22,315 research outputs found
Direct detection of galaxy stellar halos : NGC 3957 as a test case
We present a direct detection of the stellar halo of the edge-on S0 galaxy
NGC 3957, using ultra-deep VLT/VIMOS V and R images. This is achieved with a
sky subtraction strategy based on infrared techniques. These observations allow
us to reach unprecedented high signal-to-noise ratios up to 15 kpc away from
the galaxy center, rendering photon-noise negligible. The 1 sigma detection
limits are R = 30.6 mag/arcsec^2 and V = 31.4 mag/arcsec^2. We conduct a
thorough analysis of the possible sources of systematic errors that could
affect the data: flat-fielding, differences in CCD responses, scaling of the
sky background, the extended halo itself, and PSF wings. We conclude that the
V-R colour of the NGC 3957 halo, calculated between 5 and 8 kpc above the disc
plane where the systematic errors are modest, is consistent with an old and
preferentially metal-poor normal stellar population, like that revealed in
nearby galaxy halos from studies of their resolved stellar content. We do not
find support for the extremely red colours found in earlier studies of diffuse
halo emission, which we suggest might have been due to residual systematic
errors.Comment: Accepted for publication in A&A - "language edited
On the accretion origin of a vast extended stellar disk around the Andromeda galaxy
We present the discovery of an inhomogenous, low-surface brightness, extended
disk-like structure around the Andromeda galaxy (M31) based on a large
kinematic survey of more than 2800 stars with the Keck/DEIMOS spectrograph. The
stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out
to R ~ 70 kpc. The constituent stars lag the expected velocity of circular
orbits in the plane of the M31 disk by ~40 kms and have a velocity dispersion
of ~30 kms. The color range on the upper RGB shows a large spread indicative of
a population with a significant range of metallicity. The mean metallicity of
the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 +/-
0.2. The morphology of the structure is irregular at large radii, and shows a
wealth of substructures which must be transitory in nature, and are almost
certainly tidal debris. The presence of these substructures indicates that the
global entity was formed by accretion. This extended disk follows smoothly on
from the central parts of M31 disk with an exponential density law of
scale-length of 5.1 +/- 0.1 kpc, similar to that of the bright inner disk. The
population possesses similar kinematic and abundance properties over the entire
region where it is detected in the survey. We estimate that the structure
accounts for approximately 10% of the total luminosity of the M31 disk, and
given the huge scale, contains ~30% of the total disk angular momentum. This
finding indicates that at least some galactic stellar disks are vastly larger
than previously thought and are formed, at least in their outer regions,
primarily by accretion. [abridged]Comment: 20 pages, 30 figures, ApJ submitte
The Andromeda Stream
The existence of a stream of tidally stripped stars from the Sagittarius
Dwarf galaxy demonstrates that the Milky Way is still in the process of
accreting mass. More recently, an extensive stream of stars has been uncovered
in the halo of the Andromeda galaxy (M31), revealing that it too is
cannibalizing a small companion. This paper reports the recent observations of
this stream, determining it spatial and kinematic properties, and tracing its
three-dimensional structure, as well as describing future observations and what
we may learn about the Andromeda galaxy from this giant tidal stream.Comment: 3 Pages. Refereed contribution to the 5th Galacto Chemodynamics
conference held in Swinburne, July 2003. Accepted for publication in PAS
ACS photometry of extended, luminous globular clusters in the outskirts of M31
A new population of extended, luminous globular clusters has recently been
discovered in the outskirts of M31. These objects have luminosities typical of
classical globular clusters, but much larger half-light radii. We report the
first results from deep ACS imaging of four such clusters, one of which is a
newly-discovered example lying at a projected distance of ~60 kpc from M31. Our
F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the
horizontal branch level, and clearly demonstrate, for the first time, that all
four clusters are composed of >10 Gyr old, metal-poor stellar populations. No
evidence for multiple populations is observed. From a comparison with Galactic
globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]
< -1.8. The observed horizontal branch morphologies show a clear second
parameter effect between the clusters. Preliminary radial luminosity profiles
suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median
value of the globular cluster luminosity function. Our results confirm that
these four objects are bona fide old, metal-poor globular clusters, albeit with
combined structures and luminosities unlike those observed for any other
globular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter
Energy gap measurement of nanostructured thin aluminium films for use in single Cooper-pair devices
Within the context of superconducting gap engineering, Al-\alox-Al tunnel
junctions have been used to study the variation in superconducting gap,
, with film thickness. Films of thickness 5, 7, 10 and 30 nm were used
to form the small area superconductor-insulator-superconductor (SIS) tunnel
junctions. In agreement with previous measurements we have observed an increase
in the superconducting energy gap of aluminium with a decrease in film
thickness. In addition, we find grain size in small area films with thickness
\textbf{} 10 nm has no appreciable effect on energy gap. Finally, we
utilize 7 and 30 nm films in a single Cooper-pair transistor, and observe the
modification of the finite bias transport processes due to the engineered gap
profile
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