We study the effects of the external tidal field on the violent relaxation phase of star clusters dynamical evolution, with particular attention to the kinematical properties of the equilibrium ...configurations emerging at the end of this phase. We show that star clusters undergoing the process of violent relaxation in the tidal field of their host galaxy can acquire significant internal differential rotation and are characterized by a distinctive radial variation of the velocity anisotropy. These kinematical properties are the result of the symmetry breaking introduced by the external tidal field in the collapse phase and of the action of the Coriolis force on the orbit of the stars. The resulting equilibrium configurations are characterized by differential rotation, with a peak located between one and two half-mass radii. As for the anisotropy, similar to clusters evolving in isolation, the systems explored in this Letter are characterized by an inner isotropic core, followed by a region of increasing radial anisotropy. However, for systems evolving in an external tidal field, the degree of radial anisotropy reaches a maximum in the cluster intermediate regions and then progressively decreases, with the cluster outermost regions being characterized by isotropy or a mild tangential anisotropy. Young or old but less-relaxed dynamically young star clusters may keep memory of these kinematical fingerprints of their early dynamical evolution.
ABSTRACT
We investigate archival Hubble Space Telescope ACS/SBC F140LP observations of NGC 1399 to search for evidence of multiple stellar populations in extragalactic globular clusters. Enhanced ...far-ultraviolet (FUV) populations are thought to be indicators of He-enhanced second generation populations in globular clusters, specifically extreme/blue horizontal branch stars. Out of 149 globular clusters in the field of view, 58 have FUV counterparts with magnitudes brighter than 28.5. Six of these FUV-detected globular clusters are also detected in X-rays, including one ultraluminous X-ray source (LX > 1039 erg/s). While optically bright clusters corresponded to brighter FUV counterparts, we observe FUV emission from both metal-rich and metal-poor clusters, which implies that the FUV excess is not dependent on optical colour. We also find no evidence that the cluster size influences the FUV emission. The clusters with X-ray emission are not unusually FUV bright, which suggests that even the ultraluminous X-ray source does not provide significant FUV contributions. NGC 1399 is only the fourth galaxy to have its globular cluster system probed for evidence of FUV-enhanced populations, and we compare these clusters to previous studies of the Milky Way, M31, M87, and the brightest cluster in M81. These sources indicate that many globular clusters likely host extreme HB stars and/or second generation stars, and highlight the need for more complete FUV observations of extragalactic globular cluster systems.
ABSTRACT We present the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXBs) in the globular clusters (GCs) and fields of seven early-type galaxies. These galaxies are selected to have ...both deep Chandra observations, which allow their LMXB populations to be observed to X-ray luminosities of 1037-1038 erg s−1, and Hubble Space Telescope optical mosaics that enable the X-ray sources to be separated into field LMXBs, GC LMXBs, and contaminating background and foreground sources. We find that at all luminosities the number of field LMXBs per stellar mass is similar in these galaxies. This suggests that the field LMXB populations in these galaxies are not effected by the GC specific frequency, and that properties such as binary fraction and the stellar initial mass function are either similar across the sample or change in a way that does not affect the number of LMXBs. We compare the XLF of the field LMXBs to that of the GC LMXBs and find that they are significantly different with a p-value of 3 × 10−6 (equivalent to 4.7 for a normal distribution). The difference is such that the XLF of the GC LMXBs is flatter than that of the field LMXBs, with the GCs hosting relatively more bright sources and fewer faint sources. A comparison of the XLF of the metal-rich and metal-poor GCs hints that the metal-poor clusters may have more bright LMXBs, but the difference is not statistically significant.
We investigate the integrated far-ultraviolet (FUV) emission from globular clusters. We present new FUV photometry of M87's clusters based on archival Hubble Space Telescope (HST) Wide Field ...Planetary Camera 2 F170W observations. We use these data to test the reliability of published photometry based on HST space telescope imaging spectrograph FUV-MAMA observations, which are now known to suffer from significant red-leak. We generally confirm these previous FUV detections, but suggest they may be somewhat fainter. We compare the FUV emission from bright (M sub( V) < -9.0) clusters in the Milky Way, M31, M81 and M87 to each other and to the predictions from stellar populations models. Metal-rich globular clusters show a large spread in FUV - V, with some clusters in M31, M81 and M87 being much bluer than standard predictions. This requires that some metal-rich clusters host a significant population of blue/extreme horizontal branch (HB) stars. These hot HB stars are not traditionally expected in metal-rich environments, but are a natural consequence of multiple populations in clusters -- since the enriched population is observed to be He enhanced and will therefore produce bluer HB stars, even at high metallicity. We conclude that the observed FUV emission from metal-rich clusters in M31, M81 and M87 provides evidence that He-enhanced second populations, similar to those observed directly in the Milky Way, may be a ubiquitous feature of globular clusters in the local Universe. Future HST FUV photometry is required to both confirm our interpretation of these archival data and provide constraints on He-enriched second populations of stars in extragalactic globular clusters.
