54,107 research outputs found
Does the 'inverse equity hypothesis' explain how both poverty and wealth can be associated with HIV prevalence in sub-Saharan Africa?
Ionization, Kinematics, and Extent of the Diffuse Ionized Gas Halo of NGC 5775
We present key results from deep spectra of the Diffuse Ionized Gas (DIG)
halo of the edge-on galaxy NGC 5775. [NII]6583 has been detected up to about
z=13 kpc above the plane in one of two vertically oriented long slits -- making
this the spiral galaxy with the greatest spectroscopically detected halo extent
in emission. Key diagnostic line ratios have been measured up to about z=8 kpc,
allowing the source of ionization and physical state to be probed. Ionization
by a dilute radiation field from massive stars in the disk can explain some of
the line ratio behavior, but departures from this picture are clearly
indicated, most strongly by the rise of [OIII]/Halpha with z. Velocities of the
gas in both slits approach the systemic velocity of the galaxy at several kpc
above the plane. We interpret this trend as a decrease in rotation velocity
with z, with essentially no rotation at heights of several kpc. Such a trend
was observed in the edge-on galaxy NGC 891, but here much more dramatically.
This falloff is presumably due to the gravitational potential changing with z,
but will also depend on the hydrodynamic nature of the disk-halo cycling of gas
and projection effects. More detailed modeling of the ionization and kinematics
of this and other edge-ons will be presented in future papers.Comment: figures 1, 2a-d and 3 included. ApJ Letters, in pres
Appropriating the Returns from Industrial Research and Development
macroeconomics, industrial research and development, patent law
The strange evolution of the Large Magellanic Cloud Cepheid OGLE-LMC-CEP1812
Classical Cepheids are key probes of both stellar astrophysics and cosmology
as standard candles and pulsating variable stars. It is important to understand
Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and
extremely-large telescope observations. Cepheid eclipsing binary stars are
ideal tools for achieving this goal, however there are currently only three
known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions
about the evolution of classical Cepheids because of an apparent age
discrepancy between the Cepheid and its red giant companion. We show that the
Cepheid component is actually the product of a stellar merger of two main
sequence stars that has since evolved across the Hertzsprung gap of the HR
diagram. This post-merger product appears younger than the companion, hence the
apparent age discrepancy is resolved. We discuss this idea and consequences for
understanding Cepheid evolution.Comment: 5 pages, 3 figures, accepted to A&
Ionization Sources and Physical Conditions in the Diffuse Ionized Gas Halos of Four Edge-On Galaxies
Deep long-slit spectra of the diffuse ionized gas halos of the edge-on spiral
galaxies NGC 4302 and UGC 10288 are presented. These data, along with
previously presented data for NGC 5775 and NGC 891, are used to address the
issue of how DIG halos are energized. Composite photo-ionization/shock models
are generally better at explaining runs of line ratios in these galaxies than
photo-ionization models alone. Models of line ratios in NGC 5775 require a
greater contribution from shocks for filamentary regions than for
non-filamentary regions to explain the run of [OIII]/Halpha. In either case,
the [SII]/[NII] ratio is not well fit by the models. Composite models for UGC
10288 are successful at reproducing the run of [SII]/[NII] for all but the the
highest values of [NII]/Halpha; however, the run of [OIII]/Halpha vs.
[NII]/Halpha does not show any discernible trend, making it difficult to
determine whether or not shocks make a contribution.
We also examine whether the data can be explained simply by an increase in
temperature with z in a pure photo-ionization model. Runs of [SII]/Halpha,
[NII]/Halpha, and [SII]/[NII] in each of the four galaxies are consistent with
such an increase. However, the runs of [OIII]/Halpha vs. z in NGC 5775 and UGC
10288 require unusually high ionization fractions of O^{++} that can not be
explained without invoking a secondary ionization source or at the very least a
much higher temperature for the [OIII]-emitting component than for the [SII]-
and [NII]-emitting component. An increase in temperature with z is generally
more successful at explaining the [OIII]/Halpha run in NGC 891.Comment: 42 pages in aaspp4.sty format. This includes the 19 figures.
Reference added. Accepted for publication in Ap
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