5,287 research outputs found
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
MAGIQ at the W. M. Keck Observatory: initial deployment of a new acquisition, guiding, and image quality monitoring system
The W. M. Keck Observatory has completed the development and initial deployment of MAGIQ, the Multi-function Acquisition, Guiding and Image Quality monitoring system. MAGIQ is an integrated system for acquisition, guiding and image quality measurement for the Keck telescopes. This system replaces the acquisition and guiding hardware and software for existing instruments at the Observatory and is now the standard for visible wavelength band acquisition cameras for future instrumentation. In this paper we report on the final design and implementation of this new system, which includes three major components: a visible wavelength band acquisition camera, image quality measurement capability, and software for acquisition, guiding and image quality monitoring. The overall performance is described, as well as the details of our approach to integrating low order wavefront sensing capability in order to provide closed loop control of telescope focus
Stellar Population Trends in S0 Galaxies
We present stellar population age and metallicity trends for a sample of 59
S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with
optical g and r passband imaging data from the SDSS archive and stellar
population models, we obtain radial age and metallicity trends out to at least
5 effective radii for most of the galaxies in our sample. The sample covers a
range in stellar mass and light concentration. We find an average central
light-weighted age of ~ 4 Gyr and central metallicity [Z/H] ~ 0.2 dex. Almost
all galaxies show a negative metallicity gradient from the center out, with an
average value of Delta[Z/H]/Delta(log(r/Re)) = -0.6. An age increase, decrease,
and minimal change with radius is observed for 58%, 19%, and 23%, respectively,
for a mean age gradient of Delta(age)/Delta(log(r/Re)) = 2.3 Gyr dex^{-1}. For
14 out of 59 galaxies, the light-weighted age of the outer region is greater
than 10 Gyr. We find that galaxies with both lower mass and lower concentration
have younger light-weighted ages and lower light-weighted metallicities. This
mass-metallicity relation extends into the outer regions of our S0 galaxies.
Our results are consistent with the formation of S0 galaxies through the
transformation of spiral galaxy disks. Determining the structural component
that makes up the outer region of galaxies with old outksirts is a necessary
step to understand the formation history of S0 galaxies.Comment: accepted to MNRA
AEGIS: Demographics of X-ray and Optically Selected AGNs
We develop a new diagnostic method to classify galaxies into AGN hosts,
star-forming galaxies, and absorption-dominated galaxies by combining the [O
III]/Hbeta ratio with rest-frame U-B color. This can be used to robustly select
AGNs in galaxy samples at intermediate redshifts (z<1). We compare the result
of this optical AGN selection with X-ray selection using a sample of 3150
galaxies with 0.3<z<0.8 and I_AB<22, selected from the DEEP2 Galaxy Redshift
Survey and the All-wavelength Extended Groth Strip International Survey
(AEGIS). Among the 146 X-ray sources in this sample, 58% are classified
optically as emission-line AGNs, the rest as star-forming galaxies or
absorption-dominated galaxies. The latter are also known as "X-ray bright,
optically normal galaxies" (XBONGs). Analysis of the relationship between
optical emission lines and X-ray properties shows that the completeness of
optical AGN selection suffers from dependence on the star formation rate and
the quality of observed spectra. It also shows that XBONGs do not appear to be
a physically distinct population from other X-ray detected, emission-line AGNs.
