arXiv subjects

Sidney van den Bergh

Publications and source records attributed to Sidney van den Bergh.

At least 37 records · Page 2Linked to original sources

arXiv2007

Globular Clusters and Dwarf Spheroidal Galaxies

Traditionally globular clusters and dwarf spheroidal galaxies have been distinguished by using one or more of the following criteria: (1) mass, (2) luminosity, (3) size, (4) mass-to-light ratio and (5) spread in metallicity. However, a few recently discovered objects show some overlap between the domains in parameter space that are occupied by galaxies and clusters. In the present note it is shown that ellipticity can, in some cases, be used to help distinguish between globular clusters and dwarf spheroidal galaxies.

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arXiv2007

Some Musings on Galaxy Morphology

Careful inspection of large-scale photographs of Shapley-Ames galaxies seems to show a smooth transition between the morphological characteristics of galaxies located on the narrow red, and on the broad blue, sequences in the galaxian color-magnitude diagram. In other words there does not appear to be a dichotomy between blue and red galaxies. Both the colors and the morphologies of galaxies are found to correlate strongly with their environments. Red and early-type Shapley-Ames galaxies are dominant in clusters, whereas blue late-type star forming objects dominate the general field. Interestingly the colors and morphologies of galaxies in small groups resemble the field and differ from those in clusters. As noted by Baade the presence of dust and star formation are very closely correlated, except in a few galaxies that probably had unusual evolutionary histories. Over the entire range from S0 to Sc there is no significant difference between the integrated colors of normal and barred objects suggesting that the formation of a bar does not significantly affect the stellar evolutionary history of a galaxy.

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arXiv2007

Proper Motions in the Galactic Bulge: Plaut's Window

A proper motion study of a field of 20' x 20' inside Plaut's low extinction window (l,b)=(0 deg,-8 deg), has been completed. Relative proper motions and photographic BV photometry have been derived for ~21,000 stars reaching to V~20.5 mag, based on the astrometric reduction of 43 photographic plates, spanning over 21 years of epoch difference. Proper motion errors are typically 1 mas/yr and field dependent systematics are below 0.2 mas/yr. Cross-referencing with the 2MASS catalog yielded a sample of ~8,700 stars, from which predominantly disk and bulge subsamples were selected photometrically from the JH color-magnitude diagram. The two samples exhibited different proper-motion distributions, with the disk displaying the expected reflex solar motion as a function of magnitude. Galactic rotation was also detected for stars between ~2 and ~3 kpc from us. The bulge sample, represented by red giants, has an intrinsic proper motion dispersion of (sigma_l,sigma_b)=(3.39, 2.91)+/-(0.11,0.09) mas/yr, which is in good agreement with previous results, and indicates a velocity anisotropy consistent with either rotational broadening or tri-axiality. A mean distance of 6.37^{+0.87}_{-0.77} kpc has been estimated for the bulge sample, based on the observed K magnitude of the horizontal branch red clump. The metallicity [M/H] distribution was also obtained for a subsample of 60 bulge giants stars, based on calibrated photometric indices. The observed [M/H] shows a peak value at [M/H]~-0.1 with an extended metal poor tail and around 30% of the stars with supersolar metallicity. No change in proper motion dispersion was observed as a function of [M/H]. We are currently in the process of obtaining CCD UBVRI photometry for the entire proper-motion sample of ~21,000 stars.

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arXiv2007

The Luminosity Distribution of Globular Clusters in Dwarf Galaxies

The majority of the globular clusters associated with the Sagittarius dwarf galaxy are faint. In this respect it differs significantly from the globular cluster systems surrounding typical giant galaxies. The observation that most of globular clusters in the outer halo of the Galaxy are also sub-luminous may be understood by assuming that these clusters once also belonged to faint cluster-rich dwarf systems that were subsequently captured and destroyed by the Milky Way System.

