The giant spiral galaxy M101: radio observations of H II regions in external galaxies. II. Radio continuum emission from the H II regions and the nonthermal disc of M101.
Abstract
Summary. We have used the Westerbork Synthesis Radio Telescope to observe the galaxy M 101 in the continuum near 6, 21 and 49 cm. The results show: I. The flux density spectral indices of the radiation from several individual positions in the galaxy have a mean value of -0.4 between 2 I and 49 cm, similar to the integrated spectral index of M 101. The extended nonthermal radio emission is apparently partly associated with optical spiral arm features, but the con- verse is not always true at the present level of sensitivity. The continuum shows a general asymmetry in position with respect to the nucleus, in qualitative agreement with the bright optical features. The brightness temperature of the extended emission is about I K at 2tcm. 2. Six H ii region complexes have been detected with certainty; all have radio luminosities considerably greater than the largest H ii region complexes in M 33 or the Galaxy. The four brightest H ii region complexes have linear diameters of the order of I kpc on the new distance scale (D =7 Mpc) and they are therefore the largest H'ii region complexes which have been studied at radio wavelengths. The observed radio spectra show no evidence for a substantial contribution by non- thermal radiation to the total flux density of these objects. 3. For some of the detected H ii regions, at least 50 % of the radio flux density comes from the bright dense cores seen on short-exposure optical photographs. In a few cases we detect the lower-density envelopes that can be seen on long-exposure photographs and which surround the bright cores. 4. The cores have masses of the order of several x 106 M0 the envelopes have masses less than of the order of 10? - 108 M0. However, the r.m.s. electron densities of the cores are in the order of 15 - 35 while those of the envelopes are about I 5. Several hundred early-type stars are needed for the ionization of the core components. 6. Comparison with optical observations suggest the presence of large amounts of internal dust in these H ii region complexes, producing on average more than im internal extinction 7. The radio luminosity distribution function of the H II regions in M 101 is shown to differ substantially in shape from that of M 33. Key words: H II regions spiral galaxy - radio emission of galaxies - M 101
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 1975
- Bibcode:
- 1975A&A....40..421I
- Keywords:
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- Emission Spectra;
- H Ii Regions;
- Radio Sources (Astronomy);
- Spiral Galaxies;
- Astronomical Maps;
- Brightness Temperature;
- Continuous Spectra;
- Hydrogen Ions;
- Nebulae;
- Radio Emission;
- Astronomy