Table of contents

This is the second paper in a series aimed at finding high-redshift quasars from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by the Sloan Digital Sky Survey (SDSS) during its commissioning phase. In this paper, we present 22 high-redshift quasars (z > 3.6) discovered from ∼250 deg² of data in the spring Equatorial Stripe, plus photometry for two previously known high-redshift quasars in the same region of the sky. Our success rate in identifying high-redshift quasars is 68%. Five of the newly discovered quasars have redshifts higher than 4.6 (z = 4.62, 4.69, 4.70, 4.92, and 5.03). All the quasars have i* < 20.2 with absolute magnitude -28.8 < M_B < -26.1 (h = 0.5, q₀ = 0.5). Several of the quasars show unusual emission and absorption features in their spectra, including an object at z = 4.62 without detectable emission lines, and a broad absorption line (BAL) quasar at z = 4.92.

The Northern Sky Optical Cluster Survey is a project to create an objective catalog of galaxy clusters over the entire high–Galactic-latitude northern sky with well-understood selection criteria. We use the object catalogs generated from the Digitized Second Palomar Sky Survey as the basis for this survey. We apply a color criterion to select against field galaxies and use a simple adaptive kernel technique to create galaxy density maps, combined with the bootstrap technique, to make significance maps from which density peaks are selected. This survey attempts to eliminate some of the subjective criteria and assumptions of past surveys, including detection by eye and assumed luminosity functions and cluster profiles as in the Palomar Distant Cluster Survey. We also use more information (especially colors) than the most similar recent survey, the APM. This paper presents the details of our cluster-detection technique, as well as some initial results for two small areas totaling ∼60 deg². We find a mean surface density of ∼1.5 clusters deg^-2, consistent with the detection of richness class 0 and higher clusters to z ∼ 0.3. In addition, we demonstrate an effective photometric redshift estimator for our clusters.

We present high-resolution ROSAT X-ray and radio observations of three cooling flow clusters containing steep-spectrum radio sources at their cores. All three systems exhibit strong signs of interaction between the radio plasma and the hot intracluster medium. Two clusters, A133 and A2626, show enhanced X-ray emission spatially coincident with the radio source, whereas the third cluster, A2052, exhibits a large region of X-ray excess surrounding much of the radio source. Using three-dimensional numerical simulations, we show that a perturbed jet propagating through a cooling flow atmosphere can give rise to amorphous radio morphologies, particularly in the case where the jet was "turned off" and allowed to age passively. In addition, the simulated X-ray surface brightness produced both excesses and deficits as seen observationally.

We analyze the optical properties of ∼300 galaxies within and around three prominent voids of the Center for Astrophysics Redshift Survey. We determine CCD morphologies and Hαあるふぁ equivalent widths from our imaging and spectroscopic survey. We also describe a redshift survey of 250 neighboring galaxies in the imaging survey fields. We assess the morphology-density relation, EW(Hαあるふぁ)-density relation, and the effects of nearby companions for galaxies in low-density environments selected with a smoothed large-scale (5 h^-1 Mpc) galaxy number density n. Both the morphological mix and the Hαあるふぁ line width distribution of galaxies at modest underdensities, 0.5 < n/ ≤ 1, are indistinguishable from our control sample at modest overdensities, 1 < n/ ≤ 2. Both density regions contain a similar fraction of galaxies with early-type (E and S0) morphologies and with absorption-line spectra (≈35%). At the lowest densities, n/ ≤ 0.5, there is a 3 σしぐま shift in the distribution of EW(Hαあるふぁ) away from absorption-line systems (only ≈15%) and toward emission-line systems with active star formation—EW(Hαあるふぁ) ∼ 40–100 Å. There is a 2 σしぐま shift in the morphological distribution away from early types and toward irregular and peculiar morphologies.

The redshift survey of projected companions, 80% complete to m_R = 16.13, demonstrates that the incidence of a close companion in redshift space is insensitive to global density over the range we investigate (0.16 < n/ ≤ 2). However, the typical velocity separation of close pairs drops significantly (>3 σしぐま) from Δでるたcz ≳ 200 km s^-1 at 0.5 < n/ ≤ 2 down to Δでるたcz = 103 ± 20 km s^-1 at n ≤ 0.5. In the lowest density environments, galaxies with companions clearly (∼4 σしぐま) have stronger star formation than comparable galaxies at larger global density (0.5 < n/ ≤ 2). On the other hand, the distribution of EW(Hαあるふぁ) for galaxies without nearby companions (closer than ≈150 h^-1 kpc and 1000 km s^-1) varies little over the entire density range. These results, combined with the luminosity- and color-density relations of this sample (Paper I), suggest that the formation and evolution of field galaxies are insensitive to large-scale underdensity down to a threshold of roughly half the mean density. The differences in galaxy properties at the lowest global densities we can explore (n ≤ 0.5) may be explained by (1) a relative scarcity of the small-scale primordial density enhancements needed to form massive early-type/absorption-line galaxies and (2) present-day galaxy encounters that are relatively more effective because of the lower velocity dispersion on small scales (≲200 h^-1 kpc) we observe in these regions. In the voids, where the luminous galaxies presumably formed more recently, there should be more gas and dust present for active star formation triggered by nearby companions.

We have measured 492 redshifts (311 new) in the direction of the poor cluster AWM 7 and have identified 179 cluster members (73 new). We use two independent methods to derive a self-consistent mass profile, under the assumptions that the absorption-line galaxies are virialized and that they trace an underlying Navarro, Frenk, & White (NFW) dark matter profile: (1) we fit such an NFW profile to the radial distribution of galaxy positions and to the velocity dispersion profile; (2) we apply the virial mass estimator to the cluster. With these assumptions, the two independent mass estimates agree to ∼15% within 1.7 h^-1 Mpc, the radial extent of our data; we find an enclosed mass ∼(3 ± 0.5) × 10¹⁴h^-1M_⊙. The largest potential source of systematic error is the inclusion of young emission-line galaxies in the mass estimate. We investigate the behavior of the surface term correction to the virial mass estimator under several assumptions about the velocity anisotropy profile, still within the context of the NFW model, and remark on the sensitivity of derived mass profiles to outliers. We find that one must have data out to a large radius in order to determine the mass robustly, and that the surface term correction is unreliable at small radii.

