Anthony C. Danks

The On-Orbit Performance of the Space Telescope Imaging Spectrograph

The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.

HST STIS spectroscopy of the triple nucleus of M31: two nested disks in keplerian rotation around a supermassive black hole

We present Hubble Space Telescope (HST) spectroscopy of the nucleus of M31 obtained with the Space TelescopeImagingSpectrograph(STIS).SpectrathatincludetheCaiiinfraredtriplet(k ’ 85008)seeonlythered giant stars in the double brightness peaks P1 and P2. In contrast, spectra taken atk ’ 3600 51008 are sensitive to thetinybluenucleusembeddedinP2,thelowersurfacebrightnessnucleusofthegalaxy.P2 hasaK-typespectrum, but we find that the blue nucleus has an A-type spectrum: it shows strong Balmer absorption lines. Hence, the blue nucleus is blue not because of AGN light but rather because it is dominated by hot stars. We show that the spectrum is well described by A0 giant stars, A0 dwarf stars, or a 200 Myr old, single-burst stellar population. White dwarfs, in contrast, cannot fit the blue nucleus spectrum. Given the small likelihood for stellar collisions, recent star formation appears to be the most plausible origin of the blue nucleus. In stellar population, size, and velocity dispersion, the blue nucleus is so different from P1 and P2 that we call it P3 and refer to the nucleus of M31 as triple. Because P2 and P3 have very different spectra, we can make a clean decomposition of the red and blue stars and hence measure the light distribution and kinematics of eachuncontaminated by the other. The line-of-sight velocity distributions of the red stars near P2 strengthen the support for Tremaine’s eccentric disk model. Their wings indicate the presence of stars with velocities of up to 1000 km s � 1 on the anti-P1 side of P2. The kinematics of P3 are consistent with a circular stellar disk in Keplerian rotation around a supermassive black hole.If the P3 diskis perfectlythin,thentheinclination anglei ’ 55 � isidentical withinthe errorsto theinclination of the eccentric disk models for P1+P2 by Peiris & Tremaine and by Salow & Statler. Both disks rotate in the same sense and are almost coplanar. The observed velocity dispersion of P3 is largely caused by blurred rotation and has a maximum value of � ¼ 1183 � 201 km s � 1 . This is much larger than the dispersion � ’ 250 km s � 1 of the red stars along the same line of sight and is the largest integrated velocity dispersion observed in any galaxy. The rotation curve of P3 is symmetric around its center. It reaches an observed velocity of V ¼ 618 � 81 km s � 1 at radius 0B05 ¼ 0:19 pc, where the observed velocity dispersion is � ¼ 674 � 95 km s � 1 . The corresponding circular rotation velocity at this radius is � 1700 km s � 1 . We therefore confirm earlier suggestions that the central dark object

The Distance to the Vela Supernova Remnant

We have obtained high-resolution Ca II and Na I absorption-line spectra toward 68 OB stars in the direction of the Vela supernova remnant. The stars lie at distances of 190-2800 pc as determined by Hipparcos and spectroscopic parallax estimations. The presence of high-velocity absorption attributable to the remnant along some of the sight lines constrains the remnant distance to 250±30 pc. This distance is consistent with several recent investigations that suggest that the canonical remnant distance of 500 pc is too large.

A Pair of Compact Red Galaxies at Redshift 2.38, Immersed in a 100 Kiloparsec Scale Lyα Nebula*

We present Hubble Space Telescope (HST) and ground-based observations of a pair of galaxies at redshift 2.38, which are collectively known as 2142 4420 B1 (Francis et al. 1996). The two galaxies are both luminous extremely red objects (EROs), separated by 0.8 ′′ . They are embedded within a 100 kpc scale diffuse

Kinematics of the Nuclear Ionized Gas in the Radio Galaxy M84 (NGC 4374)

We present optical long-slit spectroscopy of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our spectra reveal that the nuclear gas disk seen in the Wide Field Planetary Camera 2 imaging by Bower et al. is rotating rapidly. The velocity curve has an S-shape with a peak amplitude of 400 km s−1 at 01 = 8 pc from the nucleus. To model the observed gas kinematics, we construct a thin Keplerian disk model that fits the data well if the rotation axis of the gas disk is aligned with the radio jet axis. These models indicate that the gasdynamics are driven by a nuclear compact mass of 1.5 × 109 M☉ with an uncertainty range of (0.9-2.6) × 109 M☉, and that the inclination of the disk with respect to the plane of the sky is 75°-85°. Of this nuclear mass, only ≤2 × 107 M☉ can possibly be attributed to luminous mass. Thus, we conclude that a dark compact mass (most likely a supermassive black hole) resides in the nucleus of M84.