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arXiv2006

A Comparison Between the Globular Clusters in NGC 5128 and the Galaxy

Some of the properties of the globular clusters in NGC 5128 are compared to those of Galactic globular clusters. Assuming the color- metallicity relations that hold for Galactic globular clusters then the metal-poor clusters in NGC 5128 that have [Fe/H] < -1.80 are significantly fainter than are the more metal-rich globulars in that galaxy. No such metallicity dependent luminosity difference is observed among the globular clusters associated with the Milky Way. Furthermore the NGC 5128 cluster sample contains two objects that, on the basis of their observed colors, appear to be super metal-poor. It is speculated that many of these apparently faint and metal-poor clusters in NGC 5128 are actually objects resembling intermediate-age Galactic open clusters. It is also found that large clusters with FWHM > 10 pc are typically less luminous in NGC 5128 than are their more more compact counterparts. In this respect the NGC 5128 cluster system is similar to the Galactic globular cluster system. Finally, the present data may hint at the possibility that the NGC 5128 cluster system differs from that surrounding the Milky Way, in that the NGC 5128 objects do not seem to exhibit a clear cut gap between the regions of the FWHM vs M_v plane that are occupied by globular clusters and dwarf spheroidal galaxies.

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arXiv2006

The Dwarf Satellites of M31 and the Galaxy

The satellite systems of M31 and the Galaxy are compared. It is noted that all five of the suspected stripped dSph cores of M31 companions are located within a projected distance of 40 kpc of from the nucleus of this galaxy, whereas the normal dSph companions to this object have distances > 40 kpc from the center of M31. All companions within 200 kpc < D(M31) < 600 kpc are late-type objects. The companions to the Galaxy appear to exhibit different systematics with the irregular LMC and SMC being located at small R_{gc}. It is speculated that this difference might be accounted for by assuming that the Magellanic Clouds are interlopers that were originally formed in the outer reaches of the Local Group. The radial distribution of the total sample of 40 companions of M31 and the Galaxy, which is shown in Figure 1, may hint at the possibility that these objects contain distinct populations of core (R < 25 kpc) and halo (R > 25 kpc) satellites.)

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arXiv2005

Globular Clusters in Dwarf and Giant Galaxies

The luminosity distribution of globular clusters shows a dramatic dependence on parent galaxy luminosity. Dwarf galaxies contain far more faint globulars than do luminous galaxies. This difference is significant at the 99.7% level. On the other hand the luminosity distribution of globular clusters in dwarf galaxies does not appear to depend strongly on their host's morphological type. The dichotomy of globular cluster masses occurs at a host galaxy mass of ~ 4 x 10^8 solar masses, which is almost two orders of magnitude lower than the onset of the dichotomy in globular color characteristics at ~ 3 x 10^10 solar masses that was recently noted by Forbes.

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arXiv2005

037-B327 in M31: Luminous Globular Cluster or Core of a Former Dwarf Spheroidal Companion to M31?

037-B327 is of interest because it is both the most luminous and the most highly reddened cluster known in M31. Deep observations with the Advanced Camera for Surveys on the Hubble Space Telescope provide photometric data in the F606W band, and also show that this cluster is crossed by a dust lane. We determined the structural parameters of 037-B327 by fitting the observed surface brightness distribution to a King model with r_c=0.72" (=2.69 pc), and r_t=5.87" (=21.93 pc), and a concentration index c=log(r_t/r_c)=0.91. The surface brightness profile appears to be essentially flat within 0.25" of the center and shows no signs of core collapse. Although the dust lane affects the photometry, the King model fits the surface brightness profile well except for the regions badly affected by the dust lane. We also calculate the half-light radius r_h=1.11" (=4.15 pc). Combined with previous photometry, we find that this object falls in the same region of the M_V versus log R_h diagram as do Omega Centauri, M54 and NGC 2419 in the Milky Way and the massive cluster G1 in M31. All four of these objects have been claimed to be the stripped cores of former dwarf galaxies. This suggests that 037-B327 may also be the stripped core of a former dwarf companion to M31.

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arXiv2005

Diameters of Open Star Clusters

The present paper presents a tabulation of data on all 600 Galactic open clusters for which it is presently possible to calculate linear diameters. As expected, the youngest `clusters' with ages < 15 Myr, contain a significant (greater than or equal to 20%) admixture of associations. Among intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median cluster diameter is found to increase with age. Small compact clusters are rare among objects with ages > 1.5 Gyr. Open clusters with ages > 1 Gyr appear to form what might be termed a `cluster thick disk', part of which consistst of objects that were probably captured gravitationally by the main body of the Galaxy.

