arXiv subjects

Gerry Gilmore

Publications and source records attributed to Gerry Gilmore.

34 records · Page 2Linked to original sources

arXiv2010

Binary stars as probes of dark substructures in dwarf galaxies

We use analytical and N-body methods to examine the survival of wide stellar binaries against repeated encounters with dark substructures orbiting in the dark matter haloes of dwarf spheroidal galaxies (dSphs). Our models adopt cosmologically-motivated conditions wherein dSphs are dark-matter dominated systems that form hierarchically and orbit about a host galaxy. Our analytical estimates show that wide binaries are disrupted at a rate that is proportional to the local density of dark substructures averaged over the life-time of the binary population. The fact that external tides can efficiently strip dark substructures from the outskirts of dSphs implies that the present number and distribution of binaries is strongly coupled with the mass evolution of individual galaxies. Yet we show that for the range of dynamical masses and Galactocentric distances spanned by Milky Way dSphs, a truncation in the separation function at a_max <~ 0.1 pc is expected in all these galaxies. An exception may be the Sagittarius dSph, which has lost most of is dark matter envelope to tides and is close to full disruption. Our simulations indicate that at separations larger than a_max the perturbed binary distribution scales as dN/da \propto a^{-2.1} independently of the mass and density of substructures. These results may be used to determine whether the binary separation function found in dwarf galaxies is compatible with the scale-free hierarchical picture that envisions the existence of dark substructures in all galactic haloes. We show that the ACS camera on board of the Hubble telescope may be able to test this prediction in dSphs at heliocentric distances <100 kpc, even if the binary fraction amounts only 10% of the stellar population.

astro-ph.GA↗

arXiv2006

A fixed point in Coptic Chronology: the solar eclipse of 10 March, 601

We reconsider the description of a solar eclipse in the Coptic ostracon in the Egyptian museum, Turin, confirming its identification with the solar eclipse of 10 march 601. This provides one of very few fixed dates in Coptic chronology. We comment on the rarity of such descriptions.

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arXiv2004

Chemical Abundances and Milky Way Formation

Stellar chemical element ratios have well-defined systematic trends as a function of abundance, with an excellent correlation of these trends with stellar populations defined kinematically. This is remarkable, and has significant implications for Galactic evolution. The source function, the stellar Initial Mass Function, must be nearly invariant with time, place and metallicity. Each forming star must see a well-mixed mass-averaged IMF yield, implying low star formation rates, with most star formation in at most a few regions of similar evolutionary history. These well-established results are difficult to reconcile with standard hierachical formation models, which assemble many stellar units: galaxy evolution seems to have been dominated by gas assembly, with subsequent star formation. Recent results, and some new ones, on the Galactic bulge, stellar halo and thick disk t hat justify this conclusion are presented.

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arXiv2004

GAIA -- Composition, Formation and Evolution of our Galaxy

GAIA will provide a multi-colour photometric and astrometric census of some one billion compact sources, complete to 20th magnitude. In addition, spectra for radial velocities will be obtained for about 30 million stars brighter than V=17. The high spatial resolution and astrometric precision, 0.1arcsec and 10microarcsec to V=15, will not only quantify the distribution of mass and the stellar populations in the Galaxy, but make major advances in fundamental physics, cosmology and solar system science. GAIA is an ESA mission, scheduled for launch in mid-2011. A full description of the GAIA project and science case, fairly crediting the hundreds of contributors, is available in the project `Red Book' [ESA-SCI(2000)4]. A brief overview is provided by Perryman, de Boer, Gilmore, et al (2001). The www site is http://www.rssd.esa.int/GAIA/

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arXiv2004

Galactic Structure and Evolution: a decade of surveys

Surveys of the local and distant Universe are the means to test and improve our models of galaxy formation. Substantial successes in the models are evident, while there is also considerable recent progress in identifying what remains to be learned. The key weaknesses of present models are related to merging histories, and small-scale structures, which are both significantly at variance with observations. Observers are polite, and often emphasise their agreements with models. Data are objective, and show us the way to focus future surveys, to allow improved understanding and knowledge.

