15 research outputs found
A re-interpretation of the Triangulum-Andromeda stellar clouds: a population of halo stars kicked out of the Galactic disk
The Triangulum-Andromeda stellar clouds (TriAnd1 and TriAnd2) are a pair of
concentric ring- or shell-like over-densities at large ( 30 kpc)
and ( -10 kpc) in the Galactic halo that are thought to have been
formed from the accretion and disruption of a satellite galaxy. This paper
critically re-examines this formation scenario by comparing the number ratio of
RR Lyrae to M giant stars associated with the TriAnd clouds with other
structures in the Galaxy. The current data suggest a stellar population for
these over-densities ( at 95% confidence) quite unlike
any of the known satellites of the Milky Way ( for
the very largest and for the smaller satellites) and more
like the population of stars born in the much deeper potential well inhabited
by the Galactic disk (). N-body simulations of a
Milky-Way-like galaxy perturbed by the impact of a dwarf galaxy demonstrate
that, in the right circumstances, concentric rings propagating outwards from
that Galactic disk can plausibly produce similar over-densities. These results
provide dramatic support for the recent proposal by Xu et al. (2015) that,
rather than stars accreted from other galaxies, the TriAnd clouds could
represent stars kicked-out from our own disk. If so, these would be the first
populations of disk stars to be found in the Galactic halo and a clear
signature of the importance of this second formation mechanism for stellar
halos more generally. Moreover, their existence at the very extremities of the
disk places strong constraints on the nature of the interaction that formed
them.Comment: 27 pages, 8 figures; published in MNRA
Disk Heating, Galactoseismology, and the Formation of Stellar Halos
Deep photometric surveys of the Milky Way have revealed diffuse structures
encircling our Galaxy far beyond the "classical" limits of the stellar disk.
This paper reviews results from our own and other observational programs, which
together suggest that, despite their extreme positions, the stars in these
structures were formed in our Galactic disk. Mounting evidence from recent
observations and simulations implies kinematic connections between several of
these distinct structures. This suggests the existence of collective disk
oscillations that can plausibly be traced all the way to asymmetries seen in
the stellar velocity distribution around the Sun. There are multiple
interesting implications of these findings: they promise new perspectives on
the process of disk heating, they provide direct evidence for a stellar halo
formation mechanism in addition to the accretion and disruption of satellite
galaxies, and, they motivate searches of current and near-future surveys to
trace these oscillations across the Galaxy. Such maps could be used as
dynamical diagnostics in the emerging field of "Galactoseismology", which
promises to model the history of interactions between the Milky Way and its
entourage of satellites, as well examine the density of our dark matter halo.
As sensitivity to very low surface brightness features around external galaxies
increases, many more examples of such disk oscillations will likely be
identified. Statistical samples of such features not only encode detailed
information about interaction rates and mergers, but also about long
sought-after dark matter halo densities and shapes. Models for the Milky Way's
own Galactoseismic history will therefore serve as a critical foundation for
studying the weak dynamical interactions of galaxies across the universe.Comment: 20 pages, 5 figures, accepted in for publication in a special edition
of the journal "Galaxies", reporting the proceedings of the conference "On
the Origin (and Evolution) of Baryonic Galaxy Halos", Puerto Ayora, Ecuador,
March 13-17 2017, Eds. Duncan A. Forbes and Ericson D. Lope
Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum-Andromeda Overdensity?
Thanks to modern sky surveys, over twenty stellar streams and overdensity
structures have been discovered in the halo of the Milky Way. In this paper, we
present an analysis of spectroscopic observations of individual stars from one
such structure, "A13", first identified as an overdensity using the M giant
catalog from the Two Micron All-Sky Survey. Our spectroscopic observations show
that stars identified with A13 have a velocity dispersion of 40
, implying that it is a genuine coherent structure rather
than a chance super-position of random halo stars. From its position on the
sky, distance (15~kpc heliocentric), and kinematical properties, A13 is
likely to be an extension of another low Galactic latitude substructure -- the
Galactic Anticenter Stellar Structure (also known as the Monoceros Ring) --
towards smaller Galactic longitude and farther distance. Furthermore, the
kinematics of A13 also connect it with another structure in the southern
Galactic hemisphere -- the Triangulum-Andromeda overdensity. We discuss these
three connected structures within the context of a previously proposed scenario
that one or all of these features originate from the disk of the Milky Way.Comment: 12 pages, 9 figures. Accepted for publication in Ap
Probing the Halo From the Solar Vicinity to the Outer Galaxy: Connecting Stars in Local Velocity Structures to Large-Scale Clouds
(Abridged) This paper presents the first connections made between two local
features in velocity-space found in a survey of M giant stars and stellar
spatial inhomogeneities on global scales. Comparison to cosmological,
chemodynamical stellar halo models confirm that the M giant population is
particularly sensitive to rare, recent and massive accretion events. These
events can give rise to local observed velocity sequences - a signature of a
small fraction of debris from a common progenitor, passing at high velocity
through the survey volume, near the pericenters of their eccentric orbits. The
majority of the debris is found in much larger structures, whose morphologies
are more cloud-like than stream-like and which lie at the orbital apocenters.
