.7 GHz frequencies was found to be −1.35 ± 0.07. Also, P1.4 GHz was found to be 0.77 × 1024 W Hz−1, which falls in the region where radio mini-haloes, halo upper limits and ultra-steep-spectrum (USS) haloes are found in the P1.4 GHz–LX plane. Additionally, simulations presented in ...
The Astrophysical Journal, 2000
We present Ha images of four edge-on galaxies : NGC 5775, NGC 3044, NGC 4183, and NGC 3556. Our goals are twofold : Ðrst, to continue the study of the occurrence and physical properties of extraplanar di †use ionized gas (DIG), and second, to compare extraplanar Ha, H I, and radio continuum emission. The DIG of NGC 5775 is found to be concentrated in numerous bright Ðlaments extending as high as 6 kpc o † the midplane. NGC 3044, on the other hand, has a bright, smooth region of DIG above the central part of the disk with some faint Ðlaments at higher galactocentric radius. Vertical proÐle Ðts to the extraplanar emission show that both NGC 5775 and NGC 3044 have DIG layers more extended than the Reynolds layer. In contrast, NGC 4183 shows very little extraplanar DIG. ProÐle Ðts suggest that it is very unlikely that the DIG layer of NGC 4183 is as extended as the Reynolds layer. The correlation between levels of star formation and prominence of DIG layers for these galaxies is consistent with previous observations. A prediction of the "" chimney model ÏÏ of the ISM is that extraplanar Ha features should have H I and radio continuum counterparts. Comparisons among these three tracers have revealed, for the Ðrst time, correlations on the scale of individual extraplanar Ðlaments in a galactic disk. NGC 5775 shows correlations between Ha Ðlaments, H I shells, and radio continuum spurs that are consistent with a chimney model. However, features identiÐed as possible chimneys in NGC 5775 show radio spectral indices steeper than would be expected if cosmic rays were swept into the halo via galactic winds. Calculations of cosmic-ray propagation scale lengths within the DIG Ðlaments demonstrate that di †usion may indeed be responsible for transporting cosmic rays through chimneys marked by DIG Ðlaments. Correlated emission in these three tracers is also seen in NGC 3044, where extraplanar H I and radio continuum features appear to be associated with the galaxyÏs central region of extraplanar DIG, as well as Ðlaments above the disk. Such correlations are not as prominent in NGC 3556, where extraplanar DIG detection is hampered by the galaxyÏs relatively low inclination.