Search

We present the discovery of DELVE 6, an ultra-faint stellar system identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on a maximum-likelihood fit to its structure and stellar population, we find that DELVE 6 is an old ($\tau > 9.8$ Gyr, at 95% confidence) and metal-poor ($\rm [Fe/H] < -1.17$ dex, at 95% confidence) stellar system with an absolute magnitude of $M_V = -1.5^{+0.4}_{-0.6}$ mag and an azimuthally-averaged half-light radius of $r_{1/2} =10^{+4}_{-3}$ pc. These properties are consistent with the population of ultra-faint star clusters uncovered by recent surveys. Interestingly, DELVE 6 is located at an angular separation of $\sim 10\deg$ from the center of the Small Magellanic Cloud (SMC), corresponding to a three-dimensional physical separation of $\sim 20$ kpc given the system's observed distance ($D_{\odot} = 80$ kpc). This also places the system $\sim 35$ kpc from the center of the Large Magellanic Cloud (LMC), lying within recent constraints on the size of the LMC's dark matter halo. We tentatively measure the proper motion of DELVE 6 using data from $\textit{Gaia}$, which we find supports a potential association between the system and the LMC/SMC. Although future kinematic measurements will be necessary to determine its origins, we highlight that DELVE 6 may represent only the second or third ancient ($\tau > 9$ Gyr) star cluster associated with the SMC, or one of fewer than two dozen ancient clusters associated with the LMC. Nonetheless, we cannot currently rule out the possibility that the system is a distant Milky Way halo star cluster., Comment: 13 pages, 4 figures; Submitted to AAS Journals

We perform a search for galaxy-galaxy strong lens systems using a convolutional neural network (CNN) applied to imaging data from the first public data release of the DECam Local Volume Exploration Survey (DELVE), which contains $\sim 520$ million astronomical sources covering $\sim 4,000$ $\mathrm{deg}^2$ 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. Following the methodology of similar searches using DECam data, we apply color and magnitude cuts to select a catalog of $\sim 11$ million extended astronomical sources. After scoring with our CNN, the highest scoring 50,000 images were visually inspected and assigned a score on a scale from 0 (definitely not a lens) to 3 (very probable lens). We present a list of 581 strong lens candidates, 562 of which are previously unreported. We categorize our candidates using their human-assigned scores, resulting in 55 Grade A candidates, 149 Grade B candidates, and 377 Grade C candidates. We additionally highlight eight potential quadruply lensed quasars from this sample. Due to the location of our search footprint in the northern Galactic cap ($b > 10$ deg) and southern celestial hemisphere (${\rm Dec.}<0$ deg), our candidate list has little overlap with other existing ground-based searches. Where our search footprint does overlap with other searches, we find a significant number of high-quality candidates which were previously unidentified, indicating a degree of orthogonality in our methodology. We report properties of our candidates including apparent magnitude and Einstein radius estimated from the image separation., Comment: 24 pages; published version (ApJ)

We report the discovery of six ultra-faint Milky Way satellites discovered through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes ($M_{V} \geq -3.2$ mag) and old, metal-poor stellar populations ($\tau > 10$ Gyr, [Fe/H] $< -1.4$ dex). Three of these systems are more extended ($r_{1/2} > 15$ pc), while the other three are compact ($r_{1/2} < 10$ pc). From these properties, we infer that the former three systems (Bo\"{o}tes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Bo\"{o}tes V, Leo Minor I, and DELVE 4, and tentatively detect a proper motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly-discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems., Comment: 30 pages, 12 Figures (including Appendix). Submitted to ApJ. We encourage the reader to also review Smith et al. 2022, "Discovery of a new Local Group Dwarf Galaxy Candidate in UNIONS: Bo\"{o}tes V" (arxiv: 2209.08242), who independently present the discovery of one of the candidates reported here. We are working to make code and data products available

