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Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg\\(^2\\) overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color (\\(griz\\)) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to \\(r\\)23.5 mag) powered by less massive black holes (with masses \\(10^8.5M_\\)) at high redshift for the first time. We find five candidates with significant (at \\(>\\)99.74% single-frequency significance in at least two bands with a global p-value of \\(\\)7\\(10^-4\\)--3\\(10^-3\\) accounting for the look-elsewhere effect) periodicity with observed periods of \\(\\)3--5 years (i.e., 1--2 years in rest frame) having \\(\\)4--6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of \\(0.8^+0.5_-0.3\\)% or \\(1.1^+0.7_-0.5\\) quasar per deg\\(^2\\). Our detection rate is 4--80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. We discuss implications on the future direct detection of low-frequency gravitational waves. Continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins.

We report on small-amplitude optical variability and recent dissipation of the unusually persistent broad emission lines in the blue compact dwarf galaxy PHL 293B. The galaxy's unusual spectral features (P Cygni-like profiles with \\(\\)800 km s\\(^-1\\) blueshifted absorption lines) have resulted in conflicting interpretations of the nature of this source in the literature. However, analysis of new Gemini spectroscopy reveals the broad emission has begun to fade after being persistent for over a decade prior. Precise difference imaging light curves constructed with the Sloan Digital Sky Survey and the Dark Energy Survey reveal small-amplitude optical variability of \\(\\)0.1 mag in the g band offset by \\(10021\\) pc from the brightest pixel of the host. The light curve is well-described by an active galactic nuclei (AGN)-like damped random walk process. However, we conclude that the origin of the optical variability and spectral features of PHL 293B is due to a long-lived stellar transient, likely a Type IIn supernova or non-terminal outburst, mimicking long-term AGN-like variability. This work highlights the challenges of discriminating between scenarios in such extreme environments, relevant to searches for AGNs in dwarf galaxies. This is the second long-lived transient discovered in a blue compact dwarf, after SDSS1133. Our result implies such long-lived stellar transients may be more common in metal-deficient galaxies. Systematic searches for low-level variability in dwarf galaxies will be possible with the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.

We report on SPT-CLJ2011-5228, a giant system of arcs created by a cluster at \\(z=1.06\\). The arc system is notable for the presence of a bright central image. The source is a Lyman Break galaxy at \\(z_s=2.39\\) and the mass enclosed within the 14 arc second radius Einstein ring is \\(10^14.2\\) solar masses. We perform a full light profile reconstruction of the lensed images to precisely infer the parameters of the mass distribution. The brightness of the central image demands that the central total density profile of the lens be shallow. By fitting the dark matter as a generalized Navarro-Frenk-White profile---with a free parameter for the inner density slope---we find that the break radius is \\(270^+48_-76\\) kpc, and that the inner density falls with radius to the power \\(-0.380.04\\) at 68 percent confidence. Such a shallow profile is in strong tension with our understanding of relaxed cold dark matter halos; dark matter only simulations predict the inner density should fall as \\(r^-1\\). The tension can be alleviated if this cluster is in fact a merger; a two halo model can also reconstruct the data, with both clumps (density going as \\(r^-0.8\\) and \\(r^-1.0\\)) much more consistent with predictions from dark matter only simulations. At the resolution of our Dark Energy Survey imaging, we are unable to choose between these two models, but we make predictions for forthcoming Hubble Space Telescope imaging that will decisively distinguish between them.

Host galaxy identification is a crucial step for modern supernova (SN) surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST), which will discover SNe by the thousands. Spectroscopic resources are limited, so in the absence of real-time SN spectra these surveys must rely on host galaxy spectra to obtain accurate redshifts for the Hubble diagram and to improve photometric classification of SNe. In addition, SN luminosities are known to correlate with host-galaxy properties. Therefore, reliable identification of host galaxies is essential for cosmology and SN science. We simulate SN events and their locations within their host galaxies to develop and test methods for matching SNe to their hosts. We use both real and simulated galaxy catalog data from the Advanced Camera for Surveys General Catalog and MICECATv2.0, respectively. We also incorporate \"hostless\" SNe residing in undetected faint hosts into our analysis, with an assumed hostless rate of 5%. Our fully automated algorithm is run on catalog data and matches SNe to their hosts with 91% accuracy. We find that including a machine learning component, run after the initial matching algorithm, improves the accuracy (purity) of the matching to 97% with a 2% cost in efficiency (true positive rate). Although the exact results are dependent on the details of the survey and the galaxy catalogs used, the method of identifying host galaxies we outline here can be applied to any transient survey.

