We describe the first public data release of the Dark Energy Survey, DES DR1, consisting of reduced single-epoch images, co-added images, co-added source catalogs, and associated products and services assembled over the first 3 yr of DES science operations. DES DR1 is based on optical/near-infrared imaging from 345 distinct nights (2013 August to 2016 February) by the Dark Energy Camera mounted on the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile. We release data from the DES wide-area survey covering ∼5000 deg 2 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 FWHM, a photometric precision of <1% in all bands, and an astrometric precision of 151 mas. The median co-added catalog depth for a 1 95 diameter aperture at signal-to-noise ratio (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 400 million distinct astronomical objects detected in ∼10,000 co-add tiles of size 0.534 deg 2 produced from ∼39,000 individual exposures. Benchmark galaxy and stellar samples contain ∼310 million and ∼80 million 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 co-add image visualization tool. DES DR1 constitutes the largest photometric data set to date at the achieved depth and photometric precision.

The rotation curves of spiral galaxies exhibit a diversity that has been difficult to understand in the cold dark matter (CDM) paradigm. We show that the self-interacting dark matter (SIDM) model provides excellent fits to the rotation curves of a sample of galaxies with asymptotic velocities in the 25 to 300 km/s range that exemplify the full range of diversity. We only assume the halo concentration-mass relation predicted by the CDM model and a fixed value of the self-interaction cross section. In dark matter dominated galaxies, thermalization due to self-interactions creates large cores and reduces dark matter densities. In contrast, thermalization leads to denser and smaller cores in more luminous galaxies, and naturally explains the flat rotation curves of the highly luminous galaxies. Our results demonstrate that the impact of the baryons on the SIDM halo profile and the scatter from the assembly history of halos as encoded in the concentration-mass relation can explain the diverse rotation curves of spiral galaxies.I. Introduction. The ΛCDM model, with a cosmological constant (Λ) and cold dark matter (CDM), explains the observed large-scale structure of the Universe [1] and many aspects of galaxy formation [2,3], but the diverse observed rotation curves do not have a satisfactory explanation. Observations of a number of dwarf and low surface brightness galaxies indicate that the inner halo is often badly fit by the cusped halos predicted by ΛCDM simulations [4][5][6][7][8][9][10][11][12][13]. The core densities exhibit almost an order of magnitude spread for similar total halo masses [14], and galaxies with densities at the upper end of the range are consistent with ΛCDM [15]. There is no clear explanation for the diversity in the inner rotation velocity profiles of different galaxies within similar mass halos [15].In this Letter, we demonstrate how this diversity problem [15] can be solved in the self-interacting dark matter (SIDM) framework [16], where dark matter particles exchange energy by colliding with one another in halos. Dark matter self-interactions only change the inner halo properties in accord with observations, leaving all the successes of CDM intact on large scales, e.g., [17][18][19][20]. While the original SIDM model [16] posited a cross section that is independent of velocity, constraints from galaxy clusters demand an interaction cross section that diminishes with increasing velocity [19,[21][22][23][24]. SIDM models based on a Yukawa potential [25][26][27][28][29][30][31][32] or atomic scattering [33] can naturally accommodate the required velocity dependence. Concrete particle physics realizations of such models can provide a universal solution to small-scale issues from dwarf galaxies to galaxy clusters [23,34].The diversity in the observed rotation curves is solved by a combination of interconnected features in the ΛSIDM model. In the outer parts of galaxies, the ΛSIDM model is the same as the ΛCDM model, inheriting all its successes. In the inner regions, the SIDM density profile ...

