Environmental effects are a primary driver of elevated gas-phase metallicities in galaxies around massive clusters, but the underlying physical mechanisms for this satellite metallicity enhancement (SME) are still unclear. Using the Dark Energy Spectroscopic Instrument (DESI) Data Release 1, we present the first measurement of the average SME as a function of projected cluster-centric distance. The resulting profile reveals three distinct regimes: a steep decline from the cluster center, a plateau near the cluster boundary, and an extended downturn across several cluster radii. Remarkably, the complex shape and amplitude of this observed SME profile are successfully reproduced in the EAGLE cosmological simulation. Drawing insights from EAGLE, we develop a novel satellite chemical evolution model to decompose the observed SME into physical contributions from suppressed star formation, stellar mass loss, and enriched gas inflow. Our analysis shows that continuous accretion of enriched intracluster medium dominates the SME plateau within the cluster virial radius, while mass loss and quenching jointly drive the rapid metallicity decline in the cluster core. Our method disentangles the impacts of three environmental processes on galactic chemical enrichment in the cosmic web, providing a powerful framework for understanding cluster galaxy evolution with current and future spectroscopic surveys.

We present a sample of 88,959 K-giants from DESI Milky Way Survey Year 3 data, which we use to characterize the chemo-dynamical properties of the stellar halo at Galactocentric distances of 12 to 100 kpc. Using HDBSCAN, we identify five prominent stellar halo substructures: Aleph, the Sagittarius stream, Gaia Sausage-Enceladus (GSE), Cetus-Palca and the Orphan-Chenab stream. We present the properties of each of these structures as they appear in our catalog, and examine how uncertainties on distance affect the characterization of substructure with this approach. We also examine regions associated with previously reported overdensities (such as the Virgo Overdensities and the Sagittarius spur) that we do not recover with HDBSCAN. The size and distance range of our catalog allows us to explore in detail the residual stellar halo, comprising stars that we do not associate with any substructure. We find that samples of 2000 outer halo stars with both highly prograde and highly retrograde angular momenta have similar metallicity distribution functions (MDFs), which do not resemble the MDFs of either GSE or Sagittarius. Both the prograde and retrograde residual halo MDFs are bimodal, with a metal-poor peak at [Fe/H] -2 and a metal-rich peak at [Fe/H] -1.3 (prograde) or -1.5 (retrograde). The MDF for lower angular momentum residual halo K-giants does not show clear evidence for a metal-poor peak, and broadly resembles the MDF of GSE, even at much lower binding energies than GSE itself. We discuss possible interpretations of these findings for GSE accretion scenarios.

Supplementary material to DESI+FASHI's publication "Weak Evolution of Cosmic HI Abundance in the Past 4.5 Billion Years".

Previous works of low-redshift quasar host galaxies have focused on compact quasars and found that their host galaxies are mainly star-forming galaxies. Here, we present a study of host galaxies for quasars with extended morphologies in ground-based optical images. We select a sample of more than 1000 type 1 quasars at redshift 0.1 < z < 1 that are classified as extended objects by the Dark Energy Spectroscopic Instrument (DESI). Combining high-resolution spectra from DESI and high-quality images from Subaru Hyper Suprime-Cam, we develop a spectrophotometric decomposition technique to iteratively decompose each quasar into an active galactic nucleus (AGN) component and its host galaxy. The technique can effectively break the degeneracy between the AGN and host components and capture the host spectral features. Our results show that the host galaxies of most quasars have low star formation rates (SFRs) and low specific SFRs, indicating that they are quiescent galaxies. Many of them exhibit prominent post-starburst features with the existence of significant old stellar populations. These properties are quite different from the nature of compact quasars with star-forming host galaxies. In addition, the relation between the black hole mass and stellar mass for our sample is broadly consistent with the canonical local relations. This work is complementary to the previous studies and suggests that the host galaxies of low-redshift quasars are more diverse than was thought.

