Anca Constantin

Continuum and Emission-Line Strength Relations for a Large Active Galactic Nuclei Sample

We report on the analysis of a large sample of 744 type 1 active galactic nuclei, including quasars and Seyfert 1 galaxies across the redshift from 0 z 5 and spanning nearly 6 orders of magnitude in continuum luminosity. We discuss correlations of continuum and emission-line properties in the rest-frame ultraviolet and optical spectral ranges. The well-established Baldwin effect is detected for almost all emission lines from O VI ?1034 to [O III] ?5007. Their equivalent widths are significantly anticorrelated with the continuum strength, while they are nearly independent of redshift. This is the well-known Baldwin effect. Its slope ?, measured as log W? ? log ?L?(1450 ?), shows a tendency to become steeper toward higher luminosity. The slope of the Baldwin effect also increases with the ionization energy needed to create the individual lines. In contrast to this general trend, the N V ?1240 equivalent width is nearly independent of continuum luminosity and remains nearly constant. The overall line behaviors are consistent with softer UV continuum shapes and perhaps increasing gas metallicity in more luminous active galactic nuclei.

Quasar Elemental Abundances at High Redshifts

We examine rest-frame ultraviolet spectra of 70 high-redshift quasars (z ≥ 3.5) to study the chemical enrichment history of the gas closely related to the quasars and thereby estimate the epoch of first star formation. The fluxes of several ultraviolet emission lines were investigated within the framework of the most recent photoionization models to estimate the metallicity of the gas associated with the high-redshift quasars. Standard photoionization parameters and the assumption of secondary nitrogen enrichment indicate an average abundance of Z/Z☉ 4-5 in the line-emitting gas. Assuming a timescale of τevol 0.5-0.8 Gyr for the chemical enrichment of the gas, the first major star formation for quasars with z 4 should have started at a redshift of zf 6-8, corresponding to an age of the universe of several times 108 yr (H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7). We note that this also appears to be the era of reionization of the universe. Finally, there is some evidence for a positive luminosity-metallicity relation in this high-redshift quasar sample.

The clustering of low-luminosity active galactic nuclei

We present the first multi-parameter analysis of the spatial clustering properties of narrow line emitting galaxy nuclei. We spectroscopically identify and classify active galactic nuclei (AGN) from the Sloan Digital Sky Survey (SDSS) DR2 main galaxy sample, which yields the most precise measurement to date of AGN clustering. Estimates of the two-point correlation function (CF) reveal that Seyferts are clearly less clustered than normal galaxies, while the clustering amplitude of LINERs is consistent with that of the parent galaxy population. The similarities of the distributions of host properties (color and concentration index) of Seyferts and LINERs suggest that the difference in their clustering amplitudes is not driven by the morphology-density relation. We find that the luminosity of [O I] emission shows the strongest influence on AGN clustering, with low L[OI] sources having the highest clustering amplitude, r0. This trend is much stronger than the previously detected dependence on L[OIII], which we confirm. There is a strong correspondence between the clustering patterns of objects of given spectral type and physical properties including L[OI] , L[OIII], the electron density of the emitting gas ne, and the obscuration level. LINERs, which exhibit high r0, show the lowest luminosities and obscuration levels, and relatively low ne, suggesting that these objects harbor black holes that are relatively massive yet weakly active or inefficient in their accretion, probably due to the insufficiency of their fuel supply. Seyferts, which have low r0, are very luminous in both [O I] and [O III] and show large ne, suggesting that in these systems the black holes are less massive but accrete quickly and efficiently enough to clearly dominate the ionization. Star-forming galaxies – the H ii’s – are weakly clustered; because these nuclei are hosted mainly by blue, late type hosts, this trend can be understood as a consequence of both the morphology-density and star formation rate-density relations. However, the spectral properties of the H ii’s suggest that these systems hide in their centers, amidst large amounts of obscuring material, black holes of generally low mass whose activity remains relatively feeble. Our own Milky Way may be a typical such case. Subject headings: galaxies:active—galaxies:emission lines—galaxies:clustering—galaxies:statisticsBased on the Sloan Digital Sky Survey (SDSS) DR2 sample, we present a multiparameter analysis of the spatial clustering of nearby active galactic nuclei (AGNs). Estimates of the redshift-space two-point correlation function reveal that Seyferts are less clustered than normal galaxies, while LINERs clustering amplitude (s0) is consistent with that of the parent galaxy population. This difference in clustering is not driven by the morphology-density relation, as colors and concentration indices follow similar distributions. The fact that objects of given spectral types are clustered differently seems correlated with a variety of their physical properties, including L, L, the emitting gas density ne, and the obscuration level. LINERs, which exhibit high s0, show the lowest luminosities and obscuration levels, and relatively low ne, suggesting that these objects harbor relatively massive black holes that are weakly active or inefficient in their accretion, probably due to the insufficiency of their fuel supply. Seyfert galaxies, which have low s0, are very luminous and show large ne, suggesting that their black holes are less massive but accrete quickly and efficiently enough to clearly dominate the ionization. Star-forming galaxies, the H IIs, are weakly clustered; this trend can be understood as a consequence of both the morphology-density and star formation rate-density relations. The spectral properties of the H II galaxies suggest, however, that they hide in their centers, amid large amounts of obscuring material, black holes of generally low mass whose activity remains relatively feeble. Our own Milky Way may be such a case.

