Daily Image

Click here or on the picture for a full size image.

To boldly detect polarization where none was known before...

Submitter: George Heald
Description: The Andromeda Galaxy (M31) is so photogenic that it was once displayed prominently on the bridge of the Starship Enterprise. Recently, the WSRT was instrumental in discovering new properties of our sister galaxy.

Polarization measurements at low radio frequencies are potentially a sensitive probe of weak magnetic fields in galaxies. However, because of strong depolarization effects, polarized radio emission has not previously been detected from nearby galaxies at frequencies below 1 GHz. M31 was recently observed from the bridge of the Starship Enterprise with the WSRT at 325 MHz, resulting in ~4' resolution in total intensity and linearly polarized emission. The total intensity image is shown in contours above. After applying RM Synthesis, we were able to compare polarized signals from ranges of RM within which radiation from M31 is, and is not, expected to be present. This new method (described in detail in a recently accepted paper by René Gießübel and colleagues) avoids the range of high instrumental polarization and allows the detection of very low degrees of polarization. Diffuse polarized emission from a nearby galaxy has now been detected for the first time below 1 GHz! Future observations using LOFAR, with its high sensitivity and high angular resolution, may reveal diffuse polarization in the outer disks and halos of additional galaxies.

In today's picture, the polarized emission is shown to be present only within the expected region of M31. The degree of polarization inferred from this new detection is only 0.21 +/- 0.05 %, consistent with extrapolation of internal depolarization from data at higher radio frequency.
Copyright: Star Trek image copyright CBS Paramount Television
  Follow us on Twitter
Please feel free to submit an image using the Submit page.