arXiv subjects

Fred Hamann

Publications and source records attributed to Fred Hamann.

86 records · Page 5Linked to original sources

arXiv2002

Chandra Survey of Radio-quiet, High-redshift Quasars

We observed 17 optically-selected, radio-quiet high-redshift quasars with the Chandra Observatory ACIS, and detected 16 of them. The quasars have redshift between 3.70 and 6.28 and include the highest redshift quasars known. When compared to low-redshift quasars observed with ROSAT, these high redshift quasars are significantly more X-ray quiet. We also find that the X-ray spectral index of the high redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models where the accretion flow is described by a cold, optically-thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter alpha >= 0.02. The high redshift quasars have supermassive black holes with masses ~10^{10} M_{sun}, and are accreting material at ~0.1 the Eddington limit. We detect 10 X-ray photons from the z=6.28 quasar SDS 1030+0524, which may have a Gunn-Peterson trough and be near the redshift of reionization of the intergalactic medium. The X-ray data place an upper limit on the optical depth of the intergalactic medium tau(IGM) < 10^6, compared to the lower limit from the spectrum of Lyalpha and Lybeta, which implies tau(IGM) > 20.

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arXiv2001

Metallicities and Abundance Ratios from Quasar Broad Emission Lines

The broad emission lines (BELs) of quasars and active galactic nuclei (AGNs) are important diagnostics of the relative abundances and overall metallicity in the gas. Here we present new theoretical predictions for several UV BELs. We focus specifically on the relative nitrogen abundance as a metallicity indicator, based on the expected secondary enrichment of nitrogen at metallicities Z > 0.2 Z_o. Among the lines we consider, N III] 1750/O III] 1664, N V 1240/(C IV 1549 + O VI 1034) and N V/He II 1640 are the most robust diagnostics. We argue, in particular, that the average N V BEL is not dominated by scattered Ly-alpha photons from a broad absorption line wind. We then compare our calculated line ratios with observations from the literature. The results support earlier claims that the gas-phase metallicities near quasars are typically near or several times above the solar value. We conclude that quasar activity is preceded by, or coeval with, an episode of rapid and extensive star formation in the surrounding galactic (or proto-galactic) nuclei. Chemical evolution models of these environments suggest that, to reach Z > Z_o in well-mixed interstellar gas, the star formation must have begun > 10^8 yr before the observed quasar activity.

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arXiv2001

The Metallicity of the Redshift 4.16 Quasar BR2248-1242

We estimate the metallicity in the broad emission-line region of the redshift z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to theoretical predictions. BR2248-1242 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 ~2 times solar. This result suggests that an episode of vigorous star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch. The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence for high metallicities and rapid star formation is consistent with the expected early-epoch evolution of dense galactic nuclei.

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arXiv2001

Emission-Line Properties of z > 4 Quasars

We present results of a program of high signal-to-noise spectroscopy for 44 QSOs at redshifts > 4 using the MMT and Keck observatories. The quasar spectra cover 1100 -- 1700 A in the rest frame for sources spanning a luminosity range of approximately 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.

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arXiv2001

PG 1254+047: An Intrinsically X-ray Weak, Heavily Absorbed BALQSO?

We present \textit{Chandra} observations of the radio-quiet Broad Absorption Line (BAL) QSO PG 1254+047. We find that it is a weak X-ray source, with a total of $44\pm 7$ photons measured in 36 ksec across the observed energy range $0.7 - 6.0$ keV. Its X-ray weakness is consistent with the known correlation between $α_{ox}$ and the strength of the UV absorption lines. The spectral energy distribution suggests that PG 1254+047 is intrinsically X-ray weak, in addition to being heavily X-ray absorbed. The X-ray absorption column density is $N_{H} > 10^{23}$ cm$^{-2}$ for neutral gas, while the intrinsic (unabsorbed) emission spectrum has $α_{ox} > 2$. The data are fit best by including an ionized (rather than neutral) absorber, with column density $N_{H} \gtrsim 2\times 10^{23}$ cm$^{-2}$. The degree of ionization is consistent with the UV BALs, as is the total column density if the strongest UV lines are saturated. If the X-ray absorber forms in a wind that is radiatively accelerated to the BAL velocities, then the wind must be launched from a radius of about $10^{16}$ cm with a mass loss rate of $\sim 1$ M$_\odot$ yr$^{-1}$.

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arXiv2001

High Redshift Quasars and Star Formation History

Quasars are among the most luminous objects in the universe, and they can be studied in detail up to the highest known redshift. Assuming that the gas associated with quasars is closely related to the interstellar medium of the host galaxy, quasars can be used as tracer of the star formation history in the early universe. We have observed a small sample of quasars at redshifts 3 = 10, corresponding to an age of the universe of less than 5*10^8 yrs (H_o = 65 km/s/Mpc, Omega_M = 0.3, Omega_Lambda = 0.7).

