108 research outputs found
A new approach to Highly Multiplexed Spectroscopy
The instrumentation developments within this thesis are primarily aimed at instrumentation for the next generation of telescopes: Extremely large telescopes (ELTs). In the European astronomical community, the highest priority for ground-based optical and near-IR instrumentation has been identified as high-multiplex, multi-object spectroscopy (HMS) [1]. HMS includes both simultaneous observations of multiple faint objects at the limits of detection (Multiple Object Spectroscopy: MOS) and spatially-resolved spectroscopy over contiguous fields of brighter structured objects (Integral Field Spectroscopy: IFS) and a mixture of the two (Diverse Field Spectroscopy: DFS). However, before we can start to build instrumentation for ELTs it is important to: understand fibre characteristics more thoroughly and be able to predict behaviour with the use of a theoretical model (chapter 3); look at new technologies (Photonic Crystal Fibres, chapter 4, Volume Phase Holographic Gratings, chapter 5); use fibres in different ways (MAIFU, chapter 6)
KPF: Keck Planet Finder
KPF is a fiber-fed, high-resolution, high-stability spectrometer in development at the UC Berkeley Space Sciences Laboratory for the W.M. Keck Observatory. The instrument is designed to characterize exoplanets via Doppler spectroscopy with a single measurement precision of 0.5ms_(-1) or better, however its resolution and stability will enable a wide variety of astrophysical pursuits. KPF will have a 200mm collimated beam diameter and a resolving power of >80,000. The design includes a green channel (440nm to 590 nm) and red channel (590nm to 850 nm). A novel design aspect of KPF is the use of a Zerodur optical bench, and Zerodur optics with integral mounts, to provide stability against thermal expansion and contraction effects
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
Cosmological constraints from the tomographic cross-correlation of DESI Luminous Red Galaxies and Planck CMB lensing
This is the Accepted Manuscript version of an article accepted for publication in Journal of Cosmology and Astroparticle Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1475-7516/2022/02/007We use luminous red galaxies selected from the imaging surveys that are being used for targeting by the Dark Energy Spectroscopic Instrument (DESI) in combination with CMB lensing maps from the Planck collaboration to probe the amplitude of large-scale structure over 0.4 ≤ z ≤ 1. Our galaxy sample, with an angular number density of approximately 500 deg-2 over 18,000 sq.deg., is divided into 4 tomographic bins by photometric redshift and the redshift distributions are calibrated using spectroscopy from DESI. We fit the galaxy autospectra and galaxy-convergence cross-spectra using models based on cosmological perturbation theory, restricting to large scales that are expected to be well described by such models. Within the context of ΛCDM, combining all 4 samples and using priors on the background cosmology from supernova and baryon acoustic oscillation measurements, we find S 8 = σ8(ωm/0.3)0.5 = 0.73 ± 0.03. This result is lower than the prediction of the ΛCDM model conditioned on the Planck data. Our data prefer a slower growth of structure at low redshift than the model predictions, though at only modest significanc
The DESI Sky Continuum Monitor System
The Dark Energy Spectroscopic Instrument (DESI) is an ongoing spectroscopic
survey to measure the dark energy equation of state to unprecedented precision.
We describe the DESI Sky Continuum Monitor System, which tracks the night sky
brightness as part of a system that dynamically adjusts the spectroscopic
exposure time to produce more uniform data quality and to maximize observing
efficiency. The DESI dynamic exposure time calculator (ETC) will combine sky
brightness measurements from the Sky Monitor with data from the guider system
to calculate the exposure time to achieve uniform signal-to-noise ratio (SNR)
in the spectra under various observing conditions. The DESI design includes 20
sky fibers, and these are split between two identical Sky Monitor units to
provide redundancy. Each Sky Monitor unit uses an SBIG STXL-6303e CCD camera
and supports an eight-position filter wheel. Both units have been completed and
delivered to the Mayall Telescope at the Kitt Peak National Observatory.
Commissioning results show that the Sky Monitor delivers the required
performance necessary for the ETC.Comment: 9 pages, 7 figures, 1 tabl
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
The DESI N-body simulation project – 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 × 108, 2.11 × 109, and 5.00 × 109 h−1 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 h−1 Mpc in the correlation function in redshift space. Through the resolution comparison we find that simulations run with a mass resolution of 2.1 × 109 h−1 M are sufficiently converged for systematic effects in the halo clustering to be smaller than the DESI statistical error at scales larger than 20 h−1 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 Mpc−1. 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 galaxy–halo relation will be presented in a follow-up study
Overview of the Dark Energy Spectroscopic Instrument
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 square degrees 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 present an overview of the instrumentation, the main technical requirements
and challenges, and the current status of the project.Comment: 11 pages, 4 figure
Constraining galaxy-halo connection with high-order statistics
We investigate using three-point statistics in constraining the galaxy-halo
connection. We show that for some galaxy samples, the constraints on the halo
occupation distribution parameters are dominated by the three-point function
signal (over its two-point counterpart). We demonstrate this on mock catalogs
corresponding to the Luminous Red Galaxies (LRGs), Emission-Line Galaxies
(ELG), and quasars (QSOs) targeted by the Dark Energy Spectroscopic Instrument
(DESI) Survey. The projected three-point function for triangle sides less up to
20 Mpc measured from a cubic Gpc of data can constrain the
characteristic minimum mass of the LRGs with a precision of %. For
comparison, similar constraints from the projected two-point function are
%. The improvements for the ELGs and QSOs targets are more modest. In
the case of the QSOs it is caused by the high shot-noise of the sample, and in
the case of the ELGs, this is caused by the range of halo masses of the host
halos. The most time-consuming part of our pipeline is the measurement of the
three-point functions. We adopt a tabulation method, proposed in earlier works
for the two-point function, to reduce significantly the required compute time
for the three-point analysis
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