The Astrophysical Journal (Jan 2025)
Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf
Abstract
Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2 σ width) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( ${ \mathcal O }(1)$ kpc) than are probed by its stars ( ${ \mathcal O }(10)$ pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 10 ^9 M _⊙ , the CPS's velocity dispersion will be ∼ 40 km s ^−1 larger at ϕ _1 > 20° than at ϕ _1 < 0°. If no such heating is detected, Segue 2’s mass cannot exceed 10 ^9 M _⊙ within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations.
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