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The Hubble Deep Field (HDF) gives us an unprecedented view of our universe and an opportunity to study a wide range of questions in galaxy evolution and cosmology. Here we focus on the search for unresolved faint quasars and active galactic nuclei (AGNs) in the crude combine images using a multicolor imaging analysis that has proved very successful in recent years. Quasar selection was carried out both in multicolor space and in "profile space." The latter is the multiparameter space formed by the radial profiles of the objects in the different images. By combining the dither frames available for each filter, we were able to obtain well-sampled radial profiles of the objects and measure their deviation from that of a stellar source. This procedure efficiently helps to overcome the problems caused by the WPFC2 undersampling. Furthermore, to identify areas of multicolor space where quasars might be expected, we generated synthetic quasar spectra in the range 1.0 < z < 5.5, including effects of intrinsic emission lines and absorption by Lyα forest and Lyman limit systems, and computed expected quasar colors. We also developed routines to determine the completeness of our data to point sources in the observed filters. The data are 90% complete at 26.2, 28.0, 27.8, 26.8 mag in the F300W, F450W, F606W, and F814W filters, respectively. We find 41 compact objects in the HDF: one pointlike object with colors consistent with quasars but inconsistent with that of ordinary stars; seven pointlike objects with colors consistent with quasars or stars; 18 stars; and 15 slightly resolved objects, 12 of which have colors consistent with quasars or stars. The latter objects could be resolved AGNs or regions of intense star formation. We estimate the upper limit of unresolved and slightly resolved quasars/AGNs with V₆₀₆ < 27.0 and z < 3.5 to be 20 objects (16,200 deg^-2). We independently recovered a resolved, star-forming galaxy with a spectroscopically confirmed redshift of 3.368 and five spectroscopically confirmed stars. All provide confirmation for the validity of our color and morphological modeling. We make a detailed comparison with previous studies of the HDF. We find good agreement among different authors on the number of stars in the HDF and the lack of quasar candidates with z > 3.5. We find more quasar candidates than previous work because of our more extensive modeling and use of the color information. Spectroscopic observations of the candidates are needed to determine which, if any, are quasars or AGNs.

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). The lens B2045+265 is a four-image system with a maximum separation of 19. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggest that it is the radio core of the lensing galaxy. This implies that the B2045+265 lens system consists of a flat-spectrum radio source that is being lensed by another flat-spectrum radio source. Infrared images taken with the Hubble Space Telescope and the Keck I Telescope detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with those seen at radio wavelengths. The lensing galaxy has magnitudes of J = 19.2, m_F160W = 18.8, and K = 17.6 mag in a 19 diameter aperture, which corresponds to the size of the Einstein ring of the lens. Spectra of the system taken with the Keck I Telescope reveal a lens redshift of z_l = 0.8673 and a source redshift of z_s = 1.28. The lens spectrum is typical of an Sa galaxy. The image splitting and system redshifts imply that the projected mass inside the Einstein radius of the lensing galaxy is M_E = 4.7 × 10¹¹h^-1M_⊙. An estimate of the light emitted inside the Einstein radius from the K magnitude gives a mass-to-light ratio in the rest-frame B band of (M/L_B)_E = 20 h (M/L_B)_⊙. Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation velocity for the lensing galaxy is 2–3 times higher than what is seen in nearby spiral galaxies. The large projected mass inside the Einstein ring radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies associated with the primary lensing galaxy; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H₀.

We present imaging polarimetry of the extremely luminous, redshift 2.3 IRAS source FSC 10214+4724. The observations were obtained with HST's Faint Object Camera in the F437M filter, which is free of strong emission lines. The 07 long arc is unresolved to 004 FWHM in the transverse direction and has an integrated polarization of 28% ± 3%, in good agreement with ground-based observations. The polarization position angle varies along the arc by up to 35°. The overall position angle is 62° ± 3° east of north. No counterimage is detected to B = 27.5 mag (3 σ), giving an observed arc to counterimage flux ratio greater than 250, considerably greater than the flux ratio of 100 measured previously in the I band. This implies that the configuration of the object in the source plane at the B band is different from that at I band and/or that the lensing galaxy is dusty.

We present a study of the trends in luminosity, linear size, spectral index, and redshift of classical double radio sources, from three complete samples selected at successively fainter low radio-frequency flux limits. We have been able to decouple the effects of the tight correlation between redshift and luminosity (inherent in any single flux-limited sample) which have hitherto hindered interpretation of the relationships between these four source properties. The major trends found are that (i) spectral indices increase with linear size, (ii) rest-frame spectral indices have a stronger dependence on luminosity than on redshift except at high (GHz) frequencies, and (iii) the linear sizes are smaller at higher redshifts. We reproduce the observed dependences in a model for radio sources (born throughout cosmic time according to a radio-source birth function) whose lobes are fed with a synchrotron-emitting population from compact hotspots, and which suffer inverse Compton, synchrotron, and adiabatic expansion losses. The magnetic energy density within each hotspot is proportional to the jet power, and synchrotron losses suffered in the hotspot mean that the energy spectrum of the emitting particles fed to the lobes is governed by the jet power. The axial ratios of radio sources in our model increase as the sources age, and axial ratios are higher in sources with higher jet power. In simulating the basic observed dependences, we find that there is no need to invoke any systematic change in the environments of these objects with redshift if the consequences of imposing a survey flux limit on our simulated data sets are properly included in the model. It is also necessary to include appropriate energy loss mechanisms (such as the effects of the cosmic microwave background and feeding the lobes from a compact hotspot), which cause decreasing luminosity through the life of a source. Although our study has broken the luminosity-redshift degeneracy, we present evidence that for such studies there is an unavoidable "youth-redshift degeneracy," even though radio sources are short-lived relative to the age of the universe; it is imperative to take this into account in studies that seemingly reveal correlations of source properties with redshift such as the "alignment effect."

