33 research outputs found
SDSSJ103913.70+533029.7: A Super Star Cluster in the Outskirts of a Galaxy Merger
We describe the serendipitous discovery in the spectroscopic data of the
Sloan Digital Sky Survey of a star-like object, SDSSJ103913.70+533029.7, at a
heliocentric radial velocity of +1012 km/s. Its proximity in position and
velocity to the spiral galaxy NGC 3310 suggests an association with the galaxy.
At this distance, SDSSJ103913.70+533029.7 has the luminosity of a super star
cluster and a projected distance of 17 kpc from NGC 3310. Its spectroscopic and
photometric properties imply a mass of > 10^6 solar masses and an age close to
that of the tidal shells seen around NGC 3310, suggesting that it formed in the
event which formed the shells.Comment: Accepted by AJ: 4 figures (1 color
The ACS Survey of Galactic Globular Clusters XI: The Three-Dimensional Orientation of the Sagittarius Dwarf Spheroidal Galaxy and its Globular Clusters
We use observations from the ACS study of Galactic globular clusters to
investigate the spatial distribution of the inner regions of the disrupting
Sagittarius dwarf spheroidal galaxy (Sgr). We combine previously published
analyses of four Sgr member clusters located near or in the Sgr core (M54, Arp
2, Terzan 7 and Terzan 8) with a new analysis of diffuse Sgr material
identified in the background of five low-latitude Galactic bulge clusters (NGC
6624, 6637, 6652, 6681 and 6809) observed as part of the ACS survey. By
comparing the bulge cluster CMDs to our previous analysis of the M54/Sgr core,
we estimate distances to these background features. The combined data from four
Sgr member clusters and five Sgr background features provides nine independent
measures of the Sgr distance and, as a group, provide uniformly measured and
calibrated probes of different parts of the inner regions of Sgr spanning
twenty degrees over the face of the disrupting dwarf. This allows us, for the
first time, to constrain the three dimensional orientation of Sgr's disrupting
core and globular cluster system and compare that orientation to the
predictions of an N-body model of tidal disruption. The density and distance of
Sgr debris is consistent with models that favor a relatively high Sgr core mass
and a slightly greater distance (28-30 kpc, with a mean of 29.4 kpc). Our
analysis also suggests that M54 is in the foreground of Sgr by ~2 kpc,
projected on the center of the Sgr dSph. While this would imply a remarkable
alignment of the cluster and the Sgr nucleus along the line of sight, we can
not identify any systematic effect in our analysis that would falsely create
the measured 2 kpc separation. Finally, we find that the cluster Terzan 7 has
the most discrepant distance (25 kpc) among the four Sgr core clusters, which
may suggest a different dynamical history than the other Sgr core clusters.Comment: 41 pages, 16 figures, accepted to Ap
Direct Constraints on Minimal Supersymmetry from Fermi-LAT Observations of the Dwarf Galaxy Segue 1
The dwarf galaxy Segue 1 is one of the most promising targets for the
indirect detection of dark matter. Here we examine what constraints 9 months of
Fermi-LAT gamma-ray observations of Segue 1 place upon the Constrained Minimal
Supersymmetric Standard Model (CMSSM), with the lightest neutralino as the dark
matter particle. We use nested sampling to explore the CMSSM parameter space,
simultaneously fitting other relevant constraints from accelerator bounds, the
relic density, electroweak precision observables, the anomalous magnetic moment
of the muon and B-physics. We include spectral and spatial fits to the Fermi
observations, a full treatment of the instrumental response and its related
uncertainty, and detailed background models. We also perform an extrapolation
to 5 years of observations, assuming no signal is observed from Segue 1 in that
time. Results marginally disfavour models with low neutralino masses and high
annihilation cross-sections. Virtually all of these models are however already
disfavoured by existing experimental or relic density constraints.Comment: 22 pages, 5 figures; added extra scans with extreme halo parameters,
expanded introduction and discussion in response to referee's comment
The Curious Case of Palomar 13: The Influence of the Orbital Phase on the Appearance of Galactic Satellites
We investigate the dynamical status of the low-mass globular cluster Palomar
13 by means of N-body computations to test whether its unusually high
mass-to-light ratio of about 40 and its peculiarly shallow surface density
profile can be caused by tidal shocking. Alternatively, we test - by varying
the assumed proper motion - if the orbital phase of Palomar 13 within its orbit
about the Milky Way can influence its appearance and thus may be the origin of
these peculiarities, as has been suggested by Kuepper et al. (2010). We find
that, of these two scenarios, only the latter can explain the observed
mass-to-light ratio and surface density profile. We note, however, that the
particular orbit that best reproduces those observed parameters has a proper
motion inconsistent with the available literature value. We discuss this
discrepancy and suggest that it may be caused by an underestimation of the
observational uncertainties in the proper motion determination. We demonstrate
that Palomar 13 is most likely near apogalacticon, which makes the cluster
appear supervirial and blown-up due to orbital compression of its tidal debris.
