Browsing by Subject "quasars: emission lines"
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Item A Candidate Dual Active Galactic Nucleus At Z=1.175(2012-01) Barrows, R. Scott; Stern, Daniel; Madsen, Kristin; Harrison, Fiona; Assef, Roberto J.; Comerford, Julia M.; Cushing, Michael C.; Fassnacht, Christopher D.; Gonzalez, Anthony H.; Griffith, Roger; Hickox, Ryan; Kirkpatrick, J. Davy; Lagattuta, David J.; Comerford, Julia M.The X-ray source CXOXBJ142607.6+353351 (CXOJ1426+35), which was identified in a 172 ks Chandra image in the Bootes field, shows double-peaked rest-frame optical/UV emission lines, separated by 0.''69 (5.5 kpc) in the spatial dimension and by 690 km s(-1) in the velocity dimension. The high excitation lines and emission line ratios indicate both systems are ionized by an active galactic nucleus (AGN) continuum, and the double-peaked profile resembles that of candidate dual AGNs. At a redshift of z = 1.175, this source is the highest redshift candidate dual AGN yet identified. However, many sources have similar emission line profiles for which other interpretations are favored. We have analyzed the substantial archival data available in this field as well as acquired near-infrared (NIR) adaptive optics (AO) imaging and NIR slit spectroscopy. The X-ray spectrum is hard, implying a column density of several 10(23) cm(-2). Though heavily obscured, the source is also one of the brightest in the field, with an absorption-corrected 2-10 keV luminosity of similar to 10(45) erg s(-1). Outflows driven by an accretion disk may produce the double-peaked lines if the central engine accretes near the Eddington limit. However, we may be seeing the narrow line regions of two AGNs following a galactic merger. While the AO image reveals only a single source, a second AGN would easily be obscured by the significant extinction inferred from the X-ray data. Understanding the physical processes producing the complex emission line profiles seen in CXOJ1426+35 and related sources is important for interpreting the growing population of dual AGN candidates.Item HST-COS Observations Of AGNs. II. Extended Survey Of Ultraviolet Composite Spectra From 159 Active Galactic Nuclei(2014-10) Stevans, Matthew L.; Shull, J. Michael; Danforth, Charles W.; Tilton, Evan M.; Stevans, Matthew L.The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emissionline spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGNs at redshifts 0.001 < Z(AGN) < 1.476 and construct a composite spectrum from 475 to 1875 A. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the HI Ly alpha forest, 7 Lyman-limit systems (N-HI, 10(17.2) Cm-2) and 214 partial Lyman-limit systems (14.5 < log N-HI < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F-V, proportional to v(proportional to v)(4, typically with -2 <= alpha(v), <= 0 and no discernible continuum edges at 912 angstrom (HI) or 504 angstrom (He I). The composite rest-frame continuum shows a gradual break at lambda(br) approximate to 1000 angstrom, with mean spectral index alpha(v) = -0.83 +/- 0.09 in the FUV (1200-2000 angstrom) steepening to alpha(v), = -1.41 +/- 0.15 in the EUV (500-1000 angstrom). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.Item Identification Of Outflows And Candidate Dual Active Galactic Nuclei In SDSS Quasars At Z=0.8-1.6(2013-06) Barrows, R. Scott; Lacy, Claud H. Sandberg; Kennefick, Julia; Comerford, Julia M.; Kennefick, Daniel; Berrier, Joel C.; Comerford, Julia M.We present a sample of 131 quasars from the Sloan Digital Sky Survey at redshifts 0.8 < z < 1.6 with double peaks in either of the high-ionization narrow emission lines [Ne V]lambda 3426 or [Ne III]lambda 3869. These sources were selected with the intention of identifying high-redshift analogs of the z < 0.8 active galactic nuclei (AGNs) with double-peaked [O III]lambda 5007 lines, which might represent AGN outflows or dual AGNs. Lines of high ionization potential are believed to originate in the inner, highly photoionized portion of the narrow line region, and we exploit this assumption to investigate the possible kinematic origins of the double-peaked lines. For comparison, we measure the [Ne V]lambda 3426 and [Ne III]lambda 3869 double peaks in low-redshift (z < 0.8) [O III]-selected sources. We find that [Ne V]lambda 3426 and [Ne III]lambda 3869 show a correlation between line splitting and line width similar to that of [O III]lambda 5007 in other studies, and the velocity splittings are correlated with the quasar Eddington ratio. These results suggest an outflow origin for at least a subset of the double peaks, allowing us to study the high-ionization gas kinematics around quasars. However, we find that a non-negligible fraction of our sample show no evidence for an ionization stratification. For these sources, the outflow scenario is less compelling, leaving the dual AGN scenario as a viable possibility. Finally, we find that our sample shows an anti-correlation between the velocity-offset ratio and luminosity ratio of the components, which is a potential dynamical argument for the presence of dual AGNs. Therefore, this study serves as a first attempt at extending