151 research outputs found
Cosmological Reionization by Stellar Sources
I use cosmological simulations that incorporate a physically motivated
approximation to three-dimensional radiative transfer that recovers correct
asymptotic ionization front propagation speeds for some cosmologically relevant
density distributions transfer to investigate the process of the reionization
of the universe by ionizing radiation from proto-galaxies. Reionization
proceeds in three stages and occupies a large redshift range from z~15 until
z~5. During the first, ``pre-overlap'' stage, HII regions gradually expand into
the low density IGM, leaving behind neutral high density protrusions. During
the second, ``overlap'' stage, that occurs in about 10% of the Hubble time, HII
regions merge and the ionizing background rises by a large factor. During the
third, ``post-overlap'' stage, remaining high density regions are being
gradually ionized as the required ionizing photons are being produced.
Residual fluctuations in the ionizing background reach significant (more than
10%) levels for the Lyman-alpha forest absorption systems with column densities
above 10^14 - 10^15 cm^-2 at z=3 to 4.Comment: Revised version accepted for publication in ApJ. Color versions of
Fig. 3a-h in GIF format, full (unbinned) versions of Fig. 5, 6, and 13, as
well as MPEG animations are available at
http://casa.colorado.edu/~gnedin/GALLERY/rei_p.htm
Revisiting The First Galaxies: The epoch of Population III stars
We investigate the transition from primordial Population III (Pop III) star
formation to normal Pop II star formation in the first galaxies using new
cosmological hydrodynamic simulations. We find that while the first stars seed
their host galaxies with metals, they cannot sustain significant outflows to
enrich the intergalactic medium, even assuming a top-heavy initial mass
function. This means that Pop III star formation could potentially continue
until z~6 in different unenriched regions of the universe, before being
ultimately shut off by cosmic reionization. Within an individual galaxy, the
metal production and stellar feedback from Pop II stars overtake Pop III stars
in 20-200 Myr, depending on galaxy mass.Comment: 9 pages, 7 figures, published in Ap
Revisiting The First Galaxies: The effects of Population III stars on their host galaxies
We revisit the formation and evolution of the first galaxies using new
hydrodynamic cosmological simulations with the ART code. Our simulations
feature a recently developed model for H2 formation and dissociation, and a
star formation recipe that is based on molecular rather than atomic gas. Here,
we develop and implement a recipe for the formation of metal-free Population
III stars in galaxy-scale simulations that resolve primordial clouds with
sufficiently high density. We base our recipe on the results of prior zoom-in
simulations that resolved the protostellar collapse in pre-galactic objects. We
find the epoch during which Pop III stars dominated the energy and metal budget
of the first galaxies to be short-lived. Galaxies which host Pop III stars do
not retain dynamical signatures of their thermal and radiative feedback for
more than 10^8 yr after the lives of the stars end in pair-instability
supernovae, even when we consider the maximum reasonable efficiency of the
feedback. Though metals ejected by the supernovae can travel well beyond the
virial radius of the host galaxy, they typically begin to fall back quickly,
and do not enrich a large fraction of the intergalactic medium. Galaxies with
total mass in excess of 3 x 10^6 Msun re-accrete most of their baryons and
transition to metal-enriched Pop II star formation.Comment: 13 pages, 9 figures, published in Ap
Testing multiflavored ULDM models with SPARC
We perform maximum likelihood estimates (MLEs) for single and double flavor
ultralight dark matter (ULDM) models using the Spitzer Photometry and Accurate
Rotation Curves (SPARC) database. These estimates are compared to MLEs for
several commonly used cold dark matter (CDM) models. By comparing various CDM
models we find, in agreement with previous studies, that the Burkert and
Einasto models tend to perform better than other commonly used CDM models. We
focus on comparisons between the Einasto and ULDM models and analyze cases for
which the ULDM particle masses are: free to vary; and fixed. For each of these
analyses, we perform fits assuming the soliton and halo profiles are: summed
together; and matched at a given radius. When we let the particle masses vary,
we find a negligible preference for any particular range of particle masses,
within , when assuming the
summed models. For the matched models, however, we find that almost all
galaxies prefer particles masses in the range . For both double flavor models we find that most
galaxies prefer approximately equal particle masses. We find that the summed
models give much larger variances with respect to the soliton-halo (SH)
relation than the matched models. When the particle masses are fixed, the
matched models give median and mean soliton and halo values that fall within
the SH relation bounds, for most masses scanned. When the particle masses are
fixed in the fitting procedure, we find the best fit results for the particle
mass (for the single flavor models) and
, for the double flavor,
matched model. We discuss how our study will be furthered using a reinforcement
learning algorithm.Comment: 36 pages, 25 figures, 2 appendice
Feedback from Galaxy Formation: Production and Photodissociation of Primordial Molecular Hydrogen
We use one-dimensional radiative transfer simulations to study the evolution
of H_2 gas-phase (H^- catalyzed) formation and photo-dissociation regions in
the primordial universe. We find a new positive feedback mechanism capable of
producing shells of H_2 in the intergalactic medium, which are optically thick
in some Lyman-Werner bands. While these shells exist, this feedback effect is
important in reducing the H_2 dissociating background flux and the size of
photo-dissociation spheres around each luminous object. The maximum background
opacity of the IGM in the H_2 Lyman-Werner bands is \tau_{H_2} ~ 1-2 for a
relic molecular fraction x_{H_2}=2 x 10^{-6}, about 6 times greater than found
by Haiman, Abel & Rees. Therefore, the relic molecular hydrogen can decrease
the photo-dissociation rate by about an order of magnitude. The problem is
relevant to the formation of small primordial galaxies with masses M_{DM} <
10^8 M_\odot, that rely on molecular hydrogen cooling to collapse.