We present 1210 Johnson/Cousins B, V, R, and I photometric observations of 22 recent Type Ia supernovae (SNe Ia): SNe 1993ac, 1993ae, 1994M, 1994S, 1994T, 1994Q, 1994ae, 1995D, 1995E, 1995al, 1995ac, 1995ak, 1995bd, 1996C, 1996X, 1996Z, 1996ab, 1996ai, 1996bk, 1996bl, 1996bo, and 1996bv. Most of the photometry was obtained at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics in a cooperative observing plan aimed at improving the database for SNe Ia. The redshifts of the sample range from cz = 1200 to 37,000 km s^-1 with a mean of cz = 7000 km s^-1.

A low-dispersion Keck I spectrum of SN 1980K taken in 1995 August (t = 14.8 yr after explosion) and a spectrum taken in 1997 November (t = 17.0 yr) at the MDM Observatory show broad 5500 km s^-1 emission lines of Hα, [O I] 6300, 6364 Å, and [O II] 7319, 7330 Å. Weaker but similarly broad lines detected include [Fe II] 7155 Å, [S II] 4068, 4072 Å, and a blend of [Fe II] lines at 5050–5400 Å. The presence of strong [S II] 4068, 4072 Å emission but a lack of [S II] 6716, 6731 Å emission suggests electron densities of 10⁵–10⁶ cm^-3. From the 1997 spectrum, we estimate an Hα flux of (1.3 ± 0.2) × 10^-15 ergs cm^-2 s^-1, indicating a 25% decline from the 1987–1992 levels during the period 1994 to 1997, possibly related to a reported decrease in its nonthermal radio emission. A 1993 May, Multiple Mirror Telescope spectrum of SN 1979C (t = 14.0 yr) shows a somewhat different spectrum from that of SN 1980K. Broad, 6000 km s^-1 emission lines are also seen but with weaker Hα, stronger [O III] 4959, 5007 Å, more highly clumped [O I] and [O II] line profiles, no detectable [Fe II] 7155 Å emission, and a faint but very broad emission feature near 5750 Å. A 1997 Hubble Space Telescope Faint Object Spectrograph, near-UV spectrum (2200–4500 Å) shows strong lines of C II] 2324, 2325 Å, [O II] 2470 Å, and Mg II 2796, 2803 Å, along with weak [Ne III] 3969 Å, [S II] 4068, 4072 Å, and [O III] 4363 Å emissions. The UV emission lines show a double-peak profile with the blueward peak substantially stronger than the red, suggesting dust extinction within the expanding ejecta [E(B-V) = 0.11–0.16 mag]. The lack of detectable [O II] 3726, 3729 Å emission, together with [O III] λλ(4959 + 5007)/λ4363 ≃ 4, implies electron densities 10⁶–10⁷ cm^-3. These Type II linear supernovae (SNe II-L) spectra show general agreement with the lines expected in a circumstellar interaction model, but the specific models that are available show several differences with the observations. High electron densities (10⁵–10⁷ cm^-3) result in stronger collisional de-excitation than assumed in the models, thereby explaining the absence of several moderate to strong predicted lines such as [O II] 3726, 3729 Å, [N II] 6548, 6583 Å, and [S II] 6716, 6731 Å. Interaction models are needed that are specifically suited to these supernovae. We review the overall observed range of late-time SNe II-L properties and briefly discuss their properties relative to young, ejecta-dominated Galactic supernova remnants.

We present spectroscopic and photometric results of SN 1996cb. The supernova was independently discovered in NGC 3510 by M. Aoki, T. Cho, & K. Toyama of Japan and Qiao et al. of Beijing Astronomical Observatory on 1996 December 15 and 18, respectively. The results cover about 6 months following the discovery. The first few spectra showed strong Balmer lines with obvious P Cygni profiles, offering evidence of a Type II supernova. The emergence of He I lines could be inferred in these spectra. That the He I lines became quite prominent in the spectra near optical maximum confirmed that SN 1996cb was definitely a Type IIb supernova, like SN 1987K and SN 1993J.

The photometric results showed that the B-I color evolution was very similar to that of SN 1993J. Comparing two color curves, we were able to estimate that the explosion of SN 1996cb occurred on UT 1996 December 12. Although the overall light curves resembled that of SN 1993J, they showed some differences, especially for the B band. SN 1996cb had a broad peak in the light curves, and it declined somewhat slowly and, compared with SN 1993J, exhibited a plateau-like shape between 20 and 50 days after the maximum for the B and V bands. This indicates that there was relatively more hydrogen in the outer envelope of the progenitor of SN 1996cb.

