374 research outputs found
Robust self-replication of combinatorial information via crystal growth and scission
Understanding how a simple chemical system can accurately replicate combinatorial information, such as a sequence, is an important question for both the study of life in the universe and for the development of evolutionary molecular design techniques. During biological sequence replication, a nucleic acid polymer serves as a template for the enzyme-catalyzed assembly of a complementary sequence. Enzymes then separate the template and complement before the next round of replication. Attempts to understand how replication could occur more simply, such as without enzymes, have largely focused on developing minimal versions of this replication process. Here we describe how a different mechanism, crystal growth and scission, can accurately replicate chemical sequences without enzymes. Crystal growth propagates a sequence of bits while mechanically-induced scission creates new growth fronts. Together, these processes exponentially increase the number of crystal sequences. In the system we describe, sequences are arrangements of DNA tile monomers within ribbon-shaped crystals. 99.98% of bits are copied correctly and 78% of 4-bit sequences are correct after two generations; roughly 40 sequence copies are made per growth front per generation. In principle, this process is accurate enough for 1,000-fold replication of 4-bit sequences with 50% yield, replication of longer sequences, and Darwinian evolution. We thus demonstrate that neither enzymes nor covalent bond formation are required for robust chemical sequence replication. The form of the replicated information is also compatible with the replication and evolution of a wide class of materials with precise nanoscale geometry such as plasmonic nanostructures or heterogeneous protein assemblies
Introducing Change in Public Service Organizations under Austerity: The Complex Case of the Governance of the Defence in the United Kingdom
Introducing, managing, and sustaining change in public service organizations is challenging for policy makers to implement and for scholars to theorize. In 2010, the U.K. Government introduced policy changes to help bring down the national deficit. The executive's planned reforms aimed to deliver a soâcalled battleâwinning military force, a smaller and more professional Ministry of Defence, and an affordable overall defence organization. The article borrows from theories of management and public policy to help enlighten our understanding of change under New Public Management and governance approaches. The article's central claim is that the U.K. Government sought to correct costâefficiency processes in public service organizations trying to reshape organizational and managerial structures dependent on many internal and external pressures. The article examines the executive's purpose in developing a need for change and the ways to implement it. I question whether the U.K. Government's prescriptive and hierarchical approach to organizational change in public administration is sustainable in the long term
The VANDELS survey: A strong correlation between Ly equivalent width and stellar metallicity at
We present the results of a new study investigating the relationship between
observed Ly equivalent width ((Ly)) and the
metallicity of the ionizing stellar population () for a sample of
star-forming galaxies at drawn from the VANDELS survey.
Dividing our sample into quartiles of rest-frame (Ly)
across the range -58 \unicode{xC5} \lesssim (Ly)
\lesssim 110 \unicode{xC5} we determine from full spectral
fitting of composite far-ultraviolet (FUV) spectra and find a clear
anti-correlation between (Ly) and . Our results
indicate that decreases by a factor between the lowest
(Ly) quartile
((Ly)\rangle=-18\unicode{xC5}) and the highest
(Ly) quartile
((Ly)\rangle=24\unicode{xC5}). Similarly,
galaxies typically defined as Lyman Alpha Emitters (LAEs;
(Ly) >20\unicode{xC5}) are, on average, metal poor with
respect to the non-LAE galaxy population ((Ly)
\leq20\unicode{xC5}) with
. Finally, based on the best-fitting stellar models, we
estimate that the increasing strength of the stellar ionizing spectrum towards
lower is responsible for of the observed variation
in (Ly) across our sample, with the remaining contribution
() being due to a decrease in the HI/dust covering fractions in
low galaxies.Comment: 10 pages, 6 figures, MNRAS accepte
A first look at JWST CEERS: massive quiescent galaxies from 3 < z < 5
We report a robust sample of 9 massive quiescent galaxies at redshift, , selected using the first data from the JWST CEERS programme. Three of these
galaxies are at , constituting the best evidence to date for
quiescent galaxies significantly before . These extreme galaxies have
stellar masses in the range logM, and formed
the bulk of their mass at , with two objects having star-formation
histories that suggest they had already reached logM
by . We report number densities for our sample, demonstrating that
previous work underestimated the number of quiescent galaxies at by
at least a factor of , due to a lack of ultra-deep imaging data at
m. This result deepens the existing tension between
observations and theoretical models, which already struggle to reproduce
previous estimates of quiescent galaxy number densities. Upcoming
wider-area JWST imaging surveys will provide larger samples of such galaxies,
as well as providing opportunities to search for quiescent galaxies at .
