279 research outputs found
Black Hole Formation in the First Stellar Clusters
The early Universe was composed almost entirely of hydrogen and helium, with
only trace amounts of heavy elements. It was only after the first generation of
star formation that the Universe became sufficiently polluted to produce a
second generation (Population II) of stars which are similar to those in our
local Universe. Evidence of massive star cluster formation is nearly ubiquitous
among the observed galaxy population and if this mode of star formation
occurred at early enough epochs, the higher densities in the early Universe may
have caused many of the stars in the cluster to strongly interact. In this
scenario, it may be possible to form a very massive star by repeated stellar
collisions that may directly collapse into a black hole and form a supermassive
black hole seed. In this chapter, we will explore this scenario in detail to
understand the dynamics which allow for this process to ensue and measure the
probability for this type of seed to represent the supermassive black hole
population observed at z > 6.Comment: Preprint of the chapter "Black Hole Formation in the First Stellar
Clusters", to be published in the review volume "Formation of the First Black
Holes", Latif, M. and Schleicher, D. R. G., eds., World Scientific Publishing
Company, 2018, pp 125-143 [see
https://www.worldscientific.com/worldscibooks/10.1142/10652
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
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
Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust Content of Selected Galaxies
The presence of spectroscopically confirmed Balmer breaks in galaxy spectral
energy distributions (SEDs) at provides one of the best probes of the
assembly history of the first generations of stars in our Universe. Recent
observations of the gravitationally lensed source, MACS 1149_JD1 (JD1),
indicate that significant amounts of star formation likely occurred at
redshifts as high as . The inferred stellar mass, dust mass, and
assembly history of JD1, or any other galaxy at these redshifts that exhibits a
strong Balmer break, can provide a strong test of our best theoretical models
from high-resolution cosmological simulations. In this work, we present the
results from a cosmological radiation-hydrodynamics simulation of the region
surrounding a massive Lyman-break galaxy. For two of our most massive systems,
we show that dust preferentially resides in the vicinity of the young stars
thereby increasing the strength of the measured Balmer break such that the
simulated SEDs are consistent with the photometry of JD1 and two other
systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high
redshift. We find strong variations in the shape and luminosity of the SEDs of
galaxies with nearly identical stellar and halo masses, indicating the
importance of morphology, assembly history, and dust distribution in making
inferences on the properties of individual galaxies at high redshifts. Our
results stress the importance that dust may play in modulating the observable
properties of galaxies, even at the extreme redshifts of .Comment: 16 pages, 13 Figures, Accepted to MNRA
The baryonic Tully-Fisher relation for different velocity definitions and implications for galaxy angular momentum
We study the baryonic Tully-Fisher relation (BTFR) at z=0 using 153 galaxies
from the SPARC sample. We consider different definitions of the characteristic
velocity from HI and H-alpha rotation curves, as well as HI line-widths from
single-dish observations. We reach the following results: (1) The tightest BTFR
is given by the mean velocity along the flat part of the rotation curve. The
orthogonal intrinsic scatter is extremely small (6%) and the best-fit slope is
3.85+/-0.09, but systematic uncertainties may drive the slope from 3.5 to 4.0.
