176 research outputs found
A framework to measure the properties of intergalactic metal systems with two-point flux statistics
The abundance, temperature, and clustering of metals in the intergalactic
medium are important parameters for understanding their cosmic evolution and
quantifying their impact on cosmological analysis with the Ly forest.
The properties of these systems are typically measured from individual quasar
spectra redward of the quasar's Ly emission line, yet that approach
may provide biased results due to selection effects. We present an alternative
approach to measure these properties in an unbiased manner with the two-point
statistics commonly employed to quantify large-scale structure. Our model
treats the observed flux of a large sample of quasar spectra as a continuous
field and describes the one-dimensional, two-point statistics of this field
with three parameters per ion: the abundance (column density distribution),
temperature (Doppler parameter) and clustering (cloud-cloud correlation
function). We demonstrate this approach on multiple ions (e.g., C IV, Si IV, Mg
II) with early data from the Dark Energy Spectroscopic Instrument (DESI) and
high-resolution spectra from the literature. Our initial results show some
evidence that the C IV abundance is higher than previous measurements and
evidence for abundance evolution over time. The first full year of DESI
observations will have over an order of magnitude more quasar spectra than this
study. In a future paper we will use those data to measure the growth of
clustering and its impact on the Ly forest, as well as test other DESI
analysis infrastructure such as the pipeline noise estimates and the resolution
matrix.Comment: 15 pages, 14 figure
Planting a Lyman alpha forest on AbacusSummit
The full-shape correlations of the Lyman alpha (Ly α) forest contain a wealth of cosmological information through the Alcock-Paczyński effect. However, these measurements are challenging to model without robustly testing and verifying the theoretical framework used for analysing them. Here, we leverage the accuracy and volume of the N-body simulation suite AbacusSummit to generate high-resolution Ly α skewers and quasi-stellar object (QSO) catalogues. One of the main goals of our mocks is to aid in the full-shape Ly α analysis planned by the Dark Energy Spectroscopic Instrument (DESI) team. We provide optical depth skewers for six of the fiducial cosmology base-resolution simulations (, N = 69123) at z = 2.5. We adopt a simple recipe based on the Fluctuating Gunn-Peterson Approximation (FGPA) for constructing these skewers from the matter density in an N-body simulation and calibrate it against the 1D and 3D Ly α power spectra extracted from the hydrodynamical simulation IllustrisTNG (TNG;, N = 25003). As an important application, we study the non-linear broadening of the baryon acoustic oscillation (BAO) peak and show the cross-correlation between DESI-like QSOs and our Ly α forest skewers. We find differences on small scales between the Kaiser approximation prediction and our mock measurements of the Ly α × QSO cross-correlation, which would be important to account for in upcoming analyses. The AbacusSummit Ly α forest mocks open up the possibility for improved modelling of cross-correlations between Ly α and cosmic microwave background (CMB) lensing and Ly α and QSOs, and for forecasts of the 3-point Ly α correlation function. Our catalogues and skewers are publicly available on Globus via the National Energy Research Scientific Computing Center (NERSC) (full link under the section 'Data Availability')
A data compression and optimal galaxy weights scheme for Dark Energy Spectroscopic Instrument and weak lensing data sets
Combining different observational probes, such as galaxy clustering and weak lensing, is a promising technique for unveiling the physics of the Universe with upcoming dark energy experiments. The galaxy redshift sample from the Dark Energy Spectroscopic Instrument (DESI) will have a significant overlap with major ongoing imaging surveys specifically designed for weak lensing measurements: The Kilo-Degree Survey (KiDS), the Dark Energy Survey (DES), and the Hyper Suprime-Cam (HSC) survey. In this work, we analyse simulated redshift and lensing catalogues to establish a new strategy for combining high-quality cosmological imaging and spectroscopic data, in view of the first-year data assembly analysis of DESI. In a test case fitting for a reduced parameter set, we employ an optimal data compression scheme able to identify those aspects of the data that are most sensitive to cosmological information and amplify them with respect to other aspects of the data. We find this optimal compression approach is able to preserve all the information related to the growth of structures
A data compression and optimal galaxy weights scheme for Dark Energy Spectroscopic Instrument and weak lensing datasets
Combining different observational probes, such as galaxy clustering and weak
lensing, is a promising technique for unveiling the physics of the Universe
with upcoming dark energy experiments. The galaxy redshift sample from the Dark
Energy Spectroscopic Instrument (DESI) will have a significant overlap with
