Search Results: 1 - 10 of 100 matches for " "
All listed articles are free for downloading (OA Articles)
Page 1 /100
Display every page Item
Free-Free Absorption and the Unified Scheme  [PDF]
Seiji Kameno,Makoto Inoue,Kiyoaki Wajima,Satoko Sawada-Satoh,Zhi-Qinag Shen
Physics , 2003, DOI: 10.1071/AS03037
Abstract: We report Very-Long-Baseline Array (VLBA) observations at 2.3, 8.4, and 15.4 GHz towards nine GHz-Peaked Spectrum (GPS) sources. One Seyfert 1 galaxy, one Seyfert 2 galaxy, three radio galaxies, and four quasars were included in our survey. We obtained spatial distributions of the Free-Free Absorption (FFA) opacity with milliarcsec resolutions for all sources. It is found that type-1 (Seyfert 1 and quasars) and type-2 (Seyfert 2 and radio galaxies) sources showed different distributions of the FFA opacities. The type-1 sources tend to show more asymmetric opacity distributions towards a double lobe, while those of the type-2 sources are rather symmetric. Our results imply that the different viewing angle of the jet causes the difference of FFA opacity along the external absorber. This idea supports the unified scheme between quasars and radio galaxies, proposed by Barthel (1989).
Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disk  [PDF]
Pawan Kumar,Jarrett L. Johnson
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.16437.x
Abstract: We consider the effects of supernovae (SNe) on accretion and star formation in a massive gaseous disk in a large primeval galaxy. The gaseous disk we envisage, roughly 1 kiloparsec (kpc) in size with >~ 10^8 M_Sun of gas, could have formed as a result of galaxy mergers where tidal interactions removed angular momentum from gas at larger radius and thereby concentrated it within the central ~ 1 kpc region. We find that SNe lead to accretion in the disk at a rate of roughly 0.1 - 1 M_Sun per year and induce star formation at a rate of ~ 10 - 100 M_Sun per year which contributes to the formation of a bulge; a part of the stellar velocity dispersion is due to the speed of SNa shells from which stars are formed and a part due to the repeated action of the stochastic gravitational field of the network of SNa remnants on stars. The rate of SNe in the inner kpc is shown to be self- regulating, and it cycles through phases of low and high activity. The supernova-assisted accretion transports gas from about one kpc to within a few pc of the center. If this accretion were to continue down to the central black hole (BH) then the resulting ratio of BH mass to the stellar mass in the bulge would be of order ~ 10^-2 - 10^-3, in line with the observed Magorrian relation.
Free-Free Absorption on Parsec Scales in Seyfert Galaxies  [PDF]
A. L. Roy,J. S. Ulvestad,A. S. Wilson,E. J. M. Colbert,C. G. Mundell,J. M. Wrobel,R. P. Norris,H. Falcke
Physics , 1999,
Abstract: Seyfert galaxies come in two main types (types 1 and 2) and the difference is probably due to obscuration of the nucleus by a torus of dense molecular material. The inner edge of the torus is expected to be ionized by optical and ultraviolet emission from the active nucleus, and will radiate direct thermal emission (e.g. NGC 1068) and will cause free-free absorption of nuclear radio components viewed through the torus (e.g. Mrk 231, Mrk 348, NGC 2639). However, the nuclear radio sources in Seyfert galaxies are weak compared to radio galaxies and quasars, demanding high sensitivity to study these effects. We have been making sensitive phase referenced VLBI observations at wavelengths between 21 and 2 cm where the free-free turnover is expected, looking for parsec-scale absorption and emission. We find that free-free absorption is common (e.g. in Mrk 348, Mrk 231, NGC 2639, NGC 1068) although compact jets are still visible, and the inferred density of the absorber agrees with the absorption columns inferred from X-ray spectra (Mrk 231, Mrk 348, NGC 2639). We find one-sided parsec-scale jets in Mrk 348 and Mrk 231, and we measure low jet speeds (typically <= 0.1 c). The one-sidedness probably is not due to Doppler boosting, but rather is probably free-free absorption. Plasma density required to produce the absorption is N_e >= 2 x 10^5 cm^-3 assuming a path length of 0.1 pc, typical of that expected at the inner edge of the obscuring torus.
From CM-finite to CM-free  [PDF]
Fan Kong,Pu Zhang
Mathematics , 2012,
Abstract: The aim of this paper is twofold. On one hand, we prove a slight generalization of the stability for Gorenstein categories in [SWSW] and [Huang]; and show that the relative Auslander algebra of a CM-finite algebra is CM-free. On the other hand, we describe the bounded derived category, and the Gorenstein defect category introduced in [BJO], via Gorenstein-projective objects; and we show that the Gorenstein defect category of a CM-finite algebra is triangle-equivalent to the singularity category of its relative Auslander algebra.
