158 research outputs found
The stellar mass-halo mass relation of isolated field dwarfs: a critical test of CDM at the edge of galaxy formation
We fit the rotation curves of isolated dwarf galaxies to directly measure the
stellar mass-halo mass relation () over the mass range . By accounting for cusp-core
transformations due to stellar feedback, we find a monotonic relation with
little scatter. Such monotonicity implies that abundance matching should yield
a similar if the cosmological model is correct. Using the 'field
galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the
halo mass function from the Cold Dark Matter Bolshoi simulation, we
find remarkable agreement between the two. This holds down to M, and to M if we
assume a power law extrapolation of the SDSS stellar mass function below M.
However, if instead of SDSS we use the stellar mass function of nearby galaxy
groups, then the agreement is poor. This occurs because the group stellar mass
function is shallower than that of the field below M,
recovering the familiar 'missing satellites' and 'too big to fail' problems.
Our result demonstrates that both problems are confined to group environments
and must, therefore, owe to 'galaxy formation physics' rather than exotic
cosmology.
Finally, we repeat our analysis for a Warm Dark Matter cosmology,
finding that it fails at 68% confidence for a thermal relic mass of keV, and keV if we use the power law extrapolation
of SDSS. We conclude by making a number of predictions for future surveys based
on these results.Comment: 22 pages; 2 Tables; 10 Figures. This is the version accepted for
publication in MNRAS. Key changes: (i) added substantially more information
on the surveys used to measure the stellar mass functions; (ii) added tests
of the robustness of our results. Results and conclusions unchanged from
previously. Minor typos corrected from previous versio
Dark matter cores all the way down
We use high resolution simulations of isolated dwarf galaxies to study the
physics of dark matter cusp-core transformations at the edge of galaxy
formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell
size; ~250 Msun per particle) at which the impact from individual supernovae
explosions can be resolved, becoming insensitive to even large changes in our
numerical 'sub-grid' parameters. We find that our dwarf galaxies give a
remarkable match to the stellar light profile; star formation history;
metallicity distribution function; and star/gas kinematics of isolated dwarf
irregular galaxies. Our key result is that dark matter cores of size comparable
to the stellar half mass radius (r_1/2) always form if star formation proceeds
for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun
and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter
'coreNFW' fitting function that captures this dark matter core growth as a
function of star formation time and the projected stellar half mass radius.
Our results have several implications: (i) we make a strong prediction that
if LCDM is correct, then 'pristine' dark matter cusps will be found either in
systems that have truncated star formation and/or at radii r > r_1/2; (ii)
complete core formation lowers the projected velocity dispersion at r_1/2 by a
factor ~2, which is sufficient to fully explain the 'too big to fail problem';
and (iii) cored dwarfs will be much more susceptible to tides, leading to a
dramatic scouring of the subhalo mass function inside galaxies and groups.Comment: 20 pages; 9 figures; final version to appear in MNRAS including typos
corrected in proo
Resolving mixing in smoothed particle hydrodynamics
Standard formulations of smoothed particle hydrodynamics (SPH) are unable to resolve mixing at fluid boundaries. We use an error and stability analysis of the generalized SPH equations of motion to prove that this is due to two distinct problems. The first is a leading order error in the momentum equation. This should decrease with an increasing neighbour number, but does not because numerical instabilities cause the kernel to be irregularly sampled. We identify two important instabilities: the clumping instability and the banding instability, and we show that both are cured by a suitable choice of kernel. The second problem is the local mixing instability (LMI). This occurs as particles attempt to mix on the kernel scale, but are unable to due to entropy conservation. The result is a pressure discontinuity at boundaries that pushes fluids of different entropies apart. We cure the LMI by using a weighted density estimate that ensures that pressures are single-valued throughout the flow. This also gives a better volume estimate for the particles, reducing errors in the continuity and momentum equations. We demonstrate mixing in our new optimized smoothed particle hydrodynamics (OSPH) scheme using a Kelvin-Helmholtz instability (KHI) test with a density contrast of 1:2, and the ‘blob test'- a 1:10 density ratio gas sphere in a wind tunnel - finding excellent agreement between OSPH and Eulerian code
The roles of stellar feedback and galactic environment in star-forming molecular clouds
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Feedback from massive stars is thought to play an important role in the evolution of
molecular clouds. In this work we analyse the effects of stellar winds and supernovae
(SNe) in the evolution of two massive (∼ 106 M ) giant molecular clouds (GMCs): one
gravitationally bound collapsing cloud and one unbound cloud undergoing disruption
by galactic shear. These two clouds have been extracted from a large scale galaxy
model and are re-simulated at a spatial resolution of ∼ 0.01 pc, including feedback
from winds, SNe, and the combined effect of both. We find that stellar winds stop
accretion of gas onto sink particles, and can also trigger star formation in the shells
formed by the winds, although the overall effect is to reduce the global star formation
rate of both clouds. Furthermore, we observe that winds tend to escape through the
corridors of diffuse gas. The effect of SNe is not so prominent and the star formation
rate is similar to models neglecting stellar feedback. We find that most of the energy
injected by the SNe is radiated away, but overdense areas are created by multiple
and concurrent SN events especially in the most virialised cloud. Our results suggest
that the impact of stellar feedback is sensitive to the morphology of star forming
clouds, which is set by large scale galactic flows, being of greater importance in clouds
undergoing gravitational collapse.The calculations for this paper were performed on the supercomputer
at Exeter, which is jointly funded by STFC, the Large
Facilities Capital Fund of BIS and the University of Exeter. RRR
and CLD acknowledge funding from the European Research Council
for the FP7 ERC starting grant project LOCALSTAR. OA and
RRR would like to acknowledge support from STFC consolidated
grant ST/M000990/1. Figs 1, 2, 5, and 3 were produced using SPLASH
(Price 2007)
Thin, thick and dark discs in ΛCDM
In a Λ cold dark matter (ΛCDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with vmax > 80 km s−1; seven with vmax > 60 km s−1 and 15 with vmax > 40 km s−1 merge at redshift z≳ 1. Assuming isotropic accretion, a third of these merge at an impact angle θ 20° are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax≳ 80 km s−1) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ∼50-90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z= 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a ΛCDM cosmolog
The Source of Ionization along the Magellanic Stream
Since its discovery in 1996, the source of the bright H-alpha emission (up to
750 mR) along the Magellanic Stream has remained a mystery. There is no
evidence of ionising stars within the HI stream, and the extended hot halo is
far too tenuous to drive strong shocks into the clouds. We now present a
hydrodynamical model that explains the known properties of the H-alpha emission
and provides new insights on the lifetime of the Stream clouds. The upstream
clouds are gradually disrupted due to their interaction with the hot halo gas.
