The number and size of subhalo-induced gaps in stellar streams

Abstract

Ample observational capabilities exist today to detect the small density perturbations that low-mass dark matter subhaloes impart on stellar streams from disrupting Galactic satellites. In anticipation of these observations, we investigate the expected number and size of gaps by combining an analytic prescription for gap evolution on circular orbits with the flux of subhaloes near the stream. We explore the distribution of gap sizes and depths for a typical cold stream around the Milky Way and find that for a given stream age and gap depth, each subhalo mass produces a characteristic gap size. For a stream with an age of a few Gyr, orbiting at a distance of 10-20 kpc from the Galactic center, even modest subhaloes with a mass of 10 6 − 10 7 M produce gaps with sizes that are on the order of several degrees. We consider the number and distribution of gap sizes created by subhaloes with masses 10 5 − 10 9 M , accounting for the expected depletion of subhaloes by the Milky Way disk, and present predictions for six cold streams around the Milky Way. For Pal 5, we forecast 0.7 gaps with a density depletion of at least 25% and a typical gap size of 8 • . Thus, there appears to be no tension between the recent non-detection of density depletions in the Pal 5 tidal tails and ΛCDM expectations. These predictions can be used to guide the scale of future gap searches.

Stellar Streams Discovered in the Dark Energy Survey

“…We use the relation between the stream width and the progenitor mass derived in Erkal et al (2016b). More precisely, we use Equation (27) from Erkal et al (2016a), where the progenitor mass is given in terms of the stream width as viewed from the Galactic center and the enclosed mass of the Milky Way at the stream's location. For the mass of the Milky Way, we use the best-fit model in McMillan (2017), who used a range of data to constrain the Milky Way potential.…”

“…While the existence of this underdensity remains uncertain, if it is real, it could be due to perturbations by subhalos around the Milky Way (e.g., Ibata et al 2002;Johnston et al 2002). Erkal et al (2016a) estimated the typical size and number of gaps in the ATLAS stream due to subhalos and found a characteristic gap size of~ 4 with 0.1 gaps expected. However, this prediction depends on the length and orbital trajectory of the ATLAS stream; therefore, given the increased length of the ATLAS stream detected in this work and in Pan-STARRS (Bernard et al 2016), as well as the uncertainty in its trajectory, the predicted number of gaps is likely an underestimate.…”

Fluctuations of the gravitational field generated by a random population of extended substructures

“…II dark matter-only simulations (Erkal et al 2016;Diemand et al 2008) indicates that the largest fluctuations of the tidal field of a galaxy will be caused by the smallest & most-abundant subhaloes. Given that ultra-high-resolution ΛCDM simulations of structure formation predict subshaloes with masses as low as M1 ∼ 10 −6 M (Ishiyama et al 2010 and references therein), we expect microhaloes to induce a significant tidal heating of self-gravitating objects moving across galaxies like the Milky Way, where the upper limit of the subhalo mass function is M2 ∼ 10 11 M ∼ 10 17 M1, an issue that we study in more detail below.…”

“…Issues introduced by low-number statistics can be largely removed by choosing an upper-mass limit M2 10 −3 M200, which yields N 100 for M2 M1. Following Erkal et al (2016) let us adopt a power-law size function (30) with c0 = 0.53 kpc and β = 0.5, which matches the relation between bound mass and scale radius of the subhaloes found in the Via Lactea II simulation (Diemand et al 2008). The average size of subhaloes in a mass bin M ∈ (M1, M2) is…”

Velocity Variations in the Phoenix–hermus Star Stream

“…A sub-halo crosses a stream of stars so quickly that the orbital changes of the stars in the vicinity can be derived from the impact approximation. Over about an orbital period, a gap opens up in the stream (Carlberg 2012;Erkal & Belokurov 2015a, 2015bErkal et al 2016;Sanders et al 2016). With time, the gaps are blurred out as the differential angular momentum in the stream stars cause stars to move into the gap in configuration space, with the blurring being faster for relatively more eccentric orbits.…”