We present the second public data release (DR2) from the DECam Local Volume Exploration survey (DELVE). DELVE DR2 combines new DECam observations with archival DECam data from the Dark Energy Survey, the DECam Legacy Survey, and other DECam community programs. DELVE DR2 consists of ~160,000 exposures that cover >21,000 deg^2 of the high Galactic latitude (|b| > 10 deg) sky in four broadband optical/near-infrared filters (g, r, i, z). DELVE DR2 provides point-source and automatic aperture photometry for ~2.5 billion astronomical sources with a median 5{\sigma} point-source depth of g=24.3, r=23.9, i=23.5, and z=22.8 mag. A region of ~17,000 deg^2 has been imaged in all four filters, providing four-band photometric measurements for ~618 million astronomical sources. DELVE DR2 covers more than four times the area of the previous DELVE data release and contains roughly five times as many astronomical objects. DELVE DR2 is publicly available via the NOIRLab Astro Data Lab science platform., Comment: 23 pages, 10 figures, 5 tables; to be submitted to AAS Journals; public data release at https://datalab.noirlab.edu/delve/. arXiv admin note: substantial text overlap with arXiv:2103.07476

We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a compact, ultra-faint stellar system ($r_{1/2} = 41^{+8}_{-6}$ pc; $M_V = -4.25 \pm 0.2$ mag) located at a heliocentric distance of $90^{+4}_{-6}$ kpc. Based on spectra of seven non-variable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV's velocity dispersion, measuring $\sigma_{v} = 3.3^{+1.7}_{-1.1} \text{ km s}^{-1}$ (after excluding three velocity outliers); this implies a mass-to-light ratio of $M_{1/2}/L_{V,1/2} = 167^{+224}_{-99} M_{\odot}/L_{\odot}$ for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of $\rm [Fe/H] = -2.67^{+0.25}_{-0.29}$ dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a non-zero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV's proper motion using data from Gaia Early Data Release 3, finding ($\mu_{\alpha*}, \mu_{\delta}) = (0.33\pm 0.07, -0.21 \pm 0.08) \text{ mas yr}^{-1}$. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than ten half-light radii away from the system's centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc., Comment: 22 pages, 6 figures + 2 page Appendix; submitting to AAS journals

We use the SMASH survey to obtain unprecedented deep photometry reaching down to the oldest main sequence turn-offs in the colour-magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) and quantitatively derive its star formation history (SFH) using CMD fitting techniques. We identify five distinctive peaks of star formation in the last 3.5 Gyr, at $\sim $3, $\sim$2, $\sim$1.1, $\sim $0.45 Gyr ago, and one presently. We compare these to the SFH of the Large Magellanic Cloud (LMC) finding unequivocal synchronicity, with both galaxies displaying similar periods of enhanced star formation over the past $\sim$3.5 Gyr. The parallelism between their SFHs indicates that tidal interactions between the MCs have recurrently played an important role in their evolution for at least the last $\sim$3.5 Gyr, tidally truncating the SMC and shaping the LMC's spiral arm. We show, for the first time, an SMC-LMC correlated SFH at recent times in which enhancements of star formation are localised in the northern spiral arm of the LMC, and globally across the SMC. These novel findings should be used to constrain not only the orbital history of the MCs but also how star formation should be treated in simulations., Comment: 6 pages, 4 figures. Accepted for publication to MNRAS Letters

The detection of sulphur species in the Martian atmosphere would be a strong indicator of volcanic outgassing from the surface of Mars. We wish to establish the presence of SO2, H2S, or OCS in the Martian atmosphere or determine upper limits on their concentration in the absence of a detection. We perform a comprehensive analysis of solar occultation data from the mid-infrared channel of the Atmospheric Chemistry Suite instrument, on board the ExoMars Trace Gas Orbiter, obtained during Martian years 34 and 35. For the most optimal sensitivity conditions, we determine 1-sigma upper limits of SO2 at 20 ppbv, H2S at 15 ppbv, and OCS at 0.4 ppbv; the last value is lower than any previous upper limits imposed on OCS in the literature. We find no evidence of any of these species above a 3-sigma confidence threshold. We therefore infer that passive volcanic outgassing of SO2 must be below 2 ktons/day., Comment: 13 pages, 12 figures, received for production in A&A on the 19th of November 2021

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., Comment: 9 pages, 4 figures (+1 page appendix); updated to match version published in ApJL

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., Comment: 30 pages, 14 figures, 6 tables; updated to match published version; public data release at https://datalab.noirlab.edu/delve