The galaxies we report are excellent potential targets for JWST NIRSpec
spectroscopy, which will be required to understand in detail their physical
properties, providing deeper insights into the processes responsible for
quenching star formation during the first billion years.Comment: 11 pages, 6 figures, submitted to MNRA
A surprising abundance of massive quiescent galaxies at 3 < z < 5 in the first data from JWST CEERS
A census of star formation histories of massive galaxies at 0.6 < z < 1 from spectro-photometric modeling using Bagpipes and Prospector
We present individual star-formation histories of massive galaxies
(log() > 10.5) from the Large Early Galaxy Astrophysics
Census (LEGA-C) spectroscopic survey at a lookback time of 7 billion
years and quantify the population trends leveraging 20hr-deep integrated
spectra of these 1800 star-forming and 1200 quiescent galaxies at
0.6 < < 1.0. Essentially all galaxies at this epoch contain stars of age <
3 Gyr, in contrast with older massive galaxies today, facilitating better
recovery of previous generations of star formation at cosmic noon and earlier.
We conduct spectro-photometric analysis using parametric and non-parametric
Bayesian SPS modeling tools - Bagpipes and Prospector to constrain the median
star-formation histories of this mass-complete sample and characterize
population trends. A consistent picture arises for the late-time stellar mass
growth when quantified as and , corresponding to the age of
the universe when galaxies formed 50\% and 90\% of their total stellar mass,
although the two sets of models disagree at the earliest formation times (e.g.
). Our results reveal trends in both stellar mass and stellar velocity
dispersion as in the local universe - low-mass galaxies with shallower
potential wells grow their stellar masses later in cosmic history compared to
high-mass galaxies. Unlike local quiescent galaxies, the median duration of
late-time star-formation ( = - ) does not
consistently depend on the stellar mass. This census sets a benchmark for
future deep spectro-photometric studies of the more distant universe.Comment: 25 pages, 12 figures, 4 tables, submitted to Ap
Structural and optical properties on thulium-doped LHPG-grown Ta2O5 fibres
Structural, spectroscopic and dielectric properties of thulium-doped laser-heated pedestal Ta2O5 as-grown fibres were studied. Undoped samples grow preferentially with a single crystalline monoclinic structure. The fibre with the lowest thulium content (0.1 at%) also shows predominantly a monoclinic phase and no intra-4f12 Tm3+ recombination was observed. For sample with the highest thulium amount (1.0 at%), the appearance of a dominant triclinic phase as well as intraionic optical activation was observed. The dependence of photoluminescence on excitation energy allows identification of different site locations of Tm3+ ions in the lattice. The absence of recombination between the first and the ground-state multiplets as well as the temperature dependence of the observed transitions was justified by an efficient energy transfer between the Tm3+ ions. Microwave dielectric properties were investigated using the small perturbation theory. At a frequency of 5 GHz, the undoped material exhibits a dielectric permittivity of 21 and for thulium-doped Ta2O5 samples it decreases to 18 for the highest doping concentration. Nevertheless, the dielectric losses maintain a very low value.FCT - PTDC/CTM/66195/2006FCT - PTDC/FIS/66262/2006FCT - PTDC/FIS/72843/200
The VANDELS survey: Global properties of CIII]lambda 1908 angstrom emitting star-forming galaxies at z similar to 3
Context. Strong nebular emission is ubiquitous in galaxies that contribute to cosmic reionization at redshift za a 6. High-ionization UV metal lines, such as CIII]λ1908 A, show high equivalent widths (EW) in these early galaxies, suggesting harder radiation fields at low metallicity than low-z galaxies of similar stellar mass. Understanding the physical properties driving the observed UV nebular line emission at high-z requires large and very deep spectroscopic surveys, which are now only accessible out to za a a 4. Aims. We study the mean properties of a large representative sample of 217 galaxies showing CIII] emission at 2a <a za <a 4, selected from a parent sample of a 750 main-sequence star-forming galaxies in the VANDELS survey. These CIII] emitters have a broad range of UV luminosities, allowing for a detailed stacking analysis to characterize their stellar mass, star formation rate (SFR), and metallicity as a function of the UV emission line ratios, EWs, and the carbon-to-oxygen (C/O) abundance ratio. Methods. Stacking provides unprecedented high signal-to-noise (S/N) spectra for CIII] emitters over more than three decades in luminosity, stellar mass, and SFR. This enables a full spectral fitting to derive stellar metallicities for each stack. Moreover, we use diagnostics based on photoionization models and UV line ratios to constrain the ionization sources of