Abstract
We use multiband archival Hubble Space Telescope (HST) observations to measure the photometric and structural parameters of the M81 globular cluster (GC) that hosts the fast radio burst FRB ...20200120E. Our best-fitting King model has an effective radius
r
h
= 3.06 pc with a moderate King model concentration of
c
= 53, and an inferred core radius of 0.81 pc. We revisit the exact astrometric location of the FRB within the cluster, and find that FRB 20200120E is located 1.92 pc from the center, but within the projected half-light radius. We estimate the relative encounter rate of the FRB host, along with the corresponding rates of 210 other GCs in M81, and compare these values with the encounter rates of Galactic GCs. The FRB resides in a GC with an encounter rate that is moderately higher than the median stellar encounter rate in our two comparison samples. While the estimated encounter rate of the FRB host cluster (e.g., ∼50% of a cluster like 47 Tuc) is sufficient to allow the possibility that the FRB formed dynamically, our results do not place strong constraints on this scenario due to the limitations of the available HST data and the possible systematic uncertainties and selection effects in the comparison data.
ABSTRACT
RZ2109 is the first of several extragalactic globular clusters shown to host an ultraluminous X-ray source. RZ2109 is particularly notable because optical spectroscopy shows it has broad, ...luminous O iii λλ4959,5007 emission, while also having no detectable hydrogen emission. The X-ray and optical characteristics of the source in RZ2109 make it a good candidate for being a stellar mass black hole accreting from a white dwarf donor (i.e. an ultracompact black hole X-ray binary). In this paper we present optical spectroscopic monitoring of the O iii5007 emission line from 2007 to 2018. We find that the flux of the emission line is significantly lower in recent observations from 2016 to 2018 than it was in earlier observations in 2007–2011. We also explore the behaviour of the emission line shape over time. Both the core and the wings of the emission line decline over time, with some evidence that the core declines more rapidly than the wings. However, the most recent observations (in 2019) unexpectedly show the emission line core rebrightening
A black hole in a globular cluster Maccarone, Thomas J; Kundu, Arunav; Zepf, Stephen E ...
Nature,
01/2007, Letnik:
445, Številka:
7124
Journal Article
Recenzirano
Odprti dostop
Globular star clusters contain thousands to millions of old stars packed within a region only tens of light years across. Their high stellar densities make it very probable that their member stars ...will interact or collide. There has accordingly been considerable debate about whether black holes should exist in these star clusters. Some theoretical work suggests that dynamical processes in the densest inner regions of globular clusters may lead to the formation of black holes of ∼1,000 solar masses. Other numerical simulations instead predict that stellar interactions will eject most or all of the black holes that form in globular clusters. Here we report the X-ray signature of an accreting black hole in a globular cluster associated with the giant elliptical galaxy NGC 4472 (in the Virgo cluster). This object has an X-ray luminosity of about 4 × 1039 erg s-1, which rules out any object other than a black hole in such an old stellar population. The X-ray luminosity varies by a factor of seven in a few hours, which excludes the possibility that the object is several neutron stars superposed.
ABSTRACT
Ultraluminous X-ray sources (ULXs) in globular clusters (GCs) are low-mass X-ray binaries that achieve high X-ray luminosities through a currently uncertain accretion mechanism. Using ...archival Chandra and Hubble Space Telescope observations, we perform a volume-limited search (≲70 Mpc) of 21 of the most massive ($\gt 10^{11.5} \, \mathrm{M}_\odot$) early-type galaxies to identify ULXs hosted by GC candidates. We find a total of 34 ULX candidates above the expected background within five times the effective radius of each galaxy, with 10 of these ($\sim 29.4{{\ \rm per\ cent}}$) potentially hosted by a GC. A comparison of the spatial and luminosity distributions of these new candidate GC ULXs with previously identified GC ULXs shows that they are similar: both samples peak at LX ∼ a few × 1039 erg s−1 and are typically located within a few effective radii of their host galaxies.
ABSTRACT
We have identified seven ultraluminous X-ray sources (ULXs) that are coincident with globular cluster candidates (GC) associated with M87. ULXs in the old GC environment represent a new ...population of ULXs, and ones likely to be black holes. In this study, we perform detailed X-ray spectroscopic follow-up to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 yr. This study brings the total known sample of GC ULXs to 17. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the X-ray spectra of the majority of GC ULXs are best fit by single-component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two-component model. We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples. We find that the clusters that host ULXs in M87 have metallicities from g − z = 1.01 to g − z = 1.70. The best-fitting power-law indices of the X-ray spectra range from Γ = 1.37 to 2.21, and the best-fitting inner blackbody disc temperatures range from kT = 0.56 to 1.90 keV.
ABSTRACT
The globular cluster ultraluminous X-ray source, RZ 2109, is a complex and unique system that has been detected at X-ray, ultraviolet, and optical wavelengths. Based on almost 20 yr of ...Chandra and XMM–Newton observations, the X-ray luminosity exhibits order of magnitude variability, with the peak flux lasting on the order of a few hours. We perform robust time series analysis on the archival X-ray observations and find that this variability is periodic on a time-scale of 1.3 ± 0.04 d. The source also demonstrates broad O iii λ5007 emission, which has been observed since 2004, suggesting a white dwarf donor and therefore an ultra-compact X-ray binary. We present new spectra from 2020 and 2022, marking 18 yr of observed O iii emission from this source. Meanwhile, we find that the globular cluster counterpart is unusually bright in the NUV/UVW2 band. Finally, we discuss RZ 2109 in the context of the eccentric Kozai–Lidov mechanism and show that the observed 1.3 d periodicity can be used to place constraints on the tertiary configuration, ranging from 20 min (for a 0.1 M⊙ companion) to approximately 95 min (for a 1 M⊙ companion), if the eccentric Kozai–Lidov mechanism is at the origin of the periodic variability.
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