首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 375 毫秒
1.
This is the third paper of a series devoted to the study of the global properties of Joguet's sample of 79 nearby galaxies observable from the southern hemisphere, of which 65 are Seyfert 2 galaxies. We use the population synthesis models of Paper II to derive 'pure' emission-line spectra for the Seyfert 2 galaxies in the sample, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission-line clouds suffer substantially more extinction than the starlight, and we confirm the correlations between stellar and nebular velocity dispersions and between emission-line luminosity and velocity dispersions, although with substantial scatter. Nuclear luminosities correlate with stellar velocity dispersions, but Seyferts with conspicuous star-forming activity deviate systematically towards higher luminosities. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation,   L ∝σ4★  , consistent with the Faber–Jackson law.
Emission-line ratios indicative of the gas excitation such as [O  iii ]/Hβ and [O  iii ]/[O  ii ] are statistically smaller for Seyferts with significant star formation, implying that ionization by massive stars is responsible for a substantial and sometimes even a dominant fraction of the Hβ and [O  ii ] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden active galactic nucleus (AGN). We correlate this fraction with classical indicators of AGN photoionization (i.e. X-ray luminosity and nebular excitation), but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays.  相似文献   

2.
[OIII] images and blue spectra of the emission-line dwarf galaxy F348 are presented. In [OIII] light, the object contains two knots about 9” NE of the nucleus and one large extended knot 11” to the SW. The nuclear region is hundred times less luminous in emission-line light than the knots. Despite the presence of line intensity ratios [OIII]λ5007/Hβ > 3 the prior classification as a Seyfert-2 object cannot be upheld. This clinches an earlier suggestion by Veron-Cetty & Veron (1986). In particular, we show that the line spectra can be modeled with photoionization models employing stellar input continua. Also, the line luminosities of the extranuclear knots are typical for giant HII regions. There is neither evidence for tidal tails nor for high velocity differences between the knots. In addition, the linear arrangement of the knots does not support interaction. It rather suggests self-propagating star formation In this picture, the faintness of the nuclear region can be understood by an edge-on view. In addition, the nuclear starburst appears to be fading in contrast to the young extranuclear star formation regions. Within the scheme of Melnick (1987), F348 has to be classified as a multiple-system HII galaxy.  相似文献   

3.
We present a new analysis of the main empirical calibrators of oxygen abundance for ionized gas nebulae. With that aim we have compiled an extensive sample of objects with emission-line data including the near-infrared [S  iii ] lines and the weak auroral lines which allow for the determination of the gas electron temperature. For all the objects the oxygen abundances have been derived in a homogenous way, using the most recent sets of atomic coefficients and taking into the account the effect of particle density on the temperature of O+. The residuals between directly and empirically derived abundances as a function of abundance have been studied. A grid of photoionization models, covering the range of physical properties of the gas, has been used to explain the origin of the uncertainties affecting each abundance calibrator. The range of validity for each abundance parameter has been identified and its average uncertainty has been quantified.  相似文献   

4.
We present near-infrared 2.1-μm continuum and Brγ and 1–0S(1) line images of three blue compact dwarf galaxies: II Zw 40, NGC 5253 and He 2-10. Comparison of the morphologies of the emission-line regions and continuum together with the line ratios shows that in these cases the starbursts are typified by strongly peaked Brγ and weak diffuse H2 emission, indicating the presence of one or more compact nuclei, and also tidal tails. We consider the possible origins of the H2 emission, and conclude that shock excitation in cloud collisions, and hence dynamical processes such as interactions or mergers, is important. By comparing models of the equivalent width of Brγ to other age constraints from the literature we show that the star formation must have occurred not only recently but in a short-duration burst. The current centres of star formation in all three galaxies are of a similar age but, whereas the hotspots in He 2-10 are coeval, we find phase differences between those in NGC 5253. We compare the masses and sizes of the star formation sites to those of present-day globular clusters.  相似文献   

