2,795 research outputs found
Building Teen Futures with Underwater Robotics
Preparing young Americans with science and technology skills has been on the forefront of educational reform for several years, and Extension has responded. Robotics projects have become a natural fit for 4-H clubs, with members\u27 experiences ranging from using Lego® Mindstorms® and other purchase and assemble robotics kits to building and programming robots from raw materials. This article addresses one such club\u27s impacts on educational engagement, career trajectory, and life skills development. An annotated resource list for engaging youth in building underwater robots is provided in the Tools of the Trade article Resources for Underwater Robotics Education in this issue of the Journal of Extension
Resources for Underwater Robotics Education
4-H clubs can build and program underwater robots from raw materials. An annotated resource list for engaging youth in building underwater remotely operated vehicles (ROVs) is provided. This article is a companion piece to the Research in Brief article Building Teen Futures with Underwater Robotics in this issue of the Journal of Extension
The Distribution of Metallicity in the IGM at z~2.5: OVI and CIV Absorption in the Spectra of 7 QSOs
We present a direct measurement of the metallicity distribution function for
the high redshift intergalactic medium. We determine the shape of this function
using survival statistics, which account for both detections and non-detections
of OVI and CIV associated with HI absorption in quasar spectra. Our OVI sample
probes the metal content of ~50% of all baryons at z~2.5. We find a median
intergalactic abundance of [O,C/H]=-2.82; the differential abundance
distribution is approximately lognormal with mean ~-2.85 and
\sigma=0.75 dex. Some 60-70% the Lya forest lines are enriched to observable
levels ([O,C/H]>-3.5) while the remaining ~30% of the lines have even lower
abundances. Thus we have not detected a universal metallicity floor as has been
suggested for some Population III enrichment scenaria. In fact, we argue that
the bulk of the intergalactic metals formed later than the first stars that are
thought to have triggered reionization. We do not observe a significant trend
of decreasing metallicity toward the lower density IGM, at least within regions
that would be characterized as filaments in numerical simulations. However, an
[O/H] enhancement may be present at somewhat high densities. We estimate that
roughly half of all baryons at these redshifts have been enriched to
[O/H]>=-3.5. We develop a simple model for the metallicity evolution of the
IGM, to estimate the chemical yield of galaxies formed prior to z~2.5. We find
that the typical galaxy recycled 0.1-0.4% of its mass back into the IGM as
heavy elements in the first 3 Gyr after the Big Bang.Comment: 23 pages in emulateapj, 19 figures. Accepted to ApJ, pending review
of new changes. Revised comparison between our results and Schaye et al
(2003
Discovery of excess O I absorption towards the z = 6.42 QSO SDSS J1148+5251
We present a search for O I in the spectra of nine 4.9 < z_qso < 6.4 QSOs
taken with Keck/HIRES. We detect six systems with N(O I) > 10^13.7 cm^{-2} in
the redshift intervals where O I 1302 falls redward of the Ly-alpha forest.
Four of these lie towards SDSS J1148+5251 (z_qso = 6.42). This imbalance is
unlikely to arise from variations in sensitivity among our data or from a
statistical fluctuation. The excess O I occurs over a redshift interval that
also contains transmission in Ly-alpha and Ly-beta. Therefore, if these O I
systems represent pockets of neutral gas, then they must occur within or near
regions of the IGM that are highly ionized. In contrast, no O I is detected
towards SDSS J1030+0524 (z_qso = 6.30), whose spectrum shows complete
absorption in Ly-alpha and Ly-beta over \Delta z ~ 0.2. Assuming no ionization
corrections, we measure mean abundance ratios = -0.04 +/- 0.06,
= -0.31 +/- 0.09, and = -0.34 +/- 0.07 (2 sigma), which are
consistent with enrichment dominated by Type II supernovae. The O/Si ratio
limits the fraction of silicon in these systems contributed by metal-free very
massive stars to < 30%, a result which is insensitive to ionization
corrections. The ionic comoving mass densities along the z_qso > 6.2
sightlines, including only the detected systems, are \Omega(O I) = (7.0 +/-
0.6) * 10^{-8}, \Omega(Si II) = (9.6 +/- 0.9) * 10^{-9}, and \Omega(C II) =
(1.5 +/- 0.2) * 10^{-8}.Comment: Submitted to ApJ, with changes to reflect referee's comment
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
Observations of Chemically Enriched QSO Absorbers near z ~ 2.3 Galaxies: Galaxy-Formation Feedback Signatures in the IGM
We present a study of galaxies and intergalactic gas toward the z=2.73 quasar
HS1700+6416, to explore the effects of galaxy formation feedback on the IGM.
Our observations and ionization simulations indicate that the volume within
100-200 h_71^{-1} physical kpc of high-redshift galaxies contains very small,
dense, and metal-rich absorption-line regions. These systems often contain
shock-heated gas seen in OVI, and may exhibit [Si/C] abundance enhancements
suggestive of Type II supernova enrichment. We argue that the absorbers
resemble thin sheets or bubbles, whose physical properties can be explained
with a simple model of radiatively efficient shocks propegating through the
IGM. Their high metallicities suggest that these shocks are being expelled
from--rather than falling into--star forming galaxies. There is a dropoff in
the IGM gas density at galaxy impact parameters beyond ~300 physical kpc that
may trace boundaries of gas structures where the galaxies reside. The local
heavy-element enhancement covers 100-200 kpc; beyond this the observed
abundances blend into the general IGM. Supernova-driven winds or dynamical
stripping of interstellar gas appears to affect the IGM near massive galaxies,
even at R>~100 kpc. However, these feedback systems represent only a few
percent of the Lya forest mass at z~2.5. Their mass could be larger if the more
numerous metal-poor CIV systems at >~200 kpc are tepid remnants of very
powerful winds. Based on present observations it is not clear that this
scenario is to be favored over one involving pre-enrichment by smaller galaxies
at z>~6.Comment: Accepted for publication in the Astrophysical Journal. 26 pages
emulateapj, incl. 5 pages tables, 15 figure
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