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Abstract We use a suite of hydrodynamics simulations of the interstellar medium (ISM) within a galactic disk, which includes radiative transfer, a nonequilibrium model of molecular hydrogen, and a realistic model for star formation and feedback, to study the structure of the ISM and H₂abundance as a function of local ISM properties. We show that the star formation rate and structure of the ISM are sensitive to the metallicity of the gas with a progressively smoother density distribution with decreasing metallicity. In addition to the well-known trend of the HI–H₂transition shifting to higher densities with decreasing metallicity, the maximum achieved molecular fraction in the ISM drops drastically atZ≲ 0.2Z_⊙as the formation time of H₂becomes much longer than a typical lifetime of dense regions of the ISM. We present accurate fitting formulae for both volumetric and projected $f_{{\mathrm{H}}_2}$measured on different scales as a function of gas metallicity, UV radiation field, and gas density. We show that when the formulae are applied to the patches in the simulated galaxy, the overall molecular gas mass is reproduced to better than a factor of ≲1.5 across the entire range of metallicities and scales. We also show that the presented fit is considerably more accurate than any of the previous $f_{{\mathrm{H}}_2}$models and fitting formulae in the low-metallicity regime. The fit can thus be used for modeling molecular gas in low-resolution simulations and semi-analytic models of galaxy formation in the dwarf and high-redshift regimes. 

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. 

ABSTRACT We investigate the formation (spin-up) of galactic discs in the artemis simulations of Milky Way (MW)-mass galaxies. In almost all galaxies, discs spin up at higher [Fe/H] than the MW. Those galaxies that contain an analogue of the Gaia Sausage-Enceladus (GSE) spin up at a lower average metallicity than those without. We identify six galaxies with spin-up metallicity similar to that of the MW, which formed their discs ∼8–11 Gyr ago. Five of these experience a merger similar to the GSE. The spin-up times correlate with the halo masses at early times: galaxies with early spin-up have larger virial masses at a lookback time tL = 12 Gyr. The fraction of stars accreted from outside the host galaxy is smaller in galaxies with earlier spin-ups. Accreted fractions small enough to be comparable to the MW are only found in galaxies with the earliest disc formation and large initial virial masses (M200c ≈ 2 × 1011 M⊙ at tL = 12 Gyr). We find that discs form when the halo’s virial mass reaches a threshold of M200c ≈ (6 ± 3) × 1011 M⊙, independent of the spin-up time. However, the failure to form a disc in other galaxies appears to be instead related to mergers at early times. We also find that discs form when the central potential is not particularly steep. Our results indicate that the MW assembled its mass and formed its disc earlier than the average galaxy of a similar mass. 

ABSTRACT Anomalously high nitrogen-to-oxygen abundance ratios [N/O] are observed in globular clusters (GCs), among the field stars of the Milky Way (MW), and even in the gas in a z ≈ 11 galaxy. Using data from the APOGEE Data Release 17 and the Gaia Data Release 3, we present several independent lines of evidence that most of the MW’s high-[N/O] stars were born in situ in massive bound clusters during the early, pre-disc evolution of the Galaxy. Specifically, we show that distributions of metallicity [Fe/H], energy, the angular momentum Lz, and distance of the low-metallicity high-[N/O] stars match the corresponding distributions of stars of the Aurora population and of the in situ GCs. We also show that the fraction of in situ field high-[N/O] stars, fN/O, increases rapidly with decreasing metallicity. During epochs when metallicity evolves from $$\rm [Fe/H]=-1.5$$ to $$\rm [Fe/H]=-0.9$$, the Galaxy spins up and transitions from a turbulent Aurora state to a coherently rotating disc. This transformation is accompanied by many qualitative changes. In particular, we show that high N/O abundances similar to those observed in GN-z11 were common before the spin-up ($$\rm [Fe/H]\lesssim -1.5$$) when up to $$\approx 50~{{\ \rm per\ cent}}-70~{{\ \rm per\ cent}}$$ of the in situ stars formed in massive bound clusters. The dramatic drop of fN/O at $$\rm [Fe/H]\gtrsim -0.9$$ indicates that after the disc emerges the fraction of stars forming in massive bound clusters decreases by two orders of magnitude. 