We present the second public data release of the Dark Energy Survey, DES DR2, based on optical/near-infrared imaging by the Dark Energy Camera mounted on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. DES DR2 consists of reduced single-epoch and coadded images, a source catalog derived from coadded images, and associated data products assembled from 6 years of DES science operations. This release includes data from the DES wide-area survey covering ~5000 deg2 of the southern Galactic cap in five broad photometric bands, grizY. DES DR2 has a median delivered point-spread function full-width at half maximum of g= 1.11, r= 0.95, i= 0.88, z= 0.83, and Y= 0.90 arcsec photometric uniformity with a standard deviation of < 3 mmag with respect to Gaia DR2 G-band, a photometric accuracy of ~10 mmag, and a median internal astrometric precision of ~27 mas. The median coadded catalog depth for a 1.95 arcsec diameter aperture at S/N= 10 is g= 24.7, r= 24.4, i= 23.8, z= 23.1 and Y= 21.7 mag. DES DR2 includes ~691 million distinct astronomical objects detected in 10,169 coadded image tiles of size 0.534 deg2 produced from 76,217 single-epoch images. After a basic quality selection, benchmark galaxy and stellar samples contain 543 million and 145 million objects, respectively. These data are accessible through several interfaces, including interactive image visualization tools, web-based query clients, image cutout servers and Jupyter notebooks. DES DR2 constitutes the largest photometric data set to date at the achieved depth and photometric precision., Comment: Accepted version. Copyright AAS. Reproduced with permission. 29 pages, 13 figures. Visit https://des.ncsa.illinois.edu/releases/dr2

We report the discovery of a new ultra-faint stellar system found near the Magellanic Clouds in the DECam Local Volume Exploration (DELVE) Survey. This new system, DELVE J0155$-$6815 (DELVE 2), is located at a heliocentric distance of $D_{\odot} = 71 \pm 4\text{ kpc}$, which places it at a 3D physical separation of 12 kpc from the center of Small Magellanic Cloud (SMC) and 28 kpc from the center of the Large Magellanic Cloud (LMC). DELVE 2 is identified as a resolved overdensity of old ($\tau > 13.3\text{ Gyr}$) and metal-poor (${\rm [Fe/H]} = -2.0_{-0.5}^{+0.2}$ dex) stars with a projected half-light radius of $r_{1/2} = 21^{+4}_{-3}\text{ pc}$ and an absolute magnitude of $M_V = -2.1^{+0.4}_{-0.5}\text{ mag}$. The size and luminosity of DELVE 2 are consistent with both the population of recently discovered ultra-faint globular clusters and the smallest ultra-faint dwarf galaxies. However, its age and metallicity would place it among the oldest and most metal-poor globular clusters in the Magellanic system. DELVE 2 is detected in Gaia DR2 with a clear proper motion signal, with multiple blue horizontal branch stars near the centroid of the system with proper motions consistent with the systemic mean. We measure the system proper motion to be $(\mu_{\alpha} \cos \delta, \mu_{\delta})= (1.02_{-0.25}^{+0.24}, -0.85_{-0.19}^{+0.18})$ mas yr$^{-1}$. We compare the spatial position and proper motion of DELVE 2 with simulations of the accreted satellite population of the LMC and find that it is very likely to be associated with the LMC., Comment: 17 pages, 5 figures, submitted to AAS journals

The Large Magellanic Cloud (LMC) is the closest and most studied example of an irregular galaxy. Among its principal defining morphological features, its off-centred bar and single spiral arm stand out, defining a whole family of galaxies known as the Magellanic spirals (Sm). These structures are thought to be triggered by tidal interactions and possibly maintained via gas accretion. However, it is still unknown whether they are long-lived stable structures. In this work, by combining photometry that reaches down to the oldest main sequence turn-off in the colour-magnitude diagrams (CMD, up to a distance of $\sim$4.4 kpc from the LMC centre) from the SMASH survey and CMD fitting techniques, we find compelling evidence supporting the long-term stability of the LMC spiral arm, dating the origin of this structure to more than 2~Gyr ago. The evidence suggests that the close encounter between the LMC and the Small Magellanic Cloud (SMC) that produced the gaseous Magellanic Stream and its Leading Arm (LA) also triggered the formation of the LMC's spiral arm. Given the mass difference between the Clouds and the notable consequences of this interaction, we can speculate that this should have been one of their closest encounters. These results set important constraints on the timing of LMC-SMC collisions, as well as on the physics behind star formation induced by tidal encounters., Comment: 6 pages, 3 figures, accepted for publication in A&A letters

