Joseph C. Shields

We present Space Telescope Imaging Spectrograph observations of 14 nearby low-luminosity active galactic nuclei, including 13 LINERs and 1 Seyfert, taken at multiple parallel slit positions centered on the galaxy nuclei and covering the Hα spectral region. For each galaxy, we measure the emission-line velocities, line widths, and strengths to map out the inner narrow-line region structure—typically within ~100 pc from the galaxy nucleus. There is a wide diversity among the velocity fields: in a few galaxies the gas is clearly in disk-like rotation, while in other galaxies the gas kinematics either appear chaotic or are dominated by radial flows with multiple velocity components. In most objects, the emission-line surface brightness distribution is very centrally peaked. The [S II] line ratio indicates a radial stratification in gas density, with a sharp increase within the inner 10-20 pc, in the majority of the Type 1 (broad-lined) objects. The electron-density gradients of the Type 1 objects exhibit a similar shape that is well fit by a power law of the form n _e = n ₀(r/1 pc)^α, where α = –0.60 ± 0.13. We examine how the [N II] λ6583 line width varies as a function of the aperture size over a range of spatial scales, extending from scales comparable to the black hole's sphere of influence to scales dominated by the host galaxy's bulge. For most galaxies in the sample, we find that the emission-line velocity dispersion is largest within the black hole's gravitational sphere of influence, and decreases with increasing aperture size toward values similar to the bulge stellar velocity dispersion measured within ground-based apertures. We construct models of gas disks in circular rotation and show that this behavior can be consistent with virial motion, although for some combinations of disk parameters we show that the line width can increase as a function of aperture size, as observed in NGC 3245. Future dynamical modeling to determine black hole masses for a few objects in this sample may be worthwhile, although disorganized motion will limit the accuracy of the mass measurements.

(Abridged) We present STIS observations of 14 nearby low-luminosity active galactic nuclei, including 13 LINERs and 1 Seyfert, taken at multiple parallel slit positions centered on the galaxy nuclei and covering the H-alpha spectral region. For each galaxy, we measure the emission-line velocities, line widths, and strengths, to map out the inner narrow-line region structure. There is a wide diversity among the velocity fields: in a few galaxies the gas is clearly in disk-like rotation, while in other galaxies the gas kinematics appear chaotic or are dominated by radial flows with multiple velocity components. The [S II] line ratio indicates a radial stratification in gas density, with a sharp increase within the inner 10-20 pc, in the majority of the Type 1 objects. We examine how the [N II] 6583 line width varies as a function of aperture size over a range of spatial scales, extending from scales comparable to the black hole's sphere of influence to scales dominated by the host galaxy's bulge. For most galaxies in the sample, we find that the emission-line velocity dispersion is largest within the black hole's gravitational sphere of influence, and decreases with increasing aperture size toward values similar to the bulge stellar velocity dispersion measured within ground-based apertures. Future dynamical modeling in order to determine black hole masses for a few objects in this sample may be worthwhile, although disorganized motion will limit the accuracy of the mass measurements.

The unification model for AGNs posits that Seyfert 2 galaxies are intrinsically like Seyfert 1 galaxies, but that their broad-line regions (BLRs) are hidden from our view. A Seyfert 2 nucleus that truly lacked a BLR, instead of simply having it hidden, would be a so-called true Seyfert 2. No object has as yet been conclusively proven to be one. We present a detailed analysis of four of the best true Seyfert 2 candidates discovered to date: IC 3639, NGC 3982, NGC 5283, and NGC 5427. None of the four have a broad Hα emission line, in either direct or polarized light. All four have rich, high-excitation spectra, blue continua, and HST images showing them to be unresolved sources with no host-galaxy obscuration. To check for possible obscuration on scales smaller than that resolvable by HST, we obtained X-ray observations using Chandra. All four objects show evidence of obscuration and therefore could have hidden BLRs. The picture that emerges is of moderate to high, but not necessarily Compton-thick, obscuration of the nucleus, with extranuclear soft emission extended on the hundreds of parsec scale that may originate in the narrow-line region. Since the extended soft emission compensates, in part, for the nuclear soft emission lost to absorption, both absorption and luminosity are likely to be severely underestimated unless the X-ray spectrum is of sufficient quality to distinguish the two components. This is of special concern where the source is too faint to produce a large number of counts, or where the source is too far away to resolve the extended soft X-ray-emitting region.

