314 research outputs found
How Very Massive Metal Free Stars Start Cosmological Reionization
(Abridged) Using ab initio cosmological Eulerian adaptive mesh refinement
radiation hydrodynamical calculations, we discuss how very massive stars start
the process of cosmological reionization. The models include non-equilibrium
primordial gas chemistry and cooling processes and accurate radiation transport
in the Case B approximation using adaptively ray traced photon packages,
retaining the time derivative in the transport equation. Supernova feedback is
modeled by thermal explosions triggered at parsec scales. All calculations
resolve the local Jeans length by at least 16 grid cells at all times and as
such cover a spatial dynamic range of ~10^6. These first sources of
reionization are highly intermittent and anisotropic and first photoionize the
small scales voids surrounding the halos they form in, rather than the dense
filaments they are embedded in. As the merging objects form larger, dwarf sized
galaxies, the escape fraction of UV radiation decreases and the HII regions
only break out on some sides of the galaxies making them even more anisotropic.
In three cases, SN blast waves induce star formation in overdense regions that
were formed earlier from ionization front instabilities. These stars form tens
of parsecs away from the center of their parent DM halo. Approximately 5
ionizing photons are needed per sustained ionization when star formation in
10^6 M_sun halos are dominant in the calculation. As the halos become larger
than ~10^7 M_sun, the ionizing photon escape fraction decreases, which in turn
increases the number of photons per ionization to 15-50, in calculations with
stellar feedback only. Supernova feedback in these more massive halos creates a
more diffuse medium, allowing the stellar radiation to escape more easily and
maintaining the ratio of 5 ionizing photons per sustained ionization.Comment: 16 pages, 15 figures, accepted to ApJ. Final version. High resolution
images and movies available at
http://www.slac.stanford.edu/~jwise/research/Reionizatio
Proceedings of the Project Management Innovation Annual Conference
The Project Management Innovation Conference brings together professionals, leaders, visionaries, researchers, and students to engage in the topics related to the future of project management. Attendees represent all fields and industries, including government, corporate, and private. Attendee demographics include practitioners, front-line staff, and executive management
Human Centered Projects and Survivorship Bias The Perceived Success of Project Outcomes?
Due to the bias that process design demands, we forget the example given by failed projects. The tendency to focus on success was first noted during WWII and named Survivorship Bias. This means that as project professionals, we tend to look at projects that completed successfully to identify desirable patterns for repeatability in process and practice. This leads project professionals to seek out patterns based on successful past practice modeled by successful leaders. Project Managers are trained and heavily focused on Scope, Schedule, and Cost. Perhaps the common constraints and practices of project management may be positively impacted by human centered management practices regardless of the success of the process design
Dwarf Galaxies with Ionizing Radiation Feedback. I: Escape of Ionizing Photons
We describe a new method for simulating ionizing radiation and supernova
feedback in the analogues of low-redshift galactic disks. In this method, which
we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing
technique to solve the radiative transfer equation for ultraviolet photons
emitted by thousands of distinct particles on the fly. Joined with high
numerical resolution of 3.8 pc, the realistic description of stellar feedback
helps to self-regulate star formation. This new feedback scheme also enables us
to study the escape of ionizing photons from star-forming clumps and from a
galaxy, and to examine the evolving environment of star-forming gas clumps. By
simulating a galactic disk in a halo of 2.3e11 Msun, we find that the average
escape fraction from all radiating sources on the spiral arms (excluding the
central 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period with
a mean value of 1.1%. The flux of escaped photons from these sources is not
strongly beamed, but manifests a large opening angle of more than 60 degree
from the galactic pole. Further, we investigate the escape fraction per SFMC
particle, f_esc(i), and how it evolves as the particle ages. We discover that
the average escape fraction f_esc is dominated by a small number of SFMC
particles with high f_esc(i). On average, the escape fraction from a SFMC
particle rises from 0.27% at its birth to 2.1% at the end of a particle
lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas
clumps in which they were born, and because the gas around the star-forming
clumps is dispersed by ionizing radiation and supernova feedback. The framework
established in this study brings deeper insight into the physics of photon
escape fraction from an individual star-forming clump, and from a galactic
disk.Comment: 15 pages, 12 figures, Accepted for publication in the Astrophysical
Journal, Image resolution reduced, High-resolution version of this article is
available at http://www.jihoonkim.org/index/research.html#sfm
Dwarf Galaxies with Ionizing Radiation Feedback. II: Spatially-resolved Star Formation Relation
We investigate the spatially-resolved star formation relation using a
galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation.
Our new implementation of stellar feedback includes ionizing radiation as well
as supernova explosions, and we handle ionizing radiation by solving the
radiative transfer equation rather than by a subgrid model. Photoheating by
stellar radiation stabilizes gas against Jeans fragmentation, reducing the star
formation rate. Because we have self-consistently calculated the location of
ionized gas, we are able to make spatially-resolved mock observations of star
formation tracers, such as H-alpha emission. We can also observe how stellar
feedback manifests itself in the correlation between ionized and molecular gas.
