6,592 research outputs found
A Successful Broad-band Survey for Giant Lya Nebulae I: Survey Design and Candidate Selection
Giant Lya nebulae (or Lya "blobs") are likely sites of ongoing massive galaxy
formation, but the rarity of these powerful sources has made it difficult to
form a coherent picture of their properties, ionization mechanisms, and space
density. Systematic narrow-band Lya nebula surveys are ongoing, but the small
redshift range covered and the observational expense limit the comoving volume
that can be probed by even the largest of these surveys and pose a significant
problem when searching for such rare sources. We have developed a systematic
search technique designed to find large Lya nebulae at 2<z<3 within deep
broad-band imaging and have carried out a survey of the 9.4 square degree NOAO
Deep Wide-Field Survey (NDWFS) Bootes field. With a total survey comoving
volume of ~10^8 h^-3_70 Mpc^3, this is the largest volume survey for Lya
nebulae ever undertaken. In this first paper in the series, we present the
details of the survey design and a systematically-selected sample of 79
candidates, which includes one previously discovered Lya nebula.Comment: Accepted to ApJ after minor revision; 25 pages in emulateapj format;
18 figures, 3 table
On the Automated and Objective Detection of Emission Lines in Faint-Object Spectroscopy
Modern spectroscopic surveys produce large spectroscopic databases, generally
with sizes well beyond the scope of manual investigation. The need arises,
therefore, for an automated line detection method with objective indicators for
detection significance. In this paper, we present an automated and objective
method for emission line detection in spectroscopic surveys and apply this
technique to 1574 spectra, obtained with the Hectospec spectrograph on the MMT
Observatory (MMTO), to detect Lyman alpha emitters near z ~ 2.7. The basic idea
is to generate on-source (signal plus noise) and off-source (noise only) mock
observations using Monte Carlo simulations, and calculate completeness and
reliability values, (C, R), for each simulated signal. By comparing the
detections from real data with the Monte Carlo results, we assign the
completeness and reliability values to each real detection. From 1574 spectra,
we obtain 881 raw detections and, by removing low reliability detections, we
finalize 649 detections from an automated pipeline. Most of high completeness
and reliability detections, (C, R) ~ (1.0, 1.0), are robust detections when
visually inspected; the low C and R detections are also marginal on visual
inspection. This method at detecting faint sources is dependent on the accuracy
of the sky subtraction.Comment: 19 pages, 16 figures, submitted to MNRA
Spatially Resolved Gas Kinematics within a Ly Nebula: Evidence for Large-scale Rotation
We use spatially extended measurements of Ly as well as less
optically thick emission lines from an 80 kpc Ly nebula at
to assess the role of resonant scattering and to disentangle
kinematic signatures from Ly radiative transfer effects. We find that
the Ly, CIV, HeII, and CIII] emission lines all tell a similar story in
this system, and that the kinematics are broadly consistent with large-scale
rotation. First, the observed surface brightness profiles are similar in extent
in all four lines, strongly favoring a picture in which the Ly photons
are produced in situ instead of being resonantly scattered from a central
source. Second, we see low kinematic offsets between Ly and the less
optically thick HeII line (100-200 km s), providing further
support for the argument that the Ly and other emission lines are all
being produced within the spatially extended gas. Finally, the full velocity
field of the system shows coherent velocity shear in all emission lines:
500 km s over the central 50 kpc of the nebula. The
kinematic profiles are broadly consistent with large-scale rotation in a gas
disk that is at least partially stable against collapse. These observations
suggest that the Ly nebula represents accreting material that is
illuminated by an offset, hidden AGN or distributed star formation, and that is
undergoing rotation in a clumpy and turbulent gas disk. With an implied mass of
M(<R=20 kpc) , this system may represent the
early formation of a large Milky Way mass galaxy or galaxy group.Comment: Accepted to ApJ; 25 pages in emulateapj format; 15 figures, 4 table
Preterm infants have deficient monocyte and lymphocyte cytokine responses to Group B Streptococcus
Group B streptococcus GBS) is an important cause of early-and late-onset sepsis in the newborn. Preterm infants have markedly increased susceptibility and worse outcomes, but their immunological responses to GBS are poorly defined. We compared mononuclear cell and whole-blood cytokine responses to heat-killed GBS HKGBS) of preterm infants gestational age [GA], 26 to 33 weeks), term infants, and healthy adults. We investigated the kinetics and cell source of induced cytokines and quantified HKGBS phagocytosis. HKGBS-induced tumor necrosis factor TNF) and interleukin 6 (IL-6) secretion was significantly impaired in preterm infants compared to that in term infants and adults. These cytokines were predominantly monocytic in origin, and production was intrinsically linked to HKGBS phagocytosis. Very preterm infants GA, < 30 weeks) had fewer cytokine-producing monocytes, but nonopsonic phagocytosis ability was comparable to that for term infants and adults. Exogenous complement supplementation increased phagocytosis in all groups, as well as the proportion of preterm monocytes producing IL-6, but for very preterm infants, responses were still deficient. Similar defective preterm monocyte responses were observed in fresh whole cord blood stimulated with live GBS. Lymphocyte-associated cytokines were significantly deficient for both preterm and term infants compared to levels for adults. These findings indicate that a subset of preterm monocytes do not respond to GBS, a defect compounded by generalized weaker lymphocyte responses in newborns. Together these deficient responses may increase the susceptibility of preterm infants to GBS infection
Is the short-latency dopamine response too short to signal reward error?
Unexpected stimuli that are behaviourally significant have the capacity to elicit a short-latency, short-duration burst of firing in mesencephalic dopaminergic neurones. An influential interpretation of the experimental data that characterize this response proposes that dopaminergic neurones have a crucial role in reinforcement learning because they signal error in the prediction of future reward. In this article we propose a different functional role for this ‘short-latency dopamine response’ in the mechanisms that underlie associative learning. We suggest that the initial burst of dopaminergic-neurone firing could represent an essential component in the process of switching attentional and behavioural selections to unexpected, behaviourally important stimuli. This switching response could be a crucial prerequisite for associative learning and might be part of a general short-latency response that is mediated by catecholamines and prepares the organism for an appropriate reaction to biologically significant events.
Any act which in a given situation produces satisfaction becomes associated with that situation so that when the situation recurs the act is more likely than before to recur also. E.L. Thorndike (1911)
The basal ganglia: A vertebrate solution to the selection problem?
A selection problem arises whenever two or more competing systems seek simultaneous access
to a restricted resource. Consideration of several selection architectures suggests there are significant
advantages for systems which incorporate a central switching mechanism. We propose that the vertebrate
basal ganglia have evolved as a centralized selection device, specialized to resolve conflicts over access to
limited motor and cognitive resources. Analysis of basal ganglia functional architecture and its position
within a wider anatomical framework suggests it can satisfy many of the requirements expected of an
efficient selection mechanism
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