major ongoing imaging surveys specifically designed for weak lensing
measurements: the Kilo-Degree Survey (KiDS), the Dark Energy Survey (DES) and
the Hyper Suprime-Cam (HSC) survey. In this work we analyse simulated redshift
and lensing catalogues to establish a new strategy for combining high-quality
cosmological imaging and spectroscopic data, in view of the first-year data
assembly analysis of DESI. In a test case fitting for a reduced parameter set,
we employ an optimal data compression scheme able to identify those aspects of
the data that are most sensitive to the cosmological information, and amplify
them with respect to other aspects of the data. We find this optimal
compression approach is able to preserve all the information related to the
growth of structure; we also extend this scheme to derive weights to be applied
to individual galaxies, and show that these produce near-optimal results.Comment: 14 pages, 12 Figures, DESI collaboration articl
Detecting and Characterizing Mg ii Absorption in DESI Survey Validation Quasar Spectra
We present findings of the detection of Magnesium II (Mg ii, λ = 2796, 2803 Å) absorbers from the early data release of the Dark Energy Spectroscopic Instrument (DESI). DESI is projected to obtain spectroscopy of approximately 3 million quasars (QSOs), of which over 99% are anticipated to be at redshifts greater than z > 0.3, such that DESI would be able to observe an associated or intervening Mg ii absorber illuminated by the background QSO. We have developed an autonomous supplementary spectral pipeline that detects these systems through an initial line-fitting process and then confirms the line properties using a Markov Chain Monte Carlo sampler. Based upon a visual inspection of the resulting systems, we estimate that this sample has a purity greater than 99%. We have also investigated the completeness of our sample in regard to both the signal-to-noise properties of the input spectra and the rest-frame equivalent width (W 0) of the absorber systems. From a parent catalog containing 83,207 quasars, we detect a total of 23,921 Mg ii absorption systems following a series of quality cuts. Extrapolating from this occurrence rate of 28.8% implies a catalog at the completion of the five-year DESI survey that will contain over eight hundred thousand Mg ii absorbers. The cataloging of these systems will enable significant further research because they carry information regarding circumgalactic medium environments, the distribution of intervening galaxies, and the growth of metallicity across the redshift range 0.3 ≤ z < 2.5
Baryon Acoustic Oscillations in the Ly{\alpha} forest of BOSS DR11 quasars
We report a detection of the baryon acoustic oscillation (BAO) feature in the
flux-correlation function of the Ly{\alpha} forest of high-redshift quasars
with a statistical significance of five standard deviations. The study uses
137,562 quasars in the redshift range from the Data Release
11 (DR11) of the Baryon Oscillation Spectroscopic Survey (BOSS) of SDSS-III.
This sample contains three times the number of quasars used in previous
studies. The measured position of the BAO peak determines the angular distance,
and expansion rate, , both on a scale set by the sound
horizon at the drag epoch, . We find
and
where . The optimal
combination, is determined with a precision of
. For the value , consistent with the CMB power
spectrum measured by Planck, we find
and . Tests with mock
catalogs and variations of our analysis procedure have revealed no systematic
uncertainties comparable to our statistical errors. Our results agree with the
previously reported BAO measurement at the same redshift using the
quasar-Ly{\alpha} forest cross-correlation. The auto-correlation and
cross-correlation approaches are complementary because of the quite different
impact of redshift-space distortion on the two measurements. The combined
constraints from the two correlation functions imply values of and
that are, respectively, 7% low and 7% high compared to the
predictions of a flat CDM cosmological model with the best-fit Planck
parameters. With our estimated statistical errors, the significance of this
discrepancy is .Comment: Accepted for publication in A&A. 17 pages, 18 figure
Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. I. Sample from the Early Data
\ua9 2024. The Author(s). Published by the American Astronomical Society.Changing-look active galactic nuclei (CL AGNs) can be generally confirmed by the emergence (turn-on) or disappearance (turn-off) of broad emission lines (BELs), associated with a transient timescale (about 100 ∼ 5000 days) that is much shorter than predicted by traditional accretion disk models. We carry out a systematic CL AGN search by crossmatching the spectra coming from the Dark Energy Spectroscopic Instrument and the Sloan Digital Sky Survey. Following previous studies, we identify CL AGNs based on Hα, Hβ, and Mg ii at z ≤ 0.75 and Mg ii, C iii], and C iv at z > 0.75. We present 56 CL AGNs based on visual inspection and three selection criteria, including 2 Hα, 34 Hβ, 9 Mg ii, 18 C iii], and 1 C iv CL AGN. Eight cases show simultaneous appearances/disappearances of two BELs. We also present 44 CL AGN candidates with significant flux variation of BELs, but remaining strong broad components. In the confirmed CL AGNs, 10 cases show additional CL candidate features for different lines. In this paper, we find: (1) a 24:32 ratio of turn-on to turn-off CL AGNs; (2) an upper-limit transition timescale ranging from 330 to 5762 days in the rest frame; and (3) the majority of CL AGNs follow the bluer-when-brighter trend. Our results greatly increase the current CL census (∼30%) and would be conducive to exploring the underlying physical mechanism