On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all
emission-line AGNs at L_bol>10^44 erg/s in our sample are not detected in our
200 ks Chandra images, most likely due to moderate or heavy absorption by gas
near the AGN. The 2--7 keV detection rate of Seyfert 2s at z~0.6 suggests that
their column density distribution and Compton-thick fraction are similar to
that of local Seyferts. Multiple sample selection techniques are needed to
obtain as complete a sample as possible.Comment: 24 pages, 14 figures, submitted to ApJ. Version 2 matches the ApJ
accepted version. Sec 3 was reorganized and partly rewritten with one
additional figure (Fig.3
Ordering and the micromechanics of Ti-7Al
The evolution of intergranular lattice strain in the α titanium alloy Ti–7Al wt% was characterised using in situ time-of-flight (TOF) neutron diffraction during room temperature tensile loading. Samples were aged to promote ordering and the formation of nanometre-scale α2 (Ti3Al). On ageing, at 550°C and 625°C, dislocations were observed to travel in pairs, and in planar arrays, which has been attributed to the presence of ordering. A slight change in c/a was observed, from 1.6949 to 1.6945, and a slight increase in the macroscopic modulus. However, no changes were observed in the residual lattice strains, which are the grain-orientation average elastic strains produced by plasticity. Therefore it is inferred that the changes in deformation mechanisms caused by ordering that result in an enhanced vulnerability to dwell fatigue affect primarily the extent of slip localisation. The overall strain distributions between grains in different orientations are not changed
Nuclear Shadowing in Electro-Weak Interactions
Shadowing is a quantum phenomenon leading to a non-additivity of electroweak
cross sections on nucleons bound in a nucleus. It occurs due to destructive
interference of amplitudes on different nucleons. Although the current
experimental evidence for shadowing is dominated by charged-lepton nucleus
scattering, studies of neutrino nucleus scattering have recently begun and
revealed unexpected results.Comment: 77 pages, 57 figures. To be published in "Progress in Particle and
Nuclear Physics" 201
Uncovering Blue Diffuse Dwarf Galaxies
Extremely metal poor (XMP) galaxies are known to be very rare, despite the large numbers of low-mass galaxies predicted by the local galaxy luminosity function. This paper presents a sub-sample of galaxies that were selected via a morphology-based search on SDSS images with the aim of finding these elusive XMP galaxies. By using the recently discovered extremely metal-poor galaxy, Leo P, as a guide, we obtained a collection of faint, blue systems, each with isolated H ii regions embedded in a diffuse continuum, that have remained undetected until now. Here we show the first results from optical spectroscopic follow-up observations of 12 of ∼100 of these blue, diffuse dwarf (BDD) galaxies yielded by our search algorithm. Oxygen abundances were obtained via the direct method for eight galaxies, and found to be in the range 7.45 < 12 + log (O/H) < 8.0, with two galaxies being classified as XMPs. All BDDs were found to currently have a young star-forming population (< 10 Myr) and rela-tively high ionisation parameters of their H ii regions. Despite their low luminosities (−11. MB. −18) and low surface brightnesses ( ∼ 23–25 mag arcsec−2), the galax-ies were found to be actively star-forming, with current star-formation rates between 0.0003 and 0.078 M yr−1. From our current subsample, BDD galaxies appear to be a population of non-quiescent dwarf irregular (dIrr) galaxies, or the diffuse counterparts to blue compact galaxies (BCDs) and as such may bridge the gap between these two populations. Our search algorithm demonstrates that morphology-based searches are successful in uncovering more diffuse metal-poor star-forming galaxies, which tradi-tional emission-line based searches overlook
MAGIQ at the W. M. Keck Observatory: initial deployment of a new acquisition, guiding, and image quality monitoring system
The W. M. Keck Observatory has completed the development and initial deployment of MAGIQ, the Multi-function Acquisition, Guiding and Image Quality monitoring system. MAGIQ is an integrated system for acquisition, guiding and image quality measurement for the Keck telescopes. This system replaces the acquisition and guiding hardware and software for existing instruments at the Observatory and is now the standard for visible wavelength band acquisition cameras for future instrumentation. In this paper we report on the final design and implementation of this new system, which includes three major components: a visible wavelength band acquisition camera, image quality measurement capability, and software for acquisition, guiding and image quality monitoring. The overall performance is described, as well as the details of our approach to integrating low order wavefront sensing capability in order to provide closed loop control of telescope focus
- …