161 research outputs found
Antiparticles
Nearly a half century after the discovery of the antiproton the study of cosmic-ray antimatter continues to be an exciting and fertile field. Sensitive searches for heavy cosmic-ray antimatter continue, although in recent years their value as a probe of universal baryon symmetry has all but evaporated. Antiprotons and positrons have opened new windows on the origin and history of cosmic rays. The rarity of antimatter as compared to ordinary cosmic-ray species has posed substantial experimental challenges. Early reports of significant enhancements of antiprotons and high-energy positrons fueled speculation that non-baryonic dark matter had been found. A new generation of balloon-borne magnetic spectrometers employing powerful particle identification techniques to eliminate background have finally managed to uncover the true antimatter signal. These new measurements support simple models of secondary production but also suggest the possibility of a small yet interesting primary component.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43807/1/11214_2004_Article_382988.pd
Dark Energy Spectroscopic Instrument (DESI) Fiber Positioner Production
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the Universe using the Baryon Acoustic
Oscillation technique. The spectra of 35 million galaxies and quasars over
14000 sq deg will be measured during the life of the experiment. A new prime
focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber
optic positioners. The fibers in turn feed ten broad-band spectrographs. We
will describe the production and manufacturing processes developed for the 5000
fiber positioner robots mounted on the focal plane of the Mayall telescope.Comment: SPIE 201
Cosmic-Ray Positrons: Are There Primary Sources?
Cosmic rays at the Earth include a secondary component originating in
collisions of primary particles with the diffuse interstellar gas. The
secondary cosmic rays are relatively rare but carry important information on
the Galactic propagation of the primary particles. The secondary component
includes a small fraction of antimatter particles, positrons and antiprotons.
In addition, positrons and antiprotons may also come from unusual sources and
possibly provide insight into new physics. For instance, the annihilation of
heavy supersymmetric dark matter particles within the Galactic halo could lead
to positrons or antiprotons with distinctive energy signatures. With the
High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have
measured the abundances of positrons and electrons at energies between 1 and 50
GeV. The data suggest that indeed a small additional antimatter component may
be present that cannot be explained by a purely secondary production mechanism.
Here we describe the signature of the effect and discuss its possible origin.Comment: 15 pages, Latex, epsfig and aasms4 macros required, to appear in
Astroparticle Physics (1999
Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy
Observations have found black holes spanning ten orders of magnitude in mass
across most of cosmic history. The Kerr black hole solution is however
provisional as its behavior at infinity is incompatible with an expanding
universe. Black hole models with realistic behavior at infinity predict that
the gravitating mass of a black hole can increase with the expansion of the
universe independently of accretion or mergers, in a manner that depends on the
black hole's interior solution. We test this prediction by considering the
growth of supermassive black holes in elliptical galaxies over
. We find evidence for cosmologically coupled mass growth among
these black holes, with zero cosmological coupling excluded at 99.98%
confidence. The redshift dependence of the mass growth implies that, at
, black holes contribute an effectively constant cosmological
energy density to Friedmann's equations. The continuity equation then requires
that black holes contribute cosmologically as vacuum energy. We further show
that black hole production from the cosmic star formation history gives the
value of measured by Planck while being consistent with
constraints from massive compact halo objects. We thus propose that stellar
remnant black holes are the astrophysical origin of dark energy, explaining the
onset of accelerating expansion at .Comment: 10 pages, 3 figures, published in ApJ Letter
The DESI N-body simulation project â I. Testing the robustness of simulations for the DESI dark time survey
Analysis of large galaxy surveys requires confidence in the robustness of numerical simulation methods. The simulations are used to construct mock galaxy catalogues to validate data analysis pipelines and identify potential systematics. We compare three N-body simulation codes, ABACUS, GADGET-2, and SWIFT, to investigate the regimes in which their results agree. We run N-body simulations at three different mass resolutions, 6.25 Ă 108, 2.11 Ă 109, and 5.00 Ă 109 hâ1 M, matching phases to reduce the noise within the comparisons. We find systematic errors in the halo clustering between different codes are smaller than the Dark Energy Spectroscopic Instrument (DESI) statistical error for s > 20 hâ1 Mpc in the correlation function in redshift space. Through the resolution comparison we find that simulations run with a mass resolution of 2.1 Ă 109 hâ1 M are sufficiently converged for systematic effects in the halo clustering to be smaller than the DESI statistical error at scales larger than 20 hâ1 Mpc. These findings show that the simulations are robust for extracting cosmological information from large scales which is the key goal of the DESI survey. Comparing matter power spectra, we find the codes agree to within 1 per cent for k †10 h Mpcâ1. We also run a comparison of three initial condition generation codes and find good agreement. In addition, we include a quasi-N-body code, FastPM, since we plan use it for certain DESI analyses. The impact of the halo definition and galaxyâhalo relation will be presented in a follow-up study
2-point statistics covariance with fewer mocks
We present an approach for accurate estimation of the covariance of 2-point
correlation functions that requires fewer mocks than the standard mock-based
covariance. This can be achieved by dividing a set of mocks into jackknife
regions and fitting the correction term first introduced in Mohammad & Percival
(2022), such that the mean of the jackknife covariances corresponds to the one
from the mocks. This extends the model beyond the shot-noise limited regime,
allowing it to be used for denser samples of galaxies. We test the performance
of our fitted jackknife approach, both in terms of accuracy and precision,
using lognormal mocks with varying densities and approximate EZmocks mimicking
the DESI LRG and ELG samples in the redshift range of z = [0.8, 1.2].
