4,512 research outputs found
The Influence of Black Hole Mass and Accretion Rate on the FRI/FRII Radio Galaxy Dichotomy
We use medium resolution optical spectra of 3CR radio galaxies to estimate
their black hole masses and accretion rates. Black hole masses are found from
central stellar velocity dispersions and accretion rates are derived from
narrow emission-line luminosities. The sample covers both Fanaroff-Riley (FR)
classes; the more powerful FRIIs and the less powerful FRIs. We find that FRIs
and FRIIs separate in diagrams of black hole mass versus radio luminosity and
narrow-line luminosity. This suggests that, at a given black hole mass, the
FRIIs accrete more efficiently, or accrete more matter, than FRIs.Comment: 2 pages, 1 figure. Contribution to the proceedings of the conference
"Growing Black Holes", Garching, Germany, June 21-24, 2004. Eds. A. Merloni,
S. Nayakshin, R. Sunyae
The influence of AGN nuclear parameters on the FRI/FRII dichotomy
We have investigated the influence of nuclear parameters such as black hole mass and photoionizing luminosity on the FRI/FRII transition in a sample of nearby (z<0.2) radio galaxies from the 3CR catalogue. The sample was observed with medium-resolution, optical spectroscopy and contains some galaxies with unpublished velocity dispersion measurements and emission-line fluxes. The measured velocity dispersions for the sample lie in the range 130-340 km s^(-1) with a mean of 216 km s^(-1). Using the M-Ï relation, we convert to black hole mass and find that the black hole mass distribution is identical for FRI and FRII galaxies, with a mean of â 2.5 Ă 10^8 M_â. We determine narrow emission-line luminosities from [O III] and [O III] in our spectra, as well as from the literature, and convert them to photoionizing luminosities under the assumption that the gas is ionized by the nuclear UV continuum. Most of the galaxies with FRI morphology and/or low-excitation emission-line spectra have progressively lower black hole masses at lower photoionizing (and jet) luminosities. This agrees with the well-known Ledlow-Owen relation which states that the radio luminosity at the FRI/FRII transition depends on the optical luminosity of the host, L_(radio) â L^(1.8)_(optical), because these two luminosities relate to AGN nuclear parameters. When recasting the Ledlow-Owen relation into black hole mass versus photoionizing luminosity and jet luminosity, we find that the recasted relation describes the sample quite well. Furthermore, the FRI/FRII transition is seen to occur at approximately an order of magnitude lower luminosity relative to the Eddington luminosity than the soft-to-hard transition in X-ray binaries. This difference is also consistent with the Ledlow-Owen relation, which predicts a weak black hole mass dependence in the transition luminosity in Eddington units. We conclude that the FRI/FRII dichotomy is caused by a combination of external and nuclear factors, with the latter dominating
Nuclear embedded star clusters in NGC 7582
We report on the discovery of several compact regions of mid-infrared
emission in the starforming circum nuclear disk of the starburst/Seyfert2
galaxy NGC7582. The compact sources do not have counterparts in the optical and
near-infrared, suggesting that they are deeply embedded in dust. We use the
[NeII]12.8 micron line emission to estimate the emission measure of the ionized
gas, which in turn is used to assess the number of ionizing photons. Two of the
brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the
fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst,
we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number
of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that
the compact mid-infrared sources are likely to be young, embedded star
clusters, of which only a few are known so far. Our observation highlights the
need for high resolution mid-infrared imaging to discover and study embedded
star clusters in the proximity of active galactic nuclei.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
QSO environments at intermediate redshifts
We have made a survey of quasar environments at 0.5 < z < 0.8, using a sample
of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our
observations include images of background control fields to provide a good
determination of the field galaxy counts. About 10 per cent of the quasars
appear to live in rich clusters, whereas approximately 45 per cent live in
environments similar to that of field galaxies.
The richness of galaxies within a 0.5 Mpc radius around the radio-quiet
quasars is found to be indistinguishable from the richness around the
radio-loud quasars, corresponding on average to groups or poorer clusters of
galaxies. Comparing the galaxy richness in the radio-loud quasar fields with
quasar fields in the literature, we find no evidence of an evolution in the
environment with epoch. Instead, a weak, but significant correlation between
quasar radio luminosity and environmental richness is present. It is thus
possible that the environments of quasars, at least the powerful ones, do not
evolve much between the present epoch and z \approx 0.8.Comment: 6 pages, 4 figures, to appear in the proc. of the workshop `QSO hosts
and their environments', IAA, Granada 10-12 Jan, 200
Outburst Research
The two books published by Dr. Ripu Lama (1995) and Dr. Lama and Dr. Jakob Bodziony (1996) are outstanding collections of literature, observations and original work in the field of gas outburst research and management. They provide a very solid base for further effort and developments; but they also serve to indicate that the problem is one of extreme complexity for which no âsilver bulletâ solution is likely to be found. Since 1996, research has continued in several locations in Australia and overseas, generally stimulated by outburst events, or the perceived risk of occurrence in new areas or as mines go deeper. This short presentation includes a bibliography of various publications in the recent outburst research literature (Appendix 1). These publications indicate that some good progress has recently been made both in Australia and elsewhere in developing a better understanding of outburst mechanisms, and in the authorâs own experience, in developing quantitative models of those mechanisms. It is hoped that the bibliography provides a reasonably up-to- date introduction for the enquiring reader. It does not cover gas drainage, alleviation and outburst management. More comprehensive information can be found in the reference lists of the catalogued papers, for example in the ACARP report by Wold and Choi (1999) which contains a detailed assessment of research on modelling of outburst mechanisms. The main body of this presentation comprises an overview of a research project that CSIRO Petroleum has recently commenced with support by ACARP. The project represents an attempt to provide a practical advance for the assessment of safe mining criteria for outburst prone conditions
- âŠ