743 research outputs found
Velocities from Cross-Correlation: A Guide for Self-Improvement
The measurement of Doppler velocity shifts in spectra is a ubiquitous theme
in astronomy, usually handled by computing the cross-correlation of the
signals, and finding the location of its maximum. This paper addresses the
problem of the determination of wavelength or velocity shifts among multiple
spectra of the same, or very similar, objects. We implement the classical
cross-correlation method and experiment with several simple models to determine
the location of the maximum of the cross-correlation function. We propose a new
technique, 'self-improvement', to refine the derived solutions by requiring
that the relative velocity for any given pair of spectra is consistent with all
others. By exploiting all available information, spectroscopic surveys
involving large numbers of similar objects may improve their precision
significantly. As an example, we simulate the analysis of a survey of G-type
stars with the SDSS instrumentation. Applying 'self-improvement' refines
relative radial velocities by more than 50% at low signal-to-noise ratio. The
concept is equally applicable to the problem of combining a series of
spectroscopic observations of the same object, each with a different Doppler
velocity or instrument-related offset, into a single spectrum with an enhanced
signal-to-noise ratio.Comment: 7 pages, 3 figures, uses emulateapj.cls; to appear in the
Astronomical Journal; see http://hebe.as.utexas.edu/stools/ to obtain the
companion softwar
The Hierarchical Formation of the Galactic Disk
I review the results of recent cosmological simulations of galaxy formation
that highlight the importance of satellite accretion in the formation of
galactic disks. Tidal debris of disrupted satellites may contribute to the disk
component if they are compact enough to survive the decay and circularization
of the orbit as dynamical friction brings the satellite into the disk plane.
This process may add a small but non-negligible fraction of stars to the thin
and thick disks, and reconcile the presence of very old stars with the
protracted merging history expected in a hierarchically clustering universe. I
discuss various lines of evidence which suggest that this process may have been
important during the formation of the Galactic disk.Comment: paper to be read at the "Penetrating Bars through Masks of Cosmic
Dust" conference in South Afric
Can WIMP Spin Dependent Couplings explain DAMA data, in light of Null Results from Other Experiments?
We examine whether the annual modulation found by the DAMA dark matter
experiment can be explained by Weakly Interacting Massive Particles (WIMPs), in
light of new null results from other experiments. CDMS II has already ruled out
most WIMP-nucleus spin-independent couplings as an explanation for DAMA data.
Hence we here focus on spin-dependent (axial vector; SD) couplings of WIMPs to
nuclei. We expand upon previous work by (i) considering the general case of
coupling to both protons and neutrons and (ii) incorporating bounds from all
existing experiments. We note the surprising fact that CMDS II places one of
the strongest bounds on the WIMP-neutron cross-section, and show that SD
WIMP-neutron scattering alone is excluded. We also show that SD WIMP-proton
scattering alone is allowed only for WIMP masses in the 5-13 GeV range. For the
general case of coupling to both protons and neutrons, we find that, for WIMP
masses above 13 GeV and below 5 GeV, there is no region of parameter space that
is compatible with DAMA and all other experiments. In the range (5-13) GeV, we
find acceptable regions of parameter space, including ones in which the
WIMP-neutron coupling is comparable to the WIMP-proton coupling.Comment: 11 pages, 5 figures. v2: added Baksan results, added references,
other minor changes. v3: minor changes to match PRD versio
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