ORCID Profile
0000-0001-8993-101X
Current Organisation
University College London
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Astronomical and Space Sciences | Astronomy And Astrophysics | Cosmology and Extragalactic Astronomy
Expanding Knowledge in the Physical Sciences | Physical sciences |
Publisher: Oxford University Press (OUP)
Date: 08-2006
Publisher: Elsevier BV
Date: 2008
Publisher: American Astronomical Society
Date: 18-07-2006
DOI: 10.1086/506519
Publisher: Oxford University Press (OUP)
Date: 2005
Publisher: EDP Sciences
Date: 2007
DOI: 10.1051/EAS:2007036
Publisher: Oxford University Press (OUP)
Date: 03-2005
Publisher: Cambridge University Press (CUP)
Date: 06-2008
DOI: 10.1017/S1743921308027956
Abstract: To date, fully cosmological hydrodynamic disk simulations to redshift zero have only been undertaken with particle-based codes, such as GADGET , Gasoline , or GCD+ . In light of the (supposed) limitations of traditional implementations of smoothed particle hydrodynamics (SPH), or at the very least, their respective idiosyncrasies, it is important to explore complementary approaches to the SPH paradigm to galaxy formation. We present the first high-resolution cosmological disk simulations to redshift zero using an adaptive mesh refinement (AMR)-based hydrodynamical code, in this case, RAMSES . We analyse the temporal and spatial evolution of the simulated stellar disks' vertical heating, velocity ellipsoids, stellar populations, vertical and radial abundance gradients (gas and stars), assembly/infall histories, warps/lopsideness, disk edges/truncations (gas and stars), ISM physics implementations, and compare and contrast these properties with our s le of cosmological SPH disks, generated with GCD+ . These preliminary results are the first in our long-term Galactic Archaeology Simulation program.
Publisher: Oxford University Press (OUP)
Date: 10-2005
Publisher: Oxford University Press (OUP)
Date: 10-2004
Publisher: Oxford University Press (OUP)
Date: 21-01-2010
Publisher: Oxford University Press (OUP)
Date: 24-06-2011
Publisher: American Astronomical Society
Date: 10-03-2003
DOI: 10.1086/374306
Publisher: Oxford University Press (OUP)
Date: 09-2004
Publisher: American Astronomical Society
Date: 14-06-2005
DOI: 10.1086/432157
Publisher: American Astronomical Society
Date: 10-09-2006
DOI: 10.1086/506247
Publisher: Oxford University Press (OUP)
Date: 21-11-2003
Publisher: EDP Sciences
Date: 11-04-2005
Publisher: Oxford University Press (OUP)
Date: 11-03-2023
Abstract: We employ our Bayesian Machine Learning framework BINGO (Bayesian INference for Galactic archaeOlogy) to obtain high-quality stellar age estimates for 68,360 red giant and red clump stars present in the 17th data release of the Sloan Digital Sky Survey, the APOGEE-2 high-resolution spectroscopic survey. By examining the denoised age-metallicity relationship of the Galactic disc stars, we identify a drop in metallicity with an increase in [Mg/Fe] at an early epoch, followed by a chemical enrichment episode with increasing [Fe/H] and decreasing [Mg/Fe]. This result is congruent with the chemical evolution induced by an early-epoch gas-rich merger identified in the Milky Way-like zoom-in cosmological simulation Auriga. In the initial phase of the merger of Auriga 18 there is a drop in metallicity due to the merger diluting the metal content and an increase in the [Mg/Fe] of the primary galaxy. Our findings suggest that the last massive merger of our Galaxy, the Gaia-Sausage-Enceladus, was likely a significant gas-rich merger and induced a starburst, contributing to the chemical enrichment and building of the metal-rich part of the thick disc at an early epoch.