Active Galactic Nuclei in Void Regions

We present a comprehensive study of accretion activity in the most underdense environments in the universe, the voids, based on the SDSS DR2 data. Based on investigations of multiple void regions, we show that active galactic nuclei (AGNs) are definitely common in voids, but that their occurrence rate and properties differ from those in walls. AGNs are more common in voids than in walls, but only among moderately luminous and massive galaxies (Mr < − 20, log M*/M☉ < 10.5), and this enhancement is more pronounced for the relatively weak accreting systems (i.e., L[O III] < 1039 erg s−1). Void AGNs hosted by moderately massive and luminous galaxies are accreting at equal or lower rates than their wall counterparts, show lower levels of obscuration than in walls, and have similarly aged stellar populations. The very few void AGNs in massive bright hosts accrete more strongly, are more obscured, and are associated with younger stellar emission than wall AGNs. These trends suggest that the accretion strength is connected to the availability of fuel supply, and that accretion and star formation coevolve and rely on the same source of fuel. Nearest neighbor statistics indicate that the weak accretion activity (LINER-like) usually detected in massive systems is not influenced by the local environment. However, H II galaxies, Seyferts, and transition objects are preferentially found among more grouped small-scale structures, indicating that their activity is influenced by the rate at which galaxies interact with each other. These trends support a potential H II→ Seyfert/transition object→ LINER evolutionary sequence that we show is apparent in many properties of actively line-emitting galaxies, in both voids and walls. The subtle differences between void and wall AGNs might be explained by a longer, less disturbed duty cycle of these systems in voids.

Emission-Line Properties of z > 4 Quasars* **

High-redshift QSOs provide important opportunities for advancing our understanding of the astrophysics of galaxy formation and evolution. Although a growing number of these sources are now known to exist at redshifts beyond 4, systematic studies of the emission-line properties of these objects are quite limited. We present results of a program of high signal-to-noise spectroscopy for 44 QSOs using the Multiple Mirror Telescope Observatory and the W. M. Keck Observatory. The quasar spectra cover 1100-1700 A in the rest frame for sources spanning a luminosity range of ~2 orders of magnitude. Comparisons between these data and spectra of lower redshift quasars reveal a high degree of similarity, although differences are present in the profiles and the strengths of some emission features. An examination of the luminosity dependence of the emission lines reveals evidence for a weak or absent Baldwin effect among z 4 QSOs. We compare measurements for objects in our sample with results from other high-redshift surveys characterized by different selection techniques. Distributions of equivalent widths for these different ensembles are consistent with a common parent population, suggesting that our sample is not strongly biased, or in any case, subject to selection effects that are not significantly different from other surveys, including the Sloan Digital Sky Survey. Based on this comparison, we tentatively conclude that the trends identified here are representative of high z QSOs. In particular, the data bolster indications of supersolar metallicities in these luminous, high-z sources, which support scenarios that assume substantial star formation at epochs preceding or concurrent with the QSO phenomena.

