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Sidney van den Bergh

Publications and source records attributed to Sidney van den Bergh.

At least 91 records · Page 5Linked to original sources
arXiv1998

Star and Cluster Formation in the Large Magellanic Cloud

A great burst of cluster formation increased the rate at which open clusters were formed in the Large Magellanic Cloud 3-5 Gyr ago by at least an order of magnitude. On the other hand the rate of star formation ~ 4 Gyr appears to have increased by a factor of only 2-4. This shows that the rate of cluster formation is not a good tracer of the rate at which stars are formed. Normalized to the same rate of star formation, the Large Cloud is presently forming / 600 times more star clusters than the Local Group dwarf irregular IC 1613. The high rate of cluster formation in merging gas-rich galaxies suggests that strong shocks might favor the formation of clusters.

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arXiv1998

Ages for Globular Clusters in the Outer Galactic Halo: The Second Parameter Clusters Palomar~3, Palomar~4 and Eridanus

We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$α$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present.

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arXiv1998

The Globular Clusters of Dwarf Spheroidal Galaxies

The possibility that the Sagittarius dwarf spheroidal might have formed as a Searle-Zinn fragment in the outer halo of the Galaxy is discussed. Arguments in favor of this hypothesis are: (1) The luminosity distribution of globular clusters in both Sagittarius and in the outer halo (Rgc > 80 kpc), appear to be bimodal with peaks near MV ~ -5 and MV ~ -10, and (2) the globular clusters in both Sgr, and in the outer halo, have a significantly larger age spread than do the globulars in the inner halo of the Galaxy. However, a counter argument is that only one of the four globulars associated with Sgr has the large half-light diameter that is diagnostic of outer halo clusters. The absence of globular clusters from all Local Group dwarf spheroidal galaxies fainter than MV = -12 shows that their specific globular cluster frequency must be lower than it is in the Fornax dwarf spheroidal. This result suggests that Fornax may have had an unusual evolutionary history.

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arXiv1998

The Binary Galaxies NGC 147 And NGC 185

Contrary to a previously published claim it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M 31. It is noted that the only two Local Group galaxy pairs, in which the components have comparable masses, also have similar morphological types. NGC 147 and NGC 185 are both spheroidals, while the LMC and SMC are both irregulars. This suggests that protogalaxies of similar mass that are spawned in similar environments evolve into objects having similar morphologies.

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arXiv1998

Young Galactic Globular Clusters

Nine presently known "young" Galactic globular clusters, which have ages / 3 Gyr smaller than the oldest clusters of similar metallicity, all have RGC > 15 kpc. Furthermore these young globulars are found to have below-average luminosities. These results suggest that (1) globular cluster formation extended over a considerable period of time in the outermost part of the Galactic halo, and (2) that typical luminosities (masses) of such outer halo clusters decreased with time.

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arXiv1997

Globular Cluster Systems of Early-Type Galaxies

Properties of 53 globular cluster systems are investigated. Strong correlations are found between parent galaxy luminosity and both the slope of the radial density profile for clusters and the width of the cluster color (metallicity) distribution. These correlations are in the sense that the most luminous early-type galaxies are embedded in cluster systems that have the shallowest radial gradients and exhibit the broadest color distributions. The data suggest a scenario in which luminous early-type galaxies have a more complex evolutionary history than fainter ones. A problem with the interpretation of the present data is that it is difficult (or impossible) to disentangle the strongly correlated effects of high parent galaxy luminosity, presence of a core or boxy isophotes, and shallow radial cluster density gradients.

