1,469 research outputs found
A High-Resolution Hubble Space Telescope Study of Apparent Lyman Continuum Leakers at
We present follow-up observations of 16
candidate LyC emitters in the HS1549+1919 field. With these data, we
obtain high spatial-resolution photometric redshifts of all sub-arcsecond
components of the LyC candidates in order to eliminate foreground contamination
and identify robust candidates for leaking LyC emission. Of the 16 candidates,
we find one object with a robust LyC detection that is not due to foreground
contamination. This object (MD5) resolves into two components; we refer to the
LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\
flux-density ratio of , compatible with
predictions from stellar population synthesis models. Assuming minimal IGM
absorption, this ratio corresponds to a relative (absolute) escape fraction of
% (%). The stellar
population fit to MD5b indicates an age of Myr, which is in the
youngest 10% of the sample and the youngest third of typical
Lyman break galaxies, and may be a contributing factor to its LyC detection. We
obtain a revised, contamination-free estimate for the comoving specific
ionizing emissivity at , indicating (with large uncertainties) that
star-forming galaxies provide roughly the same contribution as QSOs to the
ionizing background at this redshift. Our results show that foreground
contamination prevents ground-based LyC studies from obtaining a full
understanding of LyC emission from star-forming galaxies. Future
progress in direct LyC searches is contingent upon the elimination of
foreground contaminants through high spatial-resolution observations, and upon
acquisition of sufficiently deep LyC imaging to probe ionizing radiation in
high-redshift galaxies.Comment: 31 pages, 5 tables, 19 figures. Accepted to ApJ. Version with
full-resolution figures is available at:
http://www.astro.ucla.edu/~aes/Mostardi_HST_LyC.pd
Energetic Oxygen and Sulfur in the Jovian Magnetosphere
This paper reports measurements made by the cosmic ray subsystem onboard Voyager 1 and 2 in the Jovian magnetosphere. Energy spectra of oxygen ions in the energy range 1–20 MeV/nuc between 5 and 20 R_J are presented and used to calculate phase space densities. There is a steep positive radial gradient in the phase space density of the energetic oxygen ions in this region, indicating an inward diffusive flow. Solutions of the diffusion equation assuming a diffusion coefficient D and loss lifetime τ of the forms D=D_0L^n and τ=τ_0L^m, where D_0, τ_0, n, and m are constants, and L is the McIlwain parameter, are fit to the radial phase space density profile of oxygen ions with magnetic moments of 680 MeV/nuc-G. The best fits are found to have n + m ≈ 6 and 3 400 MeV/nuc-G diffuse inward across 10 R_J is 5 × 10^(21±1) ions s^(−1). The observations suggest that oxygen and sulfur ions in the Io plasma torus diffuse radially outward, are nonadiabatically accelerated in some region outside 17 R_J and then diffuse inward and outward from the acceleration region
Interplanetary MeV electrons of Jovian origin
Observations of low energy electron increases observed in interplanetary space on Pioneer 10 are reported as it approached Jupiter. These discrete bursts were several hundred times the normal quiet-time electron flux, and became more frequent as one approached Jupiter resulting in the quasi-continuous presence of large fluxes of these electrons in interplanetary space. It is noted that the integrated flux from quiet-time electrons is comparable to the integrated ambient electron flux itself. In addition, the spectrum of electrons observed in Jupiter's magnetosphere, on Pioneer 10 in interplanetary space near Jupiter, for the quiet-time increases near the earth, and for the ambient electron spectrum are all remarkably similar. These two lines of evidence suggest the possibility that Jupiter could be the source of most of the ambient electrons at low energies
The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at
We present results of a deep spectroscopic survey designed to quantify the
statistics of the escape of ionizing photons from star-forming galaxies at z~3.
We measure the ratio of ionizing to non-ionizing UV flux density
_obs, where f900 is the mean flux density evaluated over the range
[880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux
density by analyzing high-S/N composite spectra formed from sub-samples with
common observed properties and numbers sufficient to reduce the statistical
uncertainty in the modeled IGM+CGM correction to obtain precise values of
_out, including a full-sample average
_out=. We further show that _out
increases monotonically with Ly rest equivalent width, inducing an
inverse correlation with UV luminosity as a by-product. We fit the composite
spectra using stellar spectral synthesis together with models of the ISM in
which a fraction f_c of the stellar continuum is covered by gas with column
density N(HI). We show that the composite spectra simultaneously constrain the
intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI),
E(B-V), and , the absolute escape fraction of ionizing photons. We
find a sample-averaged , and that subsamples fall
along a linear relation . We use the FUV luminosity function, the distribution function
, and the relationship between and
_out to estimate the total ionizing emissivity of
star-forming galaxies with Muv < -19.5:
ergs/s/Hz/Mpc, exceeding the contribution of QSOs by a factor of ,
and accounting for % of the total at
estimated using indirect methods.Comment: 45 pages, 31 figures, ApJ, in pres
The equivalence of fluctuation scale dependence and autocorrelations
We define optimal per-particle fluctuation and correlation measures, relate
fluctuations and correlations through an integral equation and show how to
invert that equation to obtain precise autocorrelations from fluctuation scale
dependence. We test the precision of the inversion with Monte Carlo data and
compare autocorrelations to conditional distributions conventionally used to
study high- jet structure.Comment: 10 pages, 9 figures, proceedings, MIT workshop on correlations and
fluctuations in relativistic nuclear collision
A High Fraction of Ly-alpha-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2
Star-forming galaxies form a sequence in the [OIII]/H-beta vs. [NII]/H-alpha
diagnostic diagram, with low metallicity, highly ionized galaxies falling in
the upper left corner. Drawing from a large sample of UV-selected star-forming
galaxies at z~2 with rest-frame optical nebular emission line measurements from
Keck-MOSFIRE, we select the extreme ~5% of the galaxies lying in this upper
left corner, requiring log([NII]/H-alpha) =
0.75. These cuts identify galaxies with 12 + log(O/H) <~ 8.0, when oxygen
abundances are measured via the O3N2 diagnostic. We study the Ly-alpha
properties of the resulting sample of 14 galaxies. The mean (median) rest-frame
Ly-alpha equivalent width is 39 (36) A, and 11 of the 14 objects (79%) are
Ly-alpha-emitters (LAEs) with W_Lya > 20 A. We compare the equivalent width
distribution of a sample of 522 UV-selected galaxies at 2.0<z<2.6 identified
without regard to their optical line ratios; this sample has mean (median)
Ly-alpha equivalent width -1 (-4) A, and only 9% of these galaxies qualify as
LAEs. The extreme galaxies typically have lower attenuation at Ly-alpha than
those in the comparison sample, and have ~50% lower median oxygen abundances.
