SBb Galaxy Research Articles

Orbital decomposition of CALIFA spiral galaxies

Schwarzschild orbit-based dynamical models are widely used to uncover the internal dynamics of early-type galaxies and globular clusters. Here we present for the first time the Schwarzschild models of late-type galaxies: an SBb galaxy NGC 4210 and an S0 galaxy NGC 6278 from the CALIFA survey. The mass profiles within $2\,R_e$ are constrained well with $1\sigma$ statistical error of $\sim 10\%$. The luminous and dark mass can be disentangled with uncertainties of $\sim 20\%$ and $\sim 50\%$ respectively. From $R_e$ to $2\,R_e$, the dark matter fraction increases from $14\pm10\%$ to $18\pm10\%$ for NGC 4210 and from $15\pm10\%$ to $30\pm20\%$ for NGC 6278. The velocity anisotropy profiles of both $\sigma_r/\sigma_t$ and $\sigma_z/\sigma_R$ are well constrained. The inferred internal orbital distributions reveal clear substructures. The orbits are naturally separated into three components: a cold component with near circular orbits, a hot component with near radial orbits, and a warm component in between. The photometrically-identified exponential disks are predominantly made up of cold orbits only beyond $\sim 1\,R_e$, while they are constructed mainly with the warm orbits inside. Our dynamical hot components are concentrated in the inner regions, similar to the photometrically-identified bulges. The reliability of the results, especially the orbit distribution, are verified by applying the model to mock data.

Census of H ii regions in NGC 6754 derived with MUSE: Constraints on the metal mixing scale

We present a study of the HII regions in the galaxy NGC 6754 from a two pointing mosaic comprising 197,637 individual spectra, using Integral Field Spectrocopy (IFS) recently acquired with the MUSE instrument during its Science Verification program. The data cover the entire galaxy out to ~2 effective radii (re ), sampling its morphological structures with unprecedented spatial resolution for a wide-field IFU. A complete census of the H ii regions limited by the atmospheric seeing conditions was derived, comprising 396 individual ionized sources. This is one of the largest and most complete catalogue of H ii regions with spectroscopic information in a single galaxy. We use this catalogue to derive the radial abundance gradient in this SBb galaxy, finding a negative gradient with a slope consistent with the characteristic value for disk galaxies recently reported. The large number of H ii regions allow us to estimate the typical mixing scale-length (rmix ~0.4 re ), which sets strong constraints on the proposed mechanisms for metal mixing in disk galaxies, like radial movements associated with bars and spiral arms, when comparing with simulations. We found evidence for an azimuthal variation of the oxygen abundance, that may be related with the radial migration. These results illustrate the unique capabilities of MUSE for the study of the enrichment mechanisms in Local Universe galaxies.

GLOBULAR CLUSTER SYSTEMS OF SPIRAL AND S0 GALAXIES: RESULTS FROM WIYN IMAGING OF NGC 1023, NGC 1055, NGC 7332, AND NGC 7339

We present results from a study of the globular cluster (GC) systems of four spiral and S0 galaxies imaged as part of an ongoing wide-field survey of the GC systems of giant galaxies. The target galaxies -- the SB0 galaxy NGC1023, the SBb galaxy NGC1055, and an isolated pair comprised of the Sbc galaxy NGC7339 and the S0 galaxy NGC7332 -- were observed in BVR filters with the WIYN 3.5-m telescope and Minimosaic camera. For two of the galaxies, we combined the WIYN imaging with previously-published data from the Hubble Space Telescope and the Keck Observatory to help characterize the GC distribution in the central few kiloparsecs. We determine the radial distribution (surface density of GCs versus projected radius) of each galaxy's GC system and use it to calculate the total number of GCs (N_GC). We find N_GC = 490+/-30, 210+/-40, 175+/-15, and 75+/-10 for NGC1023, NGC1055, NGC7332, and NGC7339, respectively. We also calculate the GC specific frequency (N_GC normalized by host galaxy luminosity or mass) and find values typical of those of the other spiral and E/S0 galaxies in the survey. The two lenticular galaxies have sufficient numbers of GC candidates for us to perform statistical tests for bimodality in the GC color distributions. We find evidence at a high confidence level (>95%) for two populations in the B-R distribution of the GC system of NGC1023. We find weaker evidence for bimodality (>81% confidence) in the GC color distribution of NGC7332. Finally, we identify eight GC candidates that may be associated with the Magellanic dwarf galaxy NGC1023A, a satellite of NGC1023.

