Browsing by Autor "S. Bailey"

Now showing 1 - 4 of 4

DESI DR2 results. I. Baryon acoustic oscillations from the Lyman alpha forest
(American Physical Society, 2025) M. Abdul Karim; J. Aguilar; S. Ahlen; C. Allende Prieto; O. Alves; A. Anand; U. Andrade; E. Armengaud; A. Aviles; S. Bailey
We present the baryon acoustic oscillation (BAO) measurements with the Lyman-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>α</a:mi></a:math> (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>Ly</c:mi><c:mi>α</c:mi></c:math>) forest from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) survey. Our BAO measurements include both the autocorrelation of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>Ly</e:mi><e:mi>α</e:mi></e:math> forest absorption observed in the spectra of high-redshift quasars and the cross-correlation of the absorption with the quasar positions. The total sample size is approximately a factor of 2 larger than the DR1 dataset, with forest measurements in over 820,000 quasar spectra and the positions of over 1.2 million quasars. We describe several significant improvements to our analysis in this paper, and two supporting papers describe improvements to the synthetic datasets that we use for validation and how we identify damped <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>Ly</g:mi><g:mi>α</g:mi></g:math> absorbers. Our main result is that we have measured the BAO scale with a statistical precision of 1.1% along and 1.3% transverse to the line of sight, for a combined precision of 0.65% on the isotropic BAO scale at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>z</i:mi><i:mi>eff</i:mi></i:msub><i:mo>=</i:mo><i:mn>2.33</i:mn></i:math>. This excellent precision, combined with recent theoretical studies of the BAO shift due to nonlinear growth, motivated us to include a systematic error term in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>Ly</k:mi><k:mi>α</k:mi></k:math> BAO analysis for the first time. We measure the ratios <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>D</m:mi><m:mi>H</m:mi></m:msub><m:mo stretchy="false">(</m:mo><m:msub><m:mi>z</m:mi><m:mi>eff</m:mi></m:msub><m:mo stretchy="false">)</m:mo><m:mo>/</m:mo><m:msub><m:mi>r</m:mi><m:mi>d</m:mi></m:msub><m:mo>=</m:mo><m:mn>8.632</m:mn><m:mo>±</m:mo><m:mn>0.098</m:mn><m:mo>±</m:mo><m:mn>0.026</m:mn></m:math> and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msub><q:mi>D</q:mi><q:mi>M</q:mi></q:msub><q:mo stretchy="false">(</q:mo><q:msub><q:mi>z</q:mi><q:mi>eff</q:mi></q:msub><q:mo stretchy="false">)</q:mo><q:mo>/</q:mo><q:msub><q:mi>r</q:mi><q:mi>d</q:mi></q:msub><q:mo>=</q:mo><q:mn>38.99</q:mn><q:mo>±</q:mo><q:mn>0.52</q:mn><q:mo>±</q:mo><q:mn>0.12</q:mn></q:math>, where <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:msub><u:mi>D</u:mi><u:mi>H</u:mi></u:msub><u:mo>=</u:mo><u:mi>c</u:mi><u:mo>/</u:mo><u:mi>H</u:mi><u:mo stretchy="false">(</u:mo><u:mi>z</u:mi><u:mo stretchy="false">)</u:mo></u:math> is the Hubble distance, <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:msub><y:mi>D</y:mi><y:mi>M</y:mi></y:msub></y:math> is the transverse comoving distance, <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:msub><ab:mi>r</ab:mi><ab:mi>d</ab:mi></ab:msub></ab:math> is the sound horizon at the drag epoch, and we quote both the statistical and the theoretical systematic uncertainty. The companion paper presents the BAO measurements at lower redshifts from the same dataset and the cosmological interpretation.
