210 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
The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage
The current velocity of the Smith Cloud indicates that it has undergone at
least one passage of the Galactic disc. Using hydrodynamic simulations we
examine the present day structure of the Smith Cloud. We find that a dark
matter supported cloud is able to reproduce the observed present day neutral
hydrogen mass, column density distribution and morphology. In this case the
dark matter halo becomes elongated, owing to the tidal interaction with the
Galactic disc. Clouds in models neglecting dark matter confinement are
destroyed upon disc passage, unless the initial cloud mass is well in excess of
what is observed today. We then determine integrated flux upper limits to the
gamma-ray emission around such a hypothesised dark matter core in the Smith
Cloud. No statistically significant core or extended gamma-ray emission are
detected down to a 95% confidence level upper limit of ph
cm s in the 1-300 GeV energy range. For the derived distance of
12.4 kpc, the Fermi upper limits set the first tentative constraints on the
dark matter cross sections annihilating into and
for a high-velocity cloud.Comment: 10 pages, 8 figures. Submitted to MNRA
Constraining churning and blurring in the Milky Way using large spectroscopic surveys -- an exploratory study
We have investigated the possibilities to quantify how much stars move in the
Milky Way stellar disk due to diffuse processes (i.e. so called blurring) and
due to influences from spiral arms and the bar (i.e. so called churning). To
this end we assume that it is possible to infer the formation radius of a star
if we know their elemental abundances and age as well as the metallicity
profile of the interstellar medium at the time of the formation of the star.
Using this information, coupled with orbital information derived from Gaia DR2
data and radial velocities from large spectroscopic surveys, we show that it is
possible to isolate stellar samples such that we can start to quantify how
important the role of churning is. We use data from APOGEE DR14, parallaxes
from Gaia and stellar ages based on C and N elemental abundances in the stars.
In our sample, we find that about half of the stars have experienced some sort
of radial migration (based solely on their orbital properties), 10 % have
likely have suffered only from churning, whilst a modest 5-7 % of stars have
never experienced either churning or blurring making them ideal tracers of the
original properties of the cool stellar disk. Our investigation shows that it
is possible to put up a framework where we can begin to quantify churning and
blurring an important. Important aspects for future work would include to
investigate how selection effects should be accounted for.Comment: Paper submitted to MNRAS. Comments are welcom
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
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
Modelling discontinuities and Kelvin-Helmholtz instabilities in SPH
In this paper we discuss the treatment of discontinuities in Smoothed
Particle Hydrodynamics (SPH) simulations. In particular we discuss the
difference between integral and differential representations of the fluid
equations in an SPH context and how this relates to the formulation of dissip
ative terms for the capture of shocks and other discontinuities.
This has important implications for many problems, in particular related to
recently highlighted problems in treating Kelvin-Helmholtz instabilities across
entropy gradients in SPH. The specific problems pointed out by Agertz et al.
(2007) are shown to be related in particular to the (lack of) treatment of
contact discontinuities in standard SPH formulations which can be cured by the
simple application of an artificial thermal conductivity term. We propose a new
formulation of artificial thermal conductivity in SPH which minimises
dissipation away from discontinuities and can therefore be applied quite
generally in SPH calculations.Comment: 31 pages, 10 figures, submitted to J. Comp. Phys. Movies + hires
version available at http://www.astro.ex.ac.uk/people/dprice/pubs/kh/ . v3:
modified as per referee's comments - comparison with Ritchie & Thomas
formulation added, quite a few typos fixed. No major change in metho
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