Sarah Connoran Gallagher

CHANDRA Detects Relativistic Broad Absorption Lines from APM 08279+5255

We report the discovery of X-ray broad absorption lines (BALs) from the BAL quasar APM 08279+5255 originating from material moving at relativistic velocities with respect to the central source. The large flux magnification by a factor of ~100 provided by the gravitational lens effect combined with the large redshift (z = 3.91) of the quasar have facilitated the acquisition of the first high signal-to-noise X-ray spectrum of a quasar containing X-ray BALs. Our analysis of the X-ray spectrum of APM 08279+5255 places the rest-frame energies of the two observed absorption lines at 8.1 and 9.8 keV. The detection of each of these lines is significant at a greater than 99.9% confidence level based on the F-test. Assuming that the absorption lines are from Fe XXV Kα, the implied bulk velocities of the X-ray BALs are ~0.2c and ~0.4c, respectively. The observed high bulk velocities of the X-ray BALs combined with the relatively short recombination timescales of the X-ray-absorbing gas imply that the absorbers responsible for the X-ray BALs are located at radii of 2 × 1017 cm, within the expected location of the UV absorber. With this implied geometry, the X-ray gas could provide the necessary shielding to prevent the UV absorber from being completely ionized by the central X-ray source, consistent with hydrodynamical simulations of line-driven disk winds. Estimated mass-outflow rates for the gas creating the X-ray BALs are typically less than a solar mass per year. Our spectral analysis also indicates that the continuum X-ray emission of APM 08279+5255 is consistent with that of a typical radio-quiet quasar with a spectral slope of Γ = 1.72.

XMM-Newton Reveals the Quasar Outflow in PG 1115+080

We report on an observation of the broad absorption line (BAL) quasar PG 1115+080 performed with the XMM-Newton observatory. Spectral analysis reveals the second case of a relativistic X-ray-absorbing outflow in a BAL quasar. The first case was revealed in a recent observation of APM 08279+5255 with the Chandra X-Ray Observatory. As in the case of APM 08279+5255, the observed flux of PG 1115+080 is greatly magnified by gravitational lensing. The relatively high redshift (z = 1.72) of the quasar places the redshifted energies of resonant absorption features in a sensitive portion of the XMM-Newton spectral response. The spectrum indicates the presence of complex low-energy absorption in the 0.2-0.6 keV observed energy band and high-energy absorption in the 2-5 keV observed energy band. The high-energy absorption is best modeled by two Gaussian absorption lines with rest-frame energies of 7.4 and 9.5 keV. Assuming that these two lines are produced by resonant absorption due to Fe XXV Kα, we infer that the X-ray absorbers are outflowing with velocities of ~ 0.10c and ~ 0.34c, respectively. We have detected significant variability of the energies and widths of the X-ray BALs in PG 1115+080 and APM 08279+5255 over timescales of 19 and 1.8 weeks (proper time), respectively. The BAL variability observed from APM 08279+5255 supports our earlier conclusion that these absorbers are most likely launched at relatively small radii of 1016(Mbh/M8)1/2 cm. A comparison of the ionization properties and column densities of the low-energy and high-energy absorbers indicates that these absorbers are likely distinct; however, higher spectral resolution is needed to confirm this result. Finally, we comment on prospects for constraining the kinematic and ionization properties of these X-ray BALs with the next generation of X-ray observatories.

X-Ray Spectroscopy of Quasi-Stellar Objects with Broad Ultraviolet Absorption Lines

For the population of quasi-stellar objects (QSOs) with broad ultraviolet absorption lines, we are just beginning to accumulate X-ray observations with enough counts for spectral analysis at CCD resolution. From a sample of eight QSOs (including four broad absorption line [BAL] QSOs and three mini-BAL QSOs) with ASCA or Chandra spectra with more than 200 counts, general patterns are emerging. Their power-law X-ray continua are typical of normal QSOs with Γ ≈ 2.0, and the signatures of a significant column density [NH ≈ (0.1-4) × 1023 cm-2] of intrinsic, absorbing gas are clear. Correcting the X-ray spectra for intrinsic absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that the spectral energy distributions of this population are not inherently anomalous. In addition, a large fraction of our sample shows significant evidence for complexity in the absorption. The subset of BAL QSOs with broad Mg II absorption apparently suffers from Compton-thick absorption completely obscuring the direct continuum in the 2-10 keV X-ray band, complicating any measurement of their intrinsic X-ray spectral shapes.

