Mayall II = G1 in M31: Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy ?

(Abridged version) Mayall II = G1 is one of the brightest globular clusters belonging to M31, the Andromeda galaxy. Our observations with HST/WFPC2 provide data for the (I vs. V-I) and (V vs. V-I) color-magnitude diagrams. From model fitting, we determine a rather high mean metallicity of [Fe/H] = --0.95 +- 0.09, somewhat similar to 47 Tucanae. We find a larger spread in V-I than can be explained by the measurement errors, and we attribute this to an intrinsic metallicity dispersion amongst the stars of G1. So far, only omega Centauri, the giant Galactic globular cluster, has been known to exhibit such an intrinsic metallicity dispersion. Three estimates of the total mass of this globular cluster can be obtained: King mass = 15 x 10^6 with M/Lv ~ 7.5, Virial mass = 7.3 x 10^6 with M/Lv ~ 3.6, and King-Michie mass range from 14 to 17 x 10^6. Although uncertain, all of these mass estimates make G1 more than twice as massive as omega Centauri. Such large masses relate to the metallicity spread whose origin is still unknown (either self-enrichment, an inhomogeneous proto-cluster cloud, or remaining core of a dwarf galaxy). When considering the positions of G1 in the different diagrams defined by Kormendy (1985), G1 always appears on the sequence defined by globular clusters, and definitely away from the other sequences defined by elliptical galaxies, bulges, and dwarf spheroidal galaxies. The same is true for omega Centauri and for the nucleus of the dwarf elliptical NGC 205. This does not prove that all (massive) globular clusters are the remnant cores of nucleated dwarf galaxies.Comment: 24 pages, 5 figures, accepted for publication in AJ (August 2001

Palomar 13: An Unusual Stellar System in the Galactic Halo

We have measured Keck/HIRES radial velocities for 30 candidate red giants in the direction of Palomar 13: an object traditionally cataloged as a compact, low-luminosity globular cluster. From a sample of 21 confirmed members, we find a systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersion of 2.2 km/s. Although small, this dispersion is several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (in solar units) based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears to be anomalous among Galactic globular clusters -- depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of "extra-tidal" stars, or it is far more spatially extended than previously suspected. The full surface density profile is equally well-fit by a King-Michie model having a high concentration and large tidal radius, or by a NFW model. We examine -- and tentatively reject -- a number of possible explanations for the observed characteristics of Palomar 13 (e.g., velocity "jitter" among the red giants, spectroscopic binary stars, non-standard mass functions, modified Newtonian dynamics), and conclude that the two most plausible scenarios are either catastrophic heating during a recent perigalacticon passage, or the presence of a massive dark halo. Thus, the available evidence suggests that Palomar 13 is either a globular cluster which is now in the process of dissolving into the Galactic halo, or a faint, dark-matter-dominated stellar system (ABRIDGED).Comment: 31 pages, 13 postscript figures and 1 color gif image. Also available at http://www.physics.rutgers.edu/ast/ast-rap.html. Accepted for publication in the Astrophysical Journa

The low-mass Initial Mass Function in the 30 Doradus starburst cluster

We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56*57" (14pc*14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayears. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 Msun and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range 1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 Msun at a distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.Comment: Accepted in ApJ. Abstract abridge

Massive perturbers and the efficient merger of binary massive black holes

We show that dynamical relaxation in the aftermath of a galactic merger and the ensuing formation and decay of a binary massive black hole (MBH), are dominated by massive perturbers (MPs) such as giant molecular clouds or clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body stellar relaxation alone, and efficiently scatter stars into the binary MBH's orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the point where energy losses from the emission of gravitational waves (GW) lead to rapid coalescence. We model this process based on observed and simulated MP distributions and take into account the decreased efficiency of the star-binary MBH interaction due to acceleration in the galactic potential. We show that mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a few percent of the typical gas mass in a post-merger nucleus to coalesce in a Hubble time. The fate of a binary MBH in a gas poor galactic merger is less certain, although massive stellar structures (e.g. clusters, stellar rings) could likewise lead to efficient coalescence. These coalescence events are observable by their strong GW emission. MPs thus increase the cosmic rate of such GW events, lead to a higher mass deficit in the merged galactic core and suppress the formation of triple MBH systems and the resulting ejection of MBHs into intergalactic space.Comment: 14 pages, 4 figures, 3 tables. More detailed explanations and changes in structure. Section on hypervelocity stars moved to another paper (in preparation). Results and conclusions unchanged. Accepted to Ap

An Error Analysis of the Geometric Baade-Wesselink Method

We derive an analytic solution for the minimization problem in the geometric Baade-Wesselink method. This solution allows deriving the distance and mean radius of a pulsating star by fitting its velocity curve and angular diameter measured interferometrically. The method also provide analytic solutions for the confidence levels of the best fit parameters, and accurate error estimates for the Baade-Wesselink solution. Special care is taken in the analysis of the various error sources in the final solution, among which the uncertainties due to the projection factor, the limb darkening and the velocity curve. We also discuss the importance of the phase shift between the stellar lightcurve and the velocity curve as a potential error source in the geometric Baade-Wesselink method. We finally discuss the case of the Classical Cepheid zeta Gem, applying our method to the measurements derived with the Palomar Testbed Interferometer. We show how a careful treatment of the measurement errors can be potentially used to discriminate between different models of limb darkening using interferometric techniques.Comment: 24 pages, to be published on the Astrophysical Journal, vol. 603 March 200

