206 research outputs found
The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content
We report on new imaging observations of the Lyman alpha emission line (Lya),
performed with the Hubble Space Telescope, that comprise the backbone of the
Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies
at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far
ultraviolet continuum. We show that Lya is emitted on scales that
systematically exceed those of the massive stellar population and recombination
nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are
larger than those of Halpha by factors ranging from 1 to 3.6, with an average
of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much
of the Lya light is pushed to large radii by resonance scattering. Defining the
"Relative Petrosian Extension" of Lya compared to Halpha, \xi_ext = RP20_Lya /
RP20_Ha, we find \xi_ext to be uncorrelated with total Lya luminosity. However
\xi_ext is strongly correlated with quantities that scale with dust content, in
the sense that a low dust abundance is a necessary requirement (although not
the only one) in order to spread Lya photons throughout the interstellar medium
and drive a large extended Lya halo.Comment: Published in ApJ Letters ~~ 6 pages using emulateapj, 4 figures ~~
Higher-resolution, larger, nicer jpeg versions of Figures 1 and 2 can be
found here: http://xayes.org/pub/press_lars.htm
The Lyman Alpha Reference Sample: V. The impact of neutral ISM kinematics and geometry on Lyman Alpha escape
We present high-resolution far-UV spectroscopy of the 14 galaxies of the
Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low
redshifts (). We compare the derived properties to global
properties derived from multi band imaging and 21 cm HI interferometry and
single dish observations, as well as archival optical SDSS spectra. Besides the
Lyman line, the spectra contain a number of metal absorption features
allowing us to probe the kinematics of the neutral ISM and evaluate the optical
depth and and covering fraction of the neutral medium as a function of
line-of-sight velocity. Furthermore, we show how this, in combination with
precise determination of systemic velocity and good Ly spectra, can be
used to distinguish a model in which separate clumps together fully cover the
background source, from the "picket fence" model named by Heckman et al.
(2011). We find that no one single effect dominates in governing Ly
radiative transfer and escape. Ly escape in our sample coincides with a
maximum velocity-binned covering fraction of and bulk outflow
velocities of km s, although a number of galaxies show
these characteristics and yet little or no Ly escape. We find that
Ly peak velocities, where available, are not consistent with a strong
backscattered component, but rather with a simpler model of an intrinsic
emission line overlaid by a blueshifted absorption profile from the outflowing
wind. Finally, we find a strong anticorrelation between H equivalent
width and maximum velocity-binned covering factor, and propose a heuristic
explanatory model.Comment: 28 pages, 19 figures, 5 table
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
We present new H I imaging and spectroscopy of the 14 UV-selected
star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a
detailed study of the processes governing the production, propagation, and
escape of Ly photons. New H I spectroscopy, obtained with the 100m
Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the
14 observed LARS galaxies (although the profiles of two of the galaxies are
likely confused by other sources within the GBT beam); the three highest
redshift galaxies are not detected at our current sensitivity limits. The GBT
profiles are used to derive fundamental H I line properties of the LARS
galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS
galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging
localizes the H I gas and provides a measurement of the total H I mass in each
galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal
an enormous tidal structure that extends over 160 kpc from the main interacting
systems and that contains 10 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly escape fraction are coupled with the new direct
measurements of H I mass and significantly disturbed H I velocities. Our
robustly detected sample reveals that both total H I mass and linewidth are
tentatively correlated with key Ly tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68
We present HI spectral-line imaging of the extremely metal-poor galaxy DDO
68. This system has a nebular oxygen abundance of only 3% Z, making
it one of the most metal-deficient galaxies known in the local volume.
Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal
content, making it a significant outlier in the mass-metallicity and
luminosity-metallicity relationships. The origin of such a low oxygen abundance
in DDO 68 presents a challenge for models of the chemical evolution of
galaxies. One possible solution to this problem is the infall of pristine
neutral gas, potentially initiated during a gravitational interaction. Using
archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large
Array, we have discovered a previously unknown companion of DDO 68. This
low-mass (M 2.810 M), recently
star-forming (SFR 1.410 M yr,
SFR 710 M yr) companion has
the same systemic velocity as DDO 68 (V 506 km s; D
12.740.27 Mpc) and is located at a projected distance of 42 kpc. New HI
maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide
evidence that DDO 68 and this companion are gravitationally interacting at the
present time. Low surface brightness HI gas forms a bridge between these
objects.Comment: Accepted for publication in the Astrophysical Journal Letter
The Lyman alpha reference sample. VII. Spatially resolved H kinematics
We present integral field spectroscopic observations with the Potsdam Multi
Aperture Spectrophotometer of all 14 galaxies in the Lyman Alpha
Reference Sample (LARS). We produce 2D line of sight velocity maps and velocity
dispersion maps from the Balmer (H) emission in our data
cubes. These maps trace the spectral and spatial properties of the LARS
galaxies' intrinsic Ly radiation field. We show our kinematic maps
spatially registered onto the Hubble Space Telescope H and Lyman
(Ly) images. Only for individual galaxies a causal connection
between spatially resolved H kinematics and Ly photometry can
be conjectured. However, no general trend can be established for the whole
sample. Furthermore, we compute non-parametric global kinematical statistics --
intrinsic velocity dispersion , shearing velocity ,
and the ratio -- from our kinematic maps. In
general LARS galaxies are characterised by high intrinsic velocity dispersions
(54\,km\,s median) and low shearing velocities (65\,km\,s
median). values range from 0.5 to 3.2 with an
average of 1.5. Noteworthy, five galaxies of the sample are dispersion
dominated systems with and are thus
kinematically similar to turbulent star forming galaxies seen at high redshift.
