arXiv subjects

Gary A. Mamon

Publications and source records attributed to Gary A. Mamon.

At least 91 records · Page 5Linked to original sources

arXiv2005

Velocity moments of dark matter haloes

Using cosmological N-body simulations we study the line-of-sight velocity distribution of dark matter haloes focusing on the lowest-order even moments, dispersion and kurtosis, and their application to estimate the mass profiles of cosmological structures. For each of the ten massive haloes selected from the simulation box we determine the virial mass, concentration and the anisotropy parameter. In order to emulate observations from each halo we choose randomly 300 particles and project their velocities and positions along the line of sight and on the surface of the sky, respectively. After removing interlopers we calculate the profiles of the line-of-sight velocity moments and fit them with the solutions of the Jeans equations. The estimates of virial mass, concentration parameter and velocity anisotropy obtained in this way are in good agreement with the values found from the full 3D analysis.

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arXiv2005

Dark matter distribution in the Draco dwarf from velocity moments

We study the distribution of dark matter in the Draco dwarf spheroidal galaxy by modelling the moments of the line-of-sight velocity distribution of stars obtained from new velocity data of Wilkinson et al. The luminosity distribution is approximated by a Sersic profile fitted to the data by Odenkirchen et al. We assume that the dark matter density profile is given by a formula with an inner cusp and an outer exponential cut-off, as recently proposed by Kazantzidis et al. as a result of simulations of tidal stripping of dwarfs by the potential of the Milky Way. The dark matter distribution is characterized by the total dark mass and the cut-off radius. The models have arbitrary velocity anisotropy parameter assumed to be constant with radius. We estimate the three parameters by fitting both the line-of-sight velocity dispersion and kurtosis profiles, which allows us to break the degeneracy between the mass distribution and velocity anisotropy. The results of the fitting procedure turn out to be very different depending on the stellar sample considered, that is on our choice of stars with discrepant velocities to be discarded as interlopers. For our most reliable sample, the model parameters remain weakly constrained, but the robust result is the preference for weakly tangential stellar orbits and high mass-to-light ratios. The best-fitting total mass is then 7 10^7 M_sun, much lower than recent estimates, while the mass-to-light ratio is M/L_V = 300 and almost constant with radius. If the binary fraction in the stellar population of Draco turns out to be significant, the kurtosis of the global velocity distribution will be smaller and the orbits inferred will be more tangential, while the resulting mass estimate lower.

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arXiv2005

Radial velocity moments of dark matter haloes

Using cosmological N-body simulations we study the radial velocity distribution in dark matter haloes focusing on the lowest-order even moments, dispersion and kurtosis. We determine the properties of ten massive haloes in the simulation box approximating their density distribution by the NFW formula characterized by the virial mass and concentration. We also calculate the velocity anisotropy parameter of the haloes and find it mildly radial and increasing with distance from the halo centre. The radial velocity dispersion of the haloes shows a characteristic profile with a maximum, while the radial kurtosis profile decreases with distance starting from a value close to Gaussian near the centre. We therefore confirm that dark matter haloes possess intrinsically non-Gaussian, flat-topped velocity distributions. We find that the radial velocity moments of the simulated haloes are quite well reproduced by the solutions of the Jeans equations obtained for the halo parameters with the anisotropy measured in the simulations. We also study the radial velocity moments for a composite cluster made of ten haloes out to ten virial radii. In this region the velocity dispersion decreases systematically to reach the value of the background, while kurtosis increases from below to above the Gaussian value of 3 signifying a transition from a flat-topped to a strongly peaked velocity distribution with respect to the Gaussian, which can be interpreted as the dominance of ordered flow with a small dispersion. We illustrate the transition by showing explicitly the velocity distribution of the composite cluster in a few radial bins.