We report the discovery of two ultra-faint stellar systems found in early data from the DECam Local Volume Exploration survey (DELVE). The first system, Centaurus I (DELVE J1238-4054), is identified as a resolved overdensity of old and metal-poor stars with a heliocentric distance of ${\rm D}_{\odot} = 116.3_{-0.6}^{+0.6}$ kpc, a half-light radius of $r_h = 2.3_{-0.3}^{+0.4}$ arcmin, an age of $\tau > 12.85$ Gyr, a metallicity of $Z = 0.0002_{-0.0002}^{+0.0001}$, and an absolute magnitude of $M_V = -5.55_{-0.11}^{+0.11}$ mag. This characterization is consistent with the population of ultra-faint satellites, and confirmation of this system would make Centaurus I one of the brightest recently discovered ultra-faint dwarf galaxies. Centaurus I is detected in Gaia DR2 with a clear and distinct proper motion signal, confirming that it is a real association of stars distinct from the Milky Way foreground; this is further supported by the clustering of blue horizontal branch stars near the centroid of the system. The second system, DELVE 1 (DELVE J1630-0058), is identified as a resolved overdensity of stars with a heliocentric distance of ${\rm D}_{\odot} = 19.0_{-0.6}^{+0.5} kpc$, a half-light radius of $r_h = 0.97_{-0.17}^{+0.24}$ arcmin, an age of $\tau = 12.5_{-0.7}^{+1.0}$ Gyr, a metallicity of $Z = 0.0005_{-0.0001}^{+0.0002}$, and an absolute magnitude of $M_V = -0.2_{-0.6}^{+0.8}$ mag, consistent with the known population of faint halo star clusters. Given the low number of probable member stars at magnitudes accessible with Gaia DR2, a proper motion signal for DELVE 1 is only marginally detected. We compare the spatial position and proper motion of both Centaurus I and DELVE 1 with simulations of the accreted satellite population of the Large Magellanic Cloud (LMC) and find that neither is likely to be associated with the LMC., Comment: 17 pages, 6 figures, 1 table; updated to match published version; updated to address erratum

We present a deep Blanco/DECam colour-magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = -2.0 and [$\alpha$/Fe] = +0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: $\mu_{\alpha}\cos \delta$=-0.14 $\pm$ 0.07 , $\mu_{\delta}$=-0.10 $\pm$ 0.04 mas yr$^{-1}$, approximately perpendicular to the semi-major axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime., Comment: 13 pages, 9 figures, 3 tables. Accepted for publication in MNRAS

We propose an extension of the LSST survey to cover the northern sky to DEC < +30 (accessible at airmass <1.8). This survey will increase the LSST sky coverage by ~9,600 square degrees from 18,900 to 28,500 square degrees (a 50% increase) but use only 0.6-2.5% of the time depending on the synergies with other surveys. This increased area addresses a wide range of science cases that enhance all of the primary LSST science goals by significant amounts. The science enabled includes: increasing the area of the sky accessible for follow-up of multi-messenger transients including gravitational waves, mapping the milky way halo and halo dwarfs including discovery of RR Lyrae stars in the outer galactic halo, discovery of z>7 quasars in combination Euclid, enabling a second generation DESI and other spectroscopic surveys, and enhancing all areas of science by improving synergies with Euclid, WFIRST, and unique northern survey facilities. This white paper is the result of the Tri-Agency Working Group (TAG) appointed to develop synergies between missions and presents a unified plan for northern coverage. The range of time estimates reflects synergies with other surveys. If the modified DESC WFD survey, the ecliptic plane mini survey, and the north galactic spur mini survey are executed this plan would only need 0.6% of the LSST time, however if none of these are included the overall request is 2.5% of the 10 year survey life. In other words, the majority of these observations are already suggested as part of these other surveys and the intent of this white paper is to propose a unified baseline plan to carry out a broad range of objectives to facilitate a combination of multiple science objectives. A companion white paper gives Euclid specific science goals, and we support the white papers for southern extensions of the LSST survey., Comment: LSST Survey White Paper Submitted in Dec 2018

