6,580 research outputs found
EDGE: The sensitivity of ultra-faint dwarfs to a metallicity-dependent initial mass function
Motivated by the observed bottom-light initial mass function (IMF) in faint dwarfs, we study how a metallicity-dependent IMF affects the feedback budget and observables of an ultra-faint dwarf galaxy. We model the evolution of a low-mass (≈8×108M⊙) dark matter halo with cosmological, zoomed hydrodynamical simulations capable of resolving individual supernovae explosions, which we complement with an empirically motivated subgrid prescription for systematic IMF variations. In this framework, at the low gas metallicities typical of faint dwarfs, the IMF of newborn stellar populations becomes top-heavy, increasing the efficiency of supernova and photoionization feedback in regulating star formation. This results in a 100-fold reduction of the final stellar mass of the dwarf compared to a canonical IMF, at fixed dynamical mass. The increase in the feedback budget is none the less met by increased metal production from more numerous massive stars, leading to nearly constant iron content at z = 0. A metallicity-dependent IMF therefore provides a mechanism to produce low-mass (M⋆∼103M⊙), yet enriched ([Fe/H]≈−2) field dwarf galaxies, thus opening a self-consistent avenue to populate the plateau in [Fe/H] at the faintest end of the mass–metallicity relation
VINTERGATAN III: how to reset the metallicity of the Milky Way
Using the cosmological zoom simulation VINTERGATAN, we present a new scenario
for the onset of star formation at the metal-poor end of the low-[/Fe]
sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by
two distinct gas flows. One is enriched by outflows from massive galaxies, but
not the other. While the former feeds the inner galactic region, the latter
fuels an outer gas disk, inclined with respect to the main galactic plane, and
with a significantly poorer chemical content. The first passage of the last
major merger galaxy triggers tidal compression in the outer disk, which
increases the gas density and eventually leads to star formation, at a
metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of
the low-[/Fe] sequence. These in situ stars have halo-like kinematics,
similarly to what is observed in the Milky Way, due to the inclination of the
outer disk which eventually aligns with the inner one via gravitational
torques. We show that this tilting disk scenario is likely to be common in
Milky-Way like galaxies. This process implies that the low-[/Fe]
sequence is populated in situ, simultaneously from two formation channels, in
the inner and the outer galaxy, with distinct metallicities. This contrasts
with purely sequential scenarios for the assembly of the Milky Way disk and
could be tested observationally.Comment: MNRAS in press. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
EDGE: The sensitivity of ultra-faint dwarfs to a metallicity-dependent initial mass function
We study how an observationally-motivated, metallicity-dependent initial mass
function (IMF) affects the feedback budget and observables of an ultra-faint
dwarf galaxy. We model the evolution of a low-mass () dark matter halo with cosmological, zoomed
hydrodynamical simulations capable of resolving individual supernovae
explosions. We complement the EDGE galaxy formation model from Agertz et al.
(2020) with a new prescription for IMF variations according to Geha et al.
(2013). At the low metallicities typical of faint dwarf galaxies, the IMF
becomes top-heavy, increasing the efficiency of supernova and photo-ionization
feedback in regulating star formation. This results in a 100-fold reduction of
the final stellar mass of the dwarf compared to a canonical IMF, at fixed
dynamical mass. The increase in the feedback budget is nonetheless met by
increased metal production from more numerous massive stars, leading to nearly
constant iron content at . A metallicity-dependent IMF therefore provides
a mechanism to produce low-mass (), yet
enriched () field dwarf galaxies, thus opening a
self-consistent avenue to populate the plateau in at the faintest
end of the mass-metallicity relation.Comment: Main text 7 pages. Submitted to MNRAS, comments welcom
VINTERGATAN II: the history of the Milky Way told by its mergers
Using the VINTERGATAN cosmological zoom simulation, we explore the
contributions of the in situ and accreted material, and the effect of galaxy
interactions and mergers in the assembly of a Milky Way-like galaxy. We find
that the initial growth phase of galaxy evolution, dominated by repeated major
mergers, provides the necessary physical conditions for the assembly of a
thick, kinematically hot disk populated by high-[/Fe] stars, formed
both in situ and in accreted satellite galaxies. We find that the diversity of
evolutionary tracks followed by the simulated galaxy and its progenitors leads
to very little overlap of the in situ and accreted populations for any given
chemical composition. At a given age, the spread in [/Fe] abundance
ratio results from the diversity of physical conditions in VINTERGATAN and its
satellites, with an enhancement in [/Fe] found in stars formed during
starburst episodes. Later, the cessation of the merger activity promotes the in
situ formation of stars in the low-[/Fe] regime, in a radially
extended, thin and overall kinematically colder disk, thus establishing
chemically bimodal thin and thick disks, in line with observations. We draw
links between notable features in the [Fe/H] - [/Fe] plane with their
physical causes, and propose a comprehensive formation scenario explaining
self-consistently, in the cosmological context, the main observed properties of
the Milky Way.Comment: MNRAS in press. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
EDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses
We demonstrate how the least luminous galaxies in the Universe, ultra-faint
dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic
reionization. We select a low-mass () dark matter halo from a cosmological volume, and perform
