D. Michael Crenshaw

Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope

We present a study of the intrinsic absorption lines in the ultraviolet spectra of Seyfert 1 galaxies. The study is based on spectra from the Hubble Space Telescope and includes the Seyfert 1 galaxies observed with the Faint Object Spectrograph and Goddard High-Resolution Spectrograph at spectral resolutions of λ/Δλ ≈1000-20,000 with good signal-to-noise ratios. We find that the fraction of Seyfert 1 galaxies that show intrinsic absorption associated with their active nuclei is more than one-half (10 of 17), which is much higher than previous estimates (3%-10%) based on IUE data. There is a one-to-one correspondence between Seyfert galaxies that show intrinsic UV absorption and X-ray warm absorbers, indicating that these two phenomena are related. Although our sample is not complete, we conclude that intrinsic absorption represents an important component that needs to be integrated into our overall physical picture of active galaxies. The intrinsic UV absorption is generally characterized by high ionization: C IV and N V are seen in all 10 Seyfert galaxies with detected absorption (in addition to Lyα), whereas Si IV is present in only four of these Seyfert galaxies, and Mg II absorption is detected only in NGC 4151. The absorption lines are blueshifted (or in a few cases at rest) with respect to the narrow emission lines, indicating that the absorbing gas is undergoing net radial outflow. At high resolution, the absorption often splits into distinct kinematic components that show a wide range in widths (20-400 km s-1 FWHM), indicating macroscopic motions (e.g., radial velocity subcomponents or turbulence) within a component. The strong absorption components have cores that are much deeper than the continuum flux levels, indicating that the regions responsible for these components lie completely outside of the broad emission-line regions. Additional information on the covering factors and column densities can be derived from the absorption profiles in the high-resolution spectra. The covering factor of the absorbing gas in the line of sight, relative to the total underlying emission, is Clos ≥ 0.86, on average. The global covering factor, which is the fraction of emission intercepted by the absorber averaged over all lines of sight, is Cglobal ≥ 0.5. Thus, structures covering large solid angles as seen by the central continuum source (e.g., spherical shells, sheets, or cones with large opening angles) are required. The individual absorptioncomponents show a wide range in C IV column densities (0.1-14 × 1014 cm-2), and the ratio of N V to C IV column density varies significantly from one absorption component to the next, even in the same Seyfert galaxy. Thus, the intrinsic absorption in a Seyfert 1 galaxy is typically comprised of distinct kinematic components that are characterized by a range in physical conditions (e.g., ionization parameter and hydrogen column density). Finally, we show evidence for extreme variability in the intrinsic absorption lines of NGC 3783. In addition to our earlier report of the appearance of a C IV absorption doublet at -560 km s-1 (relative to the emission lines) over 11 months, we have detected the appearance of another C IV doublet at -1420 km s-1 over 15 months. On the other hand, the C IV absorption lines of NGC 3516 and NGC 4151 were very stable over periods of 6 months and 4 years, respectively. Monitoring observations of individual Seyfert galaxies at higher time resolution are needed to distinguish between different sources of variability (variable ionization, motion of gas across the line of sight) and to determine the densities and radial locations of the absorption components.

High‐Resolution X‐Ray and Ultraviolet Spectroscopy of the Complex Intrinsic Absorption in NGC 4051 with Chandra and the Hubble Space Telescope

