Detectability of weakly interacting massive particles in the Sagittarius dwarf tidal stream

Abstract

Tidal streams of the Sagittarius dwarf spheroidal galaxy (Sgr) may be showering dark matter onto the solar system and contributing ∼(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 11σ 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.Recently, the Sloan Digital Sky Survey (SDSS) and the Two Micron All Sky Survey (2MASS) surveys [1,2] have traced the tidal stream of the Sagittarius dwarf galaxy (Sgr) more than 360 • around the sky. It is believed that this tidal stream is currently moving through the solar neighborhood, and that it contains dark matter in addition to stars. In this paper we investigate the possibility of detecting WIMP dark matter in this stream via direct detection experiments.The Sagittarius galaxy, a dwarf spheroidal galaxy of roughly 10 9 M ⊙ , is a satellite of our own much larger Milky Way Galaxy, located inside the Milky Way, ∼15 kpc behind the Galactic Center and ∼6 kpc below the Galactic Plane [3]. There are two streams of matter that extend outwards from the main body of the Sgr galaxy. These streams, known as the leading and trailing tidal tails, are made of matter tidally pulled away from the Sgr galaxy. The leading tail may be showering matter down upon the solar system [2]. The flow is in the general direction orthogonal to the Galactic plane and has a speed of roughly 300 km/s (see discussion in §2 below). This speed is comparable to that of the relative speed of the Sun and the WIMPs in the general dark halo. Hence one can hope to detect the stream in direct detection experiments.The detectability depends on the density of dark matter in the stream. The mass-to-light ratio M/L in the stream is unknown, but is plausibly at least as large as that in the Sgr main body; in fact, the M/L in the stream may be significantly larger because the dark matter on the outskirts of the main body would be tidally stripped before the (more centrally located) stars. Various determinations of the M/L for the Sgr main body give values in the range 25 to 100 (see the discussion in [2] and references therein). With our assumptions, we find a density of dark matter in the stream in the range 0.3 to 23% of the local dark halo density. If the dwarf does not contain dark matter or the stream does not pass thr...

Annual Modulation of Dark Matter in the Presence of Streams

“…[24,40], allows for significant uncertainties; however, as we will primarily use this stream simply as an example of streams in general, we will ignore these uncertainties and use only the stated values. The velocity of the stream relative to the Sun is then…”

“…We write this in terms of the Earth's velocity V ⊕ relative to the Sun as [36,40] and elsewhere (the position vectors are more easily generalized to an elliptical orbit); the two bases are related byε 1 = −ê 2 andε 2 =ê 1 . In Galactic coordinates, wherex is the direction to the Galactic Center,ŷ the direction of disk rotation, andẑ the North Galactic Pole,…”

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“…those in Ref. [27,28] (even up to 30%). Thus this signature is model independent, very effective and, in addition, it allows the test of a large range of cross sections and of halo densities.…”

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“…In addition to these models that rely on the general features of galaxy formation, the local geography of our region of the galaxy might affect the dark matter density. For example, Freese, Gondolo and Newberg have proposed that the Sagittarius stream should add up to 23% to the local dark matter density, with a characteristic annual modulation [40]. These models illustrate not only that there is a large uncertainty in the value of the local dark matter flux at the earth but also that the understanding of this value relative to the overall average density of dark matter is an interesting astrophysical question.…”