UN J1025-0040 is a quasar at z = 0.6344 that shows an extremely bright post starburst population of age ∼400 Myr (Brotherton et al.). Images of UN J1025-0040 show a nearly stellar object 42 south-southwest of the quasar. We present imaging and spectroscopy that confirm that this object is a companion galaxy at redshift z = 0.6341. We estimate an age of ∼800 Myr for the dominant stellar population in the companion. The companion appears to be interacting with the quasar host galaxy, and this interaction may have triggered both the starburst and the quasar activity in UN J1025-0040.

An analysis is reported of the low-resolution optical spectra of the very low excitation emission-line object Lin 593. It is demonstrated that this object has a hot star that ionizes the surrounding nebula, thus creating a Strömgren sphere. The hot star is the source of the observed continuum, while the lines arise from the nebula ionized by the photons from the hot star. From the measurements of line emission and continuum, the chemical abundance and the physical parameters of the hot star as well as those of the ionized nebula were obtained. It is shown that the chemical abundance of Lin 593 follows the SMC abundance as derived from previous analysis of similar objects, except for oxygen, which presents a low abundance compared to the SMC average abundance pattern. It is also shown that the hot star is a main-sequence star with spectral type in the range O9.5–B0.5 and, finally, that the ionized nebula of Lin 593 forms a H II region for which several parameters were obtained, characterizing it as a class III–IV, classical-dense, H II region.

We present a novel approach to photometric redshifts, one that merges the advantages of both the template-fitting and empirical-fitting algorithms without any of their disadvantages. This technique derives a set of templates describing the spectral energy distributions of galaxies from a catalog with both multicolor photometry and spectroscopic redshifts. The algorithm is essentially using the shapes of the templates as the fitting parameters. From simulated multicolor data we show that for a small training set of galaxies we can reconstruct robustly the underlying spectral energy distributions even in the presence of substantial errors in the photometric observations. We apply these techniques to the multicolor and spectroscopic observations of the Hubble Deep Field, building a set of template spectra that reproduced the observed galaxy colors to better than 10%. Finally, we demonstrate that these improved spectral energy distributions lead to a photometric redshift relation for the Hubble Deep Field that is more accurate than standard template-based approaches.

We present a panchromatic study of nearby starburst galaxies from the ultraviolet to the visible, including narrowband Hαあるふぁ using Wisconsin-Indiana-Yale-NOAO (WIYN) and Hubble Space Telescope data, to determine how star formation processes affect the morphologies and integrated fluxes of nearby starbursts. We find that the UV and Hαあるふぁ morphologies of starbursts tend to differ, although not in a standard or predictable manner. From our sample of six nearby starbursts, three systems show good correlations between UV and Hαあるふぁ fluxes, but we find differences in UV and Hαあるふぁ morphology among the other three. Occasionally we find systems with well-defined H II regions without the corresponding brightness in the UV, and vice versa. We discuss the likely mechanisms behind these differences, which include starburst ages, dust absorption, stellar energy ejecta through supernovae and winds, and leakage of UV photons from stellar clusters. We conclude that the large-scale morphological features in starbursts are primarily due to both age and absorption from a "picket-fence" dust distribution. We further demonstrate the similarity and differences between these nearby starbursts and high-redshift star-forming galaxies. The overall morphology of our sample of starbursts changes little between UV and visible wavelengths. If high-redshift galaxies are similar to these starbursts, their morphologies should change little between rest-frame UV and optical. We also show that FIR and UV spectral energy distributions and slopes can be used to determine large-scale morphological features for extreme starbursts, with the steepest FIR slopes correlating with the most disturbed galaxies.

We present spectroscopic and photometric observations of the peculiar galaxy ESO 244-G012, the most prominent features of which are two distinct nuclei and a long asymmetric tail visible up to about 40 kpc from the center of the main body. The spectral characteristics of all the observed zones are typical of H II regions of low excitation; their N(N)/N(O) ratios indicate a comparative overabundance of nitrogen with respect to the oxygen. Around the northern nucleus, the internal reddenings, the excitations, the equivalent widths, and the derived abundances are systematically different from those of the southern one, suggesting they have had different evolutions. The equivalent width of the Hαあるふぁ +[N II] lines of the northern nucleus indicates it is a starburst; that of the southern one is compatible with a Sb galaxy. The Hαあるふぁ +[N II] equivalent width of the integrated spectrum is comparable with those observed in mergers of two disk galaxies. The starburst nature of the northern nucleus, observed in the optical range, is in agreement with the properties of ESO 244-G012 detected in the far IR and with the CO and radio observations. In this object, one of the nuclei dominates the IR emission, and the enhancement of the star formation activity is observed in only one of them. The integrated total color B - V corresponds to an Sa-Sab galaxy. The integrated colors B - V,V - R,R - I, and B - I do not show big radial variations reflecting a rather uniform population, on average, which would be between G8 and K0 types. Indicative B - V colors of the nuclei, corrected for internal absorption, give very different results, and that of the northern nucleus suggests it is an active star-forming region as found from the spectroscopic data. The velocity distribution of ESO 244-G012 has two branches, being similar to that of other system which also has two nuclei. The derived data would indicate that ESO 244-G012 is a merger system; the two distinct visible nuclei suggest that the components are still interacting.

