arXiv subjects

Gerard Gilmore

Publications and source records attributed to Gerard Gilmore.

At least 73 records · Page 4Linked to original sources

arXiv2002

Chemical Evolution Models of Local dSph Galaxies

We calculate chemical evolution models for 4 dwarf spheroidal satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal rich selective wind, which might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar population have formed. Systems showing extended star formation histories (Carina and Leo I) however, are consistent with the idea that their tidally limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems, remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor, strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.

astro-ph↗

arXiv2001

First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal

We present the first clear evidence for an extended dark matter halo in the Draco dwarf spheroidal galaxy based on a sample of new radial velocities for 159 giant stars out to large projected radii. Using a two parameter family of halo models spanning a range of density profiles and velocity anisotropies, we are able to rule out (at about the 2.5 sigma confidence level) haloes in which mass follows light. The data strongly favor models in which the dark matter is significantly more extended than the visible dwarf. However, haloes with harmonic cores larger than the light distribution are also excluded. When combined with existing measurements of the proper motion of Draco, our data strongly suggest that Draco has not been tidally truncated within ~1 kpc. We also show that the rising velocity dispersion at large radii represents a serious problem for modified gravity (MOND).

astro-ph↗

arXiv2001

GAIA: Understanding the Galaxy

The GAIA Observatory, ESA's Cornerstone 6 mission, addresses the origin and evolution of our Galaxy, and a host of other scientific challenges. GAIA will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group, about one per cent of the Galactic stellar population. Combined with astrophysical information for each star, provided by on-board multi-colour photometry, these data will have the precision and depth necessary to address the three key questions which underlie the GAIA science case: When did the stars in the Milky Way form? When and how was the Milky Way assembled? What is the distribution of dark matter in our Galaxy?

astro-ph↗

arXiv2001

OPTICON: EC Optical Infrared Coordination Network for Astronomy

OPTICON, the ICN Optical Infrared Coordination Network for Astronomy, brings together for the first time the operators of all Europe's medium to large optical-infrared telescopes, the largest corresponding data archives, and several user representatives. The OPTICON partners work with their communities to identify those major challenges for the future development of European optical-infrared astronomy which require Europe-wide collaboration. OPTICON sponsors and coordinates developments towards these goals, involving the entire astronomical community through workshops and meetings targeted towards these agreed common goals of general importance.

astro-ph↗

arXiv2001

The Thick Disk-Halo Interface

The star formation history of a galaxy, explicitely here our Milky Way Galaxy, where the most detailed information is attainable, is the convolution of two functions. One function describes the rate of formation of the stars which are today in the Galaxy. The second describes the assembly of those stars into the present Galactic potential well. There is direct evidence that this assembly continues today, with both stars and gas being assembled into, or at least rearranged in, the Galactic potential. But is this recent accretion significant? Was the last significant accretion the formation of the thick disk, some 10Gyr ago?. Recent spectroscopic studies support this unexpected result, while dynamical studies find increasing numbers of specific examples of smaller scale more recent accretion. We present early results for one specific such survey, the Anglo Australian Old Stellar populations Survey, to illustrate current studies.

astro-ph↗

arXiv2001

The Metal-rich Globular Cluster NGC6553: Observations with WFPC2, STIS, and NICMOS

We present a HST study of the metal-rich globular cluster NGC6553 using WFPC2, NICMOS and STIS. Our primary motivation is to calibrate the STIS broad-band LP magnitude against $V_{555}$ and $I_{814}$ magnitudes for stars of known metallicity and absolute (visual) magnitude, for application to our study of LMC globular clusters. NGC6553 has been shown in earlier studies to have a very unusual colour-magnitude diagram, so we also use our data to investigate the reddening, distance, luminosity function and structure of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but emphasise that very large patchy extinction on small angular scales prohibits accurate determination of the parameters of this cluster. The horizontal branch of NGC6553 in ($V,V-I$) is tilted at an angle close to that of the reddening vector. We show that extinction does not, however, explain the tilt, which is presumably a metallicity effect. The colour-magnitude diagram shows an apparent second turnoff some 1.5 magnitudes fainter than that of the cluster. We show that this is most likely the background Galactic bulge: however, in that case, the colour-magnitude diagram of NGC6553 is not a good match to that of the field bulge population. The cluster is probably more metal-rich than is the mean field bulge star.

