Reticulum II: Gamma-rays, dark matter content, and implications for dark matter physics Savvas M. Koushiappas Based on PRL 115, 081101 (2015) 1503.02320 ApJL 808 L36 (2015) 1504.03309 With: Alex Geringer-Sameth & Matthew Walker (Carnegie-Mellon U.), Sergey Koposov, Vasily Belokurov, Gabriel Torrealba & Wyn Evans (Cambridge U.) Vincent Bonnivard, Celine Combet, David Maurin (U. Grenoble-Alpes), Mario Mateo, John Bailey (U. Michigan), Eduard Olszewski (U. Arizona) On March 8, 2015 arXiv:1503.02079 [pdf, ps, other] Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds Sergey E. Koposov, Vasily Belokurov, Gabriel Torrealba, N. Wyn Evans arXiv:1503.02584 [pdf, other] Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data The DES Collaboration, K. Bechtol, A. Drlica-Wagner, E. Balbinot, A. Pieres, J. D. Simon, B. Yanny, B. Santiago, R. H. Wechsler, J. Frieman, A. R. Walker, P. Williams, E. Rozo, E. S. Rykof, A. Queiroz, E. Luque, A. Benoit-Levy, R. A. Bernstein, D. Tucker, I. Sevilla, R. A. Gruendl4, L. N. da Costa, A. Fausti Neto, M. A. G. Maia, T. Abbott, S. Allam, R. Armstrong, A. H. Bauer, G. M. Bernstein, E. Bertin, D. Brooks, E. Buckley-Geer, D. L. Burke, A. Carnero Rosell, F. J. Castander, C. B. D'Andrea, D. L. DePoy, S. Desai, H. T. Diehl, T. F. Eifler, J. Estrada, A. E. Evrard, E. Fernandez, D. A. Finley, B. Flaugher, E. Gaztanaga, D. Gerdes, L. Girardi, M. Gladders, D. Gruen, G. Gutierrez, J. Hao, K. Honscheid, B. Jain, D. James, S. Kent, R. Kron, K. Kuehn, N. Kuropatkin, O. Lahav, T. S. Li, et al. (32 additional authors not shown) Table 1 Parameters of the discovered MW satellites. Name αδSignif m−MDist⊙ MV rmaj r1/2 r1/2 ePABF(ell) [deg] [deg] [mag] [kpc] [mag] [arcmin] [arcmin] [pc] [deg] +0.03 Reticulum 2 53.9256 −54.0492 48.5 17.4 30 -2.7±0.1 3.4±0.2 3.7±0.2 32±10.58−0.03 71±1 >1000 +0.07 Eridanus 2 56.0878 −43.5332 31.5 22.9 380 -6.6±0.1 1.2±0.1 1.6±0.1 172±12 0.39−0.07 80±61113 +0.12 Horologium 1 43.8820 −54.1188 28.4 19.5 79 -3.4±0.1 0.9±0.1 1.3±0.2 30±30.16−0.12 55±50 0.35 +0.19 Pictoris 1 70.9475 −50.2830 17.3 20.3 114 -3.1±0.3 0.7±0.2 0.9±0.2 31±70.39−0.22 79±23 1.41 +0.18 Phoenix 2 354.9975 −54.4060 13.9 19.6 83 -2.8±0.2 0.9±0.2 1.1±0.2 27±50.38−0.19 150±54 1.81 +0.16 Indus 1 317.2044 −51.1656 13.7 20.0 100 -3.5±0.2 0.9±0.3 1.4±0.4 39±11 0.22−0.16 82±50 0.46 a +0.24 Grus 1 344.1765 −50.1633 10.1 20.4 120 -3.4±0.3 1.6±0.6 2.0±0.7 70±23 0.37−0.25 48±60 1.01 +0.23 Eridanus 3 35.6897 −52.2837 10.1 19.7 87 -2.0±0.3 0.6±0.3 0.7±0.3 18±80.34−0.23 89±36 0.81 +0.16 Tucana 2 342.9664 −58.5683 8.3 19.2 69 -4.4±0.1 7.3±1.2 9.9±1.4 199±28 0.31−0.17 106±22 1.29 aAs this object is located very close to the CCD chip gap,’+ its morphological properties should be treated with caution PSF magnitudes of star-like objects are given by the SDSS gri. Consequently, the extinction coefficients MAG_PSF output of SExtractor. As an indicator of star- used are those suitable for the SDSS photometric sys- galaxy separation we use the SPREAD_MODEL parame- tem, while the dust reddening maps employed are from ter provided by SExtractor. This is a metric simi- Schlegel et al. (1998). Note that the depth of the result- lar to psfmag-modelmag used by SDSS (see Fig. 1). ing catalogues varies somewhat across the DES footprint, A sensible selection threshold for bright stars would but could be approximately estimated from the source be SPREAD_MODEL < 