567 research outputs found
The scatter, residual correlations and curvature of the SPARC baryonic Tully-Fisher relation
In recent work, Lelli et al. (2016) argue that the tightness of the baryonic
Tully-Fisher relation (BTFR) of the SPARC galaxy sample, and the weakness of
the correlation of its residuals with effective radius, pose challenges to LCDM
cosmology. In this Letter we calculate the statistical significance of these
results in the framework of halo abundance matching, which imposes a canonical
galaxy-halo connection. Taking full account of sample variance among SPARC-like
realisations of the parent halo population, we find the scatter in the
predicted BTFR to be 3.6 sigma too high, but the correlation of its residuals
with galaxy size to be naturally weak. Further, we find abundance matching to
generate BTFR curvature in 3.0 sigma disagreement with the data, and a fraction
of galaxies with non-flat rotation curves somewhat larger than observed.Comment: 5 pages, 2 figures; revised to match MNRAS Letters accepted versio
Screened fifth forces in parity-breaking correlation functions
Cross-correlating two different types of galaxy gives rise to parity breaking
in the correlation function that derives from differences in the galaxies'
properties and environments. This is typically associated with a difference in
galaxy bias, describing the relation between galaxy number density and dark
matter density, although observational effects such as magnification bias also
play a role. In this paper we show that the presence of a screened fifth force
adds additional degrees of freedom to the correlation function, describing the
effective coupling of the force to the two galaxy populations. These are also
properties of the galaxies' environments, but with different dependence in
general to galaxy bias. We derive the parity-breaking correlation function
analytically as a function of fifth-force strength and the two populations'
fifth-force charges, and explore the result numerically using Hu-Sawicki
as a toy model of chameleon screening. We find that screening gives rise to an
octopole, which, in the absence of magnification bias, is not present in any
gravity theory without screening and thus is a qualitatively distinct
signature. The modification to the dipole and octopole can be
and respectively at redshift due to screening, but decreases towards lower redshift. The change
in the background power spectrum in theories induces a change in the
dipole of roughly the same size, but dominant to the effect of screening at low
. While current data is insufficient to measure the parity-breaking dipole
or octopole to the precision required to test these models, future surveys such
as DESI, Euclid and SKA have the potential to probe screened fifth forces
through the dipole.Comment: Matches published versio
The Tully-Fisher and mass-size relations from halo abundance matching
The Tully-Fisher relation (TFR) expresses the connection between rotating
galaxies and the dark matter haloes they inhabit, and therefore contains a
wealth of information about galaxy formation. We construct a general framework
to investigate whether models based on halo abundance matching are able to
reproduce the observed stellar mass TFR and mass-size relation (MSR), and use
the data to constrain galaxy formation parameters. Our model tests a range of
plausible scenarios, differing in the response of haloes to disc formation, the
relative angular momentum of baryons and dark matter, the impact of selection
effects, and the abundance matching parameters. We show that agreement with the
observed TFR puts an upper limit on the scatter between galaxy and halo
properties, requires weak or reversed halo contraction, and favours selection
effects that preferentially eliminate fast-rotating galaxies. The MSR
constrains the ratio of the disc to halo specific angular momentum to be
approximately in the range 0.6-1.2. We identify and quantify two problems that
models of this nature face. (1) They predict too large an intrinsic scatter for
the MSR, and (2) they predict too strong an anticorrelation between the TFR and
MSR residuals. We argue that resolving these problems requires introducing a
correlation between stellar surface density and enclosed dark matter mass.