We analyze resolved stellar near-infrared photometry of 21 HST fields in M31 to constrain the impact of metallicity on the formation of carbon stars. Observations of nearby galaxies show that the carbon stars are increasingly rare at higher metallicity. Models indicate that carbon star formation efficiency drops due to the decrease in dredge-up efficiency in metal-rich thermally-pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher initial abundance of oxygen. However, while models predict a metallicity ceiling above which carbon stars cannot form, previous observations have not yet pinpointed this limit. Our new observations reliably separate carbon stars from M-type TP-AGB stars across 2.6-13.7 kpc of M31's metal-rich disk using HST WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M) decreases more rapidly than extrapolations of observations in more metal-poor galaxies, resulting in a C/M that is too low by more than a factor of 10 in the innermost fields and indicating a dramatic decline in C star formation efficiency at metallicities higher than [M/H] $\approx$ -0.1 dex. The metallicity ceiling remains undetected, but must occur at metallicities higher than what is measured in M31's inner disk ([M/H] $\gtrsim$ +0.06 dex)., Comment: 16 pages, 13 Figures; text clarifications in response to the referee. Results are unchanged; accepted for publication in ApJ

We report the discovery of two ultra-faint satellites in the vicinity of the Large Magellanic Cloud (LMC) in data from the Magellanic Satellites Survey (MagLiteS). Situated 18$^{\circ}$ ($\sim 20$ kpc) from the LMC and separated from each other by only $18^\prime$, Carina~II and III form an intriguing pair. By simultaneously modeling the spatial and the color-magnitude stellar distributions, we find that both Carina~II and Carina~III are likely dwarf galaxies, although this is less clear for Carina~III. There are in fact several obvious differences between the two satellites. While both are well described by an old and metal poor population, Carina~II is located at $\sim 36$ kpc from the Sun, with $M_V\sim-4.5$ and $r_h\sim 90$ pc, and it is further confirmed by the discovery of 3 RR Lyrae at the right distance. In contrast, Carina~III is much more elongated, measured to be fainter ($M_V\sim-2.4$), significantly more compact ($r_h\sim30$ pc), and closer to the Sun, at $\sim 28$ kpc, placing it only 8 kpc away from Car~II. Together with several other systems detected by the Dark Energy Camera, Carina~II and III form a strongly anisotropic cloud of satellites in the vicinity of the Magellanic Clouds., Comment: 14 pages, 7 figures. Accepted for publication in MNRAS

We describe the first public data release of the Dark Energy Survey, DES DR1, consisting of reduced single epoch images, coadded images, coadded source catalogs, and associated products and services assembled over the first three years of DES science operations. DES DR1 is based on optical/near-infrared imaging from 345 distinct nights (August 2013 to February 2016) by the Dark Energy Camera mounted on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. We release data from the DES wide-area survey covering ~5,000 sq. deg. of the southern Galactic cap in five broad photometric bands, grizY. DES DR1 has a median delivered point-spread function of g = 1.12, r = 0.96, i = 0.88, z = 0.84, and Y = 0.90 arcsec FWHM, a photometric precision of < 1% in all bands, and an astrometric precision of 151 mas. The median coadded catalog depth for a 1.95" diameter aperture at S/N = 10 is g = 24.33, r = 24.08, i = 23.44, z = 22.69, and Y = 21.44 mag. DES DR1 includes nearly 400M distinct astronomical objects detected in ~10,000 coadd tiles of size 0.534 sq. deg. produced from ~39,000 individual exposures. Benchmark galaxy and stellar samples contain ~310M and ~ 80M objects, respectively, following a basic object quality selection. These data are accessible through a range of interfaces, including query web clients, image cutout servers, jupyter notebooks, and an interactive coadd image visualization tool. DES DR1 constitutes the largest photometric data set to date at the achieved depth and photometric precision., Comment: 30 pages, 20 Figures. Release page found at this url https://des.ncsa.illinois.edu/releases/dr1

The Dark matter Experiment using Argon Pulse-shape discrimination (DEAP) has been designed for a direct detection search for particle dark matter using a single-phase liquid argon target. The projected cross section sensitivity for DEAP-3600 to the spin-independent scattering of Weakly Interacting Massive Particles (WIMPs) on nucleons is $10^{-46}~\rm{cm}^{2}$ for a 100 GeV/$c^2$ WIMP mass with a fiducial exposure of 3 tonne-years. This paper describes the physical properties and construction of the DEAP-3600 detector., Comment: minor corrections in version 2. 70 pages, 24 figures, to be submitted to Astroparticle Physics Journal

