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Search Results: 1 - 10 of 174517 matches for " Nicolas F. Martin "
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Inferring the dynamics of stellar streams via distance gradients
Shoko Jin,Nicolas F. Martin
Physics , 2009, DOI: 10.1111/j.1745-3933.2009.00759.x
Abstract: We present a simple result in which the distance gradient along a stream can be used to derive the transverse velocity (i.e. proper motion) along it, if the line-of-sight velocity is also known. We show its application to a mock orbit to illustrate its validity and usage. For less extended objects, such as globular clusters and satellite galaxies being tidally disrupted, the same result can be applied in its small-angle approximation. The procedure does not rely on energy or angular momentum conservation and hence does not require a Galactic model in order to deduce the local velocity vector of the stream.
The Hercules satellite: a stellar stream in the Milky Way halo?
Nicolas F. Martin,Shoko Jin
Physics , 2010, DOI: 10.1088/0004-637X/721/2/1333
Abstract: We investigate the possibility that the recently discovered Hercules Milky Way satellite is in fact a stellar stream in formation, thereby explaining its very elongated shape with an axis ratio of 3 to 1. Under the assumption that Hercules is a stellar stream and that its stars are flowing along the orbit of its progenitor, we find an orbit that would have recently brought the system close enough to the Milky Way to induce its disruption and transformation from a bound dwarf galaxy into a stellar stream. The application of simple analytical techniques to the tentative radial velocity gradient observed in the satellite provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s in the Galactic Standard of Rest). Combined with its large receding velocity, the determined tangential velocity yields an orbit with a small pericentric distance (R_peri = 6^{+9}_{-2} kpc). Tidal disruption is therefore a valid scenario for explaining the extreme shape of Hercules. The increase in the mean flattening of dwarf galaxies as one considers fainter systems could therefore be the impact of a few of these satellites not being bound stellar systems dominated by dark matter but, in fact, stellar streams in formation, shedding their stars in the Milky Way's stellar halo.
Could the Hercules satellite be a stellar stream in the Milky Way halo?
Shoko Jin,Nicolas F. Martin
Physics , 2010, DOI: 10.1051/eas/1148082
Abstract: We investigate the possibility that Hercules, a recently discovered Milky Way (MW) satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules' highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body. The application of simple analytical techniques on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km/s) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6^{+9}_{-2} kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the MW's gravitational potential, inducing its transformation into a stellar stream.
Feeling the pull, a study of natural Galactic accelerometers. I: photometry of the delicate stellar stream of the Palomar 5 globular cluster
Rodrigo A. Ibata,Geraint F. Lewis,Nicolas F. Martin
Physics , 2015,
Abstract: We present an analysis of wide-field photometric surveys of the Palomar 5 globular cluster and its stellar stream, based on g- and r-band measures together with narrow-band DDO51 photometry. In this first study, we use the deep (g,r) data to measure the incidence of gaps and peaks along the stream. Examining the star-counts profile of the stream plus contaminating populations, we find no evidence for significant under-densities, and find only a single significant over-density. This is at odds with earlier studies based on matched-filter maps derived from shallower SDSS data if the contaminating population possesses plausible spatial properties. The lack of substantial sub-structure along the stream may be used in future dynamical simulations to examine the incidence of dark matter sub-halos in the Galactic halo. We also present a measurement of the relative distances along the stream which we use to create the deepest wide-field map of this system to date.
Andromeda XXVIII: A Dwarf Galaxy More Than 350 kpc from Andromeda
Colin T. Slater,Eric F. Bell,Nicolas F. Martin
Physics , 2011, DOI: 10.1088/2041-8205/742/1/L14
Abstract: We report the discovery of a new dwarf galaxy, Andromeda XXVIII, using data from the recently-released SDSS DR8. The galaxy is a likely satellite of Andromeda, and, at a separation of $365^{+17}_{-1}$ kpc, would be one of the most distant of Andromeda's satellites. Its heliocentric distance is $650^{+150}_{-80}$ kpc, and analysis of its structure and luminosity show that it has an absolute magnitude of $M_V = -8.5^{+0.4}_{-1.0}$ and half-light radius of $r_h = 210^{+60}_{-50}$ pc, similar to many other faint Local Group dwarfs. With presently-available imaging we are unable to determine if there is ongoing or recent star formation, which prevents us from classifying it as a dwarf spheroidal or dwarf irregular.
