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The properties of Milky Way satellite galaxies have important implications for galaxy formation, reionization, and the fundamental physics of dark matter. However, the population of Milky Way satellites includes the faintest known galaxies, and current observations are incomplete. To understand the impact of observational selection effects on the known satellite population, we perform rigorous, quantitative estimates of the Milky Way satellite galaxy detection efficiency in three wide-field survey datasets: the Dark Energy Survey Year 6, the DECam Local Volume Exploration Data Release 3, and the Pan-STARRS1 Data Release 1. Together, these surveys cover \\(\\sim\\)13,600 deg\\(^2\\) to \\(g \\sim 24.0\\) and \\(\\sim\\)27,700 deg\\(^2\\) to \\(g \\sim 22.5\\), spanning \\(\\sim\\)91% of the high-Galactic-latitude sky (\\(|b| \\geq 15^\\circ\\)). We apply multiple detection algorithms over the combined footprint and recover 49 known satellites above a strict census detection threshold. To characterize the sensitivity of our census, we run our detection algorithms on a large set of simulated galaxies injected into the survey data, which allows us to develop models that predict the detectability of satellites as a function of their properties. We then fit an empirical model to our data and infer the luminosity function, radial distribution, and size-luminosity relation of Milky Way satellite galaxies. Our empirical model predicts a total of \\(265^{+79}_{-47}\\) satellite galaxies with \\(-20 \\leq M_V \\leq 0\\), half-light radii of \\(15 \\leq r_{1/2} (\\rm pc) \\leq 3000\\), and galactocentric distances of \\(10 \\leq D_{\\rm GC} (\\rm kpc) \\leq 300\\). We also identify a mild anisotropy in the angular distribution of the observed galaxies, at a significance of $\\sim$$2\\sigma$, which can be attributed to the clustering of satellites associated with the LMC.

We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way (MW) satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration survey (DELVE EDR3). The low luminosity (\\(M_V = -3.56_{-0.37}^{+0.47}\\) ; \\(L_V = 2300_{-700}^{+1200} L_\\odot\\)), large size (\\(R_{1/2} = 90_{-30}^{+30}\\) pc), and large heliocentric distance (\\(D = 111_{-6}^{+9}\\) kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of \\(700_{-500}^{+1400} M_\\odot/L_\\odot\\) and a non-zero metallicity dispersion of \\(\\sigma_{[\\rm Fe/H]}=0.19_{-0.11}^{+0.14}\\), further confirming Leo VI's identity as an UFD. While the system has a highly elliptical shape, \\(\\epsilon = 0.54_{-0.29}^{+0.19}\\), we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy-angular momentum space make it unlikely that Leo VI is part of the proposed infall group.

We present the latest results of our search for planets with HARPS-N at the 3.6 m Telescopio Nazionale Galileo under the Tracking Advanced Planetary Systems project: an in-depth study of the 15 most Li abundant giants from the PennState - Toruń Planet Search sample. Our goals are first, to obtain radial velocities of the most Li-rich giants we identified in our sample to search for possible low-mass substellar companions, and second, to perform an extended spectral analysis to define the evolutionary status of these stars. Methods. This work is based on high-resolution spectra obtained with the Hobby-Eberly Telescope and its High Resolution Spectro- graph, and with the HARPS-N spectrograph at the Telescopio Nazionale Galileo. Two stars, HD 181368 and HD 188214 , were also observed with UVES at the VLT to determine beryllium abundances. We report i) the discovery of two new planetary systems around the Li-rich giant stars: HD 238914 and TYC 3318-01333- 1 (a binary system); ii) reveal a binary Li-rich giant, HD 181368 ; iii) although our current phase coverage is not complete, we suggest the presence of planetary mass companions around TYC 3663-01966-1 and TYC 3105-00152-1 ; iv) we confirm the previous result for BD+48 740 and present updated orbital parameters, and v) we find a lack of a relation between the Li enhancement and the Be abundance for the stars HD 181368 and HD 188214 , for which we acquired blue spectra. We found seven stars with stellar or potential planetary companions among the 15 Li-rich giant stars. The binary star frequency of the Li-rich giants in our sample appears to be normal, but the planet frequency is twice that of the general sample, which suggests a possible connection between hosting a companion and enhanced Li abundance in giant stars. We also found most of the companions orbits to be highly eccentric.

