Showing papers in "The Astronomical Journal in 1988"
TL;DR: In this paper, the radial-velocity measurements in 15 clusters of galaxies are used to look for substructure, and it is found that most of the clusters in the sample show significant subclustering.
Abstract: New radial-velocity measurements in 15 clusters of galaxies are used to look for substructure. With a new statistical test that looks for deviations of the local velocity mean and dispersion from the global values, it is found that most of the clusters in the sample show significant subclustering. In 30-40 percent of the cases, the subclusters contain a large fraction of the galaxies found in the main body of the cluster. This result is in good agreement with earlier work by Geller and Beers (1982) based on number counts. The large fraction of clusters with significant substructure, along with evidence based on radial-velocity dispersion profiles, suggest that many, perhaps most, rich clusters did not collapse in single, coherent events. 22 references.
524 citations
327 citations
TL;DR: In this paper, a relatively direct technique of determining extragalactic distances is described, which relies on measuring the luminosity fluctuations that arise from the counting statistics of the stars contributing the flux in each pixel of a high signal-to-noise CCD image of a galaxy.
Abstract: A relatively direct technique of determining extragalactic distances is described. The method relies on measuring the luminosity fluctuations that arise from the counting statistics of the stars contributing the flux in each pixel of a high signal-to-noise CCD image of a galaxy. The amplitude of these fluctuations is inversely proportional to the distance of the galaxy. This approach bypasses most of the successive stages of calibration required in the traditional extragalactic distance ladder; the only serious drawback to this method is that it requires an accurate knowledge of the bright end of the luminosity function. Potentially, this method can produce accurate distances of elliptical galaxies and spiral bulges at distances out to about 20 Mpc. In this paper, it is explained how to calculate the value of the fluctuations, taking into account various sources of contamination and the effects of finite spatial resolution, and the feasibility and limitations of this technique are demonstrated via simulations and CCD images of M 32 and N 3379.
315 citations
TL;DR: In this paper, relative proper motions were measured and membership probabilities were determined for over 1000 stars within 15 arcmin of the center of the Orion Nebula, M42, and about 900 of the stars are associated with the Orion complex.
Abstract: Relative proper motions were measured and membership probabilities were determined for over 1000 stars within 15-arcmin of the center of the Orion Nebula, M42. About 900 of the stars are associated with the Orion complex. The motions are of sufficient precision to measure the internal velocity dispersions. These dispersions are nearly isotropic, and depend only weakly on magnitude and distance from the Trapezium. The corresponding velocity dispersion in one coordinate is just over 2 km/s. The distribution of stars follows closely the distribution of gas as measured by CO-13. The stars appear to be, at most, only weakly bound to the molecular cloud. A search for variable stars has found that nearly half of the member stars are either known variables or showed variation on the plates. 30 references.
293 citations
TL;DR: In this article, a statistically complete sample of galaxies in 12 high-latitude fields, obtained in the BJ (360-520 nm), R (580-720 nm) and I (780-1100 nm) bands using CCD detectors on the 4m telescopes at CTIO and KPNO, are reported.
Abstract: Imaging and photometric observations of a statistically complete sample of galaxies in 12 high-latitude fields, obtained in the BJ (360-520 nm), R (580-720 nm) and I (780-1100 nm) bands using CCD detectors on the 4-m telescopes at CTIO and KPNO, are reported. The data are presented in extensive graphs and sample images and analyzed in detail with reference to theoretical models of galactic origin and evolution. The galaxy number-count slopes, d(log N)/dm, are found to be sub-Euclidean, varying from 0.34 in the I band to 0.45 in the BJ band, where the corrected counts appear to saturate at about 27 mag. The predictions of no-evolution models are shown to underpredict the counts at 25 mag (BJ) by a factor of 5-15 and the extragalactic background light from all galaxies (6.8 x 10 to the -6th erg/sq cm sec sr micron at 450 nm) by a factor greater than 2. 114 references.