Andromeda XXIX: A New Dwarf Spheroidal Galaxy 200kpc from Andromeda
Eric F. Bell,Colin T. Slater,Nicolas F. Martin
Physics , 2011, DOI: 10.1088/2041-8205/742/1/L15
Abstract: We report the discovery of a new dwarf galaxy, Andromeda XXIX, using data from the recently-released Sloan Digital Sky Survey DR8, and confirmed by Gemini North telescope Multi-Object Spectrograph imaging data. And XXIX appears to be a dwarf spheroidal galaxy, separated on the sky by a little more than 15 degrees from M31, with a distance inferred from the tip of the red giant branch of 730kpc+/-75kpc, corresponding to a three dimensional separation from M31 of between 205kpc and 227kpc (close to M31's virial radius). Its absolute magnitude, as determined by comparison to the red giant branch luminosity function of the Draco dwarf spheroidal, is M_V = -8.3+/-0.4. And XXIX's stellar populations appear very similar to Draco's; consequently, we estimate a metallicity for And XXIX of [Fe/H]\sim-1.8. The half-light radius of And XXIX is 360pc+/-60pc and its ellipticity is 0.35+/-0.06, typical of dwarf satellites of the Milky Way and M31 at this absolute magnitude range.
Does the Sagittarius Stream constrain the Milky Way halo to be triaxial?
Rodrigo Ibata,Geraint F. Lewis,Nicolas F. Martin,Michele Bellazzini,Matteo Correnti
Physics , 2012, DOI: 10.1088/2041-8205/765/1/L15
Abstract: Recent analyses of the stellar stream of the Sagittarius dwarf galaxy have claimed that the kinematics and three-dimensional location of the M-giant stars in this structure constrain the dark matter halo of our Galaxy to possess a triaxial shape that is extremely flattened, being essentially an oblate ellipsoid oriented perpendicular to the Galactic disk. Using a new stream-fitting algorithm, based on a Markov Chain Monte Carlo procedure, we investigate whether this claim remains valid if we allow the density profile of the Milky Way halo greater freedom. We find stream solutions that fit the leading and trailing arms of this structure even in a spherical halo, although this would need a rising Galactic rotation curve at large Galactocentric radius. However, the required rotation curve is not ruled out by current constraints. It appears therefore that for the Milky Way, halo triaxiality, despite its strong theoretical motivation, is not required to explain the Sagittarius stream. This degeneracy between triaxiality and the halo density profile suggests that in future endeavors to model this structure, it will be advantageous to relax the strict analytic density profiles that have been used to date.
A Deep Study of the Dwarf Satellites Andromeda XXVIII & Andromeda XXIX
Colin T. Slater,Eric F. Bell,Nicolas F. Martin,Erik J. Tollerud,Nhung Ho
Physics , 2015,
Abstract: We present the results of a deep study of the isolated dwarf galaxies Andromeda XXVIII and Andromeda XXIX with Gemini/GMOS and Keck/DEIMOS. Both galaxies are shown to host old, metal-poor stellar populations with no detectable recent star formation, conclusively identifying both of them as dwarf spheroidal galaxies (dSphs). And XXVIII exhibits a complex horizontal branch morphology, which is suggestive of metallicity enrichment and thus an extended period of star formation in the past. Decomposing the horizontal branch into blue (metal poor, assumed to be older) and red (relatively more metal rich, assumed to be younger) populations shows that the metal rich are also more spatially concentrated in the center of the galaxy. We use spectroscopic measurements of the Calcium triplet, combined with the improved precision of the Gemini photometry, to measure the metallicity of the galaxies, confirming the metallicity spread and showing that they both lie on the luminosity-metallicity relation for dwarf satellites. Taken together, the galaxies exhibit largely typical properties for dSphs despite their significant distances from M31. These dwarfs thus place particularly significant constraints on models of dSph formation involving environmental processes such as tidal or ram pressure stripping. Such models must be able to completely transform the two galaxies into dSphs in no more than two pericentric passages around M31, while maintaining a significant stellar populations gradient. Reproducing these features is a prime requirement for models of dSph formation to demonstrate not just the plausibility of environmental transformation but the capability of accurately recreating real dSphs.
