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I compare the dark matter content within stellar half-mass radius expected in a $$\Lambda$$CDM-based galaxy formation model with existing observational estimates for the observed dwarf satellites of the Milky Way and ultra-diffuse galaxies (UDGs). The model reproduces the main properties and scaling relations of dwarf galaxies, in particular their stellar mass-size relation. I show that the model also reproduces the relation between the dark matter mass within the half-mass radius, $$M_{\rm dm}( 

Although the standard Λ+cold dark matter (ΛCDM) model is well tested on large scales, the primordial power spectrum may deviate from the ΛCDM spectrum on small scales due to specific dark matter properties or alternative inflationary models. These deviations affect the formation of dark matter structure, which subsequently leads to different observable properties of galaxies. In this work, we study the impact of a blue and red tilted power spectrum on the central density of dwarf galaxies. To do this, we model densities of dwarf galaxies using a combination of high-resolution numerical simulations and galaxy formation model. The model galaxies in ΛCDM are consistent with observations of 41 faint dwarf satellite galaxies of the Milky Way. The deviations from the ΛCDM power spectrum are constrained by the central matter densities of dwarf galaxies, which set stringent constraints on the possible small-scale tilt of the primordial power spectrum, improving on the current limits. Moreover, similar analysis can be applied to test any feature in the power spectrum at small scales between k∼10–100 Mpc-1. 

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. 

We present estimates of the ultraviolet (UV) and Lyman continuum flux density contributed by galaxies of luminosities from to at redshifts 5≤z≤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 ≈40−60% at z>7 and decreases to ≈20% at z=6. Before reionization, even ultra-faint galaxies of contribute ≈10−25% of ionizing photons. If the escape fraction increases strongly for fainter galaxies, the contribution of galaxies before reionization increases to ≈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. 

We investigate the impact of bursty star formation on several galaxy scaling relations of dwarf galaxies using the $$\texttt{GRUMPY}$$ galaxy formation model. While this model reproduces the star formation rate (SFR)-stellar mass, stellar mass-gas mass, and stellar mass-metallicity relations, the scatter of these relations in the original model is smaller than observed. We explore the effects of additional stochasticity of SFR on the scaling relations using a model that reproduces the level of SFR burstiness in high-resolution zoom-in simulations. The additional SFR stochasticity increases the scatter in the SFR-stellar mass relation to a level similar to that exhibited by most nearby dwarf galaxies. The most extreme observed starbursting dwarfs, however, require higher levels of SFR stochasticity. We find that bursty star formation increases the scatter in the colour-magnitude distribution (CMD) for brighter dwarf galaxies $$(M_V < -12)$$ to the observed level, but not for fainter ones for which scatter remains significantly smaller than observed. This is due to the predominant old stellar populations in these faint model galaxies and their generally declining SFR over the past 10 Gyrs, rather than quenching caused by reionization. We examine the possibility that the colour scatter is due to scatter in metallicity, but show that the level of scatter required leads to an overestimation of scatter in the metallicity-mass relation. This illustrates that the scatter of observed scaling relations in the dwarf galaxy regime represents a powerful constraint on the properties of their star formation. 

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. 

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. 

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 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.