746 research outputs found
Four New Stellar Debris Streams in the Galactic Halo
We report on the detection of four new stellar debris streams and a new dwarf
galaxy candidate in the Sloan Digital Sky Survey. Three of the streams, ranging
between 3 and 15 kpc in distance and spanning between 37 and 84 degrees on the
sky, are very narrow and are most probably tidal streams originating in extant
or disrupted globular clusters. The fourth stream is much broader, roughly 45
kpc distant, at least 53 degrees in length, and is most likely the tidal debris
from a dwarf galaxy. As the streams each span multiple constellations, we
extend tradition and designate them the Acheron, Cocytos, Lethe, and Styx
streams. At the same distance and apparently embedded in the Styx stream is a
~1 kpc-wide concentration of stars with a similar color-magnitude distribution
which we designate Bootes III. Given its very low surface density, its location
within the stream, and its apparently disturbed morphology, we argue that
Bootes III may be the progenitor of Styx and in possibly the final throes of
tidal dissolution. While the current data do not permit strong constraints,
preliminary orbit estimates for the streams do not point to any likely
progenitors among the known globular clusters and dwarf galaxies.Comment: 10 pages, accepted for publication in the Astrophysical Journa
Substructure in tidal streams: Tributaries in the anticenter stream
We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65°-long, stellar stream complex previously identified with the Monoceros Ring. The three-stream complex varies in width from 4° to 6° along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies, and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of ≈8 km s^-1. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components that may have included a native population of globular clusters. While the color of the main-sequence turnoff is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions
Fitting orbits to tidal streams with proper motions
The Galaxy's stellar halo seems to be a tangle of disrupted systems that have
been tidally stretched out into streams. Each stream approximately delineates
an orbit in the Galactic force-field. In the first paper in this series we
showed that all six phase-space coordinates of each point on an orbit can be
reconstructed from the orbit's path across the sky and measurements of the
line-of-sight velocity along the orbit. In this paper we complement this
finding by showing that the orbit can also be reconstructed if we know proper
motions along the orbit rather than the radial velocities. We also show that
accurate proper motions of stream stars would enable distances to be determined
to points on the stream that are independent of any assumption about the
Galaxy's gravitational potential. Such "Galactic parallaxes" would be as
fundamental as conventional trigonometric parallaxes, but measureable to
distances ~70 times further.Comment: 5 pages, 2 figures, submitted to MNRAS Letter
At a Crossroads: Stellar Streams in the South Galactic Cap
We examine the distribution of old, metal-poor stars in a portion of the
recently released PanSTARRs survey. We find an interesting confluence of four
new cold stellar stream candidates that appear to converge on or pass near the
south Galactic pole. The stream candidates, which we designate Murrumbidgee,
Molonglo, Orinoco, and Kwando, lie at a distance of approximately 20 kpc and
range in length from 13 to 95 degrees, or about 5 to 33 kpc. The stream
candidates are between 100 and 300 pc in width, and are estimated to contain
between 3000 and 8000 stars each, suggesting progenitors similar to modern day
globular clusters. The trajectories of the streams imply orbits that range from
hyperbolic to nearly circular. The Molonglo stream is nearly parallel to, at
the same distance as, and offset by only 2.5 degrees from the previously
discovered ATLAS stream, suggesting a possible common origin. Orinoco and
Kwando also have similarly shaped, moderately eccentric, obliquely viewed
orbits that suggest distinct progenitors within a common, larger parent body.Comment: 9 pages, 4 figures, accepted for publication in the Astrophysical
Journal, final versio
Another Look at the Eastern Banded Structure: A Stellar Debris Stream and a Possible Progenitor
Using the Sloan Digital Sky Survey Data Release 7, we re-examine the Eastern Banded Structure (EBS), a stellar debris stream first discovered in Data Release 5 and more recently detected in velocity space by Schlaufman et al. The visible portion of the stream is 18° long, lying roughly in the Galactic Anticenter direction and extending from Hydra to Cancer. At an estimated distance of 9.7 kpc, the stream is ≈170 pc across on the sky. The curvature of the stream implies a fairly eccentric box orbit that passes close to both the Galactic center and to the Sun, making it dynamically distinct from the nearby Monoceros, Anticenter, and GD-1 streams. Within the stream is a relatively strong, 2°-wide concentration of stars with a very similar color-magnitude distribution that we designate Hydra I. Given its prominence within the stream and its unusual morphology, we suggest that Hydra I is the last vestige of EBS's progenitor, possibly already unbound or in the final throes of tidal dissolution. Though both Hydra I and the EBS have a relatively high-velocity dispersion, given the comparatively narrow width of the stream and the high frequency of encounters with the bulge and massive constituents of the disk that such an eccentric orbit would entail, we suggest that the progenitor was likely a globular cluster and that both it and the stream have undergone significant heating over time
An application of Galactic parallax: the distance to the tidal stream GD-1
We assess the practicality of computing the distance to stellar streams in
our Galaxy, using the method of Galactic parallax suggested by Eyre & Binney
(2009). We find that the uncertainty in Galactic parallax is dependent upon the
specific geometry of the problem in question. In the case of the tidal stream
GD-1, the problem geometry indicates that available proper motion data, with
individual accuracy ~4 mas/yr, should allow estimation of its distance with
about 50 percent uncertainty. Proper motions accurate to ~1 mas/yr, which are
expected from the forthcoming Pan-STARRS PS-1 survey, will allow estimation of
its distance to about 10 percent uncertainty. Proper motions from the future
LSST and Gaia projects will be more accurate still, and will allow the parallax
for a stream 30 kpc distant to be measured with ~14 percent uncertainty.