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arXiv2004

DENIS detections of highly obscured galaxies in the area around PKS1343-601

We present results of a search for galaxies around the highly obscured giant radio galaxy PKS1343-601 using the near-infrared DENIS survey. We compare our findings with surveys in the B-band, at the 21cm line emission, and with 2MASS. Recession velocities of galaxies in this area suggest a low-velocity-dispersion group or cluster of galaxies including an X-ray confirmed Seyfert 2. The colours of the galaxies have been used to examine the extinctions in this low-latitude area where IRAS/DIRBE estimates are unreliable. We find the true extinction to be roughly 15% lower than the IRAS/DIRBE extinctions.

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arXiv2004

Breaking the mass / anisotropy degeneracy in the Coma cluster

We provide the first direct lifting of the mass/anisotropy degeneracy for a cluster of galaxies, by jointly fitting the line of sight velocity dispersion and kurtosis profiles of the Coma cluster, assuming an NFW tracer density profile, a generalized-NFW dark matter profile and a constant anisotropy profile. We find that the orbits in Coma must be quasi-isotropic, and find a mass consistent with previous analyses, but a concentration parameter 50% higher than expected in cosmological N-body simulations. We then test the accuracy of our method on realistic non-spherical systems with substructure and streaming motions, by applying it to the ten most massive structures in a cosmological N-body simulation. We find that our method yields fairly accurate results on average (within 20%), although with a wide variation (factor 1.7 at 1 sigma) for the concentration parameter, with decreased accuracy and efficiency when the projected mean velocity is not constant with radius.

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arXiv2004

The 6dF Galaxy Survey: Samples, Observational Techniques and the First Data Release

The 6dF Galaxy Survey (6dFGS) aims to measure the redshifts of around 150,000 galaxies, and the peculiar velocities of a 15,000-member sub-sample, over almost the entire southern sky. When complete, it will be the largest redshift survey of the nearby universe, reaching out to about z ~ 0.15, and more than an order of magnitude larger than any peculiar velocity survey to date. The targets are all galaxies brighter than K_tot = 12.75 in the 2MASS Extended Source Catalog (XSC), supplemented by 2MASS and SuperCOSMOS galaxies that complete the sample to limits of (H, J, r_F, b_J) = (13.05, 13.75, 15.6, 16.75). Central to the survey is the Six-Degree Field (6dF) multi-fibre spectrograph, an instrument able to record 150 simultaneous spectra over the 5.7-degree field of the UK Schmidt Telescope. An adaptive tiling algorithm has been employed to ensure around 95% fibering completeness over the 17046 sq.deg of the southern sky with | b | > 10 deg. Spectra are obtained in two observations using separate V and R gratings, that together give R ~ 1000 over at least 4000 -- 7500 Angstroms and signal-to-noise ratio ~10 per pixel. The 6dFGS database is available at http://www-wfau.roe.ac.uk/6dFGS/, with public data releases occuring after the completion of each third of the survey.

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arXiv2003

How reliable is the kinematical evidence for dark matter: the effects of non-sphericity, substructure and streaming motions

Using cosmological $N$-body simulations of dark matter haloes, we study the effects of non-sphericity, substructure and streaming motions in reproducing the structure and internal kinematics of clusters of galaxies from kinematical measurements. Fitting an NFW model to the 3D density profile, we determine the virial mass, concentration parameter and velocity anisotropy of the haloes, and then calculate the profiles of projected velocity moments, as they would be measured by a distant observer. Using these mock data, we apply a Jeans analysis for spherical objects to reproduce the line-of-sight velocity dispersion and kurtosis and fit the three parameters. We find that the line-of-sight velocity dispersion and kurtosis profiles of a given halo can vary considerably with the angle of view of the observer. We show that the virial mass, concentration parameter and velocity anisotropy of the haloes can be reproduced satisfactorily independently of the halo shape, although the virial mass tends to be underestimated, the concentration parameter overestimated, while the recovered anisotropy is typically more tangential than the true one. The mass, concentration and velocity anisotropy of haloes are recovered with better precision when their mean velocity profiles are near zero.