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arXiv2000

Intrinsic AGN Absorption Lines

Strong absorption lines are common in rest-frame UV spectra of AGNs due to a variety of resonant transitions, for example the HI Lyman series lines (most notably Ly-alpha 1216) and high-ionization doublets like CIV 1549,1551. The lines are called ``intrinsic'' if the absorbing gas is physically related to the AGN, e.g. if the absorber resides broadly within the radius of the AGN's surrounding ``host'' galaxy. Intrinsic absorption lines are thus valuable probes of the kinematics, physical conditions and elemental abundances in the gas near AGNs. Studies of intrinsic absorbers have historically emphasized the broad absorption lines (BALs) in quasars. Today we recognize a wider variety of intrinsic lines in a wider range of objects. For example, we now know that Seyfert 1 galaxies (the less luminous cousins of quasars) have intrinsic absorption. We also realize that intrinsic lines can form in a range of AGN environments --- from the dynamic inner regions like the BALs, to the more quiescent outer host galaxies >10 kpc away. This article provides a brief introduction to current observational and theoretical work on intrinsic AGN absorbers.

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arXiv1999

Elemental Abundances in QSOs: Star Formation and Galactic Nuclear Evolution at High Redshifts

Quasar (or QSO) elemental abundances provide unique probes of high-redshift star formation and galaxy evolution. There is growing evidence from both the emission and intrinsic absorption lines that QSO environments have roughly solar or higher metallicities out to redshifts >4. The range is not well known, but solar to a few times solar appears to be typical. There is also evidence for higher metallicities in more luminous objects, and for generally enhanced N/C and Fe/alpha abundances compared to solar ratios. These results identify QSOs with vigorous, high-redshift star formation -- consistent with the early evolution of massive galactic nuclei or dense proto-galactic clumps. However, the QSOs offer new constraints. For example, 1) most of the enrichment and star formation must occur before the QSOs "turn on" or become observable, on time scales of ~<1 Gyr at least at the highest redshifts. 2) The tentative result for enhanced Fe/alpha suggests that the first local star formation began at least ~1 Gyr prior to the QSO epoch. 3) The star formation must ultimately be extensive in order to reach high metallicities, i.e. a substantial fraction of the local gas must be converted into stars and stellar remnants. The exact fraction depends on the shape of the initial mass function (IMF). 4) The highest derived metallicities require IMFs that are weighted slightly more toward massive stars than the in solar neighborhood. 5) High metallicities also require deep gravitational potentials. By analogy with the well-known mass--metallicity relation among low-redshift galaxies, metal-rich QSOs should reside in galaxies (or proto-galaxies) that are minimally as massive (or as tightly bound) as our own Milky Way.

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arXiv1998

Probing the High-Redshift Universe with Quasar Elemental Abundances

The heavy elements near QSOs provide unique measures of the star formation and chemical evolution in young galactic nuclei or pre-galactic condensations. Studies of quasar abundances also naturally address a variety of questions regarding the physics of QSO environments, including the Baldwin Effect emphasized at this meeting. Here I review the status of quasar abundance work.

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arXiv1998

Heavy Elements in QSOs: Star Formation and Galaxy Evolution at High Redshifts

Intrinsic emission and absorption lines of QSOs provide several independent probes of the metal abundances in QSO environments. They indicate that the metallicities are typically solar or higher out to redshifts z > 4. These results support models of galaxy evolution where galactic nuclei, or dense condensations that later become galactic nuclei, form stars and evolve quickly at redshifts higher than the QSOs themselves.

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arXiv1998

Broad P V Absorption in the BALQSO, PG 1254+047: Column Densities, Ionizations and Metal Abundances in BAL Winds

This paper discusses the detection of P V 1118,1128 and other broad absorption lines (BALs) in archival HST spectra of the low-redshift BALQSO, PG 1254+047. The P V identification is secured by excellent redshift and profile coincidences with the other BALs, such as C IV 1548,1550 and Si IV 1393,1403, and by photoionization calculations showing that other lines near this wavelength, e.g. Fe III 1123, should be much weaker than P V. The observed BAL strengths imply that either 1) there are extreme abundance ratios such as [C/H] >~ +1.0, [Si/H] >~ +1.8 and [P/C] >~ +2.2, or 2) at least some of the lines are much more optically thick than they appear. I argue that the significant presence of P V absorption indicates severe line saturation, which is disguised in the observed (moderate-strength) BALs because the absorber does not fully cover the continuum source(s) along our line(s) of sight. Computed optical depths for all UV resonance lines show that the observed BALs are consistent with solar abundances if 1) the ionization parameter is at least moderately high, log U >~ -0.6, 2) the total hydrogen column density is log N_H(cm-2) >~ 22.0, and 3) the optical depths in strong lines like C IV and O VI 1032,1038 are >~25 and >~80, respectively. These optical depths and column densities are at least an order of magnitude larger than expected from the residual intensities in the BAL troughs, but they are consistent with the large absorbing columns derived from X-ray observations of BALQSOs. The outflowing BALR, at velocities from -15,000 to -27,000 km/s in PG 1254+047, is therefore a strong candidate for the X-ray absorber in BALQSOs.

