3,500 research outputs found
High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)
The EMCCD is a type of CCD that delivers fast readout times and negligible
readout noise, making it an ideal detector for high frame rate applications
which improve resolution, like lucky imaging or shift-and-add. This improvement
in resolution can potentially improve the photometry of faint stars in
extremely crowded fields significantly by alleviating crowding. Alleviating
crowding is a prerequisite for observing gravitational microlensing in main
sequence stars towards the galactic bulge. However, the photometric stability
of this device has not been assessed. The EMCCD has sources of noise not found
in conventional CCDs, and new methods for handling these must be developed.
We aim to investigate how the normal photometric reduction steps from
conventional CCDs should be adjusted to be applicable to EMCCD data. One
complication is that a bias frame cannot be obtained conventionally, as the
output from an EMCCD is not normally distributed. Also, the readout process
generates spurious charges in any CCD, but in EMCCD data, these charges are
visible as opposed to the conventional CCD. Furthermore we aim to eliminate the
photon waste associated with lucky imaging by combining this method with
shift-and-add.
A simple probabilistic model for the dark output of an EMCCD is developed.
Fitting this model with the expectation-maximization algorithm allows us to
estimate the bias, readout noise, amplification, and spurious charge rate per
pixel and thus correct for these phenomena. To investigate the stability of the
photometry, corrected frames of a crowded field are reduced with a PSF fitting
photometry package, where a lucky image is used as a reference.
We find that it is possible to develop an algorithm that elegantly reduces
EMCCD data and produces stable photometry at the 1% level in an extremely
crowded field.Comment: Submitted to Astronomy and Astrophysic
Electron transport in single wall carbon nanotube weak links in the Fabry-Perot regime
We fabricated reproducible high transparency superconducting contacts
consisting of superconducting Ti/Al/Ti trilayers to gated single-walled carbon
nanotubes (SWCNTs). The reported semiconducting SWCNT have normal state
differential conductance up to and exhibit clear Fabry-Perot
interference patterns in the bias spectroscopy plot. We observed subharmonic
gap structure in the differential conductance and a distinct peak in the
conductance at zero bias which is interpreted as a manifestation of a
supercurrent. The gate dependence of this supercurrent as well as the excess
current are examined and compared to a coherent theory of superconducting point
contacts with good agreement.Comment: 10 pages, 4 figure
The Fundamental Plane in RX J0142.0+2131: a galaxy cluster merger at z=0.28
We present the Fundamental Plane (FP) in the z = 0.28 cluster of galaxies RX
J0142.0+2131. There is no evidence for a difference in the slope of the FP when
compared with the Coma cluster, although the internal scatter is larger. On
average, stellar populations in RX J0142.0+2131 have rest-frame V-band
mass-to-light ratios (M/L_V) 0.29+-0.03 dex lower than in Coma. This is
significantly lower than expected for a passively-evolving cluster formed at
z_f=2. Lenticular galaxies have lower average M/L_V and a distribution of M/L_V
with larger scatter than ellipticals. Lower mass-to-light ratios are not due to
recent star formation: our previous spectroscopic observations of RX
J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation
within the past ~4 Gyr. However, cluster members have enhanced alpha-element
abundance ratios, which may act to decrease M/L_V. The increased scatter in the
RX J0142.0+2131 FP reflects a large scatter in M/L_V implying that galaxies
have undergone bursts of star formation over a range of epochs. The seven
easternmost cluster galaxies, including the second brightest member, have M/L_V
consistent with passive evolution and z_f = 2. We speculate that RX
J0142.0+2131 is a cluster-cluster merger where the galaxies to the east are yet
to fall into the main cluster body or have not experienced star formation as a
result of the merger.Comment: 4 pages, 2 figures, accepted for publication in ApJ Letter
Lower bounds for the first eigenvalue of the magnetic Laplacian
We consider a Riemannian cylinder endowed with a closed potential 1-form A
and study the magnetic Laplacian with magnetic Neumann boundary conditions
associated with those data. We establish a sharp lower bound for the first
eigenvalue and show that the equality characterizes the situation where the
metric is a product. We then look at the case of a planar domain bounded by two
closed curves and obtain an explicit lower bound in terms of the geometry of
the domain. We finally discuss sharpness of this last estimate.Comment: Replaces in part arXiv:1611.0193
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
RXJ0142.0+2131: I. The galaxy content of an X-ray-luminous galaxy cluster at z=0.28
We present a photometric and spectroscopic study of stellar populations in
the X-ray-luminous cluster of galaxies RXJ0142.0+2131 at z=0.280. This paper
analyses the results of high signal-to-noise spectroscopy, as well as g'-, r'-,
and i'-band imaging, using the Gemini Multi-Object Spectrograph on Gemini
North. Of 43 spectroscopic targets, we find 30 cluster members over a range in
color. Central velocity dispersions and absorption-line strengths for lines in
the range 3700A < lambda_rest < 5800A are derived for cluster members, and are
compared with a low-redshift sample of cluster galaxies, and single stellar
population (SSP) models. We use a combination of these indicators to estimate
luminosity-weighted mean ages, metallicities ([M/H]), and alpha-element
abundance ratios ([alpha/Fe]).
RXJ0142.0+2131 is a relatively poor cluster and lacks galaxies with high
central velocity dispersions. Although the red sequence and the Faber-Jackson
relation are consistent with pure passive evolution of the early-type
population with a formation redshift of z_form = 2, the strengths of the 4000A
break and scaling relations between metal line indices and velocity dispersion
reject this model with high significance. By inverting SSP models for the
Hbeta_G, Mgb, and line indices, we calculate that, at a given velocity
dispersion and metallicity, galaxies in RXJ0142.0+2131 have luminosity-weighted
mean ages 0.14 +- 0.07 dex older than the low-redshift sample. We also find
that [alpha/Fe] in stellar populations in RXJ0142.0+2131 is 0.14 +- 0.03
greater than at low redshift. All scaling relations are consistent with these
estimated offsets. (abridged)Comment: AJ, accepted. 31 pages, 13 figures, uses emulateapj.cls.
High-resolution figures available on request from first autho
An Internet-Based Tool for Use in Assessing the Likely Effect of Intensification on Losses of Nitrogen to the Environment
The EU Nitrates, Habitat and National Emissions Ceilings directives and the Kyoto Agreement mean that agricultural losses of NO3, NH3 and N2O are under scrutiny by national and international environmental authorities. When farmers wish to intensify their operations, the authorities must then assess the likely environmental impact of the change in operation. The FARM-N internet tool was developed to help farmers and authorities agree how the farm will be structured and managed in the future, and to provide an objective assessment of the environmental losses that will result
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