327 research outputs found
MegaPipe: the MegaCam image stacking pipeline at the Canadian Astronomical Data Centre
This paper describes the MegaPipe image processing pipeline at the Canadian
Astronomical Data Centre. The pipeline combines multiple images from the
MegaCam mosaic camera on CFHT and combines them into a single output image.
MegaPipe takes as input detrended MegaCam images and does a careful astrometric
and photometric calibration on them. The calibrated images are then resampled
and combined into image stacks. The astrometric calibration of the output
images is accurate to within 0.15 arcseconds relative to external reference
frames and 0.04 arcseconds internally. The photometric calibration is good to
within 0.03 magnitudes. The stacked images and catalogues derived from these
images are available through the CADC website:Comment: Data available at
http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/megapipe/index.htm
The Next Generation Virgo Cluster Survey (NGVS). XVIII. Measurement and Calibration of Surface Brightness Fluctuation Distances for Bright Galaxies in Virgo (and Beyond)
We describe a program to measure surface brightness fluctuation (SBF)
distances to galaxies observed in the Next Generation Virgo Cluster Survey
(NGVS), a photometric imaging survey covering of the Virgo cluster
in the bandpasses with the Canada-France Hawaii Telescope. We
describe the selection of the sample galaxies, the procedures for measuring the
apparent -band SBF magnitude , and the calibration of the absolute
as a function of observed stellar population properties. The
multi-band NGVS data set provides multiple options for calibrating the SBF
distances, and we explore various calibrations involving individual color
indices as well as combinations of two different colors. Within the color range
of the present sample, the two-color calibrations do not significantly improve
the scatter with respect to wide-baseline, single-color calibrations involving
. We adopt the calibration as reference for the present
galaxy sample, with an observed scatter of 0.11 mag. For a few cases that lack
good photometry, we use an alternative relation based on a combination
of and colors, with only a slightly larger observed scatter of
0.12 mag. The agreement of our measurements with the best existing distance
estimates provides confidence that our measurements are accurate. We present a
preliminary catalog of distances for 89 galaxies brighter than
mag within the survey footprint, including members of the background M and W
Clouds at roughly twice the distance of the main body of the Virgo cluster. The
extension of the present work to fainter and bluer galaxies is in progress.Comment: ApJ accepte
Limits on Quaoar's Atmosphere
Here we present high cadence photometry taken by the Acquisition Camera on Gemini South, of a close passage by the ~540 km radius Kuiper belt object, (50000) Quaoar, of a r' = 20.2 background star. Observations before and after the event show that the apparent impact parameter of the event was 0."019 ± 0."004, corresponding to a close approach of 580 ± 120 km to the center of Quaoar. No signatures of occultation by either Quaoar's limb or its potential atmosphere are detectable in the relative photometry of Quaoar and the target star, which were unresolved during closest approach. From this photometry we are able to put constraints on any potential atmosphere Quaoar might have. Using a Markov chain Monte Carlo and likelihood approach, we place pressure upper limits on sublimation supported, isothermal atmospheres of pure N_2, CO, and CH_4. For N_2 and CO, the upper limit surface pressures are 1 and 0.7 μbar, respectively. The surface temperature required for such low sublimation pressures is ~33 K, much lower than Quaoar's mean temperature of ~44 K measured by others. We conclude that Quaoar cannot have an isothermal N_2 or CO atmosphere. We cannot eliminate the possibility of a CH_4 atmosphere, but place upper surface pressure and mean temperature limits of ~138 nbar and ~44 K, respectively
The Most Massive Ultra-Compact Dwarf Galaxy in the Virgo Cluster
We report on the properties of the most massive ultra-compact dwarf galaxy
(UCD) in the nearby Virgo Cluster of galaxies using imaging from the Next
Generation Virgo Cluster Survey (NGVS) and spectroscopy from Keck/DEIMOS. This
object (M59-UCD3) appears to be associated with the massive Virgo galaxy M59
(NGC 4621), has an integrated velocity dispersion of 78 km/s, a dynamical mass
of , and an effective radius () of 25 pc. With an
effective surface mass density of , it is the
densest galaxy in the local Universe discovered to date, surpassing the density
of the luminous Virgo UCD, M60-UCD1. M59-UCD3 has a total luminosity of
mag, and a spectral energy distribution consistent with an old
(14 Gyr) stellar population with [Fe/H]=0.0 and [/Fe]=+0.2. We also
examine deep imaging around M59 and find a broad low surface brightness stream
pointing towards M59-UCD3, which may represent a tidal remnant of the UCD
progenitor. This UCD, along with similar objects like M60-UCD1 and M59cO,
likely represents an extreme population of tidally stripped galaxies more akin
to larger and more massive compact early-type galaxies than to nuclear star
clusters in present-day dwarf galaxies.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter
The Next Generation Virgo Cluster Survey. VII. The intrinsic shapes of low-luminosity galaxies in the core of the Virgo cluster, and a comparison with the Local Group
(Abridged) We investigate the intrinsic shapes of low-luminosity galaxies in
the central 300 kpc of the Virgo cluster using deep imaging obtained as part of
the NGVS. We build a sample of nearly 300 red-sequence cluster members in the
yet unexplored magnitude range. The observed distribution of
apparent axis ratios is then fit by families of triaxial models with
normally-distributed intrinsic ellipticities and triaxialities. We develop a
Bayesian framework to explore the posterior distribution of the model
parameters, which allows us to work directly on discrete data, and to account
for individual, surface brightness-dependent axis ratio uncertainties. For this
population we infer a mean intrinsic ellipticity E=0.43, and a mean triaxiality
T=0.16. This implies that faint Virgo galaxies are best described as a family
of thick, nearly oblate spheroids with mean intrinsic axis ratios 1:0.94:0.57.
