RADIO STUDY OF WEAKLY STAR-FORMING GALAXIES
Krzysztof T. Chyży
Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
chris@oa.uj.edu.pl
Marita Krause & Rainer Beck
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Dominik Bomans
Astronomisches Institut der Ruhr-Universität, Universitätsstr. 150, 44780 Bochum, Germany
Marek Urbanik
Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
Abstract
1.
We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 4.85 GHz with the Effelsberg 100-m radio telescope, accompanied by H imaging, for the three weakly star-forming spiral galaxies
NGC 4236, NGC 4656 and IC 2574. The observed galaxies show weak magnetic fields. The mechanisms generating the regular and random magnetic field
components may be less efficient in these objects than in rapidly star-forming
spirals.
Quiet galaxies
In spiral galaxies strong magnetic fields are believed to be generated by
the mean-field dynamo (see Beck et al. 1996). This dynamo requires strong
Coriolis forces (hence rapid rotation) and an intense star formation to produce
turbulent motions of clouds and a sufficient scale height of the ionized gas.
We performed a high-sensitivity search for galaxy-scale magnetic fields in
three weakly star-forming spiral galaxies by means of radio polarimetry at
4.8 GHz with the Effelsberg 100-m radio telescope. We also obtained 1.4 GHz
maps for the spectral index estimation.
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Figure 1.
Total intensity contour map of NGC 4236 at 4.8 GHz with observed B-vectors
(their length indicates the polarized intensity) superimposed onto the H map
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Figure 2. Polarized intensity contour map of NGC 4236 at 4.8 GHz with observed B-vectors
(their length indicates the polarization degree) superimposed onto the DSS image
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Radio study of weakly star-forming galaxies
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Figure 3.
image
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Total radio intensity contours of NGC 4656 at 4.8 GHz superimposed on the H
The radio observations were accompanied by H imaging using the Calar
Alto Observatory 1.23-m telescope. The calibrated H fluxes of the galaxies
were corrected for extinction and for inclination effects according to Bottinelli
et al. (1995). Then from the H line emission the radio thermal flux at 4.8 GHz
was derived and compared with the radio thermal emission calculated from the
radio spectral index map between 1.4 GHz and 4.8 GHz. Both methods agreed
well. This allowed us to determine thermal fractions and derive magnetic field
strengths under the assumption of equipartition between magnetic field and
cosmic ray energies.
NGC 4236 shows extended radio emission closely tracing the star formation
regions (Fig. 1) with a maximum in the galaxy’s centre where strongly starforming clumps are present. The regular magnetic field strength (Fig. 2) is
G and the total magnetic field is about G. The polarization
about
percentage is rather low and amounts to 1.7%. The polarized emission in the
north-west is a background source.
The radio emission from NGC 4656 is asymmetric, its distribution resembles that of the ionized gas distribution as traced by the H map (Fig. 3). This
peculiar galaxy is tidally interacting with another galaxy and the radio emission is displaced from the centre to the northern part, possibly as a result of
this process. However, tidal effects are much weaker than in typical interact-
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Figure 4.
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Contours of total intensity of IC 2574 overlaid upon the DSS image
ing systems like NGC 4038/39 for which the disruption of the galactic disks is
much stronger (Chyży and Beck 2004).
The north-eastern part of NGC 4656 reveals a hook-like structure in the high
resolution VLA map at 1.4 GHz (Condon 1987) which corresponds to similar
features visible in the optical and hard X-ray emission from ROSAT. To the
south of this structure we detect the only spot of polarized intensity. However,
inspection of the FIRST survey data suggests it to be a background source.
Another background source is probably located in the southernmost part of the
radio emission which reveals the strongest peak in Condon’s high-resolution
VLA data.
The estimated equipartition strength of the mean total magnetic field is
G. The upper limit of the regular component obtained for the 2 rms level
in the polarized intensity map is G, thus the polarization percentage is not
higher than 1.8%.
2.
LSB galaxies
IC 2574 is a low surface brightness (LSB) galaxy for which we observe very
weak radio emission concentrated in the northern part of the galaxy (Fig. 4)
where large star-forming complexes are visible in the H map. There is no detectable polarized emission even in the region of strongest radio emission. The
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Radio study of weakly star-forming galaxies
upper limit for the regular magnetic field strength is G. The total magnetic
field strength reaches G.
The observed three galaxies seem to lie below a threshold value for generation of strong magnetic field by dynamo action. They are interesting
cases for a comparison with spiral galaxies with a higher level of star formation. Their magnetic fields are weaker than in rapidly star-forming spirals, for
which the mean value is
G (Beck et al. 1996).
Regular magnetic fields of the studied galaxies are also weaker and the field
regularity is lower, thus the mechanisms generating the regular and random
magnetic fields may be less efficient than in rapidly star-forming spirals.
Acknowledgments
This work was supported by a grant from the Polish Research Committee
(KBN), grant no. PB249/P03/2001/21.
References
Beck, R., et. al. 1996, ARAA, 34, 155
Bottinelli, L., et al. 1995, 296, 64
Chyży K.T., & Beck, R. 2004, AA, 417, 541
Condon J.J. 1987, ApJS 65, 485