More Like this

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)

   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.more »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.

   « less
2. Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ac96ed  

   Chiti, Anirudh ; Simon, Joshua D. ; Frebel, Anna ; Pace, Andrew B. ; Ji, Alexander P. ; Li, Ting S. ( November 2022 , The Astrophysical Journal)

   We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of$M_{1/2}(r_h)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V=${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in linemore »with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

   « less
3. A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. II. Phase Spirals in an Inhomogeneous Disk Galaxy with a Nonresponsive Dark Matter Halo

   https://doi.org/10.3847/1538-4357/acd641  

   Banik, Uddipan ; van den Bosch, Frank C. ; Weinberg, Martin D. ( July 2023 , The Astrophysical Journal)

   We develop a linear perturbative formalism to compute the response of an inhomogeneous stellar disk embedded in a nonresponsive dark matter (DM) halo to various perturbations like bars, spiral arms, and encounters with satellite galaxies. Without self-gravity to reinforce it, the response of a Fourier mode phase mixes away due to an intrinsic spread in the vertical (Ω_z), radial (Ω_r), and azimuthal (Ω_ϕ) frequencies, triggering local phase-space spirals. The detailed galactic potential dictates the shape of phase spirals: phase mixing occurs more slowly and thus phase spirals are more loosely wound in the outer disk and in the presence of an ambient DM halo. Collisional diffusion due to scattering of stars by structures like giant molecular clouds causes superexponential damping of the phase spiral amplitude. Thez–v_zphase spiral is one-armed (two-armed) for vertically antisymmetric (symmetric) bending (breathing) modes. Only transient perturbations with timescales (τ_P) comparable to the vertical oscillation period (τ_z∼ 1/Ω_z) can trigger vertical phase spirals. Each (n,l,m) mode of the response to impulsive (τ_P<τ= 1/(nΩ_z+lΩ_r+mΩ_ϕ)) perturbations is power-law (∼τ_P/τ) suppressed, but that to adiabatic (τ_P>τ) perturbations is exponentially weak ($\sim \exp \left[-{\left({\tau }_{\mathrm{P}}/\tau \right)}^{\alpha }\right]$) except for resonant (τ→ ∞ ) modes. Slower (τ_P>τ_z)more »perturbations, e.g., distant encounters with satellite galaxies, induce stronger bending modes. Sagittarius (Sgr) dominates the solar neighborhood response of the Milky Way (MW) disk to satellite encounters. Thus, if the Gaia phase spiral was triggered by a MW satellite, Sgr is the leading contender. However, the survival of the phase spiral against collisional damping necessitates an impact ∼0.6–0.7 Gyr ago.

   « less
4. The Diffuse Ionized Gas Halo of the Large Magellanic Cloud

   https://doi.org/10.3847/1538-4357/acc06e  

   Smart, B. M. ; Haffner, L. M. ; Barger, K. A. ; Ciampa, D. A. ; Hill, A. S. ; Krishnarao, D. ; Madsen, G. J. ( May 2023 , The Astrophysical Journal)

   The Large Magellanic Cloud (LMC) has an extensive Hαemission halo that traces an extended, warm ionized component of its interstellar medium. Using the Wisconsin HαMapper telescope, we present the first kinematic Hαsurvey of an extensive region around the LMC, from (ℓ,b) = (264.°5, − 45.°5) to (295.°5, − 19.°5), covering +150 ≤v_LSR≤ + 390 km s⁻¹. We find that ionized hydrogen exists throughout the galaxy and extends several degrees beyond detected neutral hydrogen emission$(\log \left(N_{\mathrm{H}\mathrm{I}/{\mathrm{cm}}^{-2}}\right)\approx 18.3)$as traced by 21 cm in current surveys. Using the column density structure of the neutral gas and stellar line-of-sight depths as a guide, we estimate the upper limit mass of the ionized component of the LMC to be roughlyM_ionized≈ (0.6–1.8) × 10⁹M_☉, which is comparable to the total neutral atomic gas mass in the same region (M_neutral≈ 0.76–0.85 × 10⁹M_☉). Considering only the atomic phases, we findM_ionized/M_{ionized+neutral}, to be 46%–68% throughout the LMC and its extended halo. Additionally, we find an ionized gas cloud that extends off of the LMC at (ℓ,b) ≈ (285°, − 28°) into a region previously identified as the Leading Arm complex. This gas is moving at a similarmore »line-of-sight velocity as the LMC and hasM_ionized/M_{ionized+neutral}= 13%–51%. This study, combined with previous studies of the SMC and extended structures of the Magellanic Clouds, continues to suggest that warm, ionized gas is as massive and dynamically important as the neutral gas in the Magellanic System.

   « less
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)

   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${\mathcal{H}}_{\epsilon }(p,q,I,\phi ,t)=h(I)+\sum _{i=1}^n\pm \left(\frac{1}{2}{p}_i^2+V_i(q_i)\right)+\epsilon H_1(p,q,I,\phi ,t),$where$(p,q)\in {\mathbb{R}}^n\times {\mathbb{T}}^n$,$(I,\phi )\in {\mathbb{R}}^d\times {\mathbb{T}}^d$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}.