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arXiv2003

The Selection of Tenured Astronomers in France

The organization of the recruitment of tenured astronomers and astrophysicists in France is presented and compared with the setup in other countries. The ages for getting tenure have increased from 27-28 in 1980 to 31 today. Foreign scientists constitute at least 11% of the recruits and the delay in their hiring is quantified. The large reliance on national tenure committees is justified, while the increased targeting of positions is questioned and a compromise proposed.

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arXiv2003

Dark matter distribution in the Coma cluster from galaxy kinematics: breaking the mass-anisotropy degeneracy

We study velocity moments of elliptical galaxies in the Coma cluster using Jeans equations. The dark matter distribution in the cluster is modelled by a generalised formula based upon the results of cosmological N-body simulations. Its inner slope (cuspy or flat), concentration, and mass within the virial radius are kept as free parameters, as well as the velocity anisotropy, assumed independent of position. We show that the study of line-of-sight velocity dispersion alone does not allow to constrain the parameters. By a joint analysis of the observed profiles of velocity dispersion and kurtosis we are able to break the degeneracy between the mass distribution and velocity anisotropy. We determine the dark matter distribution at radial distances larger than 3% of the virial radius and we find that the galaxy orbits are close to isotropic. Due to limited resolution, different inner slopes are found to be consistent with the data and we observe a strong degeneracy between the inner slope $α$ and concentration $c$: the best-fitting profiles have the two parameters related with $c=19 - 9.6 α$. Our best-fitting NFW profile has concentration $c=9$, which is 50% higher than standard values found in cosmological simulations for objects of similar mass. The total mass within the virial radius of $2.9 h_{70}^{-1}$ Mpc is $1.4 \times 10^{15} h_{70}^{-1} M_{\sun}$ (with 30% accuracy), 85% of which is dark. At this distance from the cluster centre, the mass-to-light ratio in the blue band is $351 h_{70}$ solar units. The total mass within the virial radius leads to estimates of the density parameter of the Universe, assuming that clusters trace the mass-to-light ratio and baryonic fraction of the Universe, with $Ω_0=0.29 \pm 0.1$.

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arXiv2002

Constraints on dark energy and quintessence with a comoving standard ruler applied to 2dF quasars

Structures on very large scales (> 100 Mpc) have negligible peculiar motions, and are thus roughly fixed in comoving space. We looked for significant peaks at very large separation in the two-point correlation function -- corrected for redshift selection effects -- of a well convered subsample of 2378 quasars of the recently released 10k sample of the 2dF quasar survey. Dividing our sample in three redshift intervals, we find a peak at 244/h Mpc, which is perfectly comoving for a restricted set of cosmological parameters, namely Omega_m = 0.25+/-0.15 and Omega_Lambda=0.65+/-0.35 (both at 95% confidence). Assuming a flat Universe, we constrain the quintessence parameter w_Q < -0.35 (95% confidence). We discuss the compatibility of our analysis with possible peaks in the power spectrum.