We study the optical \\(gri\\) photometric variability of a sample of 190 quasars within the SDSS Stripe 82 region that have long-term photometric coverage during \\(\\sim 1998-2020\\) with SDSS, PanSTARRS-1, the Dark Energy Survey, and dedicated follow-up monitoring with Blanco 4m/DECam. With on average \\(\\sim 200\\) nightly epochs per quasar per filter band, we improve the parameter constraints from a Damped Random Walk (DRW) model fit to the light curves over previous studies with 10-15 yr baselines and \\(\\lesssim 100\\) epochs. We find that the average damping timescale \\(\\tau_{\\rm DRW}\\) continues to rise with increased baseline, reaching a median value of \\(\\sim 750\\) days (\\(g\\) band) in the rest-frame of these quasars using the 20-yr light curves. Some quasars may have gradual, long-term trends in their light curves, suggesting that either the DRW fit requires very long baselines to converge, or that the underlying variability is more complex than a single DRW process for these quasars. Using a subset of quasars with better-constrained \\(\\tau_{\\rm DRW}\\) (less than 20\\% of the baseline), we confirm a weak wavelength dependence of \\(\\tau_{\\rm DRW}\\propto \\lambda^{0.51\\pm0.20}\\). We further quantify optical variability of these quasars over days to decades timescales using structure function (SF) and power spectrum density (PSD) analyses. The SF and PSD measurements qualitatively confirm the measured (hundreds of days) damping timescales from the DRW fits. However, the ensemble PSD is steeper than that of a DRW on timescales less than \\(\\sim\\) a month for these luminous quasars, and this second break point correlates with the longer DRW damping timescale.

We present the first detection of weak gravitational lensing around spectroscopically confirmed dwarf galaxies, using the large overlap between DESI DR1 spectroscopic data and DECADE/DES weak lensing catalogs. A clean dwarf galaxy sample with well-defined redshift and stellar mass cuts enables excess surface mass density measurements in two stellar mass bins (\\( M_*=[8.2, 9.2]~M_\\) and \\( M_*=[9.2, 10.2]~M_\\)), with signal-to-noise ratios of \\(5.6\\) and \\(12.4\\) respectively. This signal-to-noise drops to \\(4.5\\) and \\(9.2\\) respectively for measurements without applying individual inverse probability (IIP) weights, which mitigates fiber incompleteness from DESI's targeting. The measurements are robust against variations in stellar mass estimates, photometric shredding, and lensing calibration systematics. Using a simulation-based modeling framework with stellar mass function priors, we constrain the stellar mass-halo mass relation and find a satellite fraction of \\( 0.3\\), which is higher than previous photometric studies but \\(1.5\\) lower than \\(\\)CDM predictions. We find that IIP weights have a significant impact on lensing measurements and can change the inferred \\(f_sat\\) by a factor of two, highlighting the need for accurate fiber incompleteness corrections for dwarf galaxy samples. Our results open a new observational window into the galaxy-halo connection at low masses, showing that future massively multiplexed spectroscopic observations and weak lensing data will enable stringent tests of galaxy formation models and \\(\\)CDM predictions.

We present the discovery of a candidate ultra-faint Milky Way satellite, Eridanus IV (DELVE J0505\\(-\\)0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). Eridanus IV is a faint (\\(M_V = -4.7 \\pm 0.2\\)), extended (\\(r_{1/2} = 75^{+16}_{-13}\\) pc), and elliptical (\\(\\epsilon = 0.54 \\pm 0.1\\)) system at a heliocentric distance of \\(76.7^{+4.0}_{-6.1}\\) kpc, with a stellar population that is well-described by an old, metal-poor isochrone (age of \\(\\tau \\sim 13.0\\) Gyr and metallicity of \\({\\rm [Fe/H] \\lesssim -2.1}\\) dex). These properties are consistent with the known population of ultra-faint Milky Way satellite galaxies. Eridanus IV is also prominently detected using proper motion measurements from Gaia Early Data Release 3, with a systemic proper motion of \\((\\mu_{\\alpha} \\cos \\delta, \\mu_{\\delta}) = (+0.25 \\pm 0.06, -0.10 \\pm 0.05)\\) mas yr\\(^{-1}\\) measured from its horizontal branch and red giant branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.

