J/A+A/672/A21 M51 observed with LOFAR at 54MHz (Heesen+, 2023)
Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54 MHz.
Heesen V., de Gasperin F., Schulz S., Basu A., Beck R., Brueggen M.,
Dettmar R.-J., Stein M., Gajovic L., Tabatabaei F.S., Reichherzer P.
<Astron. Astrophys. 672, A21 (2023)>
=2023A&A...672A..21H 2023A&A...672A..21H (SIMBAD/NED BibCode)
ADC_Keywords: Surveys ; Radio continuum ; Galaxies, nearby
Keywords: cosmic rays - galaxies: magnetic fields -
galaxies: fundamental parameters - galaxies: ISM -
radio continuum: galaxies
Abstract:
The details of cosmic-ray transport have a strong impact on galaxy
evolution. The peak of the cosmic-ray energy distribution is
observable in the radio continuum using the electrons as proxy.
We measure the length that the cosmic-ray electrons (CRE) are
transported during their lifetime in the nearby galaxy M 51 across one
order of magnitude in cosmic-ray energy (approximately 1-10GeV). To
this end we use new ultra-low frequency observations from the LOw
Frequency ARay (LOFAR) at 54 MHz and ancillary data between 144 and
8350MHz.
As the the CRE originate from supernova remnants, the radio maps are
smoothed in comparison to the distribution of the star formation. By
convolving the map of the star-formation rate (SFR) surface density
with a Gaussian kernel, we can linearise the radio-SFR relation. The
best-fitting convolution kernel is then our estimate of the CRE
transport length.
We find that the CRE transport length increases at low frequencies, as
expected since the CRE have longer lifetimes. The CRE transport length
is lCRE = QSRT(4Dtsyn), where D is the isotropic diffusion coefficient
and rsyn is the CRE lifetime as given by synchrotron and inverse
Compton losses. We find that the data can be well fitted by diffusion,
where D=(2.14±0.13)x1028cm2/s. With D∝E0.001±0.185,
the diffusion coefficient is independent of the CRE energy E in the
range considered.
Our results suggest that the transport of GeV-cosmic ray electrons in
the star-forming discs of galaxies is governed by energy-independent
diffusion.
Description:
Preliminary LOFAR LBA Sky Survey map of M51 at 54MHz.
Objects:
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RA (2000) DE Designation(s)
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13 29 52.69 +47 11 42.9 M51 = UGC 8493
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 117 1 Information on fits image
fits/* . 1 fits image
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See also:
J/A+A/648/A104 : LOFAR LBA Sky Survey. I. (de Gasperin+, 2021)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 34 E7.6 Hz Freq Observed frequency
36- 38 I3 Kibyte size Size of FITS file
40- 64 A25 --- FileName Name of FITS file, in subdirectory fits
66-117 A52 --- Title Title of the FITS file
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Acknowledgements:
Volker Heesen, volker.heesen(at)hs.uni-hamburg.de
(End) Patricia Vannier [CDS] 14-Feb-2023