76 research outputs found
An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters
We present the first results of a large spectroscopic survey of candidate
globular clusters located in the extreme outskirts of the nearby M31 galaxy. We
obtained low resolution spectra of 48 targets selected from the XSC of 2MASS,
as in Galleti et al. (2005). The observed candidates have been robustly
classified according to their radial velocity and by verifying their
extended/point-source nature from ground-based optical images. Among the 48
observed candidates clusters we found 5 genuine remote globular clusters. One
of them has been already identified independently by Mackey et al. (2007),
their GC1; the other four are completely new discoveries: B516, B517, B518,
B519. The newly discovered clusters lie at projected distance 40 kpc<~R_p<~100
kpc from the center of M31, and have absolute integrated magnitude
-9.5<M_V<-7.5. For all the observed clusters we have measured the strongest
Lick indices and we have obtained spectroscopic metallicity estimates.
Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal
poor globular clusters ([Fe/H]<~ -1.3); B519 appears old but quite metal-rich
([Fe/H]~-0.5); B516 presents very strong Balmer absorption lines: if this is
indeed a cluster it should have a relatively young age (likely <2 Gyr). The
present analysis nearly doubles the number of M31 globulars at R_p> 40 kpc. At
odds with the Milky Way, M31 appears to have a significant population of very
bright globular clusters in its extreme outskirts.Comment: 16 pages including 6 pages published only in the electronic edition
of the Journal. Accepted for publication in A&
A low surface brightness halo surrounding the globular cluster NGC 5694
We report on the discovery of an extended stellar halo surrounding the
distant Galactic globular cluster NGC 5694, based on new deep (V ~ 24.5)
wide-field (24' * 20') photometry acquired with VIMOS at VLT. Stars with colour
and magnitude consistent with the Main Sequence of the cluster are clearly
identified out to r ~ 9'(~ 93 pc) from the cluster center, much beyond the
tidal radius of the King model that best fits the inner profile (r_t=3.15'). We
do not find a clear end of the structure within our field. The overall observed
profile cannot be properly fitted with either a King (1966) model, an Elson et
al. (1987) model, or a Wilson (1975) model; however it is very smooth and does
not show any sign of the break near the tidal radius that is typically observed
in stellar systems with tidal tails. The density map we derived does not show
evidence of tidal tails, within the considered field. The extra-tidal component
contains ~ 3.5% of the cluster light (mass) and has a surface density profile
falling as ~ r^{-3.2}. The possible origin of the detected structure is
discussed, as a clear-cut conclusion cannot be reached with the available data.Comment: 7 pages, 4 figures, accepted for publication in MNRA
A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII
We present the first spectroscopic analysis of the faint M31 satellite
galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data
for two further faint companions, And IX and AndXII. By combining data obtained
using the DEIMOS spectrograph mounted on the Keck II telescope with deep
photometry from the Suprime-Cam instrument on Subaru, we have calculated global
properties for the dwarfs, such as systemic velocities, metallicites and
half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both
photometrically and spectroscopically, from their stacked spectrum), and as
such, they continue to follow the luminosity-metallicity relationship
established with brighter dwarfs. We are unable to resolve a dispersion for And
XI due to small sample size and low S/N, but we set a one sigma upper limit of
sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity
dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and
derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4
(+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss
each satellite in the context of the Mateo relations for dwarf spheroidal
galaxies, and the Universal halo profiles established for Milky Way dwarfs
(Walker et al. 2009). For both galaxies, this sees them fall below the
Universal halo profiles of Walker et al. (2009). When combined with the
findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites
are twice as extended (in terms of both half-light and tidal radii) as their
Milky Way counterparts, these results suggest that the satellite population of
the Andromeda system could inhabit halos that are significantly different from
those of the Milky Way in terms of their central densities (abridged).Comment: 26 pages, 18 figures, MNRAS submitte
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
How Immunocontraception Can Contribute to Elephant Management in Small, Enclosed Reserves: Munyawana Population as a Case Study
Immunocontraception has been widely used as a management tool to reduce population growth in captive as well as wild populations of various fauna. We model the use of an individual-based rotational immunocontraception plan on a wild elephant, Loxodonta africana, population and quantify the social and reproductive advantages of this method of implementation using adaptive management. The use of immunocontraception on an individual, rotational basis stretches the inter-calving interval for each individual female elephant to a management-determined interval, preventing exposing females to unlimited long-term immunocontraception use (which may have as yet undocumented negative effects). Such rotational immunocontraception can effectively lower population growth rates, age the population, and alter the age structure. Furthermore, such structured intervention can simulate natural process such as predation or episodic catastrophic events (e.g., drought), which regulates calf recruitment within an abnormally structured population. A rotational immunocontraception plan is a feasible and useful elephant population management tool, especially in a small, enclosed conservation area. Such approaches should be considered for other long-lived, social species in enclosed areas where the long-term consequences of consistent contraception may be unknown
The old globular cluster system of the dIrr galaxy NGC1427A in the Fornax cluster
We present a study of the old globular cluster (GC) population of the dwarf
irregular galaxy NGC 1427A using multi-wavelength VLT observations in U, B, V,
I, H_alpha, J, H, and Ks bands under excellent observing conditions. We applied
color and size selection criteria to select old GC candidates and made use of
archival ACS images taken with the Hubble Space Telescope to reject
contaminating background sources and blended objects from the GC candidates'
list. The H_alpha observations were used to check for contamination due to
compact, highly reddened young star clusters whose colors and sizes could mimic
those of old GCs. After accounting for contamination we obtain a total number
