arXiv subjects

J. Magorrian

Publications and source records attributed to J. Magorrian.

15 recordsLinked to original sources

arXiv2017

Large-scale three-dimensional Gaussian process extinction mapping

Gaussian processes are the ideal tool for modelling the Galactic ISM, combining statistical flexibility with a good match to the underlying physics. In an earlier paper we outlined how they can be employed to construct three-dimensional maps of dust extinction from stellar surveys. Gaussian processes scale poorly to large datasets though, which put the analysis of realistic catalogues out of reach. Here we show how a novel combination of the Expectation Propagation method and certain sparse matrix approximations can be used to accelerate the dust mapping problem. We demonstrate, using simulated Gaia data, that the resultant algorithm is fast, accurate and precise. Critically, it can be scaled up to map the Gaia catalogue.

astro-ph.IM↗

arXiv2015

Marginal likelihoods of distances and extinctions to stars: computation and compact representation

We present a method for obtaining the likelihood function of distance and extinction to a star given its photometry. The other properties of the star (its mass, age, metallicity and so on) are marginalised assuming a simple Galaxy model. We demonstrate that the resulting marginalised likelihood function can be described faithfully and compactly using a Gaussian mixture model. For dust mapping applications we strongly advocate using monochromatic over bandpass extinctions, and provide tables for converting from the former to the latter for different stellar types.

astro-ph.GA↗

arXiv2014

Three-dimensional extinction mapping using Gaussian random fields

We present a scheme for using stellar catalogues to map the three-dimensional distributions of extinction and dust within our Galaxy. Extinction is modelled as a Gaussian random field, whose covariance function is set by a simple physical model of the ISM that assumes a Kolmogorov-like power spectrum of turbulent fluctuations. As extinction is modelled as a random field, the spatial resolution of the resulting maps is set naturally by the data available; there is no need to impose any spatial binning. We verify the validity of our scheme by testing it on simulated extinction fields and show that its precision is significantly improved over previous dust-mapping efforts. The approach we describe here can make use of any photometric, spectroscopic or astrometric data; it is not limited to any particular survey. Consequently, it can be applied to a wide range of data from both existing and future surveys.

astro-ph.IM↗

arXiv2011

Dynamical masses of early-type galaxies: a comparison to lensing results and implications for the stellar IMF and the distribution of dark matter

This work aims to study the distribution of luminous and dark matter in Coma early-type galaxies. Dynamical masses obtained under the assumption that mass follows light do not match with the masses of strong gravitational lens systems of similar velocity dispersions. Instead, dynamical fits with dark matter halos are in good agreement with lensing results. We derive mass-to-light ratios of the stellar populations from Lick absorption line indices, reproducing well the observed galaxy colours. Even in dynamical models with dark matter halos the amount of mass that follows the light increases more rapidly with galaxy velocity dispersion than expected for a constant stellar initial mass function (IMF). While galaxies around sigma ~ 200 km/s are consistent with a Kroupa IMF, the same IMF underpredicts luminous dynamical masses of galaxies with sigma ~ 300 km/s by a factor of two and more. A systematic variation of the stellar IMF with galaxy velocity dispersion could explain this trend with a Salpeter IMF for the most massive galaxies. If the IMF is instead constant, then some of the dark matter in high velocity dispersion galaxies must follow a spatial distribution very similar to that of the light. A combination of both, a varying IMF and a component of dark matter that follows the light is possible as well. For a subsample of galaxies with old stellar populations we show that the tilt in the fundamental plane can be explained by systematic variations of the total (stellar + dark) mass inside the effective radius. We tested commonly used mass estimator formulae, finding them accurate at the 20-30% level.

astro-ph.CO↗

arXiv2008

The flattening and the orbital structure of early-type galaxies and collisionless N-body binary disk mergers