The spectral evolution of SN 1996cb displayed further differences from SN 1993J. In the case of SN 1996cb, the Balmer lines showed strong P Cygni profiles at a very early time, resembling the early spectra of SN 1987A, a supernova resulting from a compact blue supergiant star. The dramatic changes of expansion velocities at early times indicated that the photosphere of SN 1996cb receded more quickly than it did in SN 1993J. The He I lines emerged much earlier and evolved more dramatically, causing SN 1996cb to display the features of a Type Ib supernova before maximum. This might be the result of a dramatic recession of the photosphere at an early time; the He I lines and [O I] lines showed conspicuous blueshifts when they emerged, resembling the blueshifts of the [O I] lines that appeared in the late spectra of SN 1993J. This is probably observational evidence of Rayleigh-Taylor instabilities occurring at the interfaces between the H and He and the He and O+C layers, respectively; Hα emission and absorption components, especially the latter, were conspicuous 100 days after the explosion. We also conclude that the outer envelope of SN 1996cb had relatively more hydrogen than was the case for SN 1993J, even though the amount remained much less than is typical of other Type II supernovae. This finding is consistent with the results of the photometry. The [O I] and O lines emerged very late and exhibited weak emission, indicating that the He-rich layer was relatively thick. Combining the analyses of photometric and spectroscopic evolution, we conclude that the progenitor of SN 1996cb, like that of SN 1993J, was a stripped massive star exhibiting some special features: it was probably a more compact star with a thick helium layer and a relatively more massive hydrogen envelope.

Hubble Space Telescope (HST) observations show that the surface brightness profiles of early-type galaxies have central cusps. I show that the observed characteristics of these cusps are consistent with the hypothesis that (1) all early-type galaxies have central black holes that grew adiabatically in homogeneous isothermal cores and (2) these "progenitor" cores followed scaling relations similar to those of the fundamental plane.

The models studied here are the ones first proposed by P. Young. They predict I ∝ r^-1/2 at asymptotically small radii, but I ∝ r^-γ at the radii observable with HST. The slope γ can take on all observed values and increases monotonically with μ = M_•/M_core. Both "core" profiles (which have a break at a resolved radius and a shallow slope inside that radius) and "power-law" profiles (which have a steep slope down to the resolution limit and no clear break) can be reproduced.

Observations show that, with few exceptions, galaxies with M_V < -22 have core profiles, and galaxies with M_V > -20.5 have power-law profiles; both profile types occur in galaxies with -22 < M_V < -20.5. For the models, the scaling relations for early-type galaxies imply that the progenitor core mass scales with luminosity as M_core ∝ L^1.5. If, as suggested by various arguments, the black hole (BH) mass M_• scales roughly linearly with luminosity, M_• ∝ L, then μ ∝ L^-0.5. This yields larger cusp slopes in lower luminosity galaxies. Models with BH masses and progenitor cores that obey established scaling relations predict (at the distance of the Virgo Cluster) that galaxies with M_V < -21.2 have core profiles and galaxies with M_V > -21.2 have power-law profiles. This reproduces both the sense and the absolute magnitude of the observed transition. Intrinsic scatter in BH and galaxy properties can explain why both types of galaxies are observed around the transition magnitude. The observed bimodality in cusp slopes may be due to a bimodality in M_•/L, with rapidly rotating disky galaxies having larger M_•/L than slowly rotating boxy galaxies.

I apply the models to 46 galaxies with published HST photometry. Both core and power-law galaxies are well fitted. The models suggest a roughly linear correlation between BH mass and V-band galaxy luminosity, log M_• ≈ -1.83 + log L in solar units (rms scatter 0.33 dex). This agrees with the average relation for nearby galaxies with kinematically determined BH masses. Photometrically and kinematically determined BH masses agree to within ∼0.25 dex rms for galaxies that have both. These results provide additional support to the hypothesis that every galaxy (spheroid) has a central BH. The BH mass distribution inferred here is consistent with quasar statistics for a BH accretion efficiency of 4%.

The proposed scenario is not a unique way to interpret the observed surface brightness cusps of galaxies, but it explains observational correlations that are otherwise unexplained, and it yields BH masses that agree with those determined kinematically.

Over 300 star-forming complexes in 11 intermediate- and late-type spiral and irregular galaxies have been observed in the B, I, and Hα bands to determine their luminosity functions, ages, sizes, and masses. The total Hα luminosity from complexes compared with the total galaxy Hα luminosity ranges from 20% to 70%, but the percentage is nearly constant at 7% in the B band for most galaxies. A comparison of the colors and luminosities of the complexes with published cluster evolutionary models suggests that the complexes range in age from a few times 10⁶ yr to nearly 10⁹ yr; the majority have ages of less than 1 to 2 × 10⁷ yr. The complex masses range from 10⁴ to 10⁷M_⊙. The luminosity functions for the complexes follow a power law with an exponent of approximately -2; late-type galaxies have slightly shallower slopes than intermediate-type galaxies. The sizes of the largest complexes in each galaxy scale approximately with the square root of the galaxy luminosity, confirming previous studies. The complexes may have a fractal size distribution that is consistent with values predicted by theoretical turbulence models of the interstellar medium.

