Table of contents

I review the development of direct N‐body codes at Cambridge over nearly 40 years, highlighting the main stepping stones. The first code (NBODY1) was based on the simple concepts of a force polynomial combined with individual time steps, where numerical problems due to close encounters were avoided by a softened potential. Fortuitously, the elegant Kustaanheimo‐Stiefel two‐body regularization soon permitted small star clusters to be studied (NBODY3). Subsequent extensions to unperturbed three‐body and four‐body regularization proved beneficial in dealing with multiple interactions. Investigations of larger systems became possible with the Ahmad‐Cohen neighbor scheme which was used more than 20 years ago for expanding universe models of 4000 galaxies (NBODY2). Combining the neighbor scheme with the regularization procedures enabled more realistic star clusters to be considered (NBODY5). After a period of simulations with no apparent technical progress, chain regularization replaced the treatment of compact subsystems (NBODY3, NBODY5). More recently, the Hermite integration method provided a major advance and has been implemented on the special‐purpose HARP computers (NBODY4) together with an alternative version for workstations and supercomputers (NBODY6). These codes also include a variety of algorithms for stellar evolution based on fast lookup functions. The treatment of primordial binaries contains efficient procedures for chaotic two‐body motion as well as tidal circularization, and special attention is paid to hierarchical systems and their stability. This family of N‐body codes constitutes a powerful tool for dynamical simulations which is freely available to the astronomical community, and the massive effort owes much to collaborators.

Many active galactic nuclei (AGNs) exhibit a highly variable luminosity. Some AGNs also show a pronounced time delay between variations seen in their optical continuum and in their emission lines. In effect, the emission lines are light echoes of the continuum. This light‐travel time delay provides a characteristic radius of the region producing the emission lines. The cross‐correlation function (CCF) is the standard tool used to measure the time lag between the continuum and line variations. For the few well‐sampled AGNs, the lag is ∼1–100 days, depending upon which line is used and the luminosity of the AGN. In the best sampled AGN, NGC 5548, the Hβ lag shows year‐to‐year changes, ranging from ∼8.7 to ∼22.9 days over a span of 8 years. In this paper it is demonstrated that, in the context of AGN variability studies, the lag estimate using the CCF is biased too low and subject to a large variance. Thus the year‐to‐year changes of the measured lag in NGC 5548 do not necessarily imply changes in the AGN structure. The bias and large variance are consequences of finite‐duration sampling and the dominance of long timescale trends in the light curves, not of noise or irregular sampling. Lag estimates can be substantially improved by removing low‐frequency power from the light curves prior to computing the CCF.

Data obtained in the 1950–1955 Palomar campaign for the discovery of classical novae in M81 are set out in detail. Positions and apparent B magnitudes are listed for the 23 novae that were found. There is modest evidence that the spatial distribution of the novae does not track the B brightness distribution of either the total light or the light beyond an isophotal radius that is 70 ^'' from the center of M81. The nova distribution is more extended than the aforementioned light, with a significant fraction of the sample appearing in the outer disk/spiral arm region. We suggest that many (perhaps a majority) of the M81 novae that are observed at any given epoch (compared with, say, 10¹⁰ years ago) are daughters of Population I interacting binaries. The conclusion that the present‐day novae are drawn from two population groups—one from low‐mass white dwarf secondaries of close binaries identified with the bulge/thick disk population, and the other from massive white dwarf secondaries identified with the outer thin disk/spiral arm population—is discussed. We conclude that the M81 data are consistent with the two population division as argued previously from (1) observational studies on other grounds of nearby galaxies, (2) Monte Carlo simulations of novae in M31 and in the Galaxy, and (3) population synthesis modeling of nova binaries. Two different methods of using M81 novae as distance indicators give a nova distance modulus for M81 as (m−M)₀ = 27.75, consistent with the Cepheid modulus that is the same value.

We report on the study of the Hubble V and Hubble X H ii regions in NGC 6822 with the Hubble Space Telescope employing emission‐line and stellar filters. Calibration was made for both data sets, providing magnitudes for the stars and surface brightnesses for the nebulae. The forms of these H ii regions are shown to be decidedly more complex than revealed by ground‐based images, both having bright cores within more extended emission. The four OB associations within the images are shown to have ages of 5–10 million years. We conclude that the H ii regions are sufficiently optically thick to be used as ultraviolet photon counters and hence indicators of the star formation rate.

From images taken with the Hubble Space Telescope's WFPC2, we have obtained photometry of a field in the Sculptor dwarf spheroidal galaxy to 3 mag below the main‐sequence turnoff. We determine an age equal to that of the earliest globular clusters for the bulk of the stars in our field of view. We attempt to constrain the star formation history of the Sculptor dwarf by examining the main‐sequence luminosity function. The presence of a half‐dozen blue straggler candidates blueward of the turnoff points to a possibly complex star formation history. However, the contribution of any intermediate‐age population is difficult to measure conclusively, because of the uncertain origin of blue stragglers and the sparseness of the photometric sample.

We have analyzed long‐slit spectra of a sample of 11 spiral galaxies. We identify several disk H ii regions in each galaxy and provide classifications for the nuclear spectra. We find seven nuclear H ii region galaxies, including four starburst nucleus galaxies, and four LINERs.

