69 research outputs found
Unravelling the Intra-Cluster Medium with Numerical Simulations and Multi-Wavelength Observations
Galaxy clusters are the largest coherent structures in the Universe today. With masses of over and hundreds to thousands of member galaxies, they represent the highest-density peaks of the Universe, sitting at the intersection of large-scale filaments as nodes of the Cosmic Web. In a CDM Universe, smaller structures collapse first, and form ever larger structures by merging with one another. The story of a galaxy cluster, then, is the story of the Universe - a story of the balance between gravitational collapse, Hubble expansion and, at later times, acceleration by dark energy. Most of a cluster\u27s mass is dark, and most of the baryonic mass is in the diffuse intracluster medium (ICM); only about 3 of the cluster mass lies in galaxies. Star formation and stellar feedback, whose energy can be comparable to the binding energy of an individual galaxy, are negligible compared to the gravitational potential of the cluster. Cluster cosmology until fairly recently has relied on the relative unimportance of baryonic processes, and treated the baryons simply as observable tracers of the total gravitational potential, dominated by the dark matter. Cluster cosmology and cluster astrophysics, therefore, have remained slightly distinct areas of research. Over the course of my PhD, I have investigated several questions that complicate this simple model. I introduce the standard model of cosmology today, CDM, and how we understand the formation of cosmic structure. I describe how optical, sub-millimeter, radio, gravitational lensing and X-ray observations probe different components of galaxy clusters, and how these relate to each other. Then, I explain the basic principles of numerical simulations, and the role of cosmological as well as idealised simulations in understanding galaxy clusters. My doctoral work then addresses three poorly understood processes - the role of Active Galactic Nuclei, mergers, and magnetic fields in the evolution of galaxy clusters, and their implications for cluster cosmology
Constraints from dwarf galaxies on black hole seeding and growth models with current and future surveys
Dwarf galaxies are considered to be potential ideal test-beds for
constraining models of the seeding and tracing of the growth of supermassive
and intermediate mass black holes (MBH) via their black hole occupation
fraction (BHOF). Disentangling seeding from the confounding effects of mass
assembly is, however, challenging. In this work, we use semi-analytical models
(SAMs) to probe how various surveys perform at teasing apart different seed and
growth scenarios. We check for differences in the measured BHOF given various
cuts to black hole mass and AGN luminosity and develop a scheme to robustly
compare SAMs, with their intrinsic uncertainties, to X-ray observations. We
demonstrate that to tell seeding models apart, we need to detect or model all
AGN brighter than in galaxies of Shallower surveys, like eRASS, cannot distinguish
between seed models even with the compensation of a much larger survey volume.
We show that the AMUSE survey strongly favours heavy seed models, growing with
empirically motivated growth models either a power-law Eddington Ratio
Distribution Function (ERDF) or one in which black hole accretion is tagged to
the star-formation rate (AGN-MS). These two growth channels in turn can then be
distinguished by the AGN luminosity function at .
The different models also predict different radio scaling relations, which we
quantify using the fundamental plane of black hole activity. We close with
recommendations for the design of upcoming multi-wavelength campaigns that can
optimally detect MBHs in dwarf galaxies.Comment: Submitted to AAS Journal
Painting baryons onto N-body simulations of galaxy clusters with image-to-image deep learning
Galaxy cluster mass functions are a function of cosmology, but mass is not a
direct observable, and systematic errors abound in all its observable proxies.
