Dark Matter Halos Of Galaxies Research Articles

Galaxy clustering at z 2 and halo radii

The amplitude of the angular two-point galaxy autocorrelation function w(θ) for galaxies at z ̃ 2 is estimated for galaxies in the Hubble Deep Field by using a U<27 complete subsample. The U-band selection ensures little contamination from z>2.5 galaxies, while photometric redshifts minimize the contribution from low redshift galaxies. It is confirmed that the amplitude of the correlation can be corrected for the integral constraint (lack of large-scale variance) without having to make assumptions about the shape of the correlation function and by avoiding the introduction of linear error terms. The estimate using this technique is w(θ≈5 arcsec)=0.10±0.09. Estimators that assume a power law of a given slope and include linear error terms would double this value. If the biases introduced in faint galaxy selection owing to obscuration by large objects are not corrected for by masking areas around them, then the estimate would be w(θ≈5 arcsec)=0.16±0.07. The effective (three-dimensional) galaxy pair separation at 5 arcsec and this redshift range is ≈25–250 h−1 kpc, so the correction to the spatial correlation function ξ(r) owing to exclusion of overlapping galaxy dark matter haloes should be considered. For stable clustering in proper units in an Ω=1,λ=0 universe, our w(5 arcsec) estimate (a) implies a present-day correlation length of r0̃2.6+1.1−1.7h−1 Mpc if halo overlapping is ignored, but (b) for a present-day correlation length of r0=5.5 h−1 Mpc implies that a typical halo exclusion radius is rhalo=70+420−30h−1 kpc. For Ω0=0.1,λ0=0.9, the corresponding values are (a) r0̃5.8+2.4−3.9h−1 Mpc and (b) rhalo<210 h−1 kpc (1σ upper limit). The decreasing correlation period (DCP) of a high initial bias in the spatial correlation function is not detected at this redshift. For an Ω=1,λ=0 universe and stable clustering in proper units, possible detections of the DCP in other work would imply that the values of ξ at redshifts greater than zt=1.7±0.9 would be [(1+z)/(1+zt)]2.1±3.6 times their values at zt, which is consistent with our lack of detection at z̃2.

Survival of substructure within dark matter haloes

Using high resolution cosmological N-body simulations, we investigate the survival of dark matter satellites falling into larger haloes. Satellites preserve their identity for some time after merging. We compute their loss of mass, energy and angular momentum as dynamical friction, tidal forces and collisions with other satellites dissolve them. We also analyse the evolution of their internal structure. Satellites with less than a few per cent the mass of the main halo may survive for several billion years, whereas larger satellites rapidly sink into the center of the main halo potential well and lose their identity. Penetrating encounters between satellites are frequent and may lead to significant mass loss and disruption. Only a minor fraction of cluster mass (10 per cent on average) is bound to substructure at most redshifts of interest. We discuss the application of these results to the survival and extent of dark matter haloes associated with cluster galaxies, and to interactions between galaxies in clusters. We find that 35-40 per cent of galaxy dark matter haloes are disrupted by the present time. The fraction of satellites undergoing close encounters is similar to the fraction of interacting or merging galaxies in clusters at moderate redshift.

Searching For Dark Matter With Gravitational Microlensing: A Report From The MACHO Collaboration

Gravitational microlensing is the most straightforward interpretation of the stellar brightenings that have been observed by our team and other experiments. These data have provided some of the most stringent limits to date on the nature of the Galaxy's dark matter halo. The number of events seen towards the LMC indicate that our Galaxy is not surrounded by a “standard” halo of MACHOs in the mass range of 10–6 to 0.3 solar masses. The observed optical depth towards the Galactic Center is an important constraint on the distribution of mass in the plane of the Galaxy.

Can MACHOs probe the shape of the galaxy halo ?

We study the prospects for the current microlensing searches, which have recently discovered several candidates, to yield useful information about the flattening of the Galaxy dark matter halo. Models of HI warps and N-body simulations of galaxy formation suggest that disks commonly form in oblate halos with a tilt between the disk and halo symmetry axes. The microlensing optical depth for the Large Magellanic Cloud depends sensitively on the disk-halo tilt angle in the Milky Way. We conclude that a much larger spread in values for $\tau(LMC)$ is consistent with rotation curve constraints than previously thought, and that the ratio $\tau(SMC)/\tau(LMC)$ of the optical depth to the Small and Large Magellanic clouds is not a clean test of halo flattening.