Adam Dobrzycki

A Uniform Analysis of the Lyα Forest at z = 0-5. II. Measuring the Mean Intensity of the Extragalactic Ionizing Background Using the Proximity Effect

Moderate-resolution data for 40 quasi-stellar objects (QSOs) at z ≈ 2 were combined with spectra of comparable resolution of 59 QSOs with redshifts greater than 1.7 found in the literature to form a large, homogeneous sample of moderate-resolution (~1 A) QSO spectra. These spectra were presented and the statistics of the Lyα forest were discussed in Paper I. In this analysis, we demonstrate that a proximity effect is present in the data; i.e., there exists a significant (5.5 σ) deficit of lines at zabs ≈ zem. Within 1.5 h-1 Mpc of the QSO emission redshift, the significance does depend on QSO luminosity, in accordance with the theory that this effect is caused by enhanced ionization of hydrogen in the vicinity of the QSO from UV photons from the QSO itself. The photoionization model of Bajtlik, Duncan, & Ostriker permits an estimate of the mean intensity of the extragalactic background radiation at the Lyman limit. We compare the results of this standard analysis with those obtained using a maximum likelihood technique. If the spectrum of the background is assumed to be identical to that of each individual QSO, and if this background is assumed to be constant over the redshift range 1.7 < z < 3.8, then the best-fit value for J(ν0) is found to be 1.4 × 10-21 ergs s-1 cm-2 Hz-1 sr-1, using QSO redshifts based on the Lyα emission line. Systemic QSO redshifts based on the [O III] λ5007 emission line for 19 objects in our sample show an average redshift of ~400 km s-1 with respect to Lyα emission. Using redshifts based on [O III] or Mg II for the 35 objects for which they are measured and adding 400 km s-1 to the remaining QSO Lyα redshifts gives a lower value of J(ν0), 7.0 × 10-22 ergs s-1 cm-2 Hz-1 sr-1. This value is in reasonable agreement with the predictions of various models of the ionizing background based on the integrated quasar luminosity function. Allowing for the fact that individual QSOs have different spectral indices that may also be different from that of the background, we use the standard methods to solve for the H I photoionization rate, Γ, and the parameters describing its evolution with redshift. The best-fit value for the H I ionization rate we derive is 1.9 × 10-12 s-1, in good agreement with models of the background that incorporate QSOs only. Finally, we use simulated Lyα forest spectra including the proximity effect to investigate curve-of-growth effects in the photoionization model used in the analysis. We find that the presence of lines on the saturated part of the curve of growth could cause our estimates of the background intensity to be overestimated by a factor of 2-3. This large absorption-line sample and these techniques for measuring the background and understanding the systematics involved allow us to place what we believe are the firmest limits on the background at these redshifts.

Chandra Discovery of a 300 Kiloparsec X-Ray Jet in the Gigahertz-peaked Spectrum Quasar PKS 1127–145

We have discovered an X-ray jet with Chandra imaging of the z=1.187 radioloud quasar PKS 1127-145. In this paper we present the Chandra X-ray data, follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray jet contains 273±5 net counts in 27 ksec and extends ∼ 30 from the quasar core, corresponding to a minimum projected linear size of ∼ 330h 50 kpc. The evaluation of the X-ray emission processes is complicated by the observed offsets between X-ray and radio brightness peaks. We discuss the problems posed by these observations to jet models. In addition, PKS 1127-145 is a Giga-Hertz Peaked Spectrum radio source, a member of the class of radio sources suspected to be young or “frustrated” versions of FRI radio galaxies. However the discovery of an X-ray and radio jet extending well outside the host galaxy of PKS 1127145 suggests that activity in this and other GPS sources may be long-lived and complex. Subject headings: Quasars: individual (PKS 1127-145) – galaxies: jets – X-Rays:

Chandra Survey of Radio-quiet, High-Redshift Quasars

We observed 17 optically selected, radio-quiet, high-redshift quasars with the Chandra ACIS and detected 16 of them. The quasars have redshifts between 3.70 and 6.28 and include the highest-redshift quasars known. When compared with 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 in which the accretion flow is described by a cold, optically thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter α ≥ 0.02. The high-redshift quasars have supermassive black holes, with masses of ~1010 M☉, and are accreting material at ~0.1 times the Eddington limit. We detect 10 X-ray photons from the z = 6.28 quasar SDSS 1030+0524, which might 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, τ(IGM) 20.

Chandra detection of X-ray absorption associated with a damped Lyα system

We have observed three quasars, PKS 1127-145, Q1331+171, and Q0054+144, with the ACIS-S aboard the Chandra X-Ray Observatory in order to measure soft X-ray absorption associated with intervening 21 cm and damped Ly? absorbers. For PKS 1127-145, we detect absorption that, if associated with an intervening zabs = 0.312 absorber, implies a metallicity of 23% solar. If the absorption is not at zabs = 0.312, then the metallicity is still constrained to be less than 23% solar. The advantage of the X-ray measurement is that the derived metallicity is insensitive to ionization, inclusion of an atom in a molecule, or depletion onto grains. The X-ray absorption is mostly due to oxygen and is consistent with the oxygen abundance of 30% solar derived from optical nebular emission lines in a foreground galaxy at the redshift of the absorber. For Q1331+171 and Q0054+144, only upper limits were obtained, although the exposure times were intentionally short, since for these two objects we were interested primarily in measuring flux levels to plan for future observations. The imaging results are presented in a companion paper.

