Daria Halkides (Bryn Mawr College)


VLBA Imaging Study of Seyfert Galaxy Mrk 348

This summer I worked with Jim Ulvestad and Alan Roy on a project concerning the radio morphologies of Seyfert galaxies and the absorption effects observed in both the cores and extended structures of these sources. Ultimately this may give evidence concerning the natures of type 1 and type 2 Seyferts (i.e. whether or not these objects are physically different from one another or if they are actually the same type of source oriented differently with respect to our line of sight) if we can distinguish between them based on what absorption effects are taking place.

Observations were made of 8 galaxies at 3 wavelengths each using the VLBA. I focused on MRK 348 (a well-known type 2 Seyfert galaxy at a redshift of 0.015) which was observed at 2, 6, and 18 cm. At 18 cm we found prominent radio jets to the north and south, confirming that the radio emission in this source is due to jets from an active nucleus, the same as in normal (larger) radio galaxies. Only the central part of the source appeared at 2 and 6 cm. At 6 cm it was unresolved, but at 2 cm it was resolved into two components (assumed to be the core with a small part of the jet visible to the north). We found the spectral indices for the whole central source between 2 and 6, and 6 and 18 cm. There was a definite change in spectral index, with a steep inverted spectrum between 6 and 18 cm, indicating absorption around the galactic center. Under the assumption that both components were synchrotron sources (with a spectral index of -0.7), we derived possible spectra and turn-over frequencies for each. These frequencies were then used to make order of magnitude estimates of the electron densities for each component, as well as the magnetic field strength which would be required for synchrotron self-absorption to be taking place.

The results showed that in order for synchrotron self-absorption to be occurring the magnetic fields would both have to be unreasonably high, whereas the electron densities were consistent with what would be necessary for free-free absorption. This means that free-free absorption is the primary absorption mechanism in this system and that synchrotron self-absorption is insignificant. The former is consistent with findings in other Seyfert galaxies, but the latter is inconsistent with at least one other Seyfert-2. While for the moment this is inconclusive, other sources observed as part if this survey are currently being analyzed, and these results will be compared with the results from those sources.