arXiv subjects

G. Gilmore

Publications and source records attributed to G. Gilmore.

At least 217 records · Page 12Linked to original sources

arXiv2006

A New Milky Way Dwarf Satellite in Canes Venatici

In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch, as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of ~220 kpc. Based on the SDSS data, we estimate an absolute magnitude of Mv ~ -7.9, a central surface brightness of mu_0,V ~ 28 mag arcsecond^-2, and a half-light radius of \~ 8.5' (~ 550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.

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arXiv2006

Outer structure of the Galactic warp and flare: explaining the Canis Major over-density

(Abridged) We derive the structure of the Galactic stellar Warp and Flare using 2MASS RC and RGB stars, selected at mean heliocentric distances of 3, 7 and 17 kpc. Our results are: (i) a clear stellar warp signature is derived for the 3 selected rings; (ii) the derived stellar warp is consistent (both in amplitude and phase-angle) with that for the Galactic interstellar dust and HI gas; (iii) the Sun seems not to fall on the line of nodes. The stellar warp phase-angle orientation (+15 degrees) is close to the orientation angle of the Galactic bar and this produces an asymmetric warp for the inner rings; (iv) a Northern/Southern warp symmetry is observed only for the ring at 17 kpc; (v) treating a mixture of thin and thick disk populations we trace the disk flaring and derive a constant scale-height (~0.65 kpc) within R(GC)~15 kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ~1.5 kpc at R(GC)~23 kpc; and (vi) these results provide further robust evidence that there is no disk radial truncation at R(GC)~14 kpc. In the particular case of the Canis Major over-density we confirm its coincidence with the Southern stellar maximum warp occurring near l=240. We present evidence to conclude that all observed parameters (e.g. number density, radial velocities, proper motion etc) of CMa are consistent with it being a normal Milky Way outer-disk population, thereby leaving no justification for a more complex interpretations of its origin. The present analysis does not provide a conclusive test of the structure or origin of the Monoceros Ring. Nevertheless, we show that a warped flared Milky Way contributes significantly at the locations of the Monoceros Ring.

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arXiv2006

The importance of tides for the Local Group dwarf spheroidals

There are two main tidal effects which can act on the Local Group dwarf spheroidals (dSphs): tidal stripping and tidal shocking. Using N-body simulations, we show that tidal stripping always leads to flat or rising projected velocity dispersions beyond a critical radius; it is $\sim 5$ times more likely, when averaging over all possible projection angles, that the cylindrically averaged projected dispersion will rise, rather than be flat. In contrast, the Local Group dSphs, as a class, show flat or falling projected velocity dispersions interior to $\sim 1$ kpc. This argues for tidal stripping being unimportant interior to $\sim 1$ kpc for most of the Local Group dSphs observed so far. These results have important implications for the formation of the dSphs and for cosmology. A tidal origin for the formation of these Local Group dSphs (in which they contain no dark matter) is strongly disfavoured. In the cosmological context, a naive solution to the missing satellites problem is to allow only the most massive substructure dark matter halos around the Milky Way to form stars. It is possible for dSphs to reside within these halos ($\sim 10^{10}$M$_\odot$) and have their velocity dispersions lowered through the action of tidal shocks, but only if they have a central density core in their dark matter, rather than a cusp. dSphs can reside within cuspy dark matter halos if their halos are less massive ($\sim 10^{9}$M$_\odot$) and therefore have smaller central velocity dispersions initially (abridged).

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arXiv2005

The investigation of ELAIS field by Vega photometry: Absolute - magnitude dependent on the Galactic model parameters

We estimate the density laws of the Galactic stellar populations as a function of absolute magnitude in a near-polar Galactic field. The density laws are determined by the direct fit to photometric parallaxes from Vega photometry in the ELAIS (l=84.27, b=+44.90; 6.571 deg^2) field both independently for each population and simultaneously for all stellar populations. Stars have been separated into different populations based on their spatial location. The thick disc and halo best fit by an exponential. However, the thin disc best fits by using a sech^2 law for stars at faint absolute magnitudes, (10,11], (11,12] and (12,13], whereas an exponential law for stars at relatively bright absolute magnitudes, (5,6], (6,7], (7,8], (8,9] and (9,10]. The scaleheights for the sech^2 density laws are the equivalent exponential scaleheights. Galactic model parameters are absolute magnitude dependent: The scaleheight for thin disc decreases monotonically from stars at bright absolute magnitudes to stars at faint absolute magnitudes in the range 363-163 pc, except the minimum H=211 pc at (9,10] where sech density law fits better. Its local density is flat at bright absolute magnitudes but it increases at faint absolute magnitudes. For thick disc, the scaleheight is flat within the uncertainties. The local space density of thick disc relative to the local space density for the thin disc is almost flat at absolute magnitude intervals (5,6] and (6,7], 7.59 and 7.41% respectively, whereas it decreases down to 3.31% at absolute magnitude interval (7,8]. The axial ratio for the halo is e=0.60, 0.73 and 0.78 for the absolute magnitude intervals (4,5], (5,6] and (6,7] respectively, and its local space density relative to the local space density for the thin disc is 0.06 and 0.04% for the intervals (5,6], and (6,7] respectively.

