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1. In-situ versus accreted Milky Way globular clusters: a new classification method and implications for cluster formation

   https://doi.org/10.1093/mnras/stad3920  

   Belokurov, Vasily; Kravtsov, Andrey (January 2024, Monthly Notices of the Royal Astronomical Society)

   NA (Ed.)

   ABSTRACT We present a new scheme for the classification of the in-situ and accreted globular clusters (GCs). The scheme uses total energy E and z-component of the orbital angular momentum and is calibrated using the [Al/Fe] abundance ratio. We demonstrate that this classification results in two GC populations with distinct spatial, kinematic, and chemical abundance distributions. The in-situ GCs are distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the Milky Way (MW) disc, while the accreted GCs have a wide distribution of distances and a spatial distribution close to spherical. In-situ and accreted GCs have different $$\rm [Fe/H]$$ distributions with the well-known bimodality present only in the metallicity distribution of the in-situ GCs. Furthermore, the accreted and in-situ GCs are well separated in the plane of $$\rm [Al/Fe]-[Mg/Fe]$$ abundance ratios and follow distinct sequences in the age–$$\rm [Fe/H]$$ plane. The in-situ GCs in our classification show a clear disc spin-up signature – the increase of median Vϕ at metallicities −1.3 < [Fe/H] < −1 similar to the spin-up in the in-situ field stars. This signature signals the MW’s disc formation, which occurred ≈11.7−12.7 Gyr ago (or at z ≈ 3.1−5.3) according to in-situ GC ages. In-situ GCs with metallicities of $$\rm [Fe/H]\gtrsim -1.3$$ were thus born in the MW disc, while lower metallicity in-situ GCs were born during early, turbulent, pre-disc stages of the evolution of the Galaxy and are part of its Aurora stellar component. 

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2. The distribution of [α/Fe] in the Milky Way disc

   https://doi.org/10.1093/mnras/stab2899  

   Vincenzo, Fiorenzo; Weinberg, David H; Miglio, Andrea; Lane, Richard R; Roman-Lopes, Alexandre (November 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $$p([\alpha /{\rm Fe}]\, |\, [{\rm Fe}/{\rm H}])$$ in the Milky Way disk for the α-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius R, we model p([α/Fe]) as a sum of two Gaussians, representing ‘low-α’ and ‘high-α’ populations with scale heights $$z_1=0.45\, {\rm kpc}$$ and $$z_2=0.95\, {\rm kpc}$$, respectively. By accounting for age-dependent and z-dependent selection effects in APOGEE, we infer the [α/Fe] distributions that would be found for a fair sample of long-lived stars covering all z. Near the Solar circle, this distribution is bimodal at sub-solar [Fe/H], with the low-α and high-α peaks clearly separated by a minimum at intermediate [α/Fe]. In agreement with previous results, we find that the high-α population is more prominent at smaller R, lower [Fe/H], and larger |z|, and that the sequence separation is smaller for Si and Ca than for Mg, O, and S. We find significant intrinsic scatter in [α/Fe] at fixed [Fe/H] for both the low-α and high-α populations, typically ∼0.04-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on R, [Fe/H], and α-element, provide a quantitative target for chemical evolution models and a test for hydrodynamic simulations of disk galaxy formation. We argue that explaining the observed bimodality will probably require one or more sharp transitions in the disk’s gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations. 

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3. From dawn till disc: Milky Way’s turbulent youth revealed by the APOGEE+ Gaia data