We find that the Mohammad-Percival correction produces a bias in the 2-point
correlation function covariance matrix that grows with number density and that
our fitted jackknife approach does not. We also study the effect of the
covariance on the uncertainty of cosmological parameters by performing a
full-shape analysis. We find that our fitted jackknife approach based on 25
mocks is able to recover unbiased and as precise cosmological parameters as the
ones obtained from a covariance matrix based on 1000 or 1500 mocks, while the
Mohammad-Percival correction produces uncertainties that are twice as large.
The number of mocks required to obtain an accurate estimation of the covariance
for 2-point correlation function is therefore reduced by a factor of 40-60.Comment: 13 pages, 14 figures, submitted to MNRA
DAHe white dwarfs from the DESI survey
A new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer
lines in emission has recently emerged. However, the physical origin of these
emission lines remains unclear. We present here a sample of 21 newly identified
DAHe systems and determine magnetic field strengths and (for a subset) periods
which span the ranges of ~ 6.5 -- 147 MG and ~ 0.4 -- 36 h respectively. All
but four of these systems were identified from the Dark Energy Spectroscopic
Instrument (DESI) survey sample of more than 47000 white dwarf candidates
observed during its first year of observations. We present detailed analysis of
the new DAHe WDJ161634.36+541011.51 with a spin period of 95.3 min, which
exhibits an anti-correlation between broadband flux and Balmer line strength
that is typically observed for this class of systems. All DAHe systems cluster
closely on the Gaia Hertzsprung-Russell diagram where they represent ~ 1 per
cent of white dwarfs within that region. This grouping further solidifies their
unexplained emergence at relatively late cooling times and we discuss this in
context of current formation theories. Nine of the new DAHe systems are
identifiable from SDSS spectra of white dwarfs that had been previously
classified as featureless DC-type systems. We suggest high S/N, unbiased
observations of DCs as a possible route for discovering additional DAHe
systems.Comment: 19 pages, 10 Figures, accepted for publication in MNRA
Intrinsic Alignment as an RSD Contaminant in the DESI Survey
We measure the tidal alignment of the major axes of Luminous Red Galaxies
(LRGs) from the Legacy Imaging Survey and use it to infer the artificial
redshift-space distortion signature that will arise from an
orientation-dependent, surface-brightness selection in the Dark Energy
Spectroscopic Instrument (DESI) survey. Using photometric redshifts to
down-weight the shape-density correlations due to weak lensing, we measure the
intrinsic tidal alignment of LRGs. Separately, we estimate the net polarization
of LRG orientations from DESI's fiber-magnitude target selection to be of order
10^-2 along the line of sight. Using these measurements and a linear tidal
model, we forecast a 0.2% fractional decrease on the quadrupole of the 2-point
correlation function for projected separations of 40-80 Mpc/h. We also use a
halo catalog from the Abacus Summit cosmological simulation suite to reproduce
this false quadrupole.Comment: 13 pages, 13 figures. Submitted to MNRAS. For an accessible summary
of this paper, see https://cmlamman.github.io/doc/fakeRSD_summary.pd
DESI Survey Validation Data in the COSMOS/Hyper Suprime-Cam Field: Cool Gas Trace Main-sequence Star-forming Galaxies at the Cosmic Noon
We present the first result in exploring the gaseous halo and galaxy correlation using the Dark Energy Spectroscopic Instrument survey validation data in the Cosmic Evolution Survey (COSMOS) and Hyper Suprime-Cam field. We obtain multiphase gaseous halo properties in the circumgalactic medium by using 115 quasar spectra (signal-to-noise ratio > 3). We detect Mg ii absorption at redshift 0.6 < z < 2.5, C iv absorption at 1.6 < z < 3.6, and H i absorption associated with the Mg ii and C iv. By crossmatching the COSMOS2020 catalog, we identify the Mg ii and C iv host galaxies in 10 quasar fields at 0.9< z < 3.1. We find that within the impact parameter of 250 kpc, a tight correlation is seen between the strong Mg ii equivalent width and the host galaxy star formation rate. The covering fraction f c of the strong Mg ii selected galaxies, which is the ratio of the absorbing galaxy in a certain galaxy population, shows significant evolution in the main-sequence galaxies and marginal evolution in all the galaxy populations within 250 kpc at 0.9 < z < 2.2. The f c increase in the main-sequence galaxies likely suggests the coevolution of strong Mg ii absorbing gas and the main-sequence galaxies at the cosmic noon. Furthermore, Mg ii and C iv absorbing gas is detected out of the galaxy virial radius, tentatively indicating the feedback produced by the star formation and/or the environmental effects
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