45 research outputs found
Towards a complete accounting of energy and momentum from stellar feedback in galaxy formation simulations
Stellar feedback plays a key role in galaxy formation by regulating star
formation, driving interstellar turbulence and generating galactic scale
outflows. Although modern simulations of galaxy formation can resolve scales of
10-100 pc, star formation and feedback operate on smaller, "subgrid" scales.
Great care should therefore be taken in order to properly account for the
effect of feedback on global galaxy evolution. We investigate the momentum and
energy budget of feedback during different stages of stellar evolution, and
study its impact on the interstellar medium using simulations of local star
forming regions and galactic disks at the resolution affordable in modern
cosmological zoom-in simulations. In particular, we present a novel subgrid
model for the momentum injection due to radiation pressure and stellar winds
from massive stars during early, pre-supernova evolutionary stages of young
star clusters. Early injection of momentum acts to clear out dense gas in star
forming regions, hence limiting star formation. The reduced gas density
mitigates radiative losses of thermal feedback energy from subsequent supernova
explosions, leading to an increased overall efficiency of stellar feedback. The
detailed impact of stellar feedback depends sensitively on the implementation
and choice of parameters. Somewhat encouragingly, we find that implementations
in which feedback is efficient lead to approximate self-regulation of global
star formation efficiency. We compare simulation results using our feedback
implementation to other phenomenological feedback methods, where thermal
feedback energy is allowed to dissipate over time scales longer than the formal
gas cooling time. We find that simulations with maximal momentum injection
suppress star formation to a similar degree as is found in simulations adopting
adiabatic thermal feedback.Comment: ApJ submitted. For a high-resolution version of the paper, see
http://kicp.uchicago.edu/~agertz
A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33
Fourier transform power spectra of azimuthal scans of the optical structure
of M33 are evaluated for B, V, and R passbands and fit to fractal models of
continuum emission with superposed star formation. Power spectra are also
determined for Halpha. The best models have intrinsic power spectra with 1D
slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov
spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the
power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of
around -1.5pm0.2, which could be from turbulence. Both cases model the optical
light as a superposition of continuous and point-like stellar sources that
follow an underlying fractal pattern. Foreground bright stars are clipped in
the images, but they are so prominent in M33 that even their residual affects
the power spectrum, making it shallower than what is intrinsic to the galaxy. A
model consisting of random foreground stars added to the best model of NGC 5055
fits the observed power spectrum of M33 as well as the shallower intrinsic
power spectrum that was made without foreground stars. Thus the optical
structure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure
MHD Turbulent Mixing Layers: Equilibrium Cooling Models
We present models of turbulent mixing at the boundaries between hot
(T~10^{6-7} K) and warm material (T~10^4 K) in the interstellar medium, using a
three-dimensional magnetohydrodynamical code, with radiative cooling. The
source of turbulence in our simulations is a Kelvin-Helmholtz instability,
produced by shear between the two media. We found, that because the growth rate
of the large scale modes in the instability is rather slow, it takes a
significant amount of time (~1 Myr) for turbulence to produce effective mixing.
We find that the total column densities of the highly ionized species (C IV, N
V, and O VI) per interface (assuming ionization equilibrium) are similar to
previous steady-state non-equilibrium ionization models, but grow slowly from
log N ~10^{11} to a few 10^{12} cm^{-2} as the interface evolves. However, the
column density ratios can differ significantly from previous estimates, with an
order of magnitude variation in N(C IV)/N(O VI) as the mixing develops.Comment: 10 pages, 10 Figures (2 in color), Accepted for publication on
Astrophysical Journa
Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
This paper explores the mapping between the observable properties of a
stellar halo in phase- and abundance-space and the parent galaxy's accretion
history in terms of the characteristic epoch of accretion and mass and orbits
of progenitor objects. The study utilizes a suite of eleven stellar halo models
constructed within the context of a standard LCDM cosmology. The results
demonstrate that coordinate-space studies are sensitive to the recent (0-8
Gyears ago) merger histories of galaxies (this timescale corresponds to the
last few to tens of percent of mass accretion for a Milky-Way-type galaxy).
Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in
substructures in the stellar halo as a function of surface brightness are
indicators of the importance of recent merging and of the luminosity function
of infalling dwarfs. The {\it morphology} of features serves as a guide to the
orbital distribution of those dwarfs. Constraints on the earlier merger history
(> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars:
within our models, dramatic differences in the dominant epoch of accretion or
luminosity function of progenitor objects leave clear signatures in the
[alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by
very early accretion have higher average [alpha/Fe], while those dominated by
high luminosity satellites have higher [Fe/H]. This intuition can be applied to
reconstruct much about the merger histories of nearby galaxies from current and
future data sets.Comment: 21 pages, 20 figures. To appear in the Astrophysical Journa
A Turbulent Origin for Flocculent Spiral Structure in Galaxies
The flocculent structure of star formation in 7 galaxies has a Fourier
transform power spectrum for azimuthal intensity scans with a power law slope
that increases systematically from -1 at large scales to -1.7 at small scales.