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arXiv2001

FLAIR-II spectroscopy of two DENIS J-band galaxy samples

As a pilot survey for the forthcoming 6dF Galaxy Redshift Survey, spectroscopy of galaxies selected in J (1.2 mu) with the DENIS survey was performed at the UKST using the FLAIR II multi-object spectroscope. 69 galaxy redshifts (z's) were obtained in a high |b| field and an additional 12 z's in a low latitude (b=-17 deg), obscured field. This study illustrates the feasibility of obtaining z's with optical spectra on galaxies selected at much longer wavelengths. It validated a very preliminary algorithm for star/galaxy separation for high |b| DENIS objects, with 99% reliability for J < 13.9. However, the FLAIR II z determinations required substantially longer integration times to achieve 90% completeness than expected from previous optical surveys at comparable depth. This is mainly due to a degradation in overall fibre throughput due to known problems with ageing of the prism-cement-fibre interface with exposure to UV light. Our low |b| (high extinction) field required 2.5 times more exposure time for less than 50% of successful z measurements. Among the J <= 13.9 galaxies with measured z's, only 37+/-6% display emission lines, in comparison with 60% of emission-line galaxies (ELGs) in optical samples of comparable depth. The ELGs are, on average, 0.5 mag bluer in B-J than non-ELGs of the same luminosity. We confirm a previous optically-based result that the fraction of ELGs increases rapidly with decreasing galaxy luminosity. The J-band luminosity function is estimated. Our high latitude field displays a concentration of galaxies at cz ~ 38000 km/s suggesting a possible supercluster. A radial velocity is reported for a galaxy lying near the projected centre of the Abell 1434 cluster of galaxies, for which no cluster z is currently available.

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arXiv2000

Lifting cosmic degeneracy within a single quasar survey

In the almost Friedmann-Lema^itre model of the Universe, the density parameter, Omega_matter, and the cosmological constant, Omega_Lambda, measure curvature. Several linearly degenerate relations between these two parameters have recently been measured. Here, large scale structure correlations at \~100-150 h^-1 Mpc are found in the comoving three-dimensional separations of redshift z \approx 2 quasars. These function as a comoving standard rod of length L_LSS \approx 130+-10 h^-1 Mpc. A local maximum in the correlation function at \approx L_LSS/2 also appears to be significant. By combining separate radial and tangential standard ruler analyses, the lifting of the Omega_matter - Omega_Lambda linear degeneracy within a single data set is demonstrated for the first time.

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arXiv2000

Properties of spherical galaxies and clusters with an NFW density profile

Using the standard dynamical theory of spherical systems, we calculate the properties of spherical galaxies and clusters whose density profiles obey the universal form first obtained in high resolution cosmological N-body simulations by Navarro, Frenk & White. We adopt three models for the internal kinematics: isotropic velocities, constant anisotropy and increasingly radial Osipkov-Merritt anisotropy. Analytical solutions are found for the radial dependence of the mass, gravitational potential, velocity dispersion, energy and virial ratio and we test their variability with the concentration parameter describing the density profile and amount of velocity anisotropy. We also compute structural parameters, such as half-mass radius, effective radius and various measures of concentration. Finally, we derive projected quantities, the surface mass density and line-of-sight as well as aperture velocity dispersion, all of which can be directly applied in observational tests of current scenarios of structure formation. On the mass scales of galaxies, if constant mass-to-light is assumed, the NFW surface density profile is found to fit well Hubble-Reynolds laws. It is also well fitted by Sersic R^(1/m) laws, for m ~ 3, but in a much narrower range of m and with much larger effective radii than are observed. Assuming in turn reasonable values of the effective radius, the mass density profiles imply a mass-to-light ratio that increases outwards at all radii.

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arXiv2000

Tangential Large Scale Structure as a Standard Ruler: Curvature Parameters from Quasars

Several observational analyses suggest that matter is spatially structured at $\approx 130h^{-1}Mpc$ at low redshifts. This peak in the power spectrum provides a standard ruler in comoving space which can be used to compare the local geometry at high and low redshifts, thereby constraining the curvature parameters. It is shown here that this power spectrum peak is present in the observed quasar distribution at $z\sim 2$: qualitatively, via wedge diagrams which clearly show a void-like structure, and quantitatively, via one-dimensional Fourier analysis of the quasars' tangential distribution. The sample studied here contains 812 quasars. The method produces strong constraints (68% confidence limits) on the density parameter $Ω_0$ and weaker constraints on the cosmological constant $λ_0$, which can be expressed by the relation $ Ω_0 = (0.24 \pm0.15) + (0.10\pm0.08) λ_0 $. Independently of $λ_0$ (in the range $λ_0 \in [0,1]$), the constraint is $0.1 < Ω_0 < 0.45$. Combination of the present results with SN Type Ia results yields $Ω_0 = (0.30\pm0.11) + (0.57\pm0.11) (λ_0-0.7),$ $0.55 < λ_0 < 0.95,$ (68% confidence limits). This strongly supports the possibility that the observable universe satisfies a nearly flat, perturbed Friedmann-Lema\^ıtre-Robertson-Walker model, independently of any cosmic microwave background observations.