We present results of a statistical analysis of the SFI catalog of peculiar velocities, a recently completed survey of spiral field galaxies with I-band Tully-Fisher distances. The velocity field statistic utilized is the velocity correlation function, ψぷさい₁(r), originally introduced by Górski et al. The analysis is performed in redshift space so as to circumvent potential ambiguities connected with inhomogeneous Malmquist bias corrections. The results from the SFI sample are compared with linear-theory predictions for a class of cosmological models. We generate a large set of mock samples, extracted from N-body simulations, which are used to assess the reliability of our analysis and to estimate the associated uncertainties. We assume a class of cold dark matter–like power spectrum models, specified by σしぐま₈, the rms fluctuation amplitude within a sphere of 8 h^-1 Mpc radius, and by the shape parameter, Γがんま. Defining ηいーた₈ = σしぐま₈Ωおめが, we find that the measured ψぷさい₁(r) implies a degenerate constraint in the (ηいーた₈, Γがんま)-plane, with ηいーた₈ = 0.3 ± 0.1(Γがんま/0.2)^0.5 at the 2 σしぐま level for the inverse Tully-Fisher (ITF) calibration presented in this paper. We investigate how much this constraint changes as we account for uncertainties in the analysis method and uncertainties in the distance indicator, and we consider alternative ITF calibrations. We find that both changing the error-weighting scheme and selecting galaxies according to different limiting line widths has a negligible effect. On the contrary, the model constraints are quite sensitive to the ITF calibration. The other ITF calibrations, by Giovanelli et al. and da Costa et al. both yield, for Γがんま = 0.2, a best-fit value of ηいーた₈ ≃ 0.6.

We discuss the morphology, kinematics, and physical conditions of the emitting gas of the interacting system IRAS 20048-6621. We present as well numerical simulations of this interacting system, discovered by David McLeish in 1946. The main galaxy (McL A) is an edge-on spiral galaxy with highly distorted NW side. On this side is also located McL B, the perturber galaxy. We determined a distance of 151 Mpc (h = 0.75) and a diameter of 70 kpc for McL A. It presents a bright nucleus with broad red emission lines (⟨FWHM⟩ ≈ 500 km s^-1). McL A has far-IR color indexes closely comparable to NGC 3628, one the few nearby edge-on galaxies which is a bright infrared emitter. Nevertheless, McL A is more luminous (in these bands) than any of the edge-on galaxies in the sample of bright infrared galaxies of Young et al. (1988). The two sides of McL A rotation curve are remarkably different. The N-body model that best reproduces McL A kinematical and morphological data (Kuijken &Dubinski 1995) gives a total mass 7 × 10¹¹M_⊙ for McL A. Numerical simulations with the TREESPH code closely reproduce the morphology and radial velocity observations. The best scenario for this system is that of a prograde encounter between McL A and B, with McL B's orbit 35° tilted with respect to the spiral disk of McL A and a perigalactic distance of 17.6 kpc. The derived mass ratio is McL B/McL A ≈ 1/26. In the last 5 × 10⁸ yr the perturber has crossed the main galaxy disk twice, in between it crossed the perigalacticon. According to our simulations, the emitting gas present in McL B has not been stripped out from the McL A disk, so that leads us to conclude that McL B is an irregular or small spiral galaxy.

We present optical broadband (B and R) observations of the Seyfert 1 nucleus NGC 3516, obtained at Wise Observatory from 1997 March to 1998 September contemporaneously with the X-ray 2–10 keV measurements of RXTE. The cross-correlation function shows a positive peak when the optical variations lead the X-rays by ∼100 days, and anticorrelation peaks at various leads and delays between the X-rays and the optical. We show that the putative correlation signal at 100 days is entirely due to the slow (≳30 day) components of the light curves. During the first year of this monitoring, smoothed versions of the light curves are nearly identical copies of each other, but scaled in amplitude and shifted in time. However, for the next 200 days, the X-ray and optical variations are clearly different. During the whole period, the more rapidly changing components of the light curves are uncorrelated at any lag. We consider the detection of these lags tentative and the significance of the correlations uncertain. If the 100 day delay is real, however, one interpretation is that the slowly varying part of the X-ray emission is an echo of the optical emission, Compton scattered from a medium located at, or extending, ∼50-100 lt-days from the optical source. We point out that a possibly analogous phenomenon, a lag between hard and soft X-rays for a given variability timescale, exists in Galactic stellar-mass accretors. Remarkably, in both cases the lag corresponds to a light-travel distance of the order of 10⁴ gravitational radii. Alternatively, the lag may not represent a physical size, but some other timescale. For example, it may be the manifestation of an instability propagating inward in an accretion flow, appearing first in the optical and then in the X-rays. In any event, we observe no strong correlation at zero lag, or at the small positive lags expected if the optical continuum were produced by reprocessing of X-rays. An energetically significant reprocessed component in the optical emission of NGC 3516 is thus ruled out by our data.

We have undertaken bidimensional spectroscopy of the central part of the nearby lenticular galaxy NGC 7280 with the Multi-Pupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical Observatory. We find a rather young stellar nucleus, with a mean population age of 1.5 ± 0.5 Gyr, which is more metal-rich than the bulge at R ≈ 1 kpc by an order of magnitude. The chemically and age-decoupled nucleus seems to be spatially resolved: the circumnuclear absorption index isolines represent ellipses elongated in P.A. ≈ 100°–110°. The same orientation, P.A. = 103°, is found for the elongated circumnuclear stellar structure, revealed from the morphological analysis of the Hubble Space Telescope (HST) WFPC2 and NICMOS images of NGC 7280 and seen best of all at R = 1''. Taking into account the stellar kinematics inside R ≈ 2'', we conclude that this structure is a circumnuclear stellar disk inclined with respect to the global plane of the galaxy. Meanwhile, both photometric and kinematical data in the radius range 2''–8'' imply the existence of an intermediate-scale bar elongated in P.A. ≈ 60°. The circumnuclear ionized gas is distributed and rotates in the plane orthogonal to the plane of the circumnuclear stellar disk.

