Globular cluster systems II: On the formation of old globular clusters and their sites of formation

We studied the metal-poor globular cluster (GC) populations of a large variety of galaxies (47 galaxies spanning about 10mag in absolute brightness) and compared their mean [Fe/H] with the properties of the host galaxies. The mean [Fe/H] of the systems lie in the -1.65<[Fe/H]<-1.20 range (74% of the population). Using only GC systems with more than 6 objects detected, 85% of the population lie within -1.65<[Fe/H]<-1.20. The relation between the mean [Fe/H] of the metal-poor GC systems and the Mv of their host galaxies presents a very low slope which includes zero. An analysis of the correlation of the mean [Fe/H] with other galaxy properties also leads to the conclusion that no strong correlation exists. The lack of correlation suggests a formation of all metal-poor GC in similar gas fragments. A weak correlation might exist between mean [Fe/H] of the metal-poor GC and host galaxy metallicity. This would imply that some fragments in which metal-poor GC formed were already embedded in the larger dark matter halo of the final galaxy (as oppose to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor GC in all galaxies, in typical fragments of masses around 10^9-10^10 solar masses with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We compared the mean [Fe/H] of the metal-poor GC populations with the typical metallicities of high-z objects. If we add the constraint that GC need a high column density of gas to form, DLAs are the most likely sites for the formation of metal-poor GC populations.Comment: accepted for publication in AJ, scheduled for the May 2001 issu

The Spin of M87 as measured from the Rotation of its Globular Clusters

We revisit the kinematical data for 204 globular clusters in the halo of M87. Beyond 3 r_eff along the major axis of the galaxy light, these globular clusters exhibit substantial rotation (~ 300 +/- 70 km/s) that translates into an equally substantial spin (lambda ~ 0.18). The present appearance of M87 is most likely the product of a single major merger, since this event is best able to account for so sizable a spin. A rotation this large makes improbable any significant accretion of material after this merger, since that would have diluted the rotation signature. We see weak evidence for a difference between the kinematics of the metal-poor and metal-rich population, in the sense that the metal-poor globular clusters appear to dominate the rotation. If, as we suspect, the last major merger event of M87 was mainly dissipationless and did not trigger the formation of a large number of globular clusters, the kinematic difference between the two could reflect their orbital properties in the progenitor galaxies; these differences would be compatible with these progenitors having formed in dissipational mergers. However, to put strong kinematic constraints on the origin of the globular clusters themselves is difficult, given the complex history of the galaxy and its last dominant merger event.Comment: 20 pages (AAS two column style, including 1 table and 7 figures) accepted in the AJ (November issue), also available at http://www.ucolick.org/~mkissler

Towards an Understanding of the Globular Cluster Over--abundance around the Central Giant Elliptical NGC 1399

We investigate the kinematics of a combined sample of 74 globular clusters around NGC 1399. Their high velocity dispersion, increasing with radius, supports their association with the gravitational potential of the galaxy cluster rather than with that of NGC 1399 itself. We find no evidence for rotation in the full sample, although some indication for rotation in the outer regions. The data do not allow us to detect differences between the kinematics of the blue and red sub-populations of globular clusters. A comparison between the globular cluster systems of NGC 1399 and those of NGC 1404 and NGC 1380 indicates that the globular clusters in all three galaxies are likely to have formed via similar mechanisms and at similar epochs. The only property which distinguishes the NGC 1399 globular cluster system from these others is that it is ten times more abundant. We summarize the evidence for associating these excess globulars with the galaxy cluster rather than with NGC 1399 itself, and suggest that the over-abundance can be explained by tidal stripping, at an early epoch, of neighboring galaxies and subsequent accumulation of globulars in the gravitational potential of the galaxy cluster.Comment: AJ accepted (March issue), 27 pages (6 figures included), AAS style, two columns. Also available at http://www.eso.org/~mkissle