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arXiv1997

Some Musings on Galaxy Classification

The present paper presents a discussion of classification systems for galaxies, with special emphasis on possible modifications of the Hubble "tuning fork" diagram, and on galaxy types not included in Hubble's original scheme. Hubble's morphological types were defined in terms of standards observed at small look-back times that were mostly located in the field, or in poor clusters. It is pointed out that it is often difficult to shoehorn galaxies located in the cores of rich clusters, or objects viewed at large look-back times, into the Hubble classification system. The evolutionary relationships between E, S0 and dSph galaxies are presently still controversial and poorly understood. It is suggested that S0 galaxies may have arrived at their present morphology along various evolutionary tracks. Late-type barred spirals are found to be systematically less luminous than normal late-type galaxies. This suggests that the dichotomy between normal and barred spirals may reflect significant differences in their evolutionary histories. Such differences might be explored by searching for systematic differences between the [O/Fe] values in normal and barred spirals. Finally it is pointed out that the Large Magellanic Cloud may have been a low surface brightness galaxy for an ~8 Gyr period that ended 3-5 Gyr ago. This suggests that some galaxies can jump from one morphological classification type to another.

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arXiv1996

Distribution of Supernovae in Spiral Galaxies

Data are presently available on 156 supernovae (SNe) of known type that have occurred in spirals of types Sa-Sc having inclinations i < 70 degrees. The surface density distribution of SN Ibc + SN II is found to be an exponential with exponent -4.5 r(SN) / R(25) . Available data suggest, but do not yet prove, that SNe Ibc may be more centrally concentrated than SNe II. It is tentatively suggested that such an effect might be due to preferential mass loss via stellar winds, in the relatively metal-rich progenitors of core-collapse supernovae (or by an enhanced mass transfer rate in binary SN progenitors) in the high metallicity central regions of spiral galaxies.

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arXiv1996

Some Integrated Properties of Galactic Globular Clusters

The luminosities of globular clusters are found to correlate with their half-light radii. The most luminous clusters have half-light radii r ~ 3 pc. Mean cluster luminosities are < Mv > = -6.64 +/- 0.26 for r < 2.0, < Mv > = -7.44 +/- 0.20 for 2.0 < r < 4.0 pc, and < Mv > = -6.57 +/- 0.21 for clusters with r > 4.0 pc. An even fainter value < Mv > = -5.85 +/- 0.36 is found for large clusters with r > 8.0 pc. These results possibly weaken confidence in the conclusion that the peak of the globular cluster luminosity distribution is a universal standard candle. In the outer Galactic halo globular clusters with red horizontal branches are fainter by about a factor of ten than are clusters with blue and intermediate color horizontal branches. Among clusters with Galactocentric distances > 10 kpc there appears to be a clear dichotomy between normal clusters (which all have [Fe/H] < -1.2) and the anomalous relatively metal-rich clusters Pal 1, Pal 12 and Ter 7, which are both unusually faint (Mv > -5) and relatively metal-rich ([Fe/H] > - 1.0). This suggests that these relatively metal-rich halo clusters may have had an unusual evolutionary history.

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arXiv1996

Formation of the Galaxy

Current ideas on the formation of the Galaxy are reviewed. Many of the observed characteristics of our Milky Way System are consistent with a scenario in which the Galaxy formed inside out, with the inner part of it evolving by rapid collapse of a single protogalaxy, while the outer halo was accreted over an extended period. A number of possible problems with this "Standard Model" are discussed and summarized in section 5.

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arXiv1996

Mergers of Globular Clusters

Globular clusters with composite color-magnitude diagrams, such as NGC 1851, NGC 2808 and Fornax No. 3, might have formed by mergers. It is suggested that each of these objects had two parent clusters, one with a red horizontal branch and another with a blue horizontal branch. Such mergers could have occurred if both ancestral objects were originally members of a single dwarf spheroidal galaxy in which the internal velocity dispersion was low.

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arXiv1996

The luminosities of supernovae of type Ia derived from Cepheid observations

Available data on the luminosities of supernovae of Type Ia (SNe Ia) that have been calibrated by Cepheids are collected and discussed. The objects in the present sample show a range of ~20 in luminosity. The data strongly confirm the suspicion that SNe Ia in early-type systems are, on average, fainter than those that occur in late-type galaxies. Historical observations of S Andromedae suggest that the maximum magnitude versus rate-of-decline relationship for SNe Ia exhibits a large intrinsic dispersion. This is not surprising because the amount of 56Ni that is produced, and other observable properties of SNe Ia, are expected to depend sensitively on how much fuel is ignited.