the galaxies and derive the C/O abundance. Results. Reliable CIII] detections (S/Na a „a 3) represent a 30% of the parent sample. However, stacked spectra of non-detections (S/Na <a 3) show weak (EW a 2 A) CIII] emission, suggesting that this line is common in normal star-forming galaxies at za a a 3. On the other hand, extreme CIII] emitters (EW(CIII]) a 8 A) are exceedingly rare (a 3%) in VANDELS. The UV line ratios of the sample suggest no ionization source other than massive stars. Stacks with larger EW(CIII]) show larger EW(Lyα) and lower metallicity, but not all CIII] emitters are Lyα emitters. The stellar metallicities of CIII] emitters are not significantly different from that of the parent sample, increasing from a 10% to a 40% solar for stellar masses log(Ma/Ma) a 9a? 10.5. The stellar mass-metallicity relation of the CIII] emitters is consistent with previous works, exhibiting a strong evolution from za =a 0 to za a a 3. The C/O abundances of the sample range between 35%a? 150% solar, with a noticeable increase with FUV luminosity and a smooth decrease with the CIII] EW. Here, we discuss the CIII] emitters in the C/Oa Fe/H and the C/Oa O/H planes and we find that they follow stellar and nebular abundance trends consistent with those of Milky Way halo and thick-disk stars and local HII galaxies, respectively. A qualitative agreement is also found with chemical evolution models, which suggests that CIII] emitters at za a a 3 are experiencing an active phase of chemical enrichment. Conclusions. Our results provide new insights into the nature of UV line emitters at za a a 2a 4, paving the way for future studies at higher z using the James Webb Space Telescope
Cosmic Vine: A z=3.44 Large-Scale Structure Hosting Massive Quiescent Galaxies
We report the discovery of a large-scale structure at z=3.44 revealed by JWST
data in the EGS field. This structure, dubbed "Cosmic Vine", consists of 20
galaxies with spectroscopic redshifts at and six galaxy
overdensities with consistent photometric redshifts, making up a vine-like
structure extending over a ~4x0.2 pMpc^2 area. The two most massive galaxies
(M*~10^10.9 Msun) of the Cosmic Vine are found to be quiescent with
bulge-dominated morphologies (). Comparisons with simulations suggest
that the Cosmic Vine would form a cluster with halo mass >10^14 Msun at z=0,
and the two massive galaxies are likely forming the brightest cluster galaxies
(BCGs). The results unambiguously reveal that massive quiescent galaxies can
form in growing large-scale structures at z>3, thus disfavoring the
environmental quenching mechanisms that require a virialized cluster core.
Instead, as suggested by the interacting and bulge-dominated morphologies, the
two galaxies are likely quenched by merger-triggered starburst or AGN feedback
before falling into a cluster core. Moreover, we found that the observed
specific star formation rates of massive quiescent galaxies in z>3 dense
environments are two orders of magnitude lower than that of the BCGs in the
TNG300 simulation. This discrepancy potentially poses a challenge to the models
of massive cluster galaxy formation. Future studies comparing a large sample
with dedicated cluster simulations are required to solve the problem.Comment: Submitted to A&
The VANDELS survey: the ionizing properties of star-forming galaxies at using deep rest-frame ultraviolet spectroscopy
To better understand the ionizing properties of galaxies in the EoR, we
investigate deep, rest-frame ultraviolet (UV) spectra of
star-forming galaxies at selected from the public ESO-VANDELS
spectroscopic survey. The absolute ionizing photon escape fraction () is derived by combining absorption line measurements with
estimates of the UV attenuation. The ionizing production efficiency
() is calculated by fitting the far-UV (FUV) stellar continuum of
the VANDELS galaxies. We find that the and
parameters increase towards low-mass, blue UV-continuum slopes and strong
Ly emitting galaxies, and both are just slightly higher-than-average
for the UV-faintest galaxies in the sample. Potential Lyman Continuum Emitters
(LCEs) and selected Lyman Alpha Emitters (LAEs) show systematically higher
( (Hz\erg) ) than non-LCEs
and non-LAEs ( (Hz\erg) ) at similar UV
magnitudes. This indicates very young underlying stellar populations () at relatively low metallicities ().
The FUV non-ionizing spectra of potential LCEs is characterized by very blue UV
slopes (), enhanced Ly emission (A), strong UV
nebular lines (e.g., high CIV1550/CIII]1908 ratios), and weak
absorption lines (A). The latter suggests the existence of low
gas-column-density channels in the interstellar medium which enables the escape
of ionizing photons. By comparing our VANDELS results against other surveys in
the literature, our findings imply that the ionizing budget in the EoR was
likely dominated by UV-faint, low-mass and dustless galaxies.Comment: 25 pages, 18 figures, 2 tables; submitted to MNRA
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