This is the same pattern as in the power spectra for azimuthal scans of HI
emission in the Large Magellanic Clouds and for flocculent dust clouds in
galactic nuclei. The steep part also corresponds to the slope of -3 for
two-dimensional power spectra that have been observed in atomic and molecular
gas surveys of the Milky Way and the Large and Small Magellanic Clouds. The
same power law structure for star formation arises in both flocculent and grand
design galaxies, which implies that the star formation process is the same in
each. Fractal Brownian motion models that include discrete stars and an
underlying continuum of starlight match the observations if all of the emission
is organized into a global fractal pattern with an intrinsic 1D power spectrum
having a slope between 1.3 and 1.8. We suggest that the power spectrum of
optical light in galaxies is the result of turbulence, and that large-scale
turbulent motions are generated by sheared gravitational instabilities which
make flocculent spiral arms first and then cascade to form clouds and clusters
on smaller scales.Comment: accepted for ApJ, 31 pg, 9 figure
On the Last 10 Billion Years of Stellar Mass Growth in Star-Forming Galaxies
The star formation rate - stellar mass relation (SFR-M*) and its evolution
(i.e., the SFR main sequence) describes the growth rate of galaxies of a given
stellar mass and at a given redshift. Assuming that present-day star-forming
galaxies were always star-forming in the past, these growth rate observations
can be integrated to calculate average star formation histories (SFHs). Using
this Main Sequence Integration (MSI) approach, we trace present-day massive
star-forming galaxies back to when they were 10-20% of their current stellar
mass. The integration is robust throughout those epochs: the SFR data
underpinning our calculations is consistent with the evolution of stellar mass
density in this regime. Analytic approximations to these SFHs are provided.
Integration-based results reaffirm previous suggestions that current
star-forming galaxies formed virtually all of their stellar mass at z<2. It
follows that massive galaxies observed at z>2 are not the typical progenitors
of star-forming galaxies today.
We also check MSI-based SFHs against those inferred from analysis of the
fossil record -- from spectral energy distributions (SEDs) of star-forming
galaxies in the Sloan Digital Sky Survey, and color magnitude diagrams (CMDs)
of resolved stars in dwarf irregular galaxies. Once stellar population age
uncertainties are accounted for, the main sequence is in excellent agreement
with SED-based SFHs (from VESPA). Extrapolating SFR main sequence observations
to dwarf galaxies, we find differences between MSI results and SFHs from CMD
analysis of ACS Nearby Galaxy Survey Treasury and Local Group galaxies.
Resolved dwarfs appear to grow much slower than main sequence trends imply, and
also slower than slightly higher mass SED-analyzed galaxies. This difference
may signal problems with SFH determinations, but it may also signal a shift in
star formation trends at the lowest stellar masses.Comment: 20 pages, 10 figures. Accepted for publication in Ap
THE AGORA HIGH-RESOLUTION GALAXY SIMULATIONS COMPARISON PROJECT. II. ISOLATED DISK TEST
Using an isolated Milky Way-mass galaxy simulation, we compare results from nine state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly formed stellar clump mass functions show more significant variation (difference by up to a factor of âŒ3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low-density region, and between more diffusive and less diffusive schemes in the high-density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes
Phylogenetic Approach Reveals That Virus Genotype Largely Determines HIV Set-Point Viral Load
HIV virulence, i.e. the time of progression to AIDS, varies greatly among patients. As for other rapidly evolving pathogens of humans, it is difficult to know if this variance is controlled by the genotype of the host or that of the virus because the transmission chain is usually unknown. We apply the phylogenetic comparative approach (PCA) to estimate the heritability of a trait from one infection to the next, which indicates the control of the virus genotype over this trait. The idea is to use viral RNA sequences obtained from patients infected by HIV-1 subtype B to build a phylogeny, which approximately reflects the transmission chain. Heritability is measured statistically as the propensity for patients close in the phylogeny to exhibit similar infection trait values. The approach reveals that up to half of the variance in set-point viral load, a trait associated with virulence, can be heritable. Our estimate is significant and robust to noise in the phylogeny. We also check for the consistency of our approach by showing that a trait related to drug resistance is almost entirely heritable. Finally, we show the importance of taking into account the transmission chain when estimating correlations between infection traits. The fact that HIV virulence is, at least partially, heritable from one infection to the next has clinical and epidemiological implications. The difference between earlier studies and ours comes from the quality of our dataset and from the power of the PCA, which can be applied to large datasets and accounts for within-host evolution. The PCA opens new perspectives for approaches linking clinical data and evolutionary biology because it can be extended to study other traits or other infectious diseases
Aag DNA Glycosylase Promotes Alkylation-Induced Tissue Damage Mediated by Parp1
Alkylating agents comprise a major class of front-line cancer chemotherapeutic compounds, and while these agents effectively kill tumor cells, they also damage healthy tissues. Although base excision repair (BER) is essential in repairing DNA alkylation damage, under certain conditions, initiation of BER can be detrimental. Here we illustrate that the alkyladenine DNA glycosylase (AAG) mediates alkylation-induced tissue damage and whole-animal lethality following exposure to alkylating agents. Aag-dependent tissue damage, as observed in cerebellar granule cells, splenocytes, thymocytes, bone marrow cells, pancreatic ÎČ-cells, and retinal photoreceptor cells, was detected in wild-type mice, exacerbated in Aag transgenic mice, and completely suppressed in Aagâ/â mice. Additional genetic experiments dissected the effects of modulating both BER and Parp1 on alkylation sensitivity in mice and determined that Aag acts upstream of Parp1 in alkylation-induced tissue damage; in fact, cytotoxicity in WT and Aag transgenic mice was abrogated in the absence of Parp1. These results provide in vivo evidence that Aag-initiated BER may play a critical role in determining the side-effects of alkylating agent chemotherapies and that Parp1 plays a crucial role in Aag-mediated tissue damage.National Institutes of Health (U.S.) (NIH grant R01-CA075576)National Institutes of Health (U.S.) (NIH grant R01-CA055042)National Institutes of Health (U.S.) (NIH grant R01-CA149261)National Institutes of Health (U.S.) (NIH grant P30-ES00002)National Institutes of Health (U.S.) (NIH grant P30-ES02109)National Center for Research Resources (U.S.) (grant number M01RR-01066)National Center for Research Resources (U.S.) (grant number UL1 RR025758, Harvard Clinical and Translational Science Center
The Physics of the B Factories
This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C