The southern galaxy IC 4214 is a bright, nearby example of a multiple ring early-type spiral galaxy with a weak bar. The galaxy shows three strong pseudoring features, including a nuclear ring, an inner ring, and a prominent R outer ring, each with distinctive characteristics. The galaxy is important because it is a classic resonance ring galaxy where the ring features can be linked to specific orbital resonances with the bar. We present in this paper a detailed UBVIH study of its photometric structure and a Fabry-Perot study of its kinematics to set the stage for a dynamical model in a separate paper. Despite the early Hubble type, ionized gas is well distributed in the inner disk regions, providing a well-sampled velocity field. Rapid rotation is found in the vicinity of the nuclear ring, where the rotation curve reaches a maximum, and the rotation velocities drop off somewhat with increasing radius in the outer regions. Noncircular motions are clearly seen in the observed velocity field. Analysis of both the velocity field and the shapes of isophotes give discrepant values for the inclination of the system, ranging from 47° to 50° from photometry to 55° to 58° from kinematics. A likely value of the inclination could be 52° if the inner and outer rings are intrinsically elongated and aligned perpendicular to each other.

The first paper in this series presented optical (UBVI) and near-infrared (H) surface photometry and Hα Fabry-Perot interferometry of IC 4214, an excellent example of a multiringed galaxy with a weak bar. In the present paper, we analyze the nonaxisymmetric gravitational potential of IC 4214, as defined by the near-infrared data. The gas behavior is modeled with two-dimensional sticky particle simulations that employ a rigidly rotating nonaxisymmetric potential. Our goals are to interpret the morphology and velocity field of IC 4214 and to obtain a model from which the bar pattern speed, halo contribution, and relative bulge and disk masses can be deduced. We also study the orbital families in the derived IC 4214 model.

A fairly good match, in both morphology and kinematics, is obtained for Ω_bar ≈ 40 ± 5 km s^-1 kpc^-1, placing the bar corotation slightly beyond the end of the bar (r_bar/r_cr ≈ 0.72 ± 0.05). The deduced bar amplitude implies that the visible mass of the disk dominates over the unseen halo (M_disk/M_halo > 3–4). Moreover, our model accounts quantitatively for the large difference in the orientation of IC 4214 as derived in our first paper with photometric and kinematic methods.

H I synthesis mapping of NGC 4532 and DDO 137, a pair of Sm galaxies on the edge of the Virgo Cluster, is used to determine rotation curves for each of the galaxies and to resolve the structure and kinematics of three distinct H I concentrations embedded in an extended envelope of diffuse H I discovered in earlier Arecibo studies of the system. The H I masses of the three concentrations do not appear to be sufficient for them to be self-gravitating; however, their H I masses and dynamical masses are very similar to those of faint Im galaxies in the Virgo Cluster. The peak H I column density of each of the clouds is close to the star formation threshold, but CCD images in B and R reveal no trace of stars or star formation. If the system is gravitationally bound and in virial equilibrium, we find its total mass to be 10 times that within the outermost H I contours of the individual galaxies and clouds. Thirty-seven percent of the total H I lies outside the boundaries of the individual galaxies as determined by the VLA mapping; 28% cannot be ascribed to either galaxy or to the discrete concentrations. One possible way to account for the velocity field and the large quantity of diffuse gas is to assume that DDO 137, the gas concentrations, and other apparent tidal features are due to gas infall into clumps within the dark matter potential of the group, and that prior to the interaction the group consisted of an irregular galaxy (NGC 4532) and a large, star-poor H I cloud like H I 1225+01.

By using bidimensional spectral data obtained at the 6 m telescope for the Virgo spirals NGC 4216 and NGC 4501, we have found chemically distinct metal-rich nuclei in these galaxies. Under the assumption of equal ages for the nuclear and bulge stellar populations, the metallicity difference between the nuclei and their environments in the galaxies is estimated as a factor of 2. But we have also found an age difference between the nucleus and the bulge in NGC 4216: age-metallicity disentangling on the diagrams (Hβ, Mg b), (Hβ, [MgFe]), and (Hβ, ⟨Fe⟩) results in an age estimate for the nucleus of 8–12 Gyr, the bulge being older by a factor of 1.5–2; the self-consistent metallicity difference estimate is then a factor of 3. The solar magnesium-to-iron ratios in the galactic nuclei show evidence for long duration of the secondary nuclear star formation bursts that produced the chemically distinct stellar subsystems. Detailed morphological and kinematic analyses made for the stellar and gaseous structures in the centers of NGC 4216 and 4501 have revealed the presence of circumnuclear stellar-gaseous disks with radii of some hundreds of parsecs that demonstrate fast axisymmetric rotation and lie exactly in the planes of the main galactic disks.

We have measured the stellar kinematic profiles of NGC 3379 along four position angles, using absorption lines in spectra obtained with the Multiple Mirror Telescope. We derive a far more detailed description of the kinematic fields through the main body of the galaxy than could be obtained from previous work. Our data extend 90'' from the center, at essentially seeing-limited resolution out to 17''. The derived mean velocities and dispersions have total errors (internal and systematic) better than ±10 km s^-1, and frequently better than 5 km s^-1, out to 55''. We find very weak (3 km s^-1) rotation on the minor axis interior to 12'' and no detectable rotation above 6 km s^-1 from 12'' to 50'' or above 16 km s^-1 out to 90'' (95% confidence limits). However, a Fourier reconstruction of the mean velocity field from all four sampled PAs does indicate a ∼5° twist of the kinematic major axis, in the direction opposite to the known isophotal twist. The h₃ and h₄ parameters are found to be generally small over the entire observed region. The azimuthally averaged dispersion profile joins smoothly at large radii with the velocity dispersions of planetary nebulae. Unexpectedly, we find sharp bends in the major axis rotation curve, also visible (though less pronounced) on the diagonal position angles. The outermost bend closely coincides in position with other sharp kinematic features: an abrupt flattening of the dispersion profile, and local peaks in h₃ and h₄. All of these features are in a photometrically interesting region in which the surface brightness profile departs significantly from an r^1/4 law. Features such as these are not generally known in elliptical galaxies owing to a lack of data at comparable resolution. Very similar behavior, however, is seen the kinematics of the edge-on S0 galaxy NGC 3115. We discuss the suggestion that NGC 3379 could be a misclassified S0 galaxy; preliminary results from dynamical modeling indicate that it may be a flattened, weakly triaxial system seen in an orientation that makes it appear round.

