D. Michael Crenshaw

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 HST 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 and a low-velocity blueshifted system at -600+/-130 km/s. 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 km/s and 30 km/s. 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 which 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.

We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3783, obtained with the medium resolution (λ/Δλ ≈ 40,000) echelle gratings of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). The spectra reveal the presence of three kinematic components of absorption in Lyα, C IV, and N V, at radial velocities of -1365, -548, and -724 km s^-1 with respect to the systemic velocity of the host galaxy (components 1, 2 and 3, respectively); component 1 also shows absorption by Si IV. Component 3 was not detected in any of the earlier Goddard High Resolution Spectrograph (GHRS) spectra, and the C IV absorption in the other components has changed since the most recent GHRS observation obtained ~5 yr earlier. Somewhat unexpectedly, each component has a covering factor (of the continuum source+broad emission line region) of ~0.6. We have calculated photoionization models to match the UV column densities in each of the three components. The models predict a zone characterized by high-ionization parameter (U = 0.65-0.80) and column density (6.4 × 10²⁰-1.5 × 10²¹ cm^-2) for each component, and a second, low-ionization (U = 0.0018) and low column density (4.9 × 10¹⁸ cm^-2) zone for component 1. Based on the model results, there should be strong absorption in the bandpass of the Far Ultraviolet Spectroscopic Explorer (FUSE), 912-1200 Å, including saturated O VI lines at each component velocity. The models also predict large O VII and O VIII column densities, but suggest that the UV absorbers cannot account for all of the X-ray absorption detected in recent Chandra spectra. Finally, there is no evidence for a correlation between the characteristics of the UV absorbers and the UV continuum flux, and, by inference, the ionizing continuum. Hence, we suggest that the variations observed in the GHRS and STIS spectra are due in a large part to changes in the column densities of the absorbers as the result of transverse motion.

We have obtained low-dispersion spectra of NGC 3227 with the Space Telescope Imaging Spectrograph (STIS) to study the intrinsic UV absorption and the reddening of the nucleus in this Seyfert 1 galaxy. The UV spectra show a wealth of absorption lines at the systemic redshift that span a wide range in ionization state (Mg I to N V). The equivalent widths of the lines are consistent with our earlier prediction that a ``lukewarm absorber'' (T= 18,000 K at the ionized face) with a substantial column of gas [N(H) = 2 x 10^21 cm^-2] is present and likely responsible for the reddening of the nucleus. The lukewarm absorber is also responsible for most of the absorption in the X-rays at energies less than 1 keV, although a more highly ionized ``warm absorber'' is needed to account for the O VII and O VIII ionization edges. In addition, we require a small column [N(H) = 5 x 10^19 cm^-2] of cold gas to match the strengths of the neutral and singly-ionized lines in the UV spectra. NGC 3227 is the first Seyfert galaxy in which a strong link between the reddening and intrinsic UV absorption has been found. By comparing our STIS UV and optical spectra with those of the unreddened Seyfert NGC 4151, we have determined a reddening curve for the nuclear continuum source in NGC 3227 over the 1150 - 10,200 A range. The reddening curve does not show a 2200 AA bump, and is steeper in the UV than reddening curves derived for the Galaxy, LMC, and SMC, suggesting a preponderence of small dust grains near the nucleus.Comment: 23 pages, 4 figures. LaTeX with encapsulated postscript figures. Accepted for publication in the Astrophysical Journa

Active galactic nuclei are powered by accretion onto supermassive black holes. In his Perspective, Crenshaw discusses recent insights into the mass outflow from these nuclei, which provide information about the dynamical forces near an active supermassive black hole.

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 HST 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 and a low-velocity blueshifted system at -600+/-130 km/s. 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 km/s and 30 km/s. 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 which 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.

We have examined the physical conditions in intrinsic UV-absorbing gas in the Seyfert galaxy NGC 4151, using echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope on 1999 July 19. We confirm the presence of the kinematic components detected in earlier Goddard High Resolution Spectrograph (GHRS) observations, all of which appear to be outflowing from the nucleus, as well as a new broad absorption feature at a radial velocity of -1680 km s^-1. The UV continuum of NGC 4151 was a factor of about 4 lower than in observations taken over the previous 2 yr, and we argue that the changes in the column density of the low-ionization absorption lines associated with the broad component at -490 km s^-1 reflect the decrease in the ionizing flux. Most of the strong absorption lines (e.g., N V, C IV, Si IV, etc.) from this component are saturated but show substantial residual flux in their cores, indicating that the absorber does not fully cover the source of emission. Our interpretation is that the unocculted light is due to scattering by free electrons from an extended region, which reflects continuum, emission lines, and absorption lines. For the first time in such a study, we have been able to constrain the densities for this kinematic component and several others based on the strength of absorption lines from metastable states of C III and Fe II and/or the ratios of ground and fine structure lines of O I, C II, and Si II. We have generated a set of photoionization models that successfully match not only the ionic column densities for each component during the present low-flux state but also those seen in previous high-flux states with the GHRS and STIS, confirming that the absorbers are photoionized and respond to the changes in the continuum flux. Based on the model parameters (ionization parameter and density), we have been able to map the relative radial positions of the absorbers. We find that the absorbing gas decreases in density with distance. Finally, none of the UV absorbers is of sufficiently large column density or high enough ionization state to account for the observed X-ray absorption, while the scatterer is too highly ionized. Hence, the X-ray absorption must arise in a separate component of circumnuclear gas.