We use oblate axisymmetric dynamical models including dark halos to determine the orbital structure of intermediate mass to massive Coma early-type galaxies. We find a large variety of orbital compositions. Averaged over all sample galaxies the unordered stellar kinetic energy in the azimuthal and the radial direction are of the same order, but they can differ by up to 40 percent in individual systems. In contrast, both for rotating and non-rotating galaxies the vertical kinetic energy is on average smaller than in the other two directions. This implies that even most of the rotating ellipticals are flattened by an anisotropy in the stellar velocity dispersions. Using three-integral axisymmetric toy models we show that flattening by stellar anisotropy maximises the entropy for a given density distribution. Collisionless disk merger remnants are radially anisotropic. The apparent lack of strong radial anisotropy in observed early-type galaxies implies that they may not have formed from mergers of disks unless the influence of dissipational processes was significant.

astro-ph↗

arXiv2008

Dark matter scaling relations and the assembly epoch of Coma early-type galaxies

Axisymmetric, orbit-based dynamical models are used to derive dark matter scaling relations for Coma early-type galaxies. From faint to bright galaxies halo core-radii and asymptotic circular velocities increase. Compared to spirals of the same brightness, the majority of Coma early-types -- those with old stellar populations -- have similar halo core-radii but more than 2 times larger asymptotic halo velocities. The average dark matter density inside 2 reff decreases with increasing luminosity and is 6.8 times larger than in disk galaxies of the same B-band luminosity. Compared at the same stellar mass, dark matter densities in ellipticals are 13.5 times higher than in spirals. Different baryon concentrations in ellipticals and spirals cannot explain the higher dark matter density in ellipticals. Instead, the assembly redshift (1+z) of Coma early-type halos is likely about two times larger than of comparably bright spirals. Assuming that local spirals typically assemble at a redshift of one, the majority of bright Coma early-type galaxy halos must have formed around z = 2-3. For about half of our Coma galaxies the assembly redshifts match with constraints derived from stellar populations. We find dark matter densities and estimated assembly redshifts of our observed Coma galaxies in reasonable agreement with recent semi-analytic galaxy formation models.

astro-ph↗

arXiv2008

Structure and dynamics of galaxies with a low surface brightness disc. I. The stellar and ionised-gas kinematics

(abridged) Photometry and long-slit spectroscopy are presented for a sample of 6 galaxies with a low surface brightness stellar disc and a bulge. The stellar and ionised-gas kinematics were measured along the major and minor axis in half of the sample galaxies, whereas the other half was observed only along two diagonal axes. Spectra along two diagonal axes were obtained also for one of the objects with major and minor axis spectra. The kinematic measurements extend in the disc region out to a surface-brightness level mu_R~24mag/arcsec^2 reaching in all cases the flat part of the rotation curve. The stellar kinematics turns out to be more regular and symmetric than the ionised-gas kinematics, which often shows the presence of non-circular, off-plane, and non-ordered motions. This raises the question about the reliability of the use of the ionised gas as the tracer of the circular velocity in the modeling of the mass distribution, in particular in the central regions of low surface brightness galaxies.

astro-ph↗

arXiv2007

Dynamical modelling of luminous and dark matter in 17 Coma early-type galaxies

Dynamical models for 17 Coma early-type galaxies are presented. The galaxy sample consists of flattened, rotating as well as non-rotating early-types including cD and S0 galaxies with luminosities between M=-18.79 and M=-22.56. Kinematical long-slit observations cover at least the major and minor axis and extend to 1-4 effective radii. Axisymmetric Schwarzschild models are used to derive stellar mass-to-light ratios and dark halo parameters. In every galaxy models with a dark matter halo match the data better than models without. The statistical significance is over 95 percent for 8 galaxies, around 90 percent for 5 galaxies and for four galaxies it is not significant. For the highly significant cases systematic deviations between observed and modelled kinematics are clearly seen; for the remaining galaxies differences are more statistical in nature. Best-fit models contain 10-50 percent dark matter inside the half-light radius. The central dark matter density is at least one order of magnitude lower than the luminous mass density. The central phase-space density of dark matter is often orders of magnitude lower than in the luminous component, especially when the halo core radius is large. The orbital system of the stars along the major-axis is slightly dominated by radial motions. Some galaxies show tangential anisotropy along the minor-axis, which is correlated with the minor-axis Gauss-Hermite coefficient H4. Changing the balance between data-fit and regularisation constraints does not change the reconstructed mass structure significantly. Model anisotropies tend to strengthen if the weight on regularisation is reduced, but the general property of a galaxy to be radially or tangentially anisotropic, respectively, does not change. (abridged)