Resolved Massive Star Clusters in the Milky Way and its Satellites: Brightness Profiles and a Catalogue of Fundamental Parameters

[Abridged]: We present a database of structural and dynamical properties for 153 spatially resolved star clusters (50 "young massive clusters" and 103 old globulars) in the Milky Way, the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal. This database complements and extends others in the literature, such as those of Harris, and Mackey & Gilmore. By fitting a number of models to the clusters' density profiles, we derive various characteristic surface brightnesses and radii; central potentials, velocity dispersions, and escape velocities; total luminosities, masses, and binding energies; phase-space densities and relaxation timescales; and ``kappa-space'' parameters. Population-synthesis models are used to predict intrinsic (B-V) colors, reddenings, and V-band mass-to-light ratios for the same 153 clusters plus another 63 globulars in the Milky Way, and we compare these predictions to the observed quantities where available. These results are intended to serve as the basis for future investigations of structural correlations and the fundamental plane of massive star clusters, including especially comparisons between the systemic properties of young and old clusters. We also address the question of what structural model fits each cluster best, and argue that the extended halos known to characterize many Magellanic Cloud clusters may be examples of the generic envelope structure of self-gravitating star clusters, not just transient features associated strictly with young age.Comment: Published in ApJS. 60 pages (more than half figures and tables) using emulateapj.sty. A full-resolution PDF version and complete, machine-readable tables can also be found at http://www.astro.le.ac.uk/~dm131/clusters.htm

Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms

We discuss the main properties of the Galactic globular cluster (GC) blue straggler stars (BSS), as inferred from our new catalog containing nearly 3000 BSS. The catalog has been extracted from the photometrically homogeneous V vs. (B-V) color-magnitude diagrams (CMD) of 56 GCs, based on WFPC2 images of their central cores. In our analysis we used consistent relative distances based on the same photometry and calibration. The number of BSS has been normalized to obtain relative frequencies (F_{BSS}) and specific densities (N_S) using different stellar populations extracted from the CMD. The cluster F_{BSS} is significantly smaller than the relative frequency of field BSS. We find a significant anti-correlation between the BSS relative frequency in a cluster and its total absolute luminosity (mass). There is no statistically significant trend between the BSS frequency and the expected collision rate. F_{BSS} does not depend on other cluster parameters, apart from a mild dependence on the central density. PCC clusters act like normal clusters as far as the BSS frequency is concerned. We also show that the BSS luminosity function for the most luminous clusters is significantly different, with a brighter peak and extending to brighter luminosities than in the less luminous clusters. These results imply that the efficiency of BSS production mechanisms and their relative importance vary with the cluster mass.Comment: 12 pages, 3 figures. accepted for publication in ApJ

Ontogenetic trajectories of body coloration reveal its function as a multicomponent nonsenescent signal

The understanding of developmental patterns of body coloration is challenging because of the multicomponent nature of color signals and the multiple selective pressures acting upon them, which further depend on the sex of the bearer and area of display. Pigmentary colors are thought to be strongly involved in sexual selection, while structural colors are thought to generally associate with conspecifics interactions and improve the discrimination of pigmentary colors. Yet, it remains unclear whether age dependency in each color component is consistent with their potential function. Here, we address lifelong ontogenetic variation in three color components (i.e. UV, pigmentary, and skin background colors) in a birth cohort of common lizards Zootoca vivipara across three ventral body regions (i.e. throat, chest, and belly). All three color components developed sexual dichromatism, with males displaying stronger pigmentary and UV colors but weaker skin background coloration than females. The development of color components led to a stronger sexual dichromatism on the concealed ventral region than on the throat. No consistent signs of late‐life decay in color components were found except for a deceleration of UV reflectance increase with age on the throat of males. These results suggest that body color components in common lizards are primarily nonsenescent sexual signals, but that the balance between natural and sexual selection may be altered by the conspicuousness of the area of display. These results further support the view that skin coloration is a composite trait constituted of multiple color components conveying multiple signals depending on age, sex, and body location

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 Tidal Tails of NGC 5466

The study of substructure in the stellar halo of the Milky Way has made a lot of progress in recent years, especially with the advent of surveys like the Sloan Digital Sky Survey. Here, we study the newly discovered tidal tails of the Galactic globular cluster NGC 5466. By means of numerical simulations, we reproduce the shape, direction and surface density of the tidal tails, as well as the structural and kinematical properties of the present-day NGC 5466. Although its tails are very extended in SDSS data (> 45 degrees), NGC 5466 is only losing mass slowly at the present epoch and so can survive for probably a further Hubble time. The effects of tides at perigalacticon and disc crossing are the dominant causes of the slow dissolution of NGC 5466, accounting for about 60 % of the mass loss over the course of its evolution. The morphology of the tails provides a constraint on the proper motion -- the observationally determined proper motion has to be refined (within the stated error margins) to match the location of the tidal tails.Comment: MNRAS, in pres