X-Raying the Ultraluminous Infrared Starburst Galaxy and Broad Absorption Line QSO Markarian 231 with Chandra

With 40 ks of Chandra ACIS-S3 exposure, new information on both the starburst and QSO components of the X-ray emission of Markarian 231, an ultraluminous infrared galaxy and broad absorption line QSO, has been obtained. The bulk of the X-ray luminosity is emitted from an unresolved nuclear point source, and the spectrum is remarkably hard, with the majority of the flux emitted above 2 keV. Most notably, significant nuclear variability (a decrease of ~45% in approximately 6 hr) at energies above 2 keV indicates that Chandra has probed within light-hours of the central black hole. Although we concur with Maloney & Reynolds that the direct continuum is not observed, this variability coupled with the 188 eV upper limit on the equivalent width of the Fe Kα emission line argues against the reflection-dominated model put forth by these authors based on their ASCA data. Instead, we favor a model in which a small, Compton-thick absorber blocks the direct X-rays, and only indirect, scattered X-rays from multiple lines of sight can reach the observer. Extended soft, thermal emission encompasses the optical extent of the galaxy and exhibits resolved structure. An off-nuclear X-ray source with a 0.35-8.0 keV luminosity of LX = 7 × 1039 ergs s-1, consistent with the ultraluminous X-ray sources in other nearby starbursts, is detected. We also present an unpublished Faint Object Spectrograph spectrum from the Hubble Space Telescope archive showing the broad C IV absorption.

From globular clusters to tidal dwarfs: Structure formation in the tidal tails of merging galaxies

Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic Toomre sequence mergers: NGC 4038/39 (Antennae), NGC 3256, NGC 3921, and NGC 7252 (Atoms for Peace) have been studied in order to explore how the star formation depends on the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2 V-I 0.9), particularly in its western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters along their tails. A significant cluster population is clearly associated with the prominent tidal dwarf candidates in the eastern and western tails of NGC 7252. The cluster-rich western tail of NGC 3256 is not distinguished from the others by its dynamical age or by its total H I mass. However, the mergers that have few clusters in the tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent tidal dwarfs. We speculate that star formation in tidal tails may manifest itself either in small structures like clusters along the tail or in large structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the highest star formation rate of the four mergers studied, which may contribute to the high number of star clusters in its tidal tails.

Hubble Space Telescope Images of Stephan’s Quintet: Star Cluster Formation in a Compact Group Environment* **

Analysis of Hubble Space Telescope Wide Field Planetary Camera 2 images of Stephans Quintet, Hickson Compact Group 92, yielded 115 candidate star clusters (with V (I \ 1.5). Unlike in merger remnants, the cluster candidates in Stephans Quintet are not clustered in the inner regions of the gal- axies; they are spread over the debris and surrounding area. Speci—cally, these sources are located in the long sweeping tail and spiral arms of NGC 7319, in the tidal debris of NGC 7318B/A, and in the intrag- roup starburst region north of these galaxies. Analysis of the colors of the clusters indicate several dis- tinct epochs of star formation that appear to trace the complex history of dynamical interactions in this compact group.

X-RAY ABSORPTION IN RADIO-QUIET QSOS

Major flows of ionized gas are thought to be present in the nuclei of most luminous QSOs, and absorption by these flows often has a significant effect upon the observed X-ray continuum from the black hole region. We briefly discuss X-ray studies of this gas and attempts to determine its geometry, dynamics, and ionization physics. Our focus is on X-ray warm absorber QSOs, Broad Absorption Line QSOs, and soft X-ray weak QSOs. We also discuss some prospects for further study with the next generation of X-ray observatories.