arXiv subjects

Gerard Gilmore

Publications and source records attributed to Gerard Gilmore.

At least 91 records · Page 5Linked to original sources

arXiv1998

Element Ratios and the Formation of the Stellar Halo

It is well-established that the vast majority of metal-poor Galactic halo stars shows evidence for enrichment by solely massive stars. Recent observations have identified more varied behavior in the pattern of elemental abundances measured for metal-rich, [Fe/H]=-1, halo stars, with both high and low values of the ratio of [alpha/Fe]. The low values are most naturally due to the incorporation of iron from Type Ia supernovae. These `low-alpha' halo stars have been interpreted as being accreted from dwarf galaxies. However, these stars are on very high-energy radial orbits, that plunge into the Galactic center, from the outer regions of the stellar halo. We demonstrate here that known dwarf galaxies could not reproduce the observations. Rather, the observations are consistent with fragmentation of a gaseous proto-halo into transient star-forming regions, some of which are sufficiently dense to survive close to the Galactic center, and self-enrich to relatively high metallicities. Those that probe the outer halo sustain star formation long enough to incorporate the ejecta from Type Ia supernovae. The values of the `Type II plateau' in the element ratios for disk stars and for halo stars are equal. This implies that the (massive) stars that enriched the early disk and halo had the same IMF. However, there is a discontinuity in the chemical enrichment history between the halo and disk, as probed by these samples, consistent with previous inferences based on angular momentum considerations.

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arXiv1998

Dynamical friction in dwarf galaxies

We present a simplified analytic approach to the problem of the spiraling of a massive body orbiting within the dark halo of a dwarf galaxy. This dark halo is treated as the core region of a King distribution of dark matter particles, in consistency with the observational result of dwarf galaxies having solid body rotation curves. Thus we derive a simple formula which provides a reliable and general first order solution to the problem, totally analogous to the one corresponding to the dynamical friction problem in an isothermal halo. This analytic approach allows a clear handling and a transparent understanding of the physics and the scaling of the problem. A comparison with the isothermal case shows that in the core regions of a King sphere, dynamical friction proceeds at a different rate, and is sensitive to the total core radius. Thus, in principle, observable consequences may result. In order to illustrate the possible effects, we apply this formula to the spiraling of globular cluster orbits in dwarf galaxies, and show how present day globular cluster systems could in principle be used to derive better limits on the structure of dark halos around dwarf galaxies, when the observational situation improves. As a second application, we study the way a massive black hole population forming a fraction of these dark halos would gradually concentrate towards the centre, with the consequent deformation of an originally solid body rotation curve. This effect allows us to set limits on the fraction/mass of any massive black hole minority component of the dark halos of dwarf galaxies. In essence, we take advantage of the way the global matter distribution fixes the local distribution function for the dark matter particles, which in turn determines the dynamical friction problem.

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arXiv1997

Near infrared star counts as a test of Galactic bar structure

We present survey data in the narrow-band L filter (nbL), taken at UKIRT, for a total area of 277 square arcmin, roughly equally divided between four regions at zero Galactic latitude and longitudes +-4.3 deg and +-2.3 deg. The 80% completeness level for these observations is at roughly magnitude 11.0. This magnitude limit, owing to the low coefficient for interstellar extinction at this wavelength (A(nbL)=0.047 A(V)), allows us to observe bulge giants. We match the nbL-magnitudes with DENIS survey K magnitudes, and find 95 per cent of nbL sources are matched to K sources. Constructing colour-magnitude diagrams, we deredden the magnitudes and find evidence for a longitude dependent asymmetry in the source counts. We find that there are ~15% and ~5% more sources at the negative longitude than at the corresponding positive longitude, for the fields at +-4.3 deg and +-2.3 deg respectively. This is compared with the predictions of some Galactic bar models. We find an asymmetry in the expected sense, which favours gas dynamical models and the recent deconvolution of surface photometry data (Binney et al. 1991; Binney, Gerhard & Spergel 1997), over earlier treatments of photometric data (e.g. Dwek et al. 1995).