Mass-free inference can bypass this challenge, but it requires large suites of
simulations spanning a range of cosmologies and models for directly observable
quantities. In this work, we devise a U-net - an image-to-image machine
learning algorithm - to ``paint'' the IllustrisTNG model of baryons onto
dark-matter-only simulations of galaxy clusters. Using 761 galaxy clusters with
from the TNG-300 simulation at , we
train the algorithm to read in maps of projected dark matter mass and output
maps of projected gas density, temperature, and X-ray flux. The models train in
under an hour on two GPUs, and then predict baryonic images for dark
matter maps drawn from the TNG-300 dark-matter-only (DMO) simulation in under
two minutes. Despite being trained on individual images, the model reproduces
the true scaling relation and scatter for the , as well as the
distribution functions of the cluster X-ray luminosity and gas mass. For just
one decade in cluster mass, the model reproduces three orders of magnitude in
. The model is biased slightly high when using dark matter maps from the
DMO simulation. The model performs well on inputs from TNG-300-2, whose mass
resolution is 8 times coarser; further degrading the resolution biases the
predicted luminosity function high. We conclude that U-net-based baryon
painting is a promising technique to build large simulated cluster catalogs
which can be used to improve cluster cosmology by combining existing
full-physics and large -body simulations.Comment: Accepted to MNRA
Weak-lensing mass bias in merging galaxy clusters
Although weak lensing (WL) is a powerful method to estimate a galaxy cluster
mass without any dynamical assumptions, a model bias can arise when the cluster
density profile departs from the assumed model profile. In a merging system,
the bias is expected to become most severe because the constituent halos
undergo significant structural changes. In this study, we investigate WL mass
bias in binary cluster mergers using a suite of idealized hydrodynamical
simulations. Realistic WL shear catalogs are generated by matching the source
galaxy properties, such as intrinsic shape dispersion, measurement noise,
source densities, etc., to those from Subaru and {\it Hubble Space Telescope}
observations. We find that, with the typical mass-concentration (-)
relation and the Navarro-Frenk-White (NFW) profile, the halo mass bias depends
on the time since the first pericenter passage and increases with the mass of
the companion cluster. The time evolution of the mass bias is similar to that
of the concentration, indicating that, to first order, the mass bias is
modulated by the concentration change. For a collision between two
clusters, the maximum bias amounts to . This
suggests that previous WL studies may have significantly overestimated the mass
of the clusters in some of the most massive mergers. Finally, we apply our
results to three merger cases: Abell 2034, MACS J1752.0+4440, and ZwCl
1856.8+6616, and report their mass biases at the observed epoch, as well as
their pre-merger masses, utilizing their merger shock locations as tracers of
the merger phases.Comment: 14 pages, 11 figures, submitted to Ap
Turbulent magnetic fields in merging clusters: a case study of Abell 2146
Kelvin-Helmholtz instabilities (KHI) along contact discontinuities in galaxy clusters have been used to constrain the strength of magnetic fields in galaxy clusters, following the assumption that, as magnetic field lines drape around the interface between the cold and hot phases, their magnetic tension resists the growth of perturbations. This has been observed in simulations of rigid objects moving through magnetized media and sloshing galaxy clusters, and then applied in interpreting observations of merger cold fronts. Using a suite of magnetohydrodynamic (MHD) simulations of binary cluster mergers, we show that even magnetic field strengths stronger than yet observed (Ξ² = Pth/PB = 50) show visible KHI features. This is because our initial magnetic field is tangled, producing AlfvCrossed D sig
Evidence for heavy seed origin of early supermassive black holes from a z~10 X-ray quasar
Observations of quasars reveal that many supermassive black holes (BHs) were
in place less than 700 million years after the Big Bang. However, the origin of
the first BHs remains a mystery. Seeds of the first BHs are postulated to be
either light (i.e., , remnants of the first stars or
heavy (i.e., , originating from the direct collapse
of gas clouds. Harnessing recent data from the Chandra X-ray Observatory, we
report the detection of an X-ray-luminous massive BH in a
gravitationally-lensed galaxy identified by JWST at behind the
cluster lens Abell 2744. This heavily-obscured quasar with a bolometric
luminosity of harbors a
BH assuming accretion at the Eddington
limit. This mass is comparable to the inferred stellar mass of its host galaxy,
in contrast to what is found in the local Universe wherein the BH mass is