astro-ph↗

arXiv2000

HST Color-Magnitude Data for Globular Clusters: I. Transformations Between STIS LP Magnitudes and WFPC2 F606W and F814W Magnitudes

We present deep Hubble Space Telescope optical imaging observations of two Galactic globular clusters, 47 Tuc and M15, using the Space Telescope Imaging Spectrograph (STIS) longpass (LP) filter and the Wide Field and Planetary Camera 2 (WFPC2) F606W and F814W filters. These globular clusters have very different metallicities ([Fe/H] = -0.7 dex for 47 Tuc, and [Fe/H] = -2.2 dex for M15), essentially spanning the metallicity distribution of the entire Milky Way globular cluster system, and were chosen to investigate the relationship between magnitudes in the non-standard STIS LP system and the better characterized WFPC2 magnitudes. We determine this relationship by combining our new STIS data with archival WFPC2 data, taking care to provide a robust and reliable transformation. Examining the 47 Tuc and M15 data separately, there is no evidence for a metallicity dependence in the transformation relations. This allows us to combine the data for both clusters, and also include the stars in the Small Magellanic Cloud that lie within the field of view of the 47 Tuc images, to determine final transformations that are valid for main sequence stars with colors in the range 0.3 < V606-I814 < 1.8. These final relations predict STIS magnitudes within 0.05 mag (1 sigma) of those measured for stars in both clusters. In an appendix, we discuss the differences between our empirical transformation relations and those predicted by synthetic photometry.

astro-ph↗

arXiv2000

The White Dwarf Cooling Age of the Open Cluster NGC 2420

We have used deep HST WFPC2 observations of two fields in NGC 2420 to produce a cluster CMD down to V = 27. After imposing morphological selection criteria we find eight candidate white dwarfs in NGC 2420. Our completeness estimates indicate that we have found the terminus of the WD cooling sequence. We argue that the cluster distance modulus is likely to be close to 12.10 with E(B-V) = 0.04. With these parameters we find a white dwarf cooling age for NGC 2420 of 2.0 +- 0.20 (1 sigma) Gyrs. The 0.20 Gyr uncertainty includes errors in the photometry, sequence fitting, precursor time scales, and theoretical white dwarf cooling time scales. Comparing the cluster white dwarf cooling age to ages derived from stellar isochrone fitting we find a preference for ages derived from models incorporating convective overshoot.

astro-ph↗

arXiv2000

The recent star formation history of the Hipparcos solar neighbourhood

We use data from the Hipparcos catalogue to construct colour-magnitude diagrams for the solar neighbourhood, which are then treated using advanced Bayesian analysis techniques to derive the star formation history, $SFR(t)$, of this region over the last 3 Gyr. The method we use allows the recovery of the underlying $SFR(t)$ without the need of assuming any {\it a priori} structure or condition on $SFR(t)$, and hence yields a highly objective result. The remarkable accuracy of the data permits the reconstruction of the local $SFR(t)$ with an unprecedented time resolution of $\approx 50 $ Myr. A $SFR(t)$ having an oscillatory component of period $\approx 0.5$ Gyr is found, superimposed on a small level of constant star formation activity. Problems arising from the non-uniform selection function of the Hipparcos satellite are discussed and treated. Detailed statistical tests are then performed on the results, which confirm the inferred $SFR(t)$ to be compatible with the observed distribution of stars.

astro-ph↗

arXiv2000

Age and metallicity gradients in the Galactic Bulge - A differential study using HST/WFPC2

The Galactic Bulge has long been assumed to be a largely old stellar population. However, some recent studies based on observations with the HST WF/PC-1 and WFPC2 of stars in the Galactic Bulge have concluded that the old population may not make up more then 30% of the total. Other studies using HST/WFPC2 differential studies of `Bulge' globular clusters and field stars have found the bulge to be comparable in age to the Galactic Halo. A complication in all these studies is the presence of a substantial population of stars which mimic a young bulge population, but which may be, and are often assumed to be, foreground disk stars whose reddening and distance distributions happen to mimic a young bulge turnoff. We show, using number counts in HST/WFPC2 colour-magnitude diagrams of both field stars in the Bulge and of two `bulge' and one `disk' globular cluster (NGC6528, NGC6553, and NGC5927) that the stars interpreted as young in fact are foreground disk stars. Thus, we confirm that the bulk of the bulge field stars in Baade's Window are old. The existence of a young metal-rich population cannot, however, be ruled out from our data. We also test for age and metallicity gradients using the CMD of a metal-rich globular cluster as an empirical isochrone and derive a metallicity difference of <0.2 dex between Baade's window and SGR-I window.