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arXiv1997

Broad NeVIII 774 Emission From Quasars

NeVIII 774 is an important tracer of the high-ionization gas in QSOs. We examine the NeVIII emission-line properties using new HST-FOS spectra of four sources, mean spectra derived from two QSO samples in the HST archives, and new photoionization calculations. The results support our previous claim that broad NeVIIIlines are common in QSOs, with an average flux of ~42% of OVI 1034 and velocity widths that are ~2 to 5 times larger than OVI, CIV 1549 and other broad lines in the same spectra. The strongest and most reliably measured NeVIII 774 lines (in two sources) have FWHM ~ 14,500 km/s. Line profile fits in these cases show that the unusually large widths might be caused by blending with emission from NIV 765 and OIV~\lam 789. However, standard photoionization calculations indicate that NIV, OIV and all other lines near this wavelength should be too weak, leaving (very broad) NeVIII as the only viable identification for the ~774 A feature. (This conclusion might be avoided if there are large radial velocity dispersions [>~1000 km/s] in the emitting region and the resonant absorption of continuum photons enhances the flux in weaker lines.) The calculations also indicate that the NeVIII emitting regions have ionization parameters in the range 5 > U > 30, total hydrogen column densities of 10^22 < N_H < 3 x 10^23 cm-2, and an average covering factor of >30% (for solar abundances and a nominal QSO continuum shape). The NeVIII emitting region is therefore more extensive, more highly ionized, and has much higher velocities than the rest of the broad emission line region (BELR). This highly-ionized BELR component would be a strong X-ray ``warm'' absorber if it lies along our line-of-sight to the X-ray continuum source.

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arXiv1997

The Nature and Origin of z_a~z_e Absorption Lines in the Redshift 0.20 Quasar, PKS 2135-147

We use new UV and optical spectra and an archival HST-WFPC2 image to study the z_a~z_e absorber in the z_e = 0.20 QSO PKS 2135-147. The UV spectra, obtained with HST-FOS, show strong z_a~z_e absorption lines of C IV, N V, O VI, Ly-alpha and Ly-beta. The z_a~z_e line profiles are resolved, with deconvolved FWHM of 270 to 450 km/s. Lower limits on the total column densities are of order 10^15 cm-2 for all ions. If the absorber is photoionized by the QSO and the derived relative columns in C IV and H I are roughly correct, then the metallicity must be at least solar. The location of the z_a~z_e absorber remains uncertain. Two ~L_* galaxies in a small cluster centered on PKS 2135-147 lie within 36h^{-1} kpc projected distance and have redshifts consistent with causing or contributing to the z_a~z_e lines. The extensive halo of the QSO's host galaxy could also contribute. Calculations show that the QSO is bright enough to photoionize gas up to O VI in the low-density halos of the host and nearby cluster galaxies. Nonetheless, there is indirect evidence for absorption much nearer the QSO, namely (1) the derived high (albeit uncertain) metallicity, (2) the relatively strong N V absorption lines, which might be caused by a higher nitrogen abundance in the metal-rich gas, and (3) strong, lobe-dominated steep-spectrum radio emission, which is known to correlate with a much higher incidence of (probably intrinsic) z_a~z_e lines. We propose that the CIV/NV/OVI line ratios can be used as a general diagnostic of intrinsic versus intervening absorption, as long as the line saturation effects are understood.

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arXiv1997

Metal Abundances and Ionization in QSO Intrinsic Absorbers

This paper provides a general reference for deriving the ionizations and metal abundances for absorption-line clouds in photoionization equilibrium with a QSO spectrum. The results include a quantitative assessment of the theoretical uncertainties and firm lower limits on the metallicities when no ionization constraints are available. The calculations applied to the best available column densities for QSO intrinsic absorbers support previous studies; there is strong evidence for super-solar metallicities in broad absorption line (BAL) and at least some ``associated'' (z_a~z_e) absorption regions. Even in cases where the level of ionization is unknown (for example, when only H I and C IV lines are measured), firm lower limits on the metal abundances (C/H) imply Z ~> Z_o for typical BALs and some z_a~z_e absorbers. These results support the independent evidence for high metallicities in QSO broad emission-line regions. I argue that high gas-phase abundances, up to ~10 Z_o, can be expected in QSOs due to the normal enrichment by stars in the cores of young, massive (~> 10^11 M_o) galaxies. The measured column densities indicate that the gas is usually optically thin in the continuum out to energies above at least the CIV and NV ionization edges (i.e. >100 eV). The column density ratios often require a range of ionization states in the same absorber, and neighboring (blended or adjacent) systems can have ionization parameters that differ by more than an order of magnitude. These result, and the overall levels of ionization, are not consistent with single-zone models of the UV line and X-ray continuum (``warm'') absorbers.

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