of the host galaxy's stellar mass. The combination of such a high
BH mass and large BH-to-galaxy stellar mass ratio just 500 Myrs after the
Big Bang was theoretically predicted and is consistent with a picture wherein
BHs originated from heavy seeds.Comment: 27 pages, 6 figures, accepte
ICM-SHOX. Paper I: Methodology overview and discovery of a baryon--dark matter velocity decoupling in the MACS J0018.5+1626 merger
Galaxy cluster mergers are rich sources of information to test cluster
astrophysics and cosmology. However, cluster mergers produce complex projected
signals that are difficult to interpret physically from individual
observational probes. Multi-probe constraints on both the baryonic and dark
matter cluster components are necessary to infer merger parameters that are
otherwise degenerate. We present ICM-SHOX (Improved Constraints on Mergers with
SZ, Hydrodynamical simulations, Optical, and X-ray), a systematic framework to
jointly infer multiple merger parameters quantitatively via a pipeline that
directly compares a novel combination of multi-probe observables to mock
observables derived from hydrodynamical simulations. We report on a first
application of the ICM-SHOX pipeline to the MACS J0018.5+1626 system, wherein
we systematically examine simulated snapshots characterized by a wide range of
initial parameters to constrain the MACS J0018.5+1626 merger parameters. We
strongly constrain the observed epoch of MACS J0018.5+1626 to within -- Myr of the pericenter passage, and the observed viewing angle is
inclined -- degrees from the merger axis. We obtain less
precise constraints for the impact parameter (--250 kpc), the mass
ratio (--), and the initial relative velocity when the
cluster components are separated by 3 Mpc (--3000 km s).
The primary and secondary cluster components initially (at 3 Mpc) have gas
distributions that are moderately and strongly disturbed, respectively. We
further discover a velocity space decoupling of the dark matter and baryonic
distributions in MACS J0018.5+1626, which we attribute to the different
collisional natures of the two distributions.Comment: 25 pages, 13 figures; submitted to Ap
Constraints on thermal conductivity in the merging cluster Abell 2146
The cluster of galaxies Abell 2146 is undergoing a major merger and is an
ideal cluster to study ICM physics, as it has a simple geometry with the merger
axis in the plane of the sky, its distance allows us to resolve features across
the relevant scales and its temperature lies within Chandra's sensitivity. Gas
from the cool core of the subcluster has been partially stripped into a tail of
gas, which gives a unique opportunity to look at the survival of such gas and
determine the rate of conduction in the ICM. We use deep 2.4 Ms Chandra
observations of Abell 2146 to produce a high spatial resolution map of the
temperature structure along a plume in the ram-pressure stripped tail,
described by a partial cone, which is distinguishable from the hot ambient gas.
Previous studies of conduction in the ICM typically rely on estimates of the
survival time for key structures, such as cold fronts. Here we use detailed
hydrodynamical simulations of Abell 2146 to determine the flow velocities along
the stripped plume and measure the timescale of the temperature increase along
its length. We find that conduction must be highly suppressed by multiple
orders of magnitude compared to the Spitzer rate, as the energy used is about
1% of the energy available. We discuss magnetic draping around the core as a
possible mechanism for suppressing conduction.Comment: 10 pages, 3 figures, 3 tables, accepted for publication in MNRA
Tidal Disruption Event Demographics with the Zwicky Transient Facility: Volumetric Rates, Luminosity Function, and Implications for the Local Black Hole Mass Function
We conduct a systematic tidal disruption event (TDE) demographics analysis
using the largest sample of optically selected TDEs. A flux-limited,
spectroscopically complete sample of 33 TDEs is constructed using the Zwicky
Transient Facility over three years (from October 2018 to September 2021). We
infer the black hole (BH) mass () with host galaxy scaling
relations, showing that the sample ranges from
to . We developed a survey efficiency corrected maximum
volume method to infer the rates. The rest-frame -band luminosity function
(LF) can be well described by a broken power-law of , with . In the BH mass regime of , the TDE mass function follows
, which favors a flat local BH mass
function (). We confirm
the significant rate suppression at the high-mass end (), which is consistent with theoretical predictions
considering direct capture of hydrogen-burning stars by the event horizon. At a
host galaxy mass of , the average optical TDE
rate is . We constrain
the optical TDE rate to be [3.7, 7.4, and 1.6 in galaxies with red, green, and blue colors.Comment: Replaced following peer-review process. 38 pages, 23 figures.
Accepted for publication in ApJ
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