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arXiv2005

The tidal stripping of satellites

We present an improved analytic calculation for the tidal radius of satellites and test our results against N-body simulations. The tidal radius in general depends upon four factors: the potential of the host galaxy, the potential of the satellite, the orbit of the satellite and {\it the orbit of the star within the satellite}. We demonstrate that this last point is critical and suggest using {\it three tidal radii} to cover the range of orbits of stars within the satellite. In this way we show explicitly that prograde star orbits will be more easily stripped than radial orbits; while radial orbits are more easily stripped than retrograde ones. This result has previously been established by several authors numerically, but can now be understood analytically. For point mass, power-law (which includes the isothermal sphere), and a restricted class of split power law potentials our solution is fully analytic. For more general potentials, we provide an equation which may be rapidly solved numerically. Over short times ($\simlt 1-2$ Gyrs $\sim 1$ satellite orbit), we find excellent agreement between our analytic and numerical models. Over longer times, star orbits within the satellite are transformed by the tidal field of the host galaxy. In a Hubble time, this causes a convergence of the three limiting tidal radii towards the prograde stripping radius. Beyond the prograde stripping radius, the velocity dispersion will be tangentially anisotropic.

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arXiv2005

Ursa Major: a missing low-mass CDM halo?

The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidate is about five to eight times less luminous than the faintest previously known dSphs And IX, Draco, and UMi. In this Letter, we present velocity measurements of seven color-magnitude selected UMajor candidate stars. Two of them are apparent non-members based on metallicity and velocity, and the remaining five stars yield a systemic heliocentric velocity of v=-52.45 +/-4.27 km/s and a central line of sight velocity dispersion of 9.3 (+11.7 -1.2) km/s, with 95% confidence that the dispersion is >6.5 km/s. Assuming that UMajor is in dynamical equilibrium, it is clearly dark matter dominated, and cannot be a purely stellar system like a globular cluster. It has an inferred central mass-to-light ratio of M/L~500 and, based on our studies of other dSphs, may possess a much larger total mass to light ratio. UMajor is unexpectedly massive for its low luminosity -- indeed, UMajor appears to be the most dark-matter dominated galaxy yet discovered. The presence of so much dark matter in UMajor immediately suggests that it may be a member of the missing population of low-mass galaxies predicted by the Cold Dark Matter (CDM) paradigm. Given the weak correlation between dSph mass and luminosity, it is entirely likely that a population of dark dwarfs surrounds our Galaxy.

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arXiv2005

Red clump distances to the inner Galactic structures

The least well known structure of the Galaxy is its central region, because of the high extinction, the crowding and the confusion between disk and bulge sources along the line of sight. We show how the structure of the inner Galaxy and the dust distribution can be strongly constrained by using red clump stars as distance indicator. The results of this method applied on the deep near-infrared survey of the inner Galactic bulge made with the Cambridge Infrared Survey Instrument (CIRSI) are presented.

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arXiv2005

The mass of dwarf spheroidal galaxies and the missing satellite problem

We present the results from a suite of N-body simulations of the tidal stripping of two-component dwarf galaxies comprising some stars and dark matter. We show that recent kinematic data from the local group dwarf spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently massive ($10^9 - 10^{10}$M$_\odot$) that tidal stripping is of little importance for the stars. We discuss the implications of these massive dSph galaxies for cosmology and galaxy formation.

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arXiv2005

A near-infrared and optical photometric study of the Sculptor dwarf spheroidal galaxy: implications for the metallicity spread

We present here a detailed study of the Sculptor dSph galaxy red giant branch (RGB) and horizontal branch (HB) morphology, combining new near-infrared photometry from CIRSI, with optical data from the ESO WFI. For a Sculptor-like old and generally metal-poor system, the position of RGB stars on the colour-magnitude diagram is mainly metallicity dependent. The advantage of using optical-NIR colours is that the position of the RGB locus is much more sensitive to metallicity than with optical colours alone. In contrast the horizontal branch (HB) morphology is strongly dependent on both metallicity and age. Therefore a detailed study of both the RGB in optical-NIR colours and the HB can help break the age-metallicity degeneracy. Our measured photometric width of the Sculptor giant branch corresponds to a range in metallicity of 0.75 dex. We detect the RGB and AGB bumps in both the NIR and optical luminosity functions, and derive from them a mean metallicity of [M/H] = -1.3 +/- 0.1. From isochrone fitting we derive a mean metallicity of [Fe/H] = -1.42 with a dispersion of 0.2 dex. These photometric estimators are for the first time consistent with individual metallicity measurements derived from spectroscopic observations. No spatial gradient is detected in the RGB morphology within a radius of 13 arcmin, twice the core radius. On the other hand, a significant gradient is observed in the HB morphology index, confirming the `second parameter problem' present in this galaxy. These observations are consistent with an early extended period of star formation continuing in time for a few Gyr. (Abridged)