We present our 500 pc distance-limited study of stellar fares using the Dark Energy Camera as part of the Deeper, Wider, Faster Program. The data was collected via continuous 20-second cadence g band imaging and we identify 19,914 sources with precise distances from Gaia DR2 within twelve, ~3 square-degree, fields over a range of Galactic latitudes. An average of ~74 minutes is spent on each field per visit. All light curves were accessed through a novel unsupervised machine learning technique designed for anomaly detection. We identify 96 flare events occurring across 80 stars, the majority of which are M dwarfs. Integrated are energies range from \\( 10^31-10^37\\) erg, with a proportional relationship existing between increased are energy with increased distance from the Galactic plane, representative of stellar age leading to declining yet more energetic are events. In agreement with previous studies we observe an increase in flaring fraction from M0 -> M6 spectral types. Furthermore, we find a decrease in the flaring fraction of stars as vertical distance from the galactic plane is increased, with a steep decline present around ~100 pc. We find that ~70% of identified flares occur on short timescales of ~8 minutes. Finally we present our associated are rates, finding a volumetric rate of \\(2.9 0.3 10^-6\\) flares pc\\(^-3\\) hr\\(^-1\\).

\\( \\)Low density cosmic voids gravitationally lens the cosmic microwave background (CMB), leaving a negative imprint on the CMB convergence \\(\\). This effect provides insight into the distribution of matter within voids, and can also be used to study the growth of structure. We measure this lensing imprint by cross-correlating the Planck CMB lensing convergence map with voids identified in the Dark Energy Survey Year 3 data set, covering approximately 4,200 deg\\(^2\\) of the sky. We use two distinct void-finding algorithms: a 2D void-finder which operates on the projected galaxy density field in thin redshift shells, and a new code, Voxel, which operates on the full 3D map of galaxy positions. We employ an optimal matched filtering method for cross-correlation, using the MICE N-body simulation both to establish the template for the matched filter and to calibrate detection significances. Using the DES Y3 photometric luminous red galaxy sample, we measure \\(A_\\), the amplitude of the observed lensing signal relative to the simulation template, obtaining \\(A_ = 1.03 0.22\\) (\\(4.6\\) significance) for Voxel and \\(A_ = 1.02 0.17\\) (\\(5.9\\) significance) for 2D voids, both consistent with \\(\\)CDM expectations. We additionally invert the 2D void-finding process to identify superclusters in the projected density field, for which we measure \\(A_ = 0.87 0.15\\) (\\(5.9\\) significance). The leading source of noise in our measurements is Planck noise, implying that future data from the Atacama Cosmology Telescope (ACT), South Pole Telescope (SPT) and CMB-S4 will increase sensitivity and allow for more precise measurements.

We study a phenomenological class of models where dark matter converts to dark radiation in the low redshift epoch. This class of models, dubbed DMDR, characterizes the evolution of comoving dark matter density with two extra parameters, and may be able to help alleviate the observed discrepancies between early- and late-time probes of the universe. We investigate how the conversion affects key cosmological observables such as the CMB temperature and matter power spectra. Combining 3x2pt data from Year 1 of the Dark Energy Survey, ıt Planck-2018 CMB temperature and polarization data, supernovae (SN) Type Ia data from Pantheon, and baryon acoustic oscillation (BAO) data from BOSS DR12, MGS and 6dFGS, we place new constraints on the amount of dark matter that has converted to dark radiation and the rate of this conversion. The fraction of the dark matter that has converted since the beginning of the universe in units of the current amount of dark matter, \\(\\), is constrained at 68\\% confidence level to be \\(<0.32\\) for DES-Y1 3x2pt data, \\(<0.030\\) for CMB+SN+BAO data, and \\(<0.037\\) for the combined dataset. The probability that the DES and CMB+SN+BAO datasets are concordant increases from 4\\% for the \\(\\)CDM model to 8\\% (less tension) for DMDR. The tension in \\(S_8 = _8 _ m/0.3\\) between DES-Y1 3x2pt and CMB+SN+BAO is slightly reduced from \\(2.3\\) to \\(1.9\\). We find no reduction in the Hubble tension when the combined data is compared to distance-ladder measurements in the DMDR model. The maximum-posterior goodness-of-fit statistics of DMDR and \\(\\)CDM model are comparable, indicating no preference for the DMDR cosmology over \\(\\)CDM.