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arXiv1996

A Morphological Catalog of Galaxies in the Hubble Deep Field

We present a catalog of morphological and color data for galaxies with $I < 25$ mag in the {\em Hubble Deep Field} (Williams et al. 1996). Galaxies have been inspected and (when possible) independently visually classified on the MDS and DDO systems. Measurements of central concentration and asymmetry are also included in the catalog. The fraction of interacting and merging objects is seen to be significantly higher in the {\em Hubble Deep Field} than it is among nearby galaxies. Barred spirals are essentially absent from the deep sample. The fraction of early-type galaxies in the Hubble Deep Field is similar to the fraction of early-types in the Shapley-Ames Catalog, but the fraction of galaxies resembling archetypal grand-design late-type spiral galaxies is dramatically lower in the distant HDF sample.

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arXiv1996

The Extragalactic Distance Scale

Cepheid variables are used to derive a Virgo cluster distance of 16.0 +/- 1.5 Mpc. In conjunction with the Coma velocity and the well-established Coma/Virgo distance ratio, this yields a Hubble paprameter Ho = 81 +/- 8 km/s/Mpc. By combining this value with an age of the Universe of at least 16.8 +/- 2.1 Gyr, that is derived from the metal-poor globular cluster M92, one obtains f(Omega,Lambda) > 1.39 +/- 0.22. This value is only marginally consisitent with an Einstein-de Sitter universe with Omega=0 and Lambda=0, which has f=1. An Einstein-de Sitter universe with Omega=1 and Lambda=0, for which f=2/3, appears to be excluded at the 3-sigma level. It is shown that some small recent values of Ho resulted from the intrinsic dispersion in the linear diameters of galaxies, and from the fact that well-observed supernovae of Type Ia exhibit a luminosity range of ~20 at maximum light.

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arXiv1996

Age Gradient and the Second Parameter Problem in the Galactic Halo

We establish a framework for determing absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The clusters span Galactocentric distances from 4 through 100 kpc and cover a metallicity range from $[Fe/H] = -0.6$ to $-2.3$. Adopting currently plausible choices for the relation between cluster metallicity and horizontal-branch luminosity, and alpha-enhancement ratios, we find that the majority of the globular clusters form an age distribution with a dispersion $σ(t)$ about $10^9$ years, and a total age spread smaller than 4 Gyr. Clusters in the lowest metallicity group ($[Fe/H] < -1.8$) appear to be the same age to well within 1 Gyr at all locations in the Milky Way halo, suggesting that star formation began throughout the halo nearly simultaneously in its earliest stages. We find no statistically significant correlation between mean cluster age and Galactocentric distance (no age gradient) from 4 to 100 kpc. The correlation between cluster age and horizontal-branch type suggests that causes in addition to metallicity and age are required to understand the distribution of stars along the horizontal branches in globular cluster color-magnitude diagrams.

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arXiv1995

Some Constraints on Galaxy Evolution Imposed by the Specific Frequency of Globular Clusters

Early-type dwarf galaxies are found to have high specific globular cluster frequencies which are similar to the high S values that are observed in luminous galaxies of types E and S0. It appears unlikely that cluster formation during mergers between dwarf disk systems can account for the high specific globular cluster frequency that is observed in early-type dwarf galaxies. Some alternative scenarios are briefly discussed.

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arXiv1995

Are the Luminosities of RR Lyrae Stars Affected by Second Parameter Effects?

There is a serious discrepancy between the distance to the LMC derived from the Cepheid Period-Luminosity relation and that obtained by using the Galactic calibration for the luminosity of RR Lyrae stars. It is suggested that this problem might be due to the fact that second parameter effects make it inappropriate to apply Galactic calibrations to RR Lyrae variables in the Magellanic Clouds, i.e. Mv(RR) could depend on both [Fe/H] and on one or more second parameters.

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