796 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
Quantifying Kinematic Substructure in the Milky Way's Stellar Halo
We present and analyze the positions, distances, and radial velocities for
over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn
from SDSS DR8. We search for position-velocity substructure in these data, a
signature of the hierarchical assembly of the stellar halo. Using a cumulative
"close pair distribution" (CPD) as a statistic in the 4-dimensional space of
sky position, distance, and velocity, we quantify the presence of
position-velocity substructure at high statistical significance among the BHB
stars: pairs of BHB stars that are close in position on the sky tend to have
more similar distances and radial velocities compared to a random sampling of
these overall distributions. We make analogous mock-observations of 11
numerical halo formation simulations, in which the stellar halo is entirely
composed of disrupted satellite debris, and find a level of substructure
comparable to that seen in the actually observed BHB star sample. This result
quantitatively confirms the hierarchical build-up of the stellar halo through a
signature in phase (position-velocity) space. In detail, the structure present
in the BHB stars is somewhat less prominent than that seen in most simulated
halos, quite possibly because BHB stars represent an older sub-population. BHB
stars located beyond 20 kpc from the Galactic center exhibit stronger
substructure than at kpc.Comment: 29 page, 10 figures, 1 table; accepted by APJ; for related article by
another group see arXiv:1011.192
Spectroscopy of Quasar Candidates from SDSS Commissioning Data
The Sloan Digital Sky Survey has obtained images in five broad-band colors
for several hundred square degrees. We present color-color diagrams for stellar
objects, and demonstrate that quasars are easily distinguished from stars by
their distinctive colors. Follow-up spectroscopy in less than ten nights of
telescope time has yielded 22 new quasars, 9 of them at , and one with
, the second highest-redshift quasar yet known. Roughly 80% of the
high-redshift quasar candidates selected by color indeed turn out to be
high-redshift quasars.Comment: 4 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
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
The Chemistry of the Trailing arm of the Sagittarius Dwarf Galaxy
We present abundances of C, O, Ti, and Fe for eleven M-giant stars in the
trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived
by comparing synthetic spectra with high-resolution infrared spectra obtained
with the Phoenix spectrograph on the Gemini South telescope. The targeted stars
are drawn from two regions of the Sgr trailing arm separated by 66 degrees (5
stars) and 132 degrees (6 stars) from the main body of Sgr. The trailing arm
provides a more direct diagnostic of the chemical evolution of Sgr compared to
the extensively phase-mixed leading arm.
Within our restricted sample of ~2-3 Gyr old stars, we find that the stream
material exhibits a significant metallicity gradient of -(2.4\pm0.3)x10^{-3}
dex / degree (-(9.4\pm1.1)x10^{-4} dex / kpc) away from the main body of Sgr.
The tidal disruption of Sgr is a relatively recently event. We therefore
interpret the presence of a metallicity gradient in the debris as indicative of
a similar gradient in the progenitor. The fact that such a metallicity gradient
survived for almost a Hubble time indicates that the efficiency of radial
mixing was very low in the Sgr progenitor.
No significant gradient is seen to exist in the [alpha/Fe] abundance ratio
along the trailing arm. Our results may be accounted for by a radial decrease
in star formation efficiency and/or radial increase in the efficiency of
galactic wind-driven metal loss in the chemical evolution of the Sgr
progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream
are distinct from those of the Galactic halo. We conclude that the fraction of
the intermediate to metal-rich halo population contributed by the recent
dissolution (<3 Gyr) of Sgr-like dwarf galaxies can not be substantial.Comment: 22 pages, 7 figures, ApJ accepte
Imaging of the Host Galaxies of Three X-Ray Selected BL Lacertae Objects
Hubble Space Telescope (HST) WFPC-2 I-band (F814W) images of three X-ray
selected BL Lacertae objects (MS1221.8+2452, MS1407.9+5954, & MS2143.4+0704)
reveal that each of these BL Lac objects is well-centered in an extended
nebulosity that is consistent in brightness and morphology with being light
from an elliptical galaxy at the previously reported redshifts of these BL Lac
objects. Each of the detected host galaxies have radial surface brightness
profiles that are well fit by a DeVaucouleurs' law with effective radii of
between 3 to 12 kpc (H_0=50 km s^{-1} Mpc^{-1}, q_0 = 0). The absolute
magnitudes of the host galaxies fall in the range -24.7 < M_I < -23.5, in the
range of luminosities determined for other BL Lacertae object host galaxies. In
addition to allowing the measurement of the host galaxy magnitudes and radial
surface brightness profiles, the HST images allow a search for substructure in
the host galaxies and the presence of close companion galaxies at spatial
resolutions not yet achievable from the ground. While no evidence was found for
any ``bars'' or spiral arms, ``boxy'' isophotes are present in the host galaxy
of at least one of the three objects observed as part of this study
(MS2143.4+0704). The apparent magnitudes and image properties of the companions
of the BL Lac objects are catalogued as part of this work. The three BL Lacs
appear to occur in diverse environments, from being fairly isolated
(MS1221.8+2452) to possibly being a member of a rich group of galaxies
(MS1407.9+5954).Comment: Latex, 5 postscript figures, 4 jpeg figures, To appear in The
Astrophysical Journal Nov 20, 199
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