We compare the results of a large grid of N-body simulations with the surface brightness and velocity dispersion profiles of the globular clusters ω Cen and NGC 6624. Our models include clusters with varying stellar-mass black hole retention fractions and varying masses of a central intermediate-mass black hole (IMBH). We find that an ∼ 45 000 M IMBH, whose presence has been suggested based on the measured velocity dispersion profile of ω Cen, predicts the existence of about 20 fast-moving, m > 0.5 M , main-sequence stars with a (1D) velocity v > 60 km s −1 in the central 20 arcsec of ω Cen. However, no such star is present in the HST/ACS proper motion catalogue of Bellini et al. (2017), strongly ruling out the presence of a massive IMBH in the core of ω Cen. Instead, we find that all available data can be fitted by a model that contains 4.6 per cent of the mass of ω Cen in a centrally concentrated cluster of stellar-mass black holes. We show that this mass fraction in stellar-mass BHs is compatible with the predictions of stellar evolution models of massive stars. We also compare our grid of N-body simulations with NGC 6624, a cluster recently claimed to harbour a 20 000 M black hole based on timing observations of millisecond pulsars. However, we find that models with M IMBH > 1000 M IMBHs are incompatible with the observed velocity dispersion and surface brightness profile of NGC 6624, ruling out the presence of a massive IMBH in this cluster. Models without an IMBH provide again an excellent fit to NGC 6624.
Using data from the WiFeS Atlas of Galactic Globular cluster Spectra we study the behaviour of the calcium triplet (CaT), a popular metallicity indicator in extragalactic stellar population studies. A major caveat of these studies is that the potential sensitivity to other stellar population parameters such as age, calcium abundance and the initial mass function has not yet been empirically evaluated. Here we present measurements of the strength of the CaT feature for 113 globular clusters in the Milky Way and its satellite galaxies. We derive empirical calibrations between the CaT index and both the iron abundance ([Fe/H]) and calcium abundance ([Ca/H]), finding a tighter relationship for [Ca/H] than for [Fe/H]. For stellar populations 3 Gyr and older the CaT can be used to reliably measure [Ca/H] at the 0.1 dex level but becomes less reliable for ages of ∼ 2 Gyr and younger. We find that the CaT is relatively insensitive to the horizontal branch morphology. The stellar mass function however affects the CaT strengths significantly only at low metallicities. Using our newly derived empirical calibration, we convert our measured CaT indices into [Ca/H] values for the globular clusters in our sample.
Observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs -twelve of which have no previous kinematic information -using integral-field unit data from the WAGGS project. Using N-body models, we determine dynamical masses and M/L V ratios for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. We find that metal-rich clusters have M/L V more than 2 times lower than what is predicted by simple stellar population models. This confirms that the discrepant M/L-[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects.
Cosmological N-body simulations show that Milky Way–sized galaxies harbor a population of unmerged dark matter (DM) subhalos. These subhalos could shine in gamma-rays and eventually be detected in gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-Large Area Telescope Objects (UFOs) to identify them as possible tera-electron-volt-scale DM subhalo candidates. We search for very-high-energy (E ≳ 100 GeV) gamma-ray emissions using H.E.S.S. observations toward four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any data set of the four observed UFOs or in the combined UFO data set, strong constraints are derived on the product of the velocity-weighted annihilation cross section 〈σ v〉 by the J factor for the DM models. The 95% confidence level observed upper limits derived from combined H.E.S.S. observations reach 〈σ v〉J values of 3.7 × 10−5 and 8.1 × 10−6 GeV2 cm−2 s−1 in the W + W − and τ + τ − channels, respectively, for a 1 TeV DM mass. Focusing on thermal weakly interacting massive particles, the H.E.S.S. constraints restrict the J factors to lie in the range 6.1 × 1019–2.0 × 1021 GeV2 cm−5 and the masses to lie between 0.2 and 6 TeV in the W + W − channel. For the τ + τ − channel, the J factors lie in the range 7.0 × 1019–7.1 × 1020 GeV2 cm−5 and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological N-body simulations on the J-factor distribution for Milky Way–sized galaxies, the DM models with masses >0.3 TeV for the UFO emissions can be ruled out at high confidence level.
Stellar feedback plays a fundamental role in shaping the evolution of galaxies. Here we explore the use of ionised gas kinematics in young, bipolar H ii regions as a probe of early feedback in these star-forming environments. We have undertaken a multiwavelength study of a young, bipolar H ii region in the Galactic disc, G316.81-0.06, which lies at the centre of a massive (∼ 10 3 M ) infrared-dark cloud filament. It is still accreting molecular gas as well as driving a ∼ 0.2 pc ionised gas outflow perpendicular to the filament. Intriguingly, we observe a large velocity gradient (47.81 ± 3.21 km s −1 pc −1 ) across the ionised gas in a direction perpendicular to the outflow. This kinematic signature of the ionised gas shows a reasonable correspondence with the simulations of young H ii regions. Based on a qualitative comparison between our observations and these simulations, we put forward a possible explanation for the velocity gradients observed in G316.81-0.06. If the velocity gradient perpendicular to the outflow is caused by rotation of the ionised gas, then we infer that this rotation is a direct result of the initial net angular momentum in the natal molecular cloud. If this explanation is correct, this kinematic signature should be common in other young (bipolar) H ii regions. We suggest that further quantitative analysis of the ionised gas kinematics of young H ii regions, combined with additional simulations, should improve our understanding of feedback at these early stages.
Hannah Dalgleish examines ways in which astronomy can help to forge a sustainable future for life on Earth.
The International Astronomical Youth Camp has benefited thousands of lives during its 50-year history. We explore the pedagogy behind this success, review a survey taken by more than 300 previous participants, and discuss some of the challenges the camp faces in the future.Summer camps have been enriching the lives of young people for over 100 years. There are now numerous camps dedicated to space and astronomy, such as Space Camp (1982), AstroCamp (USA, 1988), Space School (UK, 1989) and the European Space Camp (1996). Established in 1969, the International Astronomical Youth Camp (IAYC) predates these camps by more than a decade and, as far as we are aware, is both the oldest and longest-running event of its kind.The first IAYC was held in Schmallenberg, West Germany in 1969, and was organised primarily by amateur astronomer Werner Liesmann1. The camp hosted 40 participants who were lectured in astronomy and took part in project work; reportedly, it was a great success and 90 people attended the following year2. Ten years later, in 1979, the International Workshop for Astronomy (IWA) was founded as the formal organisation behind the camp. The current statutes of IWA list two aims:• Promotion of international collaboration and agreement, particularly for astronomical youth work • To spread astronomical knowledge and to teach young people to work scientifically on their own This year (2019) marks the 50th anniversary of the IAYC, and the 55th camp since 1969 (some years had additional camps). Although it now lasts three weeks instead of one, the format of the IAYC is largely unchanged; for a detailed description of the camp see Dalgleish3. So far, IAYCs have been held in 15 countries ( Figure 1) and have hosted 3,468 participants (of whom we estimate at least 1,700 to be unique), aged 16-24, from 81 nationalities worldwide (Figure 2).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.