The DIffuse Galaxy Expansion SignaTures In Various Observables project: understanding the emergence of diffuse, low surface brightness galaxies and the link to their dark matter haloes

Periodic Reporting for period 1 - DIGESTIVO (The DIffuse Galaxy Expansion SignaTures In Various Observables project: understanding the emergence of diffuse, low surface brightness galaxies and the link to their dark matter haloes)

Reporting period: 2018-02-01 to 2020-01-31

The DIGESTIVO project has been a joint theoretical and observational effort alongside theoreticians
(with me, the fellow, as leader), and observers at the IAC to study the evolution and formation of Low Surface Brightness galaxies (LSBs), including the recently
discovered Ultra-Diffuse Galaxies (UDGs).
LSBs are extremely dark matter dominated, faint objects hardly distinguishable from the night sky.
In the last decade it had became clear that large numbers of LSB galaxies exist, opening a new
window on galaxy evolution and formation.
The specific scientific objectives and questions that the ‘DIGESTIVO’ project aimed to answer were:
1. What processes lead to the emergence of such diffuse galaxies? Is the existence of such galaxies
predicted within the current model of galaxy formation, the Λ cold-DM paradigm? How are they
linked to their dark matter haloes?
2. What is the DM and baryons content of low surface brightness galaxies? Do they have gas and
how much? What is their HI gas profile? Is it as extended as the one in simulations? What are their
virial masses? Are they all dwarf-like or is there evidence for some of them being Milky Way-mass
objects?
3. What is the role of environment in forming LSBs and UDGs? Are UDGs only found in dense regions
such as clusters of galaxies?
To answer these questions, I made use of state-of-the-art, sophisticated hydrodynamical numerical
simulations of galaxies, combined with new observational data of LSBs/UDGs,
some of which attained by observers at the host institution (IAC) thanks to new techniques that
pushed the frontier of LSB imaging with optical telescopes.

In this study, led by myself and done in collaboration with theoreticians at the host institutions and
in Abu Dhabi, we investigated for the first time the origin of massive Low Surface Brightness (LSB)
galaxies in hydrodynamical simulations, and explored their formation mechanism: simulated LSBs
form as a result of co-planar co-rotating mergers and aligned accretion of gas at early times, while
perpendicular mergers and mis-aligned gas accretion result in higher SB galaxies by z=0.
Interestingly, the formation scenario of such `classical' LSBs differs from the one of less massive,
Ultra-Diffuse Galaxies, the latter resulting from the effects of SNae driven gas outflows: a stellar
Mass of 109 solar masses thus represents the transition regime between a feedback dominated to
an angular momentum dominated formation scenario in the LSB realm. Observational predictions
were offered regarding spatially resolved star formation rates through LSB discs: these, together
with upcoming surveys, will be used to verify the proposed emergence scenario of LSB galaxies.

On the morphology, rotation and kinematics of UDGs.
In this work, led by Master student Cardona-Barrero under my supervision, and in collaboration
with observers at host institution, we addressed the topic of kinematical properties of UDGs by
analyzing the stellar kinematics of isolated UDGs formed in the hydrodynamical simulation suite
NIHAO. We found that UDGs cover a broad distribution, ranging from dispersion to rotation
supported galaxies, with similar abundances in both regimes. We demonstrated that the alignment
of the infalling baryons into the protogalaxy at early z is the principal driver of the z=0 stellar
kinematic state: pressure supported isolated UDGs form via mis-aligned gas accretion while rotation
supported ones build-up their baryons in an ordered manner

On the dark matter content of diffuse galaxies.
In this work, coauthored by observational and theoretical researchers at the host institution and led by Dr. Trujillo,
we haved carried out a careful analysis of all existing data and showed that for this galaxy, the data
consistently indicated a much shorter distance (13 Mpc) than previously indicated (20 Mpc). With
this revised distance, the galaxy appears to be a rather ordinary low surface brightness galaxy with
plenty of room for dark matter, with a minimum halo mass >109 Msun.

On the stellar populations, age and metallicity of LSBs/UDGs.
In this observational work led by Dr. Ruiz-Lara and colleagues at the host institution, we have performed one of the most
complete characterizations of the stellar component of UDGs to date using deep optical
spectroscopic data from OSIRIS at Gran Telescopio CANARIAS (GTC). We find that their rotation
properties are compatible with dwarf galaxies. We have concluded that the UDGs in our sample are
extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty
SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster.

On the environmental dependence of diffuse galaxies.
This work has been led by theoretical colleagues at The Hebrew University of Jerusalem (Dr.
Fangzhou and Dekel), with me as co-author. We studied ultra-diffuse galaxies (UDGs) in zoom in
cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We found
that while field UDGs arise from dwarfs in a characteristic mass range by multiple episodes of
supernova feedback (as in my original paper Di Cintio et al.2017) group UDGs may also form by
tidal puffing up and they become quiescent by ram-pressure stripping.

Aside from scientific publications, the results of the DIGESTIVO project were disseminated and
presented in 12 seminars, conferences and workshops, and in several outreach activities such as
‘The Researchers Night of Macaronesia’ (La Laguna, Spain, 2018), through virtual visits to spanish
schools during the day of girls and women in science (Habla con Ellas: Mujeres en Astronomía,
2019) and at university fairs, such as the event ‘La ciencias en porciones’, (La Laguna, Spain, 2020).
Furthermore, I have organized 5 international conferences during the fellowship period, 2 of which
directly related to the project, such as ‘The Bewildering Nature of Ultra-diffuse Galaxies’, a fully
funded workshop at Lorentz Center, Leiden, in 2018.

Beyond the state-of-the-art, we made specific predictions for how to use and exploit our results in
the light of future observational surveys. Specifically:
i) accounting for random inclination effects, we predicted that a comprehensive survey will find
nearly half of field UDGs to have rotationally supported stellar disks, when selecting UDGs with
effective radius larger than 1 kpc;
ii) observational predictions were offered regarding spatially resolved star formation rates through
LSB discs: these, together with upcoming surveys, will be used to verify the proposed emergence
scenario of LSB galaxies.

In the 2 years of fellowship I have learnt about observational data, instruments
limitations and capability, while simultaneously offered my expertise in theoretical astrophysics and
galaxy formation and simulations to observers at IAC.
The success of the transfer of knowledge and the vast impact of the project is clear from the large
number of scientific publications that I have co-authored with the host institution colleagues.
This was the first time that a joint theoretical and observational effort on the low surface brightness
universe was developed: ultimately, this project has offered invaluable insights on the formation of
such elusive galaxies and increased our understanding of the role of baryonic feedback in galaxy
formation.

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Periodic Reporting for period 1 - DIGESTIVO (The DIffuse Galaxy Expansion SignaTures In Various Observables project: understanding the emergence of diffuse, low surface brightness galaxies and the link to their dark matter haloes)

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