Jill Bechtold

Atlas of quasar energy distributions

We present an atlas of the spectral energy distributions (SEDs) of normal, nonblazar, quasars over the whole available range (radio to 10 keV X-rays) of the electromagnetic spectrum. The primary (UVSX) sample includes 47 quasars for which the spectral energy distributions include X-ray spectral indices and UV data. Of these, 29 are radio quiet, and 18 are radio loud. The SEDs are presented both in figures and in tabular form, with additional tabular material published on CD-ROM. Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated. The effects of host galaxy starlight contamination and foreground extinction on the UVSX sample are considered and the sample is used to investigate the range of SED properties. Of course, the properties we derive are influenced strongly by the selection effects induced by quasar discovery techniques. We derive the mean energy distribution (MED) for radio-loud and radio-quiet objects and present the bolometric corrections derived from it. We note, however, that the dispersion about this mean is large (approximately one decade for both the infrared and ultraviolet components when the MED is normalized at the near-infrared inflection). At least part of the dispersion in the ultraviolet may be due to time variability, but this is unlikely to be important in the infrared. The existence of such a large dispersion indicates that the MED reflects only some of the properties of quasars and so should be used only with caution.

ARE THE VARIATIONS IN QUASAR OPTICAL FLUX DRIVEN BY THERMAL FLUCTUATIONS

We analyze a sample of optical light curves for 100 quasars, 70 of which have black hole mass estimates. Our sample is the largest and broadest used yet for modeling quasar variability. The sources in our sample have z < 2.8, 1042 λL λ(5100 A) 1046, and 106 M BH/M ☉ 1010. We model the light curves as a continuous time stochastic process, providing a natural means of estimating the characteristic timescale and amplitude of quasar variations. We employ a Bayesian approach to estimate the characteristic timescale and amplitude of flux variations; our approach is not affected by biases introduced from discrete sampling effects. We find that the characteristic timescales strongly correlate with black hole mass and luminosity, and are consistent with disk orbital or thermal timescales. In addition, the amplitude of short-timescale variations is significantly anticorrelated with black hole mass and luminosity. We interpret the optical flux fluctuations as resulting from thermal fluctuations that are driven by an underlying stochastic process, such as a turbulent magnetic field. In addition, the intranight variations in optical flux implied by our empirical model are 0.02 mag, consistent with current microvariability observations of radio-quiet quasars. Our stochastic model is therefore able to unify both long- and short-timescale optical variations in radio-quiet quasars as resulting from the same underlying process, while radio-loud quasars have an additional variability component that operates on timescales 1 day.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II, An intensive study of NGC 5548 at optical wavelengths

A large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign is described. This contribution presents the data base and describes the methods used to correct for systematic differences between spectra from different sources. Optical continuum and H-beta emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The h-beta emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with H-beta lagging behind the continuum by about 20 days. This is significantly larger than the about 10 day lag deduced for Ly-alpha. 15 refs.

The discovery of Segue 2: a prototype of the population of satellites of satellites

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of M v =− 2.5, a half-light radius of 34 pc and a systemic velocity of ∼− 40 km s −1 . Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s −1 and the possible stream as ∼ 7k m s −1 . This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites – survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z> 10 and are good candidates for fossils of the reionization epoch.

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.

Confirmation of dust in damped Lyman-alpha systems

As part of a search for dust at high redshifts, we have acquired new spectra of quasars in the Wolfe et al. sample : 13 with damped Lyα systems along the lines of sight and 15 without. Our spectra cover the region 3950-6900 A at a resolution of 15 A with signal-to-noise of 10-50. We have determined spectral indices over a common range of wavelengths in the rest frames of the quasars between Lyα and C IV emission. Several internal and external checks indicate that the typical errors in the spectral indices are ±0.1.

Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer

We report the discovery of the progenitor of the recent Type IIn SN 2008S in the nearby galaxy NGC 6946. Surprisingly, it was not found in deep, preexplosion optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the SN 2008S position is clearly detected in the 4.5, 5.8, and 8.0 ?m IRAC bands, showing no evident variability in the 3 years prior to the explosion, yet is undetected at 3.6 and 24 ?m. The distinct presence of ~440 K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S was engulfed in a shroud of its own dust. The inferred luminosity of 3.5 ? 104 L? implies a modest mass of ~10 M -->?. We conclude that objects like SN 2008S are not exclusively associated with the deaths or outbursts of very massive ? Carinae-like objects. This conclusion holds based solely on the optical flux limits even if our identification of the progenitor with the mid-IR source is incorrect.

A proto-galaxy candidate at z = 2.7 discovered by its young stellar population

A protogalaxy candidate at z=2.72 has been discovered serendipitously by the CNOC cluster redshift survey. The candidate is an extremely luminous (V=20.5 mag, absolute mag -26) and well resolved disk-like galaxy. The redshift is identified from a dozen strong UV absorption lines. No emission lines are found between 1000 and 2000A (rest), including Ly alpha. The photometric data fit the spectral energy distributions of a stellar population from 400 million years to an arbitrarily young age, dependent on the amount of dust extinction. However, the presence of a strong P-Cygni profile in CIV~indicates that a very substantial component of the stellar population must be younger than ~ 10 Myr. We interpret this object as an early-type galaxy observed within about 100 million years of the initial burst of star formation which created most of its stellar mass. Because of the resolved, regular, and smooth nature of the object, it is unlikely that the high luminosity is due to gravitational lensing.

Identification of a galaxy responsible for a high-redshift Lyman-α absorption system

DAMPEDLyman-α systems are high-column-density intergalactic clouds of hydrogen, the existence of which is inferred from absorption lines appearing in the emission spectra of distant quasars. The galaxies believed to be responsible for these absorption systems have been suggested as possible progenitors of the normal disk galaxies observed in the local Universe1. Indeed, Lyman-α systems appear to contain a substantial fraction of the baryons known to exist in galaxies today2,3. Here we report the optical detection of a galaxy (designated DLA2233 + 131) associated with a known4 damped Lyman-α absorption system at a redshift of z = 3.150. The properties of this galaxy correspond closely to those expected of a young disk galaxy in the early stages of formation, and show no evidence for an active nucleus. This finding gives strong support to the idea that damped Lyman-α systems represent a population of young galaxies at high red-shifts.

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: