scholarly journals Radio Continuum Surveys of the Galaxy and Galaxies

2002 ◽  
Vol 199 ◽  
pp. 262-267
Author(s):  
Richard Wielebinski
Keyword(s):  
Magnetic Fields ◽  
Radio Emission ◽  
Linear Polarization ◽  
Angular Resolution ◽  
Low Frequency ◽  
Radio Continuum ◽  
Sky Surveys ◽  
The Galaxy ◽  
Nearby Galaxies ◽  
Radio Frequencies

Radio sky surveys give us basic information about the origin of the radio emission from the Galaxy. By mapping the sky at several radio frequencies a separation of the thermal and non-thermal emission components is possible. The major part of the low-frequency radio emission comes from the synchrotron process, the braking of relativistic electrons in magnetic fields. By mapping the linear polarization at several frequencies (required for the correction of the Faraday rotation) the orientation of the magnetic fields in the emitting regions can be deduced. Older all-sky surveys at 30, 150 and 408 MHz have now been supplemented by new observations of the Galaxy at 45 and 1420 MHz. These surveys, in addition to being important as tracers of the morphology of the magnetic fields in the Galaxy, are also required to correct for the ‘foreground’ features in cosmological studies of the COBE data and the PLANCK surveys in the future. Studies of the Galaxy in polarization have been made some years ago indicating high percentage of linear polarization in various directions. More recent work with good angular resolution has shown spectacular polarized intensity structures in selected regions. Low-frequency data with good angular resolution are urgently required for the interpretation of these features.Observations of nearby galaxies in radio continuum (both total power and polarized intensity) have given us the possibility to study magnetic fields in objects at known distances. Polarization observations of nearby galaxies have confirmed the existence of regular magnetic fields in practically every object so far studied. Originally data were obtained from large single-dish telescopes, notably from Effelsberg and Parkes. These data were greatly enhanced by the addition of higher resolution components from the VLA and ATCA respectively. These results indicate surprisingly homogeneous magnetic fields in most galaxies. High angular resolution observations with the GMRT at lower radio frequencies will add a new dimension to the data on galaxies.

scholarly journals Linearly Polarised Radio Emission from M83 (NGC 5236) and NGC 891

1990 ◽  
Vol 140 ◽  
pp. 215-218 ◽  
Author(s):  
S. Sukumar ◽  
R.J. Allen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Radio Emission ◽  
Galactic Plane ◽  
Radio Continuum ◽  
The Galaxy ◽  
The Magnetic Field

Recent VLA 20 cm radio continuum observations of the southern face-on barred spiral M83 reveal that the magnetic field is very highly aligned at the outer regions (~12 kpc radius) and totally disrupted in the inner regions (<6 kpc) of the galaxy. The RM variation suggests an axisymmetric morphology for the magnetic field. VLA 6 cm continuum polarization observations of the edge-on spiral NGC 891 reveal ordered magnetic fields at large Z-distances (~3 kpc) from the galactic plane, probably emanating from the disk through instabilities.

The GLEAM 4-Jy Sample: The brightest radio-sources in the southern sky

2019 ◽  
Vol 15 (S356) ◽  
pp. 375-375
Author(s):  
Sarah White
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Visual Inspection ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Relativistic Jets ◽  
The Galaxy ◽  
Murchison Widefield Array ◽  
Past Activity

AbstractLow-frequency radio emission allows powerful active galactic nuclei (AGN) to be selected in a way that is unaffected by dust obscuration and orientation of the jet axis. It also reveals past activity (e.g. radio lobes) that may not be evident at higher frequencies. Currently, there are too few “radio-loud” galaxies for robust studies in terms of redshift-evolution and/or environment. Hence our use of new observations from the Murchison Widefield Array (the SKA-Low precursor), over the southern sky, to construct the GLEAM 4-Jy Sample (1,860 sources at S151MHz > 4 Jy). This sample is dominated by AGN and is 10 times larger than the heavily relied-upon 3CRR sample (173 sources at S178MHz > 10 Jy) of the northern hemisphere. In order to understand how AGN influence their surroundings and the way galaxies evolve, we first need to correctly identify the galaxy hosting the radio emission. This has now been completed for the GLEAM 4-Jy Sample – through repeated visual inspection and extensive checks against the literature – forming a valuable, legacy dataset for investigating relativistic jets and their interplay with the environment.