5.
In general, H  ii regions do not show clear signs of self-enrichment in products from massive stars  ( M ≥ 8 M⊙)  . In order to explore why I modelled the contamination with Wolf–Rayet star ejecta of metal-poor  ( Z = 0.001)  H  ii regions, ionized either by a  106 M⊙  cluster of coeval stars (cluster 1) or by a cluster resulting from continuous star formation at a rate of  1 M⊙ yr−1  (cluster 2). The clusters have   Z = 0.001  and a Salpeter initial mass function from 0.1 to  120 M⊙  . Independent one-dimensional constant density simulations of the emission-line spectra of unenriched H  ii regions were computed at the discrete ages 1, 2, 3, 4 and 5 Myr, with the photoionization code cloudy , using as input, radiative and mechanical stellar feedbacks predicted by the evolutionary synthesis code starburst99 . Each H  ii region was placed at the outer radius of the adiabatically expanding superbubble of Mac Low & McCray. For models with thermal and ionization balance time-scales of less than 1 Myr, and with oxygen emission-line ratios in agreement with observations, the volume of the superbubble and the H  ii region was uniformly and instantaneously polluted with stellar ejecta predicted by starburst99 . I obtained a maximum oxygen abundance enhancement of 0.025 dex, with cluster 1, at 4 Myr. It would be unobservable.  相似文献   

6.
The 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 activity.  相似文献   

7.
Nearly ten years ago Kwan and Krolik (1979, 1981) published the firstsuccessful photoionization model of the Broad Line Region of Active Galactic Nuclei, the so-called ‘Standard Model’. Since then several efforts have been made to obtain better results using more sophisticated models. Anopen issue is that photoionization models are generally computed startingfrom the assumption that the emission line spectrum is produced by a single-slab cloud with a ‘standard’ hydrogen density nH = 109.5 cm-3, but it seems more likely that a range of densities is present in the Broad Line Region. Purpose of this paper is to review the results given by single-cloud models using the most recent photoionization code, Cloudy 84, and to investigate if the addition of one or more components with different densities does affect the line ratios. To this aim we compute the emission line ratios produced by photoionized single-slab clouds for a wide range of hydrogen densities (nH = 109.5−1013 cm−3), ionization parameters (U = 10−4− 10−0.5) and column densities (NH = 1023−1025 cm−2). Two or more populations of clouds are then combined and the resulting emission line spectrum is compared with a sample of mean observed line ratios. We find that the addition to the standard component of clouds having different densities and located at different positions from the central source introduces many changes in the line ratios, and that these changes are in the direction of a better agreement with the observed emission line ratios.  相似文献   

8.
We study the gas emission of galaxies with active star formation, consisting mostly of H  ii and starburst galaxies, as well as some Seyfert 2 galaxies, and determine chemical and physical parameters. The data consist of 19 high signal-to-noise ratio optical templates, a result of grouping 185 emission-line galaxy spectra. Underlying stellar population models (from Raimann et al.) were subtracted from the templates in order to isolate the pure emission component.
We analyse the distribution of these improved signal-to-noise ratio emission spectra in diagnostic diagrams and find that the H  ii templates show a smaller spread in log([O  iii ]/H β ) values than the individual galaxies, apparently as a result of the population subtraction and a better signal-to-noise ratio. We thus suggest the template sequence as a fiducial observational locus for H  ii galaxies which can be used as reference for models. The sequence of line ratios presented by the H  ii galaxies in the diagram log([O  iii ] λ 5007/H β ) versus log([N  ii ] λ 6584/H α ) is primarily owing to the gas metallicity, of which the log([N  ii ]/H α ) ratio is a direct estimator. We also study the properties of the starburst galaxies and those intermediate between H  ii and starburst galaxies, which are more metal rich and sit on more massive galaxies.
We discuss the present results in the frame of a recently proposed equivalent-width diagnostic diagram for emission-line galaxies (by Rola et al.) and conclude that the observed ranges in W ([O  ii ])/ W (H β ) and W (H β ) are mostly owing to the non-ionizing stellar population contribution. We propose that W (H β ) be used as an estimator of this contribution to the continuum, and briefly discuss implications to the cosmological use of H  ii galaxies.  相似文献   

