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Register a new userA dichotomy in the orientation of dust and radio jets in nearby low-power radio galaxies
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Added by
Gijs Verdoes Kleijn
Publication date
2005
fields
Physics
and research's language is
English
Authors
Gijs Verdoes Kleijn
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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 the comparison of the dust distributions in the active and quiescent galaxy samples and the relation between the radio jet and dust orientations. Our main observational conclusions are: (a) radio galaxies contain a higher fraction of regular dust ellipses compared to quiescent galaxies which contain more often irregular dust distributions; (b) 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; (c) 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. Dust morphologies can be classified as regular ellipses and filamentary 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 $theta_{rm DJ}$ 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 $theta_{rm DJ}$. We discuss two scenarios that could explain the dust/jet/galaxy orientation dichotomy. (abridged)
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We have analyzed the position angle (PA) differences between radio jets and dust distributions in the centers of Fanaroff & Riley Type 1 (FRI) radio galaxies. We model the observed PA differences to infer the three-dimensional relative orientation of jet and dust. Our main conclusion is that there is a dichotomy in dust-jet-galaxy orientation both in projection and in three-dimensional space. The orientation dichotomy can explain the contradictory results obtained in previous studies. We briefly mention scenarios that might explain the dichotomy.
An important aspect of solving the long-standing question as to what triggers various types of Active Galactic Nuclei involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (HI) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (> 10^8 M_sun) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of HI gas (with sizes up to 190 kpc and masses up to 2 x 10^10 M_sun) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in HI mass with radio source size likely indicates that these compact radio sources are either confined by large amounts of gas in the central region, or that their fuelling is inefficient and different from the fuelling process of classical FR-I radio sources. To first order, the overall HI properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies.
(abridged) We have analyzed the properties of the Na D doublet lines in a large sample of 691 radio galaxies using the Sloan Digital Sky Survey (SDSS). These radio galaxies are resolved in the FIRST survey, have redshifts less that 0.2 and radio flux densities at 1.4 GHz higher than 40 mJy. Approximately 1/3 of the sources show a significant excess (above that contributed by their stellar populations) of Na D absorption that can be robustly fitted with two Voigt profiles representing the Na D doublet. A further 1/6 of the sources show residual absorption, for which the fits were not well constrained though while ~50% of the sample show no significant residual absorption. The residual absorption is modestly blueshifted, typically by ~50 km/s, but the velocity dispersions are high, generally ~500 km/s. Assuming that the size of the absorbing region is consistent with ~1 kpc for dust lanes in a sample of generally more powerful radio sources and a continuous constant velocity flow (continuity equation), we estimate mass and energy outflow rates of about 10 Msun/yr and few x e42 erg/s. These rates are consistent with those in the literature based on HI absorption line observations of radio galaxies. The energy required to power these outflows is on the order of 1-10% of the jet mechanical power and we conclude that the radio jet alone is sufficient. The mass and energy outflow rates are consistent with what is needed to heat/expel the mass returned by the stellar populations as well as the likely amount of gas from a cooling halo. This suggests that radio-loud AGN play a key role in energizing the outflow/heating phase of the feedback cycle. The deposition of the jet mechanical energy could be important for explaining the ensemble characteristics of massive early type galaxies in the local universe.
We study a sample of 44 low-luminosity radio-loud AGN, which represent a range of nuclear radio-power spanning 5 orders of magnitude, to unveil the accretion mechanism in these galaxies. We estimate the accretion rate of gas associated with their hot coronae by analyzing archival Chandra data, to derive the deprojected density and temperature profiles in a spherical approximation. Measuring the jet power from the nuclear radio-luminosity, we find that the accretion power correlates linearly with the jet power, with an efficiency of conversion from rest mass into jet power of ~0.012. These results strengthen and extend the validity of the results obtained by Allen and collaborators for 9 radio galaxies, indicating that hot gas accretion is the dominant process in FR I radio galaxies across their full range of radio-luminosity. We find that the different levels of nuclear activity are driven by global differences in the structure of the galactic hot coronae. A linear relation links the jet power with the host X-ray surface brightness. This implies that a substantial change in the jet power must be accompanied by a global change in its ISM properties, driven for example by a major merger. This correlation provides a simple widely applicable method to estimate the jet-power of a given object by observing the intensity of its host X-ray emission. To maintain the mass flow in the jet, the fraction of gas that crosses the Bondi radius reaching the accretion disk must be > 0.002. This implies that the radiative efficiency of the disk must be < 0.005, an indication that accretion in these objects occurs not only at a lower rate, but also at lower efficiency than in standard accretion disks.
We study the orientation of accretion disks, traced by the position angle of the jet, relative to the dust disk major axis in a sample of 20 nearby Radio Galaxies. We find that the observed distribution of angles between the jet and dust disk major axis is consistent with jets homogeneously distributed over a polar cap of 77 degrees.
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