Effect of terminal accuracy requirements on temporal gaze-hand coordination during fast discrete and reciprocal pointings
Romain Terrier, Nicolas Forestier, Félix Berrigan, Mathieu Germain-Robitaille, Martin Lavallière, Normand Teasdale
Journal of NeuroEngineering and Rehabilitation , 2011, DOI: 10.1186/1743-0003-8-10
Abstract: Subjects performed fast discrete (experiment 1) and reciprocal (experiment 2) pointings with an amplitude of 50 cm and four target diameters (7.6, 3.8, 1.9 and 0.95 cm) leading to indexes of difficulty (ID = log2[2A/D]) of 3.7, 4.7, 5.7 and 6.7 bits. Gaze and hand displacements were synchronously recorded. Temporal gaze-hand coordination parameters were compared between experiments (discrete and reciprocal pointings) and IDs using analyses of variance (ANOVAs).Data showed that the magnitude of the gaze-hand lead pattern was much higher for discrete than for reciprocal pointings. Moreover, while it was constant for discrete pointings, it decreased systematically with an increasing ID for reciprocal pointings because of the longer duration of gaze anchoring on target.Overall, the temporal gaze-hand coordination analysis revealed that even for high IDs, fast reciprocal pointings could not be considered as a concatenation of discrete units. Moreover, our data clearly illustrate the smooth adaptation of temporal gaze-hand coordination to terminal accuracy requirements during fast reciprocal pointings. It will be interesting for further researches to investigate if the methodology used in the experiment 2 allows assessing the effect of sensori-motor deficits on gaze-hand coordination.The organization and control of goal-directed movements has been studied extensively using variations of the well known Fitts' task [1,2]. Within this general paradigm, the width of the target (W) and distance (A) of the movement are systematically varied across trials and subjects are asked to point at targets as rapidly and as accurately as possible. Generally, these studies have allowed to conclude that there is a linear relationship between the index of difficulty (ID = Log2 [2A/W]) and movement time (MT) (see [3,4] for reviews of this effect) with the MT increasing when the ID increases. It has been suggested the increase in MT corresponds to an increase of the amount of visual informati
What Sets the Sizes of the Faintest Galaxies?
Crystal M. Brasseur,Nicolas F. Martin,Andrea V. Macciò,Hans-Walter Rix,Xi Kang
Physics , 2011, DOI: 10.1088/0004-637X/743/2/179
Abstract: We provide a comprehensive description and offer an explanation for the sizes of the faintest known galaxies in the universe, the dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda. After compiling a consistent data set of half-light radii (r_{1/2}) and luminosities, we describe the size-luminosity relation of dSphs by a log-normal distribution in r_{1/2} with a mean size that varies as a function of luminosity. Accounting for modest number statistics, measurement uncertainties and surface brightness limitations, we find that the size-luminosity relations of the Milky Way and Andromeda dSph populations are statistically indistinguishable, and also very similar: their mean sizes at a given stellar luminosity differ by no more than 30%. In addition, we find that the mean size, slope and scatter of this log-normal size description of Local Group dSphs matches onto the relation of more massive low-concentration galaxies. This suggests that the stellar sizes of dSphs are ultimately related to their overall initial baryonic angular momentum. To test this hypothesis we perform a series of high resolution N-body simulations that we couple with a semi-analytic model of galaxy formation. These predict the same mean size and slope as observed in dSph satellites. At the same time, these models predict that the size-luminosity distributions for satellite galaxies around similar host-halos must be similar providing a natural explanation as to why the size distributions of Milky Way and Andromeda satellites are similar. Although strong rotation is currently not observed in dSphs, this may well be consistent with our angular-momentum-based explanation for their sizes if the disks of these galaxies have become sufficiently stirred through tidal interaction.
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