A approximately 10 MPC void in the Ly-alpha forest at Z = 3.17

The spectrum of the z = 3.285 quasar, Q0302−003, shows a large (∼10 Mpc) void in the Lyα forest at z ∼ 3.17. By simulating the absorption spectrum of the quasar with a range of Lyα forest cloud parameters, it is found that the probability of such a void occurring by chance is ∼ 2 × 10 −4 . Since the void is near (although not centered on) the redshift of a bright foreground quasar, the possibility that the void results from ionization of the clouds by the EUV radiation of this QSO is discussed

Sbs 0909+532: a new double gravitational lens or binary quasar?

The z = 1.377, B = 17.0 mag quasar SBS 0909+532 A, B is a double with two images separated by Δθ = 1107 ± 0006. Because the faint image has an emission line at the same wavelength as the Mg II 2798 A emission line of the quasar, and it lacks the broad Mg Ib absorption feature expected for a star with the same colors (a K star), we conclude that image B is a quasar with similar redshift to image A. The relative probabilities that the double is the smallest separation (4.7 h-1 kpc for Ω0 = 1) correlated quasar pair or a gravitational lens are ~1:106. If the object is a lens, the mean lens redshift is zl = 0.5 with 90% confidence bounds of 0.18 < zl < 0.83 for Ω0 = 1. If the lens is an elliptical galaxy, we expect it to be brighter than I < 19.5 mag. The broadband flux ratio varies with wavelength, with ΔI = 0.31, ΔR = 0.58, and ΔB = 1.29 magnitudes, which is difficult to reconcile with the lensing hypothesis.

Discovery of four X-ray quasars behind the Large Magellanic Cloud

We present the discovery of four X-ray quasars (zem = 0.26, 0.53, 0.61, and 1.63) located behind the Large Magellanic Cloud; three of them are located behind the bar of the LMC. The quasars were identified via spectroscopy of optical counterparts to X-ray sources found serendipitously by the Chandra X-Ray Observatory satellite. All four quasars have archival VI photometry from the second phase of the Optical Gravitational Lensing Experiment (OGLE-II); one of them was found by OGLE to be variable. We present the properties of the quasars and discuss their possible applications.

A Uniform Analysis of the Lyα Forest at z = 0-5. I. The Sample and Distribution of Clouds at z > 1.7

We present moderate-resolution data for 39 quasi-stellar objects (QSOs) at z ≈ 2 obtained at the Multiple Mirror Telescope. These data are combined with spectra of comparable resolution of 60 QSOs with redshifts greater than 1.7 found in the literature to investigate the distribution of Lyα forest lines in redshift and equivalent width. We find a value for γ, the parameter describing the number distribution of Lyα forest lines in redshift, of 1.88 ± 0.22 for lines stronger than a rest equivalent width of 0.32 A, in good agreement with some previous studies. The Kolmogorov-Smirnov test was applied to the data, and it is found that this single power law is a good fit over the relevant redshift ranges. Simulations of the Lyα forest were performed to determine the completeness of the line lists and to test how well the analysis recovers the underlying line statistics, given this level of completeness.

Hydrogen and metal absorption lines in PKS 0405-123 from the halos of low redshift galaxies

HST ultraviolet spectra and ground-based optical spectra of the bright quasar PKS 0405-123 are used to identify two absorption-line systems probably produced by the halos of foreground galaxies. The quasar lies in a rich galaxy field. Loose groups of galaxies are present in the field at mean z1 = 0.1669 and mean z2 = 0.3519. Ly-alpha, C IV 1549 A, and probably Mg II 2798 A resonance lines are detected in an absorption system at z = 0.1670. An absorption system consisting of only Ly-alpha is observed at z = 0.3516. We cannot be completely certain which foreground galaxy is responsible for the metal line absorption system at z1. A likely candidate is a large, luminous, early-type spiral 40 arcsec to the east of the quasar. However, the origin of the z1 system could possibly be a small undetected galaxy more precisely aligned along our line of sight to PKS 0405-123, or a very faint galaxy whose redshift has not yet been measured.

New X-ray quasars behind the Small Magellanic Cloud

We present five X-ray quasars behind the Small Magellanic Cloud, increasing the number of known quasars behind the SMC by ~40%. They were identified via follow-up spectroscopy of serendipitous sources from the Chandra X-Ray Observatory matched with objects from the OGLE database. All quasars lie behind dense parts of the SMC and could be very useful for proper-motion studies. We analyze X-ray spectral and timing properties of the quasars. We discuss applications of those and other recently discovered quasars behind the SMC to the studies of absorption properties of the Cloud, its proper motion, and for establishing the geometrical distance to the SMC.