J/A+A/689/A68 GMRT radio continuum map of M51 (Gajovic+, 2024)
A spatially resolved radio spectral study of the galaxy M 51.
Gajovic L., Adebahr B., Basu A., Heesen V., Brueggen M., de Gasperin F.,
Lara-Lopez M.A., Oonk J.B.R., Edler H.W., Bomans D.J., Paladino R.,
Garduno L.E., Lopez-Cruz O., Stein M., Fritz J., Piotrowska J., Sinha A.
<Astron. Astrophys. 689, A68 (2024)>
=2024A&A...689A..68G 2024A&A...689A..68G (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, radio ; Radio continuum
Keywords: radiation mechanisms: general - galaxies: ISM -
galaxies: individual: M51 - galaxies: spiral -
galaxies: star formation - radio continuum: galaxies
Abstract:
Radio continuum emission from galaxies at gigahertz frequencies can be
used as an extinction-free tracer of star formation. However, at
frequencies of a few hundred megahertz, there is evidence for
low-frequency spectral flattening. We wish to understand the origin of
this low-frequency flattening better, and to this end, we performed a
spatially resolved study of the nearby spiral galaxy M51. We explored
the different effects that can cause a flattening of the spectrum
towards lower frequencies, such as free-free absorption and cosmic-ray
ionisation losses. We used radio continuum intensity maps between 54
and 8350MHz at eight different frequencies, with observations at
240MHz from the Giant Metrewave Radio Telescope presented for the
first time. We corrected for the contribution from thermal free-free
emission using an Hα map that was corrected for extinction with
24um data. We fitted free-free absorption models to the radio spectra
to determine the emission measure (EM) as well as polynomial functions
to measure the non-thermal spectral curvature. We also obtained a new
extinction corrected Hα intensity map from the Metal-THINGS
survey using integral field unit spectroscopy. The non-thermal
low-frequency radio continuum spectrum between 54 and 144MHz is very
flat and even partially inverted, particularly in the spiral arms; in
contrast, the spectrum at higher frequencies is typical for a
non-thermal radio continuum spectrum. However, we did not find any
correlation between the EMs calculated from radio and from Hα
observations; instead, the non-thermal spectral curvature weakly
correlates with the HI gas-mass surface density. This suggests that
cosmic-ray ionisation losses play an important role in the
low-frequency spectral flattening. The observed spectral flattening
towards low frequencies in M51 is caused by a combination of
ionisation losses and free-free absorption. The reasons for this
flattening need to be understood in order to use sub-gigahertz
frequencies as a tracer of star formation.
Description:
Radio continuum map of M51 (NGC 5194) at 240MHz observed by the Giant
Metrewave Radio Telescope (GMRT). The map has an angular resolution of
14"x12" and an rms noise of 500uJy/beam.
Objects:
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RA (2000) DE Designation(s)
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13 29 52.69 +47 11 42.9 M51 = NGC 5194
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 107 1 Information of fits map
fits/* . 1 Fits map
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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- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 39 A10 "datime" Obs.date Observation date
41- 46 F6.2 MHz Freq Observed frequency
48- 52 I5 Kibyte size Size of FITS file
54- 65 A12 --- FileName Name of FITS file, in subdirectory fits
67-107 A41 --- Title Title of the FITS file
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History:
From Lovorka Gajovic, lovorka.gajovic(at)hs.uni-hamburg.de
Acknowledgements:
LG and MB acknowledge funding by the
Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)
under Germany's Excellence Strategy - EXC 2121 'Quantum Universe' -
390833306.
(End) Patricia Vannier [CDS] 01-Jul-2024