72 research outputs found
Helicon Plasma Injector and Ion Cyclotron Acceleration Development in the VASIMR Experiment
In the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) radio frequency (rf) waves both produce the plasma and then accelerate the ions. The plasma production is done by action of helicon waves. These waves are circular polarized waves in the direction of the electron gyromotion. The ion acceleration is performed by ion cyclotron resonant frequency (ICRF) acceleration. The Advanced Space Propulsion Laboratory (ASPL) is actively developing efficient helicon plasma production and ICRF acceleration. The VASIMR experimental device at the ASPL is called VX-10. It is configured to demonstrate the plasma production and acceleration at the 10kW level to support a space flight demonstration design. The VX-10 consists of three electromagnets integrated into a vacuum chamber that produce magnetic fields up to 0.5 Tesla. Magnetic field shaping is achieved by independent magnet current control and placement of the magnets. We have generated both helium and hydrogen high density (>10(exp 18) cu m) discharges with the helicon source. ICRF experiments are underway. This paper describes the VX-10 device, presents recent results and discusses future plans
The Vasimr Engine: Project Status and Recent Accomplishments
The development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) was initiated in the late 1970s to address a critical requirement for fast, high-power interplanetary space transportation. While not being a fusion rocket, it nevertheless borrows heavily from that technology and takes advantage of the natural topology of open-ended magnetic systems. In addition to its high power density and high exhaust velocity, VASIMR is capable of "constant power throttling" a feature, which allows in-flight mission-optimization of thrust and specific impulse to enhance performance and reduce trip time. A NASA-led, research team, involving industry, academia and government facilities is pursuing the development of this concept in the United States. The technology can be validated, in the near term, in venues such as the International Space Station, where it can also serve as both a drag compensation device and a plasma contactor for the orbital facility. Other near-Earth applications in the commercial and scientific satellite sectors are also envisioned. This presentation covers the evolution of the VASIMR concept to its present status, as well as recent accomplishments in our understanding of the physics. Approaches and collaborative programs addressing the major technical challenges will also be presented
Development of caecaloids to study host-pathogen interactions: new insights into immunoregulatory functions of Trichuris muris extracellular vesicles in the caecum.
The caecum, an intestinal appendage in the junction of the small and large intestines, displays a unique epithelium that serves as an exclusive niche for a range of pathogens including whipworms (Trichuris spp.). While protocols to grow organoids from small intestine (enteroids) and colon (colonoids) exist, the conditions to culture organoids from the caecum have yet to be described. Here, we report methods to grow, differentiate and characterise mouse adult stem cell-derived caecal organoids, termed caecaloids. We compare the cellular composition of caecaloids with that of enteroids, identifying differences in intestinal epithelial cell populations that mimic those found in the caecum and small intestine. The remarkable similarity in the intestinal epithelial cell composition and spatial conformation of caecaloids and their tissue of origin enables their use as an in vitro model to study host interactions with important caecal pathogens. Thus, exploiting this system, we investigated the responses of caecal intestinal epithelial cells to extracellular vesicles secreted/excreted by the intracellular helminth Trichuris muris. Our findings reveal novel immunoregulatory effects of whipworm extracellular vesicles on the caecal epithelium, including the downregulation of responses to nucleic acid recognition and type-I interferon signalling
New species of the catfish genus Tatia (Siluriformes:Auchenipteridae) from the rio Teles Pires, upper rio Tapajós basin, Brazil
JWST UNCOVER: Discovery of Galaxy Candidates Behind the Lensing Cluster Abell 2744
We present the results of a search for high-redshift () galaxy
candidates in the JWST UNCOVER survey, using deep NIRCam and NIRISS imaging in
7 bands over arcmin and ancillary HST observations. The NIRCam
observations reach a limiting magnitude of AB. The
identification of high candidates relies on a combination of a dropout
selection and photometric redshifts. We find 16 candidates at and 3
candidates at , eight candidates are deemed very robust. Their lensing
amplification ranges from to 11.5. Candidates have a wide range of
(lensing-corrected) luminosities and young ages, with low stellar masses
( log(M/M) ) and low star formation rates
(SFR=0.2-7 M yr), confirming previous findings in early JWST
observations of . A few galaxies at appear to show a clear
Balmer break between the F356W and F444W/F410M bands, which helps constrain