Of the almost 40 star forming galaxies at z>~5 (not counting QSOs) observed in [CII] to date, nearly half are either very faint in [CII], or not detected at all, and fall well below expectations based on locally derived relations between star formation rate (SFR) and [CII] luminosity. Combining cosmological zoom simulations of galaxies with SIGAME (SImulator of GAlaxy Millimeter/submillimeter Emission) we have modeled the multi-phased interstellar medium (ISM) and its emission in [CII], [OI] and [OIII], from 30 main sequence galaxies at z~6 with star formation rates ~3-23Msun/yr, stellar masses ~(0.7-8)x10^9Msun, and metallicities ~(0.1-0.4)xZsun. The simulations are able to reproduce the aforementioned [CII]-faintness at z>5, match two of the three existing z>~5 detections of [OIII], and are furthermore roughly consistent with the [OI] and [OIII] luminosity relations with SFR observed for local starburst galaxies. We find that the [CII] emission is dominated by the diffuse ionized gas phase and molecular clouds, which on average contribute ~66% and ~27%, respectively. The molecular gas, which constitutes only ~10% of the total gas mass is thus a more efficient emitter of [CII] than the ionized gas making up ~85% of the total gas mass. A principal component analysis shows that the [CII] luminosity correlates with the star formation activity as well as average metallicity. The low metallicities of our simulations together with their low molecular gas mass fractions can account for their [CII]-faintness, and we suggest these factors may also be responsible for the [CII]-faint normal galaxies observed at these early epochs., Comment: 24 pages, 14 figures. Accepted for publication in the Astrophysical Journal

This paper reports the first results of a direct dark matter search with the DEAP-3600 single-phase liquid argon (LAr) detector. The experiment was performed 2 km underground at SNOLAB (Sudbury, Canada) utilizing a large target mass, with the LAr target contained in a spherical acrylic vessel of 3600 kg capacity. The LAr is viewed by an array of PMTs, which would register scintillation light produced by rare nuclear recoil signals induced by dark matter particle scattering. An analysis of 4.44 live days (fiducial exposure of 9.87 tonne-days) of data taken with the nearly full detector during the initial filling phase demonstrates the detector performance and the best electronic recoil rejection using pulse-shape discrimination in argon, with leakage $<1.2\times 10^{-7}$ (90% C.L.) between 16 and 33 keV$_{ee}$. No candidate signal events are observed, which results in the leading limit on WIMP-nucleon spin-independent cross section on argon, $<1.2\times 10^{-44}$ cm$^2$ for a 100 GeV/c$^2$ WIMP mass (90% C.L.)., Comment: Accepted for publication in Physical Review Letters

The Hamamatsu R5912-HQE photomultiplier-tube (PMT) is a novel high-quantum efficiency PMT. It is currently used in the DEAP-3600 dark matter detector and is of significant interest for future dark matter and neutrino experiments where high signal yields are needed. We report on the methods developed for in-situ characterization and monitoring of DEAP's 255 R5912-HQE PMTs. This includes a detailed discussion of typical measured single-photoelectron charge distributions, correlated noise (afterpulsing), dark noise, double, and late pulsing characteristics. The characterization is performed during the detector commissioning phase using laser light injected through a light diffusing sphere and during normal detector operation using LED light injected through optical fibres.

We report the discovery of a stellar over-density 8$^{\circ}$ north of the center of the Small Magellanic Cloud (Small Magellanic Cloud Northern Over-Density; SMCNOD) using data from the first two years of the Dark Energy Survey (DES) and the first year of the MAGellanic SatelLITEs Survey (MagLiteS). The SMCNOD is indistinguishable in age, metallicity and distance from the nearby SMC stars, being primarly composed of intermediate-age stars (6 Gyr, Z=0.001), with a small fraction of young stars (1 Gyr, Z=0.01). The SMCNOD has an elongated shape with an ellipticity of 0.6 and a size of $\sim$ 6x2 deg. It has an absolute magnitude of $M_V \cong$ -7.7, $r_h = 2.1$ kpc, and $\mu_V(r

We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness ({\mu} = 28.5 mag arcsec$^{-2}$ within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45 kpc. The physical size (r$_{1/2}$ = 46 pc) and low luminosity (Mv = -3.2 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC., Comment: 7 pages, 4 figures, 1 table; updated to match published version