The Age Difference between the Globular Cluster Sub-populations in NGC 4472

The age difference between the two main globular cluster sub-populations in the Virgo giant elliptical galaxy, NGC 4472 (M 49), has been determined using HST WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main sub-populations, down to more than one magnitude below the turn-over of their luminosity functions. This allows precise determinations of both the mean colors and the turn-over magnitudes of the two main sub-populations. By comparing the data with various population synthesis models, the age-metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal-poor and the metal-rich globular clusters are found to be coeval within the errors ($\sim 3$ Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one sub-population is half as old as the other is excluded at the 99% confidence level. The different globular cluster populations are assumed to trace the galaxy's major star-formation episodes. Consequently, the vast majority of globular clusters, and by implication the majority of stars, in NGC 4472 formed at high redshifts but by two distinct mechanisms or in two episodes.Comment: 32 pages, including 12 postscript figures, accepted for publication in the Astronomical Journal, December 1999 issu

Waning Immunity and the Second Wave: Some Projections for SARS-COV-2

This paper offers projections of future transmission dynamics for SARS-CoV-2 in an SEIRS model with demographics and waning immunity. In a stylized optimal control setting calibrated to the USA, we show that the disease is endemic in steady state and that its dynamics are characterized by damped oscillations. The magnitude of the oscillations depends on how fast immunity wanes. The optimal social distancing policy both curbs peak prevalence and postpones the infection waves relative to the uncontrolled dynamics. Last, we perform sensitivity analysis with respect to the duration of immunity, the infection fatality rate and the planning horizon

The Luminosity Function of Star Clusters in Spiral Galaxies

Star clusters in 6 nearby spiral galaxies are examined using archive images from HST/WFPC2. The galaxies have previously been studied from the ground and some of them are known to possess rich populations of "young massive clusters" (YMCs). Comparison with the HST images indicates a success-rate of about 75% for the ground-based cluster detections, with typical contaminants being blends or loose groupings of several stars in crowded regions. The luminosity functions (LFs) of cluster candidates identified on the HST images are analyzed and compared with existing data for the Milky Way and the LMC. The LFs are well approximated by power-laws of the form dN(L)/dL ~ L^alpha, with slopes in the range -2.4<alpha<-2.0. The steeper slopes tend to be found among fits covering brighter magnitude intervals, although direct hints of a variation in the LF slope with magnitude are seen only at low significance in two galaxies. The surface density of star clusters at a reference magnitude of M(V)=-8 scales with the mean star formation rate per unit area, Sigma(SFR). Assuming that the LF can be generally expressed as a power-law with normalization proportional to the galaxy area (A) and Sigma(SFR), the maximum cluster luminosity expected in a galaxy from random sampling of the LF is estimated as a function of Sigma(SFR) and A. The predictions agree well with existing observations of galaxies spanning a wide range of Sigma(SFR) values, suggesting that sampling statistics play an important role in determining the maximum observed luminosities of young star clusters in galaxies.Comment: 43 pages, including 6 tables and 14 figures. Accepted for publication in A

Bulge Globular Clusters in Spiral Galaxies

There is now strong evidence that the metal-rich globular clusters (GC) near the center of our Galaxy are associated with the Galactic bulge rather than the disk as previously thought. Here we extend the concept of bulge GCs to the GC systems of nearby spiral galaxies. In particular, the kinematic and metallicity properties of the GC systems favor a bulge rather than a disk origin. The number of metal-rich GCs normalized by the bulge luminosity is roughly constant (i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for field ellipticals when only the red (metal--rich) GCs are considered. We argue that the metallicity distributions of GCs in spiral and elliptical galaxies are remarkably similar, and that they obey the same correlation of mean GC metallicity with host galaxy mass. We further suggest that the metal-rich GCs in spirals are the direct analogs of the red GCs seen in ellipticals. The formation of a bulge/spheroidal stellar system is accompanied by the formation of metal-rich GCs. The similarities between GC systems in spiral and elliptical galaxies appear to be greater than the differences.Comment: 5 pages, Latex, 2 figures, 1 table, Accepted for publication in ApJ Letter