439 research outputs found
On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds
Measurements of the star formation efficiency (SFE) of giant molecular clouds
(GMCs) in the Milky Way generally show a large scatter, which could be
intrinsic or observational. We use magnetohydrodynamic simulations of GMCs
(including feedback) to forward-model the relationship between the true GMC SFE
and observational proxies. We show that individual GMCs trace broad ranges of
observed SFE throughout collapse, star formation, and disruption. Low measured
SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall"
SFE where most stars actually form can be much larger. Very high (>>10%) values
are often artificially enhanced by rapid gas dispersal. Simulations including
stellar feedback reproduce observed GMC-scale SFEs, but simulations without
feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms
simulated. An anticorrelation of SFE with cloud mass is shown to be an
observational artifact. We also explore individual dense "clumps" within GMCs
and show that (with feedback) their bulk properties agree well with
observations. Predicted SFEs within the dense clumps are ~2x larger than
observed, possibly indicating physics other than feedback from massive (main
sequence) stars is needed to regulate their collapse.Comment: Fixed typo in the arXiv abstrac
Reproducing the CO-to-H₂ conversion factor in cosmological simulations of Milky-Way-mass galaxies
We present models of CO(1–0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1–0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H₂ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ∼3 pc in cold gas (T < 300 K) for both CO and H₂, is able to reproduce both the observed CO(1–0) luminosity and inferred CO-to-H₂ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds
Fabrication of p-type porous GaN on silicon and epitaxial GaN
Abstract : Porous GaN layers are grown on silicon from gold or platinum catalyst seed layers, and self-catalyzed on epitaxial GaN films on sapphire. Using a Mg-based precursor, we demonstrate p-type doping of the porous GaN. Electrical measurements for p-type GaN on Si show Ohmic and Schottky behavior from gold and platinum seeded GaN, respectively. Ohmicity is attributed to the formation of a Ga2Au intermetallic. Porous p-type GaN was also achieved on epitaxial n-GaN on sapphire, and transport measurements confirm a p-n junction commensurate with a doping density of 1018 cm 3. Photoluminescence and cathodoluminescence confirm emission from Mg-acceptors in porous p-type GaN
Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations
Galactic accretion interacts in complex ways with gaseous halos, including
galactic winds. As a result, observational diagnostics typically probe a range
of intertwined physical phenomena. Because of this complexity, cosmological
hydrodynamic simulations have played a key role in developing observational
diagnostics of galactic accretion. In this chapter, we review the status of
different observational diagnostics of circumgalactic gas flows, in both
absorption (galaxy pair and down-the-barrel observations in neutral hydrogen
and metals; kinematic and azimuthal angle diagnostics; the cosmological column
density distribution; and metallicity) and emission (Lya; UV metal lines; and
diffuse X-rays). We conclude that there is no simple and robust way to identify
galactic accretion in individual measurements. Rather, progress in testing
galactic accretion models is likely to come from systematic, statistical
comparisons of simulation predictions with observations. We discuss specific
areas where progress is likely to be particularly fruitful over the next few
years.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dave, to be published by
Springer. Typos correcte
A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3
We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of
ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs)
with deep imaging in the U band obtained with the Large Binocular Camera,
mounted on the prime focus of the Large Binocular Telescope. We selected 11
LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields.
We stacked the images of our sources in the R and U band, which correspond to
an effective rest-frame wavelength of 1500\AA and 900\AA respectively,
obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We derive a
1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values
found in the literature so far at z~3.3. Assuming that the upper limit for the
escape fraction that we derived from our sample holds for all galaxies at this
redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1})
is not enough to keep the IGM ionized and a substantial contribution to the UV
background by faint AGNs is required. Since our sample is clearly still limited
in size, larger z~3 LBG samples, at similar or even greater depths are
necessary to confirm these results on a more firm statistical basis.Comment: 15 pages, 2 figures, 1 table, accepted for publication in Ap
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