J/A+A/615/A98 NGC 891 LOFAR and AMI maps (Mulcahy+, 2018)
Investigation of the cosmic ray population and magnetic field strength in the
halo of NGC 891.
Mulcahy D.D., Horneffer A., Beck R., Krause M., Schmidt P., Basu A.,
Chyzy K.T., Dettmar R.-J., Haverkorn M., Heald G., Heesen V., Horellou C.,
Iacobelli M., Nikiel-wroczynski B., Paladino R., Scaife A.M.M.,
Sridhar S.S., Strom R.G., Tabatabaei F.S., Cantwell T., Carey S.H.,
Grainge K., Hickish J., Perrot Y., Razavi-ghods N., Scott P.,
Titterington D.
<Astron. Astrophys., 615, A98 (2018)>
=2018A&A...615A..98M 2018A&A...615A..98M (SIMBAD/NED BibCode)
ADC_Keywords: Stars, nearby ; Radio continuum
Keywords: galaxies: halos - galaxies: individual: NGC 891 - galaxies: ISM -
galaxies: magnetic fields - cosmic rays - radio continuum: galaxies
Abstract:
Cosmic rays and magnetic fields play an important role for the
formation and dynamics of gaseous halos of galaxies.
Low-frequency radio continuum observations of edge-on galaxies are
ideal to study cosmic-ray electrons (CREs) in halos via radio
synchrotron emission and to measure magnetic field strengths. Spectral
information can be used to test models of CRE propagation. Free-free
absorption by ionized gas at low frequencies allows us to investigate
the properties of the warm ionized medium in the disk.
We obtained new observations of the edge-on spiral galaxy NGC 891 at
129-163MHz with the LOw Frequency ARray (LOFAR) and at 13-18GHz with
the Arcminute Microkelvin Imager (AMI) and combine them with recent
high-resolution Very Large Array (VLA) observations at 1-2GHz,
enabling us to study the radio continuum emission over two orders of
magnitude in frequency.
The spectrum of the integrated nonthermal flux density can be fitted
by a power law with a spectral steepening towards higher frequencies
or by a curved polynomial. Spectral flattening at low frequencies due
to free-free absorption is detected in star-forming regions of the
disk. The mean magnetic field strength in the halo is 7±2µG. The
scale heights of the nonthermal halo emission at 146MHz are larger
than those at 1.5GHz everywhere, with a mean ratio of 1.7±0.3,
indicating that spectral ageing of CREs is important and that
diffusive propagation dominates. The halo scale heights at 146MHz
decrease with increasing magnetic field strengths which is a signature
of dominating synchrotron losses of CREs. On the other hand, the
spectral index between 146MHz and 1.5GHz linearly steepens from the
disk to the halo, indicating that advection rather than diffusion is
the dominating CRE transport process. This issue calls for refined
modelling of CRE propagation.
Free-free absorption is probably important at and below about 150MHz
in the disks of edge-on galaxies. To reliably separate the thermal and
nonthermal emission components, to investigate spectral steepening due
to CRE energy losses, and to measure magnetic field strengths in the
disk and halo, wide frequency coverage and high spatial resolution are
indispensable.
Description:
The LOFAR observations of NGC 891 were done in interleaved mode,
switching between scans on the calibrator 3C48 (at
RA(J2000)=01:34:41.3, Dec(J2000)=+33:09:35and the target NGC 891. A
total of 44 stations were used for this observation, of which 32 were
core stations and 12 were remote stations.
The AMI (Arcminute Microkelvin Imager) consists of two radio arrays,
the Small Array (SA) and the Large Array (LA), located at the Mullard
Radio Astronomy Observatory (Cambridge, UK).
The SA data were taken as a single pointing with interleaved
observations of the phase calibrator J0222+4302 and 3C286 used as a
flux density and bandpass calibrator. The LA data were taken as a
mosaic, consisting of seven pointings arranged on a hexagonal raster
centred on NGC 891. Individual pointings were cycled between the
pointing centers every 60s, switching to the phase calibrator
J0222+4302 every 10min for 2min.
Objects:
-----------------------------------------------
RA (2000) DE Designation(s)
-----------------------------------------------
02 22 32.91 +42 20 53.9 NGC 891 = UGC 1831
-----------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 148 5 List of fits files
fits/* . 5 Individual fits files
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See also:
J/MNRAS/396/1231 : Astrophotometric catalogue of NGC 891 (Rejkuba+, 2009)
J/A+A/565/A4 : NGC 891 70-500um images (Hughes+, 2014)
J/A+A/575/A17 : NGC 891 Herschel PACS and SPIRE spectroscopy (Hughes+, 2015)
J/ApJ/790/76 : Galaxies near NGC 891 properties (Schulz, 2014)
J/ApJ/832/118 : Emission-line spectroscopy in NGC 891 (Boettcher+, 2016)
J/ApJ/853/114 : Vertical population gradients in NGC 891. (Eigenbrot+, 2018)
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- 55 A26 "datime" Obs.date Observation date
57- 67 E11.6 Hz Freq Observed frequency
69- 73 I5 Kibyte size Size of FITS file
75- 99 A25 --- FileName Name of FITS file, in subdirectory fits
101-148 A48 --- Title Title of the FITS file
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Acknowledgements:
Rainer Beck, rbeck(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 31-Oct-2019