Determining dark matter content of Milky Way satellite galaxies with current and future facilities Mei-Yu Wang Carnegie Mellon University Conference on Shedding Light on the Dark Universe with Extremely Large Telescopes, Trieste, July 5th, 2018
Discoveries of Milky Way dwarf spheroidal galaxies Drlica-Wagner et al., the DES collaboration (2015) • SDSS : ~20 • DES : >16 (~5 confirmed) • Pan-STARRS : 3 (2 confirmed) • MagLiteS, SMASH, Gaia, HSC … • LSST : >100 candidates Plot credit: Keith Bechtol
Probing lower surface brightness limits Homma+(2017) Subaru Hyper Suprime-Cam (HSC) Cetus III (Mv = -2.4, D= 251 kpc) • HSC, LSST => More intrinsically faint, low velocity dispersion (~3-5 km/s or less) and distance objects • ELTs can provide efficient spectroscopic follow-up measurements
A few examples of what we can learn from dwarf spheroidal (dSph) galaxies • Probing small-scale structure formation : missing satellite problem, core/cusp problem, too-big-to-fail problem • Galaxy formation efficiency at low mass end : reionization, SN/stellar feedback, environmental effects • Dark matter model constraints : – WIMP paradigm : indirect detection of gamma-ray from DM annihilation/decay (Fermi, HAWC, CTA … ) – Non-CDM models : WDM, Fuzzy DM, Self-interacting DM …
WIMP DM annihilation cross section limits from Milky Way dwarf satellite galaxies Ackermann et al., the Fermi-LAT collaboration (2015)
Effects of “J-factor” uncertainties on DM cross section limits Annihilation cross section “J-factor”: Derived using Jeans equation from stellar kinematic measurements Dark matter particle mass Albert et al. (2017) the Fermi-LAT collaboration
Follow-up strategies and science cases • Go deeper with known dwarf galaxies that already have spectroscopic measurement ⇒ Improving precision on J-factor (e.g. down to 0.3 dex) ⇒ Cannot solve the core/cusp problem (due to mass-anisotropy degeneracy) => will need proper motion • Following up on new candidates: => Solving missing satellite problems and probing galaxy formation efficiency at the low mass end
Simulating dSph stellar kinematic measurements • Python package “ dsphsim” : Constructing mock data that simulate spectroscopic observation of dSphs and determining how measurements of velocity dispersion and J-factor depend on exposure time for different spectrographs DEIMOS spectrograph • Including: – populating stellar luminosity function using ishochron models – Stellar kinematic modeling – Exposure time calculator for several current instruments. – Predictions for future instruments on ELTs MYW, Drlica-Wagner, Li, Strigari (2018), in preparation
Current and future spectroscopic follow-up facilities GMT Keck Magellan VLT
Improving precision of J-factor for Milky Way dwarf satellite galaxies Simon+(2015) Walker+(2015) 10 min (x4) 25 min (x4) • Typical nearby UF angular size ~ 20 arcmin diameter (2 x h_r) with intrinsic velocity dispersion ~ 3-5 km/s • Will require multiple tiling for using ELT instruments • Assuming high multiplexing (> 300)
Constructing DM velocity distribution in dSphs N-body simulation Stellar kinematics Reticulum II Segue 1 2 . 5 4 .0 0 . 2 . 0 4 4.0 3 6.0 r max [kpc] r max [kpc] 6.0 0 1 . 5 8.0 . 8 10.0 2 1 . 0 12.0 0 . 0 1 14.0 1 0 . 5 12.0 16.0 8 10 12 14 16 18 20 22 10 15 20 25 30 35 V max [km / s] V max [km / s] MYW, Cherry, Strigari and Horiuchi (2018) Assuming NFW profile DM velocity distribution function (or DM velocity dispersion)
Sommerfeld-enhanced J-factor for Milky Way satellite galaxies Velocity-dependent Boddy, Kumar, Strigari, MYW (2017) cross section Interaction between DM described by a Yukawa potential Sommerfeld enhancement factor DM velocity distribution f(v)
Sommerfeld-enhancement can change the order of J-factor among satellite galaxies Boddy, Kumar, Strigari, MYW (2017) Reticulum II Coma Berenices Segue 1 10 24 10 23 J S [GeV 2 / cm 5 ] 10 22 10 21 10 20 10 19 10 18 10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 0 10 1 10 2 10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 0 10 1 10 2 10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 0 10 1 10 2 ✏ φ ✏ φ ✏ φ Draco Ursa Minor 10 24 10 23 J S [GeV 2 / cm 5 ] 10 22 10 21 10 20 10 19 10 18 10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 0 10 1 10 2 10 − 5 10 − 4 10 − 3 10 − 2 10 − 1 10 0 10 1 10 2 ✏ φ ✏ φ Non-enhanced limit Coulomb-like potential Albert et al. (2017)
Resonantly produced sterile neutrino mass bound from satellite phase space density 10 − 7 Non − Resonant 10 − 8 10 − 9 X − ray sin 2 2 θ V 10 − 10 L 6 ≥ 2500 0 10 − 11 Combined dSph limit Segue 1 10 − 12 Draco II Cetus II (Forecast) Tucana V (Forecast) 10 − 13 10 0 10 1 [keV] m µ s MYW, Cherry, Strigari and Horiuchi (2018) Liouville’s theorem: For dissipationless and collisionless Coarse-grained particles, the phase-space density cannot phase-space density increase => Fine-grained phase-space density
Summary • As new dwarf satellite galaxies continue to be discovered by current surveys and in the near future, LSST, Milky Way dwarf satellite galaxies will continue to provide powerful constraints on DM models and galaxy formation physics. • Indirect detection limits from dSphs on WIMP DM paradigm can be greatly improved if precise stellar kinematic measurements can be carried out by ELTs. • Precise stellar kinematic measurements can also help to pin down DM velocity distribution and therefore provide useful constraints on several DM models.
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