9.
We study the relation between nitrogen and oxygen abundances as a function of metallicity for a sample of emission-line objects for which a direct measurement of the metallicity has been possible. This sample is representative of the very different conditions in ionization and chemical enrichment that we can find in the Universe. We first construct the N/O versus O/H ratio diagram, and discuss its large dispersion at all metallicity regimes. Using the same sample and a large grid of photoionization models covering very different values of the N/O ratio, we then study the most widely used strong-line calibrators of metallicity based on [N  ii ] emission lines, such as N2 and O3N2. We demonstrate that these parameters underestimate the metallicity at low N/O ratios and vice versa. We also investigate the effect of the N/O ratio on different diagnostic diagrams used to discriminate narrow-line active galactic nuclei from star-forming regions, such as the [O  iii ]/Hβ versus [N  ii ]/Hα, and show that a large fraction of the galaxies catalogued as composite in this diagram can be, in fact, star-forming galaxies with a high value of the N/O ratio. Finally, using strong-line methods sensitive to the N/O abundance ratio, like N2O2 and N2S2, we investigate the relation between this ratio and the stellar mass for the galaxies of the Sloan Digital Sky Survey. We find, as in the case of the mass–metallicity relation, a correlation between these two quantities and a flattening of the relation for the most massive galaxies, which could be a consequence of the enhancement of the dispersion of N/O ratio in the high-metallicity regime.  相似文献   

10.
We present results of our ongoing study of the morphology and kinematics of the ionised gas in 48 representative nearby elliptical and lenticular galaxies using the SAURON integral-field spectrograph on the 4.2m William Herschel Telescope. Making use of a recently developed technique, emission is detected in 75% of the galaxies. The ionised-gas distributions display varied morphologies, ranging from regular gas disks to filamentary structures. Additionally, the emission-line kinematic maps show, in general, regular motions with smooth variations in kinematic position angle. In most of the galaxies, the ionised-gas kinematics is decoupled from the stellar counterpart, but only some of them present signatures of recent accretion of gaseous material. The presence of dust is very common in our sample and is usually accompanied by gas emission. Our analysis of the [Oiii]/Hβ emission-line ratios, both across the whole sample as well as within the individual galaxies, suggests that there is no unique mechanism triggering the ionisation of the gas.  相似文献   

11.
《New Astronomy Reviews》2002,46(2-7):197-201
The FR II radio galaxy PKS 0349−27 contains a luminous emission-line nebula, extending over several tens of kpc around its host galaxy. We investigate the spatial distribution of the kinematics, line ratios and total emission-line flux of the gas by means of a detailed series of long-slit spectra that cover its entire extent. We show that the physical properties of the line-emitting gas, in particular its ionization properties, excitation state, dynamics and energetics, suggest a scenario involving auto-ionizing shocks as the dominant ionization mechanism.  相似文献   

12.
We study the central (inner few hundred parsecs) stellar populations of four starburst galaxies (NGC 34, 1614, 3310 and 7714) in the near-infrared (NIR), from 0.8 to 2.4 μm, by fitting combinations of stellar population models of various ages and metallicities. The NIR spectra of these galaxies feature many absorption lines. For the first time, we fit simultaneously as much as 15 absorption features in the NIR. The observed spectra are best explained by stellar populations containing a sizable amount (20–56 per cent by mass) of ∼1-Gyr-old stellar population with thermally pulsing asymptotic giant branch stars. We found that the metallicity of the stars which dominates the light is solar. Metallicities substantially different from solar give a worse fit. Though the ages and metallicities we estimate using the NIR spectroscopy are in agreement with values from the literature based on the ultraviolet/optical, we find older ages and a larger age spread. This may be due to the fact that the optical is mostly sensitive to the last episode of star formation, while the NIR better maintains the record of previous stellar generations. Another interesting result is that the reddening estimated from the whole NIR spectrum is considerably lower than that based on emission lines. Finally, we find a good agreement of the free emission-line spectrum with photoionization models, using as input spectral energy distribution the synthetic composite template we derived as best fit.  相似文献   

