Mass-to-light ratio gradients in early-type galaxy haloes

Owing to the fact that the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterizing the halo masses from both observational and theoretical results:  ∇_ℓϒ  , the logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from Λ-cold dark matter (CDM) simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies  ε_SF  . As a pilot study, we assemble the available  ∇_ℓϒ  data from kinematics in early-type galaxies – representing the first unbiased study of halo masses in a wide range of early-type galaxy luminosities – and find a correlation between luminosity and  ∇_ℓϒ  , such that the brightest galaxies appear the most dark-matter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the ΛCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency  ε_SF > 1  . This difficulty is eased if dark haloes are not assumed to have the standard ΛCDM profiles, but lower central concentrations.     

X-ray emission from the galaxies in Abell 2634

It is difficult to detect X-ray emission associated with galaxies in rich clusters, because the X-ray images of the clusters are dominated by the emission from their hot intracluster media (ICM). Only the nearby Virgo cluster provides us with information about the X-ray properties of galaxies in clusters. Here we report on the analysis of a deep ROSAT HRI image of the moderately rich cluster Abell 2634, by which we have been able to detect the X-ray emission from the galaxies in the cluster. The ICM of Abell 2634 is an order of magnitude denser than that of the Virgo cluster, and so this analysis allows us to explore the X-ray properties of individual galaxies in the richest environment yet explored.
By stacking the X-ray images of the galaxies together, we show that the emission from the galaxies appears to be marginally resolved by the HRI. This extent is smaller than for galaxies in poorer environments, and is comparable to the size of the galaxies in optical light. These facts suggest that the detected X-ray emission originates from the stellar populations of the galaxies, rather than from extended hot interstellar media. Support for this hypothesis comes from placing the optical and X-ray luminosities of these galaxies in the L_B–L_X plane: the galaxies of Abell 2634 lie in the region of this plane where models indicate that all the X-ray emission can be explained by the usual population of X-ray binaries. It is therefore probable that ram pressure stripping has removed the hot gas component from these galaxies.     

A combined experimental study of vivianite and As (V) reactivity in the pH range 2–11

Four different sets of experiments were completed in order to constrain vivianite [Fe₃(PO₄)₂ · 8H₂O] reactivity under conditions pertinent to As(V)-bearing groundwater systems. Firstly, titration experiments were undertaken in the pH range 4-9 to determine the zero point of charge (ZPC) of vivianite; showing that the ZPC lies at a pH of approximately 5.3. Secondly, the steady state dissolution rates of vivianite far from equilibrium were measured in aqueous solutions in the pH range 2–10 at 18.5 ^°C (±3 °C) using a fluidized bed reactor. The rate of vivianite dissolution, R, is given by     

Exploring disc galaxy dynamics using integral field unit data

In order to test the basic equations believed to dictate the dynamics of disc galaxies, we present and analyse deep two-dimensional spectral data obtained using the PPAK integral field unit for the early-type spiral systems NGC 2273, NGC 2985, NGC 3898 and NGC 5533. We describe the care needed to obtain and process such data to a point where reliable kinematic measurements can be obtained from these observations, and a new more optimal method for deriving the rotational motion and velocity dispersions in such disc systems. The data from NGC 2273 and NGC 2985 show systematic variations in velocity dispersion with azimuth, as one would expect if the shapes of their velocity ellipsoids are significantly anisotropic, while the hotter discs in NGC 3898 and NGC 5533 appear to have fairly isotropic velocity dispersions. Correcting the rotational motion for asymmetric drift using the derived velocity dispersions reproduces the rotation curves inferred from emission lines reasonably well, implying that this correction is quite robust, and that the use of the asymmetric drift equation is valid. NGC 2985 is sufficiently close to face on for the data, combined with the asymmetric drift equation, to determine all three components of the velocity ellipsoid. The principal axes of this velocity ellipsoid are found to be in the ratio  σ _z : σ_φ : σ _R ≈ 0.7 : 0.7 : 1  , which shows unequivocally that this disc distribution function respects a third integral of motion. The ratio is also consistent with the predictions of epicyclic theory, giving some confidence in the application of this approximation to even fairly early-type disc galaxies.