astro-ph↗

arXiv2004

Extragalactic Integral Field Spectroscopy on the Gemini Telescopes

We have been undertaking a programme on the Gemini 8-m telescopes to demonstrate the power of integral field spectroscopy, using the optical GMOS spectrograph, and the new CIRPASS instrument in the near-infrared. Here we present some preliminary results from 3D spectroscopy of extra-galactic objects, mapping the emission lines in a 3CR radio galaxy and in a gravitationally lensed arc, exploring dark matter sub-structure through observations of an Einstein Cross gravitational lens, and the star formation time-scales of young massive clusters in the starburst galaxy NGC 1140.

astro-ph↗

arXiv2002

Early Virtual Science: some lessons for the AVO

Experience with ASTROVIRTEL, scientific analysis of current large data sets, and detailed preparation for the truly huge future missions especially GAIA, provide important lessons for the Astrophysical Virtual Observatory. They demonstrate that the science cases are impressive, specifically allowing new thresholds to be crossed. The AVO is more than just faster cheaper better, it allows the new. The example of use of pre-explosion imaging of supernova to identify progenitors is used to illustrate some general challenges. Some non-trivial technical astronomical issues arise, especially astrometry, to complement the many technical implementation challenges. A critical scientific lesson is the need to quantify data quality. How are we to ensure the Virtual Observatory produces top science, and avoids being overwhelmed with mediocre data?

astro-ph↗

arXiv2002

Optimising GAIA: How do we meet the science challenge?

GAIA has been approved to provide the data needed to quantify the formation and evolution of the Milky Way Galaxy, and its near neighbours. That requires study of all four key Galactic stellar populations: Bulge, Halo, Thick Disk, Thin Disk. The complex analysis methodologies required to model GAIA kinematic data are being developed, and in the interim applied to the relatively simple cases of the satellite dSph galaxies. These methodologies, illustrated here, show that we will be able to interpret the GAIA data. They also quantify what data GAIA must provide. It is very unlikely in the present design that GAIA will be able to provide either radial velocities or worthwhile photometry for study of two of the key science goals: the Galactic Bulge and the (inner) Galactic old disk. The implication is that the radial velocity spectrometer and the medium band photometer should be optimised for study of low density fields -- suitable for their low spatial resolution -- and the broad band photometry must be optimised for inner galaxy astrophysical studies.

astro-ph↗

arXiv2002

Galaxy Formation: Clues from the Milky Way

Stellar Populations are the fossil record of Galactic evolution. Interpretation of this record in the Local Group allows one to determine reliably the dominant physics controlling the evolution of those galaxies which are typical of the luminosity in the Universe, and is an essential prerequisite to understanding necessarily limited data at high redshifts. In our Galaxy, the key issues are the places and times of formation and merger of the oldest stellar populations : the halo, thick disk and bulge - and their overlaps and evolutionary relationships, if any. New results on studies of the stellar initial mass function at high redshift, the stellar populations of the Galactic bulge, and the merger history of the Galactic disk are reviewed.

astro-ph↗

arXiv2002

Primordial or Dynamical Mass Segregation in Young LMC Clusters?

We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud, NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions.

astro-ph↗

arXiv2001

Evidence Supporting the Universality of the IMF

The stellar initial mass function (IMF) is an underlying distribution function which determines many important observables, from the number of ionizing photons in a population of some age and metallicity, through the creation rate of various chemical elements, to the mass to light ratio of a system. This significance, together with the empirical difficulty to determine the IMF robustly, and the near complete lack of any robust theoretical predictor, has allowed investigators freedom to treat the IMF as a continuously variable parameterisation of astrophysicists' ignorance of complexity. An ability to vary a parameter in a model is not the same as a true variation in a physical system. A more instructive approach is to use available data to constrain possible variations, and thereby to allow identification of those other aspects of an observed system whose understanding can be improved. Ideally, the most sensitive physical variable, or its parameterisation, should be the best constrained. A fundamental null hypothesis, which we defend here, is that the IMF is a universal invariant function, so that all apparent variations may be ascribed to other variables, and to irreducible statistical sampling fluctuations.