Measurements of large-scale structure are interpreted using theoretical predictions for the matter distribution, including potential impacts of baryonic physics. We constrain the feedback strength of baryons jointly with cosmology using weak lensing and galaxy clustering observables (3\\(\\)2pt) of Dark Energy Survey (DES) Year 1 data in combination with external information from baryon acoustic oscillations (BAO) and Planck cosmic microwave background polarization. Our baryon modeling is informed by a set of hydrodynamical simulations that span a variety of baryon scenarios; we span this space via a Principal Component (PC) analysis of the summary statistics extracted from these simulations. We show that at the level of DES Y1 constraining power, one PC is sufficient to describe the variation of baryonic effects in the observables, and the first PC amplitude (\\(Q_1\\)) generally reflects the strength of baryon feedback. With the upper limit of \\(Q_1\\) prior being bound by the Illustris feedback scenarios, we reach \\( 20\\%\\) improvement in the constraint of \\(S_8=_8(_ m/0.3)^0.5=0.788^+0.018_-0.021\\) compared to the original DES 3\\(\\)2pt analysis. This gain is driven by the inclusion of small-scale cosmic shear information down to 2.5 arcmin, which was excluded in previous DES analyses that did not model baryonic physics. We obtain \\(S_8=0.781^+0.014_-0.015\\) for the combined DES Y1+Planck EE+BAO analysis with a non-informative \\(Q_1\\) prior. In terms of the baryon constraints, we measure \\(Q_1=1.14^+2.20_-2.80\\) for DES Y1 only and \\(Q_1=1.42^+1.63_-1.48\\) for DESY1+Planck EE+BAO, allowing us to exclude one of the most extreme AGN feedback hydrodynamical scenario at more than \\(2 \\).

Superluminous supernovae are beginning to be discovered at redshifts as early as the epoch of reionization. A number of candidate mechanisms is reviewed, together with the discovery programs.

\\( \\)Low density cosmic voids gravitationally lens the cosmic microwave background (CMB), leaving a negative imprint on the CMB convergence \\(\\). This effect provides insight into the distribution of matter within voids, and can also be used to study the growth of structure. We measure this lensing imprint by cross-correlating the Planck CMB lensing convergence map with voids identified in the Dark Energy Survey Year 3 data set, covering approximately 4,200 deg\\(^2\\) of the sky. We use two distinct void-finding algorithms: a 2D void-finder which operates on the projected galaxy density field in thin redshift shells, and a new code, Voxel, which operates on the full 3D map of galaxy positions. We employ an optimal matched filtering method for cross-correlation, using the MICE N-body simulation both to establish the template for the matched filter and to calibrate detection significances. Using the DES Y3 photometric luminous red galaxy sample, we measure \\(A_\\), the amplitude of the observed lensing signal relative to the simulation template, obtaining \\(A_ = 1.03 0.22\\) (\\(4.6\\) significance) for Voxel and \\(A_ = 1.02 0.17\\) (\\(5.9\\) significance) for 2D voids, both consistent with \\(\\)CDM expectations. We additionally invert the 2D void-finding process to identify superclusters in the projected density field, for which we measure \\(A_ = 0.87 0.15\\) (\\(5.9\\) significance). The leading source of noise in our measurements is Planck noise, implying that future data from the Atacama Cosmology Telescope (ACT), South Pole Telescope (SPT) and CMB-S4 will increase sensitivity and allow for more precise measurements.