21 research outputs found
On measuring the Galactic dark matter halo with hypervelocity stars
Hypervelocity stars (HVSs) travel from the Galactic Centre across the dark
matter halo of the Milky Way, where they are observed with velocities in excess
of the Galactic escape speed. Because of their quasi-radial trajectories, they
represent a unique probe of the still poorly constrained dark matter component
of the Galactic potential. In this paper, we present a new method to produce
such constraints. Our likelihood is based on the local HVS density obtained by
back-propagating the observed phase space position and quantifies the ejection
probability along the orbit. To showcase our method, we apply it to simulated
Gaia samples of stars in three realistic Galactic potentials with
dark matter components parametrized by spheroidal NFW profiles. We find that
individual HVSs exhibit a degeneracy in the scale mass-scale radius plane
() and are able to measure only the combination .
Likewise, a degeneracy is also present between and the spheroidal
axis-ratio . In the absence of observational errors, we show the whole
sample can nail down both parameters with {\it sub-per cent} precision (about
and for and respectively) with no systematic bias.
This remarkable power to constrain deviations from a symmetric halo is a
consequence of the Galactocentric origin of HVSs. To compare our results with
other probes, we break the degeneracy in the scale parameters and impose a
mass-concentration relation. The result is a competitive precision on the
virial mass of about .Comment: See Fig. 8 for a summar
Weak lensing constraints on splashback around massive clusters
The splashback radius separates the physical regimes of
collapsed and infalling material around massive dark matter haloes. In
cosmological simulations, this location is associated with a steepening of the
spherically averaged density profile . In this work, we measure the
splashback feature in the stacked weak gravitational lensing signal of
massive clusters from the Cluster Canadian Comparison Project with careful
control of residual systematics effects. We find that the shear introduced by
the presence of additional structure along the line of sight significantly
affects the noise at large clustercentric distances. Although we do not detect
a significant steepening, the use of a simple parametric model enables us to
measure both comoving Mpc and the value of the
logarithmic slope at this point,
.Comment: 9 pages, 5 figures. Accepted for publication in MNRA
Lensing efficiency for gravitational wave mergers
We gain insight into the effects of gravitational lensing on the estimated
distribution of merging binaries observed through gravitational waves. We
quantify the efficiency of magnification for gravitational wave events in the
geometric optics limit, and we compare it to the electromagnetic case by making
minimal assumptions about the distribution of intrinsic properties for the
source population. We show that lensing effects leave a recognizable signature
on the observed rates, and that they can be prominent only in the presence of
an extremely steep mass function (or redshift evolution) and mainly at low
inferred redshifts. We conclude that gravitational magnification does not
represent a significant systematic for gravitational wave merger studies in the
LIGO-Virgo era.Comment: 6 pages, 4 figures. Accepted for publication in MNRA
Exploring the edge
At the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The second is dark energy, an enigmatic component responsible for the accelerated expansion of the Universe. Under these two forces, matter in the Universe organizes itself in the so-called cosmic web. The nodes of this network are large dark matter haloes, and this thesis explores how their boundaries provide information about the nature of dark energy and cosmology. Chapters 3 and 4 present robust theoretical predictions for this titular edge and discuss its simple physical interpretation. Chapters 2 and 5 corroborate these results by presenting measurements of this feature in weak-lensing data. The last scientific chapter of this thesis is a collection of studies in gravitational-wave physics. This chapter explores how these spacetime ripples observed from across the cosmos can be used to detect alternative theories of gravity. Large scale structure and cosmolog
The mass-size relation of galaxy clusters
The outskirts of accreting dark matter haloes exhibit a sudden drop in
density delimiting their multi-stream region. Due to the dynamics of accretion,
the location of this physically motivated edge strongly correlates with the
halo growth rate. Using hydrodynamical zoom-in simulations of high-mass
clusters, we explore this definition in realistic simulations and find an
explicit connection between this feature in the dark matter and galaxy
profiles. We also show that the depth of the splashback feature correlates well
with the direction of filaments and, surprisingly, the orientation of the
brightest cluster galaxy. Our findings suggest that galaxy profiles and
weak-lensing masses can define an observationally viable mass-size scaling
relation for galaxy clusters, which can be used to extract cosmological
information.Comment: 9 pages, 9 figure
Predicting the hypervelocity star population in Gaia
Hypervelocity stars (HVSs) are amongst the fastest objects in our Milky Way.
These stars are predicted to come from the Galactic center (GC) and travel
along unbound orbits across the Galaxy. In the coming years, the ESA satellite
Gaia will provide the most complete and accurate catalogue of the Milky Way,
with full astrometric parameters for more than billion stars. In this
paper, we present the expected sample size and properties (mass, magnitude,
spatial, velocity distributions) of HVSs in the Gaia stellar catalogue. We
build three Gaia mock catalogues of HVSs anchored to current observations,
exploring different ejection mechanisms and GC stellar population properties.
In all cases, we predict hundreds to thousands of HVSs with precise proper
motion measurements within a few tens of kpc from us. For stars with a relative
error in total proper motion below , the mass range extends to ~ but peaks at ~ . The majority of Gaia HVSs will
therefore probe a different mass and distance range compared to the current
non-Gaia sample. In addition, a subset of a few hundreds to a few thousands of
HVSs with ~ will be bright enough to have a precise
measurement of the three-dimensional velocity from Gaia alone. Finally, we show
that Gaia will provide more precise proper motion measurements for the current
sample of HVS candidates. This will help identifying their birthplace narrowing
down their ejection location, and confirming or rejecting their nature as HVSs.
Overall, our forecasts are extremely encouraging in terms of quantity and
quality of HVS data that can be exploited to constrain both the Milky Way
potential and the GC properties.Comment: 17 pages, 18 figures, accepted for publication in MNRA
Radio Galaxy Zoo: Cosmological Alignment of Radio Sources
We study the mutual alignment of radio sources within two surveys, FIRST and
TGSS. This is done by producing two position angle catalogues containing the
preferential directions of respectively and extended
sources distributed over more than and square degrees. The
identification of the sources in the FIRST sample was performed in advance by
volunteers of the Radio Galaxy Zoo project, while for the TGSS sample it is the
result of an automated process presented here. After taking into account
systematic effects, marginal evidence of a local alignment on scales smaller
than is found in the FIRST sample. The probability of this happening
by chance is found to be less than per cent. Further study suggests that on
scales up to the alignment is maximal. For one third of the sources,
the Radio Galaxy Zoo volunteers identified an optical counterpart. Assuming a
flat CDM cosmology with , we
convert the maximum angular scale on which alignment is seen into a physical
scale in the range Mpc . This result supports recent
evidence reported by Taylor and Jagannathan of radio jet alignment in the
deg ELAIS N1 field observed with the Giant Metrewave Radio Telescope. The
TGSS sample is found to be too sparsely populated to manifest a similar signal
The mass-size relation of galaxy clusters
Large scale structure and cosmolog
Learning how to surf: reconstructing the propagation and origin of gravitational waves with Gaussian processes
Large scale structure and cosmolog