We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (CenI) and two Milky Way (MW) \\(\\delta\\) Scuti/SX Phoenicis stars based on multi-epoch \\(giz\\) DECam observations. The two RRc stars are located within 2 times the half-light radius (r\\(_h\\)) of Cen I, while the RRab star (CenI-V3) is at \\(\\sim6\\) r\\(_h\\). The presence of three distant RRL stars clustered this tightly in space represents a 4.7\\(\\sigma\\) excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of \\(\\mu_0 = 20.354 \\pm 0.002\\) mag (\\(\\sigma=0.03\\) mag), a heliocentric distance of D\\(_\\odot = 117.7 \\pm 0.1\\) kpc (\\(\\sigma=1.6\\) kpc), with systematic errors of \\(0.07\\) mag and \\(4\\) kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars (\\(f_{cd}\\)) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of \\(f_{cd} = 0.28\\). While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs (\\(f_{cd}\\) \\(\\sim\\) 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies.

The properties of Milky Way satellite galaxies have important implications for galaxy formation, reionization, and the fundamental physics of dark matter. However, the population of Milky Way satellites includes the faintest known galaxies, and current observations are incomplete. To understand the impact of observational selection effects on the known satellite population, we perform rigorous, quantitative estimates of the Milky Way satellite galaxy detection efficiency in three wide-field survey datasets: the Dark Energy Survey Year 6, the DECam Local Volume Exploration Data Release 3, and the Pan-STARRS1 Data Release 1. Together, these surveys cover \\(\\sim\\)13,600 deg\\(^2\\) to \\(g \\sim 24.0\\) and \\(\\sim\\)27,700 deg\\(^2\\) to \\(g \\sim 22.5\\), spanning \\(\\sim\\)91% of the high-Galactic-latitude sky (\\(|b| \\geq 15^\\circ\\)). We apply multiple detection algorithms over the combined footprint and recover 49 known satellites above a strict census detection threshold. To characterize the sensitivity of our census, we run our detection algorithms on a large set of simulated galaxies injected into the survey data, which allows us to develop models that predict the detectability of satellites as a function of their properties. We then fit an empirical model to our data and infer the luminosity function, radial distribution, and size-luminosity relation of Milky Way satellite galaxies. Our empirical model predicts a total of \\(265^{+79}_{-47}\\) satellite galaxies with \\(-20 \\leq M_V \\leq 0\\), half-light radii of \\(15 \\leq r_{1/2} (\\rm pc) \\leq 3000\\), and galactocentric distances of \\(10 \\leq D_{\\rm GC} (\\rm kpc) \\leq 300\\). We also identify a mild anisotropy in the angular distribution of the observed galaxies, at a significance of $\\sim$$2\\sigma$, which can be attributed to the clustering of satellites associated with the LMC.

The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ~15000 deg\\(^2\\) of high-Galactic-latitude (|b| > 10 deg) southern sky to a 5\\(\\sigma\\) depth of g,r,i,z ~ 23.5 mag. In addition, DELVE will cover a region of ~2200 deg\\(^2\\) around the Magellanic Clouds to a depth of g,r,i ~ 24.5 mag and an area of ~135 deg\\(^2\\) around four Magellanic analogs to a depth of g,i ~ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ~520 million astronomical sources covering ~5000 deg\\(^2\\) of the southern sky to a 5\\(\\sigma\\) point-source depth of g=24.3, r=23.9, i=23.3, and z=22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.

We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity (\\(M_V = -2.5^{+0.3}_{-0.5}\\); \\(L_V = 850^{+380}_{-260} \\ L_{\\odot}\\)), extended (\\(r_{1/2} = 41^{+9}_{-8}\\) pc) stellar system located in the outer halo (\\(D_{\\odot} = 85 \\pm 4\\) kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of \\(v_{\\rm sys} = -13.1^{+1.0}_{-0.9} \\ \\rm km \\ s^{-1}\\) for the system and place an upper limit of \\(\\sigma_v < 3.5 \\rm \\ km \\ s^{-1}\\) (\\(\\sigma_v < 1.6 \\rm \\ km \\ s^{-1}\\)) on its velocity dispersion at the 95% (68%) credible level. Based on Calcium-Triplet-based metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]\\( = -2.61 \\pm 0.21\\)) with a statistically-significant metallicity spread (\\(\\sigma_{\\rm [Fe/H]} = 0.46^{+0.26}_{-0.14}\\) dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with \\(Gaia\\) proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (\\(r_{\\rm peri} = 78 \\pm 7\\) kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly below \\(\\sigma_v \\lesssim 2 \\rm \\ km \\ s^{-1}\\), Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos.