CHANG-ES – XI. Circular polarization in the cores of nearby galaxies

Abstract

We detect 5 galaxies in the CHANG-ES (Continuum Halos in Nearby Galaxies --\nan EVLA Survey) sample that show circular polarization (CP) at L-band in our\nhigh resolution data sets. Two of the galaxies (NGC~4388 and NGC~4845) show\nstrong Stokes $V/I\\,\\equiv\\,m_C\\,\\sim\\,2$\\%, two (NGC~660 and NGC~3628) have\nvalues of $m_C\\sim \\,0.3$\\%, and NGC~3079 is a marginal detection at $m_C\\sim\n\\,0.2$\\%. The two strongest $m_C$ galaxies also have the most luminous X-ray\ncores and the strongest internal absorption in X-rays. We have expanded on our\nprevious Faraday conversion interpretation and analysis and provide analytical\nexpressions for the expected $V$ signal for a general case in which the cosmic\nray electron energy spectral index can take on any value. We provide examples\nas to how such expressions could be used to estimate magnetic field strengths\nand the lower energy cutoff for CR electrons. Four out of our detections are\n{\\it resolved}, showing unique structures, including a {\\it jet} in NGC~4388\nand a CP `conversion disk' in NGC~4845. The conversion disk is inclined to the\ngalactic disk but is perpendicular to a possible outflow direction. Such CP\nstructures have never before been seen in any galaxy to our knowledge. None of\nthe galaxy cores show linear polarization at L-band. Thus CP may provide a\nunique probe of physical conditions deep into radio AGNs.\n