The Narrow Fe Kα Emission Line Of AGN And Milky-Way-like Dust In Quasar Absorption Systems

Two investigations on Active Galactic Nucleus (AGN) are presented in this dis-sertation. AGN is one of the most luminous astronomical object in the Universe as far as we know. The energetic radiation raises from the gas accretion process surrounding the central super massive black hole. AGN provides us a unique tool to study the phys-ical phenomena in extreme gravitational field. In current theory of galaxy formation and evolution, active galaxy is an important phase. Thus, the investigations on AGN are very helpful for Cosmology. The strong emissions of AGN can travel extremely long distance in the Universe. The luminous high redshift AGNs are usually named Quasi-Stellar Objects (QSOs or quasars). QSO is the most abundant source among the astronomical objects that can be observed in the high redshift Universe. Between the high redshift QSOs and the Earth, there are a lot of galaxies, which are too dim to be directly detected. These galaxies impose some absorption features on the spectra of the background QSOs, if they are lying in the sight-line of QSOs. Thus, the spectroscopic data of high redshift QSOs enables us to detect and study some of the intervening dim galaxies. The studies on these dim galaxies are very useful to constrain the model of galaxy formation and evolution.The first major part of this dissertation is on the narrow Fe Kαemission line of AGN in X-ray band. In Chapter 2, we present the study of the Baldwin effect for Nar-row Fe Kαemission line of AGN. Most AGNs exhibit a narrow Fe Kαline at-6.4 keV in the X-ray spectra, due to the fluorescent emission from cold material far from the inner accretion disk. Using XMM-Newton observations, Page et al. (2004) found that the equivalent width (EW) of the narrow Fe Kαline decreases with increasing lumi-nosity (EWαL-0.17±0.08), suggesting a decrease in the covering factor of the material emitting the line (presumably the torus). By combining the archival Chandra HETG observations of 34 type 1 AGNs with XMM observations in the literature, we build a much larger sample with 101 AGNs. We find a similar X-ray Baldwin effect in the sam-ple (EW∞L-0.2015±0.0426); however, we note that the anticorrelation is dominated by the radio-loud AGNs in the sample, whose X-ray spectra might be contaminated by the relativistic jet. Excluding the radio-loud AGNs, we find a much weaker anticorrelation (EWαL-0.1019±0.0524). We present Monte Carlo simulations showing that such a weak anticorrelation can be attributed to the relative short timescale variations of the X-ray continuum. Another related project is that weighing the Super Massive Black Holes with narrow Fe Ka line, which is presented in Chapter 3. We compare the virial prod-ucts of the time lags between the V and K band flux variations and the narrow Fe Ka widths for 10 type 1 AGNs with the black hole masses from other techniques. We find the narrow Fe Ka line width is in average 2.6-0.4 +0.9 times broader than expected assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus could explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission from outer cooler part. We note the correlation between the black hole masses based on this new technique and those based on other known techniques is statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observations could help verify the origin of the narrow iron Kαline and the reliability of this new technique.The second major part of this dissertation is on dust extinction of quasar absorp-tion line systems (QALs). We are interested in searching for MilKy-Way-Like dust in QALs, which is very rarely seen in the current QALs sample. We developed an effi-cient method for detecting the Galactic 2175-A dust extinction feature with strong high redshift MgⅡabsorption lines on Sloan Digital Sky Survey (SDSS) quasar spectra. In Chapter 4, We report detections of 392175-A dust extinction bump candidates associ-ated with strong MgⅡabsorption lines at z-1-1.8 on quasar spectra in SDSS DR3. These strong Mg absorption line systems are detected among 2,951 strong Mg II ab-sorbers with the rest equivalent width Wrλ2796>1.0A at1.0<z< 1.86, which is part of a full sample of 7,421 strong MgⅡabsorbers compiled by Prochter et al. (2006). The redshift range of the absorbers is chosen to allow the 2175-A extinction features to be completely covered within the SDSS spectrograph operation wavelength range. An upper limit of the background quasar emission redshift at z=2.1 is set to prevent the Lyαforest lines from contaminating the sensitive spectral region for the 2175-A bump measurements. The FM90 (Fitzpatrick & Massa 1990) parameterization is ap-plied to model the Optical/UV extinction curve in the rest frame of MgⅡabsorbers of the 2175-A bump candidates. The simulation technique developed by us (Jiang et al. 2010a, b) is used to derive the statistical significance of the candidate 2175-A bump-s. A total of 12 absorbers are detected with 2175-A bumps at a 5σlevel of statistical significance,10 are detected at a 4σlevel and 17 are detected at a 3σlevel. Most of the candidate bumps in this work are similar to the relatively weak 2175-A bumps ob-served in the Large Magellanic Clouds (LMC) LMC2 supershell rather than the strong ones observed in the Milky Way (MW). This sample has greatly increased the total number of 2175-A extinction bumps measured on SDSS quasar spectra. Follow-up ob-servations may rule out some of possible false detections and reveal the physical and chemical natures of 2175-A quasar absorbers. In Chapter 5, we present the results of the follow-up spectroscopic measurements on two 2175-A quasar absorbers at z～1.4. We measure the column densities of heavy-elements and dust depletion of these two absorbers. Column densities are measured from low-ionization absorption lines using Apparent Optical Depth Method on the Keck/ESI spectra. We find the dust depletion patterns resemble to that of cold diffuse clouds in the Milky Way (MW). The values, [Fe/Zn]≈-1.5 and [Si/Zn]<-0.67, are among the highest dust depletion measured for quasar absorption line systems. In another 2175-A absorber at z=1.64 toward the quasar SDSS J160457.50+220300.5, Noterdaeme et al. (2009) reported a similar dust depletion measurement ([Fe/Zn]=-1.47 and [Si/Zn]=-1.07) and detected C I and CO absorption lines on its VLT/UVES spectrum. We conclude that heavy dust depletion (i.e. a characteristic of cold dense clouds in MW) is required to produce a pronounced 2175-A extinction bump. The detected 2175-A quasar absorbers are very likely to be Milky-Way-Like mature galaxies in high redshift Universe.