135 research outputs found
Dancing to ChaNGa: A Self-Consistent Prediction For Close SMBH Pair Formation Timescales Following Galaxy Mergers
We present the first self-consistent prediction for the distribution of
formation timescales for close Supermassive Black Hole (SMBH) pairs following
galaxy mergers. Using ROMULUS25, the first large-scale cosmological simulation
to accurately track the orbital evolution of SMBHs within their host galaxies
down to sub-kpc scales, we predict an average formation rate density of close
SMBH pairs of 0.013 cMpc^-3 Gyr^-1. We find that it is relatively rare for
galaxy mergers to result in the formation of close SMBH pairs with sub-kpc
separation and those that do form are often the result of Gyrs of orbital
evolution following the galaxy merger. The likelihood and timescale to form a
close SMBH pair depends strongly on the mass ratio of the merging galaxies, as
well as the presence of dense stellar cores. Low stellar mass ratio mergers
with galaxies that lack a dense stellar core are more likely to become tidally
disrupted and deposit their SMBH at large radii without any stellar core to aid
in their orbital decay, resulting in a population of long-lived 'wandering'
SMBHs. Conversely, SMBHs in galaxies that remain embedded within a stellar core
form close pairs in much shorter timescales on average. This timescale is a
crucial, though often ignored or very simplified, ingredient to models
predicting SMBH mergers rates and the connection between SMBH and star
formation activity.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Can one reconstruct masked CMB sky?
The CMB maps obtained by observations always possess domains which have to be
masked due to severe uncertainties with respect to the genuine CMB signal.
Cosmological analyses ideally use full CMB maps in order to get e.g. the
angular power spectrum. There are attempts to reconstruct the masked regions at
least at low resolutions, i.e. at large angular scales, before a further
analysis follows. In this paper, the quality of the reconstruction is
investigated for the ILC (7yr) map as well as for 1000 CMB simulations of the
LambdaCDM concordance model. The latter allows an error estimation for the
reconstruction algorithm which reveals some drawbacks. The analysis points to
errors of the order of a significant fraction of the mean temperature
fluctuation of the CMB. The temperature 2-point correlation function C(theta)
is evaluated for different reconstructed sky maps which leads to the conclusion
that it is safest to compute it on the cut-sky
Wandering Supermassive Black Holes in Milky Way Mass Halos
We present a self-consistent prediction from a large-scale cosmological
simulation for the population of `wandering' supermassive black holes (SMBHs)
of mass greater than M on long-lived, kpc-scale orbits within
Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the
Romulus25 cosmological simulation (Tremmel et al. 2017), which is uniquely able
to capture the orbital evolution of SMBHs during and following galaxy mergers.
We predict that such halos, regardless of recent merger history or morphology,
host an average of SMBHs, including their central black hole,
within 10 kpc from the galactic center and an average of SMBHs
total within their virial radius, not counting those in satellite halos.
Wandering SMBHs exist within their host galaxies for several Gyrs, often
accreted by their host halo in the early Universe. We find, with
significance, that wandering SMBHs are preferentially found outside of galactic
disks.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letter
EDGE: A new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation
We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological
galaxy formation, and study its impact on a simulated dwarf galaxy as part of the âEDGEâ project. The target galaxy has a
maximum circular velocity of 21 km sâ1 but evolves in a region that is moving at up to 90 km sâ1 relative to the hydrodynamic
grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held
in the circumgalactic medium around the galaxy for 320 Myr, delaying the onset of star formation until cooling and collapse
eventually triggers an initial starburst at z = 9. Using genetic modification, we produce âvelocity-zeroedâ initial conditions in
which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale
structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation
starting at z = 17. While the final stellar masses are nearly consistent (4.8 Ă 106 M and 4.4 Ă 106 M for unmodified and
velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due
to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation
simulations, and is especially recommended for those of small galaxies at high redshif
In-n-out: The Gas Cycle From Dwarfs To Spiral Galaxies
We examine the scalings of galactic outflows with halo mass across a suite of 20 high-resolution cosmological zoom galaxy simulations covering halo masses in the range 10^9.5-10^12\M. These simulations self-consistently generate outflows from the available supernova energy in a manner that successfully reproduces key galaxy observables, including the stellar massâhalo mass, TullyâFisher, and massâmetallicity relations. We quantify the importance of ejective feedback to setting the stellar mass relative to the efficiency of gas accretion and star formation. Ejective feedback is increasingly important as galaxy mass decreases; we find an effective mass loading factor that scales as v-circ-2.2, with an amplitude and shape that are invariant with redshift. These scalings are consistent with analytic models for energy-driven wind, based solely on the halo potential. Recycling is common: about half of the outflow mass across all galaxy masses is later reaccreted. The recycling timescale is typically ~1 Gyr, virtually independent of halo mass. Recycled material is reaccreted farther out in the disk and with typically ~2â3 times more angular momentum. These results elucidate and quantify how the baryon cycle plausibly regulates star formation and alters the angular momentum distribution of disk material across the halo mass range where most cosmic star formation occurs
Inflationary perturbations in anisotropic backgrounds and their imprint on the CMB
We extend the standard theory of cosmological perturbations to homogeneous
but anisotropic universes. We present an exhaustive computation for the case of
a Bianchi I model, with a residual isotropy between two spatial dimensions,
which is undergoing complete isotropization at the onset of inflation; we also
show how the computation can be further extended to more general backgrounds.