Applying our techniques to the disk in a galactic halo of 2.3e11 Msun, we find
that the correlation between star formation rate density (estimated from mock
H-alpha emission) and molecular hydrogen density shows large scatter,
especially at high resolutions of <~ 75 pc that are comparable to the size of
giant molecular clouds (GMCs). This is because an aperture of GMC size captures
only particular stages of GMC evolution, and because H-alpha traces hot gas
around star-forming regions and is displaced from the molecular hydrogen peaks
themselves. By examining the evolving environment around star clusters, we
speculate that the breakdown of the traditional star formation laws of the
Kennicutt-Schmidt type at small scales is further aided by a combination of
stars drifting from their birthplaces, and molecular clouds being dispersed via
stellar feedback.Comment: 16 pages, 15 figures, Accepted for publication in the Astrophysical
Journal, Image resolution greatly reduced, High-resolution version of this
article is available at http://www.jihoonkim.org/index/research.html#sfm
The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
Starlight from galaxies plays a pivotal role throughout the process of cosmic
reionisation. We present the statistics of dwarf galaxy properties at z > 7 in
haloes with masses up to 10^9 solar masses, using a cosmological radiation
hydrodynamics simulation that follows their buildup starting with their
Population III progenitors. We find that metal-enriched star formation is not
restricted to atomic cooling ( K) haloes, but can occur
in haloes down to masses ~10^6 solar masses, especially in neutral regions.
Even though these smallest galaxies only host up to 10^4 solar masses of stars,
they provide nearly 30 per cent of the ionising photon budget. We find that the
galaxy luminosity function flattens above M_UV ~ -12 with a number density that
is unchanged at z < 10. The fraction of ionising radiation escaping into the
intergalactic medium is inversely dependent on halo mass, decreasing from 50 to
5 per cent in the mass range . Using our galaxy
statistics in a semi-analytic reionisation model, we find a Thomson scattering
optical depth consistent with the latest Planck results, while still being
consistent with the UV emissivity constraints provided by Ly forest
observations at z = 4-6.Comment: 21 pages, 15 figures, 4 tables. Accepted in MNRA
Galaxy Mergers with Adaptive Mesh Refinement: Star Formation and Hot Gas Outflow
In hierarchical structure formation, merging of galaxies is frequent and
known to dramatically affect their properties. To comprehend these interactions
high-resolution simulations are indispensable because of the nonlinear coupling
between pc and Mpc scales. To this end, we present the first adaptive mesh
refinement (AMR) simulation of two merging, low mass, initially gas-rich
galaxies (1.8e10 Ms each), including star formation and feedback. With galaxies
resolved by ~2e7 total computational elements, we achieve unprecedented
resolution of the multiphase interstellar medium, finding a widespread
starburst in the merging galaxies via shock-induced star formation. The high
dynamic range of AMR also allows us to follow the interplay between the
galaxies and their embedding medium depicting how galactic outflows and a hot
metal-rich halo form. These results demonstrate that AMR provides a powerful
tool in understanding interacting galaxies.Comment: 4 pages, 5 figures, Accepted for publication in the Astrophysical
Journal Letters, Image resolution greatly reduced, High-resolution version of
this article and movies are available at
http://www.jihoonkim.org/index/research.html#merge
Resolving the Formation of Protogalaxies. III. Feedback from the First Stars
The first stars form in dark matter halos of masses ~10^6 M_sun as suggested
by an increasing number of numerical simulations. Radiation feedback from these
stars expels most of the gas from their shallow potential well of their
surrounding dark matter halos. We use cosmological adaptive mesh refinement
simulations that include self-consistent Population III star formation and
feedback to examine the properties of assembling early dwarf galaxies. Accurate
radiative transport is modeled with adaptive ray tracing. We include supernova
explosions and follow the metal enrichment of the intergalactic medium. The
calculations focus on the formation of several dwarf galaxies and their
progenitors. In these halos, baryon fractions in 10^8 solar mass halos decrease
by a factor of 2 with stellar feedback and by a factor of 3 with supernova
explosions. We find that radiation feedback and supernova explosions increase
gaseous spin parameters up to a factor of 4 and vary with time. Stellar
feedback, supernova explosions, and H_2 cooling create a complex, multi-phase
interstellar medium whose densities and temperatures can span up to 6 orders of
magnitude at a given radius. The pair-instability supernovae of Population III
stars alone enrich the halos with virial temperatures of 10^4 K to
approximately 10^{-3} of solar metallicity. We find that 40% of the heavy
elements resides in the intergalactic medium (IGM) at the end of our
calculations. The highest metallicity gas exists in supernova remnants and very
dilute regions of the IGM.Comment: 15 pages, 16 figures, accepted to ApJ. Many changes, including
estimates of metal line cooling. High resolution images and movies available
at http://www.slac.stanford.edu/~jwise/research/PGalaxies3
The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations
By definition, Population III stars are metal-free, and their protostellar
collapse is driven by molecular hydrogen cooling in the gas-phase, leading to
large characteristic masses. Population II stars with lower characteristic
masses form when the star-forming gas reaches a critical metallicity of 10^{-6}
- 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation
hydrodynamics simulation that follows the transition from Population III to II
star formation. The maximum spatial resolution of 1 comoving parsec allows for
individual molecular clouds to be well-resolved and their stellar associations
to be studied in detail. We model stellar radiative feedback with adaptive ray
tracing. A top-heavy initial mass function for the Population III stars is
considered, resulting in a plausible distribution of pair-instability
supernovae and associated metal enrichment. We find that the gas fraction
recovers from 5 percent to nearly the cosmic fraction in halos with merger
histories rich in halos above 10^7 solar masses. A single pair-instability
supernova is sufficient to enrich the host halo to a metallicity floor of
10^{-3} Z_\odot and to transition to Population II star formation. This
provides a natural explanation for the observed floor on damped Lyman alpha
(DLA) systems metallicities reported in the literature, which is of this order.
We find that stellar metallicities do not necessarily trace stellar ages, as
mergers of halos with established stellar populations can create superpositions
of t-Z evolutionary tracks. A bimodal metallicity distribution is created after
a starburst occurs when the halo can cool efficiently through atomic line
cooling.Comment: 11 pages, 7 figures; replaced with accepted version to ApJ;
additional movies and images can be found at
http://www.astro.princeton.edu/~jwise/research/GalaxyBirth.htm
- …