We additionally attempt a study of the intrinsic shapes of Local Group
satellites of similar luminosities. For the LG population we infer a slightly
larger mean intrinsic ellipticity E=0.51, and the paucity of objects with round
apparent shapes translates into more triaxial mean shapes, 1:0.76:0.49. We
finally compare the intrinsic shapes of NGVS low-mass galaxies with samples of
more massive quiescent systems, and with field, star-forming galaxies of
similar luminosities. We find that the intrinsic flattening in this
low-luminosity regime is almost independent of the environment in which the
galaxy resides--but there is a hint that objects may be slightly rounder in
denser environments. The comparable flattening distributions of low-luminosity
galaxies that have experienced very different degrees of environmental effects
suggests that internal processes are the main drivers of galaxy structure at
low masses--with external mechanisms playing a secondary role.Comment: Accepted to ApJ. 18 pages, 12 figure
New insights into lipid and fatty acid metabolism from Raman spectroscopy
One of the challenges facing biology is to understand metabolic events at a single cellular level. While approaches to examine dynamics of protein distribution or report on spatiotemporal location of signalling molecules are well-established, tools for the dissection of metabolism in single living cells are less common. Advances in Raman spectroscopy, such as stimulated Raman spectroscopy (SRS), are beginning to offer new insights into metabolic events in a range of experimental systems, including model organisms and clinical samples, and across a range of disciplines. Despite the power of Raman imaging, it remains a relatively under-used technique to approach biological problems, in part because of the specialised nature of the analysis. To raise the profile of this method, here we consider some key studies which illustrate how Raman spectroscopy has revealed new insights into fatty acid and lipid metabolism across a range of cellular systems. The powerful and non-invasive nature of this approach offers a new suite of tools for biomolecular scientists to address how metabolic events within cells informs on or underpins biological function. We illustrate potential biological applications, discuss some recent advances, and offer a direction of travel for metabolic research in this area
The Trypanosome Exocyst:A Conserved Structure Revealing a New Role in Endocytosis
Membrane transport is an essential component of pathogenesis for most infectious organisms. In African trypanosomes, transport to and from the plasma membrane is closely coupled to immune evasion and antigenic variation. In mammals and fungi an octameric exocyst complex mediates late steps in exocytosis, but comparative genomics suggested that trypanosomes retain only six canonical subunits, implying mechanistic divergence. We directly determined the composition of the Trypanosoma brucei exocyst by affinity isolation and demonstrate that the parasite complex is nonameric, retaining all eight canonical subunits (albeit highly divergent at the sequence level) plus a novel essential subunit, Exo99. Exo99 and Sec15 knockdowns have remarkably similar phenotypes in terms of viability and impact on morphology and trafficking pathways. Significantly, both Sec15 and Exo99 have a clear function in endocytosis, and global proteomic analysis indicates an important role in maintaining the surface proteome. Taken together these data indicate additional exocyst functions in trypanosomes, which likely include endocytosis, recycling and control of surface composition. Knockdowns in HeLa cells suggest that the role in endocytosis is shared with metazoan cells. We conclude that, whilst the trypanosome exocyst has novel components, overall functionality appears conserved, and suggest that the unique subunit may provide therapeutic opportunities
Cosmic strings from pseudo-anomalous Fayet-Iliopoulos U(1) in D3/D7 brane inflation
We examine the consequences of recent developments on Fayet-Iliopoulos (FI)
terms for D-term inflationary models. There is currently no known way to couple
constant FI terms to supergravity consistently; only field-dependent FI terms
are allowed. These are natural in string theory and we argue that the FI term
in D3/D7 inflation turns out to be of this type, corresponding to a
pseudo-anomalous U(1). T he anomaly is canceled by the Green-Schwarz mechanism
in 4 dimensions. Inflation proceeds as usual, except that the scale is set by
the GS parameter. Cosmic strings resulting from a pseudo-anomalous U(1) have
potentially interesting characteristics. Originally expected to be global, they
turn out to be local in the string theory context and can support currents. We
outline the nature of these strings, discuss bounds on their formation, and
summarize resulting cosmological consequences.Comment: 10 pages; minor changes to match published versio
A blind test of photometric redshift prediction
Results of a blind test of photometric redshift predictions against
spectroscopic galaxy redshifts obtained in the Hubble Deep Field with the Keck
Telescope are presented. The best photometric redshift schemes predict
spectroscopic redshifts with a redshift accuracy of |Delta-z|<0.1 for more than
68 percent of sources and with |Delta-z|<0.3 for 100 percent, when
single-feature spectroscopic redshifts are removed from consideration. This
test shows that photometric redshift schemes work well at least when the
photometric data are of high quality and when the sources are at moderate
redshifts.Comment: 14 pp., accepted for publication in A
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