A dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies

Abstract

Abstract.We examine the properties of central dust in nearby quiescent and active early-type galaxies. The active galaxies are low-power radio galaxies with Fanaroff & Riley type I or I/II radio jets. We focus on (a) the comparison of the dust distributions in the active and quiescent galaxy samples; and (b) the relation between the radio jet and dust orientations. Our main observational conclusions are: (i) in line with previous studies, the dust detection rate is higher in radio-jet galaxies than in non radio-jet galaxies; (ii) radio galaxies contain a higher fraction of regular dust "ellipses" compared to quiescent galaxies which contain more often irregular dust distributions; (iii) the morphology, size and orientation of dust ellipses and lanes in quiescent early-types and active early-types with kpc-scale radio jets is very similar; (iv) dust ellipses are aligned with the major axis of the galaxy, dust lanes do not show a preferred alignment except for large (>kpc) dust lanes which are aligned with the minor axis of the galaxy; and (v) as projected on the sky, jets do not show a preferred orientation relative to the galaxy major axis (and hence dust ellipses), but jets are preferentially perpendicular to dust lanes. We show that the dust ellipses are consistent with being nearly circular thin disks viewed at random viewing angles. The lanes are likely warped dust structures, which may be in the process of settling down to become regular disks or are being perturbed by a non-gravitational force. We use the observed dust-jet orientations to constrain the three-dimensional angle θ DJ between jet and dust. For dust-lane galaxies, the jet is approximately perpendicular to the dust structure, while for dust-ellipse galaxies there is a much wider distribution of θ DJ . We discuss two scenarios that could explain the dust/jet/galaxy orientation dichotomy. If lanes are indeed settling, then the jet orientation apparently is roughly aligned with the angular momentum of the dust before it settles. If lanes are perturbed by a jet-related force, it appears that it causes the dust to move out of its equilibrium plane in the galaxy into a plane which is perpendicular to the jet.

Understanding the Fanaroff–riley Radio Galaxy Classification

“…Van Dokkum & Franx (1995) and Verdoes Kleijn & de Zeeuw (2005) used archival HST data to study dust in elliptical galaxies. They reported a high prevalence of dust among the hosts of radio galaxies compared to normal ellipticals, which suggested a link between dust and nuclear activity.…”

“…While the dust-radio-axis relation and dust characteristics such as appearance and size have been examined in all of the above works, the location of the dust (perpendicular to the radio axis) with respect to the host major axis received little attention except in the work on normal and FR-I ellipticals by Verdoes Kleijn & de Zeeuw (2005). Their Figure 4 shows that in several cases where the dust is perpendicular to the radio axis, it lies within 25 deg of the host major axis (8 out of 14 sources).…”

“…The dust characteristics of radio galaxies being discussed here have been sourced from mainly three previous works, de Koff et al (2000), de Ruiter et al (2002), and Verdoes Kleijn & de Zeeuw (2005). All three authors have endeavored to study the effect of distance on their findings.…”

A dichotomy in radio jet orientations

“…Third, dust morphology correlates with these distinct classes of relative orientations. The appearance of dust structures in the radio galaxy sample varies from regular elliptical shapes -evoking the idea of inclined disks -via filamentary dust 'lanes' to highly irregular structures -suggesting unsettled dust (Verdoes Kleijn et al 1999;2005). The dust ellipses tend to fall in region E and the dust lanes in region L.…”

“…A detailed analysis is presented in Verdoes Kleijn & de Zeeuw (2005). This paper also compares the dust properties of radio galaxies and quiescent ellipticals.…”