196 research outputs found
A Synchrotron Mössbauer Spectroscopy Study of a Hydrated Iron-Sulfate at High Pressures
Szomolnokite is a monohydrated ferrous iron sulfate mineral, FeSO₄·H₂O, where the ferrous iron atoms are in octahedral coordination with four corners shared with SO4 and two with H₂O groups. While somewhat rare on Earth, szomolnokite has been detected on the surface of Mars along with several other hydrated sulfates and is suggested to occur near the surface of Venus. Previous measurements have characterized the local environment of the iron atoms in szomolnokite using Mössbauer spectroscopy at a range of temperatures and 1 bar. Our study represents a step towards understanding the electronic environment of iron in szomolnokite under compression at 300 K. Using a hydrostatic helium pressure-transmitting medium, we explored the pressure dependence of iron’s site-specific behavior in a synthetic szomolnokite powdered sample up to 95 GPa with time-domain synchrotron Mössbauer spectroscopy. At 1 bar, the Mössbauer spectrum is well described by two Fe²⁺-like sites and no ferric iron, consistent with select conventional Mössbauer spectra evaluations. At pressures below 19 GPa, steep gradients in the hyperfine parameters are most likely due to a structural phase transition. At 19 GPa, a fourth site is required to explain the time spectrum with increasing fractions of a low quadrupole splitting site, which could indicate the onset of another transition. Above 19 GPa we present three different models, including those with a high- to low-spin transition, that provide reasonable scenarios of electronic environment changes of the iron in szomolnokite with pressure. We summarize the complex range of Fe²⁺ spin transition characteristics at high-pressures by comparing szomolnokite with previous studies on ferrous-iron bearing phases
Strongly Anisotropic Magnesiowüstite in Earth's Lower Mantle
The juxtaposition of a liquid iron‐dominant alloy against a mixture of silicate and oxide minerals at Earth's core‐mantle boundary is associated with a wide range of complex seismological features. One category of observed structures is ultralow‐velocity zones, which are thought to correspond to either aggregates of partially molten material or solid, iron‐enriched assemblages. We measured the phonon dispersion relations of (Mg,Fe) O magnesiowüstite containing 76 mol % FeO, a candidate ultralow‐velocity zone phase, at high pressures using high‐energy resolution inelastic X‐ray scattering. From these measurements, we find that magnesiowüstite becomes strongly elastically anisotropic with increasing pressure, potentially contributing to a significant proportion of seismic anisotropy detected near the base of the mantle
Recommended from our members
CANDELS Observations Of The Structural Properties Of Cluster Galaxies At Z=1.62
We discuss the structural and morphological properties of galaxies in a z = 1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology relation: galaxies with colors of quiescent stellar populations generally have morphologies consistent with spheroids, and galaxies with colors consistent with ongoing star formation have disk-like and irregular morphologies. The size distribution of the quiescent cluster galaxies shows a deficit of compact (less than or similar to 1 kpc), massive galaxies compared to CANDELS field galaxies at z = 1.6. As a result, the cluster quiescent galaxies have larger average effective sizes compared to field galaxies at fixed mass at greater than 90% significance. Combined with data from the literature, the size evolution of quiescent cluster galaxies is relatively slow from z similar or equal to 1.6 to the present, growing as (1 + z)(-0.6 +/- 0.1). If this result is generalizable, then it implies that physical processes associated with the denser cluster region seem to have caused accelerated size growth in quiescent galaxies prior to z = 1.6 and slower subsequent growth at z < 1.6 compared to galaxies in the lower density field. The quiescent cluster galaxies at z = 1.6 have higher ellipticities compared to lower redshift samples at fixed mass, and their surface-brightness profiles suggest that they contain extended stellar disks. We argue that the cluster galaxies require dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk material and to match the relations for ellipticity, stellar mass, size, and color of early-type galaxies in z < 1 clusters.NASA NAS5-26555HST GO-12060NASA through from the Space Telescope Science Institute GO-12060European Research CouncilRoyal SocietyTexas AM UniversityGeorge P. and Cynthia Woods Institute for Fundamental Physics and AstronomyAstronom
Using Dark Energy Explorers and Machine Learning to Enhance the Hobby-Eberly Telescope Dark Energy Experiment
We present analysis using a citizen science campaign to improve the
cosmological measures from the Hobby-Eberly Telescope Dark Energy Experiment
(HETDEX). The goal of HETDEX is to measure the Hubble expansion rate, ,
and angular diameter distance, , at 2.4, each to percent-level
accuracy. This accuracy is determined primarily from the total number of
detected Lyman- emitters (LAEs), the false positive rate due to noise,
and the contamination due to [O II] emitting galaxies. This paper presents the
citizen science project, Dark Energy Explorers, with the goal of increasing the
number of LAEs, decreasing the number of false positives due to noise and the
[O II] galaxies. Initial analysis shows that citizen science is an efficient
