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The Origin of Angular Momentum in Dark Matter Halos
We propose a new explanation for the origin of angular momentum ingalaxies and their dark halos, in which the halos obtain their spinthrough the cumulative acquisition of angular momentum from satelliteaccretion. In our model, the buildup of angular momentum is a randomwalk process associated with the mass assembly history of the halo'smajor progenitor. We assume no correlation between the angular momentaof accreted objects. The main role of tidal torques in this approach isto produce the random tangential velocities of merging satellites. Usingthe extended Press-Schechter approximation, we calculate the growth ofmass, angular momentum, and spin parameter λ for many halos. Ourrandom walk model reproduces the key features of the angular momentum ofhalos found in ΛCDM N-body simulations: a lognormal distributionin λ with an average of <λ>~0.045 and dispersionσλ=0.56, independent of mass and redshift. Theevolution of the spin parameter in individual halos in this model isquite different from the steady increase with time of angular momentumin the tidal torque picture. We find both in N-body simulations and inour random walk model that the value of λ changes significantlywith time for a halo's major progenitor. It typically has a sharpincrease due to major mergers and a steady decline during periods ofgradual accretion of small satellites. The model predicts that, onaverage, the λ of ~1012 Msolar halos thathad major mergers after redshift z=3 should be substantially larger thanthe λ of those that did not. Perhaps surprisingly, this suggeststhat halos that host later forming elliptical galaxies should rotatefaster than halos of spiral galaxies.
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| The morphological catalogue of galaxies equatorial survey
We present 865 redshifts of galaxies located in the equatorial stripdelta between -17.5 deg and -2.5 deg in the right ascension rangebetween 20 h and 5 h. Redshifts have been obtained for the completesample of all 833 galaxies in the Morphological Catalog of Galaxies withmagnitudes brighter than m = 14.5 (corresponding approximately tom(Zwicky) = 15.0). This sample also includes three galaxies from othersources with more reliable magnitudes, satisfying this limit, and 29fainter galaxies, usually companions of the galaxies in the magnitudelimited sample. Our maps of a very large volume of nearby spacedemonstrate a variety of coherent large scale structures which includelarge voids, 20-50/h Mpc in diameter and large walls at least 70/h Mpcacross.
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