zoom hydrodynamical simulations with multiple alternative histories using
"genetically modified" initial conditions. Earlier forming ultra-faints have
higher stellar mass today, due to a longer period of star formation before
their quenching by reionization. Our histories all converge to the same final
dynamical mass, demonstrating the existence of extended scatter ( 1 dex)
in stellar masses at fixed halo mass due to the diversity of possible
histories. One of our variants builds less than 2 % of its final dynamical mass
before reionization, rapidly quenching in-situ star formation. The bulk of its
final stellar mass is later grown by dry mergers, depositing stars in the
galaxy's outskirts and hence expanding its effective radius. This mechanism
constitutes a new formation scenario for highly diffuse (, ), metal-poor (), ultra-faint ()
dwarf galaxies within the reach of next-generation low surface brightness
surveys.Comment: Minor edits to match the published ApJL version. Results unchange
VINTERGATAN I: The origins of chemically, kinematically and structurally distinct discs in a simulated Milky Way-mass galaxy
Spectroscopic surveys of the Milky Way's stars have revealed spatial,
chemical and kinematical structures that encode its history. In this work, we
study their origins using a cosmological zoom simulation, VINTERGATAN, of a
Milky Way-mass disc galaxy. We find that in connection to the last major merger
at , cosmological accretion leads to the rapid formation of an
outer, metal-poor, low-[/Fe] gas disc around the inner, metal-rich
galaxy containing the old high-[/Fe] stars. This event leads to a
bimodality in [/Fe] over a range of [Fe/H]. A detailed analysis of how
the galaxy evolves since is presented. We demonstrate the way in
which inside-out growth shapes the radial surface density and metallicity
profile and how radial migration preferentially relocates stars from the inner
to the outer disc. Secular disc heating is found to give rise to increasing
velocity dispersions and scaleheights with stellar age, which together with
disc flaring explains several trends observed in the Milky Way, including
shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy
formation scenario imprints non-trivial mappings between structural
associations (i.e. thick and thin discs), velocity dispersions,
-enhancements, and ages of stars, e.g. the most metal-poor stars in the
low-[/Fe] sequence are found to have a scaleheight comparable to old
high-[/Fe] stars. Finally, we illustrate how at low spatial resolution,
comparable to the thickness of the galaxy, the proposed pathway to distinct
sequences in [/Fe]-[Fe/H] cannot be captured.Comment: 20 pages, MNRAS submitted, comments welcome. Movies available at
http://www.astro.lu.se/~florent/vintergatan.ph
Exact S-matrices for supersymmetric sigma models and the Potts model
We study the algebraic formulation of exact factorizable S-matrices for
integrable two-dimensional field theories. We show that different formulations
of the S-matrices for the Potts field theory are essentially equivalent, in the
sense that they can be expressed in the same way as elements of the
Temperley-Lieb algebra, in various representations. This enables us to
construct the S-matrices for certain nonlinear sigma models that are invariant
under the Lie ``supersymmetry'' algebras sl(m+n|n) (m=1,2; n>0), both for the
bulk and for the boundary, simply by using another representation of the same
algebra. These S-matrices represent the perturbation of the conformal theory at
theta=pi by a small change in the topological angle theta. The m=1, n=1 theory
has applications to the spin quantum Hall transition in disordered fermion
systems. We also find S-matrices describing the flow from weak to strong
coupling, both for theta=0 and theta=pi, in certain other supersymmetric sigma
models.Comment: 32 pages, 8 figure
EDGE: The shape of dark matter haloes in the faintest galaxies
Collisionless Dark Matter Only (DMO) structure formation simulations predict
that Dark Matter (DM) haloes are prolate in their centres and triaxial towards
their outskirts. The addition of gas condensation transforms the central DM
shape to be rounder and more oblate. It is not clear, however, whether such
shape transformations occur in `ultra-faint' dwarfs, which have extremely low
baryon fractions. We present the first study of the shape and velocity
anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of
. These dwarfs are drawn from the
Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high
resolution simulations that allow us to resolve DM halo shapes within the half
light radius (pc). We show that gas-poor ultra-faints (M; ) retain
their pristine prolate DM halo shape even when gas, star formation and feedback
are included. This could provide a new and robust test of DM models. By
contrast, gas-rich ultra-faints (M;
) become rounder and more oblate within half
light radii. Finally, we find that most of our simulated dwarfs have
significant radial velocity anisotropy that rises to at
. The one exception is a dwarf that forms a rotating
gas/stellar disc because of a planar, major merger. Such strong anisotropy
should be taken into account when building mass models of gas-poor
ultra-faints.Comment: 16 pages and 11 figures (excluding appendices), accepted by MNRA
Truthmakers and modality
This paper attempts to locate, within an actualist ontology, truthmakers for modal truths: truths of the form or . In section 1 I motivate the demand for substantial truthmakers for modal truths. In section 2 I criticise Armstrong’s account of truthmakers for modal truths. In section 3 I examine essentialism and defend an account of what makes essentialist attributions true, but I argue that this does not solve the problem of modal truth in general. In section 4 I discuss, and dismiss, a theistic account of the source of modal truth proposed by Alexander Pruss. In section 5 I offer a means of (dis)solving the problem
- …