201 research outputs found
HI in the Outskirts of Nearby Galaxies
The HI in disk galaxies frequently extends beyond the optical image, and can
trace the dark matter there. I briefly highlight the history of high spatial
resolution HI imaging, the contribution it made to the dark matter problem, and
the current tension between several dynamical methods to break the disk-halo
degeneracy. I then turn to the flaring problem, which could in principle probe
the shape of the dark halo. Instead, however, a lot of attention is now devoted
to understanding the role of gas accretion via galactic fountains. The current
cold dark matter theory has problems on galactic scales, such as
the core-cusp problem, which can be addressed with HI observations of dwarf
galaxies. For a similar range in rotation velocities, galaxies of type Sd have
thin disks, while those of type Im are much thicker. After a few comments on
modified Newtonian dynamics and on irregular galaxies, I close with statistics
on the HI extent of galaxies.Comment: 38 pages, 17 figures, invited review, book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Mechanisms of base selection by human single-stranded selective monofunctional uracil-DNA glycosylase
hSMUG1 (human single-stranded selective monofunctional uracil-DNA glyscosylase) is one of three glycosylases encoded within a small region of human chromosome 12. Those three glycosylases, UNG (uracil-DNA glycosylase), TDG (thymine-DNA glyscosylase), and hSMUG1, have in common the capacity to remove uracil from DNA. However, these glycosylases also repair other lesions and have distinct substrate preferences, indicating that they have potentially redundant but not overlapping physiological roles. The mechanisms by which these glycosylases locate and selectively remove target lesions are not well understood. In addition to uracil, hSMUG1 has been shown to remove some oxidized pyrimidines, suggesting a role in the repair of DNA oxidation damage. In this paper, we describe experiments in which a series of oligonucleotides containing purine and pyrimidine analogs have been used to probe mechanisms by which hSMUG1 distinguishes potential substrates. Our results indicate that the preference of hSMUG1 for mispaired uracil over uracil paired with adenine is best explained by the reduced stability of a duplex containing a mispair, consistent with previous reports with Escherichia coli mispaired uracil-DNA glycosylase. We have also extended the substrate range of hSMUG1 to include 5-carboxyuracil, the last in the series of damage products from thymine methyl group oxidation. The properties used by hSMUG1 to select damaged pyrimidines include the size and free energy of solvation of the 5-substituent but not electronic inductive properties. The observed distinct mechanisms of base selection demonstrated for members of the uracil glycosylase family help explain how considerable diversity in chemical lesion repair can be achieved
Evidence for universal structure in galactic halos
The late infall of dark matter onto a galaxy produces structure (such as
caustics) in the distribution of dark matter in the halo. We argue that such
structure is likely to occur generically on length scales proportional to , where is the age of the universe and is the
rotation velocity of the galaxy. A set of 32 extended galactic rotation curves
is analyzed. For each curve, the radial coordinate is rescaled according to
, where we choose . A
linear fit to each rescaled rotation curve is subtracted, and the residuals are
binned and averaged. The sample shows significant features near and . This is consistent with the predictions of the
self-similar caustic ring model of galactic halos.Comment: 4 pages, LaTeX, 1 epsf figur
Paramagnetic liposomes as thermosensitive probes for MRI-guided thermal treatment: In vitro feasibility studies
In this work the potential of thermosensitive paramagnetic liposomes for in vitro temperature monitoring during radiofrequency heating has been assessed. Two thermosensitive liposome formulations with different phase-transition properties were investigated. Temperature-dependent spin-lattice (T 1) relaxivity measurements were performed at 0.24 T. Magnetic resonance imaging was performed at 2 T in liposome-containing phantom models and T 1 relaxation rates (R 1) were quantified as a function of temperature. Independent temperature measurements were performed using both thermocouple and magnetic-resonance-based methods (proton resonance frequency and diffusion-based thermometry). The relaxometric measurements showed that the T 1 relaxivity increased from low values (about 0.3 s -1mM -1 at 35 °C) to about 4 s -1mM -1 when the temperature approached and exceeded the phase-transition temperature (T c) of the liposome preparations. These data correlated well to the imaging data where an increased signal intensity was observed on T 1-weighted images at temperatures above T c. The derived R 1 maps reflected the measured liposomal temperature sensitivity and temperature quantification was possible on the basis of the measured linear temperature versus R 1 correlation in the transition range of the liposomes. The studies have therefore shown that thermosensitive paramagnetic liposomes exhibit the required temperature sensitivity to allow for an accurate mapping of the temperature changes in an in vitro imaging model. © 2008 Springer-Verlag