astro-ph↗

arXiv2000

Non-parametric star formation histories for 5 dwarf spheroidal galaxies of the local group

We use recent HST colour-magnitude diagrams of the resolved stellar populations of a sample of local dSph galaxies (Carina, LeoI, LeoII, Ursa Minor and Draco) to infer the star formation histories of these systems, $SFR(t)$. Applying a new variational calculus maximum likelihood method which includes a full Bayesian analysis and allows a non-parametric estimate of the function one is solving for, we infer the star formation histories of the systems studied. This method has the advantage of yielding an objective answer, as one need not assume {\it a priori} the form of the function one is trying to recover. The results are checked independently using Saha's $W$ statistic. The total luminosities of the systems are used to normalize the results into physical units and derive SN type II rates. We derive the luminosity weighted mean star formation history of this sample of galaxies.

astro-ph↗

arXiv1999

The Low-Mass Stellar IMF at High Redshift: Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Low-mass stars, those with main-sequence lifetimes that are of order the age of the Universe, provide unique constraints on the Initial Mass Function (IMF) when they formed. Star counts in systems with simple star-formation histories are particularly straightforward to interpret, and those in old systems allow one to determine the low-mass stellar IMF at large look-back times and thus at high redshift. We present the faint stellar luminosity function (based on optical HST data) in an external galaxy, the Ursa Minor dwarf Spheroidal (dSph). This relatively-nearby (distance 70kpc) companion galaxy to the Milky Way has a stellar population with narrow distributions of age and of metallicity, remarkably similar to that of a classical halo globular cluster such as M92 or M15, i.e. old and metal-poor. Contrasting with globular clusters, the internal velocity dispersion of the Ursa Minor dSph indicates the presence of significant amounts of dark matter. We find that the main sequence stellar luminosity function of the Ursa Minor dSph, and implied IMF, down to 0.4 of a solar mass is indistinguishable from that of the halo globular clusters M92 and M15. Thus the low mass stellar IMF for stars that formed at high redshift is invariant in going from a low-surface-brightness, dark-matter-dominated external galaxy, to a globular cluster within the Milky Way.

astro-ph↗

arXiv1999

Star Formation Histories of the Galactic Satellites

Late accretion models for formation of the Galactic halo require that many Galactic satellite galaxies have been cannibalised into the halo field. Comparison of the metallicity and age distribution function of stars in the surviving satellites with the apparently exclusively old stars in the field halo can constrain the importance of any such process. We have developed a new objective technique to determine star formation histories in dSph galaxies. We apply this technique to the surviving Galactic satellites, deducing an approximately uniform distribution of ages for the constituents, quite unlike the halo field stars. Thus, late accretion did not play a substantial part in Galactic halo formation.

astro-ph↗

arXiv1999

Objective Determination of Star Formation Histories

Objective determination of a star formation history from a colour-magnitude diagram, independently of assumed parametric descriptions, is necessary to determine the evolutionary history of a galaxy. We introduce a new method for solving maximum likelihood problems through variational calculus, and apply it to the case of recovering an unknown star formation history, SFR(t), from a colour-magnitude diagram. This method provides a non-parametric solution with the advantage of requiring no initial assumptions on the SFR(t). As a full maximum likelihood statistical model is used, one may exploit all the information available in the photometric data which can be explained by available isochrones. Extensive tests of the method show it to be reliable under noise conditions comparable to those appropriate for real observations. Implications for future surveys, such as the GAIA mission are significant: accurate determination of the evolutionary history of the Milky Way is limited only by adequate spectroscopy and/or photometry to provide a determination of stellar metallicity and temperature for a sufficient sample of stars of luminosities at and above an old main-sequence turnoff.

astro-ph↗

arXiv1999

The Faint Optical Stellar Luminosity Function in the Ursa Minor Dwarf Spheroidal Galaxy

