Jonathan C. McDowell

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 Previously unpublished observational data for a second set of quasars excluded from the primary sample are also tabulated.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.

Absorption in X-ray spectra of high-redshift quasars

We present evidence that X-ray absorption is common in high-redshift quasars. We have studied six high-redshift (z approximately 3) quasars with the ROSAT Position Sensitive Proportional Counter (PSPC) of which four are in directions of low Galactic N(sub H). Three out of these four show excess absorption, while only three in approximately 50 z approximately less than 0.4 quasars do, indicating that such absorption must be common, but not ubiquitous, at high redshifts, and that the absorbers must lie at z greater than 0.4. The six quasars were: S5 0014+81, Q0420-388, PKS 0438-436, S4 0636+680. PKS 2000-330, PKS 2126-158, which have redshifts between 2.85 and 3.78. PKS 0438-436 and PKS 2126-158 show evidence for absorption above the local Galactic value at better than 99.999% confidence level. If the absorber is at the redshift of the quasar, then values of N(sub H) = (0.86(+0.49, -0.28)) x 10(exp 22) atoms/sq cm for PKS 0438-436, and N(sub H) = (1.45(+1.20, -0.64)) x 10(exp 22) atoms/ sq cm for PKS 2126-158, are implied, assuming solar abundances. The spectrum of S4 0636+680 also suggests the presence of a similarly large absorption column density at the 98% confidence level. This absorption reverses the trend for the most luminous active galactic nuclei (AGN) to have the least X-ray absorption, so a new mechanism is likely to be responsible. Intervening absorption due to damped Lyman(alpha) systems is a plausible cause. We also suggest, as an intrinsic model, that intracluster material, e.g., a cooling flow, around the quasar could account for both the X-ray spectrum and other properties of these quasars. All the quasars are radio-loud and three are gigahertz peaked (two of the three showing absorption). No excess absorption above the Galactic value is seen toward Q0420-388. This quasar has two damped Lyman(alpha) systems at z = 3.08. The limit on the X-ray column density implies a low ionization fraction, N(H I)/N(H) approximately greater than 4 x 10(exp -3) (3 (sigma)), for solar abundances, for these systems, and can set a weak limit on the size of the absorber. In the emitted frame these PSPC spectra cover the band approximately 0.5-10 keV, which has been well observed for low-redshift quasars and AGN. Comparison of high and low-redshift spectra in this emitted band shows no change of mean spectral index greater than Delta alpha(sub E) greater than 0.3 (99% confidence) with either redshift or luminosity, for radio-loud quasars.

THE METALLICITY PROFILE OF M31 FROM SPECTROSCOPY OF HUNDREDS OF H II REGIONS AND PNe

The oxygen abundance gradients among nebular emission line regions in spiral galaxies have been used as important constraints for models of chemical evolution. We present the largest-ever full-wavelength optical spectroscopic sample of emission line nebulae in a spiral galaxy (M31). We have collected spectra of 253 H II regions and 407 planetary nebulae (PNe) with the Hectospec multi-fiber spectrograph of the MMT. We measure the line-of-sight extinction for 199 H II regions and 333 PNe; we derive oxygen abundance directly, based on the electron temperature, for 51 PNe; and we use strong-line methods to estimate oxygen abundance for 192 H II regions and nitrogen abundance for 52 H II regions. The relatively shallow oxygen abundance gradient of the more extended H II regions in our sample is generally in agreement with the result of Zaritsky et al., based on only 19 M31 H II regions, but varies with the strong-line diagnostic employed. Our large sample size demonstrates that there is significant intrinsic scatter around this abundance gradient, as much as ~3 times the systematic uncertainty in the strong-line diagnostics. The intrinsic scatter is similar in the nitrogen abundances, although the gradient is significantly steeper. On small scales (deprojected distance <0.5 kpc), H II regions exhibit local variations in oxygen abundance that are larger than 0.3 dex in 33% of neighboring pairs. We do not identify a significant oxygen abundance gradient among PNe, but we do find a significant gradient in the [N II] ratio that varies systematically with surface brightness. Our results underscore the complex and inhomogeneous nature of the interstellar medium of M31, and our data set illustrates systematic effects relevant to future studies of the metallicity gradients in nearby spiral galaxies.