Gaseous Galactic Halos and QSO Absorption Line Systems  [PDF]
H. J. Mo,J. Miralda-Escude
Physics , 1996, DOI: 10.1086/177808
Abstract: A model of Lyman limit QSO absorption systems is investigated where they are produced in gaseous galactic halos with a two-phase structure: a hot phase at the halo virial temperature in approximate pressure equilibrium, and a cold, photoionized phase in the form of clouds confined by the pressure of the hot medium, and falling through it to the halo center. We show that the masses of these clouds must be over a relatively narrow range so that they are stable against gravitational collapse and evaporation. These masses, near $10^6 \msun$, also lead to a covering factor near unity. The hot phase is required to have a core radius such that its cooling time in the core is equal to the age of the halo, and the mass in the cold phase is determined by the rate at which the hot gas cools. We calculate the number of Lyman limit systems arising in halos of different masses in a CDM model and their impact parameters implied by these assumptions, and find them to be in reasonable agreement with observations. The observed low ionization systems such as MgII are well reproduced as arising from the photoionized phase, and are mostly in halos around massive galaxies, while CIV selected systems are predicted to be more commonly associated with lower mass galaxies and larger impact parameters. The hot phase may give rise to detectable absorption lines in OVI, while the column densities predicted for other highly ionized species are low and difficult to observe.
The Gaseous Extent of Galaxies and The Origin of QSO Absorption Line Systems  [PDF]
H. -W. Chen,K. M. Lanzetta,J. K. Webb,X. Barcons,A. Fernández-Soto
Physics , 1998,
Abstract: We present results of an ongoing program to study the gaseous extent of galaxies and the origin of QSO absorption line systems. For \lya absorption systems, we find that absorption equivalent width depends strongly on galaxy impact parameter and galaxy $B$-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy $B$-band luminosity as $r\propto L_B^{0.40\pm0.09}$. Applying the results to galaxies in the Hubble Deep Field to calculate the predicted number density of \lya absorption lines as a function of redshift and comparing it with observations, we find that at least 50% and perhaps as much as 100% of \lya absorption systems with $W\apg0.32$ \AA can be explained by the extended gaseous envelops of normal galaxies. The anti-correlation analysis has also been performed on \civ absorption line systems, and the results show that the ionized gas cross section scales with galaxy $B$-band luminosity as $r\propto L_B^{0.76\pm0.26}$.
QSO Absorption Lines, Gaseous Infall and Star Formation  [PDF]
Milan M. Cirkovic,Srdjan Samurovic,Vesna Milosevic-Zdjelar
Physics , 2000,
Abstract: We investigate the impact of recent observational advances in research on the gaseous content of the universe on our knowledge of star formation histories of disk galaxies. Several contemporary surveys indicate that large quantities of gas which has not been astrated or has been astrated only weakly are bound to galaxies at later epochs ($z\le 1$).
Morphology of AGN in the central kiloparsec  [PDF]
Paul Martini
Physics , 2001,
Abstract: Hubble Space Telescope observations of the central kiloparsec of AGN have revealed a wealth of structure, particularly nuclear bars and spirals, that are distinct from analogous features in the disks of spiral galaxies. WFPC2 and NICMOS images of a large sample of AGN observed at high spatial resolution make it possible to quantify the frequency and detailed properties of these structures. Nearly all AGN have nuclear spiral dust lanes in the central kiloparsec, while only a small minority contain nuclear bars. If these nuclear dust spirals trace shocks in the circumnuclear, gaseous disks, they may dissipate sufficient angular momentum to fuel the active nucleus.
The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems at z < 1  [PDF]
H. -W. Chen,K. M. Lanzetta,J. K. Webb,X. Barcons
Physics , 1997,
Abstract: We present initial results of a program to obtain and analyze HST WFPC2 images of galaxies in fields of HST spectroscopic target QSOs. The goal of the program is to investigate how the properties of \lya absorption systems observed in the spectra of background QSOs vary with the properties of intervening galaxies. We found that \lya absorption equivalent width depends strongly on galaxy impact parameter and galaxy B-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy B-band luminosity by $r\propto L_B^{0.35\pm0.10}$.
Gaseous Galactic Halos and QSO Absorption Line Systems  [PDF]
H. J. Mo
Physics , 1997,
Abstract: Recent observations have shown that (some) metal line absorption systems in QSO spectra arise from gas in galactic halos with radii much larger than the optical radii of galaxies. We show that the observed galaxy-absorber connections are natural results of galaxy formation in hierarchical cosmogonies, where galaxies form from gas cooling and condensation in dark matter halos.
Page 1 /100
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.