We present the results from simultaneous observations of the narrow-line Seyfert 1 galaxy NGC 4051 with the Chandra High Energy Transmission Grating Spectrometer and the Hubble Space Telescope Space Telescope Imaging Spectrograph. The X-ray grating spectrum reveals absorption and emission lines from hydrogen-like and helium-like ions of O, Ne, Mg, and Si. We resolve two distinct X-ray absorption systems: a high-velocity blueshifted system at -2340 ± 130 km s-1 and a low-velocity blueshifted system at -600 ± 130 km s-1. In the UV spectrum we detect strong absorption, mainly from C IV, N V, and Si IV, that is resolved into as many as nine different intrinsic absorption systems with velocities between -650 and 30 km s-1. Although the low-velocity X-ray absorption is consistent in velocity with many of the UV absorption systems, the high-velocity X-ray absorption seems to have no UV counterpart. In addition to the absorption and emission lines, we also observe rapid X-ray variability and a state of low X-ray flux during the last ≈15 ks of the observation. NGC 4051 has a soft X-ray excess that we fit in both the high and low X-ray flux states. The high-resolution X-ray spectrum directly reveals that the soft excess is not composed of narrow emission lines and that it has significant spectral curvature. A power-law model fails to fit it, while a blackbody produces a nearly acceptable fit. We compare the observed spectral variability with the results of previous studies of NGC 4051.

HST STIS Observations of PG 0946+301: The Highest Quality UV Spectrum of a BALQSO

We describe deep (40 orbits) Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph observations of the broad absorption line (BAL) quasi-stellar object (QSO) PG 0946+301 and make them available to the community. These observations are the major part of a multiwavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEAs) in BALQSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflows metallicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least 10 times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metallicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses and present the significant temporal changes that are detected between these spectra and those taken by the HST Faint Object Spectrograph in 1992.

Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. III. Physical Conditions in the Emission-Line Gas*

The physical conditions in the inner narrow-line region (NLR) of the Seyfert 2 galaxy NGC 1068 are examined using ultraviolet and optical spectra and photoionization models. The spectra were taken with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS), through the 01 ? 520 slit, covering the full STIS 1200-10000 ? waveband. The slit was centered on a position 014 north of the optical continuum peak (or hot spot) at a position angle of 202?, bisecting the brighter part of the biconical emission-line region. We have measured the emission-line fluxes for a region extending 38 northeast (~270 pc) to 18 southwest (~130 pc) of this point. The emission lines on each side show evidence of two principal kinematic components, one blueshifted with respect to the systemic velocity and the other redshifted (the kinematics were discussed in a separate paper). Based on the photoionization modeling results, we find that the physical conditions vary among these four quadrants. (1) The emission-line gas in the blueshifted northeast quadrant is photoionized by the hidden central source out to ~100 pc, at which point we find evidence of another source of ionizing radiation, which may be due to fast (~1000 km s-1) shocks resulting from the interaction of the emission-line knots and the interstellar medium. Interestingly, this occurs at approximately the location where the knots begin to show signs of deceleration. (2) The gas in the redshifted northeast quadrant is photoionized by continuum radiation that has been heavily absorbed by gas within ~30 pc of the central source. We find no strong evidence of the effects of shocks in this component. (3) The redshifted emission-line gas in the southwest quadrant is photoionized by unabsorbed continuum from the central source, similar to that in the inner ~100 pc of the blueshifted northeast quadrant. Finally, (4) the emission-line spectrum of the blueshifted southwest quadrant appears to be the superposition of highly ionized, tenuous component within the ionization cone and gas outside the cone, the latter photoionized by scattered continuum radiation. There are several implications of this complicated physical scenario. First, the hidden active nucleus is the dominant source of ionizing radiation in the inner NLR. The absorption of continuum radiation along the line of sight to the redshifted northeast quadrant may result from the intersection of the ionization cone and the plane of the host galaxy. Finally, the evidence for shock-induced continuum radiation at the point where the emission-line knots begin to decelerate indicates that the deceleration is due to the interaction of emission-line knots with slower moving gas, such as the interstellar medium of NGC 1068.