In preparation for the first cosmological measurements from the full shape of the Lyman-$\alpha$ (Ly $\alpha$) forest from Dark Energy Spectroscopic Instrument (DESI), we must carefully model all relevant systematics that might bias our analysis. It has been shown that random quasar redshift errors produce a smoothing effect on the mean quasar continuum in the Ly $\alpha$ forest region. This, in turn, gives rise to spurious features in the Ly $\alpha$ autocorrelation and its cross-correlation with quasars. Using synthetic data sets based on the DESI survey, we confirm that the impact on baryon acoustic oscillation measurements is small, but that a bias is introduced to parameters which depend on the full shape of our correlations. We combine a model of this contamination in the cross-correlation with a new model we introduce here for the autocorrelation. These are parametrized by three parameters, which, when included in a joint fit to both correlation functions, successfully eliminate any impact of redshift errors on our full-shape constraints. We also present a strategy for removing this contamination from real data, by removing $\sim$0.3 per cent of correlating pairs.

Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey, we measure the shape, orientation, radial profile, and density anisotropies of the Milky Way (MW) stellar halo over 8 kpc < r _GC < 200 kpc. We identify a triaxial stellar halo (axis ratio 10:8:7), 43° tilted from the disk, showing two break radii at ∼16 and ∼76 kpc, likely associated with Gaia-Sausage/Enceladus and the Large Magellanic Cloud (LMC), respectively. The inner stellar halo (<30 kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the vast polar structure of MW satellites. The twisted halo may arise from the disk−halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful reidentification of the Hercules-Aquila Cloud North/South (HAC-N/S) overdensity and the Virgo overdensity (VOD). Break radii are found at 15 and 30 kpc for VOD and HAC-N/S, respectively. We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at ∼100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.

We present a systematic census of dual and offset active galactic nuclei (AGN) using spectroscopic data from the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI). After correcting for observational systematics, our final sample contains >7,000 dual AGN and 27,000 galaxy pairs containing one AGN over the redshift range 0 ≲ z ≲ 3.6. This sample expands the known dual AGN sample by ∼ 1-2 orders of magnitude at 0.2 ≲ z ≲ 0.4, includes ∼ 50 dwarf dual AGN candidates in a regime where only a handful were previously known, and triples the census at z>2. Dual AGN are preferentially found at small separations, consistent with merger-driven triggering of AGN activity. The two members of a pair differ in their star formation response: the more massive (primary) host changes little with separation, while the less massive (secondary) lies ∼ 0.3 dex above matched inactive and one-AGN companions at the same projected separation in main-sequence offset. Using ASTRID simulations, we predict that the fraction of DESI dual AGN whose central black holes will merge by z ∼ 0 increases with redshift, reaching ∼ 76% by z ∼ 2, while the fraction producing LISA-detectable mergers peaks at ∼ 37% near z ∼ 0.9. These results provide the largest uniformly selected spectroscopic sample of kpc-scale dual and offset AGN candidates from a single survey, connecting their host-galaxy and AGN demographics to the progenitor population of massive black hole mergers detectable by LISA.

Here, we present the results of our search through the early data release (EDR) of the Dark Energy Spectroscopic Instrument (DESI) for extreme coronal line emitters (ECLEs) - a rare classification of galaxies displaying strong, high-ionization iron coronal emission lines within their spectra. With the requirement of a strong X-ray continuum to generate the coronal emission, ECLEs have been linked to both active galactic nuclei (AGNs) and tidal disruption events (TDEs). We focus our search on identifying TDE-linked ECLEs. We identify three such objects within the EDR sample, highlighting DESI's effectiveness for discovering new nuclear transients, and determine a galaxy-normalized TDE-linked ECLE rate of R-G = 5(-3 )(+5)& times; 10(-6 )galaxy(-1 )yr(-1) at a median redshift of z = 0.2 - broadly consistent with previous work. Additionally, we identify more than 200 AGNs displaying coronal emission lines, which serve as the primary astrophysical contaminants in searches for TDE-related events. We also include an outline of the custom python code developed for this search.

