458 research outputs found
Ultrafast relaxation of hot phonons in Graphene-hBN Heterostructures
Fast carrier cooling is important for high power graphene based devices.
Strongly Coupled Optical Phonons (SCOPs) play a major role in the relaxation of
photoexcited carriers in graphene. Heterostructures of graphene and hexagonal
boron nitride (hBN) have shown exceptional mobility and high saturation
current, which makes them ideal for applications, but the effect of the hBN
substrate on carrier cooling mechanisms is not understood. We track the cooling
of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast
pump-probe spectroscopy. We find that the carriers cool down four times faster
in the case of graphene on hBN than on a silicon oxide substrate thus
overcoming the hot phonon (HP) bottleneck that plagues cooling in graphene
devices.Comment: Pages 1-12: Main manuscript. Pages 13-18: Supplementary materia
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
Broadband Application of High Impedance Ground Planes
Electrical conductors have long been the only materials available to antenna designers for reflecting structures. However, recently reported high impedance ground plane (HIGP) structures offer an alternative by creating image currents and reflections, within a limited frequency stop-band, that are in-phase with a source rather than out-of-phase as for a perfect electric conducting (PEC) surface. Also, the high impedance structures suppress surface waves while surface waves propagate on PEC surfaces. This research explores broadband antenna applications for HIGP structures. A broadband surface mount antenna is applied to both a homogeneous narrowband HIGP and a spatially varying broadband HIGP design. Measurements reveal the ground plane alters the fundamental radiating modes of the antenna and show high frequency regions of the ground plane short out low frequency energy in the antenna. Novel broadband integrated antenna/ground-plane structures are also introduced and analyzed. Basic high impedance elements are linked to form larger broadband antenna elements within the ground plane itself. The structure provides a passive-receive capability over a 9 to 1 bandwidth, is very light and thin, and offers straightforward flush-mounted integration on PEC surfaces
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
The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?
We study the nature and the origin of the dust arch in the halo of the
edge-on galaxy NGC 4631 detected by Neininger & Dumke (1999). We present CO
observations made using the new On-The-Fly mapping mode with the FCRAO 14m
telescope, and find no evidence for CO emission associated with the dust arch.
Our examination of previously published HI data shows that if previous
assumptions about the dust temperature and gas/dust ratio are correct, then
there must be molecular gas associated with the arch, below our detection
threshold. If this is true, then the molecular mass associated with the dust
arch is between 1.5 x 10^8 M(sun)and 9.7 x 10^8 M(sun), and likely towards the
low end of the range. A consequence of this is that the maximum allowed value
for the CO-to-H_2 conversion factor is 6.5 times the Galactic value, but most
likely closer to the Galactic value. The kinematics of the HI apparently
associated with the dust arch reveal that the gas here is not part of an
expanding shell or outflow, but is instead two separate features (a tidal arm
and a plume of HI sticking out into the halo) which are seen projected together
and appear as a shell. Thus there is no connection between the dust "arch" and
the hot X-ray emitting gas that appears to surround the galaxy Wang et al.
(2001).Comment: 14 pages, including 4 figures. Accepted by A.J. for March 200
Evidence for an Additional Heat Source in the Warm Ionized Medium of Galaxies
Spatial variations of the [S II]/H-Alpha and [N II]/H-Alpha line intensity
ratios observed in the gaseous halo of the Milky Way and other galaxies are
inconsistent with pure photoionization models. They appear to require a
supplemental heating mechanism that increases the electron temperature at low
densities n_e. This would imply that in addition to photoionization, which has
a heating rate per unit volume proportional to n_e^2, there is another source
of heat with a rate per unit volume proportional to a lower power of n_e. One
possible mechanism is the dissipation of interstellar plasma turbulence, which
according to Minter & Spangler (1997) heats the ionized interstellar medium in
the Milky Way at a rate ~ 1x10^-25 n_e ergs cm^-3 s^-1. If such a source were
present, it would dominate over photoionization heating in regions where n_e <
0.1 cm^-3, producing the observed increases in the [S II]/H-Alpha and [N
II]/H-Alpha intensity ratios at large distances from the galactic midplane, as
well as accounting for the constancy of [S II]/[N II], which is not explained
by pure photoionization. Other supplemental heating sources, such as magnetic
reconnection, cosmic rays, or photoelectric emission from small grains, could
also account for these observations, provided they supply to the warm ionized
medium ~ 10^-5 ergs s^-1 per cm^2 of Galactic disk.Comment: 10 pages, 1 figur
WHAM Observations of H-Alpha, [S II], and [N II] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium
A large portion of the Galaxy (l = 123 deg to 164 deg, b = -6 deg to -35
deg), which samples regions of the Local (Orion) spiral arm and the more
distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM)
in the H-Alpha, [S II] 6716, and [N II] 6583 lines. Several trends noticed in
emission-line investigations of diffuse gas in other galaxies are confirmed in
the Milky Way and extended to much fainter emission. We find that the [S
II]/H-Alpha and [N II]/H-Alpha ratios increase as absolute H-Alpha intensities
decrease. For the more distant Perseus arm emission, the increase in these
ratios is a strong function of Galactic latitude and thus, of height above the
Galactic plane. The [S II]/[N II] ratio is relatively independent of H-Alpha
intensity. Scatter in this ratio appears to be physically significant, and maps
of it suggest regions with similar ratios are spatially correlated. The Perseus
arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%.
With [S II]/[N II] fairly constant over a large range of H-Alpha intensities,
the increase of [S II]/H-Alpha and [N II]/H-Alpha with |z| seems to reflect an
increase in temperature. Such an interpretation allows us to estimate the
temperature and ionization conditions in our large sample of observations. We
find that WIM temperatures range from 6,000 K to 9,000 K with temperature
increasing from bright to faint H-Alpha emission (low to high [S II]/H-Alpha
and [N II]/H-Alpha) respectively. Changes in [S II]/[N II] appear to reflect
changes in the local ionization conditions (e.g. the S+/S++ ratio). We also
measure the electron scale height in the Perseus arm to be 1.0+/-0.1 kpc,
confirming earlier, less accurate determinations.Comment: 28 pages, 10 figures. Figures 2 and 3 are full color--GIFs provided
here, original PS figures at link below. Accepted for publication in ApJ.
More information about the WHAM project can be found at
http://www.astro.wisc.edu/wham/ . REVISION: Figure 6, bottom panel now
contains the proper points. No other changes have been mad
Rezension: Jonas Botta, Datenschutz bei E‑Learning-Plattformen, Rechtliche Herausforderungen digitaler Hochschulbildung am Beispiel der Massive Open Online Courses (MOOCs), Nomos, 2020
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