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arXiv2005

The structure of the Galactic bar

We present a deep near-infrared wide-angle photometric analysis of the structure of the inner Galactic bar and central disk. The presence of a triaxial structure at the centre of the Galaxy is confirmed, consistent with a bar inclined at 22+/-5.5 deg from the Sun-Galactic centre line, extending to about 2.5 kpc from the Galactic centre and with a rather small axis ratio. A feature at l=-9.8 deg not aligned with this triaxiality suggests the existence of a second structure in the inner Galaxy, a double triaxiality or an inner ring. We argue that this is likely to be the signature of the end of the Galactic bar, at about 2.5-3 kpc, which is circumscribed by an inner pseudo-ring. No thick dust lane preceding the bar is detected, and a hole in the disc's dust distribution inside the bar radius is inferred.

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arXiv2004

Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles

Dwarf spheroidal galaxies have shallow central dark matter density profiles, low angular momentum and approximately exponential surface brightness distributions. Through N-body simulations and analytic calculations we investigate the extent to which these properties can be generated from ``typical'' $Λ$CDM galaxies, which differ in all of these properties, by the dynamical consequences of feedback. We find that, for a wide range of initial conditions, one impulsive mass loss event will naturally produce a surface brightness profile in the remaining stellar component of a dwarf spheroidal galaxy (dSph) which is well fit over many scale lengths by an exponential, in good qualitative agreement with observations of Local Group dSphs. Furthermore, two impulsive mass loss phases, punctuated by significant gas re-accretion, are found to be sufficient to transform a central density cusp in the dark matter profile into a near-constant density core. This may then provide the missing link between current cosmological simulations, which predict a central cusp in the dark matter density profile, and current observations, which find much shallower central density profiles. We also look at the angular momentum history of dSphs and demonstrate that if these galaxies have spent most of their lifetime in tidal isolation from massive galaxies then they cannot have formed from high angular momentum gas discs.

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arXiv2003

Characterising stellar micro-variability for planetary transit searches

(abridged) A method for simulating light curves containing stellar micro-variability for a range of spectral types and ages, based on parameter-by-parameter scaling of a multi-component fit to the solar irradiance power spectrum (from VIRGO/PMO6 data), and scaling laws derived from ground based observations of various stellar samples, is presented. We observe a correlation in the Sun between the amplitude of the power spectrum on weeks timescales and the BBSO Ca II K-line index of chromospheric activity. On the basis of this evidence, the chromospheric activity level, predicted from rotation period and B-V colour estimates according to the relationship first introduced by Noyes(1983) and Noyes et al. (1984), is used to predict the variability power on weeks time scale. The rotation period is estimated on the basis of a fit to the distribution of rotation period versus $B-V$ observed in the Hyades and the Skumanich (1972) spin-down law. The characteristic timescale of the variability is also scaled according to the rotation period. We use this model to estimate the impact of the target star type and age on the detection capability of missions such as Eddington and Kepler. K stars are found to be the most promising targets, while the performance drops significantly for stars earlier than G and younger than 2.0 Gyr. Simulations also show that \emph{Eddington} should detect terrestrial planets orbiting solar-age stars in most of the habitable zone for G2 types and all of it for K0 and K5 types.

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arXiv2003

A CCD Study of High Latitude Galactic Structure: Testing the Model Parameters

We interpret published CCD UBVI data to deduce the stellar density distribution and metallicity distribution function in the region from 2-8 kpc from the Galactic Plane, and compare our results to several star count models. A feature of extant star count models is degeneracy between the adopted scale heights of the thin and thick disks, and their local normalisation. We illustrate the utility of this small data set, and future larger sets (e.g. SDSS), by explicitly considering consistency between the derived density laws, and the implied solar neighbourhood luminosity function. Our data set, from Hall et al.(1996) (l=52, b=-39) contains 566 stars, selected to be consistent with stellar loci in colour-colour diagrams. Our analysis supports the parameterisation of the recent SDSS galaxy model of Chen et al.(2001), except in preferring the stellar halo axis ratio to be 0.84. Photometric metal-abundances have been derived for 329 stars using a new calibration. The vertical distance-dependent metallicity distribution function, if parameterised by a single mean value, can be described by a metallicity gradient -0.2$ dex/kpc for the thin disk and thick disk, and -0.1$ dex/kpc for the inner halo. The data are however better described as the sum of three discrete distribution functions, each of which has a small or zero internal gradient. The changing mix of thin disk, thick disk and halo populations with distance from the plane generates an illusion of a smooth gradient.

