44 research outputs found
The Influence of the Galactic Bar on the Dynamics of Globular Clusters
We make use of recent estimates for the parameters of the Milky Way's halo
globular clusters and study the influence of the galactic bar on the dynamics
of these clusters by computing their orbits. We use both an axisymmetric and
non-axisymmetric galactic potentials, which include the rotating elongated
bar/bulge structure. We account for observational errors both in the positions
and in the velocities of the globular clusters and explore the influence of the
bar on cluster's evolution. This is contained in the angular momentum-total
energy plane, (Lz,E), which is widely exploited as an indicator of the groups
of globular clusters that originated from the same accretion event. Particular
attention is devoted to the Gaia-Sausage/Enceladus and Pontus structures
identified recently as two independent accretion events. Our study shows that
it is not possible to identify GSE and Pontus as different merger events.Comment: 17 pages, 9 figure
The origin of globular cluster FSR 1758
Context. Globular clusters in the Milky Way are thought to have either an in situ origin, or to have been deposited in the Galaxy by past accretion events, like the spectacular Sagittarius dwarf galaxy merger.Aims. We probe the origin of the recently discovered globular cluster FSR 1758, often associated with some past merger event and which happens to be projected toward the Galactic bulge. We performed a detailed study of its Galactic orbit, and assign it to the most suitable Galactic component.Methods. We employed three different analytical time-independent potential models to calculate the orbit of the cluster by using the Gauss Radau spacings integration method. In addition, a time-dependent bar potential model is added to account for the influence of the Galactic bar. We ran a large suite of simulations via a Montecarlo method to account for the uncertainties in the initial conditions.Results. We confirm previous indications that the globular cluster FSR 1758 possesses a retrograde orbit with high eccentricity. The comparative analysis of the orbital parameters of star clusters in the Milky Way, in tandem with recent metallicity estimates, allows us to conclude that FSR 1758 is indeed a Galactic bulge intruder. The cluster can therefore be considered an old metal-poor halo globular cluster formed in situ that is passing right now in the bulge region. Its properties, however, can be roughly accounted for by also assuming that the cluster is part of some stream of extra-Galactic origin.Conclusions. We conclude that assessing the origin, either Galactic or extra-galactic, of globular clusters is surely a tantalising task. In any case, by using an Occam's razor argument, we tend to prefer an in situ origin for FSR 1758
A Capture Scenario for Globular Cluster Omega Centauri
We explore an accretion origin for Omega Cen by N-body modeling of the
orbital decay and disruption of a Milky-Way dwarf satellite. This work is
focused on studying a particular satellite model that aims to reproduce the
present orbit of Omega Cen, as recently determined from absolute proper
motions. The model satellite is launched from 58 kpc from the Galactic Center,
on a radial, low-inclination orbit. We find that a capture scenario can produce
an Omega Cen-like object with the current low-energy orbit of the cluster. Our
best model is a nucleated galaxy with a Hernquist density profile that has a
mass of 8 10**8 Msun, and a half-mass radius of 1.4 kpc.Comment: 15 pages, 5 figures, accepted by ApJ
In-Situ Star Formation in the Outskirts of the Large Magellanic Cloud: Gaia DR2 Confirmation
We explore the Gaia DR2 proper motions of six young, main-sequence stars,
members of the Large Magellanic Cloud (LMC) reported by Moni Bidin et al.
(2017). These stars are located in the outskirts of the disk, between 7 and 13
degrees from the LMC's center where there is very low H I content. Gaia DR2
proper motions confirm that four stars formed locally, in situ, while two are
consistent with being expelled via dynamical interactions from inner, more
gas-rich regions of the LMC. This finding establishes that recent star
formation occurred in the periphery of the LMC, where thus far only old
populations are known.Comment: Accepted for publication in Astrophysical Journal Letter
Chemical Composition of Young Stars in the Leading Arm of the Magellanic System
Chemical abundances of eight O- and B-type stars are determined from
high-resolution spectra obtained with the MIKE instrument on the Magellan 6.5m
Clay telescope. The sample is selected from 42 candidates of membership in the
Leading Arm of the Magellanic System. Stellar parameters are measured by two
independent grids of model atmospheres and analysis procedures, confirming the
consistency of the stellar parameter results. Abundances of seven elements (He,
C, N, O, Mg, Si, and S) are determined for the stars, as are their radial
velocities and estimates of distances and ages.
Among the seven B-type stars analyzed, the five that have radial velocities
compatible with membership to the LA have an average [Mg/H] of ,
significantly lower than the average of the remaining two [Mg/H] =
that are kinematical members of the Galactic disk. Among the
five LA members, four have individual [Mg/H] abundance compatible with that in
the LMC. Within errors, we can not exclude the possibility that one of these
stars has a [Mg/H] consistent with the more metal-poor, SMC-like material. The
remaining fifth star has a [Mg/H] close to MW values. Distances to the LA
members indicate that they are at the edge of the Galactic disk, while ages are
of the order of Myr, lower than the dynamical age of the LA,
suggesting a single star-forming episode in the LA. V the LA
members decreases with decreasing Magellanic longitude, confirming the results
of previous LA gas studies.Comment: 61 pages, 18 figures, 5 tables. Accepted for publication in Ap
Formation of Plumes in Head-on Collisions of Galaxies
Using N-body and SPH modeling we perform 3D numerical simulations of head-on
collisions between gas rich disk galaxies, including collisions between
counter-rotating disks and off-center collisions. Pure stellar intruders do not
produce gaseous plumes similar to those seen in the Cartwheel and VII Zw466
complexes of interacting galaxies; the presence of gas in an intruder galaxy
and radiative cooling are important for the formation of a gaseous plume
extending from the disk of a target galaxy. A noticeable plume structure can be
formed if the mass of an intruder is a few percent of the mass of the primary.
The halo of the intruder is stripped in the collision, and dispersed
particles form a broad stellar bridge connecting the two galaxies. The fraction
of the intruder's halo dispersed in the collision depends on the total mass of
the intruder, and low-mass intruders lose most of their mass.Comment: 15 pages, 14 figures in GIF. To appear ApJ. Vol. 505 #