13.
We present hydrogen emission-line profile models of magnetospheric accretion on to classical T Tauri stars. The models are computed under the Sobolev approximation using the three-dimensional Monte Carlo radiative-transfer code torus . We have calculated four illustrative models in which the accretion flows are confined to azimuthal curtains – a geometry predicted by magnetohydrodynamic simulations. Properties of the line profile variability of our models are discussed, with reference to dynamic spectra and cross-correlation images. We find that some gross characteristics of the observed line profile variability are reproduced by our models, although in general the level of variability predicted is larger than that observed. We conclude that this excessive variability probably excludes dynamical simulations that predict accretion flows with low degrees of axisymmetry.  相似文献   

14.
Multiwavelength imaging and spectroscopy of the z =0.708 radio galaxy 3C 441 and a red aligned optical/infrared component are used to show that the most striking aspect of the radio–optical 'alignment effect' in this object is due to the interaction of the radio jet with a companion galaxy in the same group or cluster. The stellar population of the red aligned continuum component is predominantly old, but with a small post-starburst population superposed, and it is surrounded by a low-surface-brightness halo, possibly a face-on spiral disc. The [O iii ] 500.7/[O ii ] 372.7 emission-line ratio changes dramatically from one side of the component to the other, with the low-ionization material apparently having passed through the bow shock of the radio source and been compressed. A simple model for the interaction is used to explain the velocity shifts in the emission-line gas, and to predict that the ISM of the interacting galaxy is likely to escape once the radio source bow shock has passed through. We also discuss another, much fainter, aligned component, and the subarcsecond-scale alignment of the radio source host galaxy. Finally, we comment on the implications of our explanation of 3C 441 on theories for the alignment effect.  相似文献   

15.
We present accurate measurements of the physical conditions in five powerful radio galaxies, as derived from deep, long-slit spectroscopic observations. All five objects show prominent extended line emission, and have X-ray luminosities similar to those of isolated elliptical galaxies. The data are high enough quality that the electron density and temperature can be measured at several positions across the emission-line nebulae.
We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [O  iii ](4959+5007)/4363 line ratio are in the range  10 000< T e<20 000 K  , whilst  [S  ii ](6716/6731)  densities fall between  100–500 cm-3.  Using these values, we find pressures within the line-emitting clouds a factor of  10–100  times higher than expected for pressure balance with the hot X-ray haloes of the host galaxies.
Previous studies of sources that show significant evidence of jet–cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects.
We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus.  相似文献   

16.
We summarize the main conclusions derived from integral field spectroscopy, with optical fibres, of six Seyfert galaxies. More than 95 spectra around the active nucleus have been obtained showing asymmetric emission-line profiles at different positions. Continuum and emission-line intensity maps and ionized gas velocity fields have been derived for these six galaxies. We also present the stellar velocity field for NGC 1068, NGC 3516 and NGC 985. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

17.
Deep long-slit spectroscopic data are presented for a sample of 14 3CR radio galaxies at redshift z ∼1, previously studied in detail using the Hubble Space Telescope , the Very Large Array, and the UK Infrared Telescope (UKIRT). Analysis of the [O  ii ] 3727 emission-line structures at ∼5 Å spectral resolution is carried out to derive the kinematic properties of the emission-line gas. In line with previous lower resolution studies, a wide variety of kinematics are seen, from gas consistent with a mean rotational motion through to complex structures with velocity dispersions exceeding 1000 km s −1. The data confirm the presence of a high-velocity gas component in 3C 265 and detached emission-line systems in 3C 356 and 3C 441, and show for the first time that the emission-line gas in the central regions of 3C 324 is composed of two kinematically distinct components. Emission-line fluxes and the colour of the continuum emission are determined down to unprecedentedly low observed wavelengths, λ <3500 Å, sufficiently short that any contribution of an evolved stellar population is negligible. An accompanying paper investigates the variation in the emission-line ratios and velocity structures within the sample, and draws conclusions as to the origin of the ionization and kinematics of these galaxies.  相似文献   

