386 research outputs found
Possible effect of collective modes in zero magnetic field transport in an electron-hole bilayer
We report single layer resistivities of 2-dimensional electron and hole gases
in an electron-hole bilayer with a 10nm barrier. In a regime where the
interlayer interaction is stronger than the intralayer interaction, we find
that an insulating state () emerges at or
lower, when both the layers are simultaneously present. This happens deep in
the metallic" regime, even in layers with , thus making
conventional mechanisms of localisation due to disorder improbable. We suggest
that this insulating state may be due to a charge density wave phase, as has
been expected in electron-hole bilayers from the Singwi-Tosi-Land-Sj\"olander
approximation based calculations of L. Liu {\it et al} [{\em Phys. Rev. B},
{\bf 53}, 7923 (1996)]. Our results are also in qualitative agreement with
recent Path-Integral-Monte-Carlo simulations of a two component plasma in the
low temperature regime [ P. Ludwig {\it et al}. {\em Contrib. Plasma Physics}
{\bf 47}, No. 4-5, 335 (2007)]Comment: 5 pages + 3 EPS figures (replaced with published version
Herschel-SPIRE-Fourier Transform Spectroscopy of the nearby spiral galaxy IC342
We present observations of the nearby spiral galaxy IC342 with the Herschel
Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform
Spectrometer. The spectral range afforded by SPIRE, 196-671 microns, allows us
to access a number of 12CO lines from J=4--3 to J=13--12 with the highest J
transitions observed for the first time. In addition we present measurements of
13CO, [CI] and [NII]. We use a radiative transfer code coupled with Bayesian
likelihood analysis to model and constrain the temperature, density and column
density of the gas. We find two 12CO components, one at 35 K and one at 400 K
with CO column densities of 6.3x10^{17} cm^{-2} and 0.4x10^{17} cm^{-2} and CO
gas masses of 1.26x10^{7} Msolar and 0.15x10^{7} Msolar, for the cold and warm
components, respectively. The inclusion of the high-J 12CO line observations,
indicate the existence of a much warmer gas component (~400 K) confirming
earlier findings from H_{2} rotational line analysis from ISO and Spitzer. The
mass of the warm gas is 10% of the cold gas, but it likely dominates the CO
luminosity. In addition, we detect strong emission from [NII] 205microns and
the {3}P_{1}->{3}P_{0} and {3}P_{2} ->{3}P_{1} [CI] lines at 370 and 608
microns, respectively. The measured 12CO line ratios can be explained by
Photon-dominated region (PDR) models although additional heating by e.g. cosmic
rays cannot be excluded. The measured [CI] line ratio together with the derived
[C] column density of 2.1x10^{17} cm^{-2} and the fact that [CI] is weaker than
CO emission in IC342 suggests that [CI] likely arises in a thin layer on the
outside of the CO emitting molecular clouds consistent with PDRs playing an
important role.Comment: 9 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS
Heating and cooling of the neutral ISM in the NGC4736 circumnuclear ring
The manner in which gas accretes and orbits within circumnuclear rings has
direct implications for the star formation process. In particular, gas may be
compressed and shocked at the inflow points, resulting in bursts of star
formation at these locations. Afterwards the gas and young stars move together
through the ring. In addition, star formation may occur throughout the ring, if
and when the gas reaches sufficient density to collapse under gravity. These
two scenarios for star formation in rings are often referred to as the `pearls
on a string' and `popcorn' paradigms. In this paper, we use new Herschel PACS
observations, obtained as part of the KINGFISH Open Time Key Program, along
with archival Spitzer and ground-based observations from the SINGS Legacy
project, to investigate the heating and cooling of the interstellar medium in
the nearby star-forming ring galaxy, NGC4736. By comparing spatially resolved
estimates of the stellar FUV flux available for heating, with the gas and dust
cooling derived from the FIR continuum and line emission, we show that while
star formation is indeed dominant at the inflow points in NGC 4736, additional
star formation is needed to balance the gas heating and cooling throughout the
ring. This additional component most likely arises from the general increase in
gas density in the ring over its lifetime. Our data provide strong evidence,
therefore, for a combination of the two paradigms for star formation in the
ring in NGC4736.Comment: accepted for publication in A&
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Far-infrared line imaging of the starburst ring in NGC 1097 with the Herschel/PACS spectrometer
NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Observatory. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 μm, [OIII] 88 μm, [NII] 122 μm, [CII] 158 μm and [NII] 205 μm. We observe that in the [OI] 63 μm, [OIII] 88 μm, and [NII] 122 μm line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220 km s-1 (inclination uncorrected) measured in all lines. The [OI] 63 μm/[CII] 158 μm ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 μm and [OIII] 88 μm maps. Variations of the [NII] 122 μm/[NII] 205 μm ratio correspond to a range in the ionized gas density between 150 and 400 cm-3
Dissecting the origin of the submillimeter emission in nearby galaxies with Herschel and LABOCA
We model the infrared to submillimeter spectral energy distribution of 11
nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and
compare model extrapolations at 870um (using different fitting techniques) with
LABOCA 870um observations. We investigate how the differences between
predictions and observations vary with model assumptions or environment. At
global scales, we find that modified blackbody models using realistic cold
emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed
emission within the uncertainties for most of the sample. Low values
(beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local
scales, we observe a systematic 870um excess when using beta_=2.0. The
beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with
observations in part of the disks. Some of the remaining excesses occur towards
the centres and can be partly or fully accounted for by non-dust contributions
such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In
three non-barred galaxies, the remaining excesses rather occur in the disk
outskirts. This could be a sign of a flattening of the submm slope (and
decrease of the effective emissivity index) with radius in these objects.Comment: 31 pages (including appendix), 7 figures, accepted for publication in
MNRA
A far-IR view of the starburst driven superwind in NGC 2146
NGC 2146, a nearby luminous infrared galaxy (LIRG), presents evidence for
outflows along the disk minor axis in all gas phases (ionized, neutral atomic
and molecular). We present an analysis of the multi-phase starburst driven
superwind in the central 5 kpc as traced in spatially resolved spectral line
observations, using far-IR Herschel PACS spectroscopy, to probe the effects on
the atomic and ionized gas, and optical integral field spectroscopy to examine
the ionized gas through diagnostic line ratios. We observe an increased ~250
km/s velocity dispersion in the [OI] 63 micron, [OIII] 88 micron, [NII] 122
micron and [CII] 158 micron fine-structure lines that is spatially coincident
with high excitation gas above and below the disk. We model this with a slow
~200 km/s shock and trace the superwind to the edge of our field of view 2.5
kpc above the disk. We present new SOFIA 37 micron observations to explore the
warm dust distribution, and detect no clear dust entrainment in the outflow.
The stellar kinematics appear decoupled from the regular disk rotation seen in
all gas phases, consistent with a recent merger event disrupting the system. We
consider the role of the superwind in the evolution of NGC 2146 and speculate
on the evolutionary future of the system. Our observations of NGC 2146 in the
far-IR allow an unobscured view of the wind, crucial for tracing the superwind
to the launching region at the disk center, and provide a local analog for
future ALMA observations of outflows in high redshift systems.Comment: 16 pages, 13 figures, accepted for publication in Ap
Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device
We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double
quantum well structure that can be configured as both an electron-hole bilayer
and a hole-hole bilayer, with an insulating barrier of only 10 nm between the
two quantum wells. The Coulomb drag resistivity is a direct measure of the
strength of the interlayer particle-particle interactions. We explore the
strongly interacting regime of low carrier densities (2D interaction parameter
up to 14). Our ambipolar device design allows comparison between the
effects of the attractive electron-hole and repulsive hole-hole interactions,
and also shows the effects of the different effective masses of electrons and
holes in GaAs.This work was financially supported by the UK Engineering and Physical Sciences Research Council. A.F.C. acknowledges financial support from Trinity College, Cambridge, and IF from Toshiba Research Europe.This is the author accepted manuscript. The final version is available from the American Institute of Physics via http://dx.doi.org/10.1063/1.494176
Linear non-hysteretic gating of a very high density 2DEG in an undoped metal-semiconductor-metal sandwich structure
Modulation doped GaAs-AlGaAs quantum well based structures are usually used
to achieve very high mobility 2-dimensional electron (or hole) gases. Usually
high mobilities () are achieved at
high densities. A loss of linear gateability is often associated with the
highest mobilites, on account of a some residual hopping or parallel conduction
in the doped regions. We have developed a method of using fully undoped
GaAs-AlGaAs quantum wells, where densities
can be achieved while maintaining fully
linear and non-hysteretic gateability. We use these devices to understand the
possible mobility limiting mechanisms at very high densities.Comment: 4 pages, 3 eps figure
The Ionized Gas in Nearby Galaxies as Traced by the [NII] 122 and 205 \mu m Transitions
The [NII] 122 and 205 \mu m transitions are powerful tracers of the ionized
gas in the interstellar medium. By combining data from 21 galaxies selected
from the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141
spatially resolved regions with a typical size of ~1 kiloparsec, with
observations of both [NII] far-infrared lines. We measure [NII] 122/205 line
ratios in the ~0.6-6 range, which corresponds to electron gas densities
~1-300 cm, with a median value of =30 cm. Variations in
the electron density within individual galaxies can be as a high as a factor of
~50, frequently with strong radial gradients. We find that increases as a
function of infrared color, dust-weighted mean starlight intensity, and star
formation rate surface density (). As the intensity of the [NII]
transitions is related to the ionizing photon flux, we investigate their
reliability as tracers of the star formation rate (SFR). We derive relations
between the [NII] emission and SFR in the low-density limit and in the case of
a log-normal distribution of densities. The scatter in the correlation between
[NII] surface brightness and can be understood as a property of
the distribution. For regions with close to or higher than the
[NII] line critical densities, the low-density limit [NII]-based SFR
calibration systematically underestimates the SFR since [NII] emission is
collisionally quenched. Finally, we investigate the relation between [NII]
emission, SFR, and by comparing our observations to predictions from the
MAPPINGS-III code.Comment: 18 pages, 9 figures, accepted for publication in The Astrophysical
Journa
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