129 research outputs found
Optimal Integrated Abundances for Chemical Tagging of Extragalactic Globular Clusters
High resolution integrated light (IL) spectroscopy provides detailed
abundances of distant globular clusters whose stars cannot be resolved.
Abundance comparisons with other systems (e.g. for chemical tagging) require
understanding the systematic offsets that can occur between clusters, such as
those due to uncertainties in the underlying stellar population. This paper
analyses high resolution IL spectra of the Galactic globular clusters 47 Tuc,
M3, M13, NGC 7006, and M15 to (1) quantify potential systematic uncertainties
in Fe, Ca, Ti, Ni, Ba, and Eu and (2) identify the most stable abundance ratios
that will be useful in future analyses of unresolved targets. When stellar
populations are well-modelled, uncertainties are ~0.1-0.2 dex based on
sensitivities to the atmospheric parameters alone; in the worst case scenarios,
uncertainties can rise to 0.2-0.4 dex. The [Ca I/Fe I] ratio is identified as
the optimal integrated [alpha/Fe] indicator (with offsets <0.1 dex), while [Ni
I/Fe I] is also extremely stable to within <0.1 dex. The [Ba II/Eu II] ratios
are also stable when the underlying populations are well modelled and may also
be useful for chemical tagging.Comment: 28 pages, 4 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (R_gc > 15 kpc) are found to be
systematically fainter than those at smaller Galactocentric distances. Within
the outer halo the compact clusters with half-light radii R_h < 10 pc are only
found at R_gc
10 pc are encountered at all Galactocentric distances. Among the compact
clusters with R_h 15 kpc, there are two objects with
surprisingly high metallicities. One of these is Terzan 7, which is a companion
of the Sagittarius dwarf. The other is Palomar 1. The data on these two objects
suggests that they might have had similar evolutionary histories. It is also
noted that, with one exception, luminous globular clusters in the outer halo
are all compact whereas faint ones may have any radius. This also holds for
globular clusters in the LMC, SMC and Fornax dwarf. The lone exception is the
large luminous globular NGC 2419. Possibly this object is not a normal globular
cluster, but the stripped core of a former dwarf spheroidal. In this respect it
may resemble Omega Centauri.Comment: 7 pages, 7 figures. Accepted for publication in MNRA
Evidence of differential tidal effects in the old globular cluster population of the Large Magellanic Cloud
We present for the first time extended stellar density and/or surface brightness radial profiles for almost all the known LargeMagellanic Cloud (LMC) old globular clusters (GCs). These were built from DECam images and reach out to ~ 4 times the GCs' tidal radii. The background subtracted radial profiles reveal that the GCs located closer than ~ 5 kpc from the LMC centre contain an excess of stars in their outermost regions with respect to the stellar density expected from a King profile. Such a residual amount of stars, not seen in GCs located farther than ~ 5 kpc from the LMC centre, as well as the GCs' dimensions, shows a clear dependence with the GCs' positions in the galaxy, in the sense that, the farther the GC from the centre of the LMC, the larger both the excess of stars in its outskirts and size. Although the masses of GCs located inside and outside ~ 5 kpc are commensurate, the outermost regions of GCs located closer than ~ 5 kpc from the LMC centre appear to have dynamically evolved more quickly. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC centre.Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Mackey, A. Dougal. The Australian National University; Australi
No sign (yet) of intergalactic globular clusters in the Local Group
We present Gemini Multi-Object Spectrograph (GMOS) imaging of 12 candidate intergalactic globular clusters (IGCs) in the Local Group, identified in a recent survey of the Sloan Digital Sky Survey (SDSS) footprint by di Tullio Zinn & Zinn. Our image quality is sufficiently high, at ∼0.4–0.7 arcsec, that we are able to unambiguously classify all 12 targets as distant galaxies. To reinforce this conclusion we use GMOS images of globular clusters in the M31 halo, taken under very similar conditions, to show that any genuine clusters in the putative IGC sample would be straightforward to distinguish. Based on the stated sensitivity of the di Tullio Zinn & Zinn search algorithm, we conclude that there cannot be a significant number of IGCs with MV ≤ −6 lying unseen in the SDSS area if their properties mirror those of globular clusters in the outskirts of M31 – even a population of 4 would have only a ≈1 per cent chance of non-detection
Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud
We present Gemini South GMOS g,i photometry of 14 intermediate-age Large
Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203,
2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a
continuing project to investigate the extended Main Sequence Turnoff (EMSTO)
phenomenon. Extensive artificial star tests were made over the observed field
of view. These tests reveal the observed behaviour of photometric errors with
magnitude and crowding. The cluster stellar density radial profiles were traced
from star counts over the extent of the observed field. We adopt clus- ter
radii and build colour-magnitude diagrams (CMDs) with cluster features clearly
identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching
of theoretical isochrones. The studied LMC clusters are confirmed to be
intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162
and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the
basis of having dual red clumps.Comment: MNRAS, accepte
Structural analysis of the Sextans dwarf spheroidal galaxy
We present wide-field g- and i-band stellar photometry of the Sextans dwarf spheroidal galaxy and its surrounding area out to four times its half-light radius (rh = 695 pc), based on images obtained with the Dark Energy Camera at the 4-m Blanco telescope at CTIO. We find clear evidence of stellar substructure associated with the galaxy, extending to a distance of 82 arcmin (2 kpc) from its centre. We perform a statistical analysis of the overdensities and find three distinct features, as well as an extended halo-like structure, to be significant at the 99.7 per cent confidence level or higher. Unlike the extremely elongated and extended substructures surrounding the Hercules dwarf spheroidal galaxy, the overdensities seen around Sextans are distributed evenly about its centre, and do not appear to form noticeable tidal tails. Fitting a King model to the radial distribution of Sextans stars yields a tidal radius rt = 83.2 arcmin ± 7.1 arcmin (2.08 ± 0.18 kpc), which implies the majority of detected substructure is gravitationally bound to the galaxy. This finding suggests that Sextans is not undergoing significant tidal disruption from the Milky Way, supporting the scenario in which the orbit of Sextans has a low eccentricity
Extended stellar substructure surrounding the Boötes I dwarf spheroidal galaxy
We present deep stellar photometry of the Boötes I dwarf spheroidal galaxy in g- and i-band filters, taken with the Dark Energy Camera at Cerro Tololo in Chile. Our analysis reveals a large, extended region of stellar substructure surrounding the dwarf, as well as a distinct overdensity encroaching on its tidal radius. A radial profile of the Boötes I stellar distribution shows a break radius indicating the presence of extra-tidal stars. These observations strongly suggest that Boötes I is experiencing tidal disruption, although not as extreme as that exhibited by the Hercules dwarf spheroidal. Combined with revised velocity dispersion measurements from the literature, we see evidence suggesting the need to review previous theoretical models of the Boötes I dwarf spheroidal galaxy
Evidence of differential tidal effects in the old globular cluster population of the Large Magellanic Cloud
We present for the first time extended stellar density and/or surface brightness radial profiles for almost all the known Large Magellanic Cloud (LMC) old globular clusters (GCs). These were built from DECam images and reach out to ∼ 4 times the GCs’ tidal radii. The background subtracted radial profiles reveal that the GCs located closer than ∼ 5 kpc from the LMC centre contain an excess of stars in their outermost regions with respect to the stellar density expected from a King profile. Such a residual amount of stars, not seen in GCs located farther than ∼ 5 kpc from the LMC centre, as well as the GCs’ dimensions, shows a clear dependence with the GCs’ positions in the galaxy, in the sense that, the farther the GC from the centre of the LMC, the larger both the excess of stars in its outskirts and size. Although the masses of GCs located inside and outside ∼ 5 kpc are commensurate, the outermost regions of GCs located closer than ∼ 5 kpc from the LMC centre appear to have dynamically evolved more quickly. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC centre
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