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arXiv1997

The Inner Galaxy resolved at IJK using DENIS data

We present the analysis of three colour optical/near-infrared images, in IJK, taken for the DENIS project. The region considered covers 17.4 square deg and lies within |l|<5 deg, |b|<1.5 deg. The adopted methods for deriving photometry and astrometry in these crowded images, together with an analysis of the deficiencies nevertheless remaining, are presented. The numbers of objects extracted in I,J and K are 748000, 851000 and 659000 respectively, to magnitude limits of 17,15 and 13. 80% completeness levels typically fall at magnitudes 16, 13 and 10 respectively, fainter by about 2 magnitudes than the usual DENIS limits due to the crowded nature of these fields. A simple model to describe the disk contribution to the number counts is constructed, and parameters for the dust layer derived. We find that a formal fit of parameters for the dust plane, from these data in limited directions, gives a scalelength and scaleheight of 3.4+-1.0 kpc and 40+-5 pc respectively, and a solar position 14.0+-2.5 pc below the plane. This latter value is likely to be affected by localised dust asymmetries. We convolve a detailed model of the systematic and random errors in the photometry with a simple model of the Galactic disk and dust distribution, to simulate expected colour-magnitude diagrams. These are in good agreement with the observed diagrams, allowing us to isolate those stars from the inner disk and bulge. After correcting for local dust-induced asymmetries, we find evidence for longitude-dependent asymmetries in the distant J and K sources, consistent with the general predictions of some Galactic bar models. We consider complementary L-band observations in a second paper.

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arXiv1997

Inferring dark halo structure from observed scaling laws of late type galaxies and LSB's

We re-examine the Fall & Efstathiou (1980) scenario for galaxy formation, including the dark halo gravitational reaction to the formation of the baryon disk, as well as continuous variations in the intrinsic halo density profile. The recently published rotation curves of LSB and dwarf galaxies together with previously known scaling relations provide sufficient information on the present day structure of late type disk galaxies to invert the problem. By requiring that the models reproduce all the observational restrictions we can fully constrain the initial conditions of galaxy formation, with a minimum of assumptions, in particular without the need to specify a CDM halo profile. This allows one to solve for all the initial conditions, in terms of the halo density profile, the baryon fraction and the total angular momentum. We find that a unique initial halo shape is sufficient to accurately reproduce the rotation curves of both LSB and normal late type spiral galaxies. This unique halo profile differs substantially from that found in standard CDM models (Navarro, Frenk & White 1996). A galactic baryon fraction of 0.065 is found. The initial value of the dimensionless angular momentum is seen to be the principal discriminator between the galaxy classes we examine. The present day scalings between structural parameters are seen to originate in the initial conditions.

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arXiv1997

Physical Parameterization of Stellar Spectra: The Neural Network Approach

We present a technique which employs artificial neural networks to produce physical parameters for stellar spectra. A neural network is trained on a set of synthetic optical stellar spectra to give physical parameters (e.g. T_eff, log g, [M/H]). The network is then used to produce physical parameters for real, observed spectra. Our neural networks are trained on a set of 155 synthetic spectra, generated using the SPECTRUM program written by Gray (Gray & Corbally 1994, Gray & Arlt 1996). Once trained, the neural network is used to yield T_eff for over 5000 B-K spectra extracted from a set of photographic objective prism plates (Bailer-Jones, Irwin & von Hippel 1997a). Using the MK classifications for these spectra assigned by Houk (1975, 1978, 1982, 1988) we have produced a temperature calibration of the MK system based on this set of 5000 spectra. It is demonstrated through the metallicity dependence of the derived temperature calibration that the neural networks are sensitive to the metallicity signature in the real spectra. With further work it is likely that neural networks will be able to yield reliable metallicity measurements for stellar spectra.

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arXiv1997

The Distribution of Dark Matter in the Milky Way Galaxy

A wealth of recent observational studies shows the dark matter in the Milky Way to have the following fundamental properties: 1) there is no detectable dark matter associated with the Galactic disk -- the dark matter is distributed in a purely halo distribution with local volume density near the Earth 0.3GeV/cc = 0.01Msun/pc3 2) The stellar mass function is both universal and convergent at low masses -- there is no significant very low luminosity baryonic dark matter associated with stellar light. Fundamentally, dark mass does not follow light. 3) The smallest scale length on which dark matter is clustered is in the Milky Way's satellite dwarf Spheroidal galaxies, where dark matter has a characteristic length scale about 1kpc = 10^{20}m, and mass density 0.05Msun/pc3 = 1.5GeV/cc.

