Increased Prevalence of Bent Lobes for Double-Lobed Radio Galaxies in Dense Environments

TL;DR

This study shows that double-lobed radio galaxies are more likely to have bent lobes in dense environments, with the bending fraction decreasing with distance from galaxy clusters, supporting ram pressure as the cause.

Contribution

It provides the first quantitative analysis of the environmental dependence of bent lobes in double-lobed radio galaxies using SDSS and VLA data.

Findings

Bent lobe fraction is 78% within 1 Mpc of galaxy systems.

Bent lobe fraction drops to 56% within 2 Mpc.

Match density decreases steeply with impact parameter, following approximately b^{-2.5}.

Abstract

Double-lobed radio galaxies (DLRGs) often have radio lobes which subtend an angle of less than 180 degrees, and these bent DLRGs have been shown to associate preferentially with galaxy clusters and groups. In this study, we utilize a catalog of DLRGs in SDSS quasars with radio lobes visible in VLA FIRST 20 cm radio data. We cross-match this catalog against three catalogs of galaxies over the redshift range $0 < z < 0.70$, obtaining 81 tentative matches. We visually examine each match and apply a number of selection criteria, eventually obtaining a sample of 44 securely detected DLRGs which are paired to a nearby massive galaxy, galaxy group, or galaxy cluster. Most of the DLRGs identified in this manner are not central galaxies in the systems to which they are matched. Using this sample, we quantify the projected density of these matches as a function of projected separation from the central galaxy, finding a very steep decrease in matches as the impact parameter increases (for $\Sigma \propto b^{-m}$ we find $m = 2.5^{+0.4}_{-0.3}$) out to $b \sim 2$ Mpc. In addition, we show that the fraction of DLRGs with bent lobes also decreases with radius, so that if we exclude DLRGs associated with the central galaxy in the system the bent fraction is 78\% within 1 Mpc and 56\% within 2 Mpc, compared to just 29\% in the field; these differences are significant at $3.6\sigma$ and $2.8\sigma$ respectively. This behavior is consistent with ram pressure being the mechanism that causes the lobes to bend.