While studying extraplanar neutral hydrogen in the disk-halo transition of the inner Galaxy we have discovered what appears to be a huge superbubble centered around l ~ 30 deg, whose top extends to latitudes > 25 deg at a distance of about 7 kpc. It is detected in both HI and Halpha. Using the Green Bank Telescope of the NRAO, we have measured more than 220,000 HI spectra at 9' angular resolution in and around this structure. The total HI mass in the system is ~ 10^6 Msol and it has an equal mass in H+. The Plume of HI capping its top is 1.2 x 0.6 kpc in l and b and contains 3 x 10^4 Msol of HI. Despite its location, (the main section is 3.4 kpc above the Galactic plane) the kinematics of the Plume appears to be dominated by Galactic rotation, but with a lag of 27 km/s from corotation. At the base of this structure there are ``whiskers'' of HI several hundreds of pc wide, reaching more than 1 kpc into the halo; they have a vertical density structure suggesting that they are the bubble walls and have been created by sideways rather than upwards motion. They resemble the vertical dust lanes seen in NGC891. From a Kompaneets model of an expanding bubble, we estimate that the age of this system is ~ 30 Myr and its total energy content ~ 10^53 ergs. It may just now be at the stage where its expansion has ceased and the shell is beginning to undergo significant instabilities. This system offers an unprecedented opportunity to study a number of important phenomena at close range, including superbubble evolution, turbulence in an HI shell, and the magnitude of the ionizing flux above the Galactic disk.

Using archival Chandra observations of 19 LINERs, we explore the X-ray properties of their inner kiloparsec to determine the origin of their nuclear X-ray emission, to investigate the presence of an AGN, and to identify the power source of the optical emission lines. The relative numbers of LINER types in our sample are similar to those in optical spectroscopic surveys. We find that diffuse, thermal emission is very common and is concentrated within the central few hundred parsecs. The average spectra of the hot gas in spiral and elliptical galaxies are very similar to those of normal galaxies. They can be fitted with a thermal plasma (kT ~ 0.5 keV) plus a power-law (photon index of 1.3-1.5) model. There are on average three detected point sources in their inner kiloparsec with 10³⁷ ergs s^-1 < L_{0.5-10 keV} < 10⁴⁰ ergs s^-1. The average cumulative luminosity functions for sources in spiral and elliptical galaxies are identical to those of normal galaxies. In the innermost circle of 25 radius in each galaxy we find an AGN in 12 of the 19 galaxies. The AGNs contribute a median of 60% of the 0.5-10 keV luminosity of the central 25 region, and they have luminosities of 10³⁷-10³⁹ ergs s^-1 (Eddington ratios of 10^-8 to 10^-5). The ionizing luminosity of the AGNs is not enough to power the observed optical emission lines in this particular sample. Thus, we suggest that the lines are powered either by the mechanical interaction of an AGN jet (or wind) with the circumnuclear gas or by stellar processes, such as photoionization by post-AGB stars from an old or intermediate-age population (in most cases) or by young stars (in a few cases).

The Unification Model for active galactic nuclei posits that Seyfert 2s are intrinsically like Seyfert 1s, but that their broad-line regions (BLRs) are hidden from our view. A Seyfert 2 nucleus that truly lacked a BLR, instead of simply having it hidden, would be a so-called "true" Seyfert 2. No object has as yet been conclusively proven to be one. We present a detailed analysis of four of the best "true" Seyfert 2 candidates discovered to date: IC 3639, NGC 3982, NGC 5283, and NGC 5427. None of the four has a broad H-alpha emission line, either in direct or polarized light. All four have rich, high-excitation spectra, blue continua, and Hubble Space Telescope (HST) images showing them to be unresolved sources with no host-galaxy obscuration. To check for possible obscuration on scales smaller than that resolvable by HST, we obtained X-ray observations using the Chandra X-ray Observatory. All four objects show evidence of obscuration and therefore could have hidden BLRs. The picture that emerges is of moderate to high, but not necessarily Compton-thick, obscuration of the nucleus, with extra-nuclear soft emission extended on the hundreds-of-parsecs scale that may originate in the narrow-line region. Since the extended soft emission compensates, in part, for the nuclear soft emission lost to absorption, both absorption and luminosity are likely to be severely underestimated unless the X-ray spectrum is of sufficient quality to distinguish the two components. This is of special concern where the source is too faint to produce a large number of counts, or where the source is too far away to resolve the extended soft X-ray emitting region.

