57 research outputs found
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 Científicas 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
The PAndAS view of the Andromeda satellite system - I. A Bayesian search for dwarf galaxies using spatial and color-magnitude information
We present a generic algorithm to search for dwarf galaxies in photometric
catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The
algorithm is developed in a Bayesian framework and, contrary to most
dwarf-galaxy-search codes, makes use of both the spatial and color-magnitude
information of sources in a probabilistic approach. Accounting for the
significant contamination from the Milky Way foreground and from the structured
stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in
the PAndAS footprint with high significance, even for the least luminous ones.
Some Andromeda globular clusters are also recovered and, in one case,
discovered. We publish a list of the 143 most significant detections yielded by
the algorithm. The combined properties of the 39 most significant isolated
detections show hints that at least some of these trace genuine dwarf galaxies,
too faint to be individually detected. Follow-up observations by the community
are mandatory to establish which are real members of the Andromeda satellite
system. The search technique presented here will be used in an upcoming
contribution to determine the PAndAS completeness limits for dwarf galaxies.
Although here tuned to the search of dwarf galaxies in the PAndAS data, the
algorithm can easily be adapted to the search for any localised overdensity
whose properties can be modeled reliably in the parameter space of any catalog.Comment: 20 pages, 16 figures, 1 table; accepted for publication in ApJ. High
res pdf available at
https://www.dropbox.com/s/7zk7pme2wunwkjv/PAndAS_dwarf_galaxies.pd
Core radius evolution of star clusters
We use N-body simulations of star clusters to investigate the possible
dynamical origins of the observed spread in core radius among intermediate-age
and old star clusters in the Large Magellanic Cloud (LMC). Two effects are
considered, a time-varying external tidal field and variations in primordial
hard binary fraction. Simulations of clusters orbiting a point-mass galaxy show
similar core radius evolution for clusters on both circular and elliptical
orbits and we therefore conclude that the tidal field of the LMC has not yet
significantly influenced the evolution of the intermediate-age clusters. The
presence of large numbers of hard primordial binaries in a cluster leads to
core radius expansion; however, the magnitude of the effect is insufficient to
explain the observations. Further, the range of binary fractions required to
produce significant core radius growth is inconsistent with the observational
evidence that all the LMC clusters have similar stellar luminosity functions.Comment: Accepted for publication in MNRA
A 10 kpc stellar substructure at the edge of the Large Magellanic Cloud::perturbed outer disk or evidence for tidal stripping?
We report the discovery of a substantial stellar overdensity in the periphery
of the Large Magellanic Cloud (LMC), found using public imaging from the first
year of the Dark Energy Survey. The structure appears to emanate from the edge
of the outer LMC disk at a radius degrees due north of its
centre, and stretches more than kpc towards the east. It is roughly
kpc wide and has an integrated -band luminosity of at least .
The stellar populations in the feature are indistinguishable from those in the
outer LMC disk. We attempt to quantify the geometry of the outer disk using
simple planar models, and find that only a disk with mild intrinsic ellipticity
can simultaneously explain the observed stellar density on the sky and the
azimuthal line-of-sight distance profile. We also see possible non-planar
behaviour in the outer disk that may reflect a warp and/or flare, as well as
deviations that resemble a ring-like structure between degrees from
the LMC centre. Based on all these observations, we conclude that our remote,
stream-like feature is likely comprised of material that has been stripped from
the outskirts of the LMC disk, although we cannot rule out that it represents a
transient overdensity in the disk itself. We conduct a simple -body
simulation to show that either type of structure could plausibly arise due to
the tidal force of the Milky Way; however we also recognize that a recent close
interaction between the LMC and the SMC may be the source of the stripping or
perturbation. Finally, we observe evidence for extremely diffuse LMC
populations extending to radii of kpc in the disk plane ( degrees on the sky), corroborating previous spectroscopic detections at
comparable distances.Comment: Accepted for publication in MNRAS; this posting updated to match the
accepted versio
A SkyMapper view of the large magellanic cloud: The dynamics of stellar populations
