2,480 research outputs found
Keck Spectroscopy of Faint 3 < z < 7 Lyman Break Galaxies: III. The Mean Ultraviolet Spectrum at z=4
We present and discuss the mean rest-frame ultraviolet spectrum for a sample
of 81 Lyman Break Galaxies (LBGs) selected to be B-band dropouts with a mean
redshift of z=3.9 and apparent magnitudes z_AB<26. Most of the individual
spectra are drawn from our ongoing survey in the GOODS fields with the Keck
DEIMOS spectrograph, and we have augmented our sample with published data taken
with FORS2 on the VLT. In general we find similar trends in the spectral
diagnostics to those found in the earlier, more extensive survey of LBGs at z=3
undertaken by Shapley et al (2003). Specifically, we find low-ionization
absorption lines which trace the presence of neutral outflowing gas are weaker
in galaxies with stronger Lyman-alpha emission, bluer UV spectral slopes, lower
stellar masses, lower UV luminosities and smaller half-light radii. This is
consistent with a physical picture whereby star formation drives outflows of
neutral gas which scatters Lyman-alpha and gives rise to strong low-ionization
absorption lines, while increasing the stellar mass, size, metallicity, and
dust content of galaxies. Typical galaxies are thus expected to have stronger
Lyman-alpha emission and weaker low-ionization absorption at earlier times
(higher redshifts). Indeed, our mean spectrum at z=4 shows somewhat weaker
low-ionization absorption lines than at z=3 and available data at higher
redshift indicates a rapid decrease in low-ionization absorption strength with
redshift. We argue that the reduced low-ionization absorption is likely caused
by a decrease in the covering fraction and/or velocity range of outflowing
neutral gas at earlier epochs. Our continuing survey will enable us to extend
these diagnostics more reliably to higher redshift and determine the
implications for the escape fraction of ionizing photons which governs the role
of early galaxies in cosmic reionization. [Abridged]Comment: 17 pages, 12 figures, submitted to ApJ. Comments welcom
Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies:- II. A High Fraction of Line Emitters at Redshift Six
As Lyman-alpha photons are scattered by neutral hydrogen, a change with
redshift in the Lyman-alpha equivalent width distribution of distant galaxies
offers a promising probe of the degree of ionization in the intergalactic
medium and hence when cosmic reionization ended. This simple test is
complicated by the fact that Lyman-alpha emission can also be affected by the
evolving astrophysical details of the host galaxies. In the first paper in this
series, we demonstrated both a luminosity and redshift dependent trend in the
fraction of Lyman-alpha emitters seen within color-selected Lyman-break
galaxies (LBGs) over the range 3<z<6; lower luminosity galaxies and those at
higher redshift show an increased likelihood of strong emission. Here we
present the results from much deeper 12.5 hour exposures with the Keck DEIMOS
spectrograph focused primarily on LBGs at z~6 which enable us to confirm the
redshift dependence of line emission more robustly and to higher redshift than
was hitherto possible. We find 54+/-11% of faint z~6 LBGs show strong (W_0>25
A) emission, an increase of 1.6x from a similar sample observed at z~4. With a
total sample of 74 z~6 LBGs, we determine the luminosity-dependent Lyman-alpha
equivalent width distribution. Assuming continuity in these trends to the new
population of z~7 sources located with the Hubble WFC3/IR camera, we predict
that unless the neutral fraction rises in the intervening 200 Myr, the success
rate for spectroscopic confirmation using Lyman-alpha emission should be high.Comment: 6 pages, 3 figures, submitted to ApJ
Line Emitting Galaxies Beyond a Redshift of 7: An Improved Method for Estimating the Evolving Neutrality of the Intergalactic Medium
The redshift-dependent fraction of color-selected galaxies revealing Lyman
alpha emission has become the most valuable constraint on the evolving
neutrality of the early intergalactic medium. However, in addition to resonant
scattering by neutral gas, the visibility of Lyman alpha is also dependent on
the intrinsic properties of the host galaxy, including its stellar population,
dust content and the nature of outflowing gas. Taking advantage of significant
progress we have made in determining the line emitting properties of galaxies, we propose an improved method, based on using the measured
slopes of the rest-frame ultraviolet continua of galaxies, to interpret the
growing body of near-infrared spectra of galaxies in order to take into
account these host galaxy dependencies. In a first application of our new
method, we demonstrate its potential via a new spectroscopic survey of
galaxies undertaken with the Keck MOSFIRE spectrograph. Together with earlier
published data our data provides improved estimates of the evolving visibility
of Lyman alpha, particularly at redshift . As a byproduct, we also
present a new line emitting galaxy at a redshift which supersedes an
earlier redshift record. We discuss the improving constraints on the evolving
neutral fraction over and the implications for cosmic reionization.Comment: To be submitted to Ap
Searching for the Sources Responsible for Cosmic Reionization: Probing 7 < z < 10 and Beyond
(Abridged) We review recent observations that suggest the star formation rate
density is declining over 37 and illustrate the challenges that this poses
in explaining the assembled stellar mass in z6 galaxies deduced from
Spitzer data. A plausible conclusion is a vigorous period of earlier star
formation. Prior to JWST and TMT, strong lensing offers a unique probe of the
extent of this earlier activity. We discuss the first results of a blind
spectroscopic survey of lensing clusters for 8.510 Ly emitters
using NIRSPEC which is achieving a limiting star formation rate of 0.1
yr. A companion HST/IRAC survey is targeting lensed and
-band dropouts and probes a 1 arcmin region 1 magnitude deeper
than the UDF/NICMOS observations. Both surveys will constrain the contribution
of early, low luminosity, sources to cosmic reionization.Comment: 12 pages, 6 figures, to appear in "First Light & Reionization", eds.
