Related papers: Glow-in-the-dark globular clusters: modelling thei…
We review the multiple population (MP) phenomenon of globular clusters (GCs): i.e., the evidence that GCs typically host groups of stars with different elemental abundances and/or distinct sequences in photometric diagrams. Most Galactic…
A year after \emph{Fermi} was launched, the number of known gamma-ray pulsars has increased dramatically. For the first time, a sizable population of pulsars has been discovered in gamma-ray data alone. For the first time, millisecond…
Gravitational lensing is brightening of background objects due to deflection of light by foreground sources. Rich clusters of galaxies are very effective lenses because they are centrally concentrated. Such natural Gravitational Telescopes…
Employing Fermi-LAT gamma ray observations, several independent groups have found excess extended gamma ray emission at the Galactic center (GC). Both, annihilating dark matter (DM) or a population of $\sim 10^3$ unresolved millisecond…
New and archival HST images of the globular cluster systems of 17 relatively nearby galaxies and 6 more distant galaxies have been used to establish the characteristics of GC subpopulations over a range of host galaxy morphological type…
The origin of the diffuse extragalactic, high-energy gamma-ray background (EGRB) filling the Universe remains unknown. The spectrum of this extragalactic radiation, as measured by the EGRET on-board CGRO, is well-fit by a power law across…
Millisecond pulsars are very likely the main source of gamma-ray emission from globular clusters. However, the relative contributions of two separate emission processes--curvature radiation from millisecond pulsar magnetospheres vs. inverse…
We report the Fermi Large Area Telescope detection of gamma-ray (>100 megaelectronvolts) pulsations from pulsar J1823-3021A in the globular cluster NGC 6624 with high significance (~7 sigma). Its gamma-ray luminosity L_gamma = (8.4 +/- 1.6)…
It is now widely established that globular clusters host robust populations of white dwarfs, neutron stars, and black holes throughout their lifetimes. Within clusters, dynamical processes enabled by stellar densities thousands to millions…
Substructure in globular cluster (GC) populations around large galaxies is expected in galaxy formation scenarios that involve accretion or merger events, and it has been searched for using direct associations between GCs and structure in…
In galaxy clusters, hierarchical assembly predicts the formation of stellar substructures and intracluster light (ICL), a diffuse stellar component tracing the global cluster potential. Because these features are extremely faint,…
Although globular clusters are generally chemically homogeneous, substantial abundance variations are sometimes seen even among unevolved main sequence stars, especially for the CNO group of elements. Multi-object intermediate-dispersion…
Pulsed emission from gamma-ray pulsars originates inside the magnetosphere, from radiation by charged particles accelerated near the magnetic poles or in the outer gaps. In polar cap models, the high energy spectrum is cut off by magnetic…
We discuss some of the key open questions regarding the formation and evolution of globular clusters (GCs) during galaxy formation and assembly within a cosmological framework. The current state-of-the-art for both observations and…
Globular clusters produce orders of magnitude more millisecond pulsars per unit mass than the Galactic disk. Since the first cluster pulsar was uncovered twenty years ago, at least 138 have been identified - most of which are binary…
More than 100 radio pulsars have been detected in 24 globular clusters. The largest observed samples are in Terzan 5 and 47 Tucanae, which together contain 45 pulsars. Accurate timing solutions, including positions in the cluster, are known…
Analysing 9 years of Fermi-LAT observations, we recently studied the spectral properties of the prominent globular cluster 47 Tuc (Brown et al. 2018). In particular, we investigated several models to explain the observed gamma-ray emission,…
Diffuse X-ray emission has been detected from a few Galactic globular clusters (GCs), whereas its nature still remains largely unclear. The GC Terzan 5 was previously found to show a significant diffuse thermal X-ray excess from its field,…
Terzan 5 is the only Galactic globular cluster that has plausibly been detected in the very-high-energy range. Stacking upper limits by H.E.S.S. on the integral $\gamma$-ray flux of a population of other globular clusters are very…
We have studied globular cluster systems (GCSs) around elliptical galaxies in Hickson compact groups using multi-band deep, high quality images from Keck, VLT and CFHT. Analyzing the luminosity functions, specific frequencies, color and…