The Variability of Hardness Ratio 1 observed by ROSAT： Narrow—Line Seyfert 1 Galaxies versus Broad—Line Seyfert 1 Galaxies

We examined the correlation between the ROSAT Hardness Ratio 1 and Count Rates eight Narrow-line Seyfert 1 Galaxies (NLS1s) and 14 Broad-line Seyfert 1 Galaxies (BLS1s). We found that six of the NLS1s show a positive HR1-CTs correlation, and seven of the BLS1s, a negative correlation. The other two NLS1s and seven BLS1s do not show any clear HR1-CTs correlation. Thus, the spectral behavior is statistically different for the NLS1s and BLS1s. The different behaviors can possibly be interpreted in terms of a stable ‘soft excess’ that is strong in NLS1s and weak in BLS1s, plus a power law component, common to both, which softens with increasing flux.     

Discovery of Five Narrow-Line Seyfert 1 Galaxies and Im- plications on the NLS1 Model

We report the discovery of five Narrow-Line Seyfert 1 galaxies (NLSls) identified from the ROSAT All-Sky Survey bright sources. One of them has a quasar-like luminosity and two, including the quasar-like one, have close companions and/or show interacting features. We calculate the central black hole masses and Eddington ratios for the five NLSls. In combination with the objects of Kaspi et al., we find that NLSls have smaller central black hole masses and higher accretion rate than normal Seyfert 1s.     

Discovery of Five Narrow－Line Seyfert 1 Galaxies and Implications on the NLS1 Model

We report the discovery of five Narrow-Line Seyfert 1 galaxies (NLSls) identified from the ROSAT All-Sky Survey bright sources. One of them has a quasarlike luminosity and two, including the quasar-like one, have close companions and/or show interacting features. We calculate the central black hole masses and Eddington ratios for the five NLSls. In combination with the objects of Kaspi et al., we find that NLSls have smaller central black hole masses and higher accretion rate than normal Seyfert ls.     

Black hole masses from power density spectra: determinations and consequences

We analyse the scaling of the X-ray power density spectra with the mass of the black hole in the examples of Cyg X-1 and the Seyfert 1 galaxy NGC 5548. We show that the high-frequency tail of the power density spectrum can be successfully used for the determination of the black hole mass. We determine the masses of the black holes in six broad-line Seyfert 1 galaxies, five narrow-line Seyfert 1 galaxies and two quasi-stellar objects (QSOs) using the available power density spectra. The proposed scaling is clearly appropriate for other Seyfert galaxies and QSOs. In all but one of the normal Seyferts, the resulting luminosity to Eddington luminosity ratio is smaller than 0.15, with the source MCG -6-15-30 being an exception. The applicability of the same scaling to a narrow-line Seyfert 1 is less clear and there may be a systematic shift between the power spectra of NLS1 and S1 galaxies of the same mass, leading to underestimation of the black hole mass. However, both the method based on variability and the method based on spectral fitting show that those galaxies have relatively low masses and a high luminosity to Eddington luminosity ratio, supporting the view of those objects as analogues of galactic sources in their high, soft or very high state, based on the overall spectral shape. The bulge masses of their host galaxies are similar to that of normal Seyfert galaxies, so they do not follow the black hole mass–bulge mass relation for Seyfert galaxies, being evolutionarily less advanced, as suggested by Mathur. The bulge mass–black hole mass relation in our sample is consistent with being linear, with the black hole to bulge ratio ∼0.03 per cent, similar to Wandel and Laor for low-mass objects, but significantly shifted from the relation of Magorrian et al. and McLure & Dunlop.     

The fraction of galaxies that contain active nuclei and their accretion rates

We investigate the relationship between the present-day optical luminosity function of galaxies and the X-ray luminosity function of Seyfert 1s to determine the fraction of galaxies that host Seyfert 1 nuclei and their Eddington ratios. The local type 1 active galactic nuclei (AGN) X-ray luminosity function is well reproduced if ∼1 per cent of all galaxies are type 1 Seyferts which have Eddington ratios of ∼10⁻³. However, in such a model the X-ray luminosity function is completely dominated by AGN in E and S0 galaxies, contrary to the observed mix of Seyfert host galaxies. To obtain a plausible mix of AGN host galaxy morphologies requires that the most massive black holes in E and S0 galaxies accrete with lower Eddington ratios, or have a lower incidence of Seyfert activity, than the central black holes of later-type galaxies.     