   https://doi.org/10.1093/mnras/stac1267  

   Belokurov, Vasily; Kravtsov, Andrey (May 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We use accurate estimates of aluminium abundance from the APOGEE Data Release 17 and Gaia Early Data Release 3 astrometry to select a highly pure sample of stars with metallicity −1.5 ≲ [Fe/H] ≲ 0.5 born in-situ in the Milky Way proper. The low-metallicity ([Fe/H]  ≲ −1.3) in-situ component we dub Aurora is kinematically hot with an approximately isotropic velocity ellipsoid and a modest net rotation. Aurora stars exhibit large scatter in metallicity and in many element abundance ratios. The median tangential velocity of the in-situ stars increases sharply with metallicity between [Fe/H] = −1.3 and −0.9, the transition that we call the spin-up. The observed and theoretically expected age–metallicity correlations imply that this increase reflects a rapid formation of the MW disc over ≈1–2 Gyr. The transformation of the stellar kinematics as a function of [Fe/H] is accompanied by a qualitative change in chemical abundances: the scatter drops sharply once the Galaxy builds up a disc during later epochs corresponding to [Fe/H] > −0.9. Results of galaxy formation models presented in this and other recent studies strongly indicate that the trends observed in the MW reflect generic processes during the early evolution of progenitors of MW-sized galaxies: a period of chaotic pre-disc evolution, when gas is accreted along cold narrow filaments and when stars are born in irregular configurations, and subsequent rapid disc formation. The latter signals formation of a stable hot gaseous halo around the MW progenitor, which changes the mode of gas accretion and allows development of coherently rotating disc. 

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4. Galactic coronae in Milky Way-like galaxies: the role of stellar feedback in gas accretion

   https://doi.org/10.1093/mnras/stad2152  

   Barbani, Filippo; Pascale, Raffaele; Marinacci, Federico; Sales, Laura_V; Vogelsberger, Mark; Torrey, Paul; Li, Hui (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Star-forming galaxies like the Milky Way are surrounded by a hot gaseous halo at the virial temperature – the so-called galactic corona – that plays a fundamental role in their evolution. The interaction between the disc and the corona has been shown to have a direct impact on accretion of coronal gas onto the disc with major implications for galaxy evolution. In this work, we study the gas circulation between the disc and the corona of star-forming galaxies like the Milky Way. We use high-resolution hydrodynamical N-body simulations of a Milky Way-like galaxy with the inclusion of an observationally motivated galactic corona. In doing so, we use SMUGGLE, an explicit interstellar medium (ISM), and stellar feedback model coupled with the moving-mesh code arepo. We find that the reservoir of gas in the galactic corona is sustaining star formation: the gas accreted from the corona is the primary fuel for the formation of new stars, helping in maintaining a nearly constant level of cold gas mass in the galactic disc. Stellar feedback generates a gas circulation between the disc and the corona (the so-called galactic fountain) by ejecting different gas phases that are eventually re-accreted onto the disc. The accretion of coronal gas is promoted by its mixing with the galactic fountains at the disc–corona interface, causing the formation of intermediate temperature gas that enhances the cooling of the hot corona. We find that this process acts as a positive feedback mechanism, increasing the accretion rate of coronal gas onto the galaxy. 

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5. The imprint of bursty star formation on alpha-element abundance patterns in Milky Way-like galaxies

   https://doi.org/10.1093/mnras/stad206  

   Parul, Hanna; Bailin, Jeremy; Wetzel, Andrew; Gurvich, Alexander B.; Faucher-Giguère, Claude-André; Hafen, Zachary; Stern, Jonathan; Snaith, Owain (January 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Milky Way-mass galaxies in the FIRE-2 simulations demonstrate two main modes of star formation. At high redshifts star formation occurs in a series of short and intense bursts, while at low redshifts star formation proceeds at a steady rate with a transition from one mode to another at times ranging from 3 to 7 Gyr ago for different galaxies. We analyse how the mode of star formation affects iron and alpha-element abundance. We find that the early bursty regime imprints a measurable pattern in stellar elemental abundances in the form of a ‘sideways chevron’ shape on the [Fe/H] – [O/Fe] plane and the scatter in [O/Fe] at a given stellar age is higher than when a galaxy is in the steady regime. That suggests that the evolution of [O/Fe] scatter with age provides an estimate of the end of the bursty phase. We investigate the feasibility of observing of this effect by adding mock observational errors to a simulated stellar survey and find that the transition between the bursty and steady phase should be detectable in the Milky Way, although larger observational uncertainties make the transition shallower. We apply our method to observations of the Milky Way from the Second APOKASC Catalogue and estimate that the transition to steady star formation in the Milky Way happened 7 – 8 Gyrs ago, earlier than transition times measured in the simulations. 

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