ABSTRACT We use the GRUMPY galaxy formation model based on a suite of zoom-in, high-resolution, dissipationless Λ Cold Dark Matter (ΛCDM) simulations of the Milky Way (MW) sized haloes to examine total matter density within the half-mass radius of stellar distribution, ρtot(< r1/2), of satellite dwarf galaxies around the MW hosts and their mass assembly histories. We compare model results to ρtot(< r1/2) estimates for observed dwarf satellites of the Milky Way spanning their entire luminosity range. We show that observed MW dwarf satellites exhibit a trend of decreasing total matter density within a half-mass radius, ρtot(< r1/2), with increasing stellar mass. This trend is in general agreement with the trend predicted by the model. None of the observed satellites are overly dense compared to the results of our ΛCDM-based model. We also show that although the halo mass of many satellite galaxies is comparable to the halo mass of the MW progenitor at z ≳ 10, at these early epochs halos that survive as satellites to z = 0 are located many virial radii away from the MW progenitors and thus do not have a chance to merge with it. Our results show that neither the densities estimated in observed Milky Way satellites nor their mass assembly histories pose a challenge to the ΛCDM model. In fact, the broad agreement between density trends with the stellar mass of the observed and model galaxies can be considered as yet another success of the model. 

ABSTRACT We present a simple regulator-type framework designed specifically for modelling formation of dwarf galaxies. Despite its simplicity, when coupled with realistic mass accretion histories of haloes from simulations and reasonable choices for model parameter values, the framework can reproduce a remarkably broad range of observed properties of dwarf galaxies over seven orders of magnitude in stellar mass. In particular, we show that the model can simultaneously match observational constraints on the stellar mass–halo mass relation, as well as observed relations between stellar mass and gas phase and stellar metallicities, gas mass, size, and star formation rate, as well as general form and diversity of star formation histories of observed dwarf galaxies. The model can thus be used to predict photometric properties of dwarf galaxies hosted by dark matter haloes in N-body simulations, such as colours, surface brightnesses, and mass-to-light ratios and to forward model observations of dwarf galaxies. We present examples of such modelling and show that colours and surface brightness distributions of model galaxies are in good agreement with observed distributions for dwarfs in recent observational surveys. We also show that in contrast with the common assumption, the absolute magnitude–halo mass relation is generally predicted to have a non-power law form in the dwarf regime, and that the fraction of haloes that host detectable ultra-faint galaxies is sensitive to reionization redshift (zrei) and is predicted to be consistent with observations for zrei ≲ 9. 

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. 

We use a well-motivated galaxy formation framework to predict stellar masses, star formation rates (SFR), and ultraviolet (UV) luminosities of galaxy populations at redshifts $$z\in 5-16$$, taking into account stochasticity of SFR in a controlled manner. We demonstrate that the model can match observational estimates of UV luminosity functions (LFs) at $5<10$ with a modest level of SFR stochasticity, resulting in the scatter of absolute UV luminosity at a given halo mass of $$\sigma_{M_{\rm UV}}\approx 0.75$$. To match the observed UV LFs at $$z\approx 11-13$$ and $$z\approx 16$$ the SFR stochasticity should increase so that $$\sigma_{M_{\rm UV}}\approx 1-1.3$$ and $$\approx 2$$, respectively. Model galaxies at $$z\approx 11-13$$ have stellar masses and SFRs in good agreement with existing measurements. The median fraction of the baryon budget that was converted into stars, $$f_\star$$, is only $$f_\star\approx 0.005-0.05$$, but a small fraction of galaxies at $z=16$ have $$f_\star>1$$ indicating that SFR stochasticity cannot be higher. We discuss several testable consequences of the increased SFR stochasticity at $z>10$. The increase of SFR stochasticity with increasing $$z$$, for example, prevents steepening of UV LF and even results in some flattening of UV LF at $$z\gtrsim 13$$. The median stellar ages of model galaxies at $$z\approx 11-16$$ are predicted to decrease from $$\approx 20-30$$ Myr for $$M_{\rm UV}\gtrsim -21$$ galaxies to $$\approx 5-10$$ Myr for brighter ones. Likewise, the scatter in median stellar age is predicted to decrease with increasing luminosity. The scatter in the ratio of star formation rates averaged over 10 and 100 Myr should increase with redshift. Fluctuations of ionizing flux should increase at $z>10$ resulting in the increasing scatter in the line fluxes and their ratios for the lines sensitive to ionization parameter. 

