Lori M. Lubin

Where Is the Dark Matter

How much dark matter is there in the universe and where is it located? These are two of the most fundamental questions in cosmology. We use in this paper optical and X-ray mass determinations of galaxies, groups, and clusters of galaxies to suggest that most of the dark matter may reside in very large halos around galaxies, typically extending to ~200 kpc for bright galaxies. We show that the mass-to-light ratio of galaxy systems does not increase significantly with linear scale beyond the very large halos suggested for individual galaxies. Rather, the total mass of large-scale systems such as groups and rich clusters of galaxies, even superclusters, can on average be accounted for by the total mass of their member galaxies, including their large halos (which may be stripped off in the dense cluster environment but still remain in the clusters) plus the mass of the hot intracluster gas. This conclusion also suggests that we may live in a low-density universe with Ω ~ 0.2-0.3.

The Baryon fraction and velocity temperature relation in galaxy clusters: Models versus observations

The observed baryon fraction and velocity--temperature relation in clusters of galaxies are compared with hydrodynamic simulations in two cosmological models : standard (Omega = 1) and a low-density flat (Omega=0.45 and lambda=0.55) CDM models, normalized to the COBE background fluctuations. The observed properties of clusters include the velocity dispersion versus temperature relation, the gas mass versus total mass relation, and the gas mass fraction versus velocity dispersion relation. We find that, while both cosmological models reproduce well the shape of these observed functions, only low-density CDM can reproduce the observed amplitudes. The cluster gas mass fraction reflects approximately the baryon fraction in the models, with a slight anti-bias. Therefore, due to the low baryon density given by nucleosynthesis, Omega = 1 models produce too few baryons in clusters compared with observations. Scaling our results as a function of Omega, we find that a low-density CDM model, with Omega approximately 0.3 - 0.4, best reproduces the observed mean baryon fraction in clusters. The observed beta parameter of clusters, beta = sigma^2/(k T/mu m_p) = 0.94 pm 0.08 discriminates less well between the models; it is consistent with that produced by low-density CDM (1.10 pm 0.22), while it is slightly larger than expected but still consistent with Omega = 1 (0.70 pm 0.14).

Is a data set distributed as a power law? A sensitive test

We present a method to help decide whether an observed sample of data is distributed as a power law. We study one simple member of a class of test statistics as an example, named the ‘bending statistic’ B. Its distribution depends only on the size of the sample, not on the parameters of the parent population, and is approximated well by a normal distribution even for modest sample sizes. The bending statistic can therefore be used quite conveniently to test whether a set of numbers was drawn from any power law parent population, which is a question that often arises in astrophysics. We apply this test to various subsamples of gamma‐ray burst brightnesses from the first‐year BATSE catalogue, and detect only a hint of the expected steepening of the log N(≳Cmax)⋅−log Cmax distribution.

Gas mass and total mass in clusters of galaxies

We examine the contributions of gas and galaxy mass to the total mass of clusters of galaxies as an indicator of the origin of the dark matter component in clusters and the mass density of the universe. First, we compare the observed X‐ray and optical relations in clusters to the results of large‐scale hydrodynamic simulations and show that low‐density CDM is more consistent with the observed gas mass fractions in clusters. Secondly, we examine the make‐up of the total cluster mass and suggest that there is no excess of dark matter in these systems; that is, their mass can be accounted for by the mass of the member galaxies (including their dark halos) and the mass of the baryonic intracluster gas. This would imply that the fractional contribution of the dark matter component does not keep increasing with scale beyond the very large halos suggested for individual galaxies and that the mass density of the universe is of the order of the mass density observed on the scale of clusters, i.e., Ω∼0.2.

Gamma ray busts redshifts: Zero or one? in search of a bend in log N‐log S

We have compared the observed distribution of peak count rates, Cmax, and Cmax/Cmin values of gamma‐ray bursts with simulated distributions. We find that some observed samples for which the data were available to us show significant deviations from simple power‐law distributions. We also find that presently available samples of gamma‐ray bursts are too small to distinguish significantly between galactic and extra‐galactic models for their origin on the basis of the log N‐log S distribution. The BATSE sample will have to increase to 1000–2000 bursts before this becomes feasible.