The standard ÃCDM model predicts that the major merger rate of galaxy-size dark matter halos rises rapidly with redshift. The average number of close companions per galaxy, N c , is often used to infer the galaxy merger rate; however, recent observational studies suggest that N c evolves very little with redshift. Here we use a ''hybrid'' N-body simulation plus analytic substructure model to predict N c directly. We identify dark matter subhalos with galaxies and show that the observed lack of close-pair count evolution arises because the high merger rate per halo at early times is counteracted by a decrease in the number of halos massive enough to host a galaxy pair. We compare our results to data compiled from the DEEP2, SSRS2, and UZC redshift surveys. Observed pair counts match our predictions if we assume a monotonic mapping between galaxy luminosity and the maximum circular velocity that each subhalo had when it was first accreted onto its host halo. This suggests that satellite galaxies are significantly more resilient to mass loss than are dissipationless dark matter subhalos. We argue that while N c does not provide a direct measure of the halo merger rate, it offers a powerful means to constrain both the halo occupation distribution and the spatial distribution of galaxies within halos. Interpreted in this way, close-pair counts provide a useful test of galaxy formation processes on $10Y100 kpc scales.

A logarithmic spiral is a prominent feature appearing in a majority of observed galaxies. This feature has long been associated with the traditional Hubble classification scheme, but historical quotes of pitch angle of spiral galaxies have been almost exclusively qualitative. We have developed a methodology, utilizing two-dimensional fast Fourier transformations of images of spiral galaxies, in order to isolate and measure the pitch angles of their spiral arms. Our technique provides a quantitative way to measure this morphological feature. This will allow comparison of spiral galaxy pitch angle to other galactic parameters and test spiral arm genesis theories. In this work, we detail our image processing and analysis of spiral galaxy images and discuss the robustness of our analysis techniques.

We study the formation of fifty-three galaxy cluster-size dark matter halos (M = 10 14.0−14.76 M ⊙ ) formed within a pair of cosmological ΛCDM N-body simulations, and track the accretion histories of cluster subhalos with masses large enough to host ∼ 0.1L * galaxies. By associating subhalos with cluster galaxies, we find the majority of galaxies in clusters experience no "pre-processing" in the group environment prior to their accretion into the cluster. On average, ∼ 70% of cluster galaxies fall into the cluster potential directly from the field, with no luminous companions in their host halos at the time of accretion; and less than ∼ 12% are accreted as members of groups with five or more galaxies. Moreover, we find that cluster galaxies are significantly less likely to have experienced a merger in the recent past ( 6 Gyr) than a field halo of the same mass. These results suggest that local, cluster processes like ram-pressure stripping, galaxy harassment, or strangulation play the dominant role in explaining the difference between cluster and field populations at a fixed stellar mass; and that pre-evolution or past merging in the group environment is of secondary importance for setting cluster galaxy properties for most clusters. The accretion times for z = 0 cluster members are quite extended, with ∼ 20% incorporated into the cluster halo more than 7 Gyr ago and ∼ 20% within the last 2 Gyr. By comparing the observed morphological fractions in cluster and field populations, we estimate an approximate timescale for late-type to early-type transformation within the cluster environment to be ∼ 6 Gyr.

We simulate the characteristics of the Galactic population of radio and c-ray pulsars using Monte Carlo techniques. At birth, neutron stars are spatially distributed in the Galactic disk, with supernovakick velocities, and randomly dispersed in age back to 109 yr. They are evolved in the Galactic gravitational potential to the present time. From a radio luminosity model, the radio Ñux is Ðltered through a selected set of radio-survey parameters. c-ray luminosities are assigned using the features of recent polar cap acceleration models invoking space-chargeÈlimited Ñow, and a pulsar death valley further attenuates the population of radio-loud pulsars. Assuming a simple emission geometry with aligned radio and c-ray beams of 1 sr solid angle, our model predicts that EGRET should have seen seven radio-loud and one radio-quiet c-ray pulsars. With much improved sensitivity, GLAST, on the other hand, is expected to observe 76 radio-loud and 74 radio-quiet c-ray pulsars, of which seven would be identiÐed as pulsed sources. We also explore the e †ect of magnetic Ðeld decay on the characteristics of the radio and c-ray pulsar populations. Including magnetic Ðeld decay on a timescale of 5 Myr improves agreement with the radio pulsar population and increases the predicted number of GL AST -detected pulsars to 90 radio-loud and 101 radio-quiet (nine pulsed) c-ray pulsars. The lower Ñux threshold allows GL AST to detect c-ray pulsars at larger distances than those observed by the radio surveys used in this study.

We present new and stronger evidence for a previously reported relationship between galactic spiral arm pitch angle P (a measure of the tightness of spiral structure) and the mass M BH of a disk galaxy's nuclear supermassive black hole (SMBH). We use an improved method to accurately measure the spiral arm pitch angle in disk galaxies to generate quantitative data on this morphological feature for 34 galaxies with directly measured black hole masses. We find a relation of log(M/M ⊙ ) = (8.21 ± 0.16) − (0.062 ± 0.009)P. This method is compared with other means of estimating black hole mass to determine its effectiveness and usefulness relative to other existing relations. We argue that such a relationship is predicted by leading theories of spiral structure in disk galaxies, including the density wave theory. We propose this relationship as a tool for estimating SMBH masses in disk galaxies. This tool is potentially superior when compared to other methods for this class of galaxy and has the advantage of being unambiguously measurable from imaging data alone.