Far-infrared line imaging of the starburst ring in NGC 1097 with theHerschel/PACS spectrometer

NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the <i>Herschel<i/> Space Observatory. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 <i>μ<i/>m, [OIII] 88 <i>μ<i/>m, [NII] 122 <i>μ<i/>m, [CII] 158 <i>μ<i/>m and [NII] 205 <i>μ<i/>m. We observe that in the [OI] 63 <i>μ<i/>m, [OIII] 88 <i>μ<i/>m, and [NII] 122 <i>μ<i/>m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220 km s<sup>-1<sup/> (inclination uncorrected) measured in all lines. The [OI] 63 <i>μ<i/>m/[CII] 158 <i>μ<i/>m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 <i>μ<i/>m and [OIII] 88 <i>μ<i/>m maps. Variations of the [NII] 122 <i>μ<i/>m/[NII] 205 <i>μ<i/>m ratio correspond to a range in the ionized gas density between 150 and 400 cm<sup>-3<sup/>.

DETECTION OF A PSEUDOBULGE HIDDEN INSIDE THE “BOX-SHAPED BULGE” OF NGC 4565

Numerical simulations show that box-shaped bulges of edge-on galaxies are not bulges: they are bars seen side-on. Therefore the two components that are seen in edge-on Sb galaxies such as NGC 4565 are a disk and a bar. But face-on SBb galaxies always show a disk, a bar, and a (pseudo)bulge. Where is the (pseudo)bulge in NGC 4565? We use archival Hubble Space Telescope H-band images and Spitzer Space Telescope 3.6 micron wavelength images, both calibrated to 2MASS K_s band, to penetrate the prominent dust lane in NGC 4565. We find a high surface brightness, central stellar component that is clearly distinct from the boxy bar and from the disk. Its brightness profile is a Sersic function with index n = 1.55 +- 0.07 along the major axis and 1.33 +- 0.12 along the minor axis. Therefore it is a pseudobulge. It is much less luminous than the boxy bar, so the true pseudobulge-to-total luminosity ratio of the galaxy is PB/T = 0.06 +- 0.01, much less than the previously believed value of B/T = 0.4 for the boxy bulge. We infer that published B/T luminosity ratios of edge-on galaxies with boxy bulges have been overestimated. Therefore, more galaxies than we thought contain little or no evidence of a merger-built classical bulge. From a formation point of view, NGC 4565 is a giant, pure-disk galaxy. This presents a challenge to our picture of galaxy formation by hierarchical clustering: it is difficult to grow galaxies as big as NGC 4565 without also making big classical bulges.

The Hαgalaxy survey

We analyse the current build-up of stellar mass within the disks and bulges of nearby galaxies through a comparison of the spatial distributions of forming and old stellar populations. H alpha and R-band imaging are used to determine the distributions of young and old stellar populations in 313 S0a - Im field galaxies out to 40 Mpc. Concentration indices and mean normalised light profiles are calculated as a function of galaxy type and bar classification. The mean profiles and concentration indices show a strong and smooth dependence on galaxy type. Apart from a central deficit due to bulge/bar light in some galaxy types, mean H alpha and R-band profiles are very similar. Mean profiles within a given type are remarkably constant even given wide ranges in galaxy luminosity and size. SBc, SBbc and particularly SBb galaxies have profiles that are markedly different from those of unbarred galaxies. H alpha emission from SBb galaxies is studied in detail; virtually all show resolved central components and concentrations of star formation at or just outside the bar-end radius. In these galaxies, star formation has the same radial distribution as R-band light, i.e. stellar mass is building at approximately constant morphology, with no strong evidence for outer truncation or inside-out disk formation. (Abridged.)

H i and CO observations of Arp 104: a spiral-elliptical interacting pair

We present data probing the spatial and kinematical distribution of both the atomic (H i) and molecular (CO) gas in NGC 5218, the late-type barred spiral galaxy in the spiral–elliptical interacting pair, Arp 104. We consider these data in conjunction with far-infrared and radio-continuum data, and N-body simulations, to study the galaxies interactions, and the star formation properties of NGC 5218. We use these data to assess the importance of the bar and tidal interaction on the evolution of NGC 5218, and the extent to which the tidal interaction may have been important in triggering the bar. The molecular gas distribution of NGC 5218 appears to have been strongly affected by the bar; the distribution is centrally condensed with a very large surface density in the central region. The N-body simulations indicate a time-scale since perigalacticon of ∼3 × 10^8 yr, which is consistent with the interaction having triggered or enhanced the bar potential in NGC 5218, leading to inflow and the large central molecular gas density observed. Whilst NGC 5218 appears to be undergoing active star formation, its star formation efficiency is comparable to a ‘normal’ SBb galaxy. We propose that this system may be on the brink of a more active phase of star formation.

The opacity of spiral galaxy disks

Dust extinction in spiral disks can be estimated from the counts of background field galaxies, provided the deleterious effects of confusion introduced by structure in the image of the foreground spiral disk can be calibrated. Gonzalez et al. (1998) developed a method for this calibration, the “Synthetic Field Method” (SFM), and applied this concept to a Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (WFPC2) image of NGC 4536. The SFM estimates the total extinction through the disk without requiring assumptions about the distribution of absorbers or disk light. The poor statistics, however, result in a large error in individual measurements. We report on improvements to and automation of the Synthetic Field Method (SFM) that render it suitable for application to large archival datasets. To illustrate the strengths and weaknesses of this new method, the results on NGC 1365, an SBb galaxy, and NGC 4536, an SABbc, are presented. The extinction estimate for NGC 1365 is AI = 0.6 −0.7 at 0.45 R25, and for NGC 4536 it is AI = 1.6 +1.0 −1.3 at 0.75 R25. The results for NGC 4536 are compared with those of Gonzalez et al. (1998). The automation is found to limit the maximum depth to which field galaxies can be found. Taking this into account, our results agree with those of Gonzalez et al. (1998). We conclude that this method can only give an inaccurate measure of extinction for a field covering a small solid angle. An improved measurement of disk extinction can be done by averaging the results over a series of HST fields, thereby improving the statistics. This can be achieved with the automated method, trading some completeness limit for speed. The results from this set of fields are reported in a companion paper (Chapter 3, Holwerda et al. (2005b)). 20 CHAPTER 2: AUTOMATING THE “SYNTHETIC FIELD METHOD”

The Opacity of Spiral Galaxy Disks. III. Automating the Synthetic Field Method

Dust extinction in spiral disks can be estimated from the counts of background field galaxies, provided the deleterious effects of confusion introduced by structure in the image of the foreground spiral disk can be calibrated. Gonzalez et al. developed a method for this calibration, the Synthetic Field Method (SFM), and applied this concept to a Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 image of NGC 4536. The SFM estimates the total extinction through the disk without requiring assumptions about the distribution of absorbers or disk light. The poor statistics, however, result in large errors in individual measurements. We report on improvements to and automation of the SFM that render it suitable for application to large archival data sets. To illustrate the strengths and weaknesses of this new method, the results on NGC 1365, an SBb galaxy, and NGC 4536, an SABbc, are presented. The extinction estimate for NGC 1365 is AI = 0.6 at 0.45R25, and for NGC 4536 it is AI = 1.6 at 0.75R25. The results for NGC 4536 are compared with those of Gonzalez et al. The automation is found to limit the maximum depth to which field galaxies can be found. Taking this into account, our results agree with those of Gonzalez et al. We conclude that this method can only give an inaccurate measure of extinction for a field covering a small solid angle. An improved measurement of disk extinction can be done by averaging the results over a series of HST fields, thereby improving the statistics. This can be achieved with the automated method, trading some completeness limit for speed. The results from this set of fields are reported in a companion paper by Holwerda et al.