their stellar mass. We estimate blue UV continuum slopes between
and , typical for early galaxies at but not as extreme as the
bluest recently discovered sources. We also find evidence for a rapid
redshift-evolution of the mass-luminosity relation and a redshift-evolution of
the UV continuum slope for a given range of intrinsic magnitude, in line with
theoretical predictions. These findings suggest that deeper JWST observations
are needed to reach the fainter galaxy population at those early epochs, and
follow-up spectroscopy will help better constrain the physical properties and
star formation histories of a larger sample of galaxies.Comment: Submitted to MNRA
A population of luminous accreting black holes with hidden mergers
Major galaxy mergers are thought to play an important part in fuelling the
growth of supermassive black holes. However, observational support for this
hypothesis is mixed, with some studies showing a correlation between merging
galaxies and luminous quasars and others showing no such association. Recent
observations have shown that a black hole is likely to become heavily obscured
behind merger-driven gas and dust, even in the early stages of the merger, when
the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations
further suggest that such obscuration and black-hole accretion peaks in the
final merger stage, when the two galactic nuclei are closely separated (less
than 3 kiloparsecs). Resolving this final stage requires a combination of
high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray
observations to detect highly obscured sources. However, large numbers of
obscured luminous accreting supermassive black holes have been recently
detected nearby (distances below 250 megaparsecs) in X-ray observations. Here
we report high-resolution infrared observations of hard-X-ray-selected black
holes and the discovery of obscured nuclear mergers, the parent populations of
supermassive-black-hole mergers. We find that obscured luminous black holes
(bolometric luminosity higher than 2x10^44 ergs per second) show a significant
(P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a
sample of inactive galaxies with matching stellar masses and star formation
rates (1.1 per cent), in agreement with theoretical predictions. Using
hydrodynamic simulations, we confirm that the excess of nuclear mergers is
indeed strongest for gas-rich major-merger hosts of obscured luminous black
holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the
authors' version of the wor
UNCOVER: Illuminating the Early Universe -- JWST/NIRSpec Confirmation of Galaxies
Observations of high-redshift galaxies provide a critical direct test to the
theories of early galaxy formation, yet to date, only four have been
spectroscopically confirmed at . Due to strong gravitational lensing over
a wide area, the galaxy cluster field Abell~2744 is ideal for searching for the
earliest galaxies. Here we present JWST/NIRSpec observations of two galaxies: a
robust detection at , and a plausible
candidate at . The galaxies are
discovered in JWST/NIRCam imaging and their distances are inferred with
JWST/NIRSpec spectroscopy, all from the JWST Cycle 1 UNCOVER Treasury survey.
Detailed stellar population modeling using JWST NIRCam and NIRSpec data
corroborates the primeval characteristics of these galaxies: low mass (), young, rapidly-assembling, metal-poor, and star-forming.
Interestingly, both galaxies are spatially resolved, having lensing-corrected
rest-UV effective radii on the order of 300-400 pc, which are notably larger
than other spectroscopically confirmed systems. The observed
dynamic range of size spans over an order of magnitude, implying
a significant scatter in the size-mass relation at early times. Deep into the
epoch of reionization, these discoveries elucidate the emergence of the first
galaxies.Comment: submitted to ApJL; 13 pages, 4 figures, 2 table
UNCOVER: The growth of the first massive black holes from JWST/NIRSpec -- spectroscopic confirmation of an X-ray luminous AGN at z=10.1
The James Webb Space Telescope is now detecting early black holes (BHs) as
they transition from "seeds" to supermassive BHs. Recently Bogdan et al. (2023)
reported the detection of an X-ray luminous supermassive BH, UHZ-1, with a
photometric redshift at . Such an extreme source at this very high
redshift provides new insights on seeding and growth models for BHs given the
short time available for formation and growth. Harnessing the exquisite
sensitivity of JWST/NIRSpec, here we report the spectroscopic confirmation of
UHZ-1 at . We find that the NIRSpec/Prism spectrum is
typical of recently discovered z~10 galaxies, characterized primarily by
star-formation features. We see no clear evidence of the powerful X-ray source
in the rest-frame UV/optical spectrum, which may suggest heavy obscuration of
the central BH, in line with the Compton-thick column density measured in the
X-rays. We perform a stellar population fit simultaneously to the new NIRSpec
spectroscopy and previously available photometry. The fit yields a stellar mass
estimate for the host galaxy that is significantly better constrained than
prior photometric estimates ().
