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Feedback to the interstellar medium from ionizing radiation, stellar winds and supernovae is central to regulating star formation in galaxies. Owing to their low mass (<10 9 solar masses), dwarf galaxies are particularly susceptible to such processes, making them ideal sites for studying the detailed physics of feedback. In this Perspective, we summarize the latest observational evidence for feedback from star-forming regions and how this feedback drives the formation of ‘superbubbles’ and galaxy-wide winds. We discuss the important role of external ionizing radiation—reionization—for the smallest galaxies, and the observational evidence that this feedback directly impacts galaxy properties such as their star formation histories, metal contents, colours, sizes, morphologies and even their inner dark matter densities. We conclude with a look to the future, summarizing the key questions that remain to be answered and listing some of the outstanding challenges for galaxy formation theories. This Perspective summarizes the latest observational evidence for star formation feedback and the important role of external ionizing radiation for the smallest galaxies, showing how this feedback directly impacts their properties, including their dark matter distribution.

Dark matter-only simulations of galaxy formation predict many more subhalos around a Milky Way-like galaxy than the number of observed satellites. Proposed solutions require the satellites to inhabit dark matter halos with masses 10 9 –10 10 Msun at the time they fell into the Milky Way. Here we use a modelling approach, independent of cosmological simulations, to obtain a pre-infall mass of Msun for one of the Milky Way’s satellites: Carina. This determination of a low halo mass for Carina can be accommodated within the standard model only if galaxy formation becomes stochastic in halos below ∼10 10 Msun. Otherwise Carina, the eighth most luminous Milky Way dwarf, would be expected to inhabit a significantly more massive halo. The implication of this is that a population of ‘dark dwarfs’ should orbit the Milky Way: halos devoid of stars and yet more massive than many of their visible counterparts. The cold dark matter paradigm predicts that Milky Way-like galaxies should have dwarf galaxies with dark matter halos as satellites. Ural et al. present a new model, independent of cosmological simulations, that constrains the pre-infall mass of the Milky Way satellite Carina to a value lower than expected.

We use spectroscopic observations of Antlia B, a distant ( d ∼ 1.35 Mpc) faint dwarf galaxy ( M V = −9.7, M * =10 5 M ⊙ ), from MUSE-Faint, to explore alternative dark matter (DM) models to Λ CDM, which are one possible explanation for the small-scale problems in the standard model. We measure line-of-sight velocities of 127 stars, and combine these with GravSphere, a Jeans modelling code, to place constraints on DM models and derive the first DM density profiles for this object. We present constraints on the nature of self-interacting DM, in which DM particles can interact with one another in the form of annihilation (we find a core radius of r c ≲ 69 pc) and on scalar field DM, comprised of ultralight bosons that form a Bose-Einstein Condensate (we find a characteristic length scale of the self-interaction of R TF ≲ 180 pc). These results suggest that we can rule out these models as an explanation for the cores in the larger dwarf galaxies in the Local Group.

We study feedback-driven cold dark matter core creation in the EDGE suite of radiation-hydrodynamical dwarf galaxy simulations. Understanding this process is crucial when using observed dwarf galaxies to constrain the particle nature of dark matter. While previous studies have shown the stellar-mass to halo-mass ratio \\((M_{\\star} / M_{200})\\) determines the extent of core creation, we find that in low-mass dwarfs there is a crucial additional effect, namely the timing of star formation relative to reionisation. Sustained post-reionisation star formation decreases central dark matter density through potential fluctuations; conversely, pre-reionisation star formation is too short-lived to have such an effect. In fact, large stellar masses accrued prior to reionisation are a strong indicator of early collapse, and therefore indicative of an increased central dark matter density. We parameterise this differentiated effect by considering \\(M_{\\star,\\mathrm{post}}/M_{\\star,\\mathrm{pre}}\\), where the numerator and denominator represent the amount of star formation after and before \\(z\\sim6.5\\), respectively. Our study covers the halo mass range \\(10^9 < M_{200} < 10^{10} M_\\odot\\) (stellar masses between \\(10^4 < M_{\\star} < 10^8 M_\\odot\\)), spanning both ultra-faint and classical dwarfs. In this regime, \\(M_{\\star,\\mathrm{post}}/M_{\\star,\\mathrm{pre}}\\) correlates almost perfectly with the central dark matter density at \\(z=0\\), even when including simulations with a substantially different variant of feedback and cooling. We provide fitting formulae to describe the newfound dependence.