Alternatively, the universe may have remained dark for several hundred million
years after the birth of the first stars, until galaxies with virial
temperature T_{vir} > 10^4 K formed.Comment: Accepted for pubblication on ApJ vol 560, October 2001 with minor
changes. 31 pages,including 12 figures and 1 table. Higher quality figures
are available at: http://casa.colorado.edu/~ricotti/papers.htm
New bounds on the neutrino magnetic moment from the plasma induced neutrino chirality flip in a supernova
The neutrino chirality-flip process under the conditions of the supernova
core is investigated in detail with the plasma polarization effects in the
photon propagator taken into account, in a more consistent way than in earlier
publications. It is shown in part that the contribution of the proton fraction
of plasma is essential. New upper bounds on the neutrino magnetic moment are
obtained: mu_nu < (0.5 - 1.1) 10^{-12} mu_B from the limit on the supernova
core luminosity for nu_R emission, and mu_nu < (0.4 - 0.6) 10^{-12} mu_B from
the limit on the averaged time of the neutrino spin-flip. The best upper bound
on the neutrino magnetic moment from SN1987A is improved by the factor of 3 to
7.Comment: 19 pages, LaTeX, 7 EPS figures, submitted to Journal of Cosmology and
Astroparticle Physic
Resolving Gas Dynamics in the Circumnuclear Region of a Disk Galaxy in a Cosmological Simulation
Using a hydrodynamic adaptive mesh refinement code, we simulate the growth
and evolution of a galaxy, which could potentially host a supermassive black
hole, within a cosmological volume. Reaching a dynamical range in excess of 10
million, the simulation follows the evolution of the gas structure from
super-galactic scales all the way down to the outer edge of the accretion disk.
Here, we focus on global instabilities in the self-gravitating, cold,
turbulence-supported, molecular gas disk at the center of the model galaxy,
which provide a natural mechanism for angular momentum transport down to sub-pc
scales. The gas density profile follows a power-law scaling as r^-8/3,
consistent with an analytic description of turbulence in a quasi-stationary
circumnuclear disk. We analyze the properties of the disk which contribute to
the instabilities, and investigate the significance of instability for the
galaxy's evolution and the growth of a supermassive black hole at the center.Comment: 16 pages (includes appendix), submitted to ApJ. Figures here are at
low resolution; for higher resolution version, download
http://casa.colorado.edu/~levinerd/ms.pd
Glucose-independent Acetate Metabolism Promotes Melanoma Cell Survival and Tumor Growth
Tumors rely on multiple nutrients to meet cellular bioenergetics and macromolecular synthesis demands of rapidly dividing cells. Although the role of glucose and glutamine in cancer metabolism is well understood, the relative contribution of acetate metabolism remains to be clarified. We show that glutamine supplementation is not sufficient to prevent loss of cell viability in a subset of glucose-deprived melanoma cells, but synergizes with acetate to support cell survival. Glucose-deprived melanoma cells depend on both oxidative phosphorylation and acetate metabolism for cell survival. Acetate supplementation significantly contributed to maintenance of ATP levels in glucose-starved cells. Unlike acetate, short chain fatty acids such as butyrate and propionate failed to prevent loss of cell viability from glucose deprivation. In vivo studies revealed that in addition to nucleo-cytoplasmic acetate assimilating enzyme ACSS2, mitochondrial ACSS1 was critical for melanoma tumor growth in mice. Our data indicate that acetate metabolism may be a potential therapeutic target for BRAF mutant melanoma
Sparsest factor analysis for clustering variables: a matrix decomposition approach
We propose a new procedure for sparse factor analysis (FA) such that each variable loads only one common factor. Thus, the loading matrix has a single nonzero element in each row and zeros elsewhere. Such a loading matrix is the sparsest possible for certain number of variables and common factors. For this reason, the proposed method is named sparsest FA (SSFA). It may also be called FA-based variable clustering, since the variables loading the same common factor can be classified into a cluster. In SSFA, all model parts of FA (common factors, their correlations, loadings, unique factors, and unique variances) are treated as fixed unknown parameter matrices and their least squares function is minimized through specific data matrix decomposition. A useful feature of the algorithm is that the matrix of common factor scores is re-parameterized using QR decomposition in order to efficiently estimate factor correlations. A simulation study shows that the proposed procedure can exactly identify the true sparsest models. Real data examples demonstrate the usefulness of the variable clustering performed by SSFA
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