Christopher Barber | Leiden University (original) (raw)

Papers by Christopher Barber

We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a ... more We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a dark matter halo on the shape and orientation of its substructure. Although tides are often assumed to enhance asphericity and to stretch subhaloes tangentially, these effects are short lived: as in earlier work, we find that subhaloes affected by tides become substantially more spherical and show a strong radial alignment toward the centre of the host halo. These results, combined with a semi-analytic model of galaxy formation, may be used to assess the effect of Galactic tides on the observed population of dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda galaxies. If, as the model suggests, the relatively low dark matter content of luminous dSphs such as Fornax and Leo I is due to tidal stripping, then their gravitational potential must be substantially more spherical than that of more heavily dark matter-dominated systems such as Draco or Carina. The model also predicts a tidally-induced statistical excess of satellites whose major axis aligns with the direction to the central galaxy. We find tantalizing evidence of this in the M31 satellite population, which suggests that tides may have played an important role in its evolution.

We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate... more We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the host; they are thus excluded from subhalo samples identified within the virial boundary, reducing the number of subhaloes on radial orbits. Two other factors contributing to the difference in orbital shape between dark and luminous subhaloes are their dynamical evolution after infall, which affects more markedly low-mass (dark) subhaloes, and a weak dependence of ellipticity on the redshift of first infall. The ellipticity distribution of luminous satellites exhibits little halo-to-halo scatter, and it may therefore be compared fruitfully with that of Milky Way satellites. Since the latter depends sensitively on the total mass of the Milky Way we can use the predicted distribution of satellite ellipticities to place constraints on this important parameter. Using the latest estimates of position and velocity of dwarfs compiled from the literature, we find that the most likely Milky Way mass lies in the range 6 × 10^11 M⊙ < M_200 < 3.1 × 10^12 M⊙, with a best-fitting value of M_200 = 1.1 × 10^12 M⊙. This value is consistent with Milky Way mass estimates based on dynamical tracers or the timing argument.

Optimized population synthesis provides an empirical method to extract the relative mix of stella... more Optimized population synthesis provides an empirical method to extract the relative mix of stellar evolutionary stages and the distribution of atmospheric parameters within unresolved stellar systems, yet a robust validation of this method is still lacking. We here provide a calibration of population synthesis via non-linear bound-constrained optimization of stellar populations based upon optical spectra of mock stellar systems and observed Galactic globular clusters (GGCs). The MILES stellar library is used as a basis for mock spectra as well as templates for the synthesis of deep GGC spectra from Schiavon et al. Optimized population synthesis applied to mock spectra recovers mean light-weighted stellar atmospheric parameters within a mean uncertainty of 240 K, 0.04 dex, and 0.03 dex for Teff, log g, and [Fe/H], respectively. We use additional information from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) deep colour-magnitude diagrams (CMDs) from Sarajedini et al. and literature metallicities to validate our optimization results on GGCs. Decompositions of both mock and GGC spectra confirm the method's ability to recover the expected mean light-weighted metallicity in dust-free conditions (E(B - V) ≲ 0.15) with uncertainties comparable to evolutionary population synthesis methods. Dustier conditions require either appropriate dust modelling when fitting to the full spectrum, or fitting only to select spectral features. We derive light-weighted fractions of stellar evolutionary stages from our population synthesis fits to GGCs, yielding on average a combined 25 ± 6 per cent from main-sequence and turn-off dwarfs, 64 ± 7 per cent from subgiant, red giant, and asymptotic giant branch stars, and 15 ± 7 per cent from horizontal branch stars and blue stragglers. Excellent agreement is found between these fractions and those estimated from deep HST/ACS CMDs. Overall, optimized population synthesis remains a powerful tool for understanding the stellar populations within the integrated light of galaxies and globular clusters.

We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a ... more We use cosmological N-body simulations from the Aquarius Project to study the tidal effects of a dark matter halo on the shape and orientation of its substructure. Although tides are often assumed to enhance asphericity and to stretch subhaloes tangentially, these effects are short lived: as in earlier work, we find that subhaloes affected by tides become substantially more spherical and show a strong radial alignment toward the centre of the host halo. These results, combined with a semi-analytic model of galaxy formation, may be used to assess the effect of Galactic tides on the observed population of dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda galaxies. If, as the model suggests, the relatively low dark matter content of luminous dSphs such as Fornax and Leo I is due to tidal stripping, then their gravitational potential must be substantially more spherical than that of more heavily dark matter-dominated systems such as Draco or Carina. The model also predicts a tidally-induced statistical excess of satellites whose major axis aligns with the direction to the central galaxy. We find tantalizing evidence of this in the M31 satellite population, which suggests that tides may have played an important role in its evolution.

We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate... more We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the host; they are thus excluded from subhalo samples identified within the virial boundary, reducing the number of subhaloes on radial orbits. Two other factors contributing to the difference in orbital shape between dark and luminous subhaloes are their dynamical evolution after infall, which affects more markedly low-mass (dark) subhaloes, and a weak dependence of ellipticity on the redshift of first infall. The ellipticity distribution of luminous satellites exhibits little halo-to-halo scatter, and it may therefore be compared fruitfully with that of Milky Way satellites. Since the latter depends sensitively on the total mass of the Milky Way we can use the predicted distribution of satellite ellipticities to place constraints on this important parameter. Using the latest estimates of position and velocity of dwarfs compiled from the literature, we find that the most likely Milky Way mass lies in the range 6 × 10^11 M⊙ < M_200 < 3.1 × 10^12 M⊙, with a best-fitting value of M_200 = 1.1 × 10^12 M⊙. This value is consistent with Milky Way mass estimates based on dynamical tracers or the timing argument.

Optimized population synthesis provides an empirical method to extract the relative mix of stella... more Optimized population synthesis provides an empirical method to extract the relative mix of stellar evolutionary stages and the distribution of atmospheric parameters within unresolved stellar systems, yet a robust validation of this method is still lacking. We here provide a calibration of population synthesis via non-linear bound-constrained optimization of stellar populations based upon optical spectra of mock stellar systems and observed Galactic globular clusters (GGCs). The MILES stellar library is used as a basis for mock spectra as well as templates for the synthesis of deep GGC spectra from Schiavon et al. Optimized population synthesis applied to mock spectra recovers mean light-weighted stellar atmospheric parameters within a mean uncertainty of 240 K, 0.04 dex, and 0.03 dex for Teff, log g, and [Fe/H], respectively. We use additional information from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) deep colour-magnitude diagrams (CMDs) from Sarajedini et al. and literature metallicities to validate our optimization results on GGCs. Decompositions of both mock and GGC spectra confirm the method's ability to recover the expected mean light-weighted metallicity in dust-free conditions (E(B - V) ≲ 0.15) with uncertainties comparable to evolutionary population synthesis methods. Dustier conditions require either appropriate dust modelling when fitting to the full spectrum, or fitting only to select spectral features. We derive light-weighted fractions of stellar evolutionary stages from our population synthesis fits to GGCs, yielding on average a combined 25 ± 6 per cent from main-sequence and turn-off dwarfs, 64 ± 7 per cent from subgiant, red giant, and asymptotic giant branch stars, and 15 ± 7 per cent from horizontal branch stars and blue stragglers. Excellent agreement is found between these fractions and those estimated from deep HST/ACS CMDs. Overall, optimized population synthesis remains a powerful tool for understanding the stellar populations within the integrated light of galaxies and globular clusters.