Many current and future dark matter and neutrino detectors are designed to measure scintillation light with a large array of photomultiplier tubes (PMTs). The energy resolution and particle identification capabilities of these detectors depend in part on the ability to accurately identify individual photoelectrons in PMT waveforms despite large variability in pulse amplitudes and pulse pileup. We describe a Bayesian technique that can identify the times of individual photoelectrons in a sampled PMT waveform without deconvolution, even when pileup is present. To demonstrate the technique, we apply it to the general problem of particle identification in single-phase liquid argon dark matter detectors. Using the output of the Bayesian photoelectron counting algorithm described in this paper, we construct several test statistics for rejection of backgrounds for dark matter searches in argon. Compared to simpler methods based on either observed charge or peak finding, the photoelectron counting technique improves both energy resolution and particle identification of low energy events in calibration data from the DEAP-1 detector and simulation of the larger MiniCLEAN dark matter detector., Comment: 16 pages, 16 figures

We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of Asymptotic Giant Branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C stars, contrary to the findings of previous C star searches in other regions of M31. We find only 1 candidate C star (plus up to 6 additional, less certain C stars candidates), resulting in an extremely low ratio of C to M stars (C/M = (3.3(+20,-0.1))x10^-4) that is 1-2 orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies., Comment: 8 pages, 8 figures, accepted for publication in ApJ

The DEAP-1 \SI{7}{kg} single phase liquid argon scintillation detector was operated underground at SNOLAB in order to test the techniques and measure the backgrounds inherent to single phase detection, in support of the \mbox{DEAP-3600} Dark Matter detector. Backgrounds in DEAP are controlled through material selection, construction techniques, pulse shape discrimination and event reconstruction. This report details the analysis of background events observed in three iterations of the DEAP-1 detector, and the measures taken to reduce them. The $^{222}$Rn decay rate in the liquid argon was measured to be between 16 and \SI{26}{\micro\becquerel\per\kilogram}. We found that the background spectrum near the region of interest for Dark Matter detection in the DEAP-1 detector can be described considering events from three sources: radon daughters decaying on the surface of the active volume, the expected rate of electromagnetic events misidentified as nuclear recoils due to inefficiencies in the pulse shape discrimination, and leakage of events from outside the fiducial volume due to imperfect position reconstruction. These backgrounds statistically account for all observed events, and they will be strongly reduced in the DEAP-3600 detector due to its higher light yield and simpler geometry.

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31's disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged), Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical Journal Supplements. Some figures slightly degraded to reduce submission size

We have combined deep photometry in the B,V and I bands from CTIO/MOSAIC of the Sculptor dwarf spheroidal galaxy, going down to the oldest Main Sequence Turn-Offs, with spectroscopic metallicity distributions of Red Giant Branch stars. This allows us to obtain the most detailed and complete Star Formation History to date, as well as an accurate timescale for chemical enrichment. The Star Formation History shows that Sculptor is dominated by old ($>$10 Gyr), metal-poor stars, but that younger, more metal-rich populations are also present. Using Star Formation Histories determined at different radii from the centre we show that Sculptor formed stars with an increasing central concentration with time. The old, metal-poor populations are present at all radii, while more metal-rich, younger stars are more centrally concentrated. We find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre, a total mass in stars of 7.8$\times10^{6}$ M$_{\odot}$ was formed, between 14 and 7 Gyr ago, with a peak at 13$-$14 Gyr ago. We use the detailed Star Formation History to determine age estimates for individual Red Giant Branch stars with high resolution spectroscopic abundances. Thus, for the first time, we can directly determine detailed timescales for the evolution of individual chemical elements. We find that the trends in alpha-elements match what is expected from an extended, relatively uninterrupted period of star formation continuing for 6$-$7 Gyr. The knee in the alpha-element distribution occurs at an age of 10.9$\pm$1Gyr, suggesting that SNe Ia enrichment began $\approx2\pm$1Gyr after the start of star formation in Sculptor., Comment: 23 pages, 23 figures