Using the refurbished 305 m Arecibo Gregorian Telescope, we detected 43 low surface brightness (LSB) galaxies from the 1997 catalog of O'Neil, Bothun, & Cornell. The detected galaxies range from 22.0 mag arcsec^-2 ≤ μみゅー_B(0) ≤ 25.0 mag arcsec^-2, with colors ranging from the blue through the first detection of very red LSB galaxies (B-V = -0.7 to 1.7). The M_HI/L_B of these galaxies ranges from 0.1–50 M_☉/L_☉, showing this sample to range from very gas poor to possibly the most gas-rich galaxies ever detected. One of the more intriguing results of this survey is that the galaxies with the highest M_HI/L_B correspond to some of the reddest (optically) galaxies in the survey, raising the question of why star formation has not continued in these galaxies. Since the average H I column density in these systems is above the threshold for massive star formation, the lack of such may indicate that these galaxies form some kind of "optical core" which traces a much more extended distribution of neutral hydrogen. Alternatively, a model in which no stars more massive than two solar masses form in these systems can explain the presence of both blue and red gas-rich LSB galaxies. Moreover, under this model the baryonic mass fraction (f_b) of LSB galaxies is the same as for galaxies of higher surface brightness, thus perhaps escaping the dilemma proposed by McGaugh & de Blok in 1997 with respect to LSB and high surface brightness galaxies defining the same Tully-Fisher relation.

A subset of the detected LSB galaxies have rotational velocities ≥200 km s^-1 and yet are at least an order of magnitude below L_* in total luminosity. As such, they represent extreme departures from the standard Tully-Fisher relation. In fact, our sample does not appear to have any significant correlation between the velocity widths and absolute magnitudes, with only 40% of the galaxies falling within the 1 σしぐま low surface brightness galaxy Tully-Fisher relation. Overall, the discovery of very red, sub-L_* but very gas-rich LSB galaxies in the nearby Universe has increased, once again, the overall parameter space occupied by disk galaxies.

Accurate and radially extended stellar kinematic data reaching R = 97'' from the center are presented for the cD galaxy of Fornax NGC 1399. The stellar rotation is small (≤30 km s^-1); the stellar velocity dispersion remains constant at 250–270 km s^-1. The deviations from Gaussian line-of-sight velocity distributions are small, at the percent level. We construct dynamical models of the galaxy, deprojecting its nearly round (E0–E1) surface brightness distribution and determining the spherical distribution function that best fits (at the 4% level) the kinematic data on a grid of parameterized potentials. We find that the stellar orbital structure is moderately radial, with βべーた = 0.3 ± 0.1 for R ≤ 60'', similar to results found for some normal giant elliptical galaxies. The gravitational potential is dominated by the luminous component out to the last data point, with a mass-to-light ratio M/L_B = 10(M/L)_⊙, although the presence of a central black hole of M ≈ 5 × 10⁸M_⊙ is compatible with the data in the inner 5''. The influence of the dark component is marginally detected starting from R ≈ 60''. Using the radial velocities of the globular clusters and planetary nebulae of the galaxy, we constrain the potential more strongly, ruling out the self-consistent case and finding that the best-fit solution agrees with X-ray determinations. The resulting total mass and mass-to-light ratio are M = 1.2–2.5 × 10¹²M_⊙ and M/L_B = 2-48(M/L)_⊙ inside R = 417'' or 35 kpc for D = 17.6 Mpc.

Radial velocities are presented for a new sample of globular clusters in the outer halo of M87 at a distance of 300''–540'' (24–43 kpc) from the center of this galaxy. These are used to augment our previously published data, and an analysis of the rotation and velocity dispersion of the M87 globular cluster system is carried out. The rotation is ∼300 km s^-1 at R = 32 kpc, at which point the velocity dispersion is also still quite high, ∼450 km s^-1. The high rotation is interesting. The outer halo of M87 is, as was found in our previous kinematic analysis, very massive.

We have obtained deep B and R-band CCD photometry for five dwarf elliptical galaxies that were previously identified on Schmidt films covering the region of the Centaurus A (Cen A) group. From a Fourier analysis of the R-band CCD images we determined the surface brightness fluctuation (SBF) magnitude_R for each stellar system. All magnitudes are similar, and, given the small color spread, they suggest that these low surface brightness galaxies lie approximately at the same distance, regardless of the assumed SBF zero point. Long-slit spectra have been acquired to derive redshifts for two of the dwarfs, ESO 269-066 and ESO 384-016. The velocities, v_⊙ = 784 km s^-1 and v_⊙ = 561 km s^-1, respectively, identify them unambiguously as Cen A group members. An age (Hδでるた)-metallicity (C₂ λらむだ4668) analysis of the spectra reveals an underlying old and metal-poor stellar population in both cases. Combining photometric and spectroscopic results we find strong evidence that indeed all dwarf galaxies are Cen A group members. Based on Cepheid, TRGB, and PNLF distances published for the two main Cen A group galaxies, NGC 5128 and NGC 5253, we adopted a mean group distance of 3.96 Mpc to calibrate the apparent fluctuation magnitudes. The resulting absolute SBF magnitudes _R of the dEs correlate with the dereddened colors (B-R)₀ as predicted by Worthey's stellar synthesis models, using the theoretical isochrones of Bertelli and collaborators. This good agreement allows a calibration of the SBF method for dwarf ellipticals in the color range 0.8 < (B-R)₀ < 1.5. However, two branches of stellar populations appear in the _R-color plane, and care has to be taken to decide which branch applies to a given observed dwarf. For dwarfs with (B-R)₀ < 1 there is very little color dependence (_R ≈ -1.2), in accord with our previous SBF analysis of faint, blue Sculptor group dEs. For red dwarfs, (B-R)₀ > 1.2, the _R-color relation is steep, and accurate colors are needed to achieve SBF distances with an uncertainty of only 10%. One of the dwarfs, ESO 219-010, is located slightly behind the core of the Cen A group at about 4.8 Mpc, while the remaining four recover the mean group distance of 3.96 Mpc that was put into the calibration. The depth of the group is only 0.5 Mpc, which identifies the Cen A group as a spatially well-isolated galaxy aggregate, in contrast to the nearby Sculptor group.

If dwarf galaxies are primeval objects in the universe, as hierarchical galaxy formation scenarios predict, they should show traces of their old stellar populations, perhaps distributed in extended, differentiated structures. The working hypothesis that such a structure could exist is tested for the case of DDO 187, a field, dwarf irregular galaxy showing a high gas fraction and low metallicity. For this purpose, the structure, star formation history, and other properties of the galaxy are analyzed using the spatial distribution of stars, the color-magnitude diagrams of about 1500 resolved stars, and the fluxes of H II regions, together with data about the gas distribution.

From the I magnitude of the tip of the red giant branch (I_TRGB), the distance of DDO 187 to the Milky Way is estimated to be 2.5 ± 0.2 Mpc. The distance to several neighbor galaxies and groups has been computed, showing that DDO 187 is probably an isolated, field galaxy. The distance of DDO 187 to the Milky Way is almost 3 times smaller than that obtained from Cepheid light curves. Considering that this is the third case in which such a large disagreement is detected, it seems clear that Cepheid distance estimates based on a few stars, as usually happens in dwarf galaxies, must be accepted with caution.

The star formation history of DDO 187 has been analyzed. The central region of DDO 187 shows an overall time decreasing star formation rate with a strong burst in its central region that happened between 20 and 100 Myr ago and a present-day star formation activity 3 times smaller than the maximum one. Besides this, a spatially extended stellar component has been found that has no young stars and exceeds the size of the gas component. In short, several results suggest that DDO 187 has a two-component, halo/disk-like structure: (1) differentiated morphologies for the inner (flat) and outer (spheroidal) stellar components, (2) a gas component less extended than the outer stellar component, and (3) an outer component lacking young stars, which are abundant in the inner component. The working hypothesis that a real halo/disk structure could be present is discussed. The conclusion is reached that the two-component hypothesis is not unrealistic, but nothing can be definitely stated until more detailed data, ideally including kinematics, are available.

Previous searches for atomic gas in our Galaxy's dwarf spheroidal companions have not been sufficiently complete to settle the question of whether or not these galaxies have H I, especially in their outer parts. We present new observations of the dwarf spheroidals Sextans, Leo I, Ursa Minor, and Draco, using the NRAO 140 ft telescope, to search much farther in radius than has been done before. The new data go out to at least 2.5 times the core radius in all cases and well beyond even the tidal radius in two cases. These observations give H I column-density limits of 2–6 × 10¹⁷ cm^-2. Unless H I is quite far from the galaxies' centers, we conclude that these galaxies do not contain significant amounts of atomic gas at the present time. We discuss whether the observations could have missed some atomic gas.

We report new photometry and main-sequence turnoff ages for seven populous star clusters in the SMC with M_V < -6 and age greater than 1 Gyr, using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In contrast to the accepted picture, these clusters appear to have formed in two brief intervals, the oldest 8 ± 2 Gyr ago and one during a more recent burst 2 ± 0.5 Gyr ago. When the ridgelines of the four clusters (NGC 339, 361, and 416 and Kron 3) in the 8 Gyr burst are aligned, the dispersion in turnoff luminosities is less than 0.2 mag, corresponding to a maximum age spread of ±0.7 Gyr. When the ridgelines of three clusters (NGC 152, 411, and 419) in the 2 Gyr burst are aligned, the maximum dispersion of 0.2 mag in turnoff luminosity corresponds to a permitted age spread of ±0.2 Gyr. Within each group of clusters, the entire cluster loci (including red giant branches and clumps) are nearly identical, consistent with a very small spread in metallicity and age. In contrast to the wide dispersion in ages previously reported in the literature, our sample with more precise photometry and age measurements supports a burst-punctuated rather than a continuous cluster formation history for the oldest SMC clusters.

We present a wide-field, high dynamic range, high-resolution, long-wavelength (λらむだ = 90 cm) VLA image of the Galactic center region. The image is centered on Sgr A, covers an area of 4° × 5° with an angular resolution of 43'', and has a rms sensitivity of ≈5 mJy beam^-1. The image was constructed from archival (1989 and 1991) VLA data of Pedlar et al. and Anantharamaiah et al. using new three-dimensional image restoration techniques. These three-dimensional imaging techniques resolve the problem of non-coplanar baselines encountered at long wavelengths and yield distortion-free imaging of far-field sources with improved sensitivity. At λらむだ = 90 cm the VLA is sensitive to both thermal and nonthermal emission and the resulting image gives an unprecedented contextual perspective of the large-scale radio structure in this unique and complicated region. We have catalogued over a hundred sources from this image and present for each source its 90 cm flux density, position, and size. For many of the small- diameter sources, we also derive the 20/90 cm spectral index. The spectral index as a function of length along several of the isolated nonthermal filaments has been estimated and found to be constant. We have found six new small-diameter sources, as well as several extended regions of emission, which are clearly distinct sources that have not been previously identified at higher frequencies. These data are presented as a first epoch of VLA observations that can be used to search for source variability in conjunction with a second epoch of observations that were recently initiated.

Results for the south Galactic cap region of the Montréal-Cambridge-Tololo survey of blue subluminous stars are presented. This region overlaps the 840 deg² region studied almost three decades ago by Slettebak & Brundage. We present a list of equatorial coordinates, photographic photometry, and spectroscopic identifications, as well as finding charts, for 188 blue objects [(U-B)_pg ≤ -0.6] brighter than B_pg = 16.5 in this area. Completeness of the survey and comparisons with other similar efforts are discussed.

Eighteen small (4' square) fields within the Orion Nebula cluster (ONC) have been photometrically monitored for one or more observing seasons between 1990 and 1999 with a CCD attached to the 0.6 m telescope at Van Vleck Observatory on the campus of Wesleyan University. Data were obtained exclusively in the Cousins I band on between 25 and 40 nights per season. Results from the first 3 yr of operation of this program were summarized and analyzed by Choi & Herbst. Here we provide an update based on an additional 6 yr of observation and the extensive optical and infrared study of the cluster by Hillenbrand et al. Rotation periods with false-alarm probabilities FAP < 1% are now available for 134 members of the ONC. Of these, 67 were detected at multiple epochs with identical periods by us, and an additional 15 were confirmed by Stassun et al. in their study of Ori OB1c and OB1d. Therefore, we have a sample of 82 stars with virtually certain rotation periods and another 52 with highly probable periods, all of which are cluster members. The bimodal period distribution for the ONC reported by Choi & Herbst is confirmed, but we also find a clear dependence of rotation period on mass. This phenomenon can be understood as an effect of deuterium burning, which temporarily slows the contraction and, therefore, spin-up of stars with M ≤ 0.25 M_⊙ and ages ∼1 Myr. Stars with M < 0.25 M_⊙ have not had time to bridge the gap in the period distribution at around 4 days. Excess H-K and I-K emissions, as well as Ca II infrared triplet equivalent widths (Hillenbrand et al.), show weak but significant correlations with rotation period among stars with M > 0.25 M_⊙. Our results provide new observational support for the importance of disks in the early rotational evolution of low-mass stars.

G296.5+10.0 is a high Galactic latitude supernova remnant (SNR), with a bilateral morphology in radio and X-rays. The compact X-ray source 1E 1207.4-5209, classified as a radio-quiet neutron star, is located very close to the remnant's center. We report on a survey of the H I distribution in a region of sky, 35 × 35, around the remnant, using the Australia Telescope Compact Array with a spatial resolution of about 35 and a velocity resolution of 0.82 km s^-1. The H I distribution is quite smooth, with no obvious large-scale features that can explain the SNR bilateral morphology based on external factors. There are, however, three clouds that we do associate directly with the remnant. Optical filaments outline two smaller features, which appear to have been overtaken by the shock front and are cooling radiatively. Also, a high-velocity cloud gives a lower limit of ∼35 km s^-1 to the expansion velocity of the shock into the H I gas and a lower limit of ∼2 × 10⁴⁹ ergs for the kinetic energy injected by the supernova explosion into the surrounding interstellar medium. We estimate a distance to the SNR of d = 2.1 kpc and a total mass of at least 1900 M_⊙ of associated H I gas. A hole in the H I column density is observed at the same position of the compact X-ray source 1E 1207.4-5209. This H I hole is at the same radial velocity as G296.5+10.0. We argue that this result constitutes concrete evidence that 1E 1207.4-5209 and G296.5+10.0 are physically associated.

We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AUえーゆー × 20,000 AUえーゆー The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 10⁴ cm^-3. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10^-5M_☉ yr^-1 are required in order to explain the size of the proplyds.

We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AUえーゆー × 20,000 AUえーゆー. The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 10⁴ cm⁻³. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10⁻⁵M_☉ yr⁻¹ are required in order to explain the size of the proplyds.

Due to these high mass-loss rates, the proplyds in NGC 3603 should only survive ≈10⁵ yr. Despite this short survival time, we detect three proplyds. This indicates that circumstellar disks must be common around young stars in NGC 3603 and that these particular proplyds have only recently been exposed to their present harsh UV environment.

We present observations of four high excitation planetary nebulae (IC 2165, Me 2-1, NGC 2440, NGC 7027) taken with the Wide Field Camera and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The high angular resolution of the instruments allows for the unambiguous identification of the central star, which was not found to be the case in most previous studies. In addition, the panchromatic nature of the data, which include several ultraviolet bandpasses, permits us to directly determine the amount of dust extinction present along the sight line to the central star. The combination of these two characteristics make it possible to delineate the spectral energy distribution of the central stars. Assuming a Planck function spectrum and a standard interstellar extinction law with R_V = 3.1, the adopted E_B-V values are 0.40 (IC 2165), 0.15 (Me 2-1), 0.15 (NGC 2440), and 1.10 (NGC 7027). The wavelength coverage of our measurements is not particularly sensitive to temperature values in the range 150,000–400,000 K. However, we do find that 200,000 K produces the best fits. Again using a Planck function to represent the wavelength dependence of the stellar flux, we determine V magnitudes of 17.47 ± 0.06 (IC 2165), 18.40 ± 0.05 (Me 2-1), 17.63 ± 0.16 (NGC 2440), and 16.04 ± 0.04 (NGC 7027). Combining these results with literature values for the emission-line fluxes and distances, we derive the Zanstra temperature, crossover magnitude, radius, and luminosities for each object. The implications of these measurements for the mass and the evolutionary state of each object is briefly discussed.

We have observed MyCn18, a young planetary nebula, with Hubble Space Telescope/Wide Field Planetary Camera 2 and have presented the observational results in an earlier paper. Here we present a detailed spatio-kinematic model of Hαあるふぁ emission from the nebula. We model the hourglass-shaped nebular lobes as thin-walled structures in the shape of modified paraboloids. Variations in the Hαあるふぁ surface brightness with radius imply abrupt changes in the exponent of the power-law gas density in the hourglass walls. These density variations might reflect temporal variations in the mass-loss rate of the progenitor asympotic giant branch (AGB) star. Alternatively, they may be a result of the complex interaction between the fast and slow winds. From the Hαあるふぁ surface brightness variations we infer that the nebula is density-bounded everywhere, except in the vicinity of the dense waist. We derive a total mass of 0.013 M_⊙ for the walls of the ionized nebula and an upper limit of 0.006 M_⊙ for the mass in the interior. This appears to be a small fraction of the total mass ejected in the AGB phase. The nebular kinematics are well modeled with a radial velocity field where velocity increases with latitude. The density and velocity structures derived from our model are consistent with general predictions of the interacting winds hypothesis for planetary nebula formation.

