We observed an early type (Sab) galaxy NGC4736 of ringed morphology (Chyy & Buta 2008) . It is an early-type object manifesting a large-scale oval, which causes outer and inner Lindblad resonances to result in 2 rings. The outer ring is very weak and diffuse. But the inner ring, of about 1kpc radius, is a very well-defined zone of star formation. The ring is well visible in this UV image, as well as in the H-alpha, infrared, and CO images. The galaxy has only a flocculent spiral pattern, so its morphology is actually dominated by the region of the inner ring. There are mainly circular gas motions with velocities of about 200km/s observed in this galaxy. Can the magnetic field be generated in these conditions? What kind of magnetic structure can we expect if there are no clear spiral arms?
The combined VLA and Effelsberg data at 8.46 GHz reveal that the total radio emission, shown below in contours, is clearly dominated by the galaxys inner ring. All the radio features correspond very well to the enhanced signal in infrared emission, shown here in colors. They both must be the result of the same intense star-formation process. The radio contours within the galaxys bulge are slightly elongated in the direction of a nuclear minibar, perpendicular to its major axis, best visible in CO emission. So all this seems to suit very well together.
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| Contours -radio emission at 8.46 GHz, colours - infrared emission (24 microm from SPITZER) |
However, the polarized emission (below) reveals a morphology radically different from the ringed one, shown below in colors in the H-alpha emission. The observed vectors of regular magnetic field are organized into a very distinct spiral pattern. Surprisingly, the inner ring hardly affects the magnetic vectors at all - they seem to cross the star-forming regions without any change of orientation, as if magnetic field would tell the ring: no thanks, I have a structure of my own! This is quite opposite behavior to that of the grand-design spiral galaxies, where the magnetic vectors typically match nearby density waves. We have observed this even in the case of the perturbed Virgo Cluster spiral. It seems that the spiral magnetic structure in NGC4736 is not supported by spiral density waves, being of a pure dynamo origin.
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| The magnetic vectors ignore the galaxy's ringed morphology! They must arise from a highly-efficient large-scale dynamo. |
It seems NGC4736 shows the most convincing evidence for a strong dynamo action in galaxies! It's surprising as it is still an early type-object. Which kind of recent concepts of MHD dynamo is able to explain our observations is to be determined.