astro-ph↗

arXiv2005

The central kinematics of NGC 1399 measured with 14pc resolution

We present near infra-red (NIR) adaptive optics assisted spectroscopic observations of the CO ($Δμ=2$) absorption bands towards the centre of the giant elliptical galaxy NGC 1399. The observations were made with NAOS-CONICA (ESO VLT) and have a FWHM resolution of 0\farcs15 (14pc). Kinematic analysis of the observations reveals a decoupled core and strongly non-Gaussian line-of-sight velocity profiles (VPs) in the central 0.2 arcsec (19pc). NIR imaging also indicates an asymmetric elongation of the central isophotes in the same region. We use spherical orbit-superposition models to interpret the kinematics, using a set of orthogonal ``eigenVPs'' that allow us to fit models directly to spectra. The models require a central black hole of mass $1.2^{+0.5}_{-0.6}\times10^9M_\odot$, with a strongly tangentially biased orbit distribution in the inner 40pc.

astro-ph↗

arXiv2005

Regularized orbit models unveiling the stellar structure and dark matter halo of the Coma elliptical NGC 4807

This is the second in a series of papers dedicated to unveil the mass structure and orbital content of a sample of flattened early-type galaxies in the Coma cluster. The ability of our orbit libraries to reconstruct internal stellar motions and the mass composition of a typical elliptical in the sample is investigated by means of Monte-Carlo simulations of isotropic rotator models. The simulations allow a determination of the optimal amount of regularization needed in the orbit superpositions. It is shown that under realistic observational conditions and with the appropriate regularization internal velocity moments can be reconstructed to an accuracy of about 15 per cent; the same accuracy can be achieved for the circular velocity and dark matter fraction. In contrast, the flattening of the halo remains unconstrained. Regularized orbit superpositions are applied to a first galaxy in our sample, NGC 4807, for which stellar kinematical observations extend to 3 Reff. The galaxy seems dark matter dominated outside 2 Reff. Logarithmic dark matter potentials are consistent with the data, as well as NFW-profiles, mimicking logarithmic potentials over the observationally sampled radial range. In both cases, the derived stellar mass-to-light ratio agrees well with independently obtained mass-to-light ratios from stellar population analysis. Kinematically, NGC 4807 is characterized by mild radial anisotropy outside r>0.5 Reff, becoming isotropic towards the center. Our orbit models hint at either a distinct stellar component or weak triaxiality in the outer parts of the galaxy.

astro-ph↗

arXiv2004

Mapping stationary axisymmetric phase-space distribution functions by orbit libraries

This is the first of a series of papers dedicated to unveil the mass composition and dynamical structure of a sample of flattened early type galaxies in the Coma cluster. We describe our modifications to the Schwarzschild code of Richstone et al. (in preparation). Applying a Voronoi tessellation in the surface of section we are able to assign accurate phase-space volumes to individual orbits and to reconstruct the full three-integral phase-space distribution function (DF) of any axisymmetric orbit library. Two types of tests have been performed to check the accuracy with which DFs can be represented by appropriate orbit libraries. First, by mapping DFs of spherical gamma-models and flattened Plummer models on to the library we show that the resulting line-of-sight velocity distributions and internal velocity moments of the library match those directly derived from the DF to a precision better than that of present day observational errors. Second, by fitting libraries to the projected kinematics of the same DFs we show that the distribution function reconstructed from the fitted library matches the input DF to a rms of about 15 per cent over a region in phase-space covering 90 per cent of the mass of the library. The achieved accuracy allows us to implement effective entropy-based regularisation to fit real, noisy and spatially incomplete data.

