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1. Substructure at High Speed. II. The Local Escape Velocity and Milky Way Mass with Gaia eDR3

   https://doi.org/10.3847/1538-4357/ac4244  

   Necib, Lina; Lin, Tongyan (February 2022, The Astrophysical Journal)

   Abstract Measuring the escape velocity of the Milky Way is critical in obtaining the mass of the Milky Way, understanding the dark matter velocity distribution, and building the dark matter density profile. In Necib & Lin, we introduced a strategy to robustly measure the escape velocity. Our approach takes into account the presence of kinematic substructures by modeling the tail of the stellar distribution with multiple components, including the stellar halo and the debris flow called the Gaia Sausage (Enceladus). In doing so, we can test the robustness of the escape velocity measurement for different definitions of the “tail” of the velocity distribution and the consistency of the data with different underlying models. In this paper, we apply this method to the Gaia eDR3 data release and find that a model with two components is preferred, although results from a single-component fit are also consistent. Based on a fit to retrograde data with two bound components to account for the relaxed halo and the Gaia Sausage, we find the escape velocity of the Milky Way at the solar position to be $v_{\mathrm{esc}}={445}_{-8}^{+25}$km s⁻¹. A fit with a single component to the same data gives $v_{\mathrm{esc}}={472}_{-12}^{+17}$km s⁻¹. Assuming a Navarro−Frenck−White dark matter profile, we find a Milky Way concentration of $c_{200}={19}_{-7}^{+11}$and a mass of $M_{200}={4.6}_{-0.8}^{+1.5}\times {10}^{11}{M}_{\odot }$, which is considerably lighter than previous measurements. 

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2. Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy

   https://doi.org/10.3847/1538-4357/ad8eba  

   Cerny, W.; Chiti, A.; Geha, M.; Mutlu-Pakdil, B.; Drlica-Wagner, A.; Tan, C_Y; Adamów, M.; Pace, A_B; Simon, J_D; Sand, D_J; et al (January 2025, The Astrophysical Journal)

   Abstract We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity ( $M_V=-{2.5}_{-0.5}^{+0.3};$ $L_V={850}_{-260}^{+380}{L}_{\odot }$), extended ( $r_{1/2}={41}_{-8}^{+9}$pc) stellar system located in the outer halo (D_⊙= 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of $v_{\mathrm{sys}}=-{13.1}_{-0.9}^{+1.0}\mathrm{km}{\mathrm{s}}^{-1}$for the system and place an upper limit ofσ_v< 3.5 km s⁻¹(σ_v< 1.6 km s⁻¹) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= − 2.61 ± 0.21) with a statistically significant metallicity spread ( ${\sigma }_{\left[\mathrm{Fe}/\mathrm{H}\right]}={0.46}_{-0.14}^{+0.26}$dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (r_peri= 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly belowσ_v≲ 2 km s⁻¹, Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos. 

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3. Quantifying the Projected Suppression of Cluster Escape Velocity Profiles

   https://doi.org/10.3847/1538-4357/ac4786  

   Halenka, Vitali; Miller, Christopher J.; Vansickle, Paige (February 2022, The Astrophysical Journal)

   Abstract The 3D radial escape-velocity profile of galaxy clusters has been suggested to be a promising and competitive tool for constraining mass profiles and cosmological parameters in an accelerating universe. However, the observed line-of-sight escape profile is known to be suppressed compared to the underlying 3D radial (or tangential) escape profile. Past work has suggested that velocity anisotropy in the phase-space data is the root cause. Instead, we find that the observed suppression is from the statistical undersampling of the phase spaces and that the 3D radial escape edge can be accurately inferred from projected data. We build an analytical model for this suppression that only requires the number of observed galaxiesNin the phase-space data within the sky-projected range 0.3 ≤r_⊥/R_200,critical≤ 1. The radially averaged suppression function is an inverse power law $〈Z_{\mathrm{v}}〉=1+{\left(N_0/N\right)}^{\lambda }$withN₀= 17.818 andλ= 0.362. We test our model withN-body simulations, using dark matter particles, subhalos, and semianalytic galaxies as the phase-space tracers, and find excellent agreement. We also assess the model for systematic biases from cosmology (Ω_Λ,H₀), cluster mass (M_200,critical), and velocity anisotropy (β). We find that varying these parameters over large ranges can impart a maximal additional fractional change in 〈Z_v〉 of 2.7%. These systematics are highly subdominant (by at least a factor of 13.7) to the suppression fromN. 

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4. Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

   https://doi.org/10.3847/2041-8213/ad955f  

   Comisso, Luca; Farrar, Glennys_R; Muzio, Marco_S (December 2024, The Astrophysical Journal Letters)

   Abstract Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 10¹⁸eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form $f_{\mathrm{cut}}\left(E,E_{\mathrm{cut}}\right)=\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$. Particle escape from the turbulent accelerating region is energy dependent, witht_esc∝E^−δandδ∼ 1/3. The resulting particle flux from the accelerator follows $\mathit{dN}/\mathit{dEdt}\propto E^{-s}\mathrm{sech}\left({\left(E/E_{\mathrm{cut}}\right)}^2\right)$, withs∼ 2.1. We fit the Pierre Auger Observatory’s spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being $s={2.1}_{-0.13}^{+0.06}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration. 

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5. Analytic genericity of diffusing orbits in a priori unstable Hamiltonian systems

   https://doi.org/10.1088/1361-6544/ac50bb  

   Chen, Qinbo; de la Llave, Rafael (March 2022, Nonlinearity)

   Abstract The genericity of Arnold diffusion in the analytic category is an open problem. In this paper, we study this problem in the followinga prioriunstable Hamiltonian system with a time-periodic perturbation ${\mathcal{H}}_{\epsilon }\left(p,q,I,\phi ,t\right)=h\left(I\right)+\sum _{i=1}^n\pm \left(\frac{1}{2}{p}_i^2+V_i\left(q_i\right)\right)+\epsilon H_1\left(p,q,I,\phi ,t\right),$where $\left(p,q\right)\in {\mathbb{R}}^n\times {\mathbb{T}}^n$, $\left(I,\phi \right)\in {\mathbb{R}}^d\times {\mathbb{T}}^d$withn,d⩾ 1,V_iare Morse potentials, andɛis a small non-zero parameter. The unperturbed Hamiltonian is not necessarily convex, and the induced inner dynamics does not need to satisfy a twist condition. Using geometric methods we prove that Arnold diffusion occurs for generic analytic perturbationsH₁. Indeed, the set of admissibleH₁isC^ωdense andC³open (a fortiori,C^ωopen). Our perturbative technique for the genericity is valid in theC^ktopology for allk∈ [3, ∞) ∪ {∞,ω}. 

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