In the prevailing theory of hierarchical structure formation the fundamental building block of large-scale structure is the dark matter halo and galaxies form and evolve within these. Yet despite the fundamental importance of dark matter in our theoretical picture of the Universe, we do not even know the shape of dark matter halos even in the closest galaxies to us – ultra-faint dwarfs (UFDs).

The shape of dark matter halos offers a profound test of dark matter (DM) physics. The standard cold dark matter (CDM) model generically predicts (c.f. [1]) that dark matter halos have a centrally cuspy density profile which diverges as 1/r at small radii and that is well-fit with a Navarro-Frenk-White (NFW) profile.

In stark contrast, observations of dwarf galaxies show clear evidence of a central core in classical dwarfs ([1,2]), on the face of it in clear tension with the predictions of CDM. These cores can be explained by outflow related to physical processes in the baryons in the more massive dwarfs, but they could also be a sign of DM differing from canonical CDM ([1,3]). The key to resolving this question and thus also place constraints on the nature of DM, is to study the density profiles of less massive dwarfs – which until now has only been possible for Draco ([4]). With this project we will enlarge the sample of galaxies for which such a study can be done by an order of magnitude, going from 1 to 12.

This lack of understanding of DM halos is also challenging for galaxy formation. This is a many-faceted physical process and while the last decade has led to the development of a broad-brush picture that gives a reasonable description of how galaxies form and develop, these have to resort to sub-grid models for crucial physics which are by no means well understood (e.g. [5]).

A key ingredient of current galaxy formation models is the notion that typical galaxies are equilibrium systems (e.g. [6]), where infall of matter onto the galaxies is on average balanced by “feedback” powered by star formation and nuclear activity. While appealing, many different parameters combinations can reproduce the integrated properties of galaxies; to break this degeneracy we need to consider the resolved properties of galaxies. In particular the distribution of heavy elements in galaxies and its evolution with cosmic time can be a sensitive probe of feedback prescriptions [7]. Unfortunately this has been a challenging quantity to measure well across the wide range of redshift needed to test models for galaxy formation.

At z=0 abundance gradients (Z-gradients) are well studied, and on average galaxies show declining abundance towards the outskirts with little scatter ([8]). At higher z samples are much smaller and the situation much less clear. At z>2 there have been claims of substantial numbers of galaxies having a lower abundance in the central regions than in the outskirts (e.g. [9]) while at 0.1<z<0.8 our=”” pioneering=”” survey=”” shows=”” a=”” mixed=”” picture=”” ([10]).<br=””>
In this project we will provide novel constraints on the DM halos of the faintest galaxies in the Universe, place state-of-the-art constraints on the properties of dark matter and constrain the physics of feedback by constructing a comprehensive census of Z-gradients in galaxies out to z=1.5 and down to a stellar mass of 10^7 Msun (at low redshift), thus bridging into the dwarf regime.

To achieve this we will leverage a 100hr VLT programme (MUSE-Faint, PI Brinchmann), 100+ nights of deep MUSE observations from the MUSE Guaranteed Time Observing (GTO) surveys, and space-based slitless spectra from the Hubble Space Telescope (HST), Euclid, and possibly JWST; the latter two are due to launch in 2022 and 2021 respectively. We build on demonstrated expertise of our team in these areas where we have published state-of-the-art work in recent years as detailed in the literature review.

The MUSE-Faint survey provides deep spectroscopy of UFDs, allowing us to carry out the first survey of density profiles in the lowest mass dark matter halos, building on recent advances in the modelling of small stellar systems – thereby providing novel constraints on DM candidates, as well as offering a detailed look at the results of galaxy formation in a
strongly DM dominated environment. The deep MUSE GTO observations are in areas with extensive HST coverage, most of which already have slitless spectra from HST surveys (e.g. [11]). The best studied region, the HST UDF will also get deep Euclid grism spectra and most likely JWST NIRISS slitless spectra. To exploit this we will develop the first rigorous methodology for combining groundbased IFS data with spatially resolved spectra from space, expanding on our work in [12], and use this to study the evolution of metal gradients in galaxies from z=1.5 to today.

Finally we will start development of the next generation integral field spectrograph (IFS) through our involvement in BlueMUSE, ensuring a fruitful future for the field.</z<0.8>