scholarly journals Measuring and calibrating galactic synchrotron emission

2008 ◽  
Vol 4 (S259) ◽  
pp. 603-612 ◽  
Author(s):  
Wolfgang Reich ◽  
Patricia Reich
Keyword(s):  
Large Scale ◽  
Angular Resolution ◽  
Galactic Plane ◽  
Synchrotron Emission ◽  
Diffuse Emission ◽  
Sky Surveys ◽  
Zero Level ◽  
The Galaxy ◽  
Scale Properties ◽  
Relative Measurements

AbstractOur position inside the Galaxy requires all-sky surveys to reveal its large-scale properties. The zero-level calibration of all-sky surveys differs from standard ‘relative’ measurements, where a source is measured in respect to its surroundings. All-sky surveys aim to include emission structures of all angular scales exceeding their angular resolution including isotropic emission components. Synchrotron radiation is the dominating emission process in the Galaxy up to frequencies of a few GHz, where numerous ground based surveys of the total intensity up to 1.4 GHz exist. Its polarization properties were just recently mapped for the entire sky at 1.4 GHz. All-sky total intensity and linear polarization maps from WMAP for frequencies of 23 GHz and higher became available and complement existing sky maps. Galactic plane surveys have higher angular resolution using large single-dish or synthesis telescopes. Polarized diffuse emission shows structures with no relation to total intensity emission resulting from Faraday rotation effects in the interstellar medium. The interpretation of these polarization structures critically depends on a correct setting of the absolute zero-level in Stokes U and Q.

scholarly journals Radio Continuum Observations of the Barred Galaxy NGC 4314

1987 ◽  
Vol 115 ◽  
pp. 626-627 ◽  
Author(s):  
J.A. García-Barreto ◽  
P. Pişmiş
Keyword(s):  
Angular Resolution ◽  
Radio Continuum ◽  
Continuum Emission ◽  
No Emission ◽  
The Galaxy ◽  
Thermal Origin ◽  
Linearly Polarized ◽  
Barred Spiral Galaxy ◽  
Compact Sources ◽  
The Continuum

VLA observations have been made of the continuum emission at 20-cm from the barred spiral galaxy NGC 4314 with an angular resolution of 3.5 arcseconds that corresponds to a linear scale of approximately 156 pc at a distance to the galaxy. This resolution was sufficient to resolve the central region into several compact sources. The radiation is linearly polarized which may indicate a non-thermal origin. No emission was detected from the extended bar to a level of 130 Jy.

scholarly journals Pulsar observations at millimetre wavelengths

2017 ◽  
Vol 13 (S337) ◽  
pp. 92-95
Author(s):  
Pablo Torne
Keyword(s):  
Radio Emission ◽  
Low Frequency ◽  
High Sensitivity ◽  
Small Sample ◽  
Galactic Centre ◽  
New Era ◽  
Radio Frequencies ◽  
Radio Band ◽  
First Time ◽  
Strong Scattering

AbstractDetecting and studying pulsars above a few GHz in the radio band is challenging due to the typical faintness of pulsar radio emission, their steep spectra, and the lack of observatories with sufficient sensitivity operating at high frequency ranges. Despite the difficulty, the observations of pulsars at high radio frequencies are valuable because they can help us to understand the radio emission process, complete a census of the Galactic pulsar population, and possibly discover the elusive population in the Galactic Centre, where low-frequency observations have problems due to the strong scattering. During the decades of the 1990s and 2000s, the availability of sensitive instrumentation allowed for the detection of a small sample of pulsars above 10 GHz, and for the first time in the millimetre band. Recently, new attempts between 3 and 1 mm (≈86 − 300 GHz) have resulted in the detections of a pulsar and a magnetar up to the highest radio frequencies to date, reaching 291 GHz (1.03 mm). The efforts continue, and the advent of new or upgraded millimetre facilities like the IRAM 30-m, NOEMA, the LMT, and ALMA, warrants a new era of high-sensitivity millimetre pulsar astronomy in the upcoming years.