Evidence of a Supermassive Black Hole in the Galaxy NGC 1023 from the Nuclear Stellar Dynamics

We analyze the nuclear stellar dynamics of the SB0 galaxy NGC 1023, utilizing observational data both from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and from the ground. The stellar kinematics measured from these long-slit spectra show rapid rotation (V ≈ 70 km s-1 at a distance of 01 = 4.9 pc from the nucleus) and increasing velocity dispersion toward the nucleus (where σ = 295 ± 30 km s-1). We model the observed stellar kinematics assuming an axisymmetric mass distribution with both two and three integrals of motion. Both modeling techniques point to the presence of a central dark compact mass (which presumably is a supermassive black hole) with confidence greater than 99%. The isotropic two-integral models yield a best-fitting black hole mass of (6.0 ± 1.4) × 107 M☉ and mass-to-light ratio (M/LV) of 5.38 ± 0.08, and the goodness of fit (χ2) is insensitive to reasonable values for the galaxys inclination. The three-integral models, which nonparametrically fit the observed line-of-sight velocity distribution as a function of position in the galaxy, suggest a black hole mass of (3.9 ± 0.4) × 107 M☉ and M/LV of 5.56 ± 0.02 (internal errors), and the edge-on models are vastly superior fits over models at other inclinations. The internal dynamics in NGC 1023 as suggested by our best-fit three-integral model shows that the velocity distribution function at the nucleus is tangentially anisotropic, suggesting the presence of a nuclear stellar disk. The nuclear line-of-sight velocity distribution has enhanced wings at velocities ≥600 km s-1 from systemic, suggesting that perhaps we have detected a group of stars very close to the central dark mass.

The Heavy-Element Enrichment of Lyα Clouds in the Virgo Supercluster*

Using high signal-to-noise ratio echelle spectra of 3C 273 obtained with the Space Telescope Imaging Spectrograph (resolution of 7 km s-1 FWHM), we constrain the metallicities of two Ly? clouds in the vicinity of the Virgo Cluster. We detect C II, Si II, and Si III absorption lines in the Ly? absorber at zabs = 0.00530. Previous observations with the Far Ultraviolet Spectroscopic Explorer have revealed Ly?-Ly? absorption lines at the same redshift, thereby accurately constraining the H I column density. We model the ionization of the gas and derive [C/H] = -1.2, [Si/C] = 0.2 ? 0.1, and log nH = -2.8 ? 0.3. The model implies a small absorber thickness, ~70 pc, and thermal pressure p/k ? 40 cm-3 K. It is most likely that the absorber is pressure confined by an external medium because gravitational confinement would require a very high ratio of dark matter to baryonic matter. Based on a sample of Milky Way sight lines in which carbon and silicon abundances have been reliably measured in the same interstellar cloud (including new measurements presented herein), we argue that it is unlikely that the overabundance of Si relative to C is due to depletion onto dust grains. Instead, this probably indicates that the gas has been predominately enriched by ejecta from Type II supernovae. Such enrichment is most plausibly provided by an unbound galactic wind, given the absence of known galaxies within a projected distance of 100 kpc and the presence of galaxies capable of driving a wind at larger distances (e.g., H I 1225+01). Such processes have been invoked to explain the observed abundances in the hot, X-ray-emitting gas in Virgo. However, the sight line to 3C 273 is more than 10? away from the X-ray emission region. We also constrain the metallicity and physical conditions of the Virgo absorber at zabs = 0.00337 in the spectrum of 3C 273 based on detections of O VI and H I and an upper limit on C IV. If this absorber is collisionally ionized, the O /C limit requires T 105.3 K in the O VI-bearing gas. For either collisional ionization or photoionization, we find that [O/H] -2.0 at zabs = 0.00337.

Space Telescope Imaging Spectrograph Echelle Observations of the Seyfert Galaxy NGC 4151: Physical Conditions in the Ultraviolet Absorbers*

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.

The Nuclear Dynamics of M32. I. Data and Stellar Kinematics

We have obtained optical long-slit spectroscopy of the nucleus of M32 using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The stellar rotation velocity and velocity dispersion, as well as the full line-of-sight velocity distribution (LOSVD), were determined as a function of position along the slit using two independent spectral deconvolution algorithms. We see three clear kinematical signatures of the nuclear black hole: a sudden upturn, at ~03 from the center, in the stellar velocity dispersions; a flat or rising rotation curve into the center; and strong, non-Gaussian wings on the central LOSVD. The central velocity dispersion is ~130 km s-1 (Gaussian fit) or 175 km s-1 (corrected for the wings). The central kinematics are consistent with the presence of a supermassive compact object in M32 with a mass in the range × 106 M☉.

The Absorption Spectrum of High-Density Stellar Ejecta in the Line of Sight to η Carinae

Using the high-dispersion near-UV (NUV) mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) to observe η Carinae, we have resolved and identified over 500 sharp circumstellar absorption lines of iron-group singly ionized and neutral elements with ≈20 velocity components ranging from -146 to -585 km s-1. These lines are from transitions originating from ground and metastable levels as high as 40,000 cm-1 above ground. The absorbing material is located either in dense inhomogeneities in the stellar wind, in the warm circumstellar gas immediately in the vicinity of η Car, or within the cooler foreground lobe of the Homunculus nebula. We have used classical curve-of-growth analysis to derive atomic level populations for Fe II at -146 km s-1 and for Ti II at -513 km s-1. These populations, plus photoionization and statistical equilibrium modeling, provide electron temperatures, Te, densities, nH, and constraints on distances from the stellar source, d. For the -146 km s-1 component, we derive Te = 6400 K, nH ≥ 107-108 cm-3, and d ≈ 1300 AU. For the -513 km s-1 component, we find a much cooler temperature, Te = 760 K, with nH ≥ 107 cm-3; we estimate d ≈ 10,000 AU. The large distances for these two components place the absorptions in the vicinity of identifiable ejecta from historical events, not near or in the dense wind of η Car. Further analysis, in parallel with obtaining improved experimental and theoretical atomic data, is underway to determine what physical mechanisms and elemental abundances could explain the large number of strong circumstellar absorption features in the spectrum of η Car.