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

Abstract

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 (2007( , AJ, 133, 2097) 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 = 0.9 × 10 −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 ∼ 10 34 erg s −1 Hz −1 at 1.4 GHz. 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.

A candidate dual AGN in a double-peaked emission-line galaxy with precessing radio jets

“…The rapid cocoon expansion along the direction of the hosts minor axis produces the X-shaped morphology. The relationship between the optical axis and wing orientation indicates that the formation of the XRSs is intimately related to the host galaxy's geometry (Gillone, Capetti & Rossi 2016). These results strengthen the interpretation that the X-shaped morphology in radio-sources has a hydrodynamical origin.…”

What Are “X-shaped” Radio Sources Telling Us? II. Properties of a Sample of 87

“…Although we are yet to establish this relationship -that of the wings extending along an axis close to the host minor axis for our entire sample of I-dev and genuine XRGs this has already been shown to be the case for 22 sources (Gillone, Capetti, &Rossi 2016) Roberts et al (2015)). We suggest that the minor axis is a favored "resting" axis for black holes in radio galaxies, or for massive elliptical galaxies in general.…”

“…Moreover, for the less spectacular but commonly seen central inversion symmetric extensions in the general radio galaxy populations, while the origin is not clear, interactions with host thermal halos have been implicated (Leahy & Williams 1984;Saripalli & Subrahmanyan 2009). Supporting the role of macro flows of synchrotron plasmas in generating the central wings are radio-optical studies that are beginning to hint at correlations between radio axis orientations and the projected axes of the host ellipticals as a function of the type, power and size of manifested AGN processes (Capetti et al 2002;Saripalli & Subrahmanyan 2009;Battye & Browne 2009;Gillone, Capetti, &Rossi 2016).…”

A Unified Framework for X-shaped Radio Galaxies

“…Although the primary jet (FRI HERG) and the secondary jet (FRII HERG) share the same diagrams for their XRG with those of Figure 4, the greater galaxy in this case has experienced a greater jet feedback effect, such that star formation has experienced some degree of suppression, and the overall excitation level has dropped. Such systems can be found both in field as well as in cluster environments and may receive a LERG classification despite the cold mode accretion onto the black hole (Gillone, Capetti & Rossi 2016).…”

“…The model therefore prescribes that XRG classified as low excitation radio galaxies (LEGs) (Gillone, Capetti & Rossi 2016), are actually transition objects that until recently still produced strong emission lines. Interestingly, some of the LEG classified XRG are close to the HEG/LEG boundary (Gillone, Capetti & Rossi 2016). Fourth, the model predicts that the merger that produced the original FRII HERG (Figures 4 and 5) occurred millions of years in the past of the XRG and that XRG are therefore not directly related to mergers, which seems compatible with observations (Landt et al 2010).…”