18.
We examine the K shell emission lines produced by isothermal and simple multiphase models of the hot gas in elliptical galaxies and galaxy clusters to determine the most effective means for constraining the width of the differential emission measure, ( T  ), in these systems which we characterize by a dimensionless parameter, . Comparison of line ratios of two-temperature  ( <1)  and cooling flow  ( 1)  models is presented in detail. We find that a two-temperature model can approximate very accurately a cooling flow spectrum over 0.510 keV.
We re-analyse the ASCA spectra of three of the brightest galaxy clusters to assess the evidence for multiphase gas in their cores: M87 (Virgo), the Centaurus cluster and the Perseus cluster. K emission-line blends of Si, S, Ar, Ca and Fe are detected in each system, as is significant Fe K emission. The Fe K /K ratios are consistent with optically thin plasma models and do not suggest resonance scattering in these systems. Consideration of both the ratios of H-like to He-like K lines and the local continuum temperatures clearly rules out isothermal gas in each case. To obtain more detailed constraints, we fitted plasma models over 1.69 keV where the emission is dominated by these K shell lines and by continuum. In each case the ASCA spectra cannot determine whether the gas emits at only two temperatures or over a continuous range of temperatures as expected for a cooling flow. The metal abundances are near-solar for all of the multiphase models. We discuss the implications of these results and examine the prospects for determining the temperature structure in these systems with upcoming X-ray missions.  相似文献   

19.
We present integral field spectroscopy of the nebular line emission in a sample of nine brightest cluster galaxies (BCGs). The sample was chosen to probe both cooling flow and non-cooling flow clusters, as well as a range of cluster X-ray luminosities. The line emission morphology and velocity gradients suggest a great diversity in the properties of the line emitting gas. While some BCGs show evidence for filamentary or patchy emission (Abell 1060, Abell 1668 and MKW 3s), others have extended emission (Abell 1204, Abell 2199), while still others have centrally concentrated emission (Abell 2052). We examine diagnostic line ratios to determine the dominant ionization mechanisms in each galaxy. Most of the galaxies show regions with active galactic nucleus like spectra, however, for two BCGs, Abell 1060 and Abell 1204, the emission line diagnostics suggest regions which can be described by the emission from young stellar populations. The diversity of emission-line properties in our sample of BCGs suggests that the emission mechanism is not universal, with different ionization processes dominating different systems. Given this diversity, there is no evidence for a clear distinction of the emission-line properties between cooling flow and non-cooling flow BCGs. It is not always cooling flow BCGs which show emission (or young stellar populations), and non-cooling flow BCGs which do not.  相似文献   

20.
Using multicomponent photoionization simulation, we investigated the impact of bubble-like structures around starbursts inside the low-metallicity H II regions on the ionization spectrum shape and emission line forming. Radial distribution of density values and other physical parameters of bubble-like structures were taken from Weaver et al. (Weaver et al., 1977, p. 377). The first and second inner components of such models describe the free expansion zone of superwind from the central starburst region and rarefied hot gas of the cavern thermalized by inverse shock wave, respectively. The gas density and electron temperature distributions into these components are obtained from the solution of the system of equations of continuity and energy transfer, including heat conductivity. The third component is a thin shell of high density gas formed from the gas surrounding a bubble and made by direct shock wind wave. The gas density in this component was obtained from isobaric condition at contact discontinuity between the second and third components. Input spectra of the ionizing radiation were obtained from the starburst evolutional models. The evolution grid of the multicomponent low-metallicity photoionization models with free parameters determining physical conditions inside the bubble-like structure was calculated. The impact of bubble-like structure on the change of ionization spectrum shape and the formation of fluxes of important emission lines in low-metallicity case was analyzed in detail.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号