We introduce a new method to calculate dark matter halo density profiles from simulations. Each particle is 'smeared' over its orbit to obtain a dynamical profile that is averaged over a dynamical time, in contrast to the traditional approach of binning particles based on their instantaneous positions. The dynamical and binned profiles are in good agreement, with the dynamical approach showing a significant reduction in Poisson noise in the innermost regions. We find that the inner cusps of the new dynamical profiles continue inward all the way to the softening radius, reproducing the central density profile of higher resolution simulations within the 95\\(\\%\\) confidence intervals, for haloes in virial equilibrium. Folding in dynamical information thus provides a new approach to improve the precision of dark matter density profiles at small radii, for minimal computational cost. Our technique makes two key assumptions: that the halo is in equilibrium (phase mixed), and that the potential is spherically symmetric. We discuss why the method is successful despite strong violations of spherical symmetry in the centres of haloes, and explore how substructures disturb equilibrium at large radii.

We use the Milky Way's nuclear star cluster (NSC) to test the existence of a dark matter 'soliton core', as predicted in ultra-light dark matter (ULDM) models. Since the soliton core size is proportional to mDM^{-1}, while the core density grows as mDM^{2}, the NSC (dominant stellar component within about 3 pc) is sensitive to a specific window in the dark matter particle mass, mDM. We apply a spherical isotropic Jeans model to fit the NSC line-of-sight velocity dispersion data, assuming priors on the precisely measured Milky Way's supermassive black hole (SMBH) mass and the well-measured NSC density profile. We find that the current observational data reject the existence of a soliton core for a single ULDM particle with mass in the range 10^{-20.4} < mDM < 10^{-18.5} eV, assuming that the soliton core structure is not affected by the Milky Way's SMBH. We test our methodology on mock data, confirming that we are sensitive to the same range in ULDM mass as for the real data. Dynamical modelling of a larger region of the Galactic centre, including the nuclear stellar disc, promises tighter constraints over a broader range of mDM. We will consider this in future work.

The low dark matter density in the Fornax dwarf galaxy is often interpreted as being due to the presence of a constant density `core', but it could also be explained by the effects of Galactic tides. The latter interpretation has been disfavoured because it is apparently inconsistent with the orbital parameters and star formation history of Fornax. We revisit these arguments with the help of the APOSTLE cosmological hydrodynamics simulations. We show that simulated dwarfs with similar properties to Fornax are able to form stars after infall, so that star formation is not necessarily a good tracer of infall time. We also examine the constraints on the pericentre of Fornax and point out that small pericentres (<50 kpc) are not currently ruled out by the data, allowing for Fornax to be tidally influenced on its current orbit. Furthermore, we find that some dwarfs with large orbital pericentres can be stripped prior to infall due to interactions with more massive galaxies. Tidal effects lead to a reduction in the dark matter density, while the profile remains cuspy. Navarro-Frenk-White profiles are consistent with the kinematic data within 3\\(\\sigma\\) in the innermost regions, while profiles with shallow cusps or cores provide a better fit. We predict that if the reduction of the dark matter density in Fornax occurs, at least in part, because of the action of Galactic tides, then tidal tails should be visible with a surface brightness limit of \\(\\sim35-36\\) mag arcsec\\(^{-2}\\) over a survey area of \\(\\gtrsim\\)100 deg\\(^2\\).

