NOVA Kapteyn Astronomical Institute

Groningen, Netherlands

NOVA Kapteyn Astronomical Institute

Groningen, Netherlands
SEARCH FILTERS
Time filter
Source Type

Sanders R.H.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society | Year: 2014

show that the lensing masses of the Sloan Lens Advanced Camera Surveys sample of strong gravitational lenses are consistent with the stellar masses determined from population synthesis models using the Salpeter initial mass function. This is true in the context of both General Relativity and modified Newtonian dynamics (MOND), and is in agreement with the expectation of MOND that there should be little classical discrepancy within the high surface brightness regions probed by strong gravitational lensing. There is also dynamical evidence from this sample supporting the claim that the mass-to-light ratio of the stellar component increases with the velocity dispersion. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Sanderson R.E.,NOVA Kapteyn Astronomical Institute | Helmi A.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society | Year: 2013

The class of tidal features around galaxies known as 'shells' or 'umbrellas' comprises debris that has arisen from high-mass-ratio mergers with low-impact parameter; the nearly radial orbits of the debris give rise to a unique morphology, a universal density profile and a tight correlation between positions and velocities of the material. As such they are accessible to analytical treatment, and can provide a relatively clean system for probing the gravitational potential of the host galaxy. In this work, we presenta simple analytical model that describes the density profile, phase-space distribution, and geometry of a shell and whose parameters are directly related to physical characteristics of the interacting galaxies. The model makes three assumptions: the orbit of the interacting galaxies is radial, the potential of the host galaxy at the shell radius is spherical and the satellite galaxy's initial velocity distribution is Maxwellian. We quantify the error introduced by the first two assumptions and show that selecting shellsby their appearance on the sky is a sufficient basis to assume that these simplifications are valid. We further demonstrate that (1) given only an image of a shell, the radial gravitational force at the shell edge and the phase-space density of the satellite are jointly constrained, (2) combining the image with measurements of either point line-of-sight velocities or integratedlight spectra will yield an independent estimate of the gravitational force at a shell and (3) an independent measurement of this force is obtained for each shell observed around a given galaxy, potentially enabling a determination of the galactic mass distribution. © 2013 The Authors, Published by Oxford University Press on behalf of the Royal Astronomical Society.


Sanders R.H.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

I argue that, despite repeated claims of Ibata et al., the globular cluster NGC 2419 does not pose a problem for modified Newtonian dynamics (MOND). I present a new polytropic model with a running polytropic index. This model provides an improved representation of the radial distribution of surface brightness while maintaining a reasonable fit to the velocity dispersion profile. Although it may be argued that the differences with these observations remain large compared to the reported random errors, there are several undetectable systematic effects which render a formal likelihood analysis irrelevant. I comment generally upon these effects and upon the intrinsic limitations of pressure-supported objects as tests of gravity. © 2012 The Author. Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Bekenstein J.D.,Hebrew University of Jerusalem | Sanders R.H.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

Wojtak, Hansen & Hjorth have recently claimed to confirm general relativity (GR) and to rule out the tensor-vector-scalar (TeVeS) gravitational theory based on an analysis of the gravitational redshifts of galaxies in 7800 clusters. However, their ubiquitous modelling of the sources of cluster gravitational fields with Navarro-Frenk-White mass profiles is neither empirically justified out to the necessary radii in clusters, nor germane in the case of TeVeS. Using MONDian (whereMONDstands for MOdified Newtonian Dynamics) isothermal sphere models consistently constructed within MOND (equivalent to TeVeS models), we can fit the determined redshifts no worse than does GR with dark haloes. Moreover, Wojtak, Hansen & Hjorth have inappropriately used the simple MOND interpolating function for the μ function of the scalar field of TeVeS; the consequent MOND effective interpolating function turns out to enhance the gravitational potential, and so contributes to the discrepancy which forms the basis of their claims. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Salvadori S.,NOVA Kapteyn Astronomical Institute | Ferrara A.,Normal School of Pisa
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

In order to characterize damped Lyα aborption systems (DLAs) potentially hosting first stars, we present a novel approach to investigate DLAs in the context of Milky Way (MW) formation, along with their connection with the most metal-poor stars and local dwarf galaxies. The merger tree method previously developed is extended to include inhomogeneous reionization and metal mixing, and it is validated by matching both the metallicity distribution function of Galactic halo stars and the Fe-luminosity relation of dwarf spheroidal galaxies. The model explains the observed N HI-Fe relation of DLAs along with the chemical abundances of [Fe/H] <-2 systems. In this picture, the recently discovered z abs ≈ 2.34 C-enhanced DLA pertains to a new class of absorbers hosting first stars along with second-generation long-living lowmass stars. These 'Population III DLAs' are the descendants of H 2-cooling minihaloes with M h ≈ 10 7M ⊙, which virialize at z > 8 in neutral, primordial regions of the MW environment and passively evolve after a short initial period of star formation. The gas in these systems is warm T g ≈ (40-1000) K, and strongly C-enriched by long-living, extremely metal-poor stars of total mass M * ≈ 10 2.-10 4M ⊙. © 2011 The Authors Monthly Notices of the Royal Astronomical Society. © 2011 RAS.


