400 research outputs found
Basic principles of SSM modelling: an examination of CATWOE from a soft perspective
This paper examines the SSM technique CATWOE, which focuses on defining necessary elements that together constitute a human activity system from a certain perspective. Despite its recognition within the literature and its numerous uses, there are few studies on how the technique can be improved. This research reflects on each of the elements both from a theoretical and a practical perspective. Findings point to the fact that some of the terms have a meaning in everyday language that differs from its definition within CATWOE. Other concepts are not well-defined. This is unfortunate and may both lead to misunderstandings and limit analysis. The paper points to a number of ways in which the use of CATWOE can be developed in order to further support the process of eliciting novel ideas for future actions. Hence, the overall conclusion is that the elements need to be rethought and some of them renamed
Non-Makovian decoherence of a two-level system weakly coupled to a bosonic bath
Bloch-Redfield equation is a common tool for studying evolution of qubit
systems weakly coupled to environment. We investigate the accuracy of the Born
approximation underlying this equation. We find that the high order terms in
the perturbative expansion contain accumulating divergences that make
straightforward Born approximation inappropriate. We develop diagrammatic
technique to formulate, and solve the improved self-consistent Born
approximation. This more accurate treatment reveals an exponential time
dependent prefactor in the non-Markovian contribution dominating the qubit
long-time relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time,
the associated dephasing is not affected and is described by the Born-Markov
approximation.Comment: To appear in EuroPhys. Let
The escape fraction of ionizing photons from high redshift galaxies
The fraction of ionizing photons which escape their host galaxy and so are
able to ionize hydrogen in the inter-galactic medium (IGM) is a critical
parameter in studies of the reionization era and early galaxy formation. In
this paper we combine observations of Lyman-alpha absorption towards high
redshift quasars with the measured UV luminosity function of high redshift
galaxies to constrain the escape fraction (f_esc) of ionizing photons from
galaxies at z ~ 5.5-6. The observed Lyman-alpha transmission constrains the
escape fraction to lie in the range f_esc ~ 10-25 % (at z ~ 5.5-6). Excluding
halos with M< 10^10 M_sun (as might be expected if galaxy formation is
suppressed due to the reionization of the IGM) implies a larger escape fraction
of f_esc ~ 20-45 %. Using the numerical results to calibrate an analytic
relation between the escape fraction and minimum galaxy halo mass we also
extrapolate our results to a mass (M~10^8 M_sun) corresponding to the hydrogen
cooling threshold. In this case we find f_esc ~ 5-10 %, consistent with
observed estimates at lower redshift. We find that the escape fraction of high
redshift galaxies must be greater than 5 % irrespepctive of galaxy mass. Based
on these results we use a semi-analytic description to model the reionization
history of the IGM, assuming ionizing sources with escape fractions suggested
by our numerical simulations. We find that the IBG observed at z ~ 5.5-6
implies a sufficient number of ionizing photons to have reionized the Universe
by z ~ 6. However, if the minimum mass for star-formation were greater than
10^9 M_sun, the IBG would be over-produced at redshifts less than z ~ 5. In
summary, our results support a scenario in which the IGM was reionized by low
mass galaxies.Comment: 14 pages, 9 figure
Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics
We present atomistic calculations of quantum coherent electron transport
through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We
predict that three difficult problems in molecular electronics with single
molecules may be solved by utilizing graphene contacts: (1) a back gate
modulating the Fermi level in the graphene leads facilitate control of the
device conductance in a transistor effect with high on/off current ratio; (2)
the size mismatch between leads and molecule is avoided, in contrast to the
traditional metal contacts; (3) as a consequence, distinct features in charge
flow patterns throughout the device are directly detectable by scanning
techniques. We show that moderate graphene edge disorder is unimportant for the
transistor function.Comment: 8 pages, 6 figure
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
A model of spectral galaxy evolution including the effects of nebular emission
This paper presents a new spectral evolutionary model of galaxies, properly
taking the effects of nebular emission and pre-main sequence evolution into
account. The impact of these features in different photometric filters is
evaluated, along with the influence that variations in the physical conditions
of the gas may have on broadband colours, line ratios and equivalent widths.
Inclusion of nebular emission is demonstrated to radically change the predicted
ultraviolet, optical and near-infrared colours during active star formation.
Pre-main sequence evolution is also seen to give a non-negligible contribution
to the luminosity in the near-infrared during the first few millions years of
evolution and should not be omitted when very young systems are being modelled.
Finally, we present a comparison of our predictions to observations and two
other recent codes of evolutionary synthesis.Comment: 13 pages, 10 figures, accepted for publication in A&
The starburst phenomenon from the optical/near-IR perspective
The optical/near-IR stellar continuum carries unique information about the
stellar population in a galaxy, its mass function and star-formation history.
Star-forming regions display rich emission-line spectra from which we can
derive the dust and gas distribution, map velocity fields, metallicities and
young massive stars and locate shocks and stellar winds. All this information
is very useful in the dissection of the starburst phenomenon. We discuss a few
of the advantages and limitations of observations in the optical/near-IR region
and focus on some results. Special attention is given to the role of
interactions and mergers and observations of the relatively dust-free starburst
dwarfs. In the future we expect new and refined diagnostic tools to provide us
with more detailed information about the IMF, strength and duration of the
burst and its triggering mechanisms.Comment: 6 pages, 3 figures, to appear in "Starbursts: from 30 Doradus to
Lyman Break Galaxies" 2005, eds. R. de Grijs and R. M. Gonzalez Delgado
(Kluwer
Efficient application of a browsing repellent: Can associational effects within and between plants be exploited?
publishedVersio
Penetrating the Deep Cover of Compton Thick Active Galactic Nuclei
We analyze observations obtained with the Chandra X-ray Observatory of bright
Compton thick active galactic nuclei (AGNs), those with column densities in
excess of 1.5 x 10^{24} cm^{-2} along the lines of sight. We therefore view the
powerful central engines only indirectly, even at X-ray energies. Using high
spatial resolution and considering only galaxies that do not contain
circumnuclear starbursts, we reveal the variety of emission AGNs alone may
produce. Approximately 1% of the continuum's intrinsic flux is detected in
reflection in each case. The only hard X-ray feature is the prominent Fe K
alpha fluorescence line, with equivalent width greater than 1 keV in all
sources. The Fe line luminosity provides the best X-ray indicator of the unseen
intrinsic AGN luminosity. In detail, the morphologies of the extended soft
X-ray emission and optical line emission are similar, and line emission
dominates the soft X-ray spectra. Thus, we attribute the soft X-ray emission to
material that the central engines photoionize. Because the resulting spectra
are complex and do not reveal the AGNs directly, crude analysis techniques such
as hardness ratios would mis-classify these galaxies as hosts of intrinsically
weak, unabsorbed AGNs and would fail to identify the luminous, absorbed nuclei
that are present. We demonstrate that a three-band X-ray diagnostic can
correctly classify Compton thick AGNs, even when significant soft X-ray line
emission is present. The active nuclei produce most of the galaxies' total
observed emission over a broad spectral range, and much of their light emerges
at far-infrared wavelengths. Stellar contamination of the infrared emission can
be severe, however, making long-wavelength data alone unreliable indicators of
the buried AGN luminosity.Comment: To appear in ApJ, September 1, 200
- …