We present a 9.3 sigma detection of the pairwise kinematic Sunyaev-Zel'dovich (kSZ) effect by combining a sample of 913,286 Luminous Red Galaxies (LRGs) from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1) catalog and coadded Atacama Cosmology Telescope (ACT DR6) and Planck cosmic microwave background (CMB) temperature maps. This represents the highest-significance pairwise kSZ measurement to date. The analysis uses three ACT CMB temperature maps: coadded 150 GHz, total frequency maps, and a component separated Internal Linear Combination (ILC) map, all of which cover 19,000 square degrees of the sky from Advanced ACTPol observations conducted between 2017 and 2022. Comparison of the results of these three maps serves as a consistency check for potential foreground contamination that may depend on the observation frequency. An estimate of the best-fit mass-averaged optical depth is obtained by comparing the pairwise kSZ curve with the linear theory prediction of the pairwise velocity under the best-fit Planck cosmology and is compared with predictions from simulations. This estimate serves as a reference point for future comparisons with thermal SZ-derived optical depth measurements for the same DESI cluster samples, which will be presented in a companion paper. Finally, we employ a machine learning approach, trained on simulations to estimate the optical depth for 456,803 DESI LRG-identified clusters within the simulated mass range (greater than or similar to 1013M circle dot). These are combined with the measured kSZ signal to infer the individual cluster peculiar velocities, providing the opportunity to constrain the behavior of gravity and the dark sector over a range of cosmic scales and epochs.

We perform a frequentist analysis using the standard profile likelihood method for clustering measurements from Data Release 1 of the Dark Energy Spectroscopic Instrument (DESI). While Bayesian inferences for Effective Field Theory models of galaxy clustering can be highly sensitive to the choice of priors for extended cosmological models, frequentist inferences are not susceptible to such effects. We compare Bayesian and frequentist constraints for the parameter set {σ_8, H_0, Ω_m, w_0, w_a} when fitting to the full-shape of the power spectrum multipoles, the post-reconstruction Baryon Acoustic Oscillation (BAO) measurements, as well as external datasets from the CMB and type Ia supernovae measurements. Bayesian prior effects are very significant for the w_0w_aCDM model; while the 1 σ frequentist confidence intervals encompass the maximum a posteriori (MAP), the Bayesian credible intervals almost always exclude the maximum likelihood estimate (MLE) and the MAP - indicating strong prior volume projection effects - unless supernovae data are included. We observe limited prior effects for the ΛCDM model, due to the reduced number of parameters. When DESI full-shape and BAO data are jointly fit, we obtain the following 1σ frequentist confidence intervals for ΛCDM (w_0w_aCDM): σ_8 = 0.867^+0.048_-0.041 , H_0 = 68.91^+0.80_-0.79 km s^-1Mpc^-1 , Ω_m = 0.3038±0.0110 (σ_8 = 0.793^+0.069_-0.048 , H_0 = 64.9^+4.8_-2.8 km s^-1Mpc^-1 , Ω_m = 0.369^+0.029_-0.059 , w_0 = -0.24^+0.17_-0.64 , w_a = -2.5^+1.9_), corresponding to 0.7σ, 0.3σ, 0.7σ (1.9σ, 3.4σ, 5.6σ, 5.5σ, 5.6σ) shifts between the MLE relative to the Bayesian posterior mean for ΛCDM (w_0w_aCDM) respectively.