Other velocity definitions lead to BTFRs with systematically higher scatters
and shallower slopes. (2) We provide statistical relations to infer the flat
rotation velocity from HI line-widths or less extended rotation curves (like
H-alpha and CO data). These can be useful to study the BTFR from large HI
surveys or the BTFR at high redshifts. (3) The BTFR is more fundamental than
the relation between angular momentum and galaxy mass (the Fall relation). The
Fall relation has about 7 times more scatter than the BTFR, which is merely
driven by the scatter in the mass-size relation of galaxies. The BTFR is
already the "fundamental plane" of galaxy discs: no value is added with a
radial variable as a third parameter.Comment: 12 pages, 6 figures, accepted for publication in MNRA
Testing Feedback-Modified Dark Matter Haloes with Galaxy Rotation Curves: Estimation of Halo Parameters and Consistency with CDM
Cosmological -body simulations predict dark matter (DM) haloes with steep
central cusps (e.g. NFW, Navarro et al. 1996). This contradicts observations of
gas kinematics in low-mass galaxies that imply the existence of shallow DM
cores. Baryonic processes such as adiabatic contraction and gas outflows can,
in principle, alter the initial DM density profile, yet their relative
contributions to the halo transformation remain uncertain. Recent high
resolution, cosmological hydrodynamic simulations (Di Cintio et al. 2014, DC14)
predict that inner density profiles depend systematically on the ratio of
stellar to DM mass (M/M). Using a Markov Chain Monte Carlo
approach, we test the NFW and the M/M-dependent DC14 halo
models against a sample of 147 galaxy rotation curves from the new {\it
Spitzer} Photometry and Accurate Rotation Curves (SPARC) data set. These
galaxies all have extended H{\small I} rotation curves from radio
interferometry as well as accurate stellar mass density profiles from
near-infrared photometry. The DC14 halo profile provides markedly better fits
to the data compared to the NFW profile. Unlike NFW, the DC14 halo parameters
found in our rotation curve fits naturally fall within two standard deviations
of the mass-concentration relation predicted by CDM and the stellar
mass-halo mass relation inferred from abundance matching with few outliers.
Halo profiles modified by baryonic processes are therefore more consistent with
expectations from cold dark matter (CDM) cosmology and
provide better fits to galaxy rotation curves across a wide range of galaxy
properties than do halo models that neglect baryonic physics. Our results offer
a solution to the decade long cusp-core discrepancy.Comment: 23 Pages, 18 Figures, MNRAS Accepte
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Numerical Simulations of Galaxy Formation During the Epoch of Reionization
This thesis considers various topics and open questions in galaxy formation during the epoch of reionization and presents multiple new computational techniques developed specifically to study this era. This work naturally divides into two main sections: 1) The formation of the first massive black holes and 2) Interpreting ALMA observations of galaxy formation during the epoch of reionization.
The first topic addresses the existence of super massive black holes (SMBHs) with M at . It is well established that stellar mass black holes are very unlikely to be able to accrete matter efficiently enough to grow to this mass at this redshift. For this reason, many alternative channels have been proposed for black hole formation that produce objects with significantly larger initial masses. In this thesis, I consider a mechanism whereby runaway stellar collisions in dense primordial star clusters form a very massive star that is likely to collapse to an intermediate mass black hole (IMBH) with M. In order to test this scenario, I added 12 species non-equilibrium chemistry to the massively parallel adaptive mesh refinement code RAMSES, and simulated, at sub-pc resolution, the collapse of the first metal-enriched halo which is likely to host a Population II star cluster. The properties of the central gas cloud in the collapsing halo were then extracted from the simulation and used to create initial conditions for the direct N-body integrator, NBODY6. These star clusters were simulated for 3.5Myr (until the first supernova is expected to occur) and it was determined that the properties of the gas clouds that form in cosmological simulations were indeed suitable to form a very massive star by collisional runaway. This suggests that this mechanism is a promising channel for forming the seeds of SMBHs at high redshift.
The second topic of this thesis aims to help interpret the plethora of recent and upcoming ALMA observations of star forming galaxies during the epoch of reionization. These observations target far-infrared lines such as [CII] and [OIII] which directly probe the interstellar medium (ISM) of these galaxies. In order to study this epoch, I employ the RAMSES-RT code, which allows for the computation of multifrequency radiative transfer on-the-fly. I modified this code in a number of ways so that it can handle radiation-coupled H non-equilibrium chemistry (including Lyman-Werner band radiation) and I developed the variable speed of light approximation which changes the speed of light in the simulation depending on the density of gas so that ionisation fronts propagate at the correct speed in all gas phases. Cosmological boxes were initialised to include galaxies with masses comparable to the observations of Maiolino et al. (2015) and run at various resolutions to test convergence properties. One of the major goals of this study was to identify the physical mechanism responsible for the spatial offset observed between [CII] and UV/Lyα in many high-redshift galaxies. (Abridged)Foundation Boustany, Cambridge Overseas Trust, Isaac Newton Studentshi
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