We perform a search for stellar streams around the Milky Way using the first 3 yr of multiband optical imaging data from the Dark Energy Survey (DES). We use DES data covering ∼5000 deg 2 to a depth of g>23.5 with a relative photometric calibration uncertainty of <1%. This data set yields unprecedented sensitivity to the stellar density field in the southern celestial hemisphere, enabling the detection of faint stellar streams to a heliocentric distance of ∼50 kpc. We search for stellar streams using a matched filter in color-magnitude space derived from a synthetic isochrone of an old, metal-poor stellar population. Our detection technique recovers four previously known thin stellar streams: Phoenix, ATLAS, Tucana III, and a possible extension of Molonglo. In addition, we report the discovery of 11 new stellar streams. In general, the new streams detected by DES are fainter, more distant, and lower surface brightness than streams detected by similar techniques in previous photometric surveys. As a by-product of our stellar stream search, we find evidence for extratidal stellar structure associated with four globular clusters: NGC 288, NGC 1261, NGC 1851, and NGC 1904. The ever-growing sample of stellar streams will provide insight into the formation of the Galactic stellar halo, the Milky Way gravitational potential, and the large-and small-scale distribution of dark matter around the Milky Way.

The population of Milky Way (MW) satellites contains the faintest known galaxies, and thus provides essential insight into galaxy formation and dark matter microphysics. Here, we combine a model of the galaxy-halo connection with newly derived observational selection functions based on searches for satellites in photometric surveys over nearly the entire high-Galactic-latitude sky. In particular, we use cosmological zoom-in simulations of MW-like halos that include realistic Large Magellanic Cloud (LMC) analogs to fit the position-dependent MW satellite luminosity function. We report decisive evidence for the statistical impact of the LMC on the MW satellite population due to an estimated 6.5 ± 1.5 observed LMC-associated satellites, consistent with the number of LMC satellites inferred from Gaia proper motion measurements, confirming the predictions of cold dark matter models for the existence of satellites within satellite halos. Moreover, we infer that the LMC fell into the MW within the last 2 Gyr at high confidence. Based on our detailed full-sky modeling, we find that the faintest observed satellites inhabit halos with peak virial masses below 2.2 × 10 8 M at 95% confidence, and we place the first robust constraints on the fraction of halos that host galaxies in this regime. We predict that the faintest detectable satellites occupy halos with peak virial masses above 10 6 M , highlighting the potential for powerful galaxy formation and dark matter constraints from future dwarf galaxy searches.

We study a suite of extremely high-resolution cosmological FIRE simulations of dwarf galaxies (M halo 10 10 M ), run to z = 0 with 30 M resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with M halo 10 8.6 M is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; M * < 10 5 M ) have their star formation truncated early (z 2), likely by reionization, while classical dwarfs (M * > 10 5 M ) continue forming stars to z < 0.5. The systems have bursty star formation (SF) histories, forming most of their stars in periods of elevated SF strongly clustered in both space and time. This allows our dwarf with M * /M halo > 10 −4 to form a dark matter core > 200 pc, while lower-mass UFDs exhibit cusps down to 100 pc, as expected from energetic arguments. Our dwarfs with M * > 10 4 M have half-mass radii (R 1/2 ) in agreement with Local Group (LG) dwarfs; dynamical mass vs. R 1/2 and the degree of rotational support also resemble observations. The lowest-mass UFDs are below surface brightness limits of current surveys but are potentially visible in next-generation surveys (e.g. LSST). The stellar metallicities are lower than in LG dwarfs; this may reflect pre-enrichment of the LG by the massive hosts or Pop-III stars. Consistency with lower resolution studies implies that our simulations are numerically robust (for a given physical model).

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We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite EridanusII (Eri II).We identify 28 member stars in EriII, from which we measure a systemic radial velocity of () 1 the bright blue stars previously suggested to be a young stellar population are not associated with EriII. The lack of gas and recent star formation in EriII is surprising given its mass and distance from the Milky Way, and may place constraints on models of quenching in dwarf galaxies and on the distribution of hot gas in the Milky Way halo. Furthermore, the large velocity dispersion of Eri II can be combined with the existence of a central star cluster to constrain massive compact halo object dark matter with mass 10  M .