| 11. |
- Penston, M. V., et al.
(författare)
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The extended narrow line region of NGC 4151. I. Emission line ratios and their implications
- 1990
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 236:1, s. 53-6262
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Tidskriftsartikel (refereegranskat)abstract
- The paper presents the first results from long-slit spectra of the Seyfert galaxy NGC 4151 which give average diagnostic ratios of weak lines in the extended narrow line region (ENLR) of that galaxy and the first direct density measurement in an ENLR. These data confirm that the ENLR is kinematically undisturbed gas in the disc of the galaxy which is illuminated by an ionizing continuum stronger by a factor of 13 than a power law interpolated between observed ultraviolet and X-ray fluxes. Explanations of this apparent excess include a hot thermal continuum, time variations and an anisotropic radiation field. The authors give reasons for favouring anisotropy which might be caused by shadowing by a thick accretion disc or by relativistic beaming. Shadowing by a molecular torus is unlikely, given the absence of an infrared signal from the reradiated flux absorbed by any torus. Anisotropy would have important implications for the bolometric luminosity and nature of active galactic nuclei
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| 12. |
- Rantakyrö, F. T., et al.
(författare)
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High dynamic-range VLBI observations of 3C 345 at 18 cm wavelength
- 1992
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Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 259:1, s. 8-16
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents very high dynamic range VLBI observations of the quasar 3C345 at 18 cm wavelength at two epochs, 1981.11 and 1985.27. We find that the direction of the jet changes drastically and the surface brightness decreases with increasing distance from the core. We show that a true three-dimensional curvature of the jet in space is needed to obtain the observed decrease of intensity with distance from the core, both in an adiabatic expansion model as well as in a model where the comoving flux is constant. Our observations confirm the detection of a previously reported low surface brightness component, C0. In addition, there is a large component extended perpendicular to the jet axis which we associate with a stationary shock at a distance consistent with the expected position of the narrow line region. Models for the intensity of the jet fitted to the observed dependence of intensity with distance from the core suggests this shocked region to be the place where the jet is expanding adiabatically after having had a constant width closer to the core. A limit on the allowed viewing angle theta less-than-or-similar-to 40-degrees is derived from the lack of any detectable counterjet.
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| 13. |
- Reipurth, Bo, et al.
(författare)
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Star formation in BOK globules and low-mass clouds. V - H-alpha emission stars near SA 101, CG13 and CG22
- 1993
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Ingår i: Astronomy and Astrophysics. - : Springer. - 0004-6361 .- 1432-0746. ; 267:2, s. 439-446
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Tidskriftsartikel (refereegranskat)abstract
- We have surveyed five fields in the Gum Nebula containing cometary globules, in a search for new regions of low-mass star formation. Seven H-alpha emission stars were found in association with the CG4/CG6/Sa101 cloud complex, and one in association with the CG13 globule. All eight stars are late-type emission line stars, and appear to be low-mass pre-main sequence stars. Low dispersion spectra and optical and IR photometry are presented for the objects, as well as for PH-alpha 92, an already known young star in CG22. Most of the globules surveyed are not associated with H-alpha emission stars. We demonstrate that star formation has occurred in the Gum Nebula over several million years, right up to very recently.
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| 14. |
- Romeo, Alessandro, 1961
(författare)
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How faithful are N-body simulations of disc galaxies?
- 1994
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 286, s. 799-806
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Tidskriftsartikel (refereegranskat)abstract
- High-softening two-dimensional models, frequently employed in N-body experiments, do not provide faithful simulations of real galactic discs. A prescription [Eqs. (17) and (18)] is given for choosing meaningful values of the softening length in standard regimes of astrophysical interest, when both the stellar and gaseous components are present. In addition, a stability criterion [Eq. (10); see also Eq. (11)] is given for choosing meaningful input values of the Toomre parameter for a given softening length. Such a criterion should also provide a key to the correct interpretation of computational results in terms of real phenomena.
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| 15. |
- Salamanca, I., et al.
(författare)
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Spectroscopic monitoring of active galactic nuclei. III. Size of the broad line region in NGC 3227
- 1994
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 282:3, s. 742-752752
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Tidskriftsartikel (refereegranskat)abstract
- For pt.II see ibid., vol.269, no.1-2, p.39-53 (1993). The authors present the results of a five-month monitoring campaign of the active galactic nucleus in NGC 3227. The Hbeta and Halpha emission lines and the optical continuum have been analysed. The optical AGN continuum and the broad-line region (BLR) emission in NGC 3227 appear to vary by about 40% on a timescale of 1.5 month. The levels of the broad-line emission and the 675.0 nm continuum are well correlated, a fact consistent with the assumption that the BLR material is photoionised by the central continuum source. Cross-correlation analyses between the light curves of the 675.0 nm AGN continuum and the BLR H emission indicate that the lag of the BLR emission with respect to the continuum variations is 17plusmn7 days. This result suggests that the BLR clouds that are affected by the changes in the flux of the central ionising source lie about 17 light days away from it
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| 16. |
- Stirpe, G. M., et al.