0.003 (Annunziatella et al. 2013), number counts in g, r, i filters. These number counts however| for faint magnitudes| this cut causes significant peak at magnitudes 23.7, 23.6, 22.9 in g, r, i correspond- incompleteness in stars. Therefore, instead we choose to ingly, indicating that the catalogues start to be signifi- require:4 cantly affected by incompleteness at somewhat brigher magnitudes g 23.5, r 23.4, i 22.7. SPREAD_MODEL < 0.003 + SPREADERR_MODEL (1) ∼ ∼ ∼ | | To illustrate the quality of the resulting catalogue, Fig- This particular cut ensures that the stellar complete- ure 3 displays the density of the Main Sequence Turn- ness remains reasonably high at faint magnitudes, while Off (MSTO; 0.2<g r<0.6) stars on the sky. The den- − the contamination is kept low at the same time. The sity of stars with 19<r<21 (corresponding to distances of behaviour of 0.003 + SPREADERR_MODEL as a function 10 25 kpc) is shown in the green channel, more distant − of magnitude shown in Figure 1 explains why a fixed stars with 21<r<22.75 (corresponding to distances of SPREADERR_MODEL threshold is suboptimal. To assess the 25 56kpc) are used for the red channel, and the nearby − levels of completeness and contamination induced by our stars with 17<r<19 (distances of 4 10 kpc) in the blue − stellar selection, we use a portion of the DES-covered channel. This map is an analog of the ”Field of Streams” area of sky overlapping with the CFHTLS Wide survey picture by Belokurov et al. (2006). The density distribu- (Hudelot et al. 2012). This is a dataset of comparable tion is very uniform thus confirming the high precision depth, for which morphological object classifications are and the stability of the photometry as well as the robust- provided. Figure 2 gives the resulting performance of the ness of the star-galaxy separation across the survey area. stellar selection procedure in which Equation 1 is applied The map also reveals some of the most obvious overden- to both g and r-band catalogues. In particular, the Fig- sities discovered in this work, at least two of which are ure gives completeness (black solid histogram) calculated visible as bright pixels in the Figure. as the fraction of objects classified as stars by CFHTLS (their CLASS_STAR>0.5) which are also classified as stars 3. SEARCH FOR STELLAR OVER-DENSITIES by our cuts applied to the DES data. Similarly, con- To uncover the locations of possible satellites lurking tamination can be gleaned from the fraction of objects in the DES data, we follow the approach described in classified as galaxies by the CFHTLS but as stars by our Koposov et al. (2008); Walsh et al. (2009). In short, the DES cuts (red dashed line). It is reassuring to observe satellite detection relies on applying a matched filter to low levels of contamination all the way to the very mag- the on-sky distributions of stars selected to correspond nitude limit of the DES survey. At the same time, com- to a single stellar population at a chosen distance. The pleteness is high across a wide range of magnitudes and matched filter is simply a difference of 2D Gaussians, only drops to 60% for objects fainter than r 22. It the broader one estimating the local background density, is also worth noting∼ that the star-galaxy separation∼ cri- while the narrow one yielding the amplitude of the den- teria employed in this work may not