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We present Hubble Space Telescope WFPC2 images of elephant trunks in the H II region M16. There are three principle results of this study. First, the morphology and stratified ionization structure of the interface between the dense molecular material and the interior of the H II region is well understood in terms of photoionization of a photoevaporative flow. Photoionization models of an empirical density profile capture the essential features of the observations, including the extremely localized region of [S II] emission at the interface and the observed offset between emission peaks in lower and higher ionization lines. The details of this structure are found to be a sensitive function both of the density profile of the interface and of the shape of the ionizing continuum. Interpretation of the interaction of the photoevaporative flow with gas in the interior of the nebula supports the view that much of the emission from H II regions may arise in such flows. Photoionization of photoevaporative flows may provide a useful paradigm for interpreting a wide range of observations of H II regions. Second, we report the discovery of a population of small cometary globules that are being uncovered as the main bodies of the elephant trunks are dispersed. Several lines of evidence connect these globules to ongoing star formation, including the association of a number of globules with stellar objects seen in IR images of M16 or in the continuum HST images themselves. We refer to these structures as evaporating gaseous globules, or "EGGs." These appear to be the same type of object as the nebular condensations seen previously in M42. The primary difference between the two cases is that in M16 we are seeing the objects from the side, while in M42 the objects are seen more nearly face-on against the backdrop of the ionized face of the molecular cloud. We find that the "evaporating globule" interpretation naturally accounts for the properties of objects in both nebulae, while avoiding serious difficulties with the competing "evaporating disk" model previously applied to the objects in M42. More generally, we find that disk-like structures are relatively rare in either nebula. Third, the data indicate that photoevaporation may have uncovered many EGGs while the stellar objects in them were still accreting mass, thereby freezing the mass distribution of the protostars at an early stage in their evolution. We conclude that the masses of stars in the cluster environment in M16 are generally determined not by the onset of stellar winds, as in more isolated regions of star formation, but rather by disruption of the star forming environment by the nearby 0 stars.

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Vand /-band HST Planetary Camera images of the great spiral galaxy in Andromeda, M31, show that its inner nucleus consists of two components separated by 0':49. The outer isophotes of the nucleus at 1 ~ 4 < r < 3 ~0 are elongated, but are concentric with the M31 bulge. The nuclear component with the lower surface brightness, P2, is also coincident with the bulge photocenter to-0~05; we argue that it is at the kinematic center of the galaxy. The brighter nuclear component, Pl, is well resolved and corresponds to the nuclear core imaged by Stratoscope II. The central V-band luminosity density for Pl is 5 X 10 4 L)p pc-3 • P2 is highly elongated and has a shallow cusp. Its central V-band luminosity density is > 2 X 10 L 0 pc-3. The total nucleus cannot be modeled as the superposition of two systems each with elliptical and concentric isophotes. The dark zone between the two peaks is deep and is either evidence for strong dust absorption or that the nuclear components are steeply truncated in this region. No strong changes in the V-I color are observed over the nucleus, however. Thus if dust absorption generates the asymmetric nuclear morphology, the dust grain size must be unusually large or the dust optical depth must be extremely high. Alternatively the higher surface brightness, off-center nuclear component Pl may be a separate stellar system. The nuclear velocity dispersion profiles of Dressler & Richstone [ApJ, 324, 701 (1988)] and Kormendy [ApJ, 325, 128 (1988)] already show that Pl cannot be a dynamically cold system such as a globular cluster. Requiring Pl to survive destruction by a central black hole in M31 may also lead to lower limits on its mass-to-light ratio. This hypothesis requires us to be looking at M31 at a special time; however, the lifetime of Pl against orbital decay may be substantially longer than naive dynamical friction arguments would indicate. Under this picture, we argue that P 1 is more likely to be a cannibalized galactic nucleus, possibly having its own central black hole.

Observations with the Wide-Field/Planetary Camera-2 of the Hubble Space Telescope (HST) are presented for three radio-loud quasars: 3C 48 (z = 0.367), B2 1425+267 (z = 0.366), and 3C 345 (z = 0.594). All three quasars have luminous (∼ 4 × L * ) galaxies as hosts, which are either elliptical (B2 1425+267 and 3C 345) or interacting (3C 48), and all hosts are 0.5 − 1.0 mag bluer in (V − I) than other galaxies with the same overall morphology at similar redshifts to the quasars. The host of 3C 48 has many H II regions and a very extended tidal tail.All nine of the radio-loud quasars studied here and in Bahcall et al. (1997) either have bright elliptical hosts or occur in interacting systems. There is a robust correlation between the radio emission of the quasar and the luminosity of host galaxy; the radio-loud quasars reside in galaxies that are on average ∼ 1 mag brighter than hosts of the radio-quiet quasars.

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Two-dimensional SIT and CCD detectors have been used to measure the surface brightness of the peculiar elliptical radio galaxy M87. Measurements were made in three broad-band colors (B, V, and R) to a distance of 80" from the nucleus, with 1" spatial resolution and photometric accuracy of the order of 1 %. The data are given in some detail and are compared with earlier photographic results. The most obvious feature of the data is a bright, barely resolved central luminosity spike, which is not seen in similar data on other nearby normal ellipticals. Also, attempts to fit isothermal or King models away from the nuclear spike show additional excess luminosity in the central regions of the galaxy (r < 10"), which cannot be fitted by such a model. A model-independent dynamical analysis, using the photometric data combined with spectrographic results by Sargent et al., shows that the nucleus of M87 contains a compact mass of low luminosity, with M = 5 x 10 9 M0 , r < 100 pc, and M/.!t' > 60. All of the existing data is well fitted by a King model containing a central black hole of mass M = 3 x 10 9 M0 and a point luminosity source. While such a model is not uniquely required by the data, it is perhaps the most plausible of several possible models considered. At present, M87 is probably the best case for a hypothetical massive black hole in a galaxy nucleus.