Sanders R.H.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

I show that, in the context of modified Newtonian dynamics (MOND), non-isothermal models, approximated by high-order polytropic spheres, are consistent with the observations of the radial distribution of the line-of-sight velocity dispersion in the distant globular cluster, NGC 2419. This calls into question the claim by Ibata et al. that the object constitutes a severe challenge for MOND. In general, the existence and properties of globular clusters are more problematic for LCDM than for MOND. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Cazaux S.,NOVA Kapteyn Astronomical Institute | Caselli P.,University of Leeds | Spaans M.,NOVA Kapteyn Astronomical Institute
Astrophysical Journal Letters | Year: 2011

Observations of star-forming environments revealed that the abundances of some deuterated interstellar molecules are markedly larger than the cosmic D/H ratio of 10-5. Possible reasons for this pointed to grain surface chemistry. However, organic molecules and water, which are both ice constituents, do not enjoy the same deuteration. For example, deuterated formaldehyde is very abundant in comets and star-forming regions, while deuterated water rarely is. In this paper, we explain this selective deuteration by following the formation of ices (using the rate equation method) in translucent clouds, as well as their evolution as the cloud collapses to form a star. Ices start with the deposition of gas-phase CO and O onto dust grains. While reaction of oxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reacts with atoms (H and D) to form H 2CO (HDCO, D2CO). As a result, the deuteration of formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes gravitational collapse, while the deuteration of water strongly depends on the dust temperature at the time of ice formation. These results reproduce well the deuterium fractionation of formaldehyde observed in comets and star-forming regions and can explain the wide spread of deuterium fractionation of water observed in these environments. © 2011. The American Astronomical Society. All rights reserved.


Sanders R.H.,NOVA Kapteyn Astronomical Institute
Monthly Notices of the Royal Astronomical Society | Year: 2010

In the context of modified Newtonian dynamics, the Fundamental Plane, as the observational signature of the Newtonian virial theorem, is defined by high-surface-brightness objects that deviate from being purely isothermal: the line-of-sight velocity dispersion should slowly decline with radius as observed in luminous elliptical galaxies. All high-surface-brightness objects (e.g. globular clusters, ultra-compact dwarfs) will lie, more or less, on the Fundamental Plane defined by elliptical galaxies, but low-surface-brightness objects (dwarf spheroidals) would be expected to deviate from this relation. This is borne out by observations. With Milgrom's modified Newtonian dynamics (MOND), the Faber-Jackson relation (L ∝ σ4), ranging from globular clusters to clusters of galaxies and including both high- and low-surface-brightness objects, is the more fundamental and universal scaling relation in spite of its larger scatter. The Faber-Jackson relation reflects the presence of an additional dimensional constant (the MOND acceleration a0) in the structure equation. © 2010 The Author. Journal compilation © 2010 RAS.


Sanders R.H.,NOVA Kapteyn Astronomical Institute
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

Hořava-Lifshitz gravity is an attempt to construct a renormalizable theory of gravity by breaking the Lorentz invariance of the gravitational action at high energies. The underlying principle is that Lorentz invariance is an approximate symmetry and its violation by gravitational phenomena is somehow hidden to present limits of observational precision. Here I point out that a simple modification of the low-energy limit of Hořava-Lifshitz gravity in its nonprojectable form can effectively camouflage the presence of a preferred frame in regions where the Newtonian gravitational field gradient is higher than cH0; this modification results in the phenomenology of modified Newtonian dynamics (MOND) at lower accelerations. As a relativistic theory of MOND, this modified Hořava-Lifshitz theory presents several advantages over its predecessors. © 2011 American Physical Society.


Battaglia G.,National institute for astrophysics | Helmi A.,NOVA Kapteyn Astronomical Institute | Breddels M.,NOVA Kapteyn Astronomical Institute
New Astronomy Reviews | Year: 2013

We review our current understanding of the internal dynamical properties of the dwarf spheroidal galaxies surrounding the Milky Way. These are the most dark matter dominated galaxies, and as such may be considered ideal laboratories to test the current concordance cosmological model, and in particular provide constraints on the nature of the dominant form of dark matter. We discuss the latest observations of the kinematics of stars in these systems, and how these may be used to derive their mass distribution. We tour through the various dynamical techniques used, with emphasis on the complementarity and limitations, and discuss what the results imply also in the context of cosmological models. Finally we provide an outlook on exciting developments in this field. © 2013 Elsevier B.V.

Loading NOVA Kapteyn Astronomical Institute collaborators
Loading NOVA Kapteyn Astronomical Institute collaborators