astro-ph↗

arXiv2001

The low-mass IMF - deep star counts in the dSph galaxy Ursa Minor

We present a new study of deep star counts in the Local Group dwarf spheroidal (dSph) in Ursa Minor. Both the luminosity function (LF) and the colour-magnitude diagram (CMD) of the unevolved stars are compared with the LF and CMD of the old, metal-poor globular cluster M92. The main sequence locations and turn-offs are identical within the errors. Since we know from the brighter evolved stars that the metallicities for these two disparate systems are the same this implies that they also have equal ages. A direct comparison of faint LFs is then equivalent to comparison of the low-mass stellar Initial Mass Functions (IMF). We find that their LFs are identical within the mass-range covered (~0.35 - 0.8 M_sun). The Ursa Minor dSph has one of the highest apparent M/L ratios known in the Local Group, and is an extremely low surface brightness external galaxy. M92 is a typical high surface brightness globular cluster, with no apparent dark matter. These results lead to the conclusion that the low-mass stellar IMF in systems that formed at high redshift is independent of environment. Indeed, it is consistent with the low-mass IMF in star-forming regions today.

astro-ph↗

arXiv2000

The Star Formation History of the Milky Way

Quantification of the Galaxy's star formation history involves both the duration and the rate of formation, with these parameters being known with different precision for different populations. The early rate of star formation is knowable from modelling chemical element data, the recent rate directly from isochrone analyses of colour-magnitude data. The field halo and globular clusters are almost exclusively old, and formed in at most a few Gyr. The outer bulge probably formed in a short period long ago -- extant data is inconsistent, while the inner bulge/disk is forming today, and has continued to form over time. Only very limited data is available on the inner disk. The outer disk near the Sun seems as old as the halo. The earliest extended disk, which forms the thick disk today, seems to have been in place very early, an observation which is not simply consistent with some galaxy formation models.

astro-ph↗

arXiv2000

OPTICON and the Virtual Observatory

The challenges of multi-wavelength astrophysics require new outlooks from those appropriate to traditional astronomy. The next generation of research scientists must be trained to exploit the potentiality now being provided for the first time. Just as importantly, the full range of available information must be indexed and made available, to avoid wasteful repeat observations, or incomplete analyses. Perhaps the greatest challenge in the immediate future is to ensure the wealth of multi-wavelength data already available, and being accumulated, is available for efficient scientific exploitation. The difference between observations in a depositary and a fully-operational data archive is the difference between waste and cutting-edge science. The EU Optical Infrared Coordination Network for Astronomy (OPTICON) provides a forum to coordinate and develop the many national and international efforts and desires leading towards an operational virtial observatory.

astro-ph↗

arXiv2000

Euler, Hipparcos and the five dwarfs : Reconstructing star formation histories in the Local Group

The unprecedented quality of recent colour-magnitude diagrams of resolved stellar populations in nearby galaxies requires state-of-the-art techniques to infer the star formation histories which gave rise to the observed distributions. We have developed a maximum likelihood technique, which coupled to a variational calculus allows us to make a robust, non-parametric reconstruction of the evolution of the star formation rate. A full Bayesian analysis is also applied to assess whether the best solutions found are also good fits to the data. Applying this new method to WFPC2 observations of five dSph galaxies of the Local Group, we find a wide variety of star formation histories, with no particular epoch being dominant. In the case of the solar neighbourhood observed by Hipparcos we infer, with an unprecedented resolution of 50 Myr, its star formation history over the past 3 Gyr. The surprising regularity of star formation episodes separated by some 0.5 Gyr could be interpreted as the result of interactions with two spiral arms or the Galactic bar. These bursts possibly trigger the formation of massive star clusters which slowly dissolve into the galactic field.

astro-ph↗

arXiv1998

GAIA: Origin and Evolution of the Milky Way

GAIA is a short-listed candidate for the ESA Cornerstone mission C5, meeting the ESA Survey Committee requirement for an observatory mission, dedicated to astrometry, providing 10 micro-arcsecond accuracy at 15th magnitude. The GAIA mission concept follows the dramatic success of the ESA HIPPARCOS mission, utilising a continuously scanning spacecraft, accurately measuring 1-D coordinates along great circles, in (at least) two simultaneous fields of view, separated by a known angle. These 1-D relative coordinates are later converted to the five astrometric parameters of position and motions in a global analysis. GAIA will provide precise astrometry and multi-colour photometry for all the one billion stars, quasars, and compact galaxies to I=20 on the sky. GAIA will additionally provide the sixth phase-space parameter, radial velocity, from a slitless spectroscopic survey of most stars brighter than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are spectacular, with greatest impact in the study of stellar populations and dynamical structure of the galaxies of our Local Group, and in providing the first complete census of the stars and massive planets in the Solar neighbourhood. GAIA will revolutionise our knowledge of the origin and evolution of our Milky Way Galaxy, and of the distribution of planetary systems around other stars.

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