287 citations
TL;DR: In this article, ground-based and IRAS optical and IR spectroscopic and photometric observations are reported for 90 stars in or near 59 Einstein Observatory X-ray error circles in the Tau-Aur region.
Abstract: Ground-based and IRAS optical and IR spectroscopic and photometric observations are reported for 90 stars in or near 59 Einstein Observatory X-ray error circles in the Tau-Aur region. The data are presented in extensive tables and sample spectra and characterized in detail, with particular attention to 28 newly discovered 'naked' T Tau stars, which are shown to be normal stars with no significant IR or UV excess and ages of 1-40 Myr. These stars are found to outnumber normal T Tau stars by a factor of 10 in an area near the Tau-Aur dark clouds, and it is argued that their evolution toward the ZAMS is typical for low-mass stars. The implications of this finding for the time scales of circumstellar-disk dissipation and planet formation are discussed.
271 citations
TL;DR: In this article, integrated-light spectroscopy has been obtained for 27 globular clusters at the Ca II IR triplet Line strengths and radial velocities have been measured from the spectra for the well-studied clusters in the sample.
Abstract: Integrated-light spectroscopy has been obtained for 27 globular clusters at the Ca II IR triplet Line strengths and radial velocities have been measured from the spectra For the well-studied clusters in the sample, the strength of the Ca II lines is very well correlated with previous metallicity estimates Thus, the triplet is useful as a metallicity indicator in globular cluster integrated-light spectra The greatly reduced effect of interstellar extinction at these wavelengths (compared to the blue region of the spectrum) has permitted observations of some of the most heavily reddened clusters in the Galaxy For several such clusters, the Ca II triplet metallicities are in poor agreement with metallicity estimates from IR photometry by Malkan (1981) The strength of an interstellar band at 8621A has been used to estimate the amount of extinction towards these clusters Using the new metallicity and radial-velocity data, the metallicity distribution, kinematics, and spatial distribution of the disk globular cluster system have been analyzed Results very similar to those of Zinn (1985) have been found The relation of the disk globulars to the stellar thick disk is discussed
266 citations
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178 citations
TL;DR: In this article, the spectral properties of SN 1987K in NGC 4651 have been obtained 0-2 weeks past the maximum brightness and 5-7 weeks past maximum brightness.
Abstract: Spectra of SN 1987K in NGC 4651 have been obtained 0-2 weeks past maximum brightness and 5-7 weeks past maximum. The results suggest that SNe II and at least some SNe Ib are closely related objects, and that SN 1987A could have been a massive star which lost a majority of its hydrogen prior to exploding. It is proposed that SN 1987A belongs to a new subclass, type IIb, of SN that have the spectral characteristics of type II near maximum brightness and those of type Ib long past maximum. 64 references.
177 citations
TL;DR: In this paper, the authors presented broadband photographic observations of 22 optically violent variable (OVV) active galactic nuclei and analyzed the light curves, some extending over 18 years, for linear trends and underlying structure using linear regression and unequal-interval Fourier transform techniques.
Abstract: Broadband photographic observations of 22 optically violent variable (OVV) active galactic nuclei are presented. Over 3100 observations made between 1968 and 1986 at Rosemary Hill Observatory are tabulated and displayed graphically. The majority of the observations were made in either the Johnson B system or the international photographic (PG) system. Multicolor data are presented for a few objects. Descriptions of the light curves include the assignment of each OVV to an arbitrary variability subclass. The light curves, some extending over 18 yr, are analyzed for linear trends and underlying structure using linear regression and unequal-interval Fourier transform techniques. The results of the analysis for each of the 22 objects are given, and models of the light variations of 3C 120, 3C 345, and 3C 446 are presented. The models of these light curves show underlying structure with rapid variations superimposed. The time scales seen in the light curves of 3C 120, 3C 345, and 3C 446 are compared with characteristic time scales found in massive-accretion-disk models. The time scales most likely to be responsible for the optical behavior are either the viscous or the thermal time scales. 50 references.
