Low-redshift Active Galactic Nuclei Research Articles

Statistics of the Big Blue Bump Feature in a Low Redshift Sample of Active Galactic Nuclei

We present a statistical analysis of the big blue bump (BBB) feature for a large heterogeneous sample of 95 optically selected and soft X-ray bright, low redshift active galactic nuclei (AGNs). This sample covers a sufficiently broad luminosity range, allowing us to test the luminosity dependence of the spectral energy distribution in the BBB region. Following the works of Zheng et al., Laor et al. and Kriss et al., we introduce the broad band spectral index from 1050 A to0.5 keV (α UV-SX ), compare its distribution with that of the soft X-ray spectral index (α SX ) obtained by ROSAT PSPC, and find that the two indices have equal average-values within 1 ∼ 2σuncertainties, whether in the whole sample, in luminosity divisions or in subsamples. These equalities also have no obvious luminosity dependence. This indicates that a single power law can describe the overall UV toX-ray spectrum in a statistical sense, or the broad band UV to soft X-ray spectrum is the soft X-ray spectral extension on an average. Thus, our results support Laor et al.'s conjecture about the BBB peak aroundFUV 1050 A from a statistical viewpoint. As we further test whether the equality holds for individual objects within measure errors, χ2 test refuse to accept it. In addition, our statistical results, from the luminosity divisions and on the correlation of spectral indices with luminosity (M B), imply that the luminosity dependence of α UV and α UV-SX is mainly due to absorption in low luminosity AGNs.

The space density of low-redshift active galactic nuclei

The space density of low-redshift active galactic nuclei

The space density of low-redshift active galactic nuclei

We present a new determination of the optical luminosity function (OLF) of active galactic nuclei (AGN) at low redshifts (z<0.15) based on Hubble Space Telescope (HST) observations of X-ray-selected AGN. The HST observations have allowed us to derive a true nuclear luminosity function for these AGN. The resulting OLF illustrates a two-power-law form similar to that derived for QSOs at higher redshifts. At bright magnitudes, M_B<-20, the OLF derived here exhibits good agreement with that derived from the Hamburg/ESO QSO survey. However, the single power law form for the OLF derived from the Hamburg/ESO survey is strongly ruled out by our data at M_B>-20. Although the estimate of the OLF is best-fit by a power law slope at M_B<-20.5 that is flatter than the slope of the OLF derived at z>0.35, the binned estimate of the low redshift OLF is still consistent with an extrapolation of the $z>0.35$ OLF based on pure luminosity evolution.

Ionized ultraviolet and soft X-ray absorption in the low-redshift active galactic nucleus PG 1126−041

We present here the analysis of ultraviolet spectra from IUE and an X-ray spectrum from ROSAT PSPC observations of the X-ray weak, far-infrared loud AGN, PG 1126-041 (Mrk 1298). The first UV spectra taken in June 1992, simultaneously with ROSAT, show strong absorption lines of NV, CIV and SiIV, extending over a velocity range from -1000 to -5000 km/s with respect to the corresponding line centre. Our analysis shows that the Broad Emission Line Region (BELR) is, at least partially, covered by the material causing these absorption lines. In the IUE spectrum taken in Jan. 1995, the continuum was a factor of two brighter and the UV absorption lines are found to be considerably weaker than in 1992, but only little variation in the emission line fluxes is found. With UV spectral indices of A_{uv} \simeq 1.82 and 1.46 for the 1992 and 1995 data, the far UV spectrum is steep. Based on the emission line ratios and the broad band spectral energy distribution, we argue that the steepness of the UV spectrum is unlikely to be due to reddening. The soft X-ray emission in the ROSAT band is weak. A simple power-law model yields a very poor fit with a UV-to-X-ray spectral index A_{uvx}=2.32. Highly ionized (warm) absorption is suggested by the ROSAT data. After correcting for a warm absorber, the optical to X-ray spectral slope is close to the average of A{uvx}\simeq 1.67 for radio quiet quasars.

Absorption in X-ray spectra of high-redshift quasars

We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z approximately 3) quasars with the ROSAT Position Sensitive Proportional Counter (PSPC) of which four are in directions of low Galactic N(sub H). Three out of these four show excess absorption, while only three in approximately 50 z approximately less than 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z greater than 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of N(sub H) = (0.86(+0.49, -0.28)) x 10(exp 22) atoms/sq cm for PKS 0438-436, and N(sub H) = (1.45(+1.20, -0.64)) x 10(exp 22) atoms/ sq cm for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyman(alpha) systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyman(alpha) systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) approximately greater than 4 x 10(exp -3) (3 (sigma)), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band approximately 0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than Delta alpha(sub E) greater than 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.