astro-ph↗

arXiv2003

Central DM density cuspiness in LSB's: a stellar kinematics approach

We present preliminary results from spectroscopic observations of a sample of 11 low surface brightness galaxies (LSB). We measured the stellar and gaseous kinematics along their major and minor axes. Such information will allow us to accurately investigate the dark matter (DM) content within their optical regions, providing further constraints on the predictions of standard CDM models. Although dynamical modeling is still in progress, our observations already show that the derived stellar kinematics is more regular than the ionized gas one, which often shows evidence for non-circular and asymmetric motions.

astro-ph↗

arXiv2000

Evidence of a Supermassive Black Hole in the Galaxy NGC 1023 from the Nuclear Stellar Dynamics

We analyze the nuclear stellar dynamics of the SB0 galaxy NGC 1023, utilizing observational data both from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and from the ground. The stellar kinematics measured from these long-slit spectra show rapid rotation (V = 70 km/s at a distance of 0.1 arcsec = 4.9 pc from the nucleus) and increasing velocity dispersion toward the nucleus (where sigma = 295 +/- 30 km/s). We model the observed stellar kinematics assuming an axisymmetric mass distribution with both two and three integrals of motion. Both modeling techniques point to the presence of a central dark compact mass (which presumably is a supermassive black hole) with confidence > 99%. The isotropic two-integral models yield a best-fitting black hole mass of (6.0 +/- 1.4) x 10^7 M_sun and mass-to-light ratio (M/L_V) of 5.38 +/- 0.08, and the goodness-of-fit (chi^2) is insensitive to reasonable values for the galaxy's inclination. The three-integral models, which non-parametrically fit the observed line-of-sight velocity distribution as a function of position in the galaxy, suggest a black hole mass of (3.9 +/- 0.4) x 10^7 M_sun and M/L_V of 5.56 +/- 0.02 (internal errors), and the edge-on models are vastly superior fits over models at other inclinations. The internal dynamics in NGC 1023 as suggested by our best-fit three-integral model shows that the velocity distribution function at the nucleus is tangentially anisotropic, suggesting the presence of a nuclear stellar disk. The nuclear line of sight velocity distribution has enhanced wings at velocities >= 600 km/s from systemic, suggesting that perhaps we have detected a group of stars very close to the central dark mass.

astro-ph↗

arXiv2000

The velocity and mass distribution of clusters of galaxies from the CNOC1 cluster redshift survey

In the context of the CNOC1 cluster survey, redshifts were obtained for galaxies in 16 clusters. The resulting sample is ideally suited for an analysis of the internal velocity and mass distribution of clusters. Previous analyses of this dataset used the Jeans equation to model the projected velocity dispersion profile. However, the results of such an analysis always yield a strong degeneracy between the mass density profile and the velocity dispersion anisotropy profile. Here we analyze the full (R,v) dataset of galaxy positions and velocities in an attempt to break this degeneracy. We build an `ensemble cluster' from the individual clusters under the assumption that they form a homologous sequence. To interpret the data we study a one-parameter family of spherical models with different constant velocity dispersion anisotropy. The best-fit model is sought using a variety of statistics, including the overall likelihood of the dataset. Although the results of our analysis depend slightly on which statistic is used to judge the models, all statistics agree that the best-fit model is close to isotropic. This result derives primarily from the fact that the observed grand-total velocity histogram is close to Gaussian, which is not expected to be the case for a strongly anisotropic model. The best-fitting models have a mass-to-number-density ratio that is approximately independent of radius over the range constrained by the data. They also have a mass-density profile that is consistent with the dark matter halo profile advocated by Navarro, Frenk & White, in terms of both the profile shape and the characteristic scale length. This adds important new weight to the evidence that clusters do indeed follow this proposed universal mass density profile. [Abridged]

astro-ph↗

arXiv1998

Supermassive Black Holes and the Evolution of Galaxies

Black holes, an extreme consequence of the mathematics of General Relativity, have long been suspected of being the prime movers of quasars, which emit more energy than any other objects in the Universe. Recent evidence indicates that supermassive black holes, which are probably quasar remnants, reside at the centers of most galaxies. As our knowledge of the demographics of these relics of a violent earlier Universe improve, we see tantalizing clues that they participated intimately in the formation of galaxies and have strongly influenced their present-day structure.

astro-ph↗