E. Barton & A. Cooray, New Astronomy Reviews, in pres
ILR Impact Brief - Knowledge, Skills, and Performance: Getting the Most From Team Training
Teams are an integral feature of the American workplace; indeed, more than 80% of the Fortune 500 companies make extensive use of work teams. Action teams, pulled together to carry out a particular time-limited function that requires the specialized expertise of its members, are becoming increasingly common. Researchers have noted that the success of these teams is often thwarted by their lack of information about teamwork in general and their insufficient mastery of basic team competencies. Most organizations train team members for the particular job at hand, so the question arises as to the utility of generic team training. In other words, would imparting knowledge and skills that could be applied in, and adapted to, any number of situations improve outcomes, and if so, what is the mechanism that facilitates this result
The Impact of Task- and Team-Generic Teamwork Skills Training on Team Effectiveness
This study examined the effects of training team members in three task- and teamgeneric teamwork skills: planning and task coordination, collaborative problem solving, and communication. We first examined the degree to which task- and team-generic teamwork skills training impacted team performance on a task unrelated to the content of the training program.We then examined whether the effects of task- and team-generic teamwork skills training on team performance were due to the transfer of skills directly related to planning and task coordination, collaborative problem solving, and communication. Results from 65 four-person project teams indicated that task- and team-generic teamwork skills training led to significantly higher levels of team performance. Results also indicated that the effects of task- and teamgeneric teamwork skills training on team performance were mediated by planning and task coordination and collaborative problem solving behavior. Although communication was positively affected by the task- and team-generic teamwork skills training, it did not mediate the relationship between task- and team-generic teamwork skills training and team performance.Theoretical and practical implications of these results are discussed, as well as possible limitations and directions for future research
A genetic basis for a postmeiotic X versus Y chromosome intragenomic conflict in the mouse.