TL;DR: In this article, a UBVRI photometry of supernova 1987A in the Large Magellanic Cloud for 813 days since outburst is presented, which is based on an improved calibration of the local standard stars and careful analysis of the transformation from the natural system to the standard photometric system.
Abstract: UBVRI photometry of supernova 1987A in the Large Magellanic Cloud for 813 days since outburst is presented. This photometry is based on an improved calibration of the photometry of the local standard stars and a careful analysis of the transformation from the natural system to the standard photometric system. Compared to the uniform exponential decline seen in UBVRI between days 125 and 400, SN 1987A began to fade more rapidly in all colors starting around day 500, at the time a strong thermal component appeared. Comparison with photometry of SN 1987A taken at other observatories shows that while the internal accuracy of any measurement approaches 0.01 mag, the systematic differences between UBVRI photometry obtained at different sites can be as large as 0.4 mag, due to the nonstellar nature of the spectral energy distribution of SN 1987A. 39 refs.
TL;DR: In this paper, the luminosity functions of galaxies of various Hubble types in the central 2.4-degradius core of the Fornax Cluster and the 6-degrithm core of Virgo Cluster have been compared.
Abstract: The luminosity functions of galaxies of various Hubble types in the central 2.4-deg-radius core of the Fornax Cluster and the 6-deg-radius core of the Virgo Cluster have been compared. Although properties such as density, velocity dispersion, and X-ray luminosity differ in the two clusters, no significant differences are noted in their luminosity functions or in their morphological mix of galaxies. The ratio of the distances to the Fornax and Virgo clusters has been obtained. Evidence is provided for a radial variation in the luminosity function of dwarf ellipticals in the two clusters.
TL;DR: In this article, an optical system with a 6 × 10 deg field of view was used to obtain B, V, and R CCD images of the SMC and LMC and found that SMC is found to have a coherent connected pattern of very recent star formation and the overall properties of star-forming dwarf elliptical in the Virgo Cluster.
Abstract: An optical system with a 6 x 10 deg field of view was used to obtain B, V, and R CCD images of the SMC and LMC. SMC is found to have a coherent connected pattern of very recent star formation and the overall properties of star-forming dwarf ellipticals in the Virgo Cluster. While the LMC shows a largely random pattern of star formation, some hierarchical clustering of different age regions is noted. Neither galaxy displays significant evidence for a globally large star-formation rate at the present epoch. It is suggested that if the mass distribution of the LMC was once exponential in form, beyond the 1/2 mass radius, approximately 30 percent of this mass has been lost. 24 references.
TL;DR: In this article, deep CCD photometry of stars in the Galactic nuclear bulge has been obtained on four lines of sight near l = 0 deg with b from -3.9 deg to -10 deg.
Abstract: Deep CCD photometry of stars in the Galactic nuclear bulge has been obtained on four lines of sight near l = 0 deg with b from -3.9 deg to -10 deg. Although the steep decline noted in the volume density of the bulge K and M giants cannot be explained by the spheroid of the two-component model, it does, however, match more complicated models that account for the rotation curve of the Galaxy by including a massive central component. For an assumed distance of 7 kpc, a mean stellar age derived from the main-sequence turnoff of 11-14 Gyr is obtained. It is suggested that the evolution of bulge stars on the giant branch may take place at bluer colors or higher luminosities than currently predicted. 111 references.
TL;DR: The most powerful radio galaxies are strong emitters in the far infrared as mentioned in this paper, which is consistent with recent results implying that the host galaxies of radio sources are not normal giant elliptical and that galaxy-galaxy interactions may power luminous radio sources.
Abstract: IRAS measurements at 25, 60, and 100 microns have been used to analyze the far-infrared properties of radio galaxies. Fifty-eight of the 131 objects surveyed were detected at the level of better than 300 mJy at 60 microns. The most powerful radio galaxies are strong emitters in the far infrared. Large infrared emission is shown to be correlated with the presence of strong emission lines in the optical spectra. The infrared colors of the strong radio galaxies are similar to those of Markarian Seyferts. The results are shown to be consistent with recent results implying that the host galaxies of radio sources are not normal giant ellipticals and that galaxy-galaxy interactions may power luminous radio sources. Finally, some similarities between powerful radio galaxies and powerful infrared galaxies are noted and briefly discussed.
