摘要:

AbstractThe availability of new observing facilities both from ground and space such as the Keck 10m telescope and the Hubble Space Telescope is casting new light on the spectroscopic investigation of emission line galaxies. In particular, it is now possible to detect spectra with a significantly improved signal to noise ratio in a very wide wavelength range, from the ultraviolet to the near infrared (HST, Keck) and beyond (ISO is unveiling the far infrared domain). As we move to high redshifts, however, it is more likely that the observed spectra are given by the contribution of different components in the galaxy: in particular, this may be the case if an Active Galactic Nucleus (AGN hereafter) emitting a power‐law continuum is surrounded by regions with strong star formation activity. The identification of the source which ionizes the line emitting regions is then complicated by the fact that we are observing the integrated spectrum from regions which are ionized by different sources. In this paper we wish to review which line ratios may be used in order to discriminate between photoionization by young stars and power‐law continuum in the wavelength range from 1200 Å to 100 μm. To this aim we used the photoionization code Cloudy (Ferland 1996) to present a series of diagnostic diagrams showing the dependence of emission line ratios on the main input parameters of photoionization models in the case of one component models with gaseous clouds ionized by (1) a stellar continuum typical of an HII region and (2) a power‐law continuum typical of an AGN: these line ratios are plotted as isoratio maps for different values of the hydrogen density, ionization parameter and slope of the power‐law continuum. We then show how the results may be affected by more realistic assumptions about the environment of the ionized gas: to this aim, we show the effects on selected line ratios of (a) the presence of two populations of clouds with different densities and (b) an AGN surrounded by regions with strong star formation

ISSN:0004-6337    

DOI:10.1002/asna.2123190602

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

摘要:

AbstractThe ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all‐sky survey at galactic latitudes |b|>30° (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point‐like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is>0.5 for most of the sources. Teh subsample could be nearly completely identified by low‐resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies, 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X‐ray source was a spurious detection. The redshift distrbution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new Seyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 – 1.8 (5 objects), two LINERs, and 4 possible narrow‐line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of

ISSN:0004-6337    

DOI:10.1002/asna.2123190603

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

摘要:

AbstractIt is shown that the lists of Shakhbazian Compact Galaxy Groups (SCGGs) are not complete. The number of the detected groups in the strip betweenb= ±30° andb= ±20° is by four to five times smaller than expected. The most probable reason is that during the search for SCGGs it was hard to distinguish images of compact galaxies from that of stars on the POSS prints in dense areas of the sky at lower galactic latitudes. There is some deficit of the detected groups between 60° and 40° of the north galactic latitudes. The surface density of SCGGs in the southern galactic hemisphere betweenb= −50° andb= −30° is by about three times less than it is expected. Obviously, the southern sky has not been searched properly. The list of Hickson's groups is complete down to galactic latitude ±30°. However, some excess of HCGs is found in the southern hemisphere, where the surface density of the found groups is by about two times higher than that of in the northe

ISSN:0004-6337    

DOI:10.1002/asna.2123190604

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

ISSN:0004-6337    

DOI:10.1002/asna.2123190605

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

摘要:

AbstractOn the bases of the new classification system introduced by Malyuto et al. (1997) a new technique to determine luminosities for M type stars has been established. Due to the lack of accurate distances and their inhomogeneity especially for supergiants the intrinsic scatter is rather high with 0.4 to 0.5 mag. The uncertainty of the zeropoint is 0.07 to 0.08 mag.Applying our new method to M supergiant members of the LMC a distance modulus of 18.34 ± 0.09 mag is derived which agrees well with recent work. The luminosities of the stars range from those of galactic Iab to those of Ia‐Ia‐0 supergiants, i.e. the upper luminosity limit does not exceed that of their galactic counterparts.Like the CaII H and K emission technique of Wilson&Bappu (1957), for which a revised calibration is given, the new method spans an extremely wide range of luminosities. It is less universal, as it can be applied to M stars only, but not to K stars. On the other hand the Wilson‐Bappu‐method requires high resolution spectroscopy so that it is restricted to bright stars in our Galaxy. Our new technique may be applied to faint stars in nearby galaxies since it requires low resolution spectrosc

ISSN:0004-6337    

DOI:10.1002/asna.2123190606

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

摘要:

AbstractIntensity images and Doppler‐velocity maps of the quiet sun in different heights are obtained from simultaneously recorded spectra of different lines. A relation between the intensity images is recognizable up to formation heights of 900 km above continuum, but the correlation coefficient changes sign above 400 km. The core of Hαshows a different pattern without any correlation to the continuum layer. Extreme Doppler velocities as well as the rms‐velocities have minima at a height of 400 km, values of about 2 km/s occur in deep photospheric layers and 2.5 km/s in a height of 900 km. The velocities in the lower and in the upper photosphere are well correlated indicating that the pattern of the velocity field is preserved up to higher layers than the intensity pattern. Hα‐velocities reach values up to 10 km/s and more, they show no correlation with the continuum intensities and almost no with the line core inte

ISSN:0004-6337    

DOI:10.1002/asna.2123190607

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

摘要:

AbstractAmong the observed characteristics of quiescent solar prominences, stability is perhaps the most important requirement for theoretical models of these coronal structures. Only stable equilibria can explain the presence of such cool and dense plasma sheets for extended periods of time. Utilizing the MHD‐Energy Principle (Bernstein et al. 1958) we investigate the linear stability properties of the two‐dimensional prominence equilibrium of Osherovich (1985). Incorporating a finite magnetic energy and a continous magnetic field, this model takes into account explicitly an external horizontal magnetic field in which the prominence is suspended. The presented numerical calculations indicate that the prominence model of Osherovich is stable and can explain the main features of reported oscillations. Instabilities occur only when model parameters are unrealistic for physical conditions in solar prominen

ISSN:0004-6337    

DOI:10.1002/asna.2123190608

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY

ISSN:0004-6337    

DOI:10.1002/asna.2123190601

出版商:WILEY‐VCH Verlag    

年代:1998

数据来源: WILEY