Using archival Chandra observations of 19 LINERs we explore the X-ray properties of their inner kiloparsec to determine the origin of their nuclear X-ray emission, to investigate the presence of an AGN, and to identify the power source of the optical emission lines. The relative numbers of LINER types in our sample are similar to those in optical spectroscopic surveys. We find that diffuse, thermal emission is very common and is concentrated within the central few hundred parsec. The average spectra of the hot gas in spirals and ellipticals are very similar to those of normal galaxies. They can be fitted with a thermal plasma (kT~0.5 keV) plus a power law (photon index of 1.3-1.5) model. There are on average 3 detected point sources in their inner kiloparsec with L(0.5-10 keV)~10^37-10^40 erg/s. The average cumulative luminosity functions for sources in spirals and ellipticals are identical to those of normal galaxies. In the innermost circle of 2.5" radius in each galaxy we find an AGN in 12 of the 19 galaxies. The AGNs contribute a median of 60% of the 0.5-10 keV luminosity of the central 2.5" region, they have luminosities of 10^37-10^39 erg/s (Eddington ratios 10^-8 to 10^-5). The ionizing luminosity of the AGNs is not enough to power the observed optical emission lines in this particular sample. Thus, we suggest that the lines are powered either by the mechanical interaction of an AGN jet (or wind) with the circumnuclear gas, or by stellar processes, e.g. photoionization by post-AGB stars or young stars.

(Abridged) We study the nuclear star clusters in spiral galaxies of various Hubble types using spectra obtained with STIS on-board HST. We observed the nuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imaging surveys. The spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. To infer the star formation history, metallicity and dust extinction, we fit weighted superpositions of single-age stellar population templates to the spectra. The luminosity-weighted age ranges from 10 Myrs to 10 Gyrs. The stellar populations of NCs are generally best fit as a mixture of populations of different ages. This indicates that NCs did not form in a single event, but instead they had additional star formation long after the oldest stars formed. On average, the sample clusters in late-type spirals have a younger luminosity-weighted mean age than those in early-type spirals (log(age/yr) = 8.37+/-0.25 vs. 9.23+/-0.21). The average cluster masses are smaller in late-type spirals than in early-type spirals (log(M/Msun) = 6.25+/-0.21 vs. 7.63+/-0.24), and exceed the masses typical of globular clusters. The cluster mass correlates strongly with both the Hubble type of the host galaxy and the luminosity of its bulge. The latter correlation has the same slope as the well-known correlation between supermassive black hole mass and bulge luminosity. The properties of both nuclear clusters and black holes are therefore intimately connected to the properties of the host galaxy.

We present light curves of the afterglow of GRB 050502A, including very early data at t - t_GRB < 60 s. The light curve is composed of unfiltered ROTSE-IIIb optical observations from 44 s to 6 hr postburst, R-band MDM observations from 1.6 to 8.4 hr postburst, and PAIRITEL JHK_s observations from 0.6 to 2.6 hr postburst. The optical light curve is fit by a broken power law, where t^α steepens from α = -1.13 ± 0.02 to -1.44 ± 0.02 at ~5700 s. This steepening is consistent with the evolution expected for the passage of the cooling frequency ν_c through the optical band. Even in our earliest observation at 44 s postburst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 σ with the observed spectral index β = -0.8 ± 0.1; the wind interpretation would imply a spectral index slightly (~2 σ) shallower than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of β. In this picture, the early optical decay, with the peak at or below 4.7 × 10¹⁴ Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball.

We present lightcurves of the afterglow of GRB050502A, including very early data at t-t_{GRB} < 60s. The lightcurve is composed of unfiltered ROTSE-IIIb optical observations from 44s to 6h post-burst, R-band MDM observations from 1.6 to 8.4h post-burst, and PAIRITEL J H K_s observations from 0.6 to 2.6h post-burst. The optical lightcurve is fit by a broken power law, where t^{alpha} steepens from alpha = -1.13 +- 0.02 to alpha = -1.44 +- 0.02 at \~5700s. This steepening is consistent with the evolution expected for the passage of the cooling frequency nu_c through the optical band. Even in our earliest observation at 44s post-burst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a Wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 sigma with the observed spectral index beta = -0.8 +- 0.1; the Wind interpretation would imply a slightly (~2 sigma) shallower spectral index than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of beta. In this picture, the early optical decay, with the peak at or below 4.7e14 Hz at 44s, requires very small electron and magnetic energy partitions from the fireball.