We have used the Hubble Space Telescope to obtain WFPC2 V, I photometry for stars in the halo of NGC 5128, the nearest giant elliptical galaxy. The resulting color-magnitude diagram (CMD) of this field, which lies ∼21 kpc from the center of the galaxy, contains more than 10,000 stars and reaches almost 3 mag down the red giant branch (RGB). From the sharply defined RGB tip at I = 24.1 ± 0.1 and M_I(tip) = -4.1, we obtain a distance to NGC 5128 of 3.9 Mpc. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population of NGC 5128 is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globular clusters at [Fe/H] ≲ -2 up to above-solar abundance. The relative contribution from any younger, bright asymptotic branch component is negligible. The shape of the metallicity distribution function (MDF), derived from the CMD by interpolation within the isochrones, can be remarkably well matched by a simple two-component model of closed-box chemical enrichment, where the first component starts with an initial abundance Z₀ = 0 and the second component with Z₀ ≃ 0.25 Z_⊙. Two-thirds of the stars belong to the metal-richer component, and one-third to the metal-poorer one; the mean metallicity of the entire sample is ⟨[Fe/H]⟩ = -0.41, consistent with the colors of the integrated halo light. The metal-rich component also coincides strikingly in mean and dispersion with the metal-rich peak of the halo globular clusters in NGC 5128, suggesting that both of these halo subsystems formed contemporaneously. A discussion of various models of E galaxy formation leads us to suggest that a basic in situ formation picture with two distinct epochs of star formation best fits the observations; other models involving major contributions from accretions or mergers are less satisfactory. The timing of the events we suggest is that the first, more metal-poor star-forming epoch took place while the protogalaxy was still in a clumpy, fragmented state, leaving most of the gas unused. The second and larger star formation epoch took place after the majority of the now preenriched gas had recollected into the fully formed potential well of the new giant elliptical galaxy.

We use the box-counting method on single H I contours and the perimeter-area method on families of contours to show that the projected H I distributions of seven members of the M81 group are fractal. All seven have fractal dimensions that cover a small range, 1.5 ≥ D ≥ 1.2. This is nearly the same dimension as that of CO clouds in the Milky Way and suggests that the ISM may have a fractal dimension that stays within a limited range at all scales. Reliable determination of fractal dimension requires that observations be made with high angular and frequency resolution. If theorems of projection of opaque fractals apply to H I, then the dimension of the deprojected H I distributions may be 1.5 ≥ D ≥ 1.2 as well. Fractal structures have much more surface area than smooth structures, allowing heating, cooling, and material exchange to proceed much more rapidly.

We have obtained V- and I-band photometry for about 17,500 stars in the field of the dwarf irregular galaxy NGC 3109, located in the outskirts of the Local Group. The photometry allows us to study the stellar populations present inside and outside the disk of this galaxy. From the VI color-magnitude diagram, we infer metallicities and ages for the stellar populations in the main body and in the halo of NGC 3109. The stars in the disk of this galaxy have a wide variety of ages, including very young stars of age ∼10⁷ yr. Our main result is to establish the presence of a halo consisting of Population II stars, extending out to about 45 (or 1.8 kpc) above and below the plane of this galaxy. For these old stars we derive an age of over 10¹⁰ yr and a metallicity of [Fe/H] = -1.8 ± 0.2. We construct a deep luminosity function, obtaining an accurate distance modulus (m - M)₀ = 25.62 ± 0.1 for this galaxy based on the I-magnitude of the red giant branch tip and adopting E(V-I) = 0.05.

We report observations of the double nucleus of M31 with the f/48 long-slit spectrograph of the Hubble Space Telescope Faint Object Camera. We obtain a total exposure of 19,000 s over seven orbits, with the 0063-wide slit along the line between the two brightness peaks (P.A. 42°). Careful correction of the raw data for detector backgrounds and geometric distortion is essential. A spectrum of Jupiter obtained with the same instrument is used as a spectral template to extract rotation and velocity dispersion profiles by cross-correlation. The rotation curve is resolved and reaches a maximum amplitude of ∼250 km s^-1 roughly 03 to either side of a rotation center lying between peaks P1 and P2, 016 ± 005 from the optically fainter P2. We find the velocity dispersion to be ≲250 km s^-1 everywhere except for a narrow "dispersion spike" centered 006 ± 003 on the anti-P1 side of P2, in which σ peaks at 440 ± 70 km s^-1. At much lower confidence, we see local disturbances to the rotation curve at P1 and P2 and an elevation in σ at P1. At very low significance we detect a weak asymmetry in the line-of-sight velocity distribution opposite to the sense usually encountered. Convolving our V and σ profiles to Canada-France-Hawaii Telescope resolution, we find good agreement with the forthcoming results of Kormendy & Bender, although there is a 20% discrepancy in the dispersion that cannot be attributed to the dispersion spike. Our results are not consistent with the location of the maximum dispersion found by Bacon et al. in 1994. Comparing with published models, we find that the more recent sinking star cluster model of Emsellem & Combes does not reproduce either the rotation curve or the dispersion profile. The eccentric-disk model proposed by Tremaine fares better and can be improved somewhat by adjusting the original parameters. However, detailed modeling will require dynamical models of significantly greater realism.

