5 research outputs found
On the fundamentality of the radial acceleration relation for late-type galaxy dynamics
Galaxies have been observed to exhibit a level of simplicity unexpected in
the complex galaxy formation scenario posited by standard cosmology. This is
particularly apparent in their dynamics, where scaling relations display much
regularity and little intrinsic scatter. However, the parameters responsible
for this simplicity have not been identified. Using the Spitzer Photometry &
Accurate Rotation Curves galaxy catalogue, we argue that the radial
acceleration relation (RAR) between galaxies' baryonic and total dynamical
accelerations is the fundamental -dimensional correlation governing the
radial (in-disk) dynamics of late-type galaxies. In particular, we show that
the RAR cannot be tightened by the inclusion of any other available galaxy
property, that it is the strongest projection of galaxies' radial dynamical
parameter space, and that all other statistical radial dynamical correlations
stem from the RAR plus the non-dynamical correlations present in our sample. We
further provide evidence that the RAR's fundamentality is unique in that the
second most significant dynamical relation does not possess any of these
features. Our analysis reveals the root cause of the correlations present in
galaxies' radial dynamics: they are nothing but facets of the RAR. These
results have important ramifications for galaxy formation theory because they
imply that to explain statistically late-type galaxy dynamics within the disk
it is necessary and sufficient to explain the RAR and lack of any significant,
partially independent correlation. While simple in some modified dynamics
models, this poses a challenge to standard cosmology.Comment: 17 pages, 9 figures. Accepted in MNRA
Evaluating the reconstruction of individual haloes in constrained cosmological simulations
Constrained cosmological simulations play an important role in modelling the
local Universe, enabling investigation of the dark matter content of local
structures and their formation histories. We introduce a method for determining
the extent to which individual haloes are reliably reconstructed between
constrained simulations, and apply it to the Constrained Simulations in BORG
(CSiBORG) suite of high-resolution realisations across the posterior
probability distribution of initial conditions from the Bayesian Origin
Reconstruction from Galaxies (BORG) algorithm. The method is based on the
overlap of the initial Lagrangian patch of a halo in one simulation with those
in another, and therefore measures the degree to which the haloes' particles
are initially coincident. By this metric we find consistent reconstructions of
haloes across the CSiBORG simulations, indicating
that the constraints from the BORG algorithm are sufficient to pin down the
masses, positions and peculiar velocities of clusters to high precision. The
effect of the constraints tapers off towards lower mass however, and the halo
spins and concentrations are largely unconstrained at all masses. We document
the advantages of evaluating halo consistency in the initial conditions,
describe how the method may be used to quantify our knowledge of the halo field
given galaxy survey data analysed through the lens of probabilistic inference
machines such as BORG, and describe applications to matched but unconstrained
simulations.Comment: 11 pages, 11 figures. To be submitted to MNRAS. Comments are welcome
The dependence of subhalo abundance matching on galaxy photometry and selection criteria
Subhalo abundance matching (SHAM) is a popular technique for assigning galaxy
mass or luminosity to haloes produced in N-body simulations. The method works
by matching the cumulative number functions of the galaxy and halo properties,
and is therefore sensitive both to the precise definitions of those properties
and to the selection criteria used to define the samples. Further dependence
follows when SHAM parameters are calibrated with galaxy clustering, which is
known to depend strongly on the manner in which galaxies are selected. In this
paper we introduce a new parametrisation for SHAM and derive the best-fit SHAM
parameters as a function of various properties of the selection of the galaxy
sample and of the photometric definition, including S\'ersic vs Petrosian
magnitudes, stellar masses vs r-band magnitudes and optical (SDSS) vs HI
(ALFALFA) selection. In each case we calculate the models' goodness-of-fit to
measurements of the projected two-point galaxy correlation function. In the
optically-selected samples we find strong evidence that the scatter in the
galaxy-halo connection increases towards the faint end, and that AM performs
better with luminosity than stellar mass. The SHAM parameters of optically- and
HI-selected galaxies are mutually exclusive, with the latter suggesting the
importance of properties beyond halo mass. We provide best-fit parameters for
the SHAM galaxy-halo connection as a function of each of our input choices,
extending the domain of validity of the model while reducing potential
systematic error in its use.Comment: 19 pages and 15 figures. Matches the journal versio
Are stellar--mass binary black hole mergers isotropically distributed?
The Advanced LIGO and Advanced Virgo gravitational wave detectors have
detected a population of binary black hole mergers in their first two observing
runs. For each of these events we have been able to associate a potential sky
location region represented as a probability distribution on the sky. Thus, at
this point we may begin to ask the question of whether this distribution agrees
with the isotropic model of the Universe, or if there is any evidence of
anisotropy. We perform Bayesian model selection between an isotropic and a
simple anisotropic model, taking into account the anisotropic selection
function caused by the underlying antenna patterns and sensitivity of the
interferometers over the sidereal day. We find an inconclusive Bayes factor of
, suggesting that the data from the first two observing runs is
insufficient to pick a preferred model. However, the first detections were
mostly poorly localised in the sky (before the Advanced Virgo joined the
network), spanning large portions of the sky and hampering detection of
potential anisotropy. It will be appropriate to repeat this analysis with
events from the recent third LIGO observational run and a more sophisticated
cosmological model.Comment: 8 pages, 8 figures. Accepted by MNRA
The scatter in the galaxy-halo connection: a machine learning analysis
We apply machine learning, a powerful method for uncovering complex
correlations in high-dimensional data, to the galaxy-halo connection of
cosmological hydrodynamical simulations. The mapping between galaxy and halo
variables is stochastic in the absence of perfect information, but conventional
machine learning models are deterministic and hence cannot capture its
intrinsic scatter. To overcome this limitation, we design an ensemble of neural
networks with a Gaussian loss function that predict probability distributions,
allowing us to model statistical uncertainties in the galaxy-halo connection as
well as its best-fit trends. We extract a number of galaxy and halo variables
from the Horizon-AGN and IllustrisTNG100-1 simulations and quantify the extent
to which knowledge of some subset of one enables prediction of the other. This
allows us to identify the key features of the galaxy-halo connection and
investigate the origin of its scatter in various projections. We find that
while halo properties beyond mass account for up to 50 per cent of the scatter
in the halo-to-stellar mass relation, the prediction of stellar half-mass
radius or total gas mass is not substantially improved by adding further halo
properties. We also use these results to investigate semi-analytic models for
galaxy size in the two simulations, finding that assumptions relating galaxy
size to halo size or spin are not successful.Comment: 20 pages, 11 figures. Accepted in MNRA