Ultraviolet and Optical Properties of Narrow‐Line Seyfert 1 Galaxies*

Narrow-line Seyfert 1 galaxies (NLS1s) are remarkable for their extreme continuum and emission-line properties, which are not well understood. New results bearing on the spectroscopic characteristics of these objects are presented here, with the aim of establishing their typical UV and optical spectral behavior. We employ Hubble Space Telescope observations of 22 NLS1s, which represent a substantial improvement over previous work in terms of data quality and sample size. High signal-to-noise ratio NLS1 composite spectra are constructed, allowing accurate measurements of the continuum shape and the strengths, ratios, and widths for lines, including weak features that are barely identifiable in other active galactic nuclei composites. We find that the NLS1 sources have redder UV-blue continua than those typically measured in other quasars and Seyfert galaxies. Objects with UV line absorption show redder spectra, suggesting that dust is important in modifying the continuum shapes. The data also permit a detailed investigation of the previously proposed link between NLS1s and z 4 quasars. Direct comparison of their composite spectra, as well as a principal component analysis, suggests that high-z quasi-stellar objects do not show a strong preference toward NLS1 behavior.

The Metallicity of the Redshift 4.16 Quasar BR 2248–1242*

We estimate the metallicity in the broad emission line region of the redshift z 4.16 quasar BR 2248[1242 by comparing line ratios involving nitrogen to theoretical predictions. BR 2248[1242 has unusually narrow emission lines with large equivalent widths, thus providing a rare opportunity to measure several line-ratio abundance diagnostics. The combined diagnostics indicate a metallicity of D2 times solar. This result suggests that an episode of vigorous star formation occurred near BR 2248[1242 prior to the observed z 4.16 epoch. The time available for this enrichment episode is only D1.5 Gyr at z 4.16 (for km s~1 Mpc~1, and This evidence for high metalH 0 65 ) m 0.3, ) [ 1). licities and rapid star formation is consistent with the expected early-epoch evolution of dense galactic nuclei.

SOURCE MATCHING IN THE SDSS AND RASS: WHICH GALAXIES ARE REALLY X-RAY SOURCES?

The current view of galaxy formation holds that all massive galaxies harbor a massive black hole at their center, but that these black holes are not always in an actively accreting phase. X-ray emission is often used to identify accreting sources, but for galaxies that are not harboring quasars (low-luminosity active galaxies), the X-ray flux may be weak, or obscured by dust. To aid in the understanding of weakly accreting black holes in the local universe, a large sample of galaxies with X-ray detections is needed. We cross-match the ROSAT All Sky Survey (RASS) with galaxies from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Because of the high SDSS source density and large RASS positional errors, the cross-matched catalog is highly contaminated by random associations. We investigate the overlap of these surveys and provide a statistical test of the validity of RASS-SDSS galaxy cross-matches. The SDSS quasars provide a test of our cross-match validation scheme, as they have a very high fraction of true RASS matches. We find that the number of true matches between the SDSS main galaxy sample and the RASS is highly dependent on the optical spectral classification of the galaxy; essentially no star-forming galaxies are detected, while more than 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies with ambiguous optical classification have a surprisingly high RASS detection fraction. This allows us to further constrain the SEDs of low-luminosity active galaxies. Our technique is quite general, and can be applied to any cross-matching between surveys with well-understood positional errors.

Linking the Power Sources of Emission‐Line Galaxy Nuclei from the Highest to the Lowest Redshifts

This dissertation searches for common grounds for the diversity of properties exhibited by the emission-line nuclei of galaxies, from large look-back times to the local universe. I present results of (1) a program of high signal-to-noise spectroscopy for 44 quasars using the MMT and Keck obz 4 servatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 narrow-line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and quasars by means of comparison of composite z 4 spectra and a principal component analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures. The lowand high-redshift quasars are found to be very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, sources, which supports z 4 scenarios that assume substantial star formation concurrent or preceding the quasar phenomena. An in-depth analysis of quasar selection effects is performed, based on comparisons of distributions of equivalent widths for quasar ensembles characterized by different detection techniques. The measurements are consistent with a common parent population, suggesting that the trends identified here are representative of high-z QSOs. The quasars are found to be more metal enriched and z 4 more spectroscopically heterogeneous than the NLS1s, indicating that a close connection between these objects remains doubtful. NLS1s reveal redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, suggesting that luminosity-dependent dust absorption may be important in modifying their continua. A recedingtorus–like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. This relatively simple framework also predicts a number of absorbed AGNs that is consistent with recent modeling and observations of the X-ray background light and extragalactic point source population. Finally, in most of the nearby emission-line nuclei, the expected increased AGN-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object and that the composite model proposed for the LINER/H ii transition nuclei (which assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. Only about 10% of the narrow-line objects reveal broad Ha features, thus changing their spectral classification from type 2 to type 1.