Astron. Astrophys. 322, 19-28 (1997)

5. Influence of star formation

A galaxy with strong star formation activity should have stronger thermal radio emission, due to the larger number of young, massive stars ionizing the interstellar hydrogen. Likewise, the larger number of massive stars increases the supernova rate which leads to a higher production of cosmic ray electrons. Therefore the synchrotron emission should also be increased. Hence, the thermal fraction of the radio emission is not directly coupled with the rate of star formation. According to Chi & Wolfendale (1990) the production and confinement of cosmic ray particles depends on the star formation rate. They expect steeper non-thermal spectra in case of enhanced star formation, owing to more efficient particle confinement.

The derived thermal fraction and the non-thermal spectral indices can be used to investigate the influence of star formation on the radio continuum emission of galaxies. The proportionality between the H and FIR-luminosity (e.g. Young et al. 1989) suggests the use of the FIR as a global measure for the star formation rate. Because a large galaxy will be more luminous in all frequency bands than a small one it is necessary to normalize the FIR luminosity. One way to do this is to use the mass of the molecular hydrogen. This bears the advantage that one deals with the quantity , the so-called star formation efficiency (SFE). For 47 galaxies was calculated using CO-line fluxes by Young et al. (1989) and by Solomon & Sage (1988). For both data sets the conversion factor used was (Bloemen et al. 1986).

Fig. 6 shows plots of and versus the SFE. No correlation is evident between the thermal fraction and the SFE. This suggests that enhanced star formation will increase both, the thermal and non-thermal emission in the same proportions and on time scales which are small compared to the time scale of energy loss of the relativistic particles. However, the distribution of the points in the diagram exhibits the existence of an upper envelope, in such way that galaxies with a low SFE show steeper non-thermal spectra on average. If one divides the galaxy sample into a subsample with flat () and steep () synchrotron spectra and calculates the mean SFE for the different subsamples, then for one obtains , and for one gets . The non-thermal spectra of actively star forming galaxies appear to be closer to the injection spectra (e.g. Völk et al. 1988). With the assumption of equilibrium between the production of cosmic ray electrons and their energy losses this result implies that in galaxies with a high SFE one sees a younger relativistic electron population. In galaxies with a lower SFE the production rate of cosmic ray electrons is lower and, hence, the spectrum of the electrons is more strongly affected by losses.

Fig. 6. The left diagram presents the plot of versus SFE, the right one shows plotted versus the SFE

Another point of interest is the efficiency of particle confinement. A galaxy that stores the cosmic ray particles very effectively should have a non-thermal spectral index 1, assuming the usual injection spectral index. According to Chi & Wolfendale (1990) the particle confinement should be efficient in the largest and most FIR-luminous galaxies. However, a correlation between and can clearly be ruled out from our data. For the 47 galaxies with known masses, HI-masses were taken from Huchtmeier & Richter (1989), and the total gas mass of the interstellar gas computed. Fig. 7 shows the plot of versus the . The diamonds represent large galaxies () and the stars small ones (). There exists a tendency from low-mass galaxies with flat spectra towards more massive galaxies with steep spectra. Also, smaller galaxies have flatter non-thermal spectra on average, indicating a less efficient particle confinement. This is consistent with the findings of Klein et al. (1991) who attribute the lack of non-thermal emission in Blue Compact Dwarf Galaxies to a high escape probability of the cosmic ray electrons in these low-mass systems. The fact that a significant number of galaxies seems to have a high escape probability for cosmic ray electrons sets also constraints on exisiting models of the radio-FIR correlation (e.g. Völk 1989, Lisenfeld et al. 1986). This subject is discussed in more detail by Niklas (1995) and Niklas & Beck (1996).

Fig. 7. The non-thermal spectral indices plotted versus the total gas mass of the atomic and molecular gas. The sample is divided into galaxies with (stars) and with (diamonds)

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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