Publisher: EDP Sciences
Date: 27-03-2012
Publisher: Oxford University Press (OUP)
Date: 20-05-2011
Publisher: Oxford University Press (OUP)
Date: 30-10-2012
DOI: 10.1093/MNRAS/STS161
Publisher: EDP Sciences
Date: 2021
DOI: 10.1051/0004-6361/202038307
Abstract: Ensemble studies of red-giant stars with exquisite asteroseismic ( Kepler ), spectroscopic (APOGEE), and astrometric ( Gaia ) constraints offer a novel opportunity to recast and address long-standing questions concerning the evolution of stars and of the Galaxy. Here, we infer masses and ages for nearly 5400 giants with available Kepler light curves and APOGEE spectra using the code PARAM , and discuss some of the systematics that may affect the accuracy of the inferred stellar properties. We then present patterns in mass, evolutionary state, age, chemical abundance, and orbital parameters that we deem robust against the systematic uncertainties explored. First, we look at age-chemical-abundances ([Fe/H] and [ α /Fe]) relations. We find a dearth of young, metal-rich ([Fe/H] 0.2) stars, and the existence of a significant population of old (8−9 Gyr), low-[ α /Fe], super-solar metallicity stars, reminiscent of the age and metallicity of the well-studied open cluster NGC 6791. The age-chemo-kinematic properties of these stars indicate that efficient radial migration happens in the thin disc. We find that ages and masses of the nearly 400 α -element-rich red-giant-branch (RGB) stars in our s le are compatible with those of an old (∼11 Gyr), nearly coeval, chemical-thick disc population. Using a statistical model, we show that the width of the observed age distribution is dominated by the random uncertainties on age, and that the spread of the inferred intrinsic age distribution is such that 95% of the population was born within ∼1.5 Gyr. Moreover, we find a difference in the vertical velocity dispersion between low- and high-[α/Fe] populations. This discontinuity, together with the chemical one in the [α/Fe] versus [Fe/H] diagram, and with the inferred age distributions, not only confirms the different chemo-dynamical histories of the chemical-thick and thin discs, but it is also suggestive of a halt in the star formation (quenching) after the formation of the chemical-thick disc. We then exploit the almost coeval α -rich population to gain insight into processes that may have altered the mass of a star along its evolution, which are key to improving the mapping of the current, observed, stellar mass to the initial mass and thus to the age. Comparing the mass distribution of stars on the lower RGB ( R 11 R ⊙ ) with those in the red clump (RC), we find evidence for a mean integrated RGB mass loss ⟨Δ M ⟩ = 0.10 ± 0.02 M ⊙ . Finally, we find that the occurrence of massive ( M ≳ 1.1 M ⊙ ) α -rich stars is of the order of 5% on the RGB, and significantly higher in the RC, supporting the scenario in which most of these stars had undergone an interaction with a companion.
Publisher: Oxford University Press (OUP)
Date: 20-09-2012
Publisher: Oxford University Press (OUP)
Date: 13-06-2011
Publisher: Cambridge University Press (CUP)
Date: 2003
DOI: 10.1071/AS03028
Abstract: We test the hypothesis that high-velocity gas cloud Complex C is actually a high-latitude spiral arm extension in the direction of the Galactic warp, as opposed to the standard interpretation — that of a once extragalactic, but now infalling, gas cloud. A parallel Tree N-body code was employed to simulate the tidal interaction of a satellite perturber with the Milky Way. We find that a model incorporating a perturber of the mass of the Large Magellanic Cloud on a south to north polar orbit, crossing the disk at ˜15 kpc, does yield a high-velocity, high-latitude extension consistent with the spatial, kinematical, and column density properties of Complex C. Unless this massive satellite remains undiscovered because of either a fortuitous alignment with the Galactic bulge (feasible within the framework of the model), or the lack of any associated baryonic component, we conclude that this alternative interpretation appears unlikely.
Publisher: Oxford University Press (OUP)
Date: 22-05-2012
Publisher: Oxford University Press (OUP)
Date: 03-2010
Publisher: American Astronomical Society
Date: 20-04-2006
DOI: 10.1086/500633
Publisher: Springer Science and Business Media LLC
Date: 2003
Publisher: Oxford University Press (OUP)
Date: 21-03-2004
Publisher: Cambridge University Press (CUP)
Date: 12-2010
DOI: 10.1017/S1743921311022885
Abstract: We performed numerical simulations of mergers between gas-rich disc galaxies, which result in the formation of late-type galaxies. Stars formed during the merger end up in a thick disc that is partially supported by velocity dispersion and has high [α/Fe] ratios at all metallicities. Stars formed later end up in a thin, rotationally supported disc which has lower [α/Fe] ratios. While the structural and kinematical properties of the merger remnants depend strongly upon the orbital parameters of the mergers, we find a clear chemical signature of gas-rich mergers.
Publisher: American Astronomical Society
Date: 20-03-2007
DOI: 10.1086/511056
Publisher: American Astronomical Society
Date: 10-12-2006
DOI: 10.1086/508487
Publisher: Oxford University Press (OUP)
Date: 11-12-2009
Publisher: American Astronomical Society
Date: 09-2005
DOI: 10.1086/431924
Publisher: Cambridge University Press (CUP)
Date: 2004
DOI: 10.1071/AS04004
Abstract: Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked up in white dwarf stellar remnants. We first construct a 10 9 M ⊙ stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low- and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (10 12 M ⊙ ) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift ( z ≈ 3) would have resulted in halos more than 100 times more luminous than those expected from conventional initial mass functions alone, and (2) any putative halo white dwarf progenitor dominated initial mass function component, regardless of its dynamical importance, would be virtually impossible to detect at the present day, due to its extremely faint surface brightness.
