Hydrodynamical cosmological simulations are increasing their level of realism by considering more physical processes, having more resolution or larger statistics. However, one usually has to either sacrifice the statistical power of such simulations or the resolution reach within galaxies. Here, we introduce the NewHorizon project where a zoom-in region of ∼(16Mpc)^3, larger than a standard zoom-in region around a single halo, embedded in a larger box is simulated at high resolution. A resolution of up to 34 pc, typical of individual zoom-in resimulated halos is reached within galaxies, allowing the simulation to capture the multi-phase nature of the interstellar medium and the clumpy nature of the star formation process in galaxies. In this introductory paper, we present several key fundamental properties of galaxies and of their black holes including the galaxy mass function, the cosmic star formation rate, the galactic metallicities, the Kennicutt-Schmidt relation, the stellar-to-halo mass relation, the galaxy sizes, their stellar kinematics and morphology, the gas content within galaxies and its kinematics, and the black hole mass and spin properties over time. The various scaling relations are broadly reproduced by NewHorizon with some differences with the standard observables. Due to its exquisite spatial resolution, NewHorizon captures the inefficient process of star formation in galaxies, which evolve over time from being more turbulent, gas-rich and star-bursting at high redshift. These high redshift galaxies are also more compact, and are more elliptical and clumpier until the level of internal gas turbulence decays enough to allow for the formation of discs. The NewHorizon simulation gives access to a broad range of galaxy physics at low-to-intermediate stellar masses, a regime that will become accessible in the near future through surveys such as the LSST.
Publications:
The origin of the disk and spheroid of
galaxies has been a key open question in understanding their
morphology. Using the high-resolution cosmological simulation New
Horizon, we explore kinematically decomposed disk and spheroidal
components of 144 field galaxies with masses greater than {10}9
{M}☉ at z = 0.7. The origins of stellar particles are classified
according to their birthplace (in situ or ex situ) and their
orbits at birth. Before disk settling, stars form mainly through
chaotic mergers between protogalaxies and become part of the
spheroidal component. When disk settling starts, we find that more
massive galaxies begin to form disk stars from earlier epochs;
massive galaxies commence to develop their disks at z ∼ 1-2, while
low-mass galaxies do after z ∼ 1. The formation of disks is
affected by accretion as well, as mergers can trigger gas
turbulence or induce misaligned gas infall that hinders galaxies
from forming corotating disk stars. The importance of accreted
stars is greater in more massive galaxies, especially in
developing massive spheroids. A significant fraction of the
spheroids come from the disk stars that are perturbed, and this
becomes more important at lower redshifts. Some (∼12.5%) of our
massive galaxies develop counter-rotating disks from the gas
infall misaligned with the existing disk plane, which can last for
more than a gigayear until they become the dominant component and
flip the angular momentum of the galaxy in the opposite direction.
The final disk-to-total ratio of a galaxy needs to be understood
in relation to its stellar mass and accretion history. We quantify
the significance of the stars with different origins and provide
them as guiding values.
Massive black hole
(MBH) coalescences are powerful sources of low-frequency
gravitational waves. To study these events in the cosmological
context we need to trace the large-scale structure and cosmic
evolution of a statistical population of galaxies, from dim
dwarfs to bright galaxies. To cover such a large range of galaxy
masses, we analyse two complementary simulations: Horizon-AGN
with a large volume and low resolution which tracks the
high-mass (> 1e7 Msun) MBH population, and NewHorizon with a
smaller volume but higher resolution that traces the low-mass
(< 1e7 Msun) MBH population. While Horizon-AGN can be used to
estimate the rate of inspirals for Pulsar Timing Arrays,
NewHorizon can investigate MBH mergers in a statistical sample
of dwarf galaxies for LISA, which is sensitive to low-mass MBHs.
After the numerical MBH merger at the resolution limit, we
post-process MBH dynamics to account for time delays mostly
determined by dynamical friction and stellar hardening. In both
simulations, MBHs typically merge long after the galaxies do, so
that the galaxy morphology at the time of the MBH merger is no
longer determined by the galaxy merger from which the MBH merger
originated. These time delays cause a loss of high-z MBH
coalescences, shifting the peak of the MBH merger rate to z~1-2.
This study shows how tracking MBH mergers in low-mass galaxies
is crucial to probing the MBH merger rate for LISA and
investigate the properties of the host galaxies.
