2016A&A...587A..25G

We analyze the properties of a sample of X-shaped radio-sources (XRSs). These objects show, in addition to the main lobes, a pair of wings that produce their peculiar radio morphology. We obtain our sample by selecting from the initial list of Cheung (2007AJ....133.2097C) the 53 galaxies with the better defined wings and with available SDSS images. We identify the host galaxies and measure their optical position angle, obtaining a positive result in 22 cases. The orientation of the secondary radio structures shows a strong connection with the optical axis, with all (but one) wing forming an angle larger than 40° with the host major axis. The probability that this is compatible with a uniform distribution is P=0.9x10^–4. For all but three sources of the sample, spectroscopic or photometric redshifts are avaliable. The radio luminosity distribution of XRSs has a high power cut-off at L∼10³⁴erg/s/Hz at 1.4GHz. Spectra are available from the SDSS for 28 XRSs. We modeled them to extract information on their emission lines and stellar population properties. The sample is formed by approximately the same number of high and low excitation galaxies (HEGs and LEGs); this classification is essential for a proper comparison with non-winged radio-galaxies. XRSs follow the same relations between radio and line luminosity defined by radio-galaxies in the 3C sample. While in HEGs a young stellar population is often present, this is not detected in the 13 LEGs, which is, again, in agreement with the properties of non-XRSs. The lack of young stars in LEGs supports the idea that they have not experienced a recent gas-rich merger. The connection between the optical axis and the wing orientation, as well as the stellar population and emission-line properties, provide further support for a hydro-dynamic origin of the radio-wings (for example, associated with the expansion of the radio cocoon in an asymmetric external medium) rather than with a change of orientation of the jet axis. In this framework, the high luminosity limit of XRSs can be interpreted as being due to high power jets being less affected by the properties of the surrounding medium.