the selection of candidate dual AGNs to higher redshifts.Item Line Shifts, Broad-Line Region Inflow, And The Feeding Of Active Galactic Nuclei(2013-05) Gaskell, C. Martin; Goosmann, Rene W.; Gaskell, C. MartinVelocity-resolved reverberation mapping suggests that the broad-line regions (BLRs) of active galactic nuclei (AGNs) can have significant net inflow. We use the STOKES radiative transfer code to show that electron and Rayleigh scattering off the BLR and torus naturally explains the blueshifted profiles of high-ionization lines and the ionization dependence of the blueshifts. This result is insensitive to the geometry of the scattering region. If correct, then this model resolves the long-standing conflict between the absence of outflow implied by velocity-resolved reverberation mapping and the need for outflow if the blueshifting is the result of obscuration. The accretion rate implied by the inflow is sufficient to power the AGN. We suggest that the BLR is part of the outer accretion disk and that similar magnetohydrodynamic processes are operating. In the scattering model, the blueshifting is proportional to the accretion rate so high-accretion-rate AGNs will show greater high-ionization line blueshifts, as is observed. Scattering can lead to systematically too high black hole mass estimates from the C IV line. We note many similarities between narrow-line region (NLR) and BLR blueshiftings, and suggest that NLR blueshiftings have a similar explanation. Our model explains the higher blueshifts of broad absorption line QSOs if they are more highly inclined. Rayleigh scattering from the BLR and torus could be more important in the UV than electron scattering for predominantly neutral material around AGNs. The importance of Rayleigh scattering versus electron scattering can be assessed by comparing line profiles at different wavelengths arising from the same emission-line region.Item The Mid-Infrared Narrow-Line Baldwin Effect Revealed By Spitzer(2009-01) Keremedjiev, Mark; Hao, Lei; Charmandaris, Vassilis; Hao, LeiWe present our discovery of a narrow-line Baldwin effect, an anticorrelation between the equivalent width (EW) of a line and the flux of the associated continuum, in 5-20 mu m mid-infrared (mid-IR) lines from a sample of 68 active galactic nuclei (AGNs), located at z < 0.5, observed with the Infrared Spectrograph on the Spitzer Space Telescope. Our analysis reveals a clear anticorrelation between the EW of the [S IV] 10.51 mu m, [Ne II] 12.81 mu m, and [Ne III] 15.56 mu m lines and their mid-IR continuum luminosities, while the Baldwin effect for [Ne v] 14.32 mu m is not as obvious. We suggest that this anticorrelation is driven by the central AGN, and not circumnuclear star formation in the host galaxy, and present a new method of analyzing this effect in mid-IR lines. We also find that the slope of the narrow-line Baldwin effect in the mid-IR does not appear to steepen with increasing ionization potential. Examining the dependence of the EW on the Eddington ratio (L/L(Edd)), we find no strong relationship for mid-IR lines. Our study indicates that the narrow-line mid-IR Baldwin effect is quite different from the broad-line optical/UV Baldwin effect, and it is possible that the two effects are unrelated. The anticorrelations discovered open new possibilities in understanding the physics of the ionizing region and the continuum reprocessing by dust.Item Physical Properties Of The Narrow-Line Region Of Low-Mass Active Galaxies(2012-09) Ludwig, Randi R.; Greene, Jenny E.; Barth, Aaron J.; Ho, Luis C.; Ludwig, Randi R.We present spectroscopic observations of 27 active galactic nuclei (AGNs) with some of the lowest black hole (BH) masses known. We use the high spectral resolution and small aperture of our Keck data, taken with the Echellette Spectrograph and Imager, to isolate the narrow-line regions (NLRs) of these low-mass BHs. We investigate their emission-line properties and compare them with those of AGNs with higher-mass BHs. While we are unable to determine absolute metallicities, some of our objects plausibly represent examples of the low-metallicity AGNs described by Groves et al., based on their [NII]/H alpha ratios and their consistency with the Kewley & Ellison mass-metallicity relation. We find tentative evidence for steeper far-UV spectral slopes in lower-mass systems. Overall, NLR emission lines in these low-mass AGNs exhibit trends similar to those seen in AGNs with higher-mass BHs, such as increasing blueshifts and broadening with increasing ionization potential. Additionally, we see evidence of an intermediate-line region whose intensity correlates with L/L-Edd, as seen in higher-mass AGNs. We highlight the interesting trend that, at least in these low-mass AGNs, the [OIII] equivalent width (EW) is highest in symmetric NLR lines with no blue wing. This trend of increasing [OIII] EW with line symmetry could be explained by a high covering factor of lower-ionization gas in the NLR. In general, low-mass AGNs preserve many well-known trends in the structure of the NLR, while exhibiting steeper ionizing continuum slopes and somewhat lower gas-phase metallicities.