We analyze high-resolution hydrodynamics simulations of an isolated disk dwarf galaxy with an explicit model for unresolved turbulence and turbulence-based star formation prescription. We examine the characteristic values of the star formation efficiency per free-fall time, ${ϵ}_{\mathrm{f}\mathrm{f}}$, and its variations with local environment properties, such as metallicity, UV flux, and surface density. We show that the star formation efficiency per free-fall time in $\approx 10$pc star-forming regions of the simulated disks has values in the range ${ϵ}_{\mathrm{f}\mathrm{f}}\approx 0.01-0.1$, similar to observational estimates, with no trend with metallicity and only a weak trend with the UV flux. Likewise, estimated using projected patches of 500 pc size does not vary with metallicity and shows only a weak trend with average UV flux and gas surface density. The characteristic values of ${ϵ}_{\mathrm{f}\mathrm{f}}\approx 0.01-0.1$arise naturally in the simulations via the combined effect of dynamical gas compression and ensuing stellar feedback that injects thermal and turbulent energy. The compression and feedback regulate the virial parameter, ${\alpha }_{\mathrm{v}\mathrm{i}\mathrm{r}}$, in star-forming regions, limiting it to ${\alpha }_{\mathrm{v}\mathrm{i}\mathrm{r}}\approx 3-10$. Turbulence plays an important role in the universality of ${ϵ}_{\mathrm{f}\mathrm{f}}$because turbulent energy and its dissipation are not sensitive to metallicity and UV flux that affect thermal energy. Our results indicate that the universality of observational estimates of ${ϵ}_{\mathrm{f}\mathrm{f}}$can be plausibly explained by the turbulence-driven and feedback-regulated properties of star-forming regions. 

We present estimates of the ultraviolet (UV) and Lyman continuum flux density contributed by galaxies of luminosities from to at redshifts $5\le z\le 10$using a galaxy formation model that reproduces properties of local dwarf galaxies down to the luminosities of the ultra-faint satellites. We characterize the UV luminosity function (LF) of galaxies and their abundance as a function of the ionizing photon emission rate predicted by our model and present accurate fitting functions describing them. Although the slope of the LF becomes gradually shallower with decreasing luminosity due to feedback-driven outflows, the UV LF predicted by the model remains quite steep at the luminosities . After reionization, the UV LF flattens at due to UV heating of intergalactic gas. However, before reionization, the slope of the LF remains steep and approximately constant from to . We show that for a constant ionizing photon escape fraction the contribution of faint galaxies with to the UV flux and ionizing photon budget is $\approx 40-60\%$at $z>7$and decreases to $\approx 20\%$at $z=6$. Before reionization, even ultra-faint galaxies of contribute $\approx 10-25\%$of ionizing photons. If the escape fraction increases strongly for fainter galaxies, the contribution of galaxies before reionization increases to $\approx 60-75\%$. Our results imply that dwarf galaxies fainter than , beyond the James Webb Space Telescope limit, contribute significantly to the UV flux density and ionizing photon budget before reionization alleviating requirements on the escape fraction of Lyman continuum photons.