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Register a new userMicrolensing in globular clusters: the first confirmed lens
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Authors
Philippe Jetzer
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Microlensing observations toward globular clusters could be very useful to probe their low mass star and brown dwarf content. Using the large set of microlensing events detected so far toward the Galactic centre we investigated whether for some of the observed events the lenses are located in the Galactic globular clusters. Indeed, we found that in four cases some events might be due to lenses located in the globular clusters themselves. Moreover, we discuss a microlensing event found in M22. Using the adaptive optics system NACO at ESO VLT it was possible to identify the lens, which turned out to be a low mass star of about 0.18 solar masses in the globular cluster M22 itself.
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We present an analysis of the large set of microlensing events detected so far toward the Galactic center with the purpose of investigating whether some of the dark lenses are located in Galactic globular clusters. We find that in four cases some events might indeed be due to lenses located in the globular clusters themselves. We also give a rough estimate for the average lens mass of the events being highly aligned with Galactic globular cluster centers and find that, under reasonable assumptions, the deflectors could most probably be either brown dwarfs, M-stars or stellar remnants.
We report the serendipitous discovery of HSC J142449-005322, a double source plane lens system in the Hyper Suprime-Cam Subaru Strategic Program. We dub the system Eye of Horus. The lens galaxy is a very massive early-type galaxy with stellar mass of ~7x10^11 Msun located at z_L=0.795. The system exhibits two arcs/rings with clearly different colors, including several knots. We have performed spectroscopic follow-up observations of the system with FIRE on Magellan. The outer ring is confirmed at z_S2=1.988 with multiple emission lines, while the inner arc and counterimage is confirmed at z_S1=1.302. This makes it the first double source plane system with spectroscopic redshifts of both sources. Interestingly, redshifts of two of the knots embedded in the outer ring are found to be offset by delta_z=0.002 from the other knots, suggesting that the outer ring consists of at least two distinct components in the source plane. We perform lens modeling with two independent codes and successfully reproduce the main features of the system. However, two of the lensed sources separated by ~0.7 arcsec cannot be reproduced by a smooth potential, and the addition of substructure to the lens potential is required to reproduce them. Higher-resolution imaging of the system will help decipher the origin of this lensing feature and potentially detect the substructure.
We present spectroscopic follow-up observations of 68 red, faint candidates from our multi-epoch, multi-wavelength, previously published survey of NGC 2264. Using near-infrared spectra from VLT/KMOS, we measure spectral types and extinction for 32 young low-mass sources. We confirm 13 as brown dwarfs in NGC 2264, with spectral types between M6 and M8, corresponding to masses between 0.02 and 0.08$M_{odot}$. These are the first spectroscopically confirmed brown dwarfs in this benchmark cluster. 19 more objects are found to be young M-type stars of NGC 2264 with masses of 0.08 to 0.3$,M_{odot}$. 7 of the confirmed brown dwarfs as well as 15 of the M-stars have IR excess caused by a disc. Comparing with isochrones, the typical age of the confirmed brown dwarfs is $<$0.5 to 5Myr. More than half of the newly identified brown dwarfs and very low mass stars have ages $<$0.5Myr, significantly younger than the bulk of the known cluster population. Based on the success rate of our spectroscopic follow-up, we estimate that NGC 2264 hosts 200-600 brown dwarfs in total (in the given mass range). This would correspond to a star-to-brown dwarf ratio between 2.5:1 and 7.5:1. We determine the slope of the substellar mass function as $alpha = 0.43^{+0.41}_{-0.56}$, these values are consistent with those measured for other young clusters. This points to a uniform substellar mass function across all star forming environments.
In the last decade, a new kind of stellar systems has been established that shows properties in between those of globular clusters (GCs) and early-type dwarf galaxies. These so-called ultra-compact dwarf galaxies (UCDs) have masses in the range 10^6 to 10^8 M_sun and half-light radii of 10-100 pc. The most massive UCDs known to date are predominantly metal-rich and reside in the cores of nearby galaxy clusters. The question arises whether UCDs are just the most massive globular clusters in rich globular cluster systems? Although UCDs and `normal GCs form a continuous sequence in several parameter spaces, there seems to be a break in the scaling laws for stellar systems with masses above ~2.5x10^6 M_sun. Unlike GCs, UCDs follow a mass-size relation and their mass-to-light ratios are about twice as large as those of GCs with comparable metallicities. In this contribution, I present the properties of the brightest globular clusters and ultra-compact dwarf galaxies and discuss whether the observed findings are compatible with a `star-cluster origin of UCDs or whether they are more likely related to dark matter dominated dwarf galaxies.
We present an estimate of the absolute age of 68 galactic globular clusters obtained by exploiting the distribution of stars in the full color-magnitude diagram. In particular, we jointly estimate the absolute age, distance, reddening, metallicity ([Fe/H]) and [$alpha$/Fe] of each cluster, imposing priors motivated by independent observations; we also estimate possible systematics from stellar modeling. Our derived distances for the globular cluster sample are in agreement with those obtained from GAIA using main-sequence dwarf stars (where available), and the inferred ages are in good agreement with those previously published. The novelty of our approach is that, with the adopted priors, we are able to estimate robustly these parameters from the globular cluster color-magnitude diagram. We find that the average age of the oldest globular clusters is $t_{rm GC}=13.32 pm 0.1 {rm (stat.)} pm 0.5 {rm (sys.)}$, at 68% confidence level, including systematic uncertainties from stellar modeling. These measurements can be used to infer the age of the Universe, largely independently of the cosmological parameters: we find an age of the Universe $t_{rm U}=13.5^{+0.16}_{-0.14} {rm (stat.)} pm 0.5 ({rm sys.})$ at 68% confidence level, accounting for the formation time of globular clusters and its uncertainty. This value is compatible with $13.8 pm 0.02$ Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the $Lambda$CDM model.
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