Both factors are likely to facilitate the escape of Ly-alpha: in less dusty
galaxies Ly-alpha photons are less likely to be absorbed during multiple
scatterings, while the harder ionizing spectrum and higher ionization parameter
associated with strong, low metallicity star formation may reduce the covering
fraction or column density of neutral hydrogen, further easing Ly-alpha escape.
The use of nebular emission line ratios may prove useful in the identification
of galaxies with low opacity to Ly-alpha photons across a range of redshifts.Comment: 12 pages, 5 figures, 2 tables. Accepted for publication in Ap
The static and dynamic conductivity of warm dense Aluminum and Gold calculated within a density functional approach
The static resistivity of dense Al and Au plsmas are calculated where all the
needed inputs are obtained from density functional theory (DFT). This is used
as input for a study of the dynamic conductivity. These calculations involve a
self-consistent determination of (i) the equation of state (EOS) and the
ionization balance, (ii) evaluation of the ion-ion, and ion-electron
pair-distribution functions, (iii) Determination of the scattering amplitudes,
and finally the conductivity. We present data for the static resistivity of Al
for compressions 0.1-2.0, and in the temperature range T= 0.1 - 10 eV. Results
for Au in the same temperature range and for compressions 0.1-1.0 is also
given. In determining the dynamic conductivity for a range of frequencies
consistent with standard laser probes, a knowledge of the electronic
eigenstates and occupancies of Al- or Au plasma becomes necessary. They are
calculated using a neutral-pseudoatom model. We examine a number of
first-principles approaches to the optical conductivity, including many-body
perturbation theory, molecular-dynamics evaluations, and simplified
time-dependent DFT. The modification to the Drude conductivity that arises from
the presence of shallow bound states in typical Al-plasmas is examined and
numerical results are given at the level of the Fermi Golden rule and an
approximate form of time-dependent DFT.Comment: 5 figures, Latex original. Cross-referencced to PLASMA PHYSIC
Elemental Composition of Solar Energetic Particles in 1977 and 1978
Measurements of the elemental composition of energetic nuclei with atomic numbers between 2 and 28 from seven major solar flares from September 1977 to May 1978 are presented. The abundance observations were made with the Low Energy Telescope systems of the cosmic ray detector systems on board the Voyager 1 and 2 spacecraft between 1 and 3 AU. Examination of the abundance ratios of the flare nuclei relative to oxygen reveals significant variations from event to event and between energetic nuclei and photospheric abundances, with an average composition, except for C and N, very similar to that of the galactic cosmic ray source. For the four flare events for which the elemental abundances exhibit no significant energy dependence in the energy range observed, it is found that the enhancement of energetic nuclei relative to their photospheric abundance are similar and not monotonic with atomic number, with the metallic nuclei showing an enhancement factor of approximately 5 and the volatiles showing one closer to 1
Narrowband Lyman-continuum Imaging of Galaxies at z ~ 2.85
We present results from a survey for z ~ 2.85 Lyman-continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z ≥ 2.82, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lyα emitters (LAEs) spectroscopically confirmed at z ≥ 2.82. Four LBGs and seven LAEs are detected in NB3420. Using V-band data probing the rest-frame nonionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and nonionizing UV emission and are characterized by extremely blue NB3420–V colors, corresponding to low ratios of nonionizing to ionizing radiation (F_(UV)/F_(LyC)) that are in tension with current stellar population synthesis models. We measure average values of (F_(UV)/F_(LyC)) for our LBG and LAE samples, correcting for foreground galaxy contamination and H I absorption in the intergalactic medium. We find (F_(UV)/F_(LyC)_(corr)^(LBG)= 82 ± 45 and (F_(UV)/F_(LyC)_(corr)^(LAE)= 7.4 ± 3.6. These flux density ratios correspond, respectively, to relative LyC escape fractions of f_(esc,rel)^(LBG) = 5%-8% and f_(esc,rel)^(LAE)=18%-49%, absolute LyC escape fractions of f_(esc)^(LBG)=1%-2% and f_(esc)^(LAE)=5%-15%, and a comoving LyC emissivity from star-forming galaxies of 8.8-15.0 × 10^(24) erg s^(–1) Hz^(–1) Mpc^(–3). In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lyα imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lyα equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These preliminary results are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy interstellar medium
- …