We report a direct detection of cold atomic hydrogen in the Magellanic Bridge using 21 cm absorption spectroscopy toward the radio source B0312-770. With a maximum absorption optical depth of τ = 0.10 and a maximum 21 cm emission brightness temperature of 1.4 K (N_{H I} = 1.2 × 10²⁰ cm^-2), this line of sight yields a spin temperature, T_s, between 20 and 40 K. H I 21 cm absorption and emission spectroscopy toward seven other low H I column density sight lines on the periphery of the LMC and SMC reveal absorption toward one additional background radio source behind the SMC with τ = 0.03. The data have typical sensitivities of σ_τ = 0.005 to 0.070 in absorption and σ_TB = 0.03 K in emission. These data demonstrate the presence of a cold atomic phase, which is probably accompanied by molecular condensations, in the tenuous interstellar medium of the Bridge region. Young OB stars observed in the Magellanic Bridge could form in situ from these cold condensations rather than migrate from regions of active star formation in the main body of the SMC. The existence of cold condensations and star formation in the Magellanic Bridge might be understood as a small-scale version of the mechanism that produces star formation in the tidal tails of interacting galaxies.

We present an analysis of the period distribution of about 1800 Cepheids in the LMC, based on data obtained by the MACHO microlensing experiment and on a previous catalog by C. H. Payne Gaposchkin. Using stellar evolution and pulsation models, we construct theoretical period-frequency distributions that are compared with the observations. These models reveal that a significant burst of star formation has occurred recently in the LMC (∼1.15 × 10⁸ yr). We also show that during the last ∼10⁸ yr, the main center of star formation has been propagating from southeast to northwest along the bar. We find that the evolutionary masses of Cepheids are still smaller than pulsation masses by ∼7% and that the red edge of the Cepheid instability strip could be slightly bluer than indicated by theory. There are approximately 600 Cepheids with periods below ∼2.5 days that cannot be explained by evolution theory. We suggest that they are anomalous Cepheids and that a number of these stars are double-mode Cepheids.

This paper concludes a series of three papers presenting ROSAT High Resolution Imager (HRI) observations of unidentified Einstein and serendipitous ROSAT X-ray sources in the direction of the Magellanic Clouds. Accurate positions and fluxes have been measured for these sources. Optical photometry and spectroscopy were obtained to search for identifications in order to determine the physical nature of these sources. The present paper includes new data for 24 objects; identifications are given or confirmed for 30 sources. For six sources, optical finding charts showing the X-ray positions are provided. The results from this program are summarized, showing that the populations of luminous X-ray sources in the Magellanic Clouds are quite different from those in the Galaxy.

We present the first CCD photometric observations of NGC 2383 and NGC 2384 in B, V, R, and I, NGC 1912, NGC 6709 in B, V, and I and NGC 1907 in B and V passbands, reaching down to a limiting magnitude of V ∼ 20 mag for ∼3300 stars put together. The results of the spectroscopic observations of 43 bright stars in the field of NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are also presented. The color-magnitude diagrams (CMDs) of the clusters in V versus B-V, V versus V-R, and V versus V-I are presented. The distances and reddening to these clusters are determined using the cluster CMDs. The distances to the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are 1785 ± 260, 1820 ± 265, 3340 ± 490, 2925 ± 430, and 1190 ± 175 pc, respectively. Some gaps in the cluster main sequence have been identified. We have compared the observed color-magnitude diagrams of these four open clusters with the synthetic ones derived from one classical and two overshoot stellar evolutionary models. Overshoot models estimate older ages for clusters when compared to the classical models. The age of the clusters estimated using the isochrones of Bertelli et al. are 400, 250, 400, 20, and 315 Myr for the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709, respectively. A comparison of the synthetic color-magnitude diagrams with the observed ones indicates that the overshoot models should be preferred. The comparison of integrated luminosity functions do not clearly indicate as to which model is to be preferred. The values of the mass function slopes estimated for the clusters are x = 1.7 ± 0.15 for NGC 1912 (mass range: 1.7–3.9 M_⊙) and NGC 6709 (1.7–3.4 M_⊙), x = 1.3 ± 0.15 for NGC 2383 (1.7–3.1 M_⊙), and x = 1.0 ± 0.15 for NGC 2384 (2.0–14.0). The present age estimates show that the closely located cluster pair NGC 1912 + NGC 1907 have similar ages, indicating that they may have born together, making them a good candidate to be a binary open cluster.

We report on the observations of nine globular clusters carried out by the Voyager ultraviolet spectrometers (UVSs). Three of the observed clusters, M13, NGC 6752, and M70, exhibit an intense far-ultraviolet (FUV) spectrum down to the Lyman limit. However, the spectrum obtained for NGC 6752 is heavily contaminated by the nearby star HD 177999 for wavelengths longward of 1200 Å. For M70 the Voyager spectrum is completely dominated by the B3 star HD 172535. The FUV spectral energy distribution of M13 confirms results from the Ultraviolet Imaging Telescope that the main contributors to the FUV emission are hot sdB stars. M13's integrated spectrum resembles that of an sdB star of T_eff ≈ 28,000 K. For two clusters, M92 and M5, UVS detected a weak signal, making it difficult to reach any conclusion on the underlying hot components of the systems. Four clusters, M15, NGC 2298, NGC 6656, and NGC 6793, were observed but not detected. In some cases it was possible to identify a weak stellar spectrum, which, however, very likely originates from foreground FUV emitters not related to the systems. For the nondetected clusters we provide upper limits on the FUV flux.

