1,671 research outputs found
New Constraints on the Escape of Ionizing Photons From Starburst Galaxies Using Ionization-Parameter Mapping
The fate of ionizing radiation in starburst galaxies is key to understanding
cosmic reionization. However, the galactic parameters on which the escape
fraction of ionizing radiation depend are not well understood.
Ionization-parameter mapping provides a simple, yet effective, way to study the
radiative transfer in starburst galaxies. We obtain emission-line ratio maps of
[SIII]/[SII] for six, nearby, dwarf starbursts: NGC 178, NGC 1482, NGC 1705,
NGC 3125, NGC 7126, and He 2-10. The narrow-band images are obtained with the
Maryland-Magellan Tunable Filter at Las Campanas Observatory. Using these data,
we previously reported the discovery of an optically thin ionization cone in
NGC 5253, and here we also discover a similar ionization cone in NGC 3125. This
latter cone has an opening angle of 40+/-5 degrees (0.4 ster), indicating that
the passageways through which ionizing radiation may travel correspond to a
small solid angle. Additionally, there are three sample galaxies that have
winds and/or superbubble activity, which should be conducive to escaping
radiation, yet they are optically thick. These results support the scenario
that an orientation bias limits our ability to directly detect escaping Lyman
continuum in many starburst galaxies. A comparison of the star-formation
properties and histories of the optically thin and thick galaxies is consistent
with the model that high escape fractions are limited to galaxies that are old
enough (> 3 Myr) for mechanical feedback to have cleared optically thin
passageways in the ISM, but young enough (< 5 Myr) that the ionizing stars are
still present.Comment: Accepted for publication in Ap
An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253
There are few observational constraints on how the escape of ionizing photons
from starburst galaxies depends on galactic parameters. Here, we report on the
first major detection of an ionization cone in NGC 5253, a nearby starburst
galaxy. This high-excitation feature is identified by mapping the emission-line
ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha
narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas
Observatory. The ionization cone appears optically thin, which is suggestive of
the escape of ionizing photons. The cone morphology is narrow with an estimated
solid angle covering just 3% of 4pi steradians, and the young, massive clusters
of the nuclear starburst can easily generate the radiation required to ionize
the cone. Although less likely, we cannot rule out the possibility of an
obscured AGN source. An echelle spectrum along the minor axis shows complex
kinematics that are consistent with outflow activity. The narrow morphology of
the ionization cone supports the scenario that an orientation bias contributes
to the difficulty in detecting Lyman continuum emission from starbursts and
Lyman break galaxies.Comment: 5 pages, 4 figures, Accepted to ApJ Letter
On small homotopies of loops
Two natural questions are answered in the negative:
(1) If a space has the property that small nulhomotopic loops bound small
nulhomotopies, then are loops which are limits of nulhomotopic loops themselves
nulhomotopic?
(2) Can adding arcs to a space cause an essential curve to become
nulhomotopic?
The answer to the first question clarifies the relationship between the
notions of a space being homotopically Hausdorff and -shape injective.Comment: 12 pages, 5 figure
Constructing near-embeddings of codimension one manifolds with countable dense singular sets
The purpose of this paper is to present, for all , very simple
examples of continuous maps from closed -manifolds
into closed -manifold such that even though the singular set
of is countable and dense, the map can nevertheless be
approximated by an embedding, i.e. is a {\sl near-embedding}
Calibration of the charge exchange recombination spectroscopy diagnostic for core poloidal rotation velocity measurements on JET
Ejecta Knot Flickering, Mass Ablation, and Fragmentation in Cassiopeia A
Ejecta knot flickering, ablation tails, and fragmentation are expected signatures associated with the gradual dissolution of high-velocity supernova (SN) ejecta caused by their passage through an inhomogeneous circumstellar medium or interstellar medium (ISM). Such phenomena mark the initial stages of the gradual merger of SN ejecta with and the enrichment of the surrounding ISM. Here we report on an investigation of this process through changes in the optical flux and morphology of several high-velocity ejecta knots located in the outskirts of the young core-collapse SN remnant Cassiopeia A using Hubble Space Telescope images. Examination of WFPC2 F675W and combined ACS F625W + F775W images taken between 1999 June and 2004 December of several dozen debris fragments in the remnant's northeast ejecta stream and along the remnant's eastern limb reveal substantial emission variations ("flickering") over timescales as short as nine months. Such widespread and rapid variability indicates knot scale lengths similar or equal to 10(15) cm and a highly inhomogeneous surrounding medium. We also identify a small percentage of ejecta knots located all around the remnant's outer periphery which show trailing emissions typically 0 ''.2-0 ''.7 in length aligned along the knot's direction of motion suggestive of knot ablation tails. We discuss the nature of these trailing emissions as they pertain to ablation cooling, knot disruption, and fragmentation, and draw comparisons to the emission "strings" seen in eta Car. Finally, we identify several tight clusters of small ejecta knots which resemble models of shock-induced fragmentation of larger SN ejecta knots caused by a high-velocity interaction with a lower density ambient medium
Photophysics of closed- and open-ring isomers of a diarylethene with a carboxylic anchor group
We study the transient photophysical properties of a diarylethene with a
carboxylic anchor group by a combination of steady-state and ultrafast
emission and absorption spectroscopy. After excitation of the closed-ring
form, fluorescence with a quantum yield of 10−5 is observed and separated into
different spectro-temporal components. The S1 state of the closed-ring form
shows a lifetime of 1.3 ps and decays mainly by internal conversion to the S0
state of this isomer. This vibrationally hot ground state cools on a time
scale of 10 ps
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