Intragenomic conflicts arise when a genetic element favours its own transmission to the detriment of others. Conflicts over sex chromosome transmission are expected to have influenced genome structure, gene regulation, and speciation. In the mouse, the existence of an intragenomic conflict between X- and Y-linked multicopy genes has long been suggested but never demonstrated. The Y-encoded multicopy gene Sly has been shown to have a predominant role in the epigenetic repression of post meiotic sex chromatin (PMSC) and, as such, represses X and Y genes, among which are its X-linked homologs Slx and Slxl1. Here, we produced mice that are deficient for both Sly and Slx/Slxl1 and observed that Slx/Slxl1 has an opposite role to that of Sly, in that it stimulates XY gene expression in spermatids. Slx/Slxl1 deficiency rescues the sperm differentiation defects and near sterility caused by Sly deficiency and vice versa. Slx/Slxl1 deficiency also causes a sex ratio distortion towards the production of male offspring that is corrected by Sly deficiency. All in all, our data show that Slx/Slxl1 and Sly have antagonistic effects during sperm differentiation and are involved in a postmeiotic intragenomic conflict that causes segregation distortion and male sterility. This is undoubtedly what drove the massive gene amplification on the mouse X and Y chromosomes. It may also be at the basis of cases of F1 male hybrid sterility where the balance between Slx/Slxl1 and Sly copy number, and therefore expression, is disrupted. To the best of our knowledge, our work is the first demonstration of a competition occurring between X and Y related genes in mammals. It also provides a biological basis for the concept that intragenomic conflict is an important evolutionary force which impacts on gene expression, genome structure, and speciation
A spectroscopic search for AGN activity in the reionization era
The ubiquity of Lyman alpha (Ly) emission in a sample of four bright
[O III]-strong star-forming galaxies with redshifts above 7 has led to the
suggestion that such luminous sources represent a distinct population compared
to their fainter, more numerous, counterparts. The presence of Ly
emission within the reionization era could indicate that these sources created
early ionized bubbles due to their unusually strong radiation, possibly due to
the presence of active galactic nuclei. To test this hypothesis we have secured
long integration spectra with XSHOOTER on the VLT for three 7 sources
selected to have similar luminosities and prominent excess fluxes in the IRAC
3.6 or 4.5m band, usually attributed to strong [O III] emission. We
secured additional spectroscopy for one of these galaxies at =7.15 using
MOSFIRE at the Keck telescope. For this, the most well-studied source in our
sample with the strongest IRAC excess, we detect prominent nebular emission
from He II and NV indicative of a non-thermal source. For the other two sources
at =6.81 and =6.85, for which no previous optical/near infrared
spectroscopy was initially available, Ly is seen in one and CIII]
emission in the other. Although a modest sample, our results further support
the hypothesis that the phenomenon of intense [O III] emission is associated
preferentially with sources lying in early ionized bubbles. However, even
though one of our sources at =7.15 clearly indicates the presence of
non-thermal radiation, such ionized bubbles may not uniquely arise in this
manner. We discuss the unique advantages of extending such challenging
diagnostic studies with JWST.Comment: 11 pages, 7 figures, submitted to Ap
The Lyman Continuum Escape Survey: Ionizing Radiation from [O III]-Strong Sources at a Redshift of 3.1
We present results from the LymAn Continuum Escape Survey (LACES), a Hubble
Space Telescope (HST) program designed to characterize the ionizing radiation
emerging from a sample of Lyman alpha emitting galaxies at redshift . As many show intense [O III] emission characteristic of
star-forming galaxies, they may represent valuable low redshift analogs of
galaxies in the reionization era. Using HST Wide Field Camera 3 / UVIS
to image Lyman continuum emission, we investigate the escape fraction of
ionizing photons in this sample. For 61 sources, of which 77% are
spectroscopically confirmed and 53 have measures of [O III] emission, we detect
Lyman continuum leakage in 20%, a rate significantly higher than is seen in
individual continuum-selected Lyman break galaxies. We estimate there is a 98%
probability that of our detections could be affected by foreground
contamination. Fitting multi-band spectral energy distributions (SEDs) to take
account of the varying stellar populations, dust extinctions and metallicities,
we derive individual Lyman continuum escape fractions corrected for foreground
intergalactic absorption. We find escape fractions of 15 to 60% for individual
objects, and infer an average 20% escape fraction by fitting composite SEDs for
our detected samples. Surprisingly however, even a deep stack of those sources
with no individual detections provides a stringent upper limit on the
average escape fraction of less than 0.5%. We examine various correlations with
source properties and discuss the implications in the context of the popular
picture that cosmic reionization is driven by such compact, low metallicity
star-forming galaxies.Comment: 26 pages, 16 figures, accepted for publication in Ap
A comprehensive study of infrared OH prompt emission in two comets. I. Observations and effective g-factors
We present high-dispersion infrared spectra of hydroxyl (OH) in comets C/2000 WM1 (LINEAR) and C/2004 Q2 (Machholz), acquired with the Near Infrared Echelle Spectrograph at the Keck Observatory atop Mauna Kea, Hawaii. Most of these rovibrational transitions result from photodissociative excitation of H_2O giving rise to OH "prompt" emission. We present calibrated emission efficiencies (equivalent g-factors, measured in OH photons s^(-1) [H_2O molecule]^(-1)) for more than 20 OH lines sampled in these two comets. The OH transitions analyzed cover a broad range of rotational excitation. This infrared database for OH can be used in two principal ways: (1) as an indirect tool for obtaining water production in comets simultaneously with the production of other parent volatiles, even when direct detections of H_2O are not available; and (2) as an observational constraint to models predicting the rotational distribution of rovibrationally excited OH produced by water photolysis
- …