Spectral analysis of the nuclear stellar population and gas emission in six nearby galaxies

Optical spectra were used to study the stellar population and gas emission, when present, in the central parts of the elliptical galaxies NGC 6958, NGC 6851 and ESO 185-G54, and the spiral galaxies NGC 5757, NGC 7412 and IC 4214. Stellar population properties were derived by comparing the equivalent widths of the absorption lines with those of galaxy templates built from star clusters, as well as by applying a method of stellar population synthesis based on the same star clusters. For the galaxies with emission lines, the underlying stellar population was subtracted to obtain the pure emission spectrum, and the resulting emission line intensities were measured to determine the mechanism responsible for the gas excitation. From this analysis we concluded that the galaxies have solar metallicity except the elliptical ESO 185-G54 that reached metallicity slightly above solar. The three ellipticals and the Sa galaxy IC 4214 have their nuclei dominated by old stellar populations, although IC 4214 appears to have some contribution from the young generation stars. Recent star formation ( t< 500 Myr) is prominent in the nucleus of the SBb galaxy NGC 5757, and is also present to a lesser degree in the Sbc NGC 7412. In these two galaxies, the recent star formation is responsible for the observed H ii type emission lines, while IC 4214 has LINER emission. Among the ellipticals, only in NGC 6958 did we detect ionized gas, whose spectra is typical of LINERs.

The external shear in the gravitationally lensed system Q 2237+0305: A two-plane lens modelling

We present one-plane lens models (with and without an external shear added) and a two-plane lens model for the gravitationally lensed system Q 2237+0305. The rst (the main) lens plane is at z =0 :0394 and the second lens plane is at z =0 :5664. We found the best lens solutions for this system up to now { 2 =0 :35 for the external shear model and 2 =0 :44 for the two-plane model. Generally, we found a nearly singular (with an upper limit on the core radius of36 pc) pseudoisothermal mass distribution for the central part of the main lens that is consistent with the available observations of the Q 2237+0305 system (except the misalignment of about 13 between the mass and the light distributions). Furthermore, the main source of the external shear is possibly the object connected with the Mg ii absorption. In any case the external perturbations should be taken into account in the future models of the gravitationally lensed system Q 2237+0305. For the two-plane lens model the blue mass-to-light ratio for the central 1 00 of the SBb galaxy-lens is estimated to be 6:8 +0:3 0:1M=L;B, and the mass of the SBb galaxy inside the mean ring of the images isM(< 0: 885) = 1:471 +0:069 0:010 10 10 M. The introduction of the second lens plane leaves the core radius, the axis ratio and the position angle of the mass distribution in the main lens almost unchanged but decreases the mass inside the mean ring of the images with 1.2% and increases the total magnication of the images by a factor of1.5. The parameters of the mass distribution in the second lens are not very well constrained. If the second lens is a single galaxy we set a lower limit on the radius of the Mg ii absorbing halo of14 kpc. The probability for the two-plane lens model is estimated to be 8: 01 0 9 .

Central Rotation Curves of Spiral Galaxies

We present high-resolution central-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies. We discuss their general characteristics, particularly their central behavior, as well as dependencies on morphological types, activity, and peculiarity. The rotation curves generally show a steep nuclear rise and high-velocity central rotation, followed by a broad maximum in the disk and then a flat rotation due to the massive halo. Since the central high velocity and steep rise are common to all massive galaxies, they cannot be due to noncircular motions. Disk rotation curves of barred galaxies show larger dispersion than those of normal galaxies, probably because of noncircular motions. Interacting galaxies often show perturbed outer rotation curves, while their central rotation shows no particular peculiarity. In addition, central activities, such as starbursts and active galactic nuclei, appear to show no particular correlation with the property of rotation curves. This would suggest that the central activities are triggered by a more local effect than the global dynamical property.