We present the first SkyMapper stellar population analysis of the Large Magellanic Cloud (hereafter LMC), including the identification of 3578 candidate Carbon Stars through their extremely red g − r colours. Coupled with Gaia astrometry, we analyse the distribution and kinematics of this Carbon Star population, finding the LMC to be centred at (RA, Dec.) = (80.90 ◦ ± 0.29, −68.74 ◦ ± 0.12), with a bulk proper motion of (μ α , μ δ ) = (1.878 ± 0.007, 0.293 ± 0.018) mas yr −1 and a disc inclination of i = 25.6 ◦ ± 1.1 at position angle θ = 135.6 ◦ ± 3.3 ◦ . We complement this study with the identification and analysis of additional stellar populations, finding that the dynamical centre for red giant branch stars is similar to that seen for the Carbon Stars, whereas for young stars the dynamical centre is significantly offset from the older populations. This potentially indicates that the young stars were formed as a consequence of a strong tidal interaction, probably with the Small Magellanic Cloud. In terms of internal dynamics, the tangential velocity profile increases linearly within ∼3 kpc, after which it maintains an approximately constant value of V rot = 83.6 ± 1.7 km s−1 until ∼7 kpc. With an asymmetric drift correction, we estimate the mass within 7 kpc to be M LMC (< 7 kpc) = (2.5 ± 0.1) × 10 10 M ⊙ and within the tidal radius (∼30 kpc) to be M LMC (< 30 kpc) = (1.06 ± 0.32) × 10 11 M ⊙ , consistent with other recent measurements.ZW gratefully acknowledges financial support through a the Dean’s
International Postgraduate Research Scholarship from the Physics
School of the University of Sydney. DM holds an Australian
Research Council (ARC) Future Fellowship (FT160100206). We
thank the anonymous reviewer for their constructive suggestions.
The national facility capability for SkyMapper has been funded
through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian
National University, Swinburne University of Technology, the
University of Queensland, the University of Western Australia,
the University of Melbourne, Curtin University of Technology,
Monash University and the Australian Astronomical Observator
N-body Models of Extended Clusters
We use direct N-body simulations to investigate the evolution of star
clusters with large size-scales with the particular goal of understanding the
so-called extended clusters observed in various Local Group galaxies, including
M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar
evolution and the tidal field of a host galaxy. We find that extended clusters
can arise naturally within a weak tidal field provided that the tidal radius is
filled at the start of the evolution. Differences in the initial tidal
filling-factor can produce marked differences in the subsequent evolution of
clusters and the size-scales that would be observed. These differences are more
marked than any produced by internal evolution processes linked to the
properties of cluster binary stars or the action of an intermediate-mass black
hole, based on models performed in this work and previous work to date. Models
evolved in a stronger tidal field show that extended clusters cannot form and
evolve within the inner regions of a galaxy such as M31. Instead our results
support the suggestion many extended clusters found in large galaxies were
accreted as members of dwarf galaxies that were subsequently disrupted. Our
results also enhance the recent suggestion that star clusters evolve to a
common sequence in terms of their size and mass.Comment: 12 pages, 8 figures, accepted by MNRA
Galactic cartography with SkyMapper - I. Population substructure and the stellar number density of the inner halo
The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHB stars from the overall population, we develop a machine-learning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute u magnitude for the BHB sample to be similar to 2 mag, varying slightly with (v - g)(0) and (u - v)(0) colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power law with a break radius of r(s) = 11.8 +/- 0.3 kpc, and inner and outer slopes of alpha(in) = -2.5 +/- 0.1 and alpha(out) = -4.5 +/- 0.3, respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning similar to 3 kpc to similar to 20 kpc in Galactocentric distance.ZW gratefully acknowledges financial support through the Dean’s International Postgraduate Research Scholarship from the Physics School of the University of Sydney. PRK is funded through Australian Research Council (ARC) grant DP140100395. ADM is grateful for support from an ARC Future Fellowship (FT160100206). SS is funded by University of Sydney Senior Fellowship made possible by the office of the Deputy Vice Chancellor of Research, and partial funding from Bland-Hawthorn’s Laureate
Fellowship from the Australian Research Council. GFL thanks the
University of Surrey for hosting him as an IAS fellow for the final
stages of the preparation of this paper. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the
University of Melbourne, Curtin University of Technology, Monash
University, and the Australian Astronomical Observatory
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