851 research outputs found
Can magnetized turbulence set the mass scale of stars?
Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the GIZMO code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs (â MâŒ10â50â ). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass Mâ
â becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find Mâ
âââŒMâMâ»ÂłÎ±_(turb)SFE^(1/3) for GMC mass Mâ, sonic Mach number Mâ , virial parameter α_(turb), and star formation efficiency SFE = Mâ/Mâ. This fit agrees well with previous IMF results from the RAMSES, ORION2, and SPHNG codes. Although Mâ
â has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict Mâ
â to be >20Mââ , an order of magnitude larger than observed (â âŒ2Mââ ), together with an excess of brown dwarfs. Moreover, Mâ
â is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF
Forged in FIRE: cusps, cores, and baryons in low-mass dwarf galaxies
We present ultra-high resolution cosmological hydrodynamic simulations of
dwarf galaxies that form within
dark matter halos. Our simulations rely on the
FIRE implementation of star formation feedback and were run with high enough
force and mass resolution to directly resolve stellar and dark matter structure
on the ~200 pc scales of interest for classical and ultra-faint dwarfs in the
Local Group. The resultant galaxies sit on the vs. relation
required to match the Local Group stellar mass function. They have bursty star
formation histories and also form with half-light radii and metallicities that
broadly match those observed for local dwarfs at the same stellar mass. We
demonstrate that it is possible to create a large (~1 kpc) dark matter core in
a cosmological simulation of an dwarf galaxy that
resides within an halo -- precisely the scale of
interest for resolving the Too Big to Fail problem. However, these large cores
are not ubiquitous and appear to correlate closely with the star formation
histories of the dwarfs: dark matter cores are largest in systems that form
their stars late (), after the early epoch of cusp building mergers
has ended. Our dwarf retains a cuspy dark matter halo
density profile that matches almost identically that of a dark-matter only run
of the same system. Despite forming in a field environment, this very low mass
dwarf has observable properties that match closely to those of ultra-faint
satellite galaxies of the Milky Way, including a uniformly old stellar
population (>10 Gyr). Though ancient, most of the stars in our ultra-faint form
after reionization; the UV field acts mainly to suppress fresh gas accretion,
not to boil away gas that is already present in the proto-dwarf.Comment: 16 pages, 13 figures, accepted in MNRA
DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
Evolution of giant molecular clouds across cosmic time
Giant molecular clouds (GMCs) are well studied in the local Universe, however, exactly how their properties vary during galaxy evolution is poorly understood due to challenging resolution requirements, both observational and computational. We present the first time-dependent analysis of GMCs in a Milky Way-like galaxy and an Large Magellanic Cloud (LMC)-like dwarf galaxy of the FIRE-2 (Feedback In Realistic Environments) simulation suite, which have sufficient resolution to predict the bulk properties of GMCs in cosmological galaxy formation self-consistently. We show explicitly that the majority of star formation outside the galactic centre occurs within self-gravitating gas structures that have properties consistent with observed bound GMCs. We find that the typical cloud bulk properties such as mass and surface density do not vary more than a factor of 2 in any systematic way after the first Gyr of cosmic evolution within a given galaxy from its progenitor. While the median properties are constant, the tails of the distributions can briefly undergo drastic changes, which can produce very massive and dense self-gravitating gas clouds. Once the galaxy forms, we identify only two systematic trends in bulk properties over cosmic time: a steady increase in metallicity produced by previous stellar populations and a weak decrease in bulk cloud temperatures. With the exception of metallicity, we find no significant differences in cloud properties between the Milky Way-like and dwarf galaxies. These results have important implications for cosmological star and star cluster formation and put especially strong constraints on theories relating the stellar initial mass function to cloud properties
A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2
We directly measure the evolution of the intergalactic Lya effective optical
depth, tau_eff, over the redshift range 2<z<4.2 from a sample of 86
high-resolution, high-signal-to-noise quasar spectra obtained with the ESI and
HIRES spectrographs on Keck, and with the MIKE spectrograph on Magellan. This
represents an improvement over previous analyses of the Lya forest from
high-resolution spectra in this redshift interval of a factor of two in the
size of the data set alone. We pay particular attention to robust error
estimation and extensively test for systematic effects. We find that our
estimates of the quasar continuum levels in the Lya forest obtained by spline
fitting are systematically biased low, with the magnitude of the bias
increasing with redshift, but that this bias can be accounted for using mock
spectra. The mean fractional error is <1% at z=2, 4% at z=3, and 12% at z=4.
Previous measurements of tau_eff at z>~3 based on directly fitting the quasar
continua in the Lya forest, which have generally neglected this effect, are
therefore likely biased low. We provide estimates of the level of absorption
arising from metals in the Lya forest based on both direct and statistical
metal removal results in the literature, finding that this contribution is
~6-9% at z=3 and decreases monotonically with redshift. The high precision of
our measurement, attaining 3% in redshift bins of width Delta z=0.2 around z=3,
indicates significant departures from the best-fit power-law redshift evolution
(tau_eff=0.0018(1+z)^3.92, when metals are left in), particularly near z=3.2.