scholarly journals Radio Continuum Emission from the Nuclei of Normal Galaxies

1980 ◽  
Vol 5 ◽  
pp. 177-184 ◽  
Author(s):  
J. M. van der Hulst
Keyword(s):  
Radio Emission ◽  
Galactic Center ◽  
Galactic Nuclei ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Thermal Source ◽  
Very Large Array ◽  
Normal Galaxies ◽  
The Galaxy ◽  
Sgr A

During the last few years detailed and sensitive observations of the radio emission from the nuclei of many normal spiral galaxies has become available. Observations from the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO1), in particular, enable us to distinguish details on a scale of ≤100 pc for galaxies at distances less than 21 Mpc. The best studied nucleus, however, still is the center of our own Galaxy (see Oort 1977 and references therein). Its radio structure is complex. It consists of an extended non-thermal component 200 × 70 pc in size, with embedded therein several giant HII regions and the central source Sgr A (˜9 pc in size). Sgr A itself consists of a thermal source, Sgr A West, located at the center of the Galaxy, and a weaker, non-thermal source, Sgr A East. Sgr A West moreover contains a weak, extremely compact (≤10 AU) source. The radio morphology of several other galactic nuclei is quite similar to that of the Galactic Center, as will be discussed in section 2. Recent reviews of the radio properties of the nuclei of normal galaxies have been given by Ekers (1978a,b) and De Bruyn (1978). The latter author, however, concentrates on galaxies with either active nuclei or an unusual radio morphology. In this paper I will describe recent results from the Westerbork Synthesis Radio Telescope (WSRT, Hummel 1979), the NRAO 3-element interferometer (Carlson, 1977; Condon and Dressel 1978), and the VLA (Heckman et al., 1979; Van der Hulst et al., 1979). I will discuss the nuclear radio morphology in section 2, the luminosities in section 3, and the spectra in section 4. In section 5 I will briefly comment upon the possible implications for the physical processes in the nuclei that are responsible for the radio emission.

scholarly journals Molecular Clouds in M51 and in the Galaxy

1986 ◽  
Vol 7 ◽  
pp. 513-518
Author(s):  
Per Friberg ◽  
Åke Hjalmarson
Keyword(s):  
Mass Fraction ◽  
Molecular Clouds ◽  
Angular Resolution ◽  
High Angular Resolution ◽  
Galaxy Type ◽  
The Galaxy ◽  
External Galaxies ◽  
Resolution Problem ◽  
Nearby Galaxies ◽  
Co Emission

Studies of molecular clouds in nearby galaxies require high angular resolution. Ten arcseconds corresponds to 0.5 kpc at the distance of M51. Typical gigant molecular clouds (GMC:s) have a size of 5-30 pc (Solomon et al. 1985). However complexes of GMC: s (Superclouds) can be several hundred parsecs (Elmegreen 1985; Rivolo et al. 1985). The higest angular resolution achived in CO(J=1-0) line observations of external galaxies is 7” (Lo et al 1984,1985). The resolution problem can be eased by observing M31 with a distance of only ⋍ 690 kpc (10” corresponds to 34 pc), which has been done by Combes et al. 1977a,b; Boulanger et al. 1984; Ryden and Stark 1985; Stark 1985; Blitz 1985; Ichikawa et al. 1985. In M31 the CO emission is strongly concetrated to the spiral arms with a arm interarm ratio of ≥ 25 (Ryden and Stark 1985; Stark 1985). The emission is caused by many small clouds unresolved with present resolution together with some larger clouds. Streaming is observed to occur across the arms. Extragalatic studies have the advantage of being more easy to interpret in terms of arm interarm contrast, noncircular motion, and galatic structure. They also make possible studies of the mass fraction of gas as a function of radius in different morphological types of galxies. Answers to questions like “Do any relation exist between galaxy type and molecular abundance?” are very important for our understanding of galatic evolution.

scholarly journals G51.04+0.07 and its environment: Identification of a new Galactic supernova remnant at low radio frequencies

2018 ◽  
Vol 616 ◽  
pp. A98 ◽  
Author(s):  
L. Supan ◽  
G. Castelletti ◽  
W. M. Peters ◽  
N. E. Kassim
Keyword(s):  
Radio Emission ◽  
Flux Density ◽  
Supernova Remnant ◽  
Low Frequency ◽  
Neutral Hydrogen ◽  
Radio Continuum ◽  
Very Large Array ◽  
Continuum Spectrum ◽  
Galactic Emission ◽  
Nonthermal Radio Emission