We have made 2 mm continuum observations of 15 bright-rimmed clouds (BRCs) associated with IRAS point sources and S140 with the Nobeyama Bolometer Array (NOBA) mounted on the 45 m telescope of Nobeyama Radio Observatory. Continuum emission was detected in five BRCs and in S140. These five BRCs are known to be associated with near-infrared YSO clusters, mostly on the side facing toward the exciting star(s) and, therefore, are likely the sites of small-scale sequential star formation. The detected emission peaks of these five sources correspond to the IRAS positions within the positional errors, and the IRAS sources are considered to be protostars (Class 0/I-like sources) that were formed most recently in the BRCs. Four of them are clearly extended more than the telescope beam, suggesting the presence of circumstellar structures. The circumstellar masses derived from the 2 mm continuum are ∼5–90 M_⊙, which are more massive than those of the nearby Class 0/I objects (≲1 M_⊙). This fact and the relatively large bolometric luminosities of these objects suggest that the mass of the cluster or star(s) most recently formed in these BRCs could be higher than those of the previously formed stars found in the near-infrared cluster. The comparisons with previous observations of Bok globules unassociated with bright rims and other objects are discussed. Most noteworthy is that the ratios of the bolometric luminosity to the circumstellar mass are significantly higher for these BRCs than for Bok globules.

We have acquired narrowband imaging of NGC 2346 in the transitions H I λらむだ6563 and λらむだ4861 and [O III] λらむだ5007. As a result, we are able to evaluate the variation of both excitation and extinction over the projected central parts of the nebular shell. Extinction appears to be surprisingly uniform, and there is (in particular) little evidence for the reddening asymmetries proposed in previous analyses. Cusplike enhancements in A_v at the periphery of the source are attributed to a layer of extincting material at the limits of the H II region, implying a value of major-axis reddening ΔでるたA_v < 0.5 mag. Similar extinctions are also deduced from an analysis of infrared and millimeter-wavelength observations. It is unclear what proportion of this reddening may be attributed to the molecular belt, although it seems likely that this feature contributes ∼0.3 mag of extinction in the northern lobe and explains the north-south asymmetries noted in optical images. Given that levels of local extinction are quite modest, it seems likely that the larger part of the observed reddening (A_v ≈ 2 mag) arises from intervening interstellar material, a conclusion that is again at variance with previous analyses. Such a presumption would explain the relative uniformity in extinction over the face of the nebula, and similarity between distances estimated from extinction and those determined through alternative analyses. If this is accepted, however, then it follows that prior estimates of central star extinction must be greatly in error. One possible origin for this error is noted from Hubble Space Telescope imagery in [N II] λらむだ6584, where it is clear that a secondary star is located close to the presumed A-type central star, a component that was not allowed for in previous analyses. It is unclear whether this star is physically associated with the core binary system.

We summarize current information concerning the photometry, distances, and extinctions for the central stars of bipolar nebulae (BPNs). These are used to place tight constraints upon the properties of binary companions and to derive upper limit mass and spectral functions for these stars. A comparison of these results with model binary systems suggests that many BPN central stars may have binary companions, although these are either unrecognized or undetected. However, the photometric limits also imply that such binaries are unlikely to undergo stable mass transfer during AGB Roche lobe overflow. Constraints upon the masses of the secondary stars also enable limits to be placed upon orbital velocities, whence it is apparent that the motion of very few of these binaries is likely to be detected at current levels of observational precision.

New observations of nine of the brightest northern O stars have been made with the Breger polarimeter on the 0.9 m telescope at McDonald Observatory and the AnyPol polarimeter on the 0.4 m telescope at Limber Observatory, using the Johnson-Cousins UBVRI broadband filter system. Comparison with earlier measurements shows no clearly defined long-term polarization variability. For all nine stars the wavelength dependence of the degree of polarization in the optical range can be fitted by a normal interstellar polarization law. The polarization position angles are practically constant with wavelength and are consistent with those of neighboring stars. Thus the simplest conclusion is that the polarization of all the program stars is primarily interstellar. The O stars chosen for this study are generally known from ultraviolet and optical spectroscopy to have substantial mass loss rates and variable winds, as well as occasional circumstellar emission. Their lack of intrinsic polarization in comparison with the similar Be stars may be explained by the dominance of radiation as a wind driving force due to higher luminosity, which results in lower density and less rotational flattening in the electron scattering inner envelopes where the polarization is produced. However, time series of polarization measurements taken simultaneously with Hαあるふぁ and UV spectroscopy during several coordinated multiwavelength campaigns suggest two cases of possible small-amplitude, periodic short-term polarization variability, and therefore intrinsic polarization, which may be correlated with the more widely recognized spectroscopic variations.

Three nights of blue and red spectroscopy of the extreme-amplitude dwarf nova GW Lib at quiescence show that the orbital period is 79.4 minutes and the semiamplitude of the Hβべーた emission-line velocity curve is 40.0 km s^-1. The emission lines are very narrow and are surrounded by absorption troughs. The spectra are roughly consistent with that of a white dwarf of temperature near 11,000 K contributing 100% of the light at a distance of 114 pc. The short orbital period and low are consistent with the general properties of extreme-amplitude dwarf novae.

We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 Å in the L0 dwarf 2MASS J0746+2000 and the L4 dwarf 2MASS J0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I λらむだ4227 and Na I λらむだλらむだ5890/5896 are extremely strong, with the latter having an equivalent width of 240 Å in the L4 dwarf. By spectral type L5, the D lines extend over ∼1000 Å and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband color. The K I resonance doublet at 7665/7699 Å increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.

