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UsingN-body simulations, we explore the effects of growing a supermassive black hole (SMBH) prior to or during the formation of a stellar bar. Keeping the final mass and growth rate of the SMBH fixed, we show that if it is introduced before or while the bar is still growing, the SMBH does not cause a decrease in bar amplitude. Rather, in most cases, it is strengthened. In addition, an early-growing SMBH always either decreases the buckling amplitude, delays buckling, or both. This weakening of buckling is caused by an increase in the disk vertical velocity dispersion at radii well beyond the nominal black hole sphere of influence. While we find considerable stochasticity and sensitivity to initial conditions, the only case where the SMBH causes a decrease in bar amplitude is when it is introduced after the bar has attained a steady state. In this case, we confirm previous findings that the decrease in bar strength is a result of scattering of bar-supporting orbits with small pericenter radii. By heating the inner disk both radially and vertically, an early-growing SMBH increases the fraction of stars that can be captured by the inner Lindblad resonance (ILR) and the vertical ILR, thereby strengthening both the bar and the boxy-peanut-shaped bulge. Using orbital frequency analysis of star particles, we show that when an SMBH is introduced early and the bar forms around it, the bar is populated by different families of regular bar-supporting orbits than when the bar forms without an SMBH.

The Milky Way (MW) stellar disk has both a thin and a thick component. The thin disk is composed mostly of younger stars (≲8 Gyr) with a lower abundance ofα-elements, while the thick disk contains predominantly older stars (≳8–12 Gyr) with a higherαabundance, giving rise to anα-bimodality most prominent at intermediate metallicities. A proposed explanation for the bimodality is an episode of clumpy star formation, where high-αstars form in massive clumps that appear in the first few billion years of the MW’s evolution, while low-αstars form throughout the disk and over a longer time span. To better understand the evolution of clumps, we track them and their constituent stars in two clumpy MW simulations that reproduce theα-abundance bimodality, one with 10% and the other with 20% supernova feedback efficiency. We investigate the paths that these clumps take in the chemical space ([O/Fe]–[Fe/H]) as well as their mass, star formation rate (SFR), formation location, lifetime, and merger history. The clumps in the simulation with lower feedback last longer on average, with several lasting hundreds of millions of years. Some of the clumps do not reach high-α, but the ones that do on average have a higher SFR, longer lifetime, greater mass, and form closer to the Galactic center than the ones that do not. Most clumps that reach high-αmerge with others and eventually spiral into the Galactic center, but shed stars along the way to form most of the thick-disk component.

Many barred galaxies exhibit upturns (shoulders) in their bar-major-axis density profile. Simulation studies have suggested that shoulders are supported by loopedx₁orbits, occur in growing bars, and can appear after bar buckling. We investigate the orbital support and evolution of shoulders via frequency analyses of orbits in simulations. We confirm that looped orbits are shoulder-supporting, and can remain so, to a lesser extent, after being vertically thickened. We show that looped orbits appear at the resonance ( Ω_φ− Ω_P)/Ω_R= 1/2 (analogous to the classical inner Lindblad resonance, and here called ILR) with vertical-to-radial frequency ratios 1 ≲ Ω_z/Ω_R≲ 3/2 (verticallywarmorbits).Coolorbits at the ILR (those with Ω_z/Ω_R> 3/2) are vertically thin and have no loops, contributing negligibly to shoulders. As bars slow and thicken, either secularly or by buckling, they populate warm orbits at the ILR. Further thickening carries these orbits toward crossing the vertical ILR [vILR, ( Ω_φ− Ω_P)/Ω_z= 1/2], where they convert in-plane motion to vertical motion, become chaotic, kinematically hotter, and less shoulder-supporting. Hence, persistent shoulders require bars to trap new stars, consistent with the need for a growing bar. Since buckling speeds up trapping on warm orbits at the ILR, it can be followed by shoulder formation, as seen in simulations. This sequence supports the recent observational finding that shoulders likely precede the emergence of BP-bulges. The python module for the frequency analysis,naif, is made available.

To understand the formation and evolution of the Milky Way disk, we must connect its current properties to its past. We explore hydrodynamical cosmological simulations to investigate how the chemical abundances of stars might be linked to their origins. Using hierarchical clustering of abundance measurements in two Milky Way–like simulations with distributed and steady star formation histories, we find that groups of chemically similar stars comprise different groups in birth place (R_birth) and time (age). Simulating observational abundance errors (0.05 dex), we find that to trace distinct groups of (R_birth, age) requires a large vector of abundances. Using 15 element abundances (Fe, O, Mg, S, Si, C, P, Mn, Ne, Al, N, V, Ba, Cr, Co), up to ≈10 groups can be defined with ≈25% overlap in (R_birth, age). We build a simple model to show that in the context of these simulations, it is possible to infer a star’s age andR_birthfrom abundances with precisions of ±0.06 Gyr and ±1.17 kpc, respectively. We find that abundance clustering is ineffective for a third simulation, where low-αstars form distributed in the disk and early high-αstars form more rapidly in clumps that sink toward the Galactic center as their constituent stars evolve to enrich the interstellar medium. However, this formation path leads to large age dispersions across the [α/Fe]–[Fe/H] plane, which is inconsistent with the Milky Way’s observed properties. We conclude that abundance clustering is a promising approach toward charting the history of our Galaxy.

We describe the Milky Way Survey (MWS) that will be undertaken with the Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4 m telescope at the Kitt Peak National Observatory. Over the next 5 yr DESI MWS will observe approximately seven million stars at Galactic latitudes ∣b∣ > 20°, with an inclusive target selection scheme focused on the thick disk and stellar halo. MWS will also include several high-completeness samples of rare stellar types, including white dwarfs, low-mass stars within 100 pc of the Sun, and horizontal branch stars. We summarize the potential of DESI to advance understanding of the Galactic structure and stellar evolution. We introduce the final definitions of the main MWS target classes and estimate the number of stars in each class that will be observed. We describe our pipelines for deriving radial velocities, atmospheric parameters, and chemical abundances. We use ≃500,000 spectra of unique stellar targets from the DESI Survey Validation program (SV) to demonstrate that our pipelines can measure radial velocities to ≃1 km s⁻¹and [Fe/H] accurate to ≃0.2 dex for typical stars in our main sample. We find the stellar parameter distributions from ≈100 deg²of SV observations with ≳90% completeness on our main sample are in good agreement with expectations from mock catalogs and previous surveys.