We present observations of the UV absorption lines in the Seyfert 1 galaxy NGC 3783, obtained with the medium resolution echelle gratings of the Space Telescope Imaging Spectrograph(STIS) on the Hubble Space Telescope (HST). The spectra reveal the presence of three kinematic components of absorption in Ly-alpha, C IV, and N V, at radial velocities of -1365, -548, and -724 km/s with respect to the systemic velocity of the host galaxy (Components 1, 2 and 3, respectively); Component 1 also shows absorption by Si IV. Component 3 was not detected in any of the earlier Goddard High Resolution Spectrograph (GHRS) spectra, and the C IV absorption in the other components has changed since the most recent GHRS observation obtained 5 yr earlier. Somewhat unexpectedly, each component has a covering factor (of the continuum source + broad emission line region) of about 0.6. We have calculated photoionization models to match the UV column densities in each of the three components. Based on the model results, there should be strong absorption in the bandpass of the Far Ultraviolet Spectroscopic Explorer (FUSE), 912 - 1200 A, including saturated O VI lines at each component velocity. The models also predict large O VII and O VIII column densities but suggest that the UV absorbers cannot account for all of the X-ray absorption detected in recent Chandra spectra. Finally, there is no evidence for a correlation between the characteristics of the UV absorbers and the UV continuum flux, which suggests that the variations observed in the GHRS and STIS spectra are due in large part to changes in the column densities of the absorbers as the result of transverse motion.

We have obtained low-dispersion spectra of NGC 3227 with the Space Telescope Imaging Spectrograph (STIS) to study the intrinsic UV absorption and the reddening of the nucleus in this Seyfert 1 galaxy. The UV spectra show a wealth of absorption lines at the systemic redshift that span a wide range in ionization state (Mg I to N V). The equivalent widths of the lines are consistent with our earlier prediction that a ``lukewarm absorber'' (T= 18,000 K at the ionized face) with a substantial column of gas [N(H) = 2 x 10^21 cm^-2] is present and likely responsible for the reddening of the nucleus. The lukewarm absorber is also responsible for most of the absorption in the X-rays at energies less than 1 keV, although a more highly ionized ``warm absorber'' is needed to account for the O VII and O VIII ionization edges. In addition, we require a small column [N(H) = 5 x 10^19 cm^-2] of cold gas to match the strengths of the neutral and singly-ionized lines in the UV spectra. NGC 3227 is the first Seyfert galaxy in which a strong link between the reddening and intrinsic UV absorption has been found. By comparing our STIS UV and optical spectra with those of the unreddened Seyfert NGC 4151, we have determined a reddening curve for the nuclear continuum source in NGC 3227 over the 1150 - 10,200 A range. The reddening curve does not show a 2200 AA bump, and is steeper in the UV than reddening curves derived for the Galaxy, LMC, and SMC, suggesting a preponderence of small dust grains near the nucleus.

We have examined the physical conditions in intrinsic UV-absorbing gas in the Seyfert galaxy NGC 4151, using echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS). We confirm the presence of the kinematic components detected in earlier GHRS observations as well as a new broad absorption feature at a radial velocity of -1680 km/s. The UV continuum of NGC 4151 decreased by a factor of 4 over the previous two years, and we argue the changes in the column density of the low ionization absorption lines associated with the broad component at -490 km/s reflect the decrease in the ionizing flux. Most of the strong absorption lines (e.g., N V, C IV, Si IV) from this component are saturated, but show substantial residual flux in their cores, indicating that the absorber does not fully cover the source of emission. Our interpretation is that the unocculted light is due to scattering by free electrons from an extended region, which reflects continuum, emission lines, and absorption lines. We have been able to constrain the densities for the kinematic components based on absorption lines from metastable states of C III and Fe II, and/or the ratios of ground and fine structure lines of O I,C II, and Si II. We have generated a set of photoionization models which match the ionic column densities for each component during the present low flux state and those seen in previous high flux states with the GHRS and STIS, confirming that the absorbers are photoionized and respond to the changes in the continuum flux. We have been able to map the relative radial positions of the absorbers, and find that the gas decreases in density with distance. None of the UV absorbers is of sufficiently large column density or high enough ionization state to account for the X-ray absorption.

We present echelle observations of the intrinsic UV absorption lines in the Seyfert galaxy NGC 4151; these observations were obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope on 1999 July 19. The UV-continuum flux at 1450 Å decreased by factor of about 4 over the previous 2 years, and there was a corresponding dramatic increase in the column densities of the low-ionization absorption lines (e.g., Si II, Fe II, and Al II), presumably as a result of a decrease in the ionizing continuum. In addition to the absorption lines seen in previous low states, we identify a large number of Fe II absorption lines that arise from metastable levels as high as 4.1 eV above the ground state, indicating high densities (>10⁶ cm^-3). We find that the transient absorption feature in the blue wing of the broad C IV emission, seen in a Goddard High Resolution Spectrograph spectrum and thought to be a high-velocity C IV component, is actually a Si II fine-structure absorption line at a radial velocity of -560 km s^-1 (relative to systemic). We also demonstrate that the "satellite" emission lines of C IV found in IUE spectra are actually regions of unabsorbed continuum plus broad emission that become prominent when the UV continuum of NGC 4151 is in a low state.