Chandra Observations of Arp 220: The Nuclear Source

We present the first results from 60 ks of observations of Arp 220 using the ACIS-S instrument on Chandra. We report the detection of several sources near the galaxys nucleus, including a point source with a hard spectrum that is coincident with the western radio nucleus B. This point source is mildly absorbed (NH ~ 3 × 1022 cm-2) and has an estimated luminosity of 4 × 1040 ergs s-1. In addition, a fainter source may coincide with the eastern nucleus A. Extended hard X-ray emission in the vicinity raises the total estimated nuclear 2-10 keV X-ray luminosity to 1.2 × 1041 ergs s-1, but we cannot rule out a hidden active galactic nucleus behind columns exceeding 5 × 1024 cm-2. We also detect a peak of soft X-ray emission to the west of the nucleus and a hard point source 2.5 kpc from the nucleus with a luminosity of 6 × 1039 ergs s-1.

Chandra Observations of Extended X-Ray Emission in ARP 220

We resolve the extended X-ray emission from the prototypical ultraluminous infrared galaxy Arp 220. Extended, faint, edge-brightened, soft X-ray lobes outside the optical galaxy are observed to a distance of 10-15 kpc on each side of the nuclear region. Bright plumes inside the optical isophotes coincide with the optical line emission and extend 11 kpc from end to end across the nucleus. The data for the plumes cannot be fitted by a single-temperature plasma and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from bright, diffuse circumnuclear emission in the inner 3 kpc centered on the Hα peak, which is displaced from the radio nuclei. There is a close morphological correspondence between the Hα and soft X-ray emission on all spatial scales. We interpret the plumes as a starburst-driven superwind and discuss two interpretations of the emission from the lobes in the context of simulations of the merger dynamics of Arp 220.

The X-ray-faint Emission of the Supermassive Nuclear Black Hole of IC 1459

Chandra observations of the supermassive black hole in the nucleus of IC 1459 show a weak (LX=8×10 40 erg s −1 , 0.3-8 keV), unabsorbed nuclear X-ray source, with a slope = 1.88 ±0.09, and no strong Fe-K line at 6.4 keV (EW<382 eV). This describes a normal AGN X-ray spectrum, but lies at 3×10 −7 below the Eddington limit. The SED of the IC 1459 nucleus is extremely radio loud compared to normal radio-loud quasars. The nucleus is surrounded by hot ISM (kT�0.50.6 keV) with an average density of 0.3 cm −3 , within the central �180 pc radius, which is comparable to the gravitational capture radius, rA �140 pc. We estimate that for a standard AGN efficiency of 10%, the Bondi accretion would correspond to a luminosity of �6×10 44 erg s −1 , nearly four orders of magnitude higher than LX. ADAF solutions can explain the X-ray spectrum, but not the high radio/X-ray ratio. A jet model fits the radio-100µm and X-ray spectra well. The total power in this jet is �10% of LBondi, implying that accretion close to the Bondi rate is needed.

The Far-Infrared Spectral Energy Distributions of X-Ray-selected Active Galaxies*

Hard X-ray selection is, arguably, the optimal method for defining a representative sample of active galactic nuclei (AGNs). Hard X-rays are unbiased by the effects of obscuration and reprocessing along the line of sight intrinsic/external to the AGN, which result in unknown fractions of the population being missed from traditional optical/soft X-ray samples. We present the far-infrared (far-IR) observations of 21 hard X-ray-selected AGNs from the HEAO 1 A2 sample observed with Infrared Space Observatory (ISO). We characterize the far-IR continua of these X-ray-selected AGNs and compare them with those of various radio and optically selected AGN samples and with models for an AGN-heated, dusty disk. The X-ray-selected AGNs show broad, warm IR continua covering a wide temperature range (~20-1000 K in a thermal emission scenario). Where a far-IR turnover is clearly observed, the slopes are less than 2.5 in all but three cases so that nonthermal emission remains a possibility, although the presence of cooler dust resulting in a turnover at wavelengths longward of the ISO range is considered more likely. The sample also shows a wider range of optical/UV shapes than the optical/radio-selected samples, extending to redder near-IR colors. The bluer objects are type 1 Seyfert galaxies, while the redder AGNs are mostly intermediate or type 2 Seyfert galaxies. This is consistent with a modified unification model in which obscuration increases as we move from a face-on toward a more edge-on line of sight. However, this relation does not extend to the mid-infrared as the 25/60 μm ratios are similar in Seyfert galaxies with differing type and optical/UV reddening. The resulting limits on the column density of obscuring material through which we are viewing the redder AGNs (NH ~ 1022 cm-2) are inconsistent with standard optically thick torus models (NH ~ 1024 cm-2) and simple unification models. Instead our results support more complex models in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wave bands and of the broad emission line region, which, in turn, results in a mixture of behaviors for AGNs with similar optical emission-line classifications. The resulting decrease in the optical depth of the obscuring material also allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass of ~109 M☉ and size of approximately a few hundred parsecs is able to generate optical-far-IR spectral energy distributions (SEDs) that reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.