The clouds that follow plough into gas ablated from the upstream clouds,
leading to shock ionisation at the leading edges of the downstream clouds.
Since the following clouds also experience ablation, and weaker H-alpha
(100-200 mR) is quite extensive, a disruptive cascade must be operating along
much of the Stream. In our model, the clouds are evolving on timescales of
100-200 Myr, such that the Stream must be replenished by the Magellanic Clouds
at a fairly constant rate. The ablated material falls onto the Galaxy as a warm
drizzle which suggests that diffuse ionized gas at 10**4 K may be an important
constituent of galactic accretion. The observed HI emission provides a new
constraint on the rate of disruption of the Stream and, consequently, the
infall rate of metal-poor gas onto the Galaxy. When the ionized component of
the Stream is fully accounted for, the rate of gas accretion is 0.4 Msun/yr,
roughly twice the rate deduced from HI observations alone.Comment: 5 pages, 4 figures; high quality preprint and simulations available
at http://www.aao.gov.au/astro/M
The rapid onset of stellar bars in the baryon-dominated centers of disk galaxies
Recent observations of high-redshift galactic disks () show a
strong negative trend in the dark matter fraction with increasing
baryonic surface density. For this to be true, the inner baryons must dominate
over dark matter in early massive galaxies, as observed in the Milky Way today.
If disks are dominant at early times, we show that stellar bars form promptly
within these disks, leading to a high bar fraction at early times. New JWST
observations provide the best evidence to date for mature stellar bars in this
redshift range. The disk mass fraction within is
the dominant factor in determining how rapidly a bar forms. Using 3D hydro
simulations of halo-disk-bulge galaxies, we confirm the "Fujii relation" for
the exponential dependence of the bar formation time as a function
of . For , the bar formation time declines
exponentially fast with increasing . This relation is a challenge to
simulators - barred models with inadequate resolution fall off this curve.
Instead of Fujii's arbitrary threshold for when a bar forms, for the first
time, we exploit the exponential growth timescale associated with a positive
feedback cycle as the bar emerges from the underlying disk. A modified,
mass-dependent trend is observed for halos relevant to systems at cosmic noon
(), where the bar onset is slower for higher mass
halos at a fixed . If baryons dominate over dark matter within , we predict that a high fraction of bars will be found in
high-redshift disks long before . Due to its widespread use in
simulations, we investigate the Efstathiou-Lake-Negroponte criterion for bar
instability: this sub-optimal parameter is inversely related to ,
with a secondary dependence on .Comment: 27 pages, 8 figures, 1 table - Astrophysical Journal, accepted (9
March 2023
Environmental regulation of cloud and star formation in galactic bars
The strong time-dependence of the dynamics of galactic bars yields a complex
and rapidly evolving distribution of dense gas and star forming regions.
Although bars mainly host regions void of any star formation activity, their
extremities can gather the physical conditions for the formation of molecular
complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical
simulation of a Milky Way-like galaxy, we probe these conditions to explore how
and where bar (hydro-)dynamics favours the formation or destruction of
molecular clouds and stars. The interplay between the kpc-scale dynamics (gas
flows, shear) and the parsec-scale (turbulence) is key to this problem. We find
a strong dichotomy between the leading and trailing sides of the bar, in term
of cloud fragmentation and in the age distribution of the young stars. After
orbiting along the bar edge, these young structures slow down at the
extremities of the bar, where orbital crowding increases the probability of
cloud-cloud collision. We find that such events increase the Mach number of the
cloud, leading to an enhanced star formation efficiency and finally the
formation of massive stellar associations, in a fashion similar to
galaxy-galaxy interactions. We highlight the role of bar dynamics in decoupling
young stars from the clouds in which they form, and discuss the implications on
the injection of feedback into the interstellar medium, in particular in the
context of galaxy formation.Comment: MNRAS accepte
EDGE: A new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation
We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological
galaxy formation, and study its impact on a simulated dwarf galaxy as part of the ‘EDGE’ project. The target galaxy has a
maximum circular velocity of 21 km s−1 but evolves in a region that is moving at up to 90 km s−1 relative to the hydrodynamic
grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held
in the circumgalactic medium around the galaxy for 320 Myr, delaying the onset of star formation until cooling and collapse
eventually triggers an initial starburst at z = 9. Using genetic modification, we produce ‘velocity-zeroed’ initial conditions in
which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale
structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation
starting at z = 17. While the final stellar masses are nearly consistent (4.8 × 106 M and 4.4 × 106 M for unmodified and
velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due
to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation
simulations, and is especially recommended for those of small galaxies at high redshif
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