TL;DR: In this paper, the IRAS data were used to search for variability on time scales of 3-9 months in 24 objects and the detection rate of variability found for blazars was nearly 40 percent, as compared with a complete absence of detected variability in data sets for quasars and Seyfert galaxies.
Abstract: This paper presents observations of 162 blazars, over one-half of which were detected with data from the IRAS satellite using a coadded survey and pointed observations. The far-IR data were combined with published data at other wavelengths to complete 93 energy distributions from 10 to the 8th to 10 to the 18th Hz. The IRAS data were used to search for variability on time scales of 3-9 months in 24 objects. The detection rate of variability found for blazars was nearly 40 percent, as compared with a complete absence of detected variability in data sets for quasars and Seyfert galaxies. The range of variability in both spectral index and flux density increases with decreasing wavelength. Half of the blazars with total luminosities less than 10 to the 12th solar luminosity show evidence for thermal IR component. The global energy budget of blazars is dominated by IR emission from 1 to 100 microns.
TL;DR: In this article, a newly developed analysis of variance technique was proposed to test for the presence of intrinsic variability without regard to its random or periodic nature, and applied to observations of the variable stars ZZ Ceti and US 943 and the active extragalactic objects OQ 530, US 211, US 844, LB 9743, and OJ 287.
Abstract: Differential photometric time series obtained from CCD frames are tested for intrinsic variability using a newly developed analysis of variance technique. In general, the objects used for differential photometry will not all be of equal magnitude, so the techniques derived here explicitly correct for differences in the measured variances due to photon statistics. Other random-noise terms are also considered. The technique tests for the presence of intrinsic variability without regard to its random or periodic nature. It is then applied to observations of the variable stars ZZ Ceti and US 943 and the active extragalactic objects OQ 530, US 211, US 844, LB 9743, and OJ 287.
TL;DR: In this article, a comparison of a large sample of nearby Seyfert galaxies with a control sample of normal spirals reveals only marginal evidence that Seyfers possess an excess of comparably sized companions.
Abstract: Comparison of a large sample of nearby Seyfert galaxies with a control sample of normal spirals reveals only marginal evidence that Seyferts possess an excess of comparably sized companions. It is shown that the excess of companions near Seyferts noted by Dahari (1984) is primarily due to low-luminosity galaxies that would have been eliminated as background by the present technique. Seyferts are found to be in sparser environments than radio galaxies both with or without strong emission lines in their nuclear spectra.
TL;DR: In this paper, the effect of redundant baselines on closure-phase observations is analyzed, and it is shown that for bright objects a nonredundant mask approach is superior to the use of the full aperture.
Abstract: The construction of diffraction-limited images with ground-based optical telescopes by means of closure-phase observations is explored. The effect of redundant baselines on closure-phase observations is analyzed, and it is shown that for bright objects a nonredundant-mask approach is superior to the use of the full aperture. Observations of closure phase with a nonredundant mask on the 200 in. Hale Telescope are presented, and compared with the predictions of atmospheric seeing theory. It is demonstrated that fairly complex images can be constructed from closure-phase observations alone.
TL;DR: In this article, the discovery of neutral hydrogen moving at high speed perpendicular to the disk of the nearby spiral galaxy M101 is reported, where the velocities appear redshifted by up to about 150 km/s with respect to the local H I disk.
Abstract: In this paper, the discovery of neutral hydrogen moving at high speed perpendicular to the disk of the nearby spiral galaxy M101 is reported. This material is found in two locations where the spiral structure itself seems particularly disturbed. The mass involved is (1-10) x 10 to the 7th solar. The velocities appear redshifted by up to about 150 km/s with respect to the local H I disk of M101, and yet they seem to connect smoothly to other features seen in that disk. The origin of these high-velocity H I structures is not clear. The observations also confirm that the H I layer of M101 extends far beyond the optical image in radius. This exended disk seems to become more symmetric in its faint outer parts, very much as hinted by previous single-dish observations. The velocity field suggests that the rotation curve of M101 has started to decline slowly in those outer regions. 20 references.