Intrinsic Absorption Lines in the Seyfert 1 Galaxy NGC 5548: Ultraviolet Echelle Spectra from the Space Telescope Imaging Spectrograph*

We present the first observations of a Seyfert galaxy with the echelle gratings on the Space Telescope Imaging Spectrograph (STIS), which provide high-resolution (λ/Δλ ≈ 40,000) coverage of the intrinsic UV absorption lines in NGC 5548. We confirm the presence of five kinematic components of absorption in Lyα, C IV, and N V at radial velocities of -160 to -1060 km s-1 with respect to the emission lines, and find an additional Lyα component near the systemic velocity, which probably arises in the interstellar medium of the host galaxy. Compared to Goddard High-Resolution Spectrograph spectra of the N V and C IV absorption obtained ~2 yr earlier, the kinematic components have not changed in radial velocity, but the ionic column densities for two components have decreased. We attribute these variations to changes in the total column of gas, but for one component we cannot rule out changes in the ionization of the gas. We have calculated photoionization models to match the UV column densities from each of the five components associated with the nucleus. In four of the components, the ionization parameters (U = 0.15-0.80) and effective hydrogen column densities (Neff = 6.0 × 1018 to 2.8 × 1020 cm-2) cannot produce the O VII and O VIII absorption edges seen in the X-ray-warm absorber. The remaining component is more highly ionized (U = 2.4, Neff = 6.5 × 1021 cm-2), and our model matches the previously observed X-ray absorption columns. This component is therefore likely to be responsible for the X-ray-warm absorber. It also has the highest outflow velocity and showed the largest variations in column density.

Integrated Ultraviolet Spectra and Line Indices of M31 Globular Clusters and the Cores of Elliptical Galaxies

We present observations of the integrated light of four M31 globular clusters (MIV, MII, K280, and K58) and of the cores of six elliptical galaxies (NGC 3605, 3608, 5018, 5831, 6127, and 7619) made with the Faint Object Spectrograph on the Hubble Space Telescope. The spectra cover the range 2200–4800 A at a resolution of 8 A with signal-to-noise ratio of more than 20 and flux accuracy of ~5%. To these data we add from the literature IUE observations of the dwarf elliptical galaxy M32, Galactic globular clusters, and Galactic stars. The stellar populations in these systems are analyzed with the aid of mid-UV and near-UV colors and absorption line strengths. Included in the measured indices is the key NH feature at 3360 A. We compare these line index measures with the 2600 - 3000 colors of these stars and stellar populations. We find that the M31 globular clusters, Galactic globular clusters/Galactic stars, and elliptical galaxies represent three distinct stellar populations, based on their behavior in color–line strength correlations involving Mg II, NH, CN, and several UV metallic blends. In particular, the M31 globular cluster MIV, as metal-poor as the Galactic globular M92, shows a strong NH 3360 A feature. Other line indices, including the 3096 A blend that is dominated by lines of Mg I and Al I, show intrinsic differences as well. We also find that the broadband line indices often employed to measure stellar population differences in faint objects, such as the 4000 A and the Mg 2800 breaks, are disappointingly insensitive to these stellar population differences. We find that the hot (T > 20,000 K) stellar component responsible for the UV upturn at shorter wavelengths can have an important influence on the mid-UV spectral range (2400–3200 A) as well. The hot component can contribute over 50% of the flux at 2600 A in some cases and affects both continuum colors and line strengths. Mid-UV spectra of galaxies must be corrected for this effect before they can be used as age and abundance diagnostics. Of the three stellar populations studied here, M31 globular clusters and elliptical galaxies are more similar to each other than either is to the Galactic stellar populations defined by globular clusters and nearby stars. Similarities between the abundance-pattern differences currently identified among these stellar populations and those among globular cluster stars (N, Al enhancements) present a curious coincidence that deserves future investigation.

Space Telescope Imaging Spectrograph Near-Ultraviolet Time-tagged Spectra of the Crab Pulsar

We present the spectrum and the pulse profile of the Crab Pulsar in the near-ultraviolet (1600-3200 A) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional near-ultraviolet Multianode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2×2 aperture and the low-dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse period is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 A dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55±0.05 leads to a good fit to a power law with slope αν = -0.3±0.2. The spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The four pulse profiles produced by breaking the spectrum into 400 A bins show the pulse profile to be stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption-line equivalent widths of Mg I, Mg II, and Fe II are lower than expected based on the implied H I column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.