14,534 research outputs found
Action-space clustering of tidal streams to infer the Galactic potential
We present a new method for constraining the Milky Way halo gravitational
potential by simultaneously fitting multiple tidal streams. This method
requires full three-dimensional positions and velocities for all stars to be
fit, but does not require identification of any specific stream or
determination of stream membership for any star. We exploit the principle that
the action distribution of stream stars is most clustered when the potential
used to calculate the actions is closest to the true potential. Clustering is
quantified with the Kullback-Leibler Divergence (KLD), which also provides
conditional uncertainties for our parameter estimates. We show, for toy
Gaia-like data in a spherical isochrone potential, that maximizing the KLD of
the action distribution relative to a smoother distribution recovers the true
values of the potential parameters. The precision depends on the observational
errors and the number of streams in the sample; using KIII giants as tracers,
we measure the enclosed mass at the average radius of the sample stars accurate
to 3% and precise to 20-40%. Recovery of the scale radius is precise to 25%,
and is biased 50% high by the small galactocentric distance range of stars in
our mock sample (1-25 kpc, or about three scale radii, with mean 6.5 kpc).
About 15 streams, with at least 100 stars per stream, are needed to obtain
upper and lower bounds on the enclosed mass and scale radius when observational
errors are taken into account; 20-25 streams are required to stabilize the size
of the confidence interval. If radial velocities are provided for stars out to
100 kpc (10 scale radii), all parameters can be determined with 10% accuracy
and 20% precision (1.3% accuracy in the case of the enclosed mass), underlining
the need for ground-based spectroscopic follow-up to complete the radial
velocity catalog for faint halo stars observed by Gaia.Comment: Accepted versio
Group finding in the stellar halo using M-giants in 2MASS: An extended view of the Pisces Overdensity?
A density based hierarchical group-finding algorithm is used to identify
stellar halo structures in a catalog of M-giants from the Two Micron All Sky
Survey (2MASS). The intrinsic brightness of M-giant stars means that this
catalog probes deep into the halo where substructures are expected to be
abundant and easy to detect. Our analysis reveals 16 structures at high
Galactic latitude (greater than 15 degree), of which 10 have been previously
identified. Among the six new structures two could plausibly be due to masks
applied to the data, one is associated with a strong extinction region and one
is probably a part of the Monoceros ring. Another one originates at low
latitudes, suggesting some contamination from disk stars, but also shows
protrusions extending to high latitudes, implying that it could be a real
feature in the stellar halo. The last remaining structure is free from the
defects discussed above and hence is very likely a satellite remnant. Although
the extinction in the direction of the structure is very low, the structure
does match a low temperature feature in the dust maps. While this casts some
doubt on its origin, the low temperature feature could plausibly be due to real
dust in the structure itself. The angular position and distance of this
structure encompass the Pisces overdensity traced by RR Lyraes in Stripe 82 of
the Sloan Digital Sky Survey (SDSS). However, the 2MASS M-giants indicate that
the structure is much more extended than what is visible with the SDSS, with
the point of peak density lying just outside Stripe 82. The morphology of the
structure is more like a cloud than a stream and reminiscent of that seen in
simulations of satellites disrupting along highly eccentric orbits.Comment: Accepted for publication in Ap
Galaxy alignments: An overview
The alignments between galaxies, their underlying matter structures, and the
cosmic web constitute vital ingredients for a comprehensive understanding of
gravity, the nature of matter, and structure formation in the Universe. We
provide an overview on the state of the art in the study of these alignment
processes and their observational signatures, aimed at a non-specialist
audience. The development of the field over the past one hundred years is
briefly reviewed. We also discuss the impact of galaxy alignments on
measurements of weak gravitational lensing, and discuss avenues for making
theoretical and observational progress over the coming decade.Comment: 43 pages excl. references, 16 figures; minor changes to match version
published in Space Science Reviews; part of a topical volume on galaxy
alignments, with companion papers at arXiv:1504.05546 and arXiv:1504.0546
Data analysis challenges in transient gravitational-wave astronomy
Gravitational waves are radiative solutions of space-time dynamics predicted
by Einstein's theory of General Relativity. A world-wide array of large-scale
and highly sensitive interferometric detectors constantly scrutinizes the
geometry of the local space-time with the hope to detect deviations that would
signal an impinging gravitational wave from a remote astrophysical source.
Finding the rare and weak signature of gravitational waves buried in
non-stationary and non-Gaussian instrument noise is a particularly challenging
problem. We will give an overview of the data-analysis techniques and
associated observational results obtained so far by Virgo (in Europe) and LIGO
(in the US), along with the prospects offered by the up-coming advanced
versions of those detectors.Comment: 7 pages, 5 figures, Proceedings of the ARENA'12 Conference, few minor
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