39 research outputs found
Some Systematics of Galactic Globular Clusters
The global properties of all known Galactic globular clusters are examined.
The relationship between the luminosities and the metallicities of Galactic
globular clusters is found to be complex. Among luminous clusters there is a
correlation in the sense that the oldest clusters are slightly more metal
deficient than are younger clusters. However, no such clear-cut relationship is
found among the faintest globular clusters. The central concentration index C
of globular clusters is seen to be independent of metallicity. The dependence
of the half-light radii of globular clusters on their Galactocentric distances
can be approximated by the relation . Clusters with
collapsed cores are mostly situated close to the Galactic nucleus. For kpc the luminosities and the radii of clusters appear to be uncorrelated.
The Galaxy differs from the LMC and the SMC in that it appears to lack highly
flattened luminous clusters. Galactic globular clusters with ages 13.0
Gyr are all of Oosterhoff type II, whereas almost all of those with ages
13.0 Gyr have been assigned to Oosterhoff type I. Globular clusters with ages
11.5 Gyr are all located in the outer Galactic halo, have below-average
luminosities and above-average radii. On the other hand the very old globular
cluster NGC 6522 is situated close to the Galactic nucleus.Comment: PASP, in pres
Galactic bulge formation as a maximum intensity starburst
Properties of normal galactic star formation, including the density
dependence, threshold density, turbulent scaling relations, and clustering
properties, are applied to the formation of galactic bulges. One important
difference is that the bulge potential well is too deep to have allowed
self-regulation or blow-out by the pressures from young stars, unlike galactic
disks or dwarf galaxies. As a result, bulge formation should have been at the
maximum rate, which is such that most of the gas would get converted into stars
in only a few dynamical time scales, or ~10^8 years. The gas accretion phase
can be longer than this, but once the critical density is reached, which
depends primarily on the total virial density from dark matter, the formation
of stars in the bulge should have been extremely rapid. Such three-dimensional
star formation should also have formed many clusters, like normal disk star
formation today. Some of these clusters may have survived as old globulars, but
most got dispersed, although they might still be observable as concentrated
streams in phase space.Comment: 10 pages, 1 figure, scheduled for ApJ, vol. 517, May 20, 199
Synchronized Formation of Sub-Galactic Systems at Cosmological Reionization: Origin of Halo Globular Clusters
Gas rich sub-galactic halos with mass Mt <= 10^7.5 Msun, while incapable of
forming stars due to lack of adequate coolants, contain a large fraction of
baryonic mass at cosmological reionization. We show that the reionization of
the universe at z=10-20 has an interesting physical effect on these halos. The
external radiation field causes a synchronous inward propagation of an
ionization front towards each halo, resulting in an inward, convergent shock.
The resident gas of mass Mb~10^4-10^7 Msun in low spin (initial dimensionless
spin parameter lambda <= 0.01) halos with a velocity dispersion sigmav <=
11km/s would be compressed by a factor of ~100 in radius and form self
-gravitating baryonic systems. Under the assumption that such compressed
gaseous systems fragment to form stars, the final stellar systems will have a
size 2-40pc, velocity dispersion 1-10km/s and a total stellar mass of M*
10^3-10^6 Msun. The characteristics of these proposed systems seem to match the
observed properties of halo globular clusters. The expected number density is
consistent with the observed number density of halo globular clusters. The
observed mass function of slope ~-2 at the high mass end is predicted by the
model. Strong correlation between velocity dispersion and luminosity (or
surface brightness) and lack of correlation between velocity dispersion and
size, in agreement with observations, are expected. Metallicity is, on average,
expected to be low and should not correlate with any other quantities of
globular clusters, except that a larger dispersion of metallicity among
globular clusters is expected for larger galaxies. The observed trend of
specific frequency with galaxy type may be produced in the model. We suggest
that these stellar systems are seen as halo globular clusters today.Comment: accepted to ApJ, 7 ApJ page
The Efficiency of Globular Cluster Formation
(Abridged): The total populations of globular cluster systems (GCSs) are
discussed in terms of their connection to the efficiency of globular cluster
formation---the mass fraction of star-forming gas that was able to form bound
stellar clusters rather than isolated stars or unbound associations---in galaxy
halos. Observed variations in GCS specific frequencies (S_N=N_gc/L_gal), both
as a function of galactocentric radius in individual systems and globally
between entire galaxies, are reviewed in this light. It is argued that trends
in S_N do not reflect any real variation in the underlying efficiency of
cluster formation; rather, they result from ignoring the hot gas in many large
ellipticals. This claim is checked and confirmed in each of M87, M49, and NGC
1399, for which existing data are combined to show that the volume density
profile of globular clusters, rho_cl, is directly proportional to the sum of
(rho_gas+rho_stars) at large radii. The constant of proportionality is the same
in each case: epsilon=0.0026 +/- 0.0005 in the mean. This is identified with
the globular cluster formation efficiency. The implication that epsilon might
have had a universal value is supported by data on the GCSs of 97 early-type
galaxies, on the GCS of the Milky Way, and on the ongoing formation of open
clusters. These results have specific implications for some issues in GCS and
galaxy formation, and they should serve as a strong constraint on more general
theories of star and cluster formation.Comment: 36 pages with 11 figures; accepted for publication in The
Astronomical Journa
The Multiplicity of Main Sequence Turnoffs in Globular Clusters
We present color-magnitude diagrams of globular clusters for models with
self-enrichment and pre-enrichment. The models with self-enrichment turn out to
have two or more main sequence turnoff points in the color-magnitude diagram if
the fraction of mass lost by the globular cluster under supernova explosions
does not exceed 95-97%. The models with pre-enrichment can have only one main
sequence turnoff point. We argue that the cluster wCen evolved according to a
self-enrichment scenario.Comment: 13 pages, 7 figure