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arXiv1997

Galactic Bulges

We review current knowledge on the structure, properties and evolution of galactic bulges, considering particularly common preconceptions in the light of recent observational results.

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arXiv1996

The Kinematics, Orbit, and Survival of the Sagittarius Dwarf Spheroidal Galaxy

The Sagittarius galaxy (Sgr), the closest satellite galaxy of the Milky Way, has survived for many orbits about the Galaxy. Extant numerical calculations modeled this galaxy as a system with a centrally-concentrated mass profile, following the light, and found that it should lose more than one-half of its mass every 2--4 orbits and be completely disrupted long before now. Apparently Sgr, and by implication other dSph galaxies, do not have a centrally-concentrated profile for their dark matter. We develop a model in which the stars of the Sgr dwarf are embedded in a constant-density dark matter halo, representing the core of a tidally-limited system, and show that this is consistent with its survival. We present new photometric and kinematic observations of Sgr and show these data are consistent with this explanation for the continued existence of this galaxy. Sgr is being tidally distorted and is tidally limited, but is not disrupted as yet. The corresponding minimum total mass is $10^9 \msun$, while the central mass to visual light ratio $\sim 50$ in Solar units. Our new photographic photometry allows the detection of main-sequence stars of Sgr over an area of $22°\times 8°$. Sgr is prolate, with axis ratios $\sim$~3:1:1. For an adopted distance of $16 \pm 2 \kpc$ from the Galactic center on the opposite side of the Galaxy to the Sun, the major axis is $\gta 9 \kpc$ long and is aligned approximately normal to the plane of the Milky Way Galaxy, roughly following the coordinate line $\ell=5^\circ$. The central velocity dispersion of giant stars which are members of Sgr is $11.4 \pm 0.7 \kms$ and is consistent with being constant over the face of the galaxy. The gradient in mean line-of-sight velocity with position along the

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arXiv1996

The Metallicity Dependence of the Stellar Luminosity and Initial Mass Functions: HST Observations of Open and Globular Clusters

Using HST and the WFPC2 we have acquired very deep V- and I-band photometry of stars in NGC 2420 and NGC 2477 to study cluster luminosity functions at approximately solar metallicity. We have determined these cluster luminosity functions down to $M_I$ = 10.5 (0.2 M$_{\odot}$) and find that the luminosity function of NGC 2420 turns over at $M_I$ $\approx$ 9.0, and possibly stops altogether by $M_I$ $\approx$ 9.5. The luminosity function of NGC 2477 may flatten at $M_I$ $\geq$ 9.5. We compare our open cluster luminosity functions to the solar neighborhood field star luminosity function of Kroupa, Tout \& Gilmore (1993) and the four published HST globular cluster luminosity functions: $ω$ Cen (Elson {\it et al.}\ 1995), 47 Tuc (De Marchi \& Paresce 1995b), M 15 (De Marchi \& Paresce 1995a), and NGC 6397 (Paresce, De Marchi \& Romaniello 1995). We find a smooth relation between the location of the luminosity function turn-over and the metallicity for all these low mass star samples which matches the expected $M_I$ versus [Fe/H] trend for a model star of $\approx$ 0.27 M$_{\odot}$ (Saumon 1995; Alexander {\it et al.}\ 1996). We interpret this smooth and systematic behavior in the cluster luminosity functions as strong evidence in favor of an invariant initial mass function and a metallicity-dependent mass-luminosity relation.