The dark matter halo shape of edge-on disk galaxies - I. HI observations
This is the first paper of a series in which we will attempt to put
constraints on the flattening of dark halos in disk galaxies. We observe for
this purpose the HI in edge-on galaxies, where it is in principle possible to
measure the force field in the halo vertically and radially from gas layer
flaring and rotation curve decomposition respectively. In this paper, we define
a sample of 8 HI-rich late-type galaxies suitable for this purpose and present
the HI observations.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12565.pd
Keck spectroscopy and Spitzer Space Telescope analysis of the outer disk of the Triangulum Spiral Galaxy M33
In an earlier study of the spiral galaxy M33, we photometrically identified
arcs or outer spiral arms of intermediate age (0.6 Gyr - 2 Gyr) carbon stars
precisely at the commencement of the HI-warp. Stars in the arcs were
unresolved, but were likely thermally-pulsing asymptotic giant branch carbon
stars. Here we present Keck I spectroscopy of seven intrinsically bright and
red target stars in the outer, northern arc in M33. The target stars have
estimated visual magnitudes as faint as V \sim 25 mag. Absorption bands of CN
are seen in all seven spectra reported here, confirming their carbon star
status. In addition, we present Keck II spectra of a small area 0.5 degree away
from the centre of M33; the target stars there are also identified as carbon
stars. We also study the non-stellar PAH dust morphology of M33 secured using
IRAC on board the Spitzer Space Telescope. The Spitzer 8 micron image attests
to a change of spiral phase at the start of the HI warp. The Keck spectra
confirm that carbon stars may safely be identified on the basis of their red
J-K_s colours in the outer, low metallicity disk of M33. We propose that the
enhanced number of carbon stars in the outer arms are an indicator of recent
star formation, fueled by gas accretion from the HI-warp reservoir.Comment: 9 pages, 5 figures, accepted in A&
Warp signatures of the Galactic disk as seen in mid infrared from Midcourse Space Experiment
The gross features in the distribution of stars as well as warm (T >~ 100 K)
interstellar dust in the Galactic disk have been investigated using the recent
mid infrared survey by Midcourse Space Experiment (MSX) at 8, 12, 14 & 21
micron bands. An attempt has been made to determine the location of the
Galactic mid-plane at various longitudes, using two approaches : (i) fitting
exponential functions to the latitude profiles and (ii) statistical
indicators.The former method is successful for the inner Galaxy (-90 < l < 90),
and quantifies characteristic angular scales along latitude, which have been
translated to linear scale heights (z_h) and radial length scales (R_l) using
geometric description of the Galactic disk. The distribution of warm dust in
the Galactic disk is found to be characterised by R_l < 6 kpc and 60 < z_h <~
100 pc, in agreement with other studies. The location of the Galactic mid-plane
as a function of longitude, for stars as well as warm dust, has been searched
for signatures of warp-like feature in their distribution, by fitting sinusoid
with phase and amplitude as parameters. In every case, the warp signature has
been detected. An identical analysis of the DIRBE/COBE data in all its ten
bands covering the entire infrared spectrum (1.25-240 micron), also leads to
detection of warp signatures with very similar phase as found from the MSX
data. Our results have been compared with those from other studies.Comment: To be published in 'Astronomy and Astrophysics' (12 pages including 9
figures & 4 tables
The alignment of molecular cloud magnetic fields with the spiral arms in M33
The formation of molecular clouds, which serve as stellar nurseries in
galaxies, is poorly understood. A class of cloud formation models suggests that
a large-scale galactic magnetic field is irrelevant at the scale of individual
clouds, because the turbulence and rotation of a cloud may randomize the
orientation of its magnetic field. Alternatively, galactic fields could be
strong enough to impose their direction upon individual clouds, thereby
regulating cloud accumulation and fragmentation, and affecting the rate and
efficiency of star formation. Our location in the disk of the Galaxy makes an
assessment of the situation difficult. Here we report observations of the
magnetic field orientation of six giant molecular cloud complexes in the
nearby, almost face-on, galaxy M33. The fields are aligned with the spiral
arms, suggesting that the large-scale field in M33 anchors the clouds.Comment: to appear in Natur
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