be ideally suited for sity peak at the location of the satellite. other studies, as they may have different requirements in We start by taking a catalogue of sources classified as terms of the balance between the completeness and the stars. A sub-set of these is then carved out with either a contamination. set of colour-magnitude cuts or with an isochrone mask In the stellar catalogues built using the procedure offset to a trial distance modulus. Then a 2D on-sky described above, the magnitudes are equivalent to the density map of the selected stars is constructed, keeping the spatial pixel sufficiently small, e.g. 1′ on a side. At 4 http://1.usa.gov/1zHCdrq the next step, the density map is convolved with a set Reticulum 2 nearest of the new DES dwarfs (30 kpc) Reticulum II in gamma-rays Gamma-rays 1-300 GeV Gamma-ray background model at 8 GeV 40 1.6 [1-300] GeV − 1.5 45 − 1.4 1.3 50 − 1.2 1.1 Relative intensity 55 Galactic latitude− [Deg] 1.0 60 0.9 − 105 100 95 90 85 80 − Galactic− − longitude− [Deg]− − BasedFar on PRLaway 115, 081101 from & ApJL known 808 L36 (2015)sources Uniform background Reticulum 2 nearest of the new DES dwarfs (30 kpc) Reticulum II in gamma-rays Gamma-rays 1-300 GeV Gamma-ray background model at 8 GeV 1.6 [1-300] GeV 40 Intensity− contrast map at 8 GeV 40 1.6 1.5 − 45 1.5 − 1.4 45 − 1.4 1.3 50 1.3 50 − 1.2 − 1.2 1.1 Relative intensity 55 1.1 Relative intensity 55 Galactic latitude [Deg] Galactic latitude− [Deg] − 1.0 1.0 60 60 0.9 0.9 − −105 100 95 90 85 80 − − 105− 100− −95 −90 85 80 Galactic− longitudeGalactic− [Deg]− longitude− [Deg]− − BasedFar on PRLaway 115, 081101 from & ApJL known 808 L36 (2015)sources Uniform background Reticulum 2 nearest of the new DES dwarfs (30 kpc) Reticulum II in gamma-rays Gamma-rays 1-300 GeV Gamma-ray background model at 8 GeV 1.6 [1-300] GeV 40 − 5 10− ] 1.5 1 − sr 1 45 1.4 − s − 2 − 6 1.3 10− 50 [GeV cm − 1.2 Relative intensity dF/dE 1.1 2 55 Galactic latitude [Deg] E − 7 10− 1.0 100 101 102 Energy [GeV] 60 0.9 − 105 100 95 90 85 80 − Galactic− − longitude− [Deg]− − Fermi-LAT isotropic diffuse background model Sum of the two Fermi-LAT Galactic diffuse background model BasedFar on PRLaway 115, 081101 from & ApJL known 808 L36 (2015)sources Uniform background Reticulum 2 nearest of the new DES dwarfs (30 kpc) Reticulum II in gamma-rays Gamma-rays 1-300 GeV Gamma-ray background model at 8 GeV Empirical 1.6 [1-300] GeV 40 − 5 10− 1.5 ] 1 − sr 1 45 1.4 − − s 2 − 6 1.3 10− 50 [GeV cm − 1.2 Relative intensity dF/dE 1.1 2 55 Galactic latitude [Deg] E − 7 10− 1.0 100 101 102 Energy [GeV] 60 0.9 − 105 100 95 90 85 80 − Galactic− − longitude− [Deg]− − Fermi-LAT isotropic diffuse background model Sum of the two Fermi-LAT Galactic diffuse background model BasedFar on PRLaway 115, 081101 from & ApJL known 808 L36 (2015)sources Uniform background Reticulum 2 nearest of the new DES dwarfs (30 kpc) Reticulum II in gamma-rays Gamma-rays 1-300 GeV Gamma-ray background model at 8 GeV A first look at the gamma-ray signal 1.6 [1-300] GeV 40 −5 10− 1.5 ] 1 1 − 10 6 sr 1 45 13390 18 11 1.4 − 1 s − 51 33 22 2 135.4 − 96.3 59.2 Events within1.3 0.5° 6 33.5 10− 19.9 11.5 50 6.5 3.9 of Ret2 2.3 1.3 [GeV cm − 0.6 1.2 0.3 0.2 0.1 0.1 Relative intensity dF/dE 1.1 2 55 Galactic latitude [Deg] E − 7 10− 1.0 100 101 102 Energy [GeV] 60 0.9 − 105 100 95 90 85 80 − Galactic− − longitude− [Deg]− − AGS et.