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arXiv2005

Globular Clusters in Dwarf Galaxies

Data are presently available on the luminosities and half-light radii of 101 globular clusters associated with low-luminosity parent galaxies. The luminosity distribution of globulars embedded in dwarf galaxies having $M_{v} > -16$ is found to differ dramatically from that for globular clusters surrounding giant host galaxies with $M_{v} < -16$. The luminosity distribution of globular clusters in giant galaxies peaks at $M_{v} \sim -7.5$, whereas that for dwarfs is found to increases monotonically down to the completeness limit of the cluster data at $M_{v} \sim -5.0$. Unexpectedly, the power law distribution of the luminosities of globular clusters hosted by dwarf galaxies is seen to be much flatter than the that of bright unevolved part of the luminosity distribution of globular clusters associated with giant galaxies. The specific frequency of globular clusters that are fainter than $M_{v} = -7.5$ is found to be particularly high in dwarf galaxies. The luminosity distribution of the LMC globular clusters is similar to that in giant galaxies, and differs from those of the globulars in dwarf galaxies. The present data appear to show no strong dependence of globular cluster luminosity on the morphological types of their parent galaxies. No attempt is made to explain the unexpected discovery that the luminosity distribution of globular clusters is critically dependent on parent galaxy luminosity (mass?), but insensitive to the morphological type of their host galaxy.

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arXiv2005

Classifications of the Host Galaxies of Supernovae, Set III

A homogeneous sample comprising host galaxies of 604 recent supernovae, including 212 objects discovered primarily in 2003 and 2004, has been classified on the David Dunlap Observatory system. Most SN 1991bg-like SNe Ia occur in E and E/Sa galaxies, whereas the majority of SN 1991T-like SNe Ia occur in intermediate-type galaxies. This difference is significant at the 99.9% level. As expected, all types of SNe II are rare in early-type galaxies, whereas normal SNe Ia occur in all Hubble types. This difference is significant at the 99.99% level. A small number of SNe II in E galaxies might be due to galaxy classification errors, or to a small young-population component in these mainly old objects. No significant difference is found between the distributions over Hubble type of SNe Ibc and SNe II. This confirms that both of these types of objects have similar (massive) progenitors. The present data show that, in order to understand the dependence of supernova type on host-galaxy population, it is more important to obtain accurate morphological classifications than it is to increase the size of the data sample.

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arXiv2005

The Globular Cluster Content of Maffei 1

Near-infrared images have been used to search for bright globular clusters near the center of the elliptical galaxy Maffei 1. The number of objects in various radial intervals from the center of Maffei 1 are compared with the number density of sources in two control fields, and we find an excess of 31 +/- 11 objects with K between 16.6 and 18.0 in an annulus between 20 and 90 arcsec from the galaxy center. At radii in excess of 90 arcsec the ability to find clusters is frustrated by statistical noise in the foreground star population. It is demonstrated that the globular clusters located within 90 arcsec of the center of Maffei 1 span a range of near-infrared colors that are at least as great as that which is seen among M31 clusters; some of the clusters appear to have colors that are consistent with them being very young. The specific globular cluster frequency within 90 arcsec of the center of Maffei 1 is S_N = 1.3 +/- 0.6 if the distance modulus is 28.1 +/- 0.2 and A_V = 4.7 +/- 0.2. The specific cluster frequency of Maffei 1 is thus similar to that of other nearby field elliptical galaxies.

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arXiv2005

The properties of Galactic globular cluster sub-systems

In this paper we compare the properties of three sub-systems of Galactic globular clusters, which are defined according to metallicity and HB morphology. We focus on cluster luminosities, structures, surface brightnesses (SB) and ellipticities. It is shown that the so-called "young" halo (YH) clusters, which may have been formed in external satellite galaxies, exhibit characteristics clearly distinct from those of the "old" halo (OH) and bulge/disk (BD) clusters, the majority of which are believed to be Galactic natives. The YH clusters have properties most similar to those of clusters belonging to several present day satellite dwarf galaxies. The OH and BD populations have apparently been modified by destructive tidal forces in the inner Galaxy. We estimate that the present population of Galactic natives may only represent some two-thirds of the original population. Several low SB clusters are highly flattened. We briefly speculate that this ellipticity may represent the intrinsic flattening of dark matter mini-halos in which such optically dim clusters might be embedded. Finally, we examine the distribution of clusters on the size vs.luminosity plane. Three objects fall above the sharp upper envelope of the main distribution on this plane: Omega Cen, M54, and NGC 2419. All three have previously, and independently, been suggested to be the stripped cores of former dwarf galaxies. This suspicion is strengthened by the observation that the massive cluster G1 in M31, plus a number of the most luminous clusters in NGC 5128 also fall in the same region of the plane. All of these objects have also previously been suggested to be the stripped cores of now-defunct dwarf galaxies.