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arXiv1995

Chemistry and Kinematics in the Solar Neighborhood: Implications for Stellar Populations and for Galaxy Evolution

The immediate Solar neighborhood should be a fair sample of the local Galaxy. However, the chemical abundance distribution of long-lived disk stars very near the Sun contains a factor of five to ten more metal-poor stars, $-1 \simlt {\rm [Fe/H]} \simlt -0.4$ dex, than is consistent with modern star-count models of larger scale Galactic structure. The metallicity distribution of complete samples of long-lived stars has long been recognised as providing unique constraints on the early stages of chemical evolution of the Galaxy, so that one would like to resolve this anomaly. We present a new derivation of the local G-dwarf metallicity distribution, based on the Third Gliese catalog combined with Olsen's (1983) Strömgren photometry. Kinematic data for these same stars, as well as for a high-precision sample studied by Edvardsson {\sl et al.} (1993), provide clear evidence that the abundance distribution below [Fe/H]$\sim -0.4$ contains two over-lapping distributions, the thick disk and the thin disk. We achieve a reliable deconvolution of the relative numbers in each population by comparing the local metallicity distribution with a recent determination (Gilmore, Wyse \& Jones 1995) of the metallicity distribution of F/G stars {\sl in situ} some 1500pc from the Sun. The gravitational sieve of the Galactic potential acts on this second sample to segregate the low velocity dispersion, thin-disk, component of the local sample, leaving predominantly the second, higher velocity dispersion component. The combination of these two datasets allows us to determine the source of the local paradox: there is a substantial tail of the thin disk (defined kinematically) metallicity distribution, which extends below ${\rm [Fe/H] \approx -0.4}$dex. This is a robust conclusion, being consistent with the sum of star count, stellar spatial

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arXiv1995

Good Abundances from Bad Spectra: I. Techniques

We have developed techniques to extract true iron abundances and surface gravities from spectra of the type provided by the multiple-object fibre-fed spectroscopic radial-velocity surveys underway with 2dF, HYDRA, NESSIE, and the forthcoming Sloan survey. Our method is optimised for low S/N, intermediate resolution blue spectra of G stars. Spectroscopic indices sensitive to iron abundance and gravity are defined from a set of narrow (few Angstrom) wavelength intervals, and calibrated using synthetic spectra. We have also defined a single abundance indicator which is able to provide useful iron abundance information from spectra having S/N ratios as low as 10 per Angstrom. The theoretical basis and calibration using synthetic spectra are described in this paper. The empirical calibration of these techniques by application to observational data is described in Jones, Wyse and Gilmore (PASP July 1995). The technique provides precise iron abundances, with zero-point correct to $\sim 0.1$ dex, and is reliable, with typical uncertainties being $\approxle 0.2$ dex. A derivation of the {\it in situ\/} thick disk metallicity distribution using these techniques is presented by Gilmore, Wyse and Jones (AJ 1995 v109 p1095).

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arXiv1995

HST Star Counts at High Galactic Latitudes

We use star counts from 13 deep HST fields imaged with the {\it Wide Field Camera - 2} in order to constrain the amount of dark matter in the Galaxy that can be in the form of low-mass main sequence stars or white-dwarfs. Based on the number of red stars found in our fields, we exclude the possibility that more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs; fairly massive ($M \sim 0.2 M_\odot$) and yet extremely faint ($M_I$ \gtsima $14.5$) stellar candidates would have to be invoked in order to make the observed number of stars compatible with that predicted by a stellar dark halo. White dwarfs must also be intrinsically very faint ($M_I$ \gtsima $14$) in order to be consistent with the observed number of faint stars in the HST fields. We also rule out an increasing or flat stellar luminosity function beyond $M_V \sim 13$. The inferred slope of the disk luminosity function is intermediary between local, volume-limited surveys and ground-based photometric ones. Finally, the magnitude counts are well fitted by existing models for the structure of the Galaxy, with only small changes in the fiducial values of the model parameters. The colour distribution, however, is not well reproduced by the models. It is unclear at present if this reflects inadequacies of the available models or uncertainties in the colour-magnitude diagrams for low metallicity stars and in the photometric calibration.

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arXiv1995

Good Abundances from Bad Spectra :II. Application and a New Stellar Color--Temperature Relation

Stellar spectra derived from current multiple-object fiber-fed spectroscopic radial-velocity surveys, of the type feasible with, among other examples, AUTOFIB, 2dF, HYDRA, NESSIE, and the Sloan survey, differ significantly from those traditionally used for determination of stellar abundances. The spectra tend to be of moderate resolution (around $1\,$Å) and signal-to-noise ratio (around 10-20 per resolution element), and cannot usually have reliable continuum shapes determined over wavelength ranges in excess of a few tens of Angstroms. Nonetheless, we have developed techniques to extract true iron abundances and surface gravities from such spectra of G-type stars in the 4000--$5000\,$Å wavelength region. The theoretical basis and calibration using synthetic spectra are described in detail in another paper. The practical application of these techniques to observational data is given in the present paper. We show that this method can provide true iron abundances, with an uncertainty of less than $ 0.2$ dex over the range of metallicity found in the Galactic thick and thin disks, from spectra obtained with fiber-fed spectrographs.