We have identified a new supernova remnant (SNR), G51.04+0.07, using observations at 74 MHz from the Very Large Array Low-Frequency Sky Survey Redux (VLSSr). Earlier, higher frequency radio continuum, recombination line, and infrared data had correctly inferred the presence of nonthermal radio emission within a larger, complex environment including ionised nebulae and active star formation. However, our observations have allowed us to redefine at least one SNR as a relatively small source (7.′5 × 3′in size) located at the southern periphery of the originally defined SNR candidate G51.21+0.11. The integrated flux density of G51.04+0.07 at 74 MHz is 6.1 ± 0.8 Jy, while its radio continuum spectrum has a slope α = −0.52 ± 0.05 (S v ∝ vα), typical of a shell-type remnant. We also measured spatial variations in the spectral index between 74 and 1400 MHz across the source, ranging from a steeper spectrum (α = −0.50 ± 0.04) coincident with the brightest emission to a flatter component (α = −0.30 ± 0.07) in the surrounding fainter region. To probe the interstellar medium into which the redefined SNR is likely evolving, we have analysed the surrounding atomic and molecular gas using the 21 cm neutral hydrogen (HI) and 13CO(J = 1 − 0) emissions. We found that G51.04+0.07 is confined within an elongated HI cavity and that its radio emission is consistent with the remains of a stellar explosion that occurred ~6300 yr ago at a distance of 7.7 ± 2.3 kpc. Kinematic data suggest that the newly discovered SNR lies in front of HII regions in the complex, consistent with the lack of a turnover in the low frequency continuum spectrum. The CO observations revealed molecular material that traces the central and northern parts of G51.04+0.07. The interaction between the cloud and the radio source is not conclusive and motivates further study. The relatively low flux density (~1.5 Jy at 1400 MHz) of G51.04+0.07 is consistent with this and many similar SNRs lying hidden along complex lines of sight towards inner Galactic emission complexes. It would also not be surprising if the larger complex studied here hosted additional SNRs.

scholarly journals LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

2018 ◽  
Vol 619 ◽  
pp. A36 ◽  
Author(s):  
K. T. Chyży ◽  
W. Jurusik ◽  
J. Piotrowska ◽  
B. Nikiel-Wroczyński ◽  
V. Heesen ◽  
...  
Keyword(s):  
Star Formation ◽  
Low Frequency ◽  
Cosmic Ray ◽  
Radio Continuum ◽  
Viewing Angle ◽  
Frequency Spectra ◽  
Nearby Star ◽  
Nearby Galaxies ◽  
5 Ghz ◽  
Thermal Absorption

Aims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies’ spectra are generally flatter at lower compared to higher frequencies: the median spectral index αlow measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one αhigh, calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100–200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way’s foreground on the spectra of all external galaxies is also expected at very low frequencies.

scholarly journals Radio Emission from Supernova Remnants

1996 ◽  
Vol 145 ◽  
pp. 333-340
Author(s):  
Richard G. Strom
Keyword(s):  
Cosmic Rays ◽  
Radio Emission ◽  
Supernova Remnants ◽  
Galactic Plane ◽  
Galactic Disk ◽  
Energy Content ◽  
Particle Energy ◽  
The Galaxy ◽  
Radio Frequencies ◽  
Shock Fronts

Most of the supernova remnants known in the Galaxy have only been detected at radio frequencies. The reason for this is absorption in the Galactic plane at both optical and X-ray wavelengths. All available evidence suggests that the shock fronts which accompany supernova remnants accelerate enough cosmic rays to GeV energies to produce readily detectable radio emission. This is fortunate, for it enables us to study remnants throughout the Galactic disk, although existing catalogues may be anywhere from 50 to 90 % incomplete. Cosmic rays and the magnetic fields in which they gyrate are the essential ingredients for producing the synchrotron radiation which is observed at radio frequencies. Various methods for estimating magnetic field strengths can be applied to a small number of remnants, and produce values not far from those based upon equipartition between the energy contents of particles and fields. From this, the particle energy content is derived for a number of objects.

Sign in / Sign up

Export Citation Format

Share Document