In presence of a single inflaton field, there are three physical perturbations
(precisely as in the isotropic case), which are obtained (i) by removing gauge
and nondynamical degrees of freedom, and (ii) by finding the combinations of
the remaining modes in terms of which the quadratic action of the perturbations
is canonical. The three perturbations, which later in the isotropic regime
become a scalar mode and two tensor polarizations (gravitational wave), are
coupled to each other already at the linearized level during the anisotropic
phase. This generates nonvanishing correlations between different modes of the
CMB anisotropies, which can be particularly relevant at large scales (and,
potentially, be related to the large scale anomalies in the WMAP data). As an
example, we compute the spectrum of the perturbations in this Bianchi I
geometry, assuming that the inflaton is in a slow roll regime also in the
anisotropic phase. For this simple set-up, fixing the initial conditions for
the perturbations appears more difficult than in the standard case, and
additional assumptions seem to be needed to provide predictions for the CMB
anisotropies.Comment: 31 pages, 3 figure
Evolution of the cosmological mass density of neutral gas from Sloan Digital Sky Survey II - Data Release 7
We present the results of a search for damped Lyman-alpha (DLA) systems in
the Sloan Digital Sky Survey II (SDSS), Data Release 7. We use a fully
automatic procedure to identify DLAs and derive their column densities. The
procedure is checked against the results of previous searches for DLAs in SDSS.
We discuss the agreements and differences and show the robustness of our
procedure. For each system, we obtain an accurate measurement of the absorber's
redshift, the HI column density and the equivalent width of associated metal
absorption lines, without any human intervention. We find 1426 absorbers with
2.15 =20, out of which 937 systems have log N(HI)>=
20.3. This is the largest DLA sample ever built, made available to the
scientific community through the electronic version of this paper. In the
course of the survey, we discovered the intervening DLA with highest HI column
density known to date with log N(HI)=22.0+/-0.1. This single system provides a
strong constraint on the high-end of the N(HI) frequency distribution now
measured with high accuracy. We show that the presence of a DLA at the blue end
of a QSO spectrum can lead to important systematic errors and propose a method
to avoid them. This has important consequences for the measurement of the
cosmological mass density of neutral gas at z~2.2 and therefore on our
understanding of galaxy evolution over the past 10 billion years. [truncated]Comment: Accepted for publication in A&
New Classes of Off-Diagonal Cosmological Solutions in Einstein Gravity
In this work, we apply the anholonomic deformation method for constructing
new classes of anisotropic cosmological solutions in Einstein gravity and/or
generalizations with nonholonomic variables. There are analyzed four types of,
in general, inhomogeneous metrics, defined with respect to anholonomic frames
and their main geometric properties. Such spacetimes contain as particular
cases certain conformal and/or frame transforms of the well known
Friedman-Robertson-Walker, Bianchi, Kasner and Godel universes and define a
great variety of cosmological models with generic off-diagonal metrics, local
anisotropy and inhomogeneity. It is shown that certain nonholonomic
gravitational configurations may mimic de Sitter like inflation scenaria and
different anisotropic modifications without satisfying any classical
false-vacuum equation of state. Finally, we speculate on perspectives when such
off-diagonal solutions can be related to dark energy and dark matter problems
in modern cosmology.Comment: latex2e, 11pt, 33 pages with table of content, a variant accepted to
IJT
Linearization of homogeneous, nearly-isotropic cosmological models
Homogeneous, nearly-isotropic Bianchi cosmological models are considered.
Their time evolution is expressed as a complete set of non-interacting linear
modes on top of a Friedmann-Robertson-Walker background model. This connects
the extensive literature on Bianchi models with the more commonly-adopted
perturbation approach to general relativistic cosmological evolution.
Expressions for the relevant metric perturbations in familiar coordinate
systems can be extracted straightforwardly. Amongst other possibilities, this
allows for future analysis of anisotropic matter sources in a more general
geometry than usually attempted.
We discuss the geometric mechanisms by which maximal symmetry is broken in
the context of these models, shedding light on the origin of different Bianchi
types. When all relevant length-scales are super-horizon, the simplest Bianchi
I models emerge (in which anisotropic quantities appear parallel transported).
Finally we highlight the existence of arbitrarily long near-isotropic epochs
in models of general Bianchi type (including those without an exact isotropic
limit).Comment: 31 pages, 2 figures. Submitted to CQ
The Simulated HI Sky at low redshift
Observations of intergalactic neutral hydrogen can provide a wealth of
information about structure and galaxy formation, potentially tracing accretion
and feedback processes on Mpc scales. Below a column density of NHI ~ 10^19
cm-2, the "edge" or typical observational limit for HI emission from galaxies,
simulations predict a cosmic web of extended emission and filamentary
structures. We study the distribution of neutral hydrogen and its 21cm emission
properties in a cosmological hydrodynamic simulation, to gain more insights
into the distribution of HI below NHI ~ 10^19 cm-2. Such Lyman Limit systems
are expected to trace out the cosmic web, and are relatively unexplored.
Beginning with a 32 h^-1 Mpc simulation, we extract the neutral hydrogen
component by determining the neutral fraction, including a post-processed
correction for self-shielding based on the thermal pressure. We take into
account molecular hydrogen, assuming an average density ratio Omega_H2 /
Omega_HI = 0.3 at z = 0. The statistical properties of the HI emission are
compared with observations, to assess the reliability of the simulation. The
simulated HI distribution robustly describes the full column density range
between NHI ~ 10^14 and NHI ~ 10^21 cm-2 and agrees very well with available
measurements from observations. Furthermore there is good correspondence in the
statistics when looking at the two-point correlation function and the HI mass
function. The reconstructed maps are used to simulate observations of existing
and future telescopes by adding noise and taking account of the sensitivity of
the telescopes. The general agreement in statistical properties of HI suggests
that neutral hydrogen as modeled in this hydrodynamic simulation is a fair
representation of that in the Universe. (abridged)Comment: 20 pages, 17 figures, Accepted for publication in A&A, figures
compressed to low resolution; high-resolution version available at:
http://www.astro.rug.nl/~popping/simulated_HI_sky.pd
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