Nuclear-to-disk rotation curves and mass-to-luminosity ratio in galaxies

High-resolution nuclear-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies generally show a steep nuclear rise and flat rotation from the disk to the halo. The high-velocity central rotation indicates massive cores within bulges. Since this characteristic is common to most galaxies, the high-velocity central rotation cannot be due to a particular orientation of non-circular motion. Using these rotation curves, we derive the distributions of surface-mass density, and compare them directly with observed surface-luminosity distributions. The mass-to-luminosity ratio (M/L) increases from the outer bulge to the disk, indicating that the outer disk is already dominated by dark-mass. It, then, increases more rapidly toward the outer optical edge, indicating the massive halo. In the central regions of some galaxies, the M/L increases steeply toward the nucleus, reaching a value an order of magnitude greater in the central 100 pc region than that in the disk, which may indicate a massive core of radius ∼ 100 parsecs and mass of ∼ 10 9 M ⊙. The core may be an object linking a bulge and a black hole at the nucleus.

Markaryan 907: An interacting system of galaxies

The results are given of a detailed spectrophotometric and morphological investigation of the galaxy Markaryan 907. The material was obtained with the 6-m telescope of the Special Astrophysical Observatory of the USSR Academy of Sciences and the 2.6-m telescope of the Byurakan Astrophysical Observatory. The relative intensities of the emission lines were used to determine the abundance of heavy elements, the masses, and the filling factors of the ionized gas, the number of 09 stars, and the rate of Star formation in six condensations in the galaxy. It is shown that Markaryan 907 is an interacting double system. The southern component of the pair is an SBb galaxy with powerful nucleus; the northern component is a blue dwarf galaxy consisting of three individual condensations. Two H II regions have been identified in the spiral galaxy.

Dynamical models - The barred spiral galaxy NGC 1300

view Abstract Citations (45) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical Models: The Barred Spiral Galaxy NGC 1300 England, Martin N. Abstract High-resolution neutral hydrogen observations are used as constraints for dynamical models of the SBb(s) galaxy NGC 1300. Near-infrared surface photometry provides constraints on the morphology of a triaxial ellipsoidal figure used to represent the stellar bar. The components considered in the dynamical models consist of (a) an axisymmetric Toomre disk of index n = 1, (b) a triaxial ellipsoid used for the perturbing bar potential, (c) an oval distortion of the axisymmetric disk, (d) a spherical halo, and (e) a central bulge. The dynamical models are projected into the same orientation as the observed galaxy and convolved with a synthetic antenna response to give a resolution directly comparable to the observations. The effect of the various components is investigated, and composite models are constructed in an attempt to reproduce the gas observations and provide dynamical information on the galaxy. The models only partially reproduce the observations but nevertheless provide bounds for various dynamical parameters. These models indicate that either the models are too simplistic or non-dynamical effects are important in the galaxy. Publication: The Astrophysical Journal Pub Date: September 1989 DOI: 10.1086/167833 Bibcode: 1989ApJ...344..669E Keywords: Astronomical Models; Barred Galaxies; Gas Dynamics; Two Dimensional Models; Astronomical Photometry; Dynamic Models; Galactic Structure; Mass Distribution; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1300; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE; RADIO SOURCES: 21 CM RADIATION full text sources ADS | data products SIMBAD (5) NED (5)

Observations of the kinematics of barred spiral galaxies. I - NGC 1300

view Abstract Citations (49) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the kinematics of barred spiral galaxies. I. NGC 1300. Peterson, C. J. ; Huntley, J. M. Abstract The observed emission-line velocity field of the SBb galaxy NGC 1300 shows strong departures from the pattern expected for purely circular motions. A comparison of the observed data with the kinematics predicted by the Huntley barred galaxy model suggests that the underlying mass structure of the galaxy is in the form of an elongated bar whose major axis lies in the same position angle as the apparent optical bar; the deviations from circular motion are due to the response of the gaseous disk to the rotation of the stellar bar. The pattern of velocities further suggests that the actual mass distribution in the galaxy is more centrally condensed than that of the dynamically self-consistent models. Publication: The Astrophysical Journal Pub Date: December 1980 DOI: 10.1086/158525 Bibcode: 1980ApJ...242..913P Keywords: Barred Galaxies; Dynamic Models; Galactic Structure; Mass Distribution; Velocity Distribution; Absorption Spectra; Astronomical Models; Dynamic Response; Electrophotometers; Interstellar Gas; Rotating Disks; Stellar Mass; Stellar Motions; Stellar Rotation; Astronomy full text sources ADS | data products SIMBAD (2) NED (1)