Four planetary nebulae (PNe) with conspicuous point-symmetric morphology are studied in this paper through high-quality imagery and long-slit echelle spectroscopy. Point symmetry is also found in the velocity space, and this is related to particular forms of bipolar collimated outflows. Morphology and kinematics together reveal the presence of collimated bipolar ejections in an episodic way with indications of rotation or displacement of the symmetry axis of the outflow. Point symmetry is currently known to occur in a wide variety of PNe and the convenience of a reevaluation of point-symmetric PNe as a main morphological class is pointed out.

We have acquired [O III] λ5007, H I λλ6563 and 4861, and [S II] λλ6717 and 6731 imaging of the bipolar outflow source NGC 6445. As a result, we find that density is preferentially enhanced within an interior optical/radio emission annulus and takes a maximum value ∼1.1 × 10³ cm^-3. Visual extinction appears also to be enhanced within this regime, taking values ∼0.8 mag greater than for the enveloping shell, a result that is shown to imply ratios A_v/N(H₂) comparable to that of NGC 7027 and at least 4 times greater than for the interstellar medium. Finally, we investigate the variation of excitation through the projected nebular envelope. While the nucleus possesses excitation properties consistent with normal (radiatively excited) sources, it appears that line ratios toward the source periphery may indicate the presence of appreciable shock activity.

We have recalibrated a method for the estimation of stellar metal abundance, parameterized as [Fe/H], based on medium-resolution (1–2 Å) optical spectra (the majority of which cover the wavelength range 3700–4500 Å). The equivalent width of the Ca II K line (3933 Å) as a function of [Fe/H] and broadband B-V color, as predicted from spectrum synthesis and model atmosphere calculations, is compared with observations of 551 stars with high-resolution abundances available from the literature (a sevenfold increase in the number of calibration stars that were previously available). A second method, based on the Fourier autocorrelation function technique first described by Ratnatunga & Freeman, is used to provide an independent estimate of [Fe/H], as calibrated by comparison with 405 standard-star abundances.

Metallicities based on a combination of the two techniques for dwarfs and giants in the color range 0.30 ≤ (B-V)₀ ≤ 1.2 exhibit an external 1 σ scatter of approximately 0.10–0.20 dex over the abundance range -4.0 ≤ [Fe/H] ≤ 0.5. Particular attention has been given to the determination of abundance estimates at the metal-rich end of the calibration, where our previous attempt suffered from a considerable zero-point offset. Radial velocities, accurate to approximately 10 km s^-1, are reported for all 551 calibration stars.

Infrared spectra at 1.9–2.5 μm and narrowband photometry of three low-mass objects, DENIS-P J0205.4-1159, J1058.7-1548, and J1228.2-1547, are presented. As shown previously by Delfosse et al., DENIS-P J0205.4-1159 shows an absorption feature at 2.2 μm. We attribute this absorption to H₂. A simple two-parameter analysis of the K-band spectrum of low-mass objects is presented in which the relative strength of the H₂O and H₂ absorption bands is found to be correlated with the effective temperature of the objects. The analysis confirms that DENIS-P J0205.4-1159 is the lowest temperature object of the three. We present narrowband photometry of these objects that provides the continuum flux level in between the deep H₂O absorption bands. These data show the continuum level accurately for the first time, and they will provide tight constraints for spectral models of these interesting objects.

We present the first UV spectral observations of six magnetic cataclysmic variables discovered by the ROSAT Wide Field Camera (WFC). Using the International Ultraviolet Explorer (IUE), 1200–3400 Å spectra were obtained of the AM Herculis stars RE 0531-46, RE 1149+28, RE 1844-74, QS Tel (RE 1938-46), and HU Aqr (RE 2107-05) and the DQ Herculis star PQ Gem (RE 0751+14). The high-state UV spectra are dominated by strong emission lines. Continuum flux distributions for these stars (from 100 to 5500 Å) reveal that over this entire range, none of the spectral energy distributions can be fitted by a single-valued blackbody. Our new UV observations and additional archival IUE spectra were used to discover a correlation between the strength of the high-state UV emission lines and the strength of the white dwarf magnetic field. Model spectral results are used to confirm the production of the UV emission lines by photoionization from X-ray and EUV photons.

Improved orbital elements for 22 binary systems are presented. For 12 systems, masses are calculated using available trigonometric parallaxes and making certain assumptions regarding the mass ratio. For the other 10 systems, provisional elements are provided that should provide relatively accurate ephemerides for the next decade.

Parallaxes and proper motions are listed for 48 stars in 14 fields in the course of the ongoing astrometric program at Wesleyan and Yale Universities. A number of resolved binary stars are included in the list.