Since the satellites of the Milky Way are on average closer to apo- than
perigalacticon, their internal dynamics may be influenced by the same effect,
and we advocate that this needs to be taken into account when interpreting
their kinematical data. Moreover, we briefly discuss the influence of a
possible binary population on such measurements.Comment: Funding acknowledgement adde
The Blue Straggler Population in Dwarf Galaxies
In this chapter I review the recent developments regarding the study of Blue
Stragglers (BSS) in dwarf galaxies. The loose density environment of dwarf
galaxies resembles that of the Galactic Halo, hence it is natural to compare
their common BSS properties. At the same time, it is unescapable to compare
with the BSS properties in Galactic Globular clusters, which constitute the
reference point for BSS studies. Admittedly, the literature on BSS in dwarf
galaxies is not plentiful. The limitation is mostly due to the large distance
to even the closest dwarf galaxies. Nevertheless, recent studies have allowed a
deeper insight on the BSS photometric properties that are worth examining.Comment: Chapter 6, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Dark Matter and Fundamental Physics with the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is a project for a next-generation
observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy,
currently in its design phase, and foreseen to be operative a few years from
now. Several tens of telescopes of 2-3 different sizes, distributed over a
large area, will allow for a sensitivity about a factor 10 better than current
instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few
tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the
following study, we investigate the prospects for CTA to study several science
questions that influence our current knowledge of fundamental physics. Based on
conservative assumptions for the performance of the different CTA telescope
configurations, we employ a Monte Carlo based approach to evaluate the
prospects for detection. First, we discuss CTA prospects for cold dark matter
searches, following different observational strategies: in dwarf satellite
galaxies of the Milky Way, in the region close to the Galactic Centre, and in
clusters of galaxies. The possible search for spatial signatures, facilitated
by the larger field of view of CTA, is also discussed. Next we consider
searches for axion-like particles which, besides being possible candidates for
dark matter may also explain the unexpectedly low absorption by extragalactic
background light of gamma rays from very distant blazars. Simulated
light-curves of flaring sources are also used to determine the sensitivity to
violations of Lorentz Invariance by detection of the possible delay between the
arrival times of photons at different energies. Finally, we mention searches
for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics
Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation
(abridged) Predictions of the Concordance Cosmological Model (CCM) of the
structures in the environment of large spiral galaxies are compared with
observed properties of Local Group galaxies. Five new most probably
irreconcilable problems are uncovered. However, the Local Group properties
provide hints that may lead to a solution of the above problems The DoS and
bulge--satellite correlation suggest that dissipational events forming bulges
are related to the processes forming phase-space correlated satellite
populations. Such events are well known to occur since in galaxy encounters
energy and angular momentum are expelled in the form of tidal tails, which can
fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star
clusters. If Local Group satellite galaxies are to be interpreted as TDGs then
the sub-structure predictions of CCM are internally in conflict. All findings
thus suggest that the CCM does not account for the Local Group observations and
that therefore existing as well as new viable alternatives have to be further
explored. These are discussed and natural solutions for the above problems
emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor
text adjustments for conformity with the published version and additional
minor changes resulting from reader's feedback. The speculation on a dark
force has been added. Also, the Fritz Zwicky Paradox is now included to agree
with the published versio
Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy
We study the dynamics of faint stellar substructures around the Umbrella Galaxy, NGC 4651, which hosts a dramatic system of streams and shells formed through the tidal disruption of a nucleated dwarf elliptical galaxy. We elucidate the basic characteristics of the system (colours, luminosities, stellar masses) using multiband Subaru/Suprime-Cam images. The implied stellar mass ratio of the ongoing merger event is ∼1:50. We identify candidate kinematic tracers (globular clusters, planetary nebulae, H ii regions) and follow up a subset with Keck/DEIMOS (DEep Imaging Multi-object Spectrograph) spectroscopy to obtain velocities. We find that 15 of the tracers are likely associated with halo substructures, including the probable stream progenitor nucleus. These objects delineate a kinematically cold feature in position–velocity phase space. We model the stream using single test particle orbits, plus a rescaled pre-existing N-body simulation. We infer a very eccentric orbit with a period of ∼0.35 Gyr and turning points at ∼2–4 and ∼40 kpc, implying a recent passage of the satellite through the disc, which may have provoked the visible disturbances in the host galaxy. This work confirms that the kinematics of low surface brightness substructures can be recovered and modelled using discrete tracers – a breakthrough that opens up a fresh avenue for unravelling the detailed physics of minor merging