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Astrophysical constraints on the identity of the dark matter

Periodic Reporting for period 2 - DMIDAS (Astrophysical constraints on the identity of the dark matter)

Reporting period: 2020-04-01 to 2021-09-30

Problem addressed:

Dark matter makes up most of the mass in the Universe and is responsible for the growth of cosmic structure. Multiple lines of evidence indicate that
it is an elementary particle made in the early phases of the Big Bang, which is different from ordinary (or baryonic) matter. The goal of
DMIDAS is to search for clues to the identity of the dark matter in astrophysical objects. Through a combination of theoretical developments
and astronomical observations, it may possible to rule out the current standard hypothesis for the nature of dark matter, known as ``Cold dark matter'' (or CDM),
or viable alternatives such as `Warm'' or ``Self-interacting'' dark matter.

The project involves studying a range of phenomena, from the gravitational lensing signals of small-mass dark matter halos
to the properties of dwarf galaxies and satellites of large galaxies, all expected to depend on the nature of the dark matter.

Importance for society:

The identity of the dark matter is a fundamental problem whose solution will have major implications for cosmology, astronomy and particle physics.
Problems of this kind have tremendous public appeal and can engage society in major scientific advances, thereby bringing out the power of a rational
approach to problems and the importance of rigorous, systematic thinking.

Basic science is the foundation of applied science and technology but the benefits many not be immediate. This project
relies on advanced massively parallel computing, including the development of novel techniques and codes, many transferable to applied
science. It also involves the design and application of statistical techniques including machine learning, and these too have applications beyond
astrophysics. An unexpected development is that the same statistical techniques that we have developed for the analysis of gravitational
lensing images can be applied to tackle a completely different problem: the treatment of cancer patients.
Finally, the project involves the design and deployment of a novel telescope concept to be operated above the atmosphere on
high-altitude, long-flight balloons. Both are important technological developments.

Objectives:

To: (i) calculate predictions, for different assumptions for the dark matter, of the properties of small-mass halos, both those that
remain dark and those that host galaxies; (ii) test these predictions against new astronomical data; (iii) predict the signals expected
in direct and indirect experimental searches for dark matter; (iv) make the theoretical predictions publicly available.. An important
byproduct will be a greater understanding of the physics of galaxy formation.
The project contains a mixture of technical developments and scientific applications. On the technical side:

1. We have made enormous progress in the development of the SWIFT cosmological simulations code. In
collaboration with colleagues at Leiden, we have nearly completed the implementation of new ``subgrid'' physics
models that make it possible to simulate the small-scale processes that are vital for this project.

2. We had a successful test flight of SuperBIT.

3. The design, construction and deployment of the DESI survey instrumentat the Mayall telescope in Arizona were
all successfully completed in budget and on time.

We have published 73 refereed papers since the start of the project, including an extensive review of dark matter subhaloes
(Zavala and Frenk 2019) and a paper in Nature (Wang et al 2020). These papers have already been cited over 1200 times,
with 16 of them having accrued over 30, and 6 over 50, citations each since publication in 2019.

Highlights include:

1. The completion of the Auriga high-resolution simulations of Milky Way-like galaxies (Grand et al. 2018). These
represent the state-of-the art in the subject and have been used for a variety of studies of the satellites (Bose et
al. 2019, Simpson et al. 2019) and stellar halo (Monachesi et al. 2019, Fattahi et al. 2019, 2020) of the Milky Way. They are
the basis of the AuriGaia stellar mock catalogues of the GAIA DR2 data (Grand et al. 2018), which we have publicly released.

2. The most accurate determination of the mass of the Milky Way to date from a combination of satellite dynamics
using GAIA data and the Auriga simulations (Callingham et al. 2019) and, by fitting physically motivated models to the
Gaia DR2 Galactic rotation curve and other data, of its spatial distribution (Cautun et al. 2020).

3. A detailed analysis, based on a systematic set of targeted cosmological hydrodynamics simulations, of the mechanisms that can
create cores in the centres of cold dark matter haloes by baryonic effects (Benitez-Llambay et al 2019, Bose et al. 2019).

4. An analysis of the neutral hydrogen rotation curves of simulated dwarf galaxies applying the same techniques as for observational data.
The ``diversity of rotation curves'' (which we uncovered in earlier work) does not imply the presence of central cores but,
instead, could be due to non-circular velocities in the gas (Oman et al. 2019, Santos-Santos et al. 2020).

5. A variety of studies relevant to dark matter detection, from the properties of decaying dark matter (Lovell et al. 2019a, 2019b)
through the effects of the galactic disc on the velocity distribution of cold dark matter particles (Bozorgnia et al. 2019a, 2019b) to the
destruction of small subhaloes as they pass through the galactic disc (Richings et al. 2020).

6. Studies of the dynamics and spatial distribution of galactic satellites, including their peculiar arrangement in a
rotating plane of satellites (Riley et al 2019, Callingham et al. 2020, Cautun et al. 2020, Shao et al. 2019, 2020).

7. A dark matter simulation that achieves a dynamic range of 30 orders of magnitude in mass and resolves the internal structure
of Earth-mass haloes. Halo density profiles are universal over the entire mass range and are well described by the Navarro-Frenk-White
formula. Halo mass and concentration are tightly related in a way that depends on cosmology and on the nature
of the dark matter. Small halos contribute about equally (per logarithmic interval) to the dark matter annihilation luminosity,
which we find to be smaller than all previous estimates by factors ranging up to one thousand (Wang et al. 2020).

8. A physical model of the formation of the first galaxies and the prediction that dark matter halos with present-day mass less than 3x10^8 Mo
are all dark, while those with mass above 5×10^9 Mo are all luminous. The results of the model are in excellent agreement with cosmological
hydrodynamic simulations (Benitez-Llambay and Frenk 2020).
The SWIFT code goes beyond the state-of-the-art in speed. The Auriga simulations are the state-of-the-art in simulations of
Milky Way-like galaxies. The simulations of dark matter halos over 30 orders of magnitude in mass are a breakthrough both in
techniques and scientific results. SuperBIT will achieve the resolution of the Hubble Space telescope at less than 1 percent of the cost.

The Covid-19 pandemic affected two elements of the project. The first is the DESI survey which has been delayed by 9 months.
The other is the first science ballon flight of the SuperBIT telescope which we now hope will take place in 2021.
Apart from these delays, the project has proceeded as planned and all our other milestones were achieved and
some surpassed.
Projected dark matter density map of a cosmological simulation and a zoom showing Earth-mass halos.