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arXiv1995

The Outer Regions of the Galactic Bulge:I Observations

An observational survey of stars selected from the region of sky in the direction of the Galactic bulge is presented. We discuss the choice of tracer populations for this study. Digitised UK Schmidt plate photometry, calibrated with CCD photometry, is obtained in 18 regions over the bulge. Stars are selected for spectroscopic follow-up from within a carefully chosen colour-magnitude selection window, optimised for efficient detection of bulge K giants. Some 1500 stellar spectra were obtained, with the AAT AUTOFIB facility, giving the largest data set yet obtained in this region of the Galaxy, by a factor of $\approx 10$. We have derived a radial velocity and metallicity for each star, and have quantified the uncertainties in these measurements. Luminosity classification has been derived both by visual classification and using an automated routine based on Principal Component Analysis. There are two basic results from this survey: the discovery of the Sagittarius dwarf galaxy, which is described by Ibata, Gilmore & Irwin (1994,1995), and a study of the Galactic Bulge, which is presented in an accompanying paper (Ibata & Gilmore 1995).

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arXiv1995

The Outer Regions of the Galactic Bulge:II Analysis

We analyse velocity, chemical abundance and spatial distribution data for some 1500 K- and M- giants in the Galactic Bulge. The bulge has a well-determined linear rotation curve over the range $\rm 700pc \le R \le 3500pc$ with amplitude ${\rm \sim 25 km s^{-1} kpc^{-1}}$. The oblate isotropic bulge model of Kent (1992) is in reasonable agreement with our data. We do not find a significant requirement for asymmetry or a bar. The metallicity distribution of K giants in a subset of our fields, using the Mg$b$ feature, has mean ${\rm [Fe/H] \approx -0.3}$ with no variation. That is, there is no detectable abundance gradient in the Galactic bulge over the galactocentric range $\rm 500pc \le R_{GC} \le 3500pc$. We derive the distribution function of specific angular momentum for the bulge from our data, and compare it with determinations for the halo, the thick disk and the thin disk from Wyse & Gilmore (1992). We confirm that the bulge and the halo have angular momentum distributions which are indistinguishable, as do the thick disk and the thin disk. The bulge-halo distribution is however very different from the thick disk-thin disk distribution. This is perhaps the strongest available clue to the evolutionary relationships between different Galactic structural components.

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arXiv1995

An Independent Calibration of Stellar Ages: HST Observations of White Dwarfs at V=25

The white dwarf luminosity function of a stellar cluster will have a sharp truncation at a luminosity which is determined by the time since formation of the first white dwarfs in that cluster. Calculation of the dependence of this limiting luminosity on age requires relatively well-understood physics and is independent of stellar evolutionary models. Thus, measurement of the termination of the white dwarf luminosity function provides an independent method to determine the age of a cluster, and thereby to calibrate stellar evolutionary ages. We have obtained HST WFPC2 data in two open clusters, identified the white dwarf sequence, and proved the feasibility of this approach, by detecting white dwarfs to V=25. Much deeper data are feasible. From our present limited data, we show that degenerate cooling ages are not consistent with some published isochrone ages for clusters with ages of order 1Gyr.

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arXiv1994

A Determination of the Thick Disk Chemical Abundance Distribution: Implications for Galaxy Evolution

We present a determination of the thick disk iron abundance distribution obtained from an in situ sample of F/G stars. These stars are faint, $15 \simlt V \simlt 18$, selected on the basis of color, being a subset of the larger survey of Gilmore and Wyse designed to determine the properties of the stellar populations several kiloparsecs from the Sun. The fields studied in the present paper probe the iron abundance distribution of the stellar populations of the Galaxy at 500--3000pc above the plane, at the solar Galactocentric distance.

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