When linking our kinematical statistics to the global LARS Ly
properties, we find that dispersion dominated systems show higher Ly
equivalent widths and higher Ly escape fractions than systems with
. Our result indicates that turbulence in
actively star-forming systems is causally connected to interstellar medium
conditions that favour an escape of Ly radiation.Comment: 26 pages, 15 figures, accepted for publication in A&
Intra- and inter-radiation therapist reproducibility of daily isocenter verification using prostatic fiducial markers
BACKGROUND: We sought to determine the intra- and inter-radiation therapist reproducibility of a previously established matching technique for daily verification and correction of isocenter position relative to intraprostatic fiducial markers (FM). MATERIALS AND METHODS: With the patient in the treatment position, anterior-posterior and left lateral electronic images are acquired on an amorphous silicon flat panel electronic portal imaging device. After each portal image is acquired, the therapist manually translates and aligns the fiducial markers in the image to the marker contours on the digitally reconstructed radiograph. The distances between the planned and actual isocenter location is displayed. In order to determine the reproducibility of this technique, four therapists repeated and recorded this operation two separate times on 20 previously acquired portal image datasets from two patients. The data were analyzed to obtain the mean variability in the distances measured between and within observers. RESULTS: The mean and median intra-observer variability ranged from 0.4 to 0.7 mm and 0.3 to 0.6 mm respectively with a standard deviation of 0.4 to 1.0 mm. Inter-observer results were similar with a mean variability of 0.9 mm, a median of 0.6 mm, and a standard deviation of 0.7 mm. When using a 5 mm threshold, only 0.5% of treatments will undergo a table shift due to intra or inter-observer error, increasing to an error rate of 2.4% if this threshold were reduced to 3 mm. CONCLUSION: We have found high reproducibility with a previously established method for daily verification and correction of isocenter position relative to prostatic fiducial markers using electronic portal imaging
ASIME 2018 White Paper. In-Space Utilisation of Asteroids: Asteroid Composition -- Answers to Questions from the Asteroid Miners
In keeping with the Luxembourg government's initiative to support the future
use of space resources, ASIME 2018 was held in Belval, Luxembourg on April
16-17, 2018.
The goal of ASIME 2018: Asteroid Intersections with Mine Engineering, was to
focus on asteroid composition for advancing the asteroid in-space resource
utilisation domain. What do we know about asteroid composition from
remote-sensing observations? What are the potential caveats in the
interpretation of Earth-based spectral observations? What are the next steps to
improve our knowledge on asteroid composition by means of ground-based and
space-based observations and asteroid rendez-vous and sample return missions?
How can asteroid mining companies use this knowledge?
ASIME 2018 was a two-day workshop of almost 70 scientists and engineers in
the context of the engineering needs of space missions with in-space asteroid
utilisation. The 21 Questions from the asteroid mining companies were sorted
into the four asteroid science themes: 1) Potential Targets, 2)
Asteroid-Meteorite Links, 3) In-Situ Measurements and 4) Laboratory
Measurements. The Answers to those Questions were provided by the scientists
with their conference presentations and collected by A. Graps or edited
directly into an open-access collaborative Google document or inserted by A.
Graps using additional reference materials. During the ASIME 2018, first day
and second day Wrap-Ups, the answers to the questions were discussed further.
New readers to the asteroid mining topic may find the Conversation boxes and
the Mission Design discussions especially interesting.Comment: Outcome from the ASIME 2018: Asteroid Intersections with Mine
Engineering, Luxembourg. April 16-17, 2018. 65 Pages. arXiv admin note:
substantial text overlap with arXiv:1612.0070
Neighborhood-Level Interventions to Improve Childhood Opportunity and Lift Children Out of Poverty
Abstract Population health is associated with the socioeconomic characteristics of neighborhoods. There is considerable scientific and policy interest in community-level interventions to alleviate child poverty. Intergenerational poverty is associated with inequitable access to opportunities. Improving opportunity structures within neighborhoods may contribute to improved child health and development. Neighborhood-level efforts to alleviate poverty for all children require alignment of cross-sector efforts, community engagement, and multifactorial approaches that consider the role of people as well as place. We highlight several accessible tools and strategies that health practitioners can engage to improve regional and local systems that influence child opportunity. The Child Opportunity Index is a population-level surveillance tool to describe community-level resources and inequities in US metropolitan areas. The case studies reviewed outline strategies for creating higher opportunity neighborhoods for pediatricians interested in working across sectors to address the impact of neighborhood opportunity on child health and well-being
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