BigBOSS is a Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with a wide-area galaxy and quasar redshift survey over 14,000 square degrees. It has been conditionally accepted by NOAO in response to a call for major new instrumentation and a high-impact science program for the 4-m Mayall telescope at Kitt Peak. The BigBOSS instrument is a robotically-actuated, fiber-fed spectrograph capable of taking 5000 simultaneous spectra over a wavelength range from 340 nm to 1060 nm, with a resolution R = 3000-4800. Using data from imaging surveys that are already underway, spectroscopic targets are selected that trace the underlying dark matter distribution. In particular, targets include luminous red galaxies (LRGs) up to z = 1.0, extending the BOSS LRG survey in both redshift and survey area. To probe the universe out to even higher redshift, BigBOSS will target bright [OII] emission line galaxies (ELGs) up to z = 1.7. In total, 20 million galaxy redshifts are obtained to measure the BAO feature, trace the matter power spectrum at smaller scales, and detect redshift space distortions. BigBOSS will provide additional constraints on early dark energy and on the curvature of the universe by measuring the Ly-alpha forest in the spectra of over 600,000 2.2 < z < 3.5 quasars. BigBOSS galaxy BAO measurements combined with an analysis of the broadband power, including the Ly-alpha forest in BigBOSS quasar spectra, achieves a FOM of 395 with Planck plus Stage III priors. This FOM is based on conservative assumptions for the analysis of broad band power (kmax = 0.15), and could grow to over 600 if current work allows us to push the analysis to higher wave numbers (kmax = 0.3). BigBOSS will also place constraints on theories of modified gravity and inflation, and will measure the sum of neutrino masses to 0.024 eV accuracy., Comment: This report is based on the BigBOSS proposal submission to NOAO in October 2010, and reflects the project status at that time with minor updates

We present wide-field photometry of resolved stars in the nearby Sculptor dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude Diagrams can be used to constrain the ages of different stellar populations, and also their spatial distribution. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old) which can be easily resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. By using spectroscopic metallicities of RGB stars in combination with our deep Main Sequence Turn-Off photometry we can constrain the ages of different stellar populations with particular accuracy. We find that the known metallicity gradient in Sculptor is well matched to an age gradient. This is the first time that this link with age has been directly quantified. This gradient has been previously observed as a variation in Horizontal Branch properties and is now confirmed to exist for Main Sequence Turn-Offs as well. It is likely the Sculptor dSph first formed an extended metal-poor population at the oldest times, and subsequent more metal-rich, younger stars were formed more towards the centre until the gas was depleted or lost roughly 7 Gyr ago. The fact that these clear radial gradients have been preserved up to the present day is consistent with the apparent lack of signs of recent tidal interactions., Comment: 28 pages, 22 figures

We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy program on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified., Comment: (43 pages, 21 figures, 4 tables including one large table out of order; to be published in MNRAS)

We present 2323 High-Amplitude \delta-Scuti (HADS) candidates discovered in the Large Magellanic Cloud (LMC) by the SuperMACHO survey (Rest et al. 2005). Frequency analyses of these candidates reveal that several are multimode pulsators, including 119 whose largest amplitude of pulsation is in the fundamental (F) mode and 19 whose largest amplitude of pulsation is in the first overtone (FO) mode. Using Fourier decomposition of the HADS light curves, we find that the period-luminosity (PL) relation defined by the FO pulsators does not show a clear separation from the PL-relation defined by the F pulsators. This differs from other instability strip pulsators such as type c RR Lyrae. We also present evidence for a larger amplitude, subluminous population of HADS similar to that observed in Fornax (Poretti et al. 2008)., Comment: 26 pages, 15 figures, AJ accepted

We report the first detection of asymmetry in a supernova (SN) photosphere based on SN light echo (LE) spectra of Cas A from the different perspectives of dust concentrations on its LE ellipsoid. New LEs are reported based on difference images, and optical spectra of these LEs are analyzed and compared. After properly accounting for the effects of finite dust-filament extent and inclination, we find one field where the He I and H alpha features are blueshifted by an additional ~4000 km/s relative to other spectra and to the spectra of the Type IIb SN 1993J. That same direction does not show any shift relative to other Cas A LE spectra in the Ca II near-infrared triplet feature. We compare the perspectives of the Cas A LE dust concentrations with recent three-dimensional modeling of the SN remnant (SNR) and note that the location having the blueshifted He I and H alpha features is roughly in the direction of an Fe-rich outflow and in the opposite direction of the motion of the compact object at the center of the SNR. We conclude that Cas A was an intrinsically asymmetric SN. Future LE spectroscopy of this object, and of other historical SNe, will provide additional insight into the connection of explosion mechanism to SN to SNR, as well as give crucial observational evidence regarding how stars explode., Comment: 13 pages, 7 figures, accepted for publication in ApJ