We have combined Hipparcos proper-motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars that may have passed (or will pass) close enough to the Sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the inner solar system, which would increase the impact hazard at Earth. We find that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by N = 3.5D^2.12 Myr^-1, less than the number predicted by a simple stellar dynamics model. However, this value is clearly a lower limit because of observational incompleteness in the Hipparcos data set. One star, Gliese 710, is estimated to have a closest approach of less than 0.4 pc 1.4 Myr in the future, and several stars come within 1 pc during a ±10 Myr interval. We have performed dynamical simulations that show that none of the passing stars perturb the Oort cloud sufficiently to create a substantial increase in the long-period comet flux at Earth's orbit.

Large particles may be present in comets in numbers sufficient to dominate the total mass of the coma. These large particles are not readily sensed by conventional (optical - infrared) techniques but are prominent at submillimeter wavelengths. Images taken using a new camera sensitive to submillimeter wavelengths reveal that comet Hale-Bopp was a prodigious source of particulate matter, releasing dust at 2000 metric tons per second when near perihelion and contributing 3 × 10¹³ kg to the interplanetary dust complex. The dust production rate exceeded that of gas (mostly water) by a factor greater than 5.

During the period from 1985 through 1990, Pluto and its satellite Charon underwent a series of transits, eclipses, and occultations, which are collectively called "mutual events." The albedo distribution of Pluto's sub-Charon hemisphere can be determined from these events with a spatial resolution that surpasses any current direct-imaging schemes. We use an iterative technique to determine a map of Pluto's sub-Charon hemisphere with resolutions down to 200 km in some areas. This map resolves a localized bright feature that may be due to condensation around a geyser or in a crater.

A new set of ephemerides for 15 of the largest asteroids has been produced for use in the Astronomical Almanac. The ephemerides cover the period from 1800 through 2100. The internal uncertainty in the mean longitude at epoch, 1997 December 18, ranges from 005 for 7 Iris through 022 for 65 Cybele, and the uncertainty in the mean motion varies from 002 per century for 4 Vesta to 014 per century for 511 Davida. This compares very favorably with the internal errors for the outer planets in recent Jet Propulsion Laboratory planetary ephemerides. However, because the asteroids have relatively little mass and are subject to perturbations by other asteroids, the actual uncertainties in their mean motions are likely to be a few tenths of an arcsecond per century. As part of the improvement to the ephemerides, new masses and densities were determined for 1 Ceres, 2 Pallas, and 4 Vesta, the three largest asteroids. These masses are as follows: Ceres = (4.39 ± 0.04) × 10^-10M_⊙, Pallas = (1.59 ± 0.05) × 10^-10M_⊙, and Vesta = (1.69 ± 0.11) × 10^-10M_⊙. The mass for Ceres is smaller than most previous determinations of its mass. This smaller mass is a direct consequence of the increase in the mass determined for Pallas. The densities found for these three asteroids are 2.00 ± 0.03 g cm^-3 for Ceres, 4.2 ± 0.3 gm cm^-3 for Pallas, and 4.3 ± 0.3 g cm^-3 for Vesta. The density for Ceres is somewhat greater than that found for the taxonomically similar 253 Mathilde.

We examine the nonlinear stability of the Wisdom-Holman (WH) symplectic mapping applied to the integration of perturbed, highly eccentric (e ≳ 0.9) two-body orbits. We find that the method is unstable and introduces artificial chaos into the computed trajectories for this class of problems, unless the step size chosen is small enough that periapse is always resolved, in which case the method is generically stable. This "radial orbit instability" persists even for weakly perturbed systems. Using the Stark problem as a fiducial test case, we investigate the dynamical origin of this instability and argue that the numerical chaos results from the overlap of step-size resonances; interestingly, for the Stark problem many of these resonances appear to be absolutely stable.

We similarly examine the robustness of several alternative integration methods: a time-regularized version of the WH mapping suggested by Mikkola; the potential-splitting (PS) method of Duncan, Levison, & Lee; and two original methods incorporating approximations based on Stark motion instead of Keplerian motion (compare Newman et al.). The two fixed point problem and a related, more general problem are used to conduct a comparative test of the various methods for several types of motion. Among the algorithms tested, the time-transformed WH mapping is clearly the most efficient and stable method of integrating eccentric, nearly Keplerian orbits in the absence of close encounters. For test particles subject to both high eccentricities and very close encounters, we find an enhanced version of the PS method—incorporating time regularization, force-center switching, and an improved kernel function—to be both economical and highly versatile. We conclude that Stark-based methods are of marginal utility in N-body type integrations. Additional implications for the symplectic integration of N-body systems are discussed.

We have performed an overall pattern comparison between the proper motions of the FK5 (Fifth Fundamental Catalogue) systems (FK5, Astrographic Catalog Reference Stars [ACRS], and PPM Catalogue) and those of Hipparcos. The global rotation (time dependent) between the two reference frames is found to be different for different stellar samples, depending on the spectral types. Taking the precessional correction Δp ≈ -3.00 ± 0.20 mas yr^-1 into account, which is given by VLBI and lunar laser ranging observations, we cannot explain the systematic difference in proper motions between the two reference systems. Analyzing the Hipparcos and ACRS proper motions of ∼24,000 K–M giants, we have found a remarkably large difference in the Galactic rotation: V₀ = 249.6 ± 7.0 km s^-1 from the Hipparcos proper motions, and V₀ = 178.3 ± 6.6 km s^-1 from the ACRS proper motions. Again it implies an inconsistency of the two proper-motion systems, even if the errors of the precession and the fictitious equinoctial motion have been corrected to the ACRS proper motions.