18 research outputs found
The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency
Antennas (HBA) and imaged in total intensity and polarisation. This observation
covered the frequencies between 115 MHz and 175 MHz. We produced an image of
total emission of M51 at the mean frequency of 151 MHz with 20 arcsec
resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy
at frequencies below 300 MHz so far. The integrated spectrum of total radio
emission is described well by a power law, while flat spectral indices in the
central region indicate thermal absorption. We observe that the disk extends
out to 16 kpc and see a break in the radial profile near the optical radius of
the disk. Our main results, the scale lengths of the inner and outer disks at
151 MHz and 1.4 GHz, arm--interarm contrast, and the break scales of the
radio--far-infrared correlations, can be explained consistently by CRE
diffusion, leading to a longer propagation length of CRE of lower energy. The
distribution of CRE sources drops sharply at about 10 kpc radius, where the
star formation rate also decreases sharply. We find evidence that thermal
absorption is primarily caused by HII regions. The non-detection of
polarisation from M51 at 151 MHz is consistent with the estimates of Faraday
depolarisation. Future searches for polarised emission in this frequency range
should concentrate on regions with low star formation rates.Comment: 20 pages, 18 figures, accepted for publication in A&
LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Accepted for publication in Astronomy and AstrophysicsAims. 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 α low measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one α high, 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.Peer reviewedFinal Accepted Versio
LOFAR MSSS: The Scaling Relation between AGN Cavity Power and Radio Luminosity at Low Radio Frequencies
This article has been accepted for publication in a forthcoming issue of Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.We present a new analysis of the widely used relation between cavity power and radio luminosity in clusters of galaxies with evidence for strong AGN feedback. We study the correlation at low radio frequencies using two new surveys - the First Alternative Data Release of the TIFR GMRT Sky Survey (TGSS ADR1) at 148 MHz and LOFAR's first all-sky survey, the Multifrequency Snapshot Sky Survey (MSSS) at 140 MHz. We find a scaling relation , with a logarithmic slope of , which is in good agreement with previous results based on data at 327 MHz. The large scatter present in this correlation confirms the conclusion reached at higher frequencies that the total radio luminosity at a single frequency is a poor predictor of the total jet power. We show that including measurements at 148 MHz alone is insufficient to reliably compute the bolometric radio luminosity and reduce the scatter in the correlation. For a subset of four well-resolved sources, we examine the detected extended structures at low frequencies and compare with the morphology known from higher frequency images and Chandra X-ray maps. In Perseus we discuss details in the structures of the radio mini-halo, while in the 2A 0335+096 cluster we observe new diffuse emission associated with multiple X-ray cavities and likely originating from past activity. For A2199 and MS 0735.6+7421, we confirm that the observed low-frequency radio lobes are confined to the extents known from higher frequencies. This new low-frequency analysis highlights the fact that existing cavity power to radio luminosity relations are based on a relatively narrow range of AGN outburst ages. We discuss how the correlation could be extended using low frequency data from the LOFAR Two-metre Sky Survey (LoTSS) in combination with future, complementary deeper X-ray observations.Peer reviewe
Magnetic field evolution in interacting galaxies
Violent gravitational interactions can change the morphologies of galaxies
and, by means of merging, transform them into elliptical galaxies. We aim to
investigate how they affect the evolution of galactic magnetic fields. We
selected 16 systems of interacting galaxies and compared their radio emission
and estimated magnetic field strengths with their star-forming activity,
far-infrared emission, and the stage of tidal interaction. We find a general
evolution of magnetic fields: for weak interactions the strength of magnetic
field is almost constant (10-15muG) as interaction advances, then it increases
up to 2x, peaks at the nuclear coalescence (25muG), and decreases again, down
to 5-6muG, for the post-merger remnants. The magnetic field strength for whole
galaxies is weakly affected by the star formation rate (SFR), while the
dependence is higher for galactic centres. We show that the morphological
distortions visible in the radio total and polarized emission do not depend
statistically on the global or local SFRs, while they do increase with the
advance of interaction. The constructed radio-far-infrared relations for
interacting and non-interacting galaxies display a similar balance between the
generation of cosmic rays, magnetic fields, and the production of the thermal
energy and dust radiation. The process of strong gravitational interactions can
efficiently magnetize the merger's surroundings, having a similar magnetizing
effect on intergalactic medium as supernova explosions or galactic winds. If
interacting galaxies generate some ultra-high energy cosmic rays (UHECRs), the
disk or magnetized outflows can deflect them (up to 23 degrees), and make an
association of the observed UHECRs with the sites of their origin very
uncertain.Comment: 17 pages, 16 figures, 5 tables. Published in Astronomy and
Astrophysics, minor changes to v
Stokes I image of M51
Item does not contain fulltextLow-frequency radio continuum observations (<300MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M51 and other extragalactic sources in the field. The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115MHz and 175MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. We produced an image of total emission of M51 at the mean frequency of 151MHz with 20'' resolution and 0.3mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151MHz, and the break is smoother at 151MHz than those observed at 1.4GHz. The arm-interarm contrast is lower at 151MHz than at 1400MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151MHz and 1400MHz and the FIR emission at 70µm reveal breaks on scales of 1.4 and 0.7kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 µG in the central region to about 10 µG at 10 kpc radius. No significant polarisation was detected from M51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M51 field of 4.1°x4.1° size. Two sources show complex structures in Faraday space. Our main results, the scale lengths of the inner and outer disks at 151MHz and 1.4GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by HII regions. The non-detection of polarisation from M51 at 151MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.nul
VizieR Online Data Catalog: Low-frequency (115-175MHz) image of M51 (Mulcahy+, 2014)
Stokes I FITS image of M51 observed with LOFAR High Frequency Antennas (HBA) with a mean frequency of 151MHz at 20-arcsec resolution. Full details of the data reduction are explained in section 2 of the paper. (2 data files)