NLS1 galaxies and estimation of their central black hole masses from the X-ray excess variance method

Black hole mass determination in active galaxies is a key issue in understanding various luminosity states. In the present paper, we try to generalize the mass determination method based on the X-ray excess variance, successfully used for typical broad line Seyfert 1 galaxies (BLS1) to narrow line Seyfert 1 (NLS1) galaxies. NLS1 galaxies differ from BLS1 with respect to several properties. They are generally more variable in 2–10 keV energy band so the natural expectation is the need to use a different scaling coefficient between the mass and the variance in these two types of sources. However, we find that such a simple approach is not enough. Although for majority of the 21 NLS1 galaxies in our sample a single scaling coefficient (larger by a factor 20) provided us with a satisfactory method of mass determination, in a small subset of NLS1 galaxies this approach failed. Variability of those objects appeared to be at the intermediate level between NLS1 and BLS1 galaxies. These exceptional NLS1 galaxies have much harder soft X-ray spectra than majority of NLS1 galaxies. We thus postulate that the division of Seyfert 1 galaxies into BLS1 and NLS1 according to the widths of the  Hβ  line is less generic than according to the soft X-ray slope.     

Evolutionary unification in composite active galactic nuclei

In this paper, we present an evolutionary unification scenario, involving supermassive black holes (SMBHs) and starbursts (SBs) with outflow (OF), that seems capable of explaining most of the observational properties (of at least part) of active galactic nuclei (AGN).
The scenario includes a nuclear/circumnuclear SB closely associated with the AGN where the narrow-line region (NLR), broad-line region (BLR) and broad absorption line (BAL) region are produced in part by the OF process with shells and in compact supernova remnants (cSNRs).
The OF process in BAL quasi-stellar objects (QSOs) with extreme infrared (IR) and Fe  ii emission is studied. In addition, the Fe  ii problem regarding the BLR of AGN is analysed. The correlations between the BAL, IR emission, Fe  ii intensity and the intrinsic properties of the AGN are not clearly understood. We suggest here that the behaviour of the BAL, IR and Fe  ii emission in AGN can be understood within an evolutionary and composite model for AGN.
In our model, strong BAL systems and Fe  ii emission are present (and intense) in young IR objects. Parameters like the BALs, IR emission, Fe  ii /Hβ intensity ratio, Fe  ii equivalent width (EW), broad-line width, [O  iii ]λ5007-Å intensity and width, NLR size, X-ray spectral slope in radio quiet (RQ) AGN plus lobe separation, and lobe to core intensity ratio in radio loud (RL) AGN are proposed to be fundamentally time-dependent variables inside time-scales of the order of 10⁸ yr. Orientation/obscuration effects take the role of a second parameter providing the segregation between Seyfert 1/Seyfert 2 galaxies (Sy1/Sy2) and broad-/narrow-line radio galaxies (BLRG/NLRG).     

Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273

We present spectrophotometric results of the Seyfert 2 galaxy NGC 2273. The presence of high-order Balmer absorption lines (H8, H9, H10) and weak equivalent widths of Call K A3933, CN A4200, G-band A4300 and MgIb 5173 clearly indicate recent star-forming activity in the nuclear region. Using a simple stellar population synthesis model, we find that for the best fit, the contributions of a power-law featureless continuum, an intermediate-age (~ 108 yr) and an old (> 109yr) stellar population to the total light at the reference normalization wavelength are 10.0%, 33.4% and 56.6%, respectively. The existence of recent starburst activity is also consistent with its high far-infrared luminosity (log LFIR/L = 9.9), its infrared color indexes [a(25,60) = -1.81 and a(60,100) = -0.79, typical values for Seyfert galaxies with circumnuclear starburst], and its q-value (2.23, ratio of infrared to radio flux, very similar to that of normal spirals and starburst galaxies). Byrd et al. have suggested that NGC 2273 mig     

The active nuclei of z < 1.0 3CRR radio sources

We combine Chandra and XMM–Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared (mid-IR) data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with   z < 1.0  from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations and the availability of more mid-IR data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type 2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered 'fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud active galactic nucleus (AGN) on such diagnostic diagrams.     

Multiwavelength study of the nuclei of a volume-limited sample of galaxies – I. X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 10³⁸ erg s⁻¹. The galaxies are selected from a complete, volume-limited sample of 46 galaxies with     for which we have extensive multiwavelength data. For an almost complete subsample with     (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ∼35 per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity – there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H  ii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population – out of 29 galaxies we find nine sources with a luminosity greater than 10³⁹ erg s⁻¹. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.     