Adopting this interpretation, the full-space motions represented by the
observed velocity features are derived under the assumption that the members
within each sequence share a common velocity. Orbit integrations are then used
to trace the past and future trajectories of these stars across the sky
revealing plausible associations with large, previously-discovered, cloud-like
structures. The connections made between nearby velocity structures and these
distant clouds represent preliminary steps towards developing coherent maps of
such giant debris systems. These maps promise to provide new insights into the
origin of debris clouds, new probes of Galactic history and structure, and new
constraints on the high-velocity tails of the local dark matter distribution
that are essential for interpreting direct detection experiments.Comment: submitted to the Astrophysical Journal, 40 pages, 13 figure
Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
Our Galaxy is thought to have undergone an active evolutionary history
dominated by star formation, the accretion of cold gas, and, in particular,
mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of
these interactions in the form of stellar streams, substructures, and
chemically distinct stellar components. The impact of dwarf galaxy mergers on
the content and morphology of the Galactic disk is still being explored. Recent
studies have identified kinematically distinct stellar substructures and moving
groups, which may have extragalactic origin. However, there is mounting
evidence that stellar overdensities at the outer disk/halo interface could have
been caused by the interaction of a dwarf galaxy with the disk. Here we report
detailed spectroscopic analysis of 14 stars drawn from two stellar
overdensities, each lying about 5 kiloparsecs above and below the Galactic
plane - locations suggestive of association with the stellar halo. However, we
find that the chemical compositions of these stars are almost identical, both
within and between these groups, and closely match the abundance patterns of
the Milky Way disk stars. This study hence provides compelling evidence that
these stars originate from the disk and the overdensities they are part of were
created by tidal interactions of the disk with passing or merging dwarf
galaxies.Comment: accepted for publication in Natur
Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
[Abridged] We present a medium-resolution spectroscopic survey of late-type
giant stars at mid-Galactic latitudes of (30),
designed to probe the properties of this population to distances of 9
kpc. Because M giants are generally metal-rich and we have limited
contamination from thin disk stars by the latitude selection, most of the stars
in the survey are expected to be members of the thick disk
(-0.6) with some contribution from the metal-rich component of
the nearby halo.
Here we report first results for 1799 stars. The distribution of radial
velocity (RV) as a function of l for these stars shows (1) the expected thick
disk population and (2) local metal-rich halo stars moving at high speeds
relative to the disk, that in some cases form distinct sequences in RV-
space. High-resolution echelle spectra taken for 34 of these "RV outliers"
reveal the following patterns across the [Ti/Fe]-[Fe/H] plane: seventeen of the
stars have abundances reminiscent of the populations present in dwarf
satellites of the Milky Way; eight have abundances coincident with those of the
Galactic disk and more metal-rich halo; and nine of the stars fall on the locus
defined by the majority of stars in the halo. The chemical abundance trends of
the RV outliers suggest that this sample consists predominantly of stars
accreted from infalling dwarf galaxies. A smaller fraction of stars in the RV
outlier sample may have been formed in the inner Galaxy and subsequently kicked
to higher eccentricity orbits, but the sample is not large enough to
distinguish conclusively between this interpretation and the alternative that
these stars represent the tail of the velocity distribution of the thick disk.
Our data do not rule out the possibility that a minority of the sample could
have formed from gas {\it in situ} on their current orbits.Comment: 43 pages, 9 figures, 4 tables, published in the Astrophysical Journa
On the Hunt for the Origins of the Orphan--Chenab Stream: Detailed Element Abundances with APOGEE and Gaia
Stellar streams in the Galactic halo are useful probes of the assembly of
galaxies like the Milky Way. Many tidal stellar streams that have been found in
recent years are accompanied by a known progenitor globular cluster or dwarf
galaxy. However, the Orphan--Chenab (OC) stream is one case where a relatively
narrow stream of stars has been found without a known progenitor. In an effort
to find the parent of the OC stream, we use astrometry from the early third
data release of ESA's Gaia mission (Gaia EDR3) and radial velocity information
from the SDSS-IV APOGEE survey to find up to 13 stars that are likely members
of the OC stream. We use the APOGEE survey to study the chemical nature (for up
to 13 stars) of the OC stream in the (O, Mg, Ca, Si, Ti, S), odd-Z
(Al, K, V), Fe-peak (Fe, Ni, Mn, Co, Cr) and neutron capture (Ce) elemental
groups. We find that the stars that make up the OC stream are not consistent
with a mono-metallic population and have a median metallicity of --1.92~dex
with a dispersion of 0.28 dex. Our results also indicate that the
-elements are depleted compared to the known Milky Way populations and
that its [Mg/Al] abundance ratio is not consistent with second generation stars
from globular clusters. The detailed chemical pattern of these stars indicates
that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy
with a mass of ~10 M.Comment: 13 Pages; 4 Figures. Submitted to AAS Journals, comments welcom