The observed downward departure is statistically consistent with a similar
feature detected in a precision statistical measurement using Sloan Digital Sky
Survey spectra by Bernardi and coworkers using an independent approach.Comment: 27 pages, including 18 figures, published in Ap
A Predicted Correlation Between Age Gradient and Star Formation History in FIRE Dwarf Galaxies
We explore the radial variation of star formation histories in dwarf galaxies
simulated with Feedback In Realistic Environments (FIRE) physics. The sample
contains 9 low-mass field dwarfs with M_ star = 10^5 - 10^7 M_sun from previous
FIRE results, and a new suite of 17 higher mass field dwarfs with M_star = 10^7
- 10^9 M_sun introduced here. We find that age gradients are common in our
dwarfs, with older stars dominant at large radii. The strength of the gradient
correlates with overall galaxy age such that earlier star formation produces a
more pronounced gradient. The relation between formation time and strength of
the gradient is driven by both mergers and star-formation feedback. Mergers can
both steepen and flatten the age gradient depending on the timing of the merger
and star formation history of the merging galaxy. In galaxies without
significant mergers, early feedback pushes stars to the outskirts at early
times. Interestingly, among galaxies without mergers, those with large dark
matter cores have flatter age gradients because these galaxies have more
late-time feedback. If real galaxies have age gradients as we predict, stellar
population studies that rely on sampling a limited fraction of a galaxy can
give a biased view of its global star formation history. We show that central
fields can be biased young by a few Gyrs while outer fields are biased old.
Fields positioned near the 2D half-light radius will provide the least biased
measure of a dwarf galaxy's global star formation history.Comment: 13 pages, 8 figures. Submitted to MNRAS, comments welcom
SIDM on FIRE: Hydrodynamical Self-Interacting Dark Matter simulations of low-mass dwarf galaxies
We compare a suite of four simulated dwarf galaxies formed in 10 haloes of collisionless Cold Dark Matter (CDM) with galaxies
simulated in the same haloes with an identical galaxy formation model but a
non-zero cross-section for dark matter self-interactions. These cosmological
zoom-in simulations are part of the Feedback In Realistic Environments (FIRE)
project and utilize the FIRE-2 model for hydrodynamics and galaxy formation
physics. We find the stellar masses of the galaxies formed in Self-Interacting
Dark Matter (SIDM) with are very similar to those in CDM
(spanning ) and all runs lie on a
similar stellar mass -- size relation. The logarithmic dark matter density
slope () in the central pc remains
steeper than for the CDM-Hydro simulations with stellar mass
and core-like in the most massive galaxy.
In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with
. Moreover, the central density profiles predicted in SIDM runs
without baryons are similar to the SIDM runs that include FIRE-2 baryonic
physics. Thus, SIDM appears to be much more robust to the inclusion of
(potentially uncertain) baryonic physics than CDM on this mass scale,
suggesting SIDM will be easier to falsify than CDM using low-mass galaxies. Our
FIRE simulations predict that galaxies less massive than provide potentially ideal targets for discriminating models,
with SIDM producing substantial cores in such tiny galaxies and CDM producing
cusps.Comment: 10 Pages, 7 figures, submitted to MNRA
Modular organization of the white spruce (Picea glauca) transcriptome reveals functional organization and evolutionary signatures
Transcript profiling has shown the molecular bases of several biological processes in plants but few studies have developed an understanding of overall transcriptome variation. We investigated transcriptome structure in white spruce (Picea glauca), aiming to delineate its modular organization and associated functional and evolutionary attributes. Microarray analyses were used to: identify and functionally characterize groups of co-expressed genes; investigate expressional and functional diversity of vascular tissue preferential genes which were conserved among Picea species, and identify expression networks underlying wood formation. We classified 22 857 genes as variable (79%; 22 coexpression groups) or invariant (21%) by profiling across several vegetative tissues. Modular organization and complex transcriptome restructuring among vascular tissue preferential genes was revealed by their assignment to coexpression groups with partially overlapping profiles and partially distinct functions. Integrated analyses of tissue-based and temporally variable profiles identified secondary xylem gene networks, showed their remodelling over a growing season and identified PgNAC-7 (no apical meristerm (NAM), Arabidopsis transcription activation factor (ATAF) and cup-shaped cotyledon (CUC) transcription factor 007 in Picea glauca) as a major hub gene specific to earlywood formation. Reference profiling identified comprehensive, statistically robust coexpressed groups, revealing that modular organization underpins the evolutionary conservation of the transcriptome structure. © 2015 The Authors
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