Finally, we explore the expected difference between the TFRs of central and
satellite galaxies, finding that in the favoured models this difference should
be detectable in a sample of ~700 galaxies.Comment: 27 pages, 10 figures; revised to match published MNRAS versio
Uncorrelated velocity and size residuals across galaxy rotation curves
The mass--velocity--size relation of late-type galaxies decouples into
independent correlations between mass and velocity (the Tully-Fisher relation),
and between mass and size. This behaviour is different to early-type galaxies
which lie on a Fundamental Plane. We study the coupling of the Tully-Fisher and
mass-size relations in observations (the SPARC sample) and in empirical galaxy
formation models based on halo abundance matching, and rotation curve fits with
a hydrodynamically motivated halo profile. We systematically investigate the
correlation coefficient between the Tully-Fisher residuals and
mass-size residuals as a function of the radius at which the
velocity is measured, and thus present the relation
across rotation curves. We find no significant correlation in either the data
or models for any , aside from where baryonic mass
dominates. We show that this implies an anticorrelation between galaxy size and
halo concentration (or halo mass) at fixed baryonic mass, and provides evidence
against the hypothesis that galaxy and halo specific angular momentum are
proportional. Finally, we study the relations produced by
the baryons and dark matter separately by fitting halo profiles to the rotation
curves. The balance between these components illustrates the "disk-halo
conspiracy" required for no overall correlation.Comment: 7 pages, 4 figures; revised to match MNRAS published versio
The Tight Empirical Relation between Dark Matter Halo Mass and Flat Rotation Velocity for Late-Type Galaxies
We present a new empirical relation between galaxy dark matter halo mass
() and the velocity along the flat portion of the rotation
curve (), derived from 120 late-type galaxies from the SPARC
database. The orthogonal scatter in this relation is comparable to the observed
scatter in the baryonic Tully-Fisher relation (BTFR), indicating a tight
coupling between total halo mass and galaxy kinematics at .
The small vertical scatter in the relation makes it an extremely competitive
estimator of total halo mass. We demonstrate that this conclusion holds true
for different priors on that give a tight BTFR, but requires
that the halo density profile follows DC14 rather than NFW. We provide
additional relations between and other velocity definitions at
smaller galactic radii (i.e. , , and ) which can be useful for estimating halo masses from kinematic
surveys, providing an alternative to abundance matching. Furthermore, we
constrain the dark matter analog of the Radial Acceleration Relation and also
find its scatter to be small, demonstrating the fine balance between baryons
and dark matter in their contribution to galaxy kinematics.Comment: 6 pages, 4 figures, Accepted to MNRAS Letter
Reconstructing the gravitational field of the local universe
Tests of gravity at the galaxy scale are in their infancy. As a first step to
systematically uncovering the gravitational significance of galaxies, we map
three fundamental gravitational variables -- the Newtonian potential,
acceleration and curvature -- over the galaxy environments of the local
universe to a distance of approximately 200 Mpc. Our method combines the
contributions from galaxies in an all-sky redshift survey, halos from an N-body
simulation hosting low-luminosity objects, and linear and quasi-linear modes of
the density field. We use the ranges of these variables to determine the extent
to which galaxies expand the scope of generic tests of gravity and are capable
of constraining specific classes of model for which they have special
significance. Finally, we investigate the improvements afforded by upcoming
galaxy surveys.Comment: 12 pages, 4 figures; revised to match MNRAS accepted versio
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
Marginalised Normal Regression: Unbiased curve fitting in the presence of x-errors
The history of the seemingly simple problem of straight line fitting in the
presence of both and errors has been fraught with misadventure, with
statistically ad hoc and poorly tested methods abounding in the literature. The
problem stems from the emergence of latent variables describing the "true"
values of the independent variables, the priors on which have a significant
impact on the regression result. By analytic calculation of maximum a
posteriori values and biases, and comprehensive numerical mock tests, we assess
the quality of possible priors. In the presence of intrinsic scatter, the only
prior that we find to give reliably unbiased results in general is a mixture of
one or more Gaussians with means and variances determined as part of the
inference. We find that a single Gaussian is typically sufficient and dub this
model Marginalised Normal Regression (MNR). We illustrate the necessity for MNR
by comparing it to alternative methods on an important linear relation in
cosmology, and extend it to nonlinear regression and an arbitrary covariance
matrix linking and . We publicly release a Python/Jax implementation of
MNR and its Gaussian mixture model extension that is coupled to Hamiltonian
Monte Carlo for efficient sampling, which we call ROXY (Regression and
Optimisation with X and Y errors).Comment: 14+6 pages, 9 figures; submitted to the Open Journal of Astrophysic
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