A search for dark matter annihilation in the newly discovered dwarf galaxy Reticulum II

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1 Note. Best-fit parameters from the maximum-likelihood fit assuming the composite isochrone described in Section 3.2. Uncertainties are calculated from the the highest density interval containing 90% of the posterior distribution. Map Sig refers to detection significance of the candidate from the stellar density map search method (Section 3.1). TS Scan refers to the significance (Equation 4) from the likelihood scan using a Plummer model spatial kernel with half-light radius rh = 0. 1 (Section 3.2). TS Fit denotes the significance of the likelihood method using the set of best-fit parameters. Ellipticities and position angles are not quoted for lower significance candidates where they are not well constrained by the data. pi is the estimated number of satellite member stars with g < 23 in the stellar catalog. A search for dark matter annihilation in the newly discovered dwarf galaxy Reticulum II RetII DES Alex Geringer-Sameth Fig. 2. Left: False color gri coadd image of the region centered on DES J Carnegie Mellon Right: Stars in the same fielduniversity of view with membership probability pi > 0.01 are marked with colored circles. In this color map, red signifies high-confidence association with DES J In collaboration with 5403 and blue indicates lower membership probability. The membership probabilities have been Matt Walker (CMU) evaluated using Equation (2) for the best-fit model parameters listed in Table 1. Savvas Koushiappas (Brown) Sergey Koposov, Vasily Belokurov, Gabriel Torrealba, Wyn Evans (Cambridge) Vincent Bonnivard, Celine Combet, David Maurin (LPSC Grenoble) arxiv: (PRL), arxiv: (PRD), arxiv: (ApJL)

2 Dark matter Annihilation h vi Weak scale Always* get gamma-rays

3 Milky Way dwarf galaxies Gamma-rays Fermi satellite Nearby Lots of dark matter Not much else > no astrophysical background

4 Reticulum II nearest of the new DES dwarfs (30 kpc) (Koposov+ 2015, Bechtol+ 2015) Detected events 40 Background model Galactic latitude [Deg] Relative intensity Galactic longitude [Deg] 0.9

5 A suggestive feature? 10 5 E 2 df/de [GeV cm 2 s 1 sr 1 ] Events within 0.5 of RetII Energy [GeV] Geringer-Sameth+ arxiv: (PRL)

6 Statistical procedure Quantify the significance of the signal (e.g. p value) Each photon gets a weight X T = w(q i ) w Q = log 1+ s Q b Q signal background i2photons sum over all observed events Geringer-Sameth, Koushiappas, Walker arxiv: (PRD)

7 Results Significance 3 2 Significance µ + µ + q q b b W + W hh Di use 1 Di use 2 Empirical 1 Empirical Mass [GeV] Mass [GeV] Local p-value < (4σ) in every channel Robust to different background spectra with trials, p global < Geringer-Sameth+ arxiv: (PRL)

8 Results Galactic latitude [Deg] Empirical background sampling Galactic longitude [Deg] Significance Relative intensity + Seg Mass [GeV] RetII Local p value of 8/3306 = (2.8σ) Global p value of 32/3306 = (2.3σ) Geringer-Sameth+ arxiv: (PRL)

9 If signal is due to dark matter annihilation what can we say about the dark matter particle? Reticulum 2 Other dwarfs J19 h vi 26 Jh vi h vi ruled out b b Mass [GeV] Geringer-Sameth+ arxiv: (PRL) hh µ + µ allowed Mass [GeV] AGS, Koushiappas, Walker (PRD 91, ) <σv> upper limits from other dwarfs gives a prediction for Ret2 s J value log 10 J & 19.6 ± 0.3

10 Measured J values Use line of sight velocities + Jeans equation to infer dark matter density profile Bonnivard et. al. arxiv: (ApJL) cm -5 ) 2 (J(α int = 0.5 )/GeV Log Ret II: Optimised setup Pi-weighted Binned analysis Bootstrap 68% CIs (+ triaxiality systematics [ ] ) Bonnivard et al. (2015b) Classical Ultrafaint d [kpc] Ret II UMa II Coma Wil I Dra UMi Seg I see also Simon et. al. arxiv: (ApJ)

11 Measured J values Use line of sight velocities + Jeans equation to infer dark matter density profile cm -5 ) 2 = 0.5 )/GeV (J(α int Log Bonnivard et. al. arxiv: (ApJL) Bonnivard et. al. arxiv: Ret II: Optimised setup Pi-weighted Binned analysis Bootstrap 68% CIs (+ triaxiality systematics [ ] ) Bonnivard et al. (2015b) Classical Ultrafaint d [kpc] Ret II UMa II Coma Wil I Dra UMi Seg I Seg1 see also Simon et. al. arxiv: (ApJ) 19.6

12 Hurdles for any dark matter interpretation 1. Gamma-ray data is inconsistent with background 2. Consistent with dark matter annihilation Energy spectrum of signal! Dark matter halo of RetII 3. Inconsistent with any other possible source Distant source coincidentally in same direction! Instrument/data, Pass 8! related to (1) Drlica-Wagner+ (Fermi,DES) (ApJL) Hooper & Linden