Detectability of Weakly Interacting Massive Particles in the Sagittarius Dwarf Tidal Stream

TL;DR

The paper analyzes whether the leading tail of the Sagittarius dwarf tidal stream contributes detectable dark matter in the Solar neighborhood and induces a step-like feature in WIMP recoil spectra. By combining empirical stellar-density measurements with a dark-matter–to–stellar-mass–ratio argument, it estimates a local stream density of $\rho_{{\rm tail},\odot}\approx[0.001,0.07]$ GeV cm$^{-3}$, corresponding to a $0.3$–$23\%$ enhancement over the standard halo density. It predicts a characteristic recoil-energy edge $E_{c}(t)$ that modulates annually, with the step phase opposite to the usual halo modulation, and provides detailed rate formulas, velocity distributions, and directional detection signatures. The results indicate that existing (e.g., CDMS, DAMA) and upcoming (XENON, CryoArray, DRIFT) experiments could detect or constrain the Sgr stream’s dark-matter component, offering a novel probe of Galactic structure and dark matter distribution in tidal streams.

Abstract

Tidal streams of the Sagittarius dwarf spheroidal galaxy (Sgr) may be showering dark matter onto the solar system and contributing approx (0.3--23)% of the local density of our Galactic Halo. If the Sagittarius galaxy contains WIMP dark matter, the extra contribution from the stream gives rise to a step-like feature in the energy recoil spectrum in direct dark matter detection. For our best estimate of stream velocity (300 km/sec) and direction (the plane containing the Sgr dwarf and its debris), the count rate is maximum on June 28 and minimum on December 27 (for most recoil energies), and the location of the step oscillates yearly with a phase opposite to that of the count rate. In the CDMS experiment, for 60 GeV WIMPs, the location of the step oscillates between 35 and 42 keV, and for the most favorable stream density, the stream should be detectable at the 11 sigma level in four years of data with 10 keV energy bins. Planned large detectors like XENON, CryoArray and the directional detector DRIFT may also be able to identify the Sgr stream.

Figures (4)

• Figure 1: Count rate of 60 GeV WIMPs in a NaI detector such as DAMA as a function of recoil energy. The dotted lines (towards the left) indicate the count rate due to Galactic halo WIMPs alone for an isothermal halo. The solid and dashed lines indicate the step in the count rate that arises if we include the WIMPs in the Sgr stream for $v_{str} = 300$ km/sec in the direction $(l,b)=(90^\circ,-76^\circ)$ with a stream velocity dispersion of 20 km/sec. The plot assumes that the Sgr stream contributes an additional 20% of the local Galactic halo density. The solid and dashed lines are for June 28 and December 27 respectively, the dates at which the annual modulation of the stream is maximized and minimized.
• Figure 2: Count rate of 60 GeV WIMPs in a ${}^{73}$Ge detector as a function of recoil energy. The dotted lines (towards the left) indicate the count rate due to Galactic halo WIMPs alone for an isothermal halo. The solid and dashed lines indicate the step in the count rate that arises if we include the WIMPs in the Sgr stream. The parameters of the curves are the same as described in Figure 1.
• Figure 3: Count rate of 60 GeV WIMPs in a CS$_2$ detector (DRIFT) as a function of recoil energy and direction of the nuclear recoil. The figure shows the count rate in a 2-dimensional slice of the 3-dimensional recoil space. The chosen slice is perpendicular to the direction of Galactic rotation and defined by a recoil energy of 20 keV in that direction. The horizontal axis represents recoils in the direction of the Galactic center (left) and Galactic anticenter (right); the vertical axis represents recoils in the direction of the North Galactic Pole (upward) and South Galactic Pole (downward). The gray scale indicates the count rate per kilogram of detector per day and per unit cell in the 3-dimensional energy space. Lighter regions correspond to higher count rates. The white band on the upper part is the location of nuclear recoil due to WIMPs in the Sgr stream. The fuzzy gray cloud at the center contains recoils due to WIMPs in the local isothermal Galactic halo. The two WIMP populations can in principle be easily separated, given a sufficient exposure.
• Figure 4: Section of figure 3 with $E\widehat{\bf q}_X=0$.