Validation of semi-analytical, semi-empirical covariance matrices for two-point correlation function for early DESI data
We present an extended validation of semi-analytical, semi-empirical covariance matrices for the two-point correlation function (2PCF) on simulated catalogs representative of luminous red galaxies (LRGs) data collected during the initial 2 months of operations of the Stage-IV ground-based Dark Energy Spectroscopic Instrument (DESI). We run the pipeline on multiple effective Zel'dovich (EZ) mock galaxy catalogs with the corresponding cuts applied and compare the results with the mock sample covariance to assess the accuracy and its fluctuations. We propose an extension of the previously developed formalism for catalogs processed with standard reconstruction algorithms. We consider methods for comparing covariance matrices in detail, highlighting their interpretation and statistical properties caused by sample variance, in particular, non-trivial expectation values of certain metrics even when the external covariance estimate is perfect. With improved mocks and validation techniques, we confirm a good agreement between our predictions and sample covariance. This allows one to generate covariance matrices for comparable data sets without the need to create numerous mock galaxy catalogs with matching clustering, only requiring 2PCF measurements from the data itself. The code used in this paper is publicly available at https://github.com/oliverphilcox/RascalC
A striking relationship between dust extinction and radio detection in DESI QSOs: evidence for a dusty blow-out phase in red QSOs
We present the first eight months of data from our secondary target program
within the ongoing Dark Energy Spectroscopic Instrument (DESI) survey. Our
program uses a mid-infrared and optical colour selection to preferentially
target dust-reddened QSOs that would have otherwise been missed by the nominal
DESI QSO selection. So far we have obtained optical spectra for 3038
candidates, of which ~70% of the high-quality objects (those with robust
redshifts) are visually confirmed to be Type 1 QSOs, consistent with the
expected fraction from the main DESI QSO survey. By fitting a dust-reddened
blue QSO composite to the QSO spectra, we find they are well-fitted by a normal
QSO with up to Av~4 mag of line-of-sight dust extinction. Utilizing radio data
from the LOFAR Two-metre Sky Survey (LoTSS) DR2, we identify a striking
positive relationship between the amount of line-of-sight dust extinction
towards a QSO and the radio detection fraction, that is not driven by
radio-loud systems, redshift and/or luminosity effects. This demonstrates an
intrinsic connection between dust reddening and the production of radio
emission in QSOs, whereby the radio emission is most likely due to low-powered
jets or winds/outflows causing shocks in a dusty environment. On the basis of
this evidence we suggest that red QSOs may represent a transitional "blow-out"
phase in the evolution of QSOs, where winds and outflows evacuate the dust and
gas to reveal an unobscured blue QSO.Comment: 21 pages, 17 figures, 6 tables, accepted by MNRA
Mock data sets for the Eboss and DESI Lyman- forest surveys
{We present a publicly-available code to generate mock Lyman- (\lya)
forest data sets. The code is based on the Fluctuating Gunn-Peterson
Approximation (FGPA) applied to Gaussian random fields and on the use of fast
Fourier transforms (FFT). The output includes spectra of lya transmitted flux
fraction, , a quasar catalog, and a catalog of high-column-density systems.
While these three elements have realistic correlations, additional code is then
used to generate realistic quasar spectra, to add absorption by
high-column-density systems and metals, and to simulate instrumental effects.
Redshift space distortions (RSD) are implemented by including the large-scale
velocity-gradient field in the FGPA resulting in a correlation function of
that can be accurately predicted. One hundred realizations have been produced
over the 14,000 deg Dark Energy Spectroscopy Instrument (DESI) survey
footprint with 100 quasars per deg, and they are being used for the
Extended Baryon Oscillation Survey (eBOSS) and DESI surveys. The analysis of
these realizations shows that the correlation of follows the prediction
within the accuracy of eBOSS survey. The most time-consuming part of the
production occurs before application of the FGPA, and the existing pre-FGPA
forests can be used to easily produce new mock sets with modified
redshift-dependent bias parameters or observational conditions.Comment: to be submitted ot JCA
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