The large-scale radio continuum structure of the edge-on spiral galaxy NGC 891.
Abstract
We present radio continuum measurements of the edge-on spiral galaxy NGC 891 at 327 MHz, 610 MHz and 1.49 GHz. The main results can be summarized as follows: (i) The radio spectrum of the total emission shows a sudden break of ~0.2 at a break frequency near 0.8 GHz. It is also consistent with a more gradual change of {DELTA}α= 0.5 centred at 0.8 GHz. Because it is the dominant component this must also be the case for the halo component. (ii) A large fraction of the total emission at 1.49 GHz (~65%) is in a halo component. The exponential scale heights are 0.8 and 1.0 kpc on the eastern and western side, respectively. The eastern side shows a peanut- shape morphology. The south-western part shows an extension with a scale height of ~2 kpc. (iii) The z-extent at the 100/0 intensity level of the radio halo changes with frequency. In the range from 327 to 1490 MHz it changes as ν^-0.3^. (iv) The low frequency (between 327 and 610 MHz) and the high frequency (between 610 and 1490 MHz) spectral indices in the plane of the galaxy show a difference of ~0.3. (v) The thin disk component is essentially unresolved in the z- direction. In the radial direction it is more extended than the halo component. It has a spectral index of ~- 0.4 compared to <- 0.9 for the radio halo. At 1.49 GHz about 30% of the total emission is in this thin disk component. (vi) Compared to NGC 4631 the radio halo of NGC 891 is less extended and has less energy stored in the relativistic electrons and B-field. The energy densities in cosmic rays and B-field are higher. The results are consistent with models for a dynamical halo in which adiabatic energy losses, caused by a galactic wind, are significant at lower energies. The comparison of NGC 891 and NGC 4631 lends further support for the importance of convection perpendicular to the plane and that this is probably related to the star formation activity in the disks.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 1991
- Bibcode:
- 1991A&A...246...10H
- Keywords:
-
- Cosmic Rays;
- Galactic Radio Waves;
- Halos;
- Radio Spectra;
- Spiral Galaxies;
- Astronomical Maps;
- Radiant Flux Density;
- Radio Astronomy;
- Radio Emission;
- Very Large Array (Vla);
- Astrophysics