Gas as a tracer of barred spiral dynamics

The gravitational field of a barred spiral galaxy is described by a simple model consisting of two variable components: a central or nuclear disk and a homogeneous triaxial spheroid: and one fixed component: an extended disk or halo. The model is characterized by five free dimensionless parameters: the ratio of bar mass to central disk mass, two axial ratios of the spheroid, the ratio of the central disk length scale to the spheroid semimajor axis, and the corotation radius in units of the spheroid semimajor axis. Two-dimensional, time-dependent gas dynamical calculations are carried out in the gravitational field resulting from this mass model in order to generate a grid of hydrodynamical models on the five-dimensional parameter space. It is found that over a well-defined domain of this parameter space, the steady-state gas density and velocity distributions resemble the observed morphology and kinematics of the gas in actual SBb galaxies.

Four-color surface photometry of Markaryan galaxies

The results are given of four-color surface photometry of the galaxy Markaryan 10. Outside the nuclear region, the galaxy is rather red compared with the Seyfert nucleus, and the mean surface brightness of this part of the galaxy is not high. The regions of the galaxy whose emission lies above the black body curve when the colors are plotted on the (U--B, B--V) diagram emit in B about 60 percent of the total radiation of the galaxy, this being due to the presence of the Seyfert nucleus. Without allowance for the nucleus, the galaxy differs from normal Sb or SBb galaxies by the small value of U--B, which is more characteristic of Sc or SBc galaxies.

Ionized Gas in Spiral and Irregular Galaxies.

view Abstract Citations (141) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ionized Gas in Spiral and Irregular Galaxies. Burbidge, E. Margaret ; Burbidge, G. R. Abstract The distribution of ionized gas as deduced from the emission lines seen with the nebular spectrograph is described for more than 30 spiral and irregular galaxies. From the comparison of estimates of the relative intensities of Ha and [N ii] X 6583, with relative intensities computed by the usual techniques, it is concluded that the major regions of emission in the outer parts of galaxies and in the spiral arms, where the Ha/[N ii] X 6583 ratio is 3, are similar to the extended H ii regions first discovered by Struve and Elvey in our own Galaxy. The investigation has substantiated the result that irregular galaxies have the highest proportion of ionized gas and that this decreases steadily through barred spirals of types SBc and SBb and normal spirals of types Sc, Sbc, Sb, and Sa. Throughout irregular galaxies and in the nuclear, as well as the spiral-arm, regions of SBc and SBb galaxies and some Sbc and Sc galaxies there must be a considerable population of massive luminous stars. The nuclei of other Sc and Sbc galaxies and the nuclei in the majority of Sb galaxies contain less ionized gas than do the spiral-arm regions. The most striking observational result is that between the outer parts and the nuclear regions in many of these latter galaxies the ratio of the strengths of Ha and [N ii] X 6583 decreases from about 3 to values in the range 10.1. This reversal is most common in Sb and Sa galaxies with large prominent nuclei and well-developed nuclear bulges, but it appears even in some Sc galaxies with small nuclei. This can be explained either by a general deficiency of hydrogen in these regions (which we think improbable) or by supposing that radiative processes are comparatively unimportant in those nuclei where the light comes mainly from K giants, and that collisional ionization and excitation take place in the gas with an electron temperature of , or with energies of a few electron volts, since such excitation may be non-thermal. These excitation conditions might arise through corpuscular radiation from . Whether such an explanation for the anomalous ratios is accepted depends on the energy balance, i.e., estimates of the energy supply required to maintain such a high electron temperature against cooling mainly by line emission. This will require absolute measurement of emission4ine intensities. Publication: The Astrophysical Journal Pub Date: May 1962 DOI: 10.1086/147313 Bibcode: 1962ApJ...135..694B full text sources ADS | data products SIMBAD (42) NED (36)