The ultraviolet emission properties of five low-redshift active galactic nuclei at high signal-to-noise ratio and spectral resolution

We analyze the ultraviolet (UV) emission line and continuum properties of five low-redshift active galactic nuclei (four luminous quasars: PKS~0405$-$123, H1821+643, PG~0953+414, and 3C273, and one bright Seyfert 1 galaxy: Mrk~205). The HST spectra have higher signal-to-noise ratios (typically $\sim 60$ per resolution element) and spectral resolution ($R = 1300$) than all previously- published UV spectra used to study the emission characteristics of active galactic nuclei. We include in the analysis ground-based optical spectra covering \hb\ and the narrow [O III] $\lambda\lambda$4959,5007 doublet. The following new results are obtained: \lyb/\lya=0.03$-$0.12 for the four quasars, which is the first accurate measurement of the long-predicted \lyb\ intensity in QSOs. The cores of \lya\ and C~IV are symmetric to an accuracy of better than 2.5% within about 2000 km s$^{-1}$ of the line peak. This high degree of symmetry of \lya\ argues against models in which the broad line cloud velocity field has a significant radial component. The observed smoothness of the \lya\ and C~IV line profiles requires at least $\sim 10^4$ individual clouds if bulk velocity is the only line-broadening mechanism. The overall similarity of the \lya\ and C IV $\lambda$1549 profiles rules out models for the broad line region (BLR) with a radial distribution of virialized....

A comparison of C IV and H-beta broad emission-line profiles in low-redshift active galactic nuclei

view Abstract Citations (29) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of C IV and H beta Broad Emission-Line Profiles in Low-Redshift Active Galactic Nuclei Corbin, Michael R. Abstract Recently published IUE spectra of low-redshift QSOs and Seyfert 1 galaxies have permitted a comparison of the profiles of the C IV λ1549 and Hβ broad emission lines. A linear correlation between the FWHM values of the profiles has been found in a sample of 19 objects and is verified to not be the result of separate correlations between these parameters and luminosity. Interpreted in terms of the geometry of the broad-line region, this result is contrary to models which predict low-ionization line emission from an optically thick, geometrically thin accretion disk surrounding the central object. Current evidence favors a model in which broad-line emission arises predominantly in a stratified ensemble of clouds moving radially relative to the central continuum source. Publication: The Astrophysical Journal Pub Date: April 1991 DOI: 10.1086/186000 Bibcode: 1991ApJ...371L..51C Keywords: Active Galactic Nuclei; Emission Spectra; H Beta Line; Hydrogen Clouds; Luminous Intensity; Red Shift; Quasars; Seyfert Galaxies; Spectral Line Width; Astrophysics; GALAXIES: NUCLEI; LINE PROFILES; QUASARS full text sources ADS | data products SIMBAD (23) NED (19) MAST (1)

The All-Sky Extragalactic X-Ray Foreground

Observations of galaxies in the IR and optical (Lynden-Bell et al. 1989) suggest that the 600 km/s peculiar velocity of the LG (Local Group of galaxies) arises mainly from a foreground of anisotropically distributed mass within z = 0.013 (i.e., HR &lt; 4000 km/s). Since the X-ray luminosity of bright extragalactic X-ray sources provides a good mass measure of the radiating objects involved and can be observed relatively free of galactic obscuration effects, such sources are likely candidates for serving as reliable tracers of the total underlying mass (i.e., dark as well as visible) responsible for the acceleration of the LG. In this connection, we note that the local gravitational dipole implied by the fifty X-ray brightest clusters of galaxies at z &gt; 0.013 considered by Lahav et al. (1989) is relatively small compared with that inferred from the only three clusters at lower redshifts. Since the local space density of AGN (Active Galactic Nuclei) is about two orders of magnitude greater than rich clusters, however, such compact sources have the potential of providing a vastly improved statistical sample for tracing mass in the low-redshift region of particular interest. Furthermore, recent dipole analysis of the X-ray flux from bright AGN observed with HEAO-1 A2 indicates that they are indeed strong tracers of this matter (Miyaji and Boldt 1990). The implications of this for the very pronounced large-scale foreground anisotropies to be measured via low-redshift AGN resolved in more sensitive all-sky surveys are explored.

Characterization of X-ray sources detected in a deep Einstein exposure of the field around 3C 295

An extensive set of optical and radio observations is presented that characterize the X-ray sources detected in an extremely deep Einstein Observatory exposure of the 3C 295 region. A total of five X-ray sources was found. This number is consistent with the only previous log N - log S result published at these flux levels. One of the sources was a galactic star. The four extragalactic sources are active galactic nuclei (three Seyferts and one QSO). Two of the sources are Seyfert galaxies in the 3C 295 cluster, one of which was previously unknown. The number of Seyfert galaxies in the cluster that is found from this complete X-ray survey is inconsistent with that expected from the fraction of low-redshift active galactic nuclei in their clusters.

A spectrophotometric classification of low-redshift quasars and active galactic nuclei

Current classifications for quasars and active galactic nuclei are based on apparent properties. We propose a spectrophotometric classification which is based on intrinsic properties by separating objects into three groups: Class A-objects with Fe II emission lines; Class B-objects without Fe II and with permitted lines broader than forbidden ones; and Class C-objects without Fe II and with permitted lines and forbidden lines having the same width.