Detection of the large-scale tidal field with galaxy multiplet alignment in the DESI Y1 spectroscopic survey
(Oxford University Press, 2024) C Lamman; Daniel J. Eisenstein; J. E. Forero-Romero; J. Aguilar; S. P. Ahlen; S. Bailey; D. Bianchi; David J. Brooks; T. Claybaugh; Axel de la Macorra
ABSTRACT We explore correlations between the orientations of small galaxy groups, or ‘multiplets’, and the large-scale gravitational tidal field. Using data from the Dark Energy Spectroscopic Instrument (DESI) Y1 survey, we detect the intrinsic alignment (IA) of multiplets to the galaxy-traced matter field out to separations of $100\,h^{-1}$ Mpc. Unlike traditional IA measurements of individual galaxies, this estimator is not limited by imaging of galaxy shapes and allows for direct IA detection beyond redshift $z=1$. Multiplet alignment is a form of higher order clustering, for which the scale-dependence traces the underlying tidal field and amplitude is a result of small-scale ($\lt 1h^{-1}$ Mpc) dynamics. Within samples of bright galaxies, luminous red galaxies (LRG) and emission-line galaxies, we find similar scale-dependence regardless of intrinsic luminosity or colour. This is promising for measuring tidal alignment in galaxy samples that typically display no IA. DESI’s LRG mock galaxy catalogues created from the A bacusS ummitN-body simulations produce a similar alignment signal, though with a 33 per cent lower amplitude at all scales. An analytic model using a non-linear power spectrum (NLA) only matches the signal down to 20 $h^{-1}$ Mpc. Our detection demonstrates that galaxy clustering in the non-linear regime of structure formation preserves an interpretable memory of the large-scale tidal field. Multiplet alignment complements traditional two-point measurements by retaining directional information imprinted by tidal forces, and contains additional line-of-sight information compared to weak lensing. This is a more effective estimator than the alignment of individual galaxies in dense, blue, or faint galaxy samples.
Identifying Anomalous DESI Galaxy Spectra with a Variational Autoencoder
(Oxford University Press, 2026) C. Nicolaou; Rowina S Nathan; O Lahav; A. Palmese; A. Saintonge; J. Aguilar; S. P. Ahlen; C. Allende Prieto; S. Bailey; S. BenZvi
ABSTRACT The tens of millions of spectra being captured by the Dark Energy Spectroscopic Instrument (DESI) provide tremendous discovery potential. In this work we show how Machine Learning, in particular Variational Autoencoders (VAE), can detect anomalies in a sample of approximately 200 000 DESI spectra comprising galaxies, quasars and stars. We demonstrate that the VAE can compress the dimensionality of a spectrum by a factor of 100, while still retaining enough information to accurately reconstruct spectral features. We detect anomalous spectra as those with high reconstruction error and those which are isolated in the VAE latent representation. The anomalies identified fall into two categories: spectra with artefacts and spectra with unique physical features. Awareness of the former could improve the DESI spectroscopic pipeline; whilst the latter could help us discover new and unusual objects. To further curate the list of outliers identified, we use the Astronomaly package which employs Active Learning to provide personalized outlier recommendations for visual inspection. In this work we also explore the VAE latent space, finding that different object classes and subclasses are separated despite being unlabelled. We inject controlled synthetic anomalies and analyse their locations in the latent space to illustrate how the VAE responds to atypical spectral features; and we demonstrate the interpretability of this latent space by identifying tracks within it that correspond to various spectral characteristics. In upcoming work we hope to apply the methods presented here to search for both systematics and astrophysically interesting objects in much larger datasets of DESI spectra.
Spectroscopic search for optical emission lines from dark matter decay
(American Physical Society, 2024) Hanyue Wang; Daniel J. Eisenstein; J. Aguilar; S. P. Ahlen; S. Bailey; David J. Brooks; T. Claybaugh; Axel de la Macorra; Peter Doel; J. E. Forero-Romero
We search for narrow-line optical emission from dark matter decay by stacking dark-sky spectra from the Dark Energy Spectroscopic Instrument (DESI) at the redshift of nearby galaxies from DESI's Bright Galaxy and Luminous Red Galaxy samples. Our search uses regions separated by 5 to 20 arcsec from the centers of the galaxies, corresponding to an impact parameter of approximately 50 kpc. No unidentified spectral line shows up in the search, and we place a line flux limit of ${10}^{\ensuremath{-}19}\text{ }\text{ }\mathrm{ergs}/\mathrm{s}/{\mathrm{cm}}^{2}/{\mathrm{arcsec}}^{2}$ on emissions in the wavelength range of $2000--9000\stackrel{\ensuremath{\circ}}{\mathrm{A}}$. This places the tightest constraints yet on the two-photon decay of dark matter in the mass range of 5 to 12 eV, with a particle lifetime exceeding $3\ifmmode\times\else\texttimes\fi{}{10}^{25}\text{ }\text{ }\mathrm{s}$. This detection limit also implies that the line surface brightness contributed from all dark matter along the line of sight is at least 2 orders of magnitude lower than the measured extragalactic background light (EBL), ruling out the possibility that narrow optical-line emission from dark matter decay is a major source of the EBL.