Feedback to the interstellar medium (ISM) from ionising radiation, stellar winds and supernovae is central to regulating star formation in galaxies. Due to their low mass (\\(M_{*} < 10^{9}\\)\\,M\\(_\\odot\\)), dwarf galaxies are particularly susceptible to such processes, making them ideal sites to study the detailed physics of feedback. In this perspective, we summarise the latest observational evidences for feedback from star forming regions and how this drives the formation of 'superbubbles' and galaxy-wide winds. We discuss the important role of external ionising radiation -- 'reionisation' -- for the smallest galaxies. And, we discuss the observational evidences that this feedback directly impacts galaxy properties such as their star formation histories, metal content, colours, sizes, morphologies and even their inner dark matter densities. We conclude with a look to the future, summarising the key questions that remain unanswered and listing some of the outstanding challenges for galaxy formation theories.

Scouring by supermassive black hole (SMBH) binaries is the most accepted mechanism for the formation of the cores seen in giant elliptical galaxies. However, an additional mechanism is required to explain the largest observed cores. Gravitational wave (GW) recoil is expected to trigger further growth of the core, as subsequent heating from dynamical friction of the merged SMBH removes stars from the central regions. We model core formation in massive elliptical galaxies from both binary scouring and heating by GW recoil and examine their unique signatures. We aim to determine if the nature of cores in 3D space density can be attributed uniquely to either process and if the magnitude of the kick can be inferred. We perform \\(N\\)-body simulations of galactic mergers of multicomponent galaxies, based on the observed parameters of four massive elliptical galaxies with cores \\(> 0.5\\) kpc. After binary scouring and hardening, the merged SMBH remnant is given a range of GW recoil kicks with \\(0.5\\)-\\(0.9\\) of the escape speed of the galaxy. We find that binary scouring alone can form the cores of NGC 1600 and A2147-BCG, which are \\(< 1.3\\) kpc in size. However, the \\(> 2\\) kpc cores in NGC 6166 and A2261-BCG require heating from GW recoil kicks of \\(< 0.5\\) of the galaxy escape speed. A unique feature of GW recoil heating is flatter cores in surface brightness, corresponding to truly flat cores in 3D space density. It also preferentially removes stars on low angular momentum orbits from the galactic nucleus.

Aims. We use the stellar line-of-sight velocities of Antlia B (Ant B), a faint dwarf galaxy in the NGC 3109 association, to derive constraints on the fundamental properties of scalar field dark matter (SFDM), which was originally proposed to solve the small-scale problems faced by cold dark matter models. Methods. We used the first spectroscopic observations of Ant B, a distant (d \\(\\sim\\) 1.35 Mpc) faint dwarf (\\(M_\\text{V} = -9.7\\), \\(M_\\star \\sim 8\\times10^5\\)M\\(_\\odot\\)), from MUSE-Faint, a survey of ultra-faint dwarfs conducted using the Multi Unit Spectroscopic Explorer. By measuring the line-of-sight velocities of stars in the \\(1'\\times 1'\\) field of view, we identified 127 stars as members of Ant B, which enabled us to model its dark matter density profile with the Jeans modelling code GravSphere. We implemented a model for SFDM into GravSphere and used this to place constraints on the self-coupling strength of this model. Results. We find a virial mass of \\({M_{200} \\approx 1.66^{+2.51}_{-0.92}\\times 10^9}\\) \\(M_\\odot\\) and a concentration parameter of \\({c_{200}\\approx 17.38^{+6.06}_{-4.20}}\\) for Ant B. These results are consistent with the mass-concentration relations in the literature. We constrain the characteristic length scale of the repulsive self-interaction \\(R_{\\text{TF}}\\) of the SFDM model to \\(R_{\\text{TF}} \\lesssim 180\\) pc (\\(68\\%\\) confidence level), which translates to a self-coupling strength of \\(\\frac{g}{m^2c^4}\\lesssim 5.2 \\times 10^{-20}\\) eV\\(^{-1}\\)cm\\(^3\\). The constraint on the characteristic length scale of the repulsive self-interaction is inconsistent with the value required to match observations of the cores of dwarf galaxies in the Local Group, suggesting that the cored density profiles of those galaxies are not caused by SFDM.