Inclination of Broad Line Region in Narrow Line and Broad Line Seyfert 1 Galaxies

The sizes of the Broad Line Region (BLR) of some Seyfert 1 galax-ies and nearby quasars can be determined by the reverberation mapping method.Combining with the observed FWHM of Hβ emission line and assuming that themotion of BLR clouds is virialized, the black hole masses of these objects have beenestimated. However, this method strongly depends on the poorly-understood geom-etry and inclination of the BLR. On the other hand, a tight correlation between theblack hole mass and the bulge velocity dispersion was recently found for both activeand nearby inactive galaxies. This may provide another method, independent of theBLR geometry, for estimating the black hole mass. Using this method for estimatingthe black hole mass and combining with the measured BLR size and FWHM of Hβemission line, we derived the BLR inclination angles for 20 Seyfert I galaxies underthe assumption that the BLR is disk-like. The derived inclination angles agree wellwith those derived previously by fitting the UV continuum and Hβ emission lineprofiles. Adopting a relation between the FWHMs of [OⅢ]λ5007 forbidden line andthe stellar velocity dispersion, we also estimated the BLR inclinations for 50 nar-row line Seyfert 1 galaxies (NLSls). We found that the inclinations of broad LineSeyfert 1 galaxies (BLS1s) are systematically greater than those of NLS1s, whichseldom exceed 30. This may be an important factor that leads to the differencesbetween NLS1s and BLS1s if the BLR of NLS1s is really disk-like.     

Identification of newly discovered radio-emitting X-ray sources: results from spectroscopy

We present optical identifications for a sample of 20 previously unknown X-ray/radio sources that are present both in the source catalogue of ROSAT PSPC pointed observations ( ROSAT SRC) and in the NRAO VLA Sky Survey (NVSS). The optical spectroscopy was carried out with the 2.1-m telescope at San Pedro Martir (Mexico) during 1995 April and September. We have identified 15 active galactic nuclei [including 12 broad-emission-line (FWHM >1000 km s⁻¹) objects, one bona fide BL Lac, one BL Lac candidate and one narrow-line (FWHM < 1000 km s⁻¹) radio galaxy] and five radio galaxies. We derive the X-ray fluxes and luminosities by analysing the PSPC exposures, and show the radio morphology from the NVSS maps.  We find that the correlation between the monochromatic X-ray luminosity at 2 keV and the core radio luminosity at 5 GHz for the radio galaxies in our sample follows that found for the 3CR radio galaxies, suggesting a possible nuclear origin for the X-ray emission in these sources. This correlation is weaker in the case of broad-line objects, indicating the presence of another (unbeamed) mechanism for the X-ray emission only weakly related to the radio emission.     

Morphology and surface brightness evolution of z∼1.1 radio galaxies

We use K '-band (2.1-μm) imaging to investigate the angular size and morphology of 10 6C radio galaxies, at redshifts 1≤ z ≤1.4. Two radio galaxies appear to be undergoing mergers, another contains, within a single envelope, two intensity peaks aligned with the radio jets, while the other seven appear consistent with being normal ellipticals in the K band.
Intrinsic half-light radii are estimated from the areas of each radio galaxy image above a series of thresholds. The 6C galaxy radii are found to be significantly smaller than those of the more radio-luminous 3CR galaxies at similar redshifts. This would indicate that the higher mean K -band luminosity of the 3CR galaxies reflects a difference in the size of the host galaxies, and not solely a difference in the power of the active nuclei.
The size–luminosity relation of the z ∼1.1 6C galaxies indicates a 1.0–1.6 mag enhancement of their rest frame R -band surface brightness relative to either local ellipticals of the same size or FRII radio galaxies at z <0.2. The 3CR galaxies at z ∼1.1 show a comparable enhancement in surface brightness. The mean radius of the 6C galaxies suggests that they evolve into ellipticals of L ∼ L * luminosity, and is consistent with their low-redshift counterparts being relatively small FRII galaxies ∼25 times lower in radio luminosity, or small FRI galaxies ∼1000 times lower in radio luminosity. Hence the 6C radio galaxies appear to undergo as much optical and radio evolution as the 3CR galaxies.     

A survey of hard spectrum ROSAT sources – II. Optical identification of hard sources

We have surveyed 188 ROSAT Position Sensitive Proportional Counter (PSPC) fields for X-ray sources with hard spectra ( α <0.5); such sources must be major contributors to the X-ray background at faint fluxes. In this paper we present optical identifications for 62 of these sources: 28 active galactic nuclei (AGN) which show broad lines in their optical spectra (BLAGN), 13 narrow emission line galaxies (NELGs), five galaxies with no visible emission lines, eight clusters and eight Galactic stars.
The BLAGN, NELGs and galaxies have similar distributions of X-ray flux and spectra. Their ROSAT spectra are consistent with their being AGN obscured by columns of 20.5< log( N _H/cm⁻²)<23 . The hard spectrum BLAGN have a distribution of X-ray to optical ratios which is similar to that found for AGN from soft X-ray surveys (1< α _OX<2) . However, a relatively large proportion (15 per cent) of the BLAGN, NELGs and galaxies are radio loud. This could be because the radio jets in these objects produce intrinsically hard X-ray emission, or if their hardness is caused by absorption, it could be because radio-loud objects are more X-ray luminous than radio-quiet objects. The eight hard sources identified as clusters of galaxies are the brightest, and softest group of sources and hence clusters are unlikely to be an important component of the hard, faint population.
We propose that BLAGN are likely to constitute a significant fraction of the faint, hard, 0.5–2 keV population and could be important to reproducing the shape of the X-ray background, because they are the most numerous type of object in our sample (comprising almost half the identified sources), and because all our high redshift ( z >1) identified hard sources have broad lines.     