of 38+/-8 GC candidates with colors consistent with an old (~10 Gyr) and
metal-poor (Z < 0.4xZ_solar) population as judged by simple stellar population
models. Our contamination analysis indicates that the density distribution of
GCs in the outskirts of the Fornax central cD galaxy NGC1399 may not be
spherically symmetric. We derive a present-day specific frequency S_N of
1.6+/-0.23 for NGC 1427A, a value significantly larger than what is observed in
the Local Group dwarf irregular galaxies and comparable with the values found
for the same galaxy types in the Virgo and Fornax clusters. Assuming a
universal globular cluster luminosity function turnover magnitude, we derive a
distance modulus to NGC 1427A of 31.01+/-0.21 mag which places it
3.2+/-2.5(statistic)+/-1.6(systematic) Mpc in front of the Fornax central cD
galaxy NGC 1399. The implications of this result for the relationship between
NGC 1427A and the cluster environment are briefly discussed.Comment: 16 pages, 2 tables, 14 figures; accepted for publication in A&
The non-peculiar velocity dispersion profile of the stellar system omega Centauri
We present the results of a survey of radial velocities over a wide region
extending from r~10 arcmin out to r~80 arcmin (~1.5 tidal radii) within the
massive star cluster omega Centauri. The survey was performed with FLAMES@VLT,
to study the velocity dispersion profile in the outer regions of this stellar
system. We derived accurate radial velocities for a sample of 2557 newly
observed stars, identifying 318 bona-fide cluster red giants. Merging our data
with those provided by Pancino et al. (2007), we assembled a final homogeneous
sample of 946 cluster members that allowed us to trace the velocity dispersion
profile from the center out to r~32 arcmin. The velocity dispersion appears to
decrease monotonically over this range, from a central value of sigma_{v}~17.2
Km/s down to a minimum value of sigma_{v}~5.2 Km/s. The observed surface
brightness profile, rotation curve, velocity dispersion profile and ellipticity
profile are simultaneously well reproduced by a simple dynamical model in which
mass follows light, within the classical Newtonian theory of gravitation. The
comparison with an N-body model of the evolution of a system mimicking omega
Cen during the last 10 orbits into the Galactic potential suggests that (a) the
rotation of stars lying in the inner ~20 arcmin of the clusters is not due to
the effects of the tidal field of the Milky Way, as hypothized by other
authors, and (b) the overall observational scenario is still compatible with
the possibility that the outer regions of the cluster are subject to some tidal
stirring.Comment: 12 pages, 12 figures, accepted for publication by MNRA
The Effects of Herbivory by a Mega- and Mesoherbivore on Tree Recruitment in Sand Forest, South Africa
Herbivory by megaherbivores on woody vegetation in general is well documented; however studies focusing on the individual browsing effects of both mega- and mesoherbivore species on recruitment are scarce. We determined these effects for elephant Loxodonta africana and nyala Tragelaphus angasii in the critically endangered Sand Forest, which is restricted to east southern Africa, and is conserved mainly in small reserves with high herbivore densities. Replicated experimental treatments (400 m2) in a single forest patch were used to exclude elephant, or both elephant and nyala. In each treatment, all woody individuals were identified to species and number of stems, diameter and height were recorded. Results of changes after two years are presented. Individual tree and stem densities had increased in absence of nyala and elephant. Seedling recruitment (based on height and diameter) was inhibited by nyala, and by elephant and nyala in combination, thereby preventing recruitment into the sapling stage. Neither nyala or elephant significantly reduced sapling densities. Excluding both elephant and nyala in combination enhanced recruitment of woody species, as seedling densities increased, indicating that forest regeneration is impacted by both mega- and mesoherbivores. The Sand Forest tree community approached an inverse J-shaped curve, with the highest abundance in the smaller size classes. However, the larger characteristic tree species in particular, such as Newtonia hildebrandtii, were missing cohorts in the middle size classes. When setting management goals to conserve habitats of key importance, conservation management plans need to consider the total herbivore assemblage present and the resulting browsing effects on vegetation. Especially in Africa, where the broadest suite of megaherbivores still persists, and which is currently dealing with the ‘elephant problem’, the individual effects of different herbivore species on recruitment and dynamics of forests and woodlands are important issues which need conclusive answers
ENIGMA and global neuroscience: A decade of large-scale studies of the brain in health and disease across more than 40 countries
This review summarizes the last decade of work by the ENIGMA (Enhancing NeuroImaging Genetics through Meta Analysis) Consortium, a global alliance of over 1400 scientists across 43 countries, studying the human brain in health and disease. Building on large-scale genetic studies that discovered the first robustly replicated genetic loci associated with brain metrics, ENIGMA has diversified into over 50 working groups (WGs), pooling worldwide data and expertise to answer fundamental questions in neuroscience, psychiatry, neurology, and genetics. Most ENIGMA WGs focus on specific psychiatric and neurological conditions, other WGs study normal variation due to sex and gender differences, or development and aging; still other WGs develop methodological pipelines and tools to facilitate harmonized analyses of "big data" (i.e., genetic and epigenetic data, multimodal MRI, and electroencephalography data). These international efforts have yielded the largest neuroimaging studies to date in schizophrenia, bipolar disorder, major depressive disorder, post-traumatic stress disorder, substance use disorders, obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, autism spectrum disorders, epilepsy, and 22q11.2 deletion syndrome. More recent ENIGMA WGs have formed to study anxiety disorders, suicidal thoughts and behavior, sleep and insomnia, eating disorders, irritability, brain injury, antisocial personality and conduct disorder, and dissociative identity disorder. Here, we summarize the first decade of ENIGMA's activities and ongoing projects, and describe the successes and challenges encountered along the way. We highlight the advantages of collaborative large-scale coordinated data analyses for testing reproducibility and robustness of findings, offering the opportunity to identify brain systems involved in clinical syndromes across diverse samples and associated genetic, environmental, demographic, cognitive, and psychosocial factors
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