To investigate the relative photometry produced by adaptive optics within the isoplanatic patch, we observed four binaries, 10 UMa, ϕ UMa, 81 Cnc, and κかっぱ UMa, with adaptive optics using natural guide stars on the 3.5 m telescope, as well as one binary, βべーた Del, with adaptive optics using a laser guide star on the 1.5 m telescope at the Starfire Optical Range. Iterative blind deconvolution (IBD) and parametric blind deconvolution (PBD) techniques were used to postprocess the data, which produced consistent results for position angles, separations, and magnitude differences. We also conducted simulations that verify the agreement between IBD and PBD and compared their measurements to truth data. From the results of both observations and simulations, we conclude that adaptive optics is well suited for providing not only position angles and separations for close binaries, but also good relative magnitudes without quadrant ambiguity. From the observations, we find that the secondary of 81 Cnc (separation = 012) appears to be 0.12 mag brighter than the primary at 0.85 μみゅーm and is, therefore, cooler. We also derive a new orbit for κかっぱ UMa (separation = 0067). Our results for βべーた Del (ADS 14073) have significantly improved precision compared with the 1998 analyses of the same data by ten Brummelaar and colleagues and by Roberts, ten Brummelaar, and Mason.

The results of a new spectroscopic analysis of HD 75289, recently reported to harbor a Jovian-mass planet, are presented. From high-resolution, high signal-to-noise ratio spectra, we derive [Fe/H] = +0.28 ± 0.05 for this star, in agreement with the spectroscopic study of Gratton et al. published 10 years ago. In addition, we present a reanalysis of the spectra of υうぷしろん And and τたう Boo; our new parameters for these two stars are now in better agreement with photometrically derived values and with the recent spectroscopic analyses of Fuhrmann et al. We also report on extended abundance analyses of 14 Her, HD 187123, HD 210277, and ρろー¹ Cnc. If we include the recent spectroscopic analyses of HD 217107 by Randich et al. and Sadakane et al., who both reported [Fe/H] ∼ +0.30 for this star, we can state that all the "hot-Jupiter" systems studied to date have metal-rich parent stars. We find that the mean [C/Fe] and [Na/Fe] values among the sample of stars with planets are smaller than the corresponding quantities among field stars of the same [Fe/H].

During orbital migration of a giant extrasolar planet via ejection of planetesimals (as studied by Murray et al. in 1998), inner mean-motion resonances can be strong enough to cause planetesimals to graze or impact the star. We integrate numerically the motions of particles which pass through the 3:1 or 4:1 mean-motion resonances of a migrating Jupiter-mass planet. We find that many particles can be trapped in the 3:1 or 4:1 resonances and pumped to high enough eccentricities that they impact the star. This implies that for a planet migrating a substantial fraction of its semimajor axis, a fraction of its mass in planetesimals could impact the star. This process may be capable of enriching the metallicity of the star at a time when the star is no longer fully convective. Upon close approaches to the star, the surfaces of these planetesimals will be sublimated. Orbital migration should cause continuing production of evaporating bodies, suggesting that this process should be detectable with searches for transient absorption lines in young stars. The remainder of the particles will not impact the star but can be ejected subsequently by the planet as it migrates further inward. This allows the planet to migrate a substantial fraction of its initial semimajor axis by ejecting planetesimals.

Distribution and evolution of particle velocities in planetary rings of one- and two-size components are investigated by a local N-body simulation with periodic boundary conditions, which includes collisions and gravitational interactions between particles. Evolution of rms eccentricities and inclinations for a system of low optical depth is found to be well predicted by the numerical results of three-body orbit calculations. The Kolmogorov-Smirnov test is performed to examine the particle velocity distribution, and it is found that the distribution of orbital eccentricities and inclinations of particles in a disk with low optical depth can be well approximated by a Rayleigh distribution when the restitution coefficient of particles is small enough (≲0.6) to achieve an equilibrium state, whereas excess of high-velocity particles is found in more elastic cases in which velocity dispersion increases monotonically. When the optical depth is larger and the disk of particles becomes gravitationally unstable, however, effects of collective wakes become important. Theoretical results based on three-body orbit calculations fail to predict the evolution in this case, and eccentricities and inclinations deviate from the Rayleigh distribution.

The 2 millimeter continuum observations of parent comets of meteor storms, P/Tempel-Tuttle and P/Giacobini-Zinner, were made with a bolometer array installed on the 45 m radio telescope at the Nobeyama Radio Observatory. P/Tempel-Tuttle was observed on 1998 January 16, near its closest approach to the earth, and P/Giacobini-Zinner was observed on 1998 November 5 and 8. The maps obtained showed no signal from these comets greater than 12.0 mJy for P/Tempel-Tuttle and 17.4 mJy for P/Giacobini-Zinner (3 σしぐま). Using the same assumptions as Jewitt & Mathews, we estimated that the mass upper limits in the 12'' beam were 7.5 × 10⁹ kg and 6.2 × 10¹⁰ kg for P/Tempel-Tuttle and P/Giacobini-Zinner, respectively.

This paper presents color transformations for Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera 2 observations to the ground-based Carnegie image tube photometric system at Cerro Tololo Inter-American Observatory (CIT/CTIO), using observations of 19 moderately bright, red stars in Baade's window in the color range 0.7 < (J-K) < 1.6. We emphasize that these transformations will be accurate only for cool giants of near-solar metallicity. We simulate both NICMOS and ground-based photometry by convolving near-IR spectra taken above the atmosphere with different filter transmission profiles. The results are consistent with our transformation.

Symplectic N-body integrators are widely used to study problems in celestial mechanics. The most popular algorithms are of second and fourth order, requiring two and six substeps per time step, respectively. The number of substeps increases rapidly with order in time step, rendering higher order methods impractical. However, symplectic integrators are often applied to systems in which perturbations between bodies are a small factor of the force due to a dominant central mass. In this case, it is possible to create optimized symplectic algorithms that require fewer substeps per time step. This is achieved by only considering error terms of order and neglecting those of order ², ³, etc. Here we devise symplectic algorithms with four and six substeps per step which effectively behave as fourth- and sixth-order integrators when is small. These algorithms are more efficient than the usual second- and fourth-order methods when applied to planetary systems.