The Redshift of the Optical Transient Associated with GRB 010222

The gamma-ray burst (GRB) 010222 is the brightest GRB detected to date by the BeppoSAX satellite. Prompt identification of the associated optical transient (OT) allowed for spectroscopy with the Tillinghast 1.5 m telescope at F. L. Whipple Observatory while the source was still relatively bright (R 18.6 mag), within 5 hr of the burst. The OT shows a blue continuum with many superposed absorption features corresponding to metal lines at z = 1.477, 1.157, and possibly also 0.928. The redshift of GRB 010222 is therefore unambiguously placed at z ? 1.477. The high number of Mg II absorbers and especially the large equivalent widths of the Mg II, Mg I, and Fe II absorption lines in the z = 1.477 system further argue either for a very small impact parameter or that the z = 1.477 system is the GRB host galaxy itself. The spectral index of the OT is relatively steep, F? ?-0.89?0.03, and this cannot be caused by dust with a standard Galactic extinction law in the z = 1.477 absorption system. This spectroscopic identification of the redshift of GRB 010222 shows that prompt and well-coordinated follow-up of bright GRBs can be successful even with telescopes of modest aperture.

Infrared to x-ray spectral energy distributions of high redshift quasars

We have observed 14 quasars with z greater than 2.8 with the ROSAT-PSPC, and detected 12 of them, including the z=4.11 quasar 0000-263. We present the first x-ray spectrum of a radio quiet quasar with z greater than 3, 1946+768. Its x-ray spectrum is consistent with a power law with spectral index alpha(sub E)=1.8(sup +2.1, sub -1.4) and no evidence for absorption in excess of the galactic column (alpha(sub E)=1.00(sup +0.28, sub -0.32) assuming N(sub H)=N(sub H)(Gal)). A Position Sensitive Proportional Counter (PSPC) hardness ratio is used to constrain the x-ray spectral properties of the quasars for which there were less than 100 photons detected. For the radio quiet quasars, (alpha(sub E)) approximately equals 1.2, if one assumes that there is no absorption in excess of the galactic column. We combine the x-ray data with new ground based optical and near-IR spectrophotometry obtained at the Steward 2.3 m and Multiple Mirror Telescope, and data from the literature. The spectral energy distributions are compared to those of low redshift objects. For the radio quiet quasars with z greater than 2.5, the mean (alpha(sub ox)) is approximately 1.8. This is larger than the mean for quasars with z less than 2.5, but consistent with the expected value for quasars with the high optical luminosities of the objects in this sample. For the radio-loud quasars, (alpha(sub ox)) is approximately 1.4, independent of redshift. This is smaller than the expected value for the optically luminous, high redshift objects in this sample, if they are mostly GHz peaked radio sources and hence comparable to steep-spectrum, compact radio sources at lower redshift. Finally, we compare the spectral energy distributions of two representative objects to the predicted spectrum of a thin accretion disk in the Kerr geometry, and discuss the uncertainties in deriving black hole masses and mass accretion rates.

A Chandra Survey of Nearby Spiral Galaxies. I. Point Source Catalogs

Emission from discrete point sources dominates the X-ray luminosity in spiral galaxies. We present results from a survey of 11 nearby, nearly face-on spiral galaxies with the Chandra X-ray Observatory. These galaxies span the Hubble sequence for spirals, allowing insights into the X-ray source population of many diverse systems. In this paper, we present source lists for the 11 galaxies along with fluxes, luminosities, X-ray colors, and variability properties. We briefly discuss X-ray luminosity functions and how they relate to star formation of the host galaxies. We also discuss source colors and variability and what these can tell us about the composition of the X-ray source population.