TL;DR: In this article, the stellar and interstellar environments of the 32 known SNRs in the LMC have been examined to determine the relative numbers of population I and II progenitors.
Abstract: The stellar and interstellar environments of the 32 known SNRs in the LMC have been examined to determine the relative numbers of population I and II progenitors. At least 2/3 of the LMC SNRs are associated with pop I environments. The data suggest that the existing SNR surveys are biased against the detection of SNRs located in evolved star-forming regions, such as supershells produced by OB asociations, and in the cores of luminous H II regions. Therefore, the fraction of SNRs in pop I environments may be even higher. The results are compared to previous studies of SN statistics in late-type galaxies.
TL;DR: In this paper, a system of weights corresponding to the precision of open cluster data is described, and some properties of open clusters can be studied more accurately than was possible earlier using these weights.
Abstract: A system of weights corresponding to the precision of open cluster data is described. Using these weights, some properties of open clusters can be studied more accurately than was possible earlier. It is clear that there are three types of objects: unbound clusters, bound clusters in the thin disk, and older bound clusters. Galactic gradients of metallicity, longevity, and linear diameter are studied. Distributions at right angles to the galactic plane are discussed in the light of the different cluster types. The clumping of clusters in complexes is studied. An estimate of the selection effects influencing the present material of open cluster data is made in order to evaluate the role played by open clusters in the history of the galactic disk. 58 references.
TL;DR: In this paper, radio identifications for 176 galaxies are made on the basis of 1.4-GHz Green Bank sky maps or 1.49-GHz observations obtained with the C configuration of the VLA in November-December 1986.
Abstract: New and previously published observational data on galaxies with declination less than +82 deg from the Uppsala General Catalog (Nilson, 1973) are compiled in extensive tables and characterized in detail. Optical positions are confirmed by measurement of Palomar Sky Survey O prints, and radio identifications for 176 galaxies are made on the basis of 1.4-GHz Green Bank sky maps or 1.49-GHz observations obtained with the C configuration of the VLA in November-December 1986; contour maps based on the latter observations are provided. Radio-selected and IR-selected galaxy populations are found to be similar (and distinct from optically selected populations), and three radio/IR criteria are developed to distinguish galaxies powered by starbursts from those with supermassive black holes or other monster energy sources. 197 references.
TL;DR: In this article, the empirical age relation for star clusters in the Large Magellanic Cloud presented by Elson and Fall (1985) are reexamined using ages based only on main-sequence turnoffs.
Abstract: The empirical age relation for star clusters in the Large Magellanic Cloud presented by Elson and Fall (1985) are reexamined using ages based only on main-sequence turnoffs. The present sample includes 57 clusters, 24 of which have color-magnitude diagrams published since 1985. The new calibration is very similar to that found previously, and the scatter in the relation corresponds to uncertainties of about a factor of 2 in age. The age distribution derived from the new calibration does not differ significantly from that derived in earlier work. It is compared with age distributions estimated by other authors for different samples of clusters, and the results are discussed.
TL;DR: In this paper, it was found that the four point sources are distinctly bluer than the galaxy, but that they do not have identical colors, and the observed configuration was well reproduced by a simple model that assumes that the objects are images of the quasar and that the lens is a constant mass-to-light ratio, elliptical, de Vaucouleurs bulge.
Abstract: Images of the gravitational lens 2237+0305 acquired in good seeing have resolved the system into at least five components within the central few arsec of the object: the galaxy nucleus and four point sources in a ringlike formation approximately centered on the galaxy. It is found that the four point sources are distinctly bluer than the galaxy, but that they do not have identical colors. The observed configuration is well reproduced by a simple model that assumes that the four objects are images of the quasar and that the lens is a constant mass-to-light ratio, elliptical, de Vaucouleurs bulge.