Publisher: Cambridge University Press (CUP)
Date: 2004
DOI: 10.1071/AS04045
Publisher: EDP Sciences
Date: 06-05-2008
Publisher: Oxford University Press (OUP)
Date: 16-09-2016
Publisher: American Astronomical Society
Date: 10-09-2004
DOI: 10.1086/422709
Publisher: Oxford University Press (OUP)
Date: 11-04-2003
Publisher: EDP Sciences
Date: 2007
DOI: 10.1051/EAS:2007020
Publisher: Oxford University Press (OUP)
Date: 02-2005
Publisher: American Astronomical Society
Date: 03-2006
DOI: 10.1086/499154
Publisher: Oxford University Press (OUP)
Date: 15-04-2020
Abstract: The Milky Way underwent its last significant merger ten billion years ago, when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and progenitor stars born prior to the merger make up the bulk of the inner halo. Even though these two main populations of halo stars have similar durations of star formation prior to their merger, they differ in [α/Fe]-[Fe/H] space, with the GES population bending to lower [α/Fe] at a relatively low value of [Fe/H]. We use cosmological simulations of a ‘Milky Way’ to argue that the different tracks of the halo stars through the [α/Fe]-[Fe/H] plane are due to a difference in their star formation history and efficiency, with the lower mass GES having its low and constant star formation regulated by feedback whilst the higher mass main progenitor has a higher star formation rate prior to the merger. The lower star formation efficiency of GES leads to lower gas pollution levels, pushing [α/Fe]-[Fe/H] tracks to the left. In addition, the increasing star formation rate maintains a higher relative contribution of Type II SNe to Type Ia SNe for the main progenitor population that formed during the same time period, thus maintaining a relatively high [α/Fe]. Thus the different positions of the downturns in the [α/Fe]-[Fe/H] plane for the GES stars are not reflective of different star formation durations, but instead reflect different star formation efficiencies.
Publisher: Springer Science and Business Media LLC
Date: 2002
Publisher: American Astronomical Society
Date: 20-05-2007
DOI: 10.1086/511514
Publisher: Oxford University Press (OUP)
Date: 08-03-2021
Abstract: We develop a Bayesian Machine Learning framework called BINGO (Bayesian INference for Galactic archaeOlogy) centred around a Bayesian neural network. After being trained on the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Kepler asteroseismic age data, BINGO is used to obtain precise relative stellar age estimates with uncertainties for the APOGEE stars. We carefully construct a training set to minimize bias and apply BINGO to a stellar population that is similar to our training set. We then select the 17 305 stars with ages from BINGO and reliable kinematic properties obtained from Gaia DR2. By combining the age and chemo-kinematical information, we dissect the Galactic disc stars into three components, namely the thick disc (old, high-[α/Fe], [α/Fe] ≳ 0.12), the thin disc (young, low-[α/Fe]), and the Bridge, which is a region between the thick and thin discs. Our results indicate that the thick disc formed at an early epoch only in the inner region, and the inner disc smoothly transforms to the thin disc. We found that the outer disc follows a different chemical evolution pathway from the inner disc. The outer metal-poor stars only start forming after the compact thick disc phase has completed and the star-forming gas disc extended outwardly with metal-poor gas accretion. We found that in the Bridge region the range of [Fe/H] becomes wider with decreasing age, which suggests that the Bridge region corresponds to the transition phase from the smaller chemically well-mixed thick to a larger thin disc with a metallicity gradient.
Publisher: Oxford University Press (OUP)
Date: 19-12-2013
Publisher: Cambridge University Press (CUP)
Date: 2004
DOI: 10.1071/AS04014
Abstract: Hierarchical clustering represents the favoured paradigm for galaxy formation throughout the Universe due to its proximity, the Magellanic system offers one of the few opportunities for astrophysicists to decompose the full six-dimensional phase-space history of a satellite in the midst of being cannibalised by its host galaxy. The availability of improved observational data for the Magellanic Stream and parallel advances in computational power has led us to revisit the canonical tidal model describing the disruption of the Small Magellanic Cloud and the consequent formation of the Stream. We suggest improvements to the tidal model in light of these recent advances.
Publisher: Oxford University Press (OUP)
Date: 11-08-2003
Publisher: Cambridge University Press (CUP)
Date: 07-2017
DOI: 10.1017/S1743921317007499
Abstract: We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a s le of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. We also find that the intrinsic dispersion in metallicity increases steadily with age. Our results are consistent with a scenario that thin disk stars formed from a flaring thin star-forming disk.
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 05-2013
End Date: 12-2016
Amount: $390,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 03-2007
End Date: 12-2011
Amount: $740,000.00
Funder: Australian Research Council
View Funded Activity