This paper presents a new strategy for observing faint galaxies with high‐order natural guide star systems. We have imaged five high Galactic latitude fields within the isoplanatic patch of bright stars (8.5 mag < R < 10.3 mag). The fields provide a rich set of faint field galaxies that are observable with a natural guide star adaptive optics (AO) system on a large telescope. Because of the small fields of many AO science cameras, these preliminary images are necessary to identify candidate galaxies. We present the photometry and positions for 78 objects (at least 40 galaxies) near five bright stars, appropriate for diffraction‐limited studies with the Keck and other AO systems on large ground‐based telescopes. The K‐band seeing conditions in each field were excellent (04–07), allowing us to identify stars and estimate galaxy sizes. We also simulate AO images of field galaxies to determine the feasibility of infrared morphological studies at the diffraction limit. With new high‐order AO systems coming on line with 8–10 m class telescopes, we believe these observations are invaluable in beginning to study faint galaxy populations at the diffraction limit.

The passbands of the MACHO blue and red photometric bands have been synthesized and tested by comparing the synthetic photometry of the Vilnius spectra with the actual photometry of the Landolt field Ru 149 taken with the MACHO telescope. Transformations between B_ma and R_ma and standard Cousins V and R were derived. The effect of interstellar reddening on the V−R and B_ma−R_ma colors of F stars has been calculated. A new calibration of V−R, T_eff, and BC_V for metal‐poor A–K stars is presented.

The telluric features redward of 6700 Å have been removed from the accurate spectrophotometric standards of Hamuy et al. to permit more reliable relative and absolute spectrophotometry to be obtained from CCD spectra. Smooth fluxes from 3300 to 10500 Å are best determined by dividing the raw spectra of all objects taken in a night by the raw spectrum of a "smooth" spectrum star before deriving the instrumental response function using the revised standard star fluxes. In this way the telluric features and any large instrumental variation with wavelength are removed from the raw data, leaving smooth spectra that need only small corrections to place them on an absolute flux scale. These small corrections with wavelength are well described by a low‐order polynomial and result in very smooth flux‐calibrated spectra.

An undersampled point‐spread function (PSF) may interact with the microstructure of a solid‐state detector such that the total flux detected can depend sensitively on where the PSF center falls within a pixel. Such intrapixel sensitivity variations will not be corrected by flat‐field calibration and may limit the accuracy of stellar photometry conducted with undersampled images, as are typical for Hubble Space Telescope observations. The total flux in a stellar image can vary by up to 0.03 mag in F555W WFC images depending on how it is sampled, for example. For NIC3, these variations are especially strong, up to 0.39 mag, strongly limiting its use for stellar photometry. Intrapixel sensitivity variations can be corrected for, however, by constructing a well‐sampled PSF from a dithered data set. The reconstructed PSF is the convolution of the optical PSF with the pixel response. It can be evaluated at any desired fractional pixel location to generate a table of photometric corrections as a function of relative PSF centroid. A caveat is that the centroid of an undersampled PSF can also be affected by the pixel response function; thus sophisticated centroiding methods, such as cross‐correlating the observed PSF with its fully sampled counterpart, are required to derive the proper photometric correction.

A new observing procedure and method of analysis of CCD observations are presented. The observing procedure maximizes the on‐source time without the need for observing separate blank sky fields. It is shown that a substantial improvement in the correction of sky and detector systematics can be achieved by explicitly separating the sky subtraction from the calibration of the spatial gain variations of the CCD, or flat‐fielding. When we perform the sky subtraction before flat‐fielding, the requirements on the latter are not as stringent. We test this method with deep I‐band images obtained under extremely challenging observing conditions. Because of the presence of illumination gradients due to moonlight, a flat field with an accuracy of better than 10% could not be obtained. Nevertheless, a sky subtraction with an accuracy of approximately 0.1% of the sky brightness was achieved over the whole mosaic.

We describe a new system for multiple‐object spectroscopy and integral field spectroscopy at near‐infrared wavelengths using optical fibers. Both modes of the SMIRFS instrument have been tested at the UK Infrared Telescope with the CGS4 infrared spectrograph. The modular system includes a common optical system to image the fiber slit onto the cold slit inside the CGS4 cryostat. The multiobject mode consists of 14 interchangeable fused silica or zirconium fluoride fibers each with a field of 4 ^''. The integral field mode consists of 72 fused silica fibers coupled with a lenslet array to give a contiguous field of 6^'' × 4^'' with 06 sampling.

We describe the performance of both modes. For the multiobject mode, the feasibility and desirability of using fluoride fibers to extend the wavelength range into the K band is discussed. For the integral field mode, the performance is compared with theoretical expectation with particular attention to the effect of focal ratio degradation in the fibers.

These results demonstrate the feasibility of multiobject and integral field spectroscopy in the near‐infrared using lenslet‐coupled fiber systems. Although SMIRFS in an experimental system working with a spectrograph not designed for this purpose, the throughput and uniformity of response are good. SMIRFS points the way forward to systems with much larger numbers of elements.