2018A&A...615A..98M


C.D.S. - SIMBAD4 rel 1.7 - 2019.10.21CEST01:02:34

2018A&A...615A..98M - Astronomy and Astrophysics, volume 615A, 98-98 (2018/7-1)

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. and TITTERINGTON D.

Abstract (from CDS):


Context. Cosmic rays and magnetic fields play an important role for the formation and dynamics of gaseous halos of galaxies.
Aims. 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.
Methods. 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.
Results. 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.
Conclusions. 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.

Abstract Copyright: © ESO 2018

Journal keyword(s): galaxies: halos - galaxies: individual: NGC 891 - galaxies: ISM - galaxies: magnetic fields - cosmic rays - radio continuum: galaxies

Simbad objects: 18

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Number of rows : 18

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2019
#notes
1 IC 10 G 00 20 23.16 +59 17 34.7   13.6 9.5     ~ 981 0
2 NGC 253 SyG 00 47 33.134 -25 17 19.68   8.03   6.94 8.1 ~ 2857 2
3 3C 48 QSO 01 37 41.2995845985 +33 09 35.079126038   16.62 16.20     ~ 2431 2
4 SN 1986J SN* 02 22 31.33 +42 19 56.4   18.4 18.4     SN.II 301 1
5 NGC 891 H2G 02 22 32.907 +42 20 53.95 11.08 10.81 9.93 7.86   ~ 1490 2
6 3C 66A BLL 02 22 39.6111630878 +43 02 07.797284539   15.71 15.21 14.5   ~ 772 1
7 UGC 1841 Sy1 02 23 11.41124 +42 59 31.3840   15.71 14.81     ~ 524 2
8 3C 65 rG 02 23 43.10 +40 00 52.0   23.0       ~ 174 1
9 M 1 SNR 05 34 31.94 +22 00 52.2           ~ 5486 4
10 M 82 IG 09 55 52.430 +69 40 46.93 9.61 9.30 8.41     ~ 5213 6
11 M 87 BiC 12 30 49.42338230 +12 23 28.0438581 10.16 9.59 8.63   7.49 ~ 6026 3
12 NGC 4631 GiP 12 42 08.009 +32 32 29.44   9.78 9.19 9.10   ~ 943 2
13 M 51 GiP 13 29 52.698 +47 11 42.93   9.26 8.36 8.40   ~ 3683 4
14 3C 286 Sy1 13 31 08.2885060664 +30 30 32.960825108   17.51 17.25     ~ 3615 1
15 NGC 5775 GiP 14 53 57.653 +03 32 40.10   13.0 11.34     ~ 383 2
16 RFS 100 Rad 17 59 46.7 -22 14 06           ~ 4 0
17 NAME Cyg A Sy2 19 59 28.35645829 +40 44 02.0966496   16.22 15.10     ~ 2057 2
18 NAME Cas A SNR 23 23 24.000 +58 48 54.00           ~ 2336 1

    Equat.    Gal    SGal    Ecl

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2019.10.21-01:02:34

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