The disc-dominated host galaxy of FR-I radio source B2 0722+30

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H  i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O  iii ] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H  i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.     

Spectropolarimetry of a complete infrared-selected sample of Seyfert 2 galaxies

We report the results of a spectropolarimetric survey of a complete far-infrared-selected sample of Seyfert 2 galaxies. We have found polarized broad H α emission in one new source, NGC 5995. In the sample as a whole, there is a clear tendency for galaxies in which we have detected broad H α in polarized light to have warm mid–far-infrared colours     in agreement with our previous results. However, a comparison of the optical, radio and hard X-ray properties of these systems leads us to conclude that this is a secondary consequence of the true mechanism governing our ability to see scattered light from the broad-line region. We find a strong trend for galaxies showing such emission to lie above a critical value of the relative luminosity of the active core to the host galaxy (as measured from the [O  iii ] 5007-Å equivalent width) which varies as a function of the obscuring column density as measured from hard X-ray observations. The warmth of the infrared colours is then largely due to a combination of the luminosity of the active core, the obscuring column and the relative importance of the host galaxy in powering the far-infrared emission, and not solely orientation as we inferred in our previous paper. Our data may also provide an explanation as to why the most highly polarized galaxies, which appear to have tori that are largely edge-on, are also the most luminous and have the most easily detectable scattered broad H α .     

Extended gas in Seyfert 2 galaxies: implications for the nuclear source

We use long-slit spectroscopic optical data to derive the properties of the extended emitting gas and the nuclear luminosity of a sample of 18 Seyfert 2 galaxies. From the emission-line luminosities and ratios we derive the density, reddening and mass of the ionized gas as a function of distance up to 2–4 kpc from the nucleus. Taking into account the geometric dilution of the nuclear radiation, we derive the radial distribution of covering factors and the minimum rate of ionizing photons emitted by the nuclear source. This number is an order of magnitude larger than that obtained from the rate of ionizing photons 'intercepted' by the gas and measured from the Hα luminosity. A calibration is proposed to recover this number from the observed luminosity. The He  ii λ4686/Hβ line ratio was used to calculate the slope of the ionizing spectral energy distribution (SED), which in combination with the number of ionizing photons allows the calculation of the hard X-ray luminosities. These luminosities are consistent with those derived from X-ray spectra in the eight cases for which such data are available and recover the intrinsic X-ray emission in Compton-thick cases. Our method can thus provide reliable estimates of the X-ray fluxes in Seyfert 2 galaxies for the cases where it is not readily available. We also use the ionizing SED and luminosity to predict the infrared luminosity under the assumption that it is dominated by reprocessed radiation from a dusty torus, and find a good agreement with the observed IRAS luminosities.     

Narrow-line Seyfert 1 galaxies and the evolution of galaxies and active galaxies

Narrow Line Seyfert 1 galaxies (NLS1s) are intriguing owing to their continuum as well as emission-line properties. The observed peculiar properties of the NLS1s are believed to be as a result of an accretion rate close to the Eddington limit. As a consequence of this, for a given luminosity, NLS1s have smaller black hole (BH) masses compared with normal Seyfert galaxies. Here we argue that NLS1s might be Seyfert galaxies in their early stage of evolution and as such may be low-redshift, low-luminosity analogues of high-redshift quasars. We propose that NLS1s may reside in rejuvenated, gas-rich galaxies. We also argue in favour of collisional ionization for production of Fe  ii in active galactic nuclei.     

The Difference between the α-disks of Seyfert 1 Galaxiesand Quasars

In a previous paper, it was suggested that contamination of the nuclear luminosity by the host galaxy plays an important role in determining the parameters of the standard a disk of AGNs. Using the nuclear absolute B band magnitude instead of the total absolute B band magnitude, we have recalculated the central black hole masses, accretion rates and disk inclinations for 20 Seyfert 1 galaxies and 17 Palomar-Green (PG) quasars. It is found that a small value of a is needed for the Seyfert 1 galaxies than for the PG quasars. This difference in a possibly leads to the different properties of Seyfert 1 galaxies and quasars. Furthermore, we find most of the objects in this sample are not accreting at super-Eddington rates if we adopt the nuclear optical luminosity in our calculation.     

MERLIN radio observations of CfA Seyferts

We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is         , and approximately 25 per cent of the components have a spectral index flatter than     . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions.