Origin of X-shaped radio-sources: further insights from the properties of their host galaxies

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 producing their peculiar radio morphology. We obtain our sample by selecting from the initial list of Cheung (2007, AJ, 133, 2097) the 53 galaxies with the better defined wings and with available SDSS images. We identified the host galaxies and measured 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 a angle larger than 40 degrees with the host major axis. The probability that this is compatible with a uniform distribution is P = 0.9 10E-4. 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 (HEG and LEG); 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, again in agreement with the properties of non XRSs. The lack of young stars in LEGs support the idea that they did not experiences a recent gas rich merger. The connection between the optical axis and the wings orientation, as well as the stellar population and emission lines properties, provide further support for an 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.Comment: Accepted for publication in A&

The evolution of galaxies in groups: how galaxy properties are affected by their group properties

It has been long known that galaxy properties are strongly connected to their environment; however, a complete picture is still missing. This work's aim is to better understand the role of environment in shaping the galaxy properties, using a sample of 25 redshift-selected galaxy groups at 0.060 < z < 0.063, for which 30 multi-wavelength parameters are available. Given the wide variety of group dynamical states, it was fundamental to try and identify different classes of groups performing a statistical clustering analysis using all the available parameters independently of their physical meaning, which resulted in two classes distinct by their mass. To move beyond mass driven correlations, a new clustering analysis was performed removing the mass dependent properties, this approach provided a categorisation in four classes with distinctive group properties. Based on this, the galaxy properties were investigated and the classes interpreted as follows: a class of field-like galaxies in the early stage of structure formation; a class of low-mass groups either still in formation phase, or evolved, but small because they are isolated; a class of massive groups with no, or very little, ongoing star formation, likely in a more evolved stage of structure formation; and a class of massive groups possibly experiencing merger events. The result obtained have shown that it is possible to distinguish between classes of groups and thus be able to study the property of galaxies in systems with homogeneous properties. The method developed applied to data sets with larger statistics and good data quality could be a powerful tool to study galaxy evolution in galaxy groups