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arXiv2003

ISOGAL: A deep survey of the obscured inner Milky Way with ISO at 7 and 15 micron and with DENIS in the near-infrared

The ISOGAL project is an infrared survey of specific regions sampling the Galactic Plane selected to provide information on Galactic structure,stellar populations,stellar mass-loss and the recent star formation history of the inner disk and Bulge of the Galaxy. ISOGAL combines 7 and 15 micron ISOCAM observations - with a resolution of 6'' at worst - with DENIS IJKs data to determine the nature of the sources and theinterstellar extinction. We have observed about 16 square degrees with a sensitivity approaching 10-20mJy, detecting ~10^5 sources,mostly AGB stars,red giants and young stars. The main features of the ISOGAL survey and the observations are summarized in this paper,together with a brief discussion of data processing and quality. The primary ISOGAL products are described briefly (a full description is given in Schuller et al. 2003, astro-ph/0304309): viz. the images and theISOGAL-DENIS five-wavelength point source catalogue. The main scientific results already derived or in progress are summarized. These include astrometrically calibrated 7 and 15um images,determining structures of resolved sources; identification and properties of interstellar dark clouds; quantification of the infrared extinction law and source dereddening; analysis of red giant and (especially) AGB stellar populations in the central Bulge,determining luminosity,presence of circumstellar dust and mass--loss rate,and source classification,supplemented in some cases by ISO/CVF spectroscopy; detection of young stellar objects of diverse types,especially in the inner Bulge with information about the present and recent star formation rate; identification of foreground sources with mid-IR excess. These results are the subject of about 25 refereed papers published or in preparation.

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arXiv2003

Chemical evolution of the M82 B fossil starburst

M82 B is an old starburst site located in the eastern part of the M82 disc. We derive the distributions of age and metallicity of the star clusters located in this region of M82 by using theoretical evolutionary population synthesis models. Our analysis is based on the comparison of the $BVIJ$ photometry obtained by de Grijs et al. (2001) with the colours of single-generation stellar populations. We show that M82 B went through a chemical enrichment phase up to super-solar metallicities around the time of the last close encounter between M82 and its large neighbour galaxy M81. We date and confirm the event triggering the enhanced cluster formation at about 1 Gyr ago. At almost the same time an additional, distinct subpopulation of metal-poor clusters formed in the part of M82 B nearest to the galactic centre. The formation of these peculiar clusters may be related to infall of circumgalactic gas onto M82 B.

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arXiv2002

Can Supermassive Black Holes alter Cold Dark Matter cusps through accretion?

We present some simple models to determine whether or not the accretion of cold dark matter by supermassive black holes is astrophysically important. Contrary to some claims in the literature, we show that supermassive black holes cannot significantly alter a power law density cusp via accretion, whether during mergers or in the steady state.

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arXiv2002

The Frequency Content of the VIRGO/SoHO Lightcurves: Implications for Planetary Transit Detection from Space

Stellar micro-variability poses a serious threat to the capacities of space-based planet-finding missions such as Kepler or Eddington. The solar lightcurves obtained by the VIRGO PMO6 and SPM instruments on board SoHO from 1996 to 2001 have been studied in order to follow variability changes through the activity cycle. In all datasets, active regions-induced variability, below 2 microHz, is closely correlated to the BBSO Ca II K-line index. The PMO6 (total irradiance) data shows evidence for a meso-granulation component around tau = 8x10^3 s, while all narrow-band SPM datasets (red, green and blue) show super-granulation (tau = 5x10^4 s) but no meso-granulation. Both actvity and granulation related components have significantly smaller amplitudes in the red than in the blue channel. These results, coupled with available stellar data, allow us to generate simulated lightcurves with enhanced variability as a testbed for pre-processing and detection methods, and influence the case for using colour information in this kind of mission.

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arXiv2001

Parallel Science with Astero-seismology missions

Eddington, COROT, MONS, KEPLER, and the other asteroseismology and planet finding missions, obtain extremely high photometric quality time-series data as their primary purpose. Similar quality data are potentially, and in some designs actually, being obtained for very many other sources in the telescope field of view, in addition to the primary mission targets. These parallel data, of exceptional quality and broad scientific interest, can be made available for scientific analysis with small system impact. This paper lists some of the most obvious of the serendipitous research and discovery opportunities which these parallel data will allow. The scientific potential is both large and unique, encouraging efforts to provide these data to the community.

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