2,110 research outputs found
Magnetic Merging in Colliding Flux Tubes
We develop an analytical theory of reconnection between colliding, twisted
magnetic flux tubes. Our analysis is restricted to direct collisions between
parallel tubes and is based on the collision dynamics worked out by Bogdan
(1984). We show that there is a range of collision velocities for which neutral
point reconnection of the Parker-Sweet type can occur, and a smaller range for
which reconnection leads to coalescence. Mean velocities within the solar
convection zone are probably significantly greater than the upper limit for
coalescence. This suggests that the majority of flux tube collisions do not
result in merging, unless the frictional coupling of the tubes to the
background flow is extremely strong.Comment: uuencoded-compressed-tarred PostScript (un-tars to 2 postscript
files), 15pp text + 3 figures. Accepted for publication in The Astrophysical
Journal. POP-00
Young red supergiants and the near infrared light appearance of disk galaxies
Disk galaxies often show prominent nonaxisymmetric features at near-infrared
wavelengths. Such features may indicate variations in the surface density of
stellar mass, contributions from young red supergiants in star forming regions,
or substantial dust obscuration. To distinguish among these possibilities, we
have searched for spatial variations in the 2.3 micron photometric CO index
within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This
index measures the strength of the absorption bands of molecular CO in stellar
atmospheres, and is strong in cool, low surface-gravity stars, reaching the
largest values for red supergiants. We observe significant spatial CO index
variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant
stellar population in the near-infrared is not everywhere the same. Central CO
index peaks are present in two galaxies; these could be due to either
metallicity gradients or recent star formation activity. In addition,
significant azimuthal CO index variations are seen in NGC 278. Because strong
azimuthal metallicity gradients are physically implausible in disk galaxies,
these features are most naturally explained by the presence of a young stellar
population. The fraction of 2 micron light due to young stellar populations in
star forming regions can be calculated from our data. Overall, young stellar
populations can contribute ~3% of a (normal) galaxy's near infrared flux.
Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the
total near infrared flux, but can be locally dominant in star forming regions,
and can bias estimates of spiral arm amplitude or other nonaxisymmetric
structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg
format. Uses AASTeX. Accepted for publication in The Astronomical Journa
Searching for GRB remnants in nearby galaxies
Gamma Ray Bursts (GRBs) are expected to leave behind GRB remnants, similar to
how ``standard'' supernovae (SN) leave behind SN remnants. The identification
of these remnants in our own and in nearby galaxies would allow a much closer
look at GRB birth sites, and possibly lead to the discovery of the compact
object left behind. It would also provide independent constraints on GRB rates
and energetics. We have initiated an observational program to search for GRB
remnants in nearby galaxies. The identification is based on specific line
ratios, such as OIII/ and HeII/, which are expected to be
unusually high in case of GRB remnants according to the theoretical predictions
of Perna et al. (2000). The observing strategies and preliminary studies from a
test run at 2.34 m VBT as well as archival data from planetary nebulae surveys
of spiral galaxies are discussed.Comment: 4 pages, uses aipproc.cls To appear in AIP, proceeding of GRB
conference '30 years after the discovery' held at Santa Fe, NM from 8-12
Sept, 200
Potential probiotic approaches to control Legionella in engineered aquatic ecosystems
Opportunistic pathogens belonging to the genus Legionella are among the most reported waterborne-associated pathogens in industrialized countries. Legionella colonize a variety of engineered aquatic ecosystems and persist in biofilms where they interact with a multitude of other resident microorganisms. In this review, we assess how some of these interactions could be used to develop a biological-driven "probiotic" control approach against Legionella. We focus on: (i) mechanisms limiting the ability of Legionella to establish and replicate within some of their natural protozoan hosts; (ii) exploitative and interference competitive interactions between Legionella and other microorganisms; and (iii) the potential of predatory bacteria and phages against Legionella. This field is still emergent, and we therefore specifically highlight research for future investigations, and propose perspectives on the feasibility and public acceptance of a potential probiotic approach.
Keywords: Legionella; antagonism; biofilm; competition; pathogen–host interaction; predation; probiotics; protozoa
First Results from the Large Area Lyman Alpha Survey
We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman
Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume
and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD
Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National
Observatory.
Well-detected sources with flux and equivalent width matching known high
redshift Lyman alpha galaxies (i.e., observed equivalent width above 80
Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in
an 80 Angstrom filter) have an observed surface density corresponding to 11000
+- 700 per square degree per unit redshift at z=4.5. Spatial variation in this
surface density is apparent on comparison between counts in 6561 and 6730
Angstrom filters.