Analyses of their internal stellar kinematics imply that the dwarf spheroidal (dSph) companion galaxies to the Milky Way are among the most dark-matter dominated systems known. Should there be significant dark matter in the form of faint stars in these systems, the stellar luminosity function must be very different from that of a similar metallicity globular cluster, for which there is no evidence for dark matter. We present the faint stellar luminosity function in the Ursa Minor dSph, down to a luminosity corresponding to roughly 0.45 M_sun, derived from new deep HST/WFPC2 data. We find a remarkable similarity between this luminosity function, and inferred initial mass function, and that of the globular cluster M92, a cluster of similar age and metallicity to the Ursa Minor dSph.

astro-ph↗

arXiv1998

Baryonic Dark Matter: Limits from HST and ISO

Recent HST and ISO observations provide very severe limits on any compact baryonic contributions to galactic (dark) halos. When combined with Milky Way Galaxy microlensing results, almost the entire plausible range of massive compact baryonic objects is excluded by direct observation. Deep direct imaging at 7mu and 15mu with ISOCAM on the ISO spacecraft directly excludes hydrogen-burning stars of any mass above the hydrogen-burning limit, and of any chemical abundance, from being the predominant explanation of the dark halos of external spiral galaxies. In the Milky Way Galaxy, HST has provided luminosity functions to the hydrogen-burning limit in several globular clusters. The resulting mass functions do not provide any support for dominance by very low-mass stars. This is consistent with field surveys for sub-stellar mass brown dwarfs, which show such objects to be relatively rare. These results are complemented by very deep HST luminosity functions in the Large Magellanic Cloud, providing strong support for the (near)-universality of the stellar mass function. Very recent HST results are available for the nearby dSph galaxy UMi. This galaxy, the most dark-matter dominated object known on kpc scales, has a normal stellar mass function at low masses. The prospects are bright for dark elementary particles.

astro-ph↗

arXiv1998

Deriving star formation histories: Inverting HR diagrams through a variational calculus maximum likelihood method

We introduce a new method for solving maximum likelihood problems through variational calculus, and apply it to the case of recovering an unknown star formation history, $SFR(t)$, from a resulting HR diagram. This approach allows a totally non-parametric solution which has the advantage of requiring no initial assumptions on the $SFR(t)$. As a full maximum likelihood statistical model is used, we take advantage of all the information available in the HR diagram, rather than concentrating on particular features such as turn off points or luminosity functions. We test the method using a series of synthetic HR diagrams produced from known $SFR(t)$, and find it to be quite successful under noise conditions comparable to those present in current observations. At this point we restrict the analysis to situations where the metallicity of the system is known, as is the case with the resolved populations of the dwarf spheroidal companions to the Milky Way or the solar neighbourhood Hipparcos data. We also include tests to quantify the way uncertainties in the assumed metallicity, binary fraction and IMF affect our inferences.

astro-ph↗

arXiv1998

Dark Halos of Spiral Galaxies: ISO photometry

We exclude hydrogen burning stars, of any mass above the hydrogen-burning limit and any metallicity, as significant contributors to the massive halos deduced from rotation curves to dominate the outer parts of spiral galaxies. We present and analyse images of 4 nearly edge-on bulgeless spiral galaxies (UGC711, NGC2915, UGC12426, UGC1459) obtained with ISOCAM (The CAMera instrument on board the Infrared Space Observatory) at 14.5micron and 6.75micron. Our sensitivity limit for detection of any diffuse infrared emission associated with the dark halos in these galaxies is a few tens of microJy per 6arcsec x 6arcsec pixel, with this limit currently set by remaining difficulties in modelling the non-linear behaviour of the detectors. All four galaxies show zero detected signal from extended non-disk emission, consistent with zero halo-like luminosity density distribution. The 95percent upper limit on any emission, for NGC2915 in particular, allows us to exclude very low mass main sequence stars (M > 0.08 Msun), and young brown dwarfs (< 1 Gyr) as significant contributors to dark matter in galactic halos. Combining our results with those of the Galactic microlensing surveys, which exclude objects with M < 0.01 Msun, excludes almost the entire possible mass range of compact baryonic objects from contributing to galactic dark matter.

astro-ph↗