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arXiv2005

Properties of Extremely Red Objects in an Overdense Region

We use a serendipitously discovered overdensity of extremely red objects (EROs) to study the morphologies and cumulative surface number density of EROs in a dense environment. Our extremely deep imaging allows us to select very faint EROs, reaching Ks=21, or ~2 magnitudes fainter than the L* of passively evolving ellipticals at z=1.5. We find that the shape of the ERO cumulative surface number density in our overdense field mimics that of the field ERO population over all magnitudes down to Ks=21 but with a factor of 3-4 higher normalization. The excellent seeing in our images (0.4" in Ks and 0.6" in R) allows for morphological classification of the brighter (Ks<19) EROs and we find a mix of morphologies including interacting systems and disks; the fraction of pure bulges (at most 38%), galaxies with disks (at least 46%), and interacting systems (at least 21%) is consistent with morphological fractions in field ERO studies. The similarity in the shape of the cumulative surface density and morphological mix between our overdense field and the field ERO population suggests that ERO galaxies in overdense regions at z~1-2 may not have had an appreciably different history than those in the field.

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arXiv2004

Globular clusters and the formation of the outer Galactic halo

Globular clusters in the outer halo (R_gc > 15 kpc) are found to be systematically fainter than those at smaller Galactocentric distances. Within the outer halo the compact clusters with half-light radii R_h < 10 pc are only found at R_gc < 40 kpc, while on the other hand the larger clusters with R_h > 10 pc are encountered at all Galactocentric distances. Among the compact clusters with R_h < 10 pc that have R_gc > 15 kpc, there are two objects with surprisingly high metallicities. One of these is Terzan 7, which is a companion of the Sagittarius dwarf. The other is Palomar 1. The data on these two objects suggests that they might have had similar evolutionary histories. It is also noted that, with one exception, luminous globular clusters in the outer halo are all compact whereas faint ones may have any radius. This also holds for globular clusters in the LMC, SMC and Fornax dwarf. The lone exception is the large luminous globular NGC 2419. Possibly this object is not a normal globular cluster, but the stripped core of a former dwarf spheroidal. In this respect it may resemble Omega Centauri.

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arXiv2004

Spectral Types of Field and Cluster O-Stars

The recent catalog of spectral types of Galactic O-type stars by Mai'z-Apella'niz et al. has been used to study the differences between the frequencies of various subtypes of O-type stars in the field, in OB associations and among runaway stars. At a high level of statistical significance the data show that O-stars in clusters and associations have earlier types (and hence presumably larger masses and/or younger ages) than those that are situated in the general field. Furthermore it is found that the distribution of spectral subtypes among runaway O-stars is indistinguishable from that among field stars, and differs significantly from that of the O-type stars that are situated in clusters and associations. The difference is in the sense that runaway O-stars, on average, have later subtypes than do those that are still located in clusters and associations.

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arXiv2004

Shapley-Ames Galaxies in the Blue and Infrared

The Shapley-Ames Catalog of 1276 galaxies with B < 12.5 is compared with the Sanders et al. all sky sample of the 629 galaxies with 60 m flux density > 5.24 Jy. The fraction of Shapley-Ames galaxies that are visible in the IR is found to increase from 0.006 for E or E/S0 galaxies to 0.384 for Sc galaxies. The subset of Shapley-Ames galaxies that are detected in the IR has a median blue luminosity that is ~0.8 mag fainter than that of all Shapley-Ames galaxies. Most of this difference is due to the fact that late-type galaxies (which contain dust and hot stars) are systematically less luminous in blue light than are early-type galaxies. Within individual stages along the Hubble sequence no significant differences are found between the luminosity distributions in blue light of galaxies that were detected in the infrared and those that were not. However, our data show a puzzling exception (significant at 99.9%) for SBc galaxies. For reasons that are not understood Shapley-Ames SBc galaxies, that are visible in the IR, are more luminous in blue light than those SBc galaxies that are not detected in the infrared. An other peculiarity of the data is that Shapley-Ames Sc galaxies galaxies are (at 99.6% confidence) more luminous than objects of type SBc .

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arXiv2003

What are S0 (0) Galaxies?

Among early-type galaxies with almost circular isophotes E0 and E1 galaxies are, at 99.3% significance, more luminous than face-on objects classified as S0 (0) and S(0) (1). This result supports the view that rotation and "diskiness" are more important in the outer regions of faint-early type galaxies than they are for more luminous galaxies of very early morphological type.

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