The effective redshift distribution n(z) of galaxies is a critical component in the study of weak gravitational lensing. Here, we introduce a new method for determining n(z) for weak lensing surveys based on high-quality redshifts and neural-network-based importance weights. Additionally, we present the first unified photometric redshift calibration of the three leading stage-III weak lensing surveys, the Dark Energy Survey (DES), the Hyper Suprime-Cam (HSC) survey, and the Kilo-Degree Survey (KiDS), with state-of-the-art spectroscopic data from the Dark Energy Spectroscopic Instrument (DESI). We verify our method using a new, data-driven approach and obtain n(z) constraints with statistical uncertainties of the order of sigma z & strns;similar to 0.01 and smaller. Our analysis is largely independent of previous photometric redshift calibrations and, thus, provides an important cross-check in light of recent cosmological tensions. Overall, we find excellent agreement with previously published results on the DES Y3 and HSC Y1 data sets, while there are some differences on the mean redshift with respect to the previously published KiDS-1000 results. We attribute the latter to mismatches in photometric noise properties in the COSMOS field compared to the wider KiDS self-organizing map-gold catalog. At the same time, the new n(z) estimates for KiDS do not significantly change estimates of cosmic structure growth from cosmic shear. Finally, we discuss how our method can be applied to future weak lensing calibrations with DESI data.

We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in Zwicky Transient Facility (ZTF), 0 .'' 83 from a z = 0.42 elliptical galaxy, the follow-up SNIFS/UH2.2 m and LRIS/Keck spectra revealed absorption lines at z = 1.371. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by similar to 0.'' 07 , and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong H alpha emission also at z = 1.371. The lightcurves and spectra are consistent with the Type II supernova source being magnified greater than or similar to 100 times, with similar to 250 required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Lightcurve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.

We present joint measurements of the pre- and post-reconstruction power spectra, P_ pre and P_ post, together with their cross-power spectrum, P_ cross, for the Luminous Red Galaxies (LRGs) in the DESI Data Release 1 (DR1). We jointly analyse these observables with an emulator-based full-shape modeling framework, thereby, for the first time, we extract complementary nonlinear information from the galaxy density field before and after reconstruction in real survey data. Specifically, including P_ post and P_ cross in addition to P_ pre (hereafter P_ all) yields an improvement of approximately 18-27% in the σ_8 constraint in both ΛCDM and wCDM, depending on the redshift bin, relative to the P_ pre-only analysis with the cosmic microwave background distance priors (hereafter CMB). In wCDM, the joint CMB+P_ all analysis can tighten the constraints on w by approximately 5-15% across the two LRG redshift bins, compared to the CMB+P_ pre combination. Further incorporating the Type Ia supernova dataset and comparing the cosmological constraints in wCDM from each individual power-spectrum component with those from the full combination, we find that P_ all consistently provides the tightest constraints. From the joint CMB+P_ all+DES-Dovekie dataset, we obtain Ω_m = 0.314 ± 0.0048 and w = -0.988 ± 0.023 for the sample, and Ω_m = 0.318 ± 0.0046 and w = -0.988 ± 0.025 for . These results demonstrate that combining pre- and post-reconstruction power spectra with their cross-correlation enables DESI to harvest additional nonlinear information, leading to tighter constraints on cosmological parameters.

Ly alpha emitters (LAEs) are valuable high-redshift cosmological probes traditionally identified using specialized narrowband photometric surveys. In ground-based spectroscopy, it can be difficult to distinguish the sharp LAE peak from residual sky emission lines using automated methods, leading to misclassified redshifts. We present a Bayesian spectral component separation technique to automatically determine spectroscopic redshifts for LAEs while marginalizing over sky residuals. We use visually inspected spectra of LAEs obtained using the Dark Energy Spectroscopic Instrument (DESI) to create a data-driven prior and can determine redshift by jointly inferring sky residual, LAE, and residual components for each individual spectrum. We demonstrate this method on 881 spectroscopically observed z = 2-4 DESI LAE candidate spectra and determine their redshifts with >90% accuracy when validated against visually inspected redshifts. Using the Delta chi(2) value from our pipeline as a proxy for detection confidence, we then explore potential survey design choices and implications for targeting LAEs with medium-band photometry. This method allows for scalability and accuracy in determining redshifts from DESI spectra, and the results provide recommendations for LAE targeting in anticipation of future high-redshift spectroscopic surveys.