(författare)
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Monitoring of active galactic nuclei. V. The Seyfert 1 galaxy Markarian 279
- 1994
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 285:3, s. 857-867867
-
Tidskriftsartikel (refereegranskat)abstract
- For pt. IV see ibid., vol.284, no.1, p.33-43 (1994). Reports on the Lovers of Active Galaxies' (LAG) monitoring of the Seyfert 1 galaxy Markarian 279 from January to June 1990. The source, which was in a very bright state, gradually weakened after the first month of monitoring: the Halpha and Hbeta flux decreased by 20% and 35% respectively, and the continuum under Halpha by 30%. The luminosity-weighted radius of the broad line region (BLR), as derived from the cross-correlation function, is of the order of 10 light days. This result is very uncertain because the features in the light curves are very shallow, but it is unlikely that the radius of the BLR is more than 1 light month. The profile variations of Halpha confirm that the prevailing motions are not radial. The data of the present campaign and those obtained in previous years, when the source was in a much weaker state, show that the red asymmetry of the Balmer lines correlates positively with the broad line flux. This new effect is briefly discussed
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| 17. |
- Thomasson, Magnus, 1961, et al.
(författare)
-
Simulations of the effect of spiral arms on the cloud-ensemble velocity dispersion
- 1991
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 250:2, s. 316-323
-
Tidskriftsartikel (refereegranskat)abstract
- N-body simulations are used to study the effect of spiral heating, viscous heating, and cooling by inelastic collisions on the molecular cloud ensemble velocity dispersion. The simulations include inelastic collisions between clouds as well as long-range gravitational forces. The theory for heating and cooling of the cloud population is described, and the numerical code and the galaxy model are examined. Spiral arms not only heat the cloud population, they also cause increased cooling due to the large number of cloud-cloud collisions in the arms. It is found that the heating dominates, in that a stronger spiral potential leads to a higher velocity dispersion. The spiral potential is stronger for smaller values of Q(star). The steady-state cloud-ensemble velocity dispersion is found to be constant with the radius. Its value for a typical simulation of a multiple-arm spiral is slightly smaller than the value observed in the Galaxy. The stellar velocity dispersion corresponds to a radius-independent Q(star), for multiple-arm galaxies regardless of the initial radial dependence.
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| 18. |
- van Groningen, E.
(författare)
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An analysis of the spectra of 3 Seyfert-1 galaxies with strong Ca II emission
- 1993
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 272:1, s. 25-3636
-
Tidskriftsartikel (refereegranskat)abstract
- High resolution spectroscopy is presented of three Seyfert-1 galaxies-I Zw 1, Akn 564 and Mkn 231. The spectra cover the optical wavelength region and the regions with the Ca II triplet and the [Ca II] lines. The broad lines in I Zw 1 and Akn 564 are relatively narrow (FWHM<1000 km s -1); and in both objects the Ca II triplet lines are considerably broader than the Balmer lines. These lines are formed at large column densities in the line emitting region; electron scattering by thermal electrons in the broad line clouds may explain the larger widths of the Ca II lines. I Zw 1 has three emission line regions. In addition to the broad line region there are two kinematically separated narrow line regions: a high excitation region with relatively broad lines (FWHM~1100 km s -1), blue shifted with regard to the systemic velocity by ~700 km s -1; and a region with very low excitation at the systemic velocity and much narrower lines (FWHM~400 km s -1)
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| 19. |
- Wanders, I., et al.
(författare)
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Seeing and aperture effects on [O III]-based flux calibration of AGN spectra. NGC 3516 : a case study
- 1992
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 266:1, s. 72-7676
-
Tidskriftsartikel (refereegranskat)abstract
- In variability studies of the broad emission lines in the spectra of active galactic nuclei, the narrow emission lines are generally used to flux calibrate spectra taken at different epochs to each other. In objects where the narrow-line region is resolved, seeing affects the ratio of narrow-line flux to broad-line and continuum flux entering the aperture since the broad-line region and the continuum source are unresolved. Thus observing under different seeing conditions introduces artificial correlated variations of the continuum and broad-line fluxes if one assumes the narrow lines to be constant in flux. The authors show that one can correct for this effect by simulating seeing variations using a narrow-band [O III] image, taken in good seeing conditions, of the nearby Seyfert 1 galaxy NGC 3516, in which the narrow-line region is clearly resolved and extends over more than 20". They also show that centring the aperture on the object consistently is very important and miscentring is also a source of artificial variability of the broad-line and continuum fluxes
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| 20. |
- Wanders, I., et al.
(författare)
-
Spectroscopic monitoring of active galactic nuclei. II. The Seyfert-1 galaxy NGC 3516
- 1993
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 269:1-2, s. 39-5353
-
Tidskriftsartikel (refereegranskat)abstract
- The authors present the results of a five month spectroscopic monitoring campaign of the Seyfert-1 galaxy NGC 3516. Using a new calibration method and applying a correction for seeing differences during the various observations they can scale the spectra to each other with a much higher accuracy than hitherto achieved. NGC 3516 shows large amplitude variations on a time scale of several weeks. Asymmetric profile variations occur on the same time scale as the continuum variations. The authors show the presence of a varying dip on the blue wing of the Hbeta profile, which is not present on the Halpha profile, and which corresponds to the absorption features previously seen in the UV emission lines of NGC 3516. From cross-correlation analysis they find a time lag of 14plusmn2 days for the Halpha and 7plusmn3 days for the Hbeta emission-line response to continuum variations
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