Early spectroscopic followup results from the Keck telescope included three
sources meeting our criteria for good Lyman alpha candidates. Of these, one is
confirmed as a z=4.52 source, while another remains consistent with either
z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide
Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies
per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The
Astrophysical Journal Letter
Ly at Cosmic Dawn with a Simulated Roman Grism Deep Field
The slitless grism on the Nancy Grace Roman Space Telescope will enable deep
near-infrared spectroscopy over a wide field of view. We demonstrate Roman's
capability to detect Ly galaxies at using a multi-position-angle
(PA) observational strategy. We simulate Roman grism data using a realistic
foreground scene from the COSMOS field. We also input fake Ly galaxies
spanning redshift z=7.5-10.5 and a line-flux range of interest. We show how a
novel data cube search technique -- CUBGRISM -- originally developed for GALEX
can be applied to Roman grism data to produce a Ly flux-limited sample
without the need for continuum detections. We investigate the impact of
altering the number of independent PAs and exposure time. A deep Roman grism
survey with 25 PAs and a total exposure time of hrs can achieve Ly
line depths comparable to the deepest narrow-band surveys
(erg s). Assuming a null result,
where the opacity of the intergalactic medium (IGM) remains unchanged from
, this level of sensitivity will detect deg Ly
emitters from . A decline from this expected number density is the
signature of an increasing neutral hydrogen fraction and the onset of
reionization. Our simulations indicate that a deep Roman grism survey has the
ability to measure the timing and magnitude of this decline, allowing us to
infer the ionization state of the IGM and helping us to distinguish between
models of reionization.Comment: 11 pages, 7 figures, Submitted to Ap
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
embargo until publishe
Probing Patchy Reionization with the Void Probability Function of Lyman- Emitters
We probe what constraints for the global ionized hydrogen fraction the Void
Probability Function (VPF) clustering can give for the Lyman-Alpha Galaxies in
the Epoch of Reionization (LAGER) narrowband survey as a function of area.
Neutral hydrogen acts like a fog for Lyman-alpha emission, and measuring the
drop in the luminosity function of Lyman- emitters (LAEs) has been used
to constrain the ionization fraction in narrowband surveys. However, the
clustering of LAEs is independent from the luminosity function's inherent
evolution, and can offer additional constraints for reionization under
different models. The VPF measures how likely a given circle is to be empty. It
is a volume-averaged clustering statistic that traces the behavior of higher
order correlations, and its simplicity offers helpful frameworks for planning
surveys.
Using the \citet{Jensen2014} simulations of LAEs within various amount of
ionized intergalactic medium, we predict the behavior of the VPF in one
(301x150.5x30 Mpc), four (5.44 Mpc), or eight (1.1 Mpc) fields of LAGER imaging. We examine the VPF at 5 and 13
arcminutes, corresponding to the minimum scale implied by the LAE density and
the separation of the 2D VPF from random, and the maximum scale from the
8-field 15.5 deg LAGER area. We find that even a single DECam field of
LAGER (2-3 deg) could discriminate between mostly neutral vs. ionized.
Additionally, we find four fields allows the distinction between 30, 50, and 95
percent ionized; and that eight fields could even distinguish between 30, 50,
73, and 95 percent ionized.Comment: 14 pages, 5 figure
Identifying high redshift AGNs using X-ray hardness
The X-ray color (hardness ratio) of optically undetected X-ray sources can be
used to distinguish obscured active galactic nuclei (AGNs) at low and
intermediate redshift from viable high-redshift (i.e., z>5) AGN candidates.
This will help determine the space density, ionizing photon production, and
X-ray background contribution of the earliest detectable AGNs. High redshift
AGNs should appear soft in X-rays, with hardness ratio HR ~ -0.5, even if there
is strong absorption by a hydrogen column density N_H up to 10^23 cm^-2, simply
because the absorption redshifts out of the soft X-ray band in the observed
frame. Here the X-ray hardness ratio is defined as HR= (H-S)/(H+S), where S and
H are the soft and hard band net counts detected by Chandra. High redshift AGNs
that are Compton thick (N_H>~10^24 cm^-2) could have HR~0.0 at z>5. However,
these should be rare in deep Chandra images, since they have to be >~10 times
brighter intrinsically, which implies >~100 times drop in their space density.
Applying the hardness criterion (HR<0.0) can filter out about 50% of the
candidate high redshift AGNs selected from deep Chandra images.Comment: 13 pages, including 3 figures, ApJ letter in pres
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