and effective tool for classification most accurately done by the human eye,
especially in combination with unsupervised machine learning. Three aspects
from the citizen science campaign that have the most impact are 1) identifying
individual problems with detections, 2) providing a clean sample with 100%
visual identification above a signal-to-noise cut, and 3) providing labels for
machine learning efforts. Since the end of 2022, Dark Energy Explorers has
collected over three and a half million classifications by 11,000 volunteers in
over 85 different countries around the world. By incorporating the results of
the Dark Energy Explorers we expect to improve the accuracy on the and
parameters at 2.4 by 10 - 30%. While the primary goal is to
improve on HETDEX, Dark Energy Explorers has already proven to be a uniquely
powerful tool for science advancement and increasing accessibility to science
worldwide.Comment: 14 pages, 6 figures, accepted for publication in The Astrophysical
Journa
The HETDEX Pilot Survey. IV. The Evolution of [O II] Emitting Galaxies from z ~ 0.5 to z ~ 0
We present an analysis of the luminosities and equivalent widths of the 284 z
< 0.56 [O II]-emitting galaxies found in the 169 square arcmin pilot survey for
the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). By combining
emission-line fluxes obtained from the Mitchell spectrograph on the McDonald
2.7-m telescope with deep broadband photometry from archival data, we derive
each galaxy's de-reddened [O II] 3727 luminosity and calculate its total star
formation rate. We show that over the last ~5 Gyr of cosmic time there has been
substantial evolution in the [O II] emission-line luminosity function, with L*
decreasing by ~0.6 +/-0.2 dex in the observed function, and by ~0.9 +/-0.2 dex
in the de-reddened relation. Accompanying this decline is a significant shift
in the distribution of [O II] equivalent widths, with the fraction of high
equivalent-width emitters declining dramatically with time. Overall, the data
imply that the relative intensity of star formation within galaxies has
decreased over the past ~5 Gyr, and that the star formation rate density of the
universe has declined by a factor of ~2.5 between z ~ 0.5 and z ~ 0. These
observations represent the first [O II]-based star formation rate density
measurements in this redshift range, and foreshadow the advancements which will
be generated by the main HETDEX survey.Comment: 11 pages with 9 figures and 1 table; accepted for publication in the
Astrophysical Journa
Probing the Star Formation History and Initial Mass Function of the z~2.5 Lensed Galaxy SMM J163554.2+661225 with Herschel
We present the analysis of Herschel SPIRE far-infrared (FIR) observations of
the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and
500 micron observations with existing data, we make an improved fit to the FIR
spectral energy distribution (SED) of this galaxy. We find a total infrared
(IR) luminosity of L(8--1000 micron) = 6.9 +/- 0.6x10^11 Lsol; a factor of 3
more precise over previous L_IR estimates for this galaxy, and one of the most
accurate measurements for any galaxy at these redshifts. This FIR luminosity
implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10
Msol per yr, which is a factor of 1.9 +/- 0.35 lower than the SFR derived from
the nebular Pa-alpha emission line (a 2.5-sigma discrepancy). Both SFR
indicators assume identical Salpeter initial mass functions (IMF) with slope
Gamma=2.35 over a mass range of 0.1 - 100 Msol, thus this discrepancy suggests
that more ionizing photons may be necessary to account for the higher
Pa-alpha-derived SFR. We examine a number of scenarios and find that the
observations can be explained with a varying star formation history (SFH) due
to an increasing star formation rate (SFR), paired with a slight flattening of
the IMF. If the SFR is constant in time, then larger changes need to be made to
the IMF by either increasing the upper-mass cutoff to ~ 200 Msol, or a
flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These
scenarios result in up to double the number of stars with masses above 20 Msol,
which produce the requisite increase in ionizing photons over a Salpeter IMF
with a constant SFH.Comment: 9 pages, 4 figures, 1 table; Accepted for publication in the
Astrophysical Journa
Recommended from our members
The Active Galactic Nuclei in the Hobby-Eberly Telescope Dark Energy Experiment Survey (HETDEX). III. A Red Quasar with Extremely High Equivalent Widths Showing Powerful Outflows
We report an active galactic nucleus (AGN) with an extremely high equivalent width (EW), EWLyα+N V,rest ≳921 Å, in the rest frame, at z ∼ 2.24 in the Hobby-Eberly Telescope Dark Energy Experiment Survey (HETDEX), as a representative case of the high-EW AGN population. The continuum level is a nondetection in the HETDEX spectrum; thus the measured EW is a lower limit. The source is detected with significant emission lines (>7σ) at Lyα + N v λ1241, C iv λ1549, and a moderate emission line (∼4σ) at He ii λ1640 within the wavelength coverage of HETDEX (3500-5500 Å). The r-band magnitude is 24.57 from the Hyper Suprime-Cam-HETDEX joint survey with a detection limit of r = 25.12 at 5σ. The Lyα emission line spans a clearly resolved region of ∼10″ (85 kpc) in diameter. The Lyα line profile is strongly double peaked. The spectral decomposed blue gas and red gas Lyα emission are separated by ∼1.″2 (10.1 kpc) with a line-of-sight velocity offset of ∼1100 km s−1. This source is probably an obscured AGN with powerful winds
- …