The broad line region of quasars has long been probed by reverberation mapping techniques that measure time lags between continuum and broad emission line variations. Stacked reverberation mapping has been proposed as a less observationally expensive alternative to traditional methods. This ensemble approach also reduces biases from small-number statistics. The Dark Energy Spectroscopic Instrument (DESI) is conducting the most extensive spectroscopic survey of quasars to date. We create mock light curves emulating expected DESI quasar observations at redshifts 1.48<z<5.2 and luminosities 44.68 ≤log L_1350 λ/ erg s^-1≤ 45.99 to test stacked reverberation mapping feasibility using sparse spectroscopic data paired with well-sampled photometric data. The pipeline, using the lag estimation code JAVELIN, successfully recovers the simulated C IV lags within one sigma of the true values using spectroscopic light curves composed of only a few spectral epochs (2-10) with irregular cadences. We investigate how observational factors, including C IV flux error magnitude, number of stacked quasars, and spectral epoch count, affect performance. This work motivates a pathway for future stacked reverberation mapping projects with large scale spectroscopic surveys of quasars having ≥ 2 spectroscopic observations. Our results suggest an economical alternative for constraining and extending the radius-luminosity relation to higher redshifts and luminosities. Subsequently, this relation can be employed more reliably in single-epoch black hole mass measurements and quasar cosmology in these distant regimes.

We present a detailed study of the 3D morphology of two extended associations of multiple protoclusters at z=3.1. These protocluster `complexes', designated COSMOS-z3.1-A and COSMOS-z3.1-C, are the most prominent overdensities of z=3.1 Lyα emitters (LAEs) identified in the COSMOS field by the One-hundred-deg^2 DECam Imaging in Narrowbands (ODIN) survey. These protocluster complexes have been followed up with extensive spectroscopy from Keck, Gemini, and DESI. Using a probabilistic method that combines photometrically selected and spectroscopically confirmed LAEs, we reconstruct the 3D structure of these complexes on scales of ≈50 cMpc. We validate our reconstruction method using the IllustrisTNG300-1 cosmological hydrodynamical simulation and show that it consistently outperforms approaches relying solely on spectroscopic data. The resulting 3D maps reveal that both complexes are irregular and elongated along a single axis, emphasizing the impact of sightline on our perception of structure morphology. The complexes consist of multiple density peaks, ten in COSMOS-z3.1-A and four in COSMOS-z3.1-C. The former is confirmed to be a proto-supercluster, similar to Hyperion at z=2.4 but observed at an even earlier epoch. Multiple `tails' connected to the cores of the density peaks are seen, likely representing cosmic filaments feeding into these extremely overdense regions. The 3D reconstructions further provide strong evidence that Lyα blobs preferentially reside in the outskirts of the highest density regions. Descendant mass estimates of the density peaks suggest that COSMOS-z3.1-A and COSMOS-z3.1-C will evolve to become ultra-massive structures by z=0, with total masses log(M/M_⊙) ≳ 15.3, exceeding that of Coma.

Stellar streams are excellent probes of the gravitational potential in which they evolve. In the Milky Way (MW), globular cluster (GC) streams are routinely used to infer properties about time-dependent perturbations of the underlying potential. This implies that streams with Galactocentric radii small enough to be perturbed by the MW bar should offer constraints on it, such as its pattern speed, which currently has a wide range of values reported in the literature and is important when studying stellar kinematics. The GC M92 has a small pericentre and should be affected by the bar. It has a diffuse stellar stream, but confirming stream members has previously been hindered by a lack of spectroscopic data. In this paper, we use Dark Energy Spectroscopic Instrument (DESI) observations together with photometric and astrometric data to obtain spectroscopic members of the M92 stream for the first time. We identify a clear spatial distribution and gradients in distance moduli, proper motions, and radial velocities that confirm the stream's existence. We compare the observed stream to mock streams generated in different barred potentials and estimate the MW bar's pattern speed Ω= 29.1^+0.7_-0.4 km s^-1 kpc^-1 and Ω̇= 0.7^+3.5_-2.3 km s^-1 kpc^-1 Gyr^-1. This is the first time a stellar stream is used to probabilistically infer these bar properties, and it opens up an exciting realm of inner Galactic potential characterisation using stellar streams.