Dwarf galaxies (M*<10^9 Msun) are key
drivers of mass assembly in high mass galaxies, but relatively
little is understood about the assembly of dwarf galaxies
themselves. Using the New Horizon cosmological simulation (40 pc
spatial resolution), we investigate how mergers and fly-bys drive
the mass assembly and structural evolution of around 1000 field
and group dwarfs up to z=0.5. We find that, while dwarf galaxies
often exhibit disturbed morphologies (30 and 50 per cent are
disturbed at z=1 and z=3 respectively), only a small proportion of
the morphological disturbances seen in dwarf galaxies are driven
by mergers at any redshift (for 10^9 Msun, mergers drive only 20
per cent morphological disturbances). They are instead primarily
the result of interactions that do not end in a merger
(e.g. fly-bys). Given the large fraction of apparently
morphologically disturbed dwarf galaxies which are not, in fact,
merging, this finding is particularly important to future studies
identifying dwarf mergers and post-mergers morphologically at
intermediate and high redshifts. Dwarfs typically undergo one
major and one minor merger between z=5 and z=0.5, accounting for
10 per cent of their total stellar mass. Mergers can also drive
moderate star formation enhancements at lower redshifts (up to 5
times at z=1), but this only accounts for 5 per cent of stellar
mass in the dwarf regime given their infrequency. Non-merger
interactions drive significantly smaller star formation
enhancements (around two times), but their preponderance relative
to mergers means they account for 10 per cent of stellar mass in
the dwarf regime.
Low-surface-brightness galaxies (LSBGs) -- defined as systems that are fainter than the surface-brightness limits of past wide-area surveys -- form the overwhelming majority of galaxies in the dwarf regime (M* < 10^9 MSun). Using New Horizon, a high-resolution cosmological simulation, we study the origin of LSBGs and explain why LSBGs at similar stellar mass show the large observed spread in surface brightness. New Horizon galaxies populate a well-defined locus in the surface brightness -- stellar mass plane, with a spread of ~3 mag arcsec^-2, in agreement with deep SDSS Stripe data. Galaxies with fainter surface brightnesses today are born in regions of higher dark-matter density. This results in faster gas accretion and more intense star formation at early epochs. The stronger resultant supernova feedback flattens gas profiles at a faster rate which, in turn, creates shallower stellar profiles (i.e. more diffuse systems) more rapidly. As star formation declines towards late epochs (z<1), the larger tidal perturbations and ram pressure experienced by these systems (due to their denser local environments) accelerate the divergence in surface brightness, by increasing their effective radii and reducing star formation respectively. A small minority of dwarfs depart from the main locus towards high surface brightnesses, making them detectable in past wide surveys. These systems have anomalously high star-formation rates, triggered by recent, fly-by or merger-driven starbursts. We note that objects considered extreme/anomalous at the depth of current datasets, e.g. `ultra-diffuse galaxies', actually dominate the predicted dwarf population and will be routinely visible in future surveys like LSST.
Ever since the thick disk was proposed to explain the vertical distribution of the Milky Way disk
stars, its origin has been a recurrent question. We aim to answer this question by inspecting 19
disk galaxies with stellar mass greater than 10^10 M in recent cosmological high-resolution zoom-in
simulations: Galactica and NewHorizon. The thin and thick disks are reproduced by the simulations
with scale heights and luminosity ratios that are in reasonable agreement with observations. When we
spatially classify the disk stars into thin and thick disks by their heights from the galactic plane, the
“thick” disk stars are older, less metal-rich, kinematically hotter, and higher in accreted star fraction
than the “thin” disk counterparts. However, both disks are dominated by stellar particles formed in
situ. We find that approximately half of the in-situ stars in the thick disks are formed even before the
galaxies develop their disks, and the other half are formed in spatially and kinematically thinner disks
and then thickened with time by heating. We thus conclude from our simulations that the thin and
thick disk components are not entirely distinct in terms of formation processes, but rather markers
of the evolution of galactic disks. Moreover, as the combined result of the thickening of the existing
disk stars and the continued formation of young thin-disk stars, the vertical distribution of stars does
not change much after the disks settle, pointing to the modulation of both orbital diffusion and star
formation by the same confounding factor: the proximity of galaxies to marginal stability.
In the standard Lambda-CDM paradigm, dwarf galaxies are expected to be dark-matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter contents appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark-matter deficient, even though their initial dark-matter fractions are normal. The process of dark matter stripping is responsible for the large scatter in the stellar-to-halo mass relation in the dwarf regime. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs which exhibit stellar-to-halo mass ratios as low as unity, consistent with the findings of recent observational studies. Given their close orbits, a significant fraction of DM deficient dwarfs merge with their massive companions (e.g. ~70 per cent merge over timescales of ~3.5 Gyrs), with the DM deficient population being constantly replenished by new interactions between dwarfs and massive companions. The creation of these galaxies is, therefore, a natural by-product of galaxy evolution and the existence of these systems is not in tension with the standard paradigm..