Emission-lines of quasars and similar objects

Astronom

The Iron Unresolved Transition Array in Active Galactic Nuclei

The unresolved transition array (UTA) of iron M-shell ions is a prominent absorption feature in the X-ray spectrum of many active galactic nuclei (AGNs). Modeling photoionized plasmas in attempt to match the observed silicon and oxygen lines fail to predict the level of ionization of iron as inferred by this feature. It is suggested that the discrepancy is due to underestimation of the low-temperature dielectronic recombination rates for iron M-shell ions. Modified ionization balance calculations, based on new (guessed) atomic data, support this idea. The results are shown and compared to the global properties of several observed UTAs. Implications for AGN absorbing gas are discussed including an analysis of the ionization parameter distribution in such sources. The need for real calculations of such atomic data is stressed.Comment: 5 pages, 4 figures, accepted by Ap

Complex Optical-X-ray Correlations in the Narrow-Line Seyfert 1 Galaxy NGC 4051

This paper presents the results of a dense and intensive X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051 carried out in 2000. Results of the optical analysis are consistent with previous measurements. The amplitude of optical emission line variability is a factor of two larger than that of the underlying optical continuum, but part or all of the difference can be due to host-galaxy starlight contamination or due to the lines being driven by the unseen UV continuum, which is more variable than the optical continuum. We measured the lag between optical lines and continuum and found a lower, more accurate broad line region size of 3.0+-1.5 light days in this object. The implied black hole mass is M_BH=5(+6,-3)x10^5 M_sun; this is the lowest mass found, so far, for an active nucleus. We find significant evidence for an X-ray-optical (XO) correlation with a peak lag of about <1 day, although the centroid of the asymmetric correlation function reveals that part of the optical flux varies in advance of the X-ray flux by 2.4+-1.0 days. This complex XO correlation is explained as a possible combination of X-ray reprocessing and perturbations propagating from the outer (optically emitting) parts of the accretion disc into its inner (X-ray emitting) region.Comment: 8 pages, 4 figures included, LaTeX mn.sty, accepted for publication in MNRA

Estimating Black Hole Masses in Active Galaxies Using the Halpha Emission Line

It has been established that virial masses for black holes in low-redshift active galaxies can be estimated from measurements of the optical continuum strength and the width of the broad Hbeta line. Under various circumstances, however, both of these quantities can be challenging to measure or can be subject to large systematic uncertainties. To mitigate these difficulties, we present a new method for estimating black hole masses. From analysis of a new sample of broad-line active galactic nuclei, we find that Halpha luminosity scales almost linearly with optical continuum luminosity and that a strong correlation exists between Halpha and Hbeta line widths. These two empirical correlations allow us to translate the standard virial mass system to a new one based solely on observations of the broad Halpha emission line.Comment: to appear in Apj; 8 pages; 5 figures; uses emulateapj5.st

Near Infrared Spectroscopy of High Redshift Active Galactic Nuclei. II. Disappearing Narrow Line Regions and the Role of Accretion

We present new near infrared spectroscopic measurements for 29 luminous high-z quasars and use the data to discuss the size and other properties of the NLRs in those sources. The high resolution spectra have been used to carefully model the Fe II blends and to provide reliable [O III], Fe II and Hb measurements. We find that about 2/3 of all high luminosity sources show strong [O III] lines while the remaining objects show no or very weak such line. While weak [O III] emitters are also found among lower luminosity AGN, we argue that the implications for very high luminosity objects are different. In particular, we suggest that the averaging of these two populations in other works gave rise to claims of a Baldwin relationship in [O III] which is not confirmed by our data. We also argue that earlier proposed relations of the type R_NLR \propto L_[O III]^{1/2}, where R_NLR is the NLR radius, are theoretically sound yet they must break down for R_NLR exceeding a few kpc. This suggests that the NLR properties in luminous sources are different from those observed in nearby AGN. In particular, we suggest that some sources lost their very large, dynamically unbound NLR while others are in a phase of violent star-forming events that produce a large quantity of high density gas in the central kpc. This gas is ionized and excited by the central radiation source and its spectroscopic properties may be different from those observed in nearby, lower luminosity NLRs. We also discuss the dependence of EW(Hb) and Fe II/Hb on L, M_BH, and accretion rate for a large sample of AGNs. The strongest dependence of the two quantities is on the accretion rate and the Fe II/Hb correlation is probably due to the EW(Hb) dependence on accretion rate. We show the most extreme values measured so far of Fe II/Hb and address its correlation with EW([O III]).Comment: 10 pages (emulateapj), 9 figures. Accepted by Ap

Analyzing X-ray variability by State Space Models

In recent years, autoregressive models have had a profound impact on the description of astronomical time series as the observation of a stochastic process. These methods have advantages compared with common Fourier techniques concerning their inherent stationarity and physical background. If autoregressive models are used, however, it has to be taken into account that real data always contain observational noise often obscuring the intrinsic time series of the object. We apply the technique of a Linear State Space Model which explicitly models the noise of astronomical data and allows to estimate the hidden autoregressive process. As an example, we have analysed a sample of Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a relationship between the relaxation timescale and the spectral hardness.Comment: 4 pages, Latex, uses Kluwer Style file crckapb.cls To appear in Proc. of Astronomical Time Series, Tel Aviv, 199

Locating Star-Forming Regions in Quasar Host Galaxies

We present a study of the morphology and intensity of star formation in the host galaxies of eight Palomar-Green quasars using observations with the Hubble Space Telescope. Our observations are motivated by recent evidence for a close relationship between black hole growth and the stellar mass evolution in its host galaxy. We use narrow-band [O II] $\lambda$3727, H$\beta$, [O III] $\lambda$5007 and Pa$\alpha$ images, taken with the WFPC2 and NICMOS instruments, to map the morphology of line-emitting regions, and, after extinction corrections, diagnose the excitation mechanism and infer star-formation rates. Significant challenges in this type of work are the separation of the quasar light from the stellar continuum and the quasar-excited gas from the star-forming regions. To this end, we present a novel technique for image decomposition and subtraction of quasar light. Our primary result is the detection of extended line-emitting regions with sizes ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus, powered primarily by star formation. We determine star-formation rates of order a few tens of M$_\odot$/yr. The host galaxies of our target quasars have stellar masses of order $10^{11}$ M$_\odot$ and specific star formation rates on a par with those of M82 and luminous infrared galaxies. As such they fall at the upper envelope or just above the star-formation mass sequence in the specific star formation vs stellar mass diagram. We see a clear trend of increasing star formation rate with quasar luminosity, reinforcing the link between the growth of the stellar mass of the host and the black hole mass found by other authors.Comment: Accepted for publication in M.N.R.A.