Given the predicted BH mass (), the resulting
ratio of remains two to three orders of magnitude higher than
local values, thus lending support to the heavy seeding channel for the
formation of supermassive BHs within the first billion years of cosmic
evolution.Comment: 9 pages, 4 figures, submitted to ApJL. Minor text correction
UNCOVER: Candidate Red Active Galactic Nuclei at 3<z<7 with JWST and ALMA
The James Webb Space Telescope (JWST) is revolutionizing our knowledge of
galaxies and their actively accreting black holes. Using the JWST Cycle 1
Treasury program Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of
Reionization (UNCOVER) in the lensing field Abell 2744, we report the
identification of a sample of little red dots at that
likely contain highly-reddened accreting supermassive black holes. Using a
NIRCam-only selection to F444W mag, we find 26 sources over the
arcmin field that are blue in F115WF200W (or for ), red in F200WF444W =
(), and are dominated by a point-source like
central component. Of the 20 sources with deep ALMA 1.2-mm coverage, none are
detected individually or in a stack. For the majority of the sample, SED fits
to the JWST+ALMA observations prefer models with hot dust rather than obscured
star-formation to reproduce the red NIRCam colors and ALMA 1.2-mm
non-detections. While compact dusty star formation can not be ruled out, the
combination of extremely small sizes ( pc after
correction for magnification), red rest-frame optical slopes, and hot dust can
by explained by reddened broad-line active galactic nuclei (AGNs). Our targets
have faint mag but inferred
bolometric luminosities of erg/s, reflecting
their obscured nature. If the candidates are confirmed as AGNs with upcoming
UNCOVER spectroscopy, then we have found an abundant population of reddened
luminous AGN that are at least ten times more numerous than UV-luminous AGN at
the same intrinsic bolometric luminosity.Comment: submitted to Ap
UNCOVER: A NIRSpec Identification of a Broad-line AGN at z = 8.50
Deep observations with the James Webb Space Telescope (JWST) have revealed an emerging population of red pointlike sources that could provide a link between the postulated supermassive black hole seeds and observed quasars. In this work, we present a JWST/NIRSpec spectrum from the JWST Cycle 1 UNCOVER Treasury survey of a massive accreting black hole at z = 8.50 displaying a clear broad-line component as inferred from the Hβ line with FWHM = 3439 ± 413 km s−1, typical of the broad-line region of an active galactic nucleus (AGN). The AGN nature of this object is further supported by high ionization, as inferred from emission lines, and a point-source morphology. We compute a black hole mass of
and a bolometric luminosity of Lbol ∼ 6.6 × 1045 erg s−1. These values imply that our object is accreting at ∼40% of the Eddington limit. Detailed modeling of the spectral energy distribution in the optical and near-infrared, together with constraints from ALMA, indicate an upper limit on the stellar mass of , which would lead to an unprecedented ratio of black hole to host mass of at least ∼30%. This is orders of magnitude higher compared to the local QSOs but consistent with recent AGN studies at high redshift with JWST. This finding suggests that a nonnegligible fraction of supermassive black holes either started out from massive seeds and/or grew at a super-Eddington rate at high redshift. Given the predicted number densities of high-z faint AGN, future NIRSpec observations of larger samples will allow us to further investigate galaxy–black hole coevolution in the early Universe
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