We demonstrate the feasibility of the method and show that our uncertainty
estimates are accurate by computing Galactic parallax using simulated data for
the GD-1 stream. We also apply the method to actual data for the GD-1 stream,
published by Koposov et al. (2009). With the exception of one datum, the
distances estimated using Galactic parallax match photometric estimates with
less than 1 kpc discrepancy. The scatter in the distances recovered using
Galactic parallax is very low, suggesting that the proper motion uncertainty
reported by Koposov et al. (2009) is in fact over-estimated.
We conclude that the GD-1 stream is (8 +/- 1) kpc distant, on a retrograde
orbit inclined 37 deg to the plane, and that the visible portion of the stream
is likely to be near pericentre.Comment: 9 pages, 6 figures. Submitted to Monthly Notice
Dark Matter Sub-Halo Counts via Star Stream Crossings
Dark matter sub-halos create gaps in the stellar streams orbiting in the
halos of galaxies. We evaluate the sub-halo stream crossing integral with the
guidance of simulations to find that the linear rate of gap creation, R_gap, in
a typical Cold Dark Matter (CDM) galactic halo at 100 kpc is R_gap = 0.0066
M_8^-0.35 kpc^-1 Gyr^-1$, where M_8 = M/10^8 M_sun, is the minimum mass halo
that creates a visible gap. The relation can be recast entirely in terms of
observables, as R_gap= 0.059 w^-0.85 kpc^-1 Gyr^-1, for w in kpc, normalized at
100 kpc. Using published data, the density of gaps is estimated for M31's NW
stream and the Milky Way Pal 5 stream, Orphan stream, and Eastern Banded
Structure. The estimated rates of gap creation all have errors of 50% or more
due to uncertain dynamical ages and the relatively noisy stream density
measurements. The gap rate-width data are in good agreement with the CDM
predicted relation. The high density of gaps in the narrow streams require a
total halo population of 10^5 sub-halos above a minimum mass of 10^5 M_sun.Comment: ApJ accepted and copy-edite
Substructure in Tidal Streams; Tributaries in the Anticenter Ring
We report on the detection in Sloan Digital Sky Survey data of at least
three, roughly parallel components in a 65 degree-long stellar stream complex
previously identified with the Anticenter or Monoceros Ring. The three-stream
complex varies in width from 4 to 6 degrees along its length and appears to be
made up of two or more narrow substreams as well as a broader, diffuse
component. The width and complexity of the stream indicate that the progenitor
was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc
distant and is oriented almost perpendicularly to our line of sight. The
visible portion of the stream does not pass near any known dwarf galaxies and a
preliminary orbit does not point to any viable progenitor candidates. Orbits
for the narrower substreams can be modeled with velocity offsets from the broad
component of about 8 km/s. We suggest that the broad component is likely to be
the remains of a dwarf galaxy, while the narrower streams constitute the
remnants of dynamically distinct components which may have included a native
population of globular clusters. While the color of the main sequence turn-off
is not unlike that for the Monoceros Ring, neither the visible stream nor any
reasonable projection of its orbit passes through Monoceros or Canis Major, and
we conclude that this stream is probably unrelated to the overdensities found
in these regions.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
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