29,641 research outputs found

    ACS photometry of extended, luminous globular clusters in the outskirts of M31

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    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

    On the accretion origin of a vast extended stellar disk around the Andromeda galaxy

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    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

    A Minor Axis Surface Brightness Profile for M31

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    We use data from the Isaac Newton Telescope Wide Field Camera survey of M31 to determine the surface brightness profile of M31 along the south-east minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4 degrees (~55 kpc) where the V-band surface brightness is 32 mag per square arcsec; this is the first time the M31 minor axis profile has been mapped over such a large radial distance using a single dataset. We confirm the finding by Pritchet & van den Bergh (1994) that the minor axis profile can be described by a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power-law of index -2.3 or an exponential of scalelength 14 kpc. The fraction of the total M31 luminosity contained in this component is ~2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean colour of the stellar population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.Comment: 11 pages, 3 figures, ApJ Letters in press, extremely minor modification

    Energy gap measurement of nanostructured thin aluminium films for use in single Cooper-pair devices

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    Within the context of superconducting gap engineering, Al-\alox-Al tunnel junctions have been used to study the variation in superconducting gap, Δ\Delta, 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{≥\geq} 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

    Tidal Remnants and Intergalactic HII Regions

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    We report the discovery of two small intergalactic HII regions in the loose group of galaxies around the field elliptical NGC 1490. The HII regions are located at least 100 kpc from any optical galaxy but are associated with a number of large HI clouds that are lying along an arc 500 kpc in length and that have no optical counterpart on the Digital Sky Survey. The sum of the HI masses of the clouds is almost 10^10 M_sun and the largest HI cloud is about 100 kpc in size. Deep optical imaging reveals a very low surface brightness counterpart to this largest HI cloud, making this one of the HI richest optical galaxies known (M_HI/L_V~200). Spectroscopy of the HII regions indicates that the abundance in these HII regions is only slightly sub-solar, excluding a primordial origin of the HI clouds. The HI clouds are perhaps remnants resulting from the tidal disruption of a reasonably sized galaxy, probably quite some time ago, by the loose group to which NGC1490 belongs. Alternatively, they are remnants of the merger that created the field elliptical NGC1490. The isolated HII regions show that star formation on a very small scale can occur in intergalactic space in gas drawn from galaxies by tidal interactions. Many such intergalactic small star formation regions may exist near tidally interacting galaxies.Comment: To appear in the proceedings of the IAU Symposium #217, Recycling Intergalactic and Interstellar Matter, eds. P.-A. Duc, J. Braine, and E. Brinks, 6 pages with low resolution figures. The full paper with high resolution images can be downloaded from http://www.astron.nl/~morganti/Papers/cloud.ps.g

    Young accreted globular clusters in the outer halo of M31

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    We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.Comment: 14 pages, 8 figures, accepted for publication in MNRA

    The Andromeda Stream

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    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
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