Using the Dark Energy Spectroscopic Instrument Milky Way Survey (DESI MWS), we examine the 6D space of the anticenter region of the stellar disk (150^∘ < Galactic longitude < 220^∘) using 61,883 main sequence turn-off stars. We focus on two well-known stellar overdensities in the anticenter, the Monoceros Ring (MRi) and Anticenter Stream (ACS). We find that the MRi overdensity has kinematics consistent with a tidally induced spiral arm, a type of dynamic spiral arm created by an interaction with a satellite galaxy, most likely the Sagittarius Dwarf Spheroidal galaxy (Sgr). We use the kinematics of the MRi to calculate the two most recent passage times of Sgr are 0.25 ± 0.09 Gyrs and 1.10 ± 0.23 Gyrs from the present day. We validate that the ACS is kinematically decoupled from the MRi because they are moving in opposite radial and vertical directions. We find that the kinematics associated with the ACS are not confined to our defined overdensity. The features we see in the ACS region are likely part of a broader distribution of stars with the same kinematic signature as detected in other places, like the vertical wave in the outer disk and phase spiral.

We utilize multiepoch line-of-sight velocity measurements from the Milky Way Survey of the Dark Energy Spectroscopic Instrument to estimate the binary fraction for member stars in the dwarf spheroidal galaxy Ursa Minor (UMi). Our dataset comprises 670 distinct member stars, with a total of more than 2000 observations collected over approximately one year. We constrain the binary fraction for UMi to be 0.61(-0.20)(+0.16) and 0.69-(+0.19)(0.17) , with the binary orbital parameter distributions based on solar neighborhood observation from A. Duquennoy & M. Mayor and M. Moe & R. Di Stefano, respectively. Furthermore, by dividing our data into two subsamples at the median metallicity, we identify that the binary fraction for the metal-rich ([Fe/H] > -2.14) population is slightly higher than that of the metal-poor ([Fe/H] < -2.14) population. Based on M. Moe & R. Di Stefano's model, the best-constrained binary fractions for metal-rich and metal-poor populations in UMi are 0.86(-0.24)(+0.14) and 0.48(-0.19)(+0.26) , respectively. After a thorough examination, we find that this offset cannot be attributed to sample selection effects. We also divide our data into two subsamples according to their projected radius to the center of UMi, and find that the more centrally concentrated population in a denser environment has a lower binary fraction of 0.33(-0.20)(+0.30) , compared with 1.00(-0.32)(+0.00) for the subsample in the outskirts.

Quasars acting as strong gravitational lenses offer a rare opportunity to probe the redshift evolution of scaling relations between supermassive black holes and their host galaxies, particularly the M _BH – M _host relation. Using these powerful probes, the mass of the host galaxy can be precisely inferred from the Einstein radius θ _E . Using 812,118 quasars from DESI DR1 (0.03 ≤ z ≤ 1.8), we searched for quasars lensing higher-redshift galaxies by identifying background emission-line features in their spectra. To detect these rare systems, we trained a convolutional neural network (CNN) on mock lenses constructed from real DESI spectra of quasars and emission-line galaxies (ELGs), achieving a high classification performance (AUC = 0.99). We also trained a regression network to estimate the redshift of the background ELG. Applying this pipeline, we identified seven high-quality (Grade A) lens candidates, each exhibiting a strong [O II ] doublet at a higher redshift than the foreground quasar; four candidates additionally show H β , [O III ] λ 4959, and [O III ] λ 5007 emission. These results significantly expand the sample of quasar lens candidates beyond the 12 identified and 3 confirmed in previous work and demonstrate the potential for scalable, data-driven discovery of quasars as strong lenses in upcoming spectroscopic surveys.