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Astrophysical constraints on the identity of the dark matter

Periodic Reporting for period 1 - DMIDAS (Astrophysical constraints on the identity of the dark matter)

Reporting period: 2018-10-01 to 2020-03-31

Problem being 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 the dark matter is an elementary particle made
in the early phases of the Big Bang, which is different from ordinary
(or baryonic) matter. The goal of the DMIDAS project is to search for
clues to the identity of the dark matter in astrophysical
objects and phenomena. In particular, 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 astrophysical 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.

Like many research programmes in basic science, the benefits many not be immediate.
Firstly, basic science is the foundation of applied science and technology. This project
relies on advanced massively parallel computing, including the development of novel,
powerful techniques and codes, many transferable to problems in applied
science. Secondly, the project involves the design and application of novel statistical
techniques including machine learning, and these too have applications beyond
astrophysics. 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 as well as use of the state-of-the-art survey
telescope. Both are important technological developments.

Overall objectives

The objectives of DMIDAS are 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 in order to identify excluded and viable candidates;
(iii) predict the signals expected in direct and indirect
experimental searches for dark matter; (iv) make the theoretical
predictions publicly available through mock catalogues tailored to
real surveys. An important byproduct will be a greater understanding
of the physics of galaxy formation on small scales.
The project contains a mixture of technical developments and
scientific applications. On the technical side:

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

(ii) We had a successful test flight for SuperBIT.

(iii) The design, construction and deployment of the DESI survey instrument
at the Mayall telescope in Arizona were all successfully completed in
budget and on time.

On the science side, we have published 50 refereed papers since the start of
the grant, including an extensive review of dark matter subhaloes
(Zavala and Frenk 2019) and a paper soon to be published in Nature
(Wang et al 2020). These papers have already been cited 550 times,
with 6 of them having accrued over 30 citations each since publication in 2019.

Highlights include:

(i) The completion and publication of the Auriga high-resolution
simulations of Milky Way-like galaxies (Grand et al. 2018). These
simulations 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 have been publicly released.

(ii) 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 the development of a
new model for the mass distribution of the Milky Way (Cautun et
al. 2020).

(iii) 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).

(iv) A detailed analysis of the neutral hydrogen rotation curves of
simulated dwarf galaxies applying exactly the same techniques as used in
the analysis of observational data. The conclusion is that the
``diversity of rotation curves'' (which we uncovered in earlier work)
does not necessarily 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).

(v) 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 (the targets of strong gravitational
lensing studies) as they pass through the galactic disc (Richings et
al. 2020) and the possible confusing effects of globular clusters (He
et al. 2018).

(vi) 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).

(vii) A series of dark matter simulations that achieve a dynamic range
of 30 orders of magnitude in mass and resolve the internal structure
of hundreds of Earth-mass haloes. They show that halo density profiles are
universal over the entire mass range and are well described by simple
two-parameter fitting formulae. 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).
The SWIFT cosmological simulations code goes beyond the
state-of-the-art in speed. The Auriga simulations represent the new
state-of-the-art in simulations of Milky Way-like galaxies. The
of simulations of dark matter halos over 30 orders of magnitude in
mass are a breakthrough both from the point of view of techniques and
scientific results. SuperBIT is a unique telescope shown to achieve
the same resolution as the Hubble Space telescope at less than 1
percent of the cost. The DESI survey instrument is the most advanced
of its kind deployed so far in astronomy.

We are on track to achieve the results detailed in the original proposal.
Projected dark matter density map of a cosmological simulation and a zoom showing Earth-mass halos.