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arXiv1999

Theory of galaxy dynamics in clusters and groups

Analytical estimates of the mass and radial dependence of the rates of galaxy mergers and of tidal interactions are derived for clusters and groups of galaxies, taking into account the tides from the system potential that limit the sizes of galaxies. Only high mass galaxies undergo significant major merging before being themselves cannibalized by more massive galaxies. Strong tides from the group/cluster potential severely limit the merger/tide cross-sections in the central regions, and while tides are most efficient at the periphery, one should see merging encounters further inside rich clusters.

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arXiv1999

Merging History Trees for Dark Matter Halos: Tests of the Merging Cell Model in a CDM Cosmology

The merging history of dark matter halos is computed with the Merging Cell Model proposed by Rodrigues & Thomas (1996). While originally discussed in the case of scale-free power spectra, it is developed and tested here in the framework of the cold dark matter cosmology. The halo mass function, the mass distribution of progenitors and child halos, as well as the probability distribution of formation times, have been computed and compared to the available analytic predictions. The halo auto-correlation function has also been obtained (a first for a semi-analytic merging tree), and tested against analytic formulae. An overall good agreement is found between results of the model, and the predictions derived from the Press & Schechter theory and its extensions. More severe discrepancies appear when formulae that better describe N-body simulations are used for comparison. In many instances, the model can be a useful tool for following the hierarchical growth of structures. In particular, it is suitable for addressing the issue of the formation and evolution of galaxy clusters, as well as the population of Lyman-break galaxies at high redshift, and their clustering properties.

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arXiv1999

Dark Matter Halos Merging Trees: the Merging Cell Model in a CDM Cosmology

We have constructed the merging history of dark matter halos in a SCDM cosmology, by means of a the {``Merging Cell Model''} (Rodrigues & Thomas 1996). It is based on the linear theory of growth of density fluctuations, and the Top-Hat model, and it takes into account mergers between halos through simplified criteria. The halo mass function, the progenitors and children mass distribution, the distribution of formation times, and the halo autocorrelation function (in the Lagrangian space of initial conditions) have been computed. To test the reliability of the model, results have been compared to the analytical predictions of the extended Press & Schechter theory, that in turn well describes results of N-body simulations. A general good agreement has been found, thus providing us with a reliable tool for rapidly addressing the issue of galaxy formation and evolution in hierarchical scenarios. The model is particularly suitable for studying galaxy clusters at low redshift, and the population of Lyman-break galaxies at high z.

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arXiv1998

The Wide-Field DENIS Near-IR Imaging Survey and 6dF Redshift and Peculiar Velocity Surveys

The DENIS survey is currently imaging the entire southern sky in the I, J, and K wavebands. The current star/galaxy separation algorithm is presented and the galaxy counts are nearly perfectly Euclidean. 95% complete and reliable galaxy samples with better than 0.2 magnitude photometry should include 50,000 (K < 12.0), 500,000 (J < 14.8), and 900,000 (I < 16.5) galaxies, respectively, over the full hemisphere. Two spectroscopic followups of DENIS and 2MASS galaxies are planned on the 6dF robotic multi-object spectroscopic unit, currently under construction at the AAO, and which will be mounted on the UKST Schmidt telescope: a redshift survey of roughly 120,000 NIR selected galaxies and a peculiar velocity survey of roughly 15,000 galaxies (both early-types and inclined spirals) at cz < 10,000 km/s.

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