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We report on a V=11.2 early K dwarf, XO-2 (GSC 03413-00005), that hosts a Rp=0.98+0.03/-0.01 Rjup, Mp=0.57+/-0.06 Mjup transiting extrasolar planet, XO-2b, with an orbital period of 2.615857+/-0.000005 days. XO-2 has high metallicity, [Fe/H]=0.45+/-0.02, high proper motion, mu_tot=157 mas/yr, and has a common proper motion stellar companion with 31" separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. Due to the high metallicity, these early K dwarf stars have a mass and radius close to solar, Ms=0.98+/-0.02 Msolar and Rs=0.97+0.02/-0.01 Rsolar. The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, Rper<4 kpc) well confined to the Galactic disk (Zmax~100 pc). In addition, the phase space position of XO-2 is near the Hercules dynamical stream, which points to an origin of XO-2 in the metal-rich, inner Thin Disk and subsequent dynamical scattering into the solar neighborhood. We describe an efficient Markov Chain Monte Carlo algorithm for calculating the Bayesian posterior probability of the system parameters from a transit light curve., Comment: 14 pages, 10 Figures, Accepted in ApJ. Negligible changes to XO-2 system properties. Removed Chi^2 light curve analysis section, and simplified MCMC light curve analysis discussion

The angular momentum of galaxies is routinely ascribed to a process of tidal torques acting during the early stages of gravitational collapse, and is predicted from the initial mass distribution using second-order perturbation theory and the Zel'dovich approximation. We have tested this theory for a flat hierarchical cosmogony using a large N-body simulation with sufficient dynamic range to include tidal fields, allow resolution of individual galaxies, and thereby expand on previous studies. We find relatively good correlation between the predictions of linear theory and actual galaxy evolution. While structure formation from early times is a complex history of hierarchical merging, salient features are well described by the simple spherical-collapse model. Most notably, we test several methods for determining the turnaround epoch, and find that turnaround is succesfully described by the spherical collapse model. The angular momentum of collapsing structures grows linearly until turnaround, as predicted, and continues quasi-linearly until shell crossing. The predicted angular momentum for well-resolved galaxies at turnaround overestimates the true turnaround and final values by a factor of ~3 with a scatter of ~70 percent, and only marginally yields the correct direction of the angular momentum vector. We recover the prediction that final angular momentum scales as mass to the 5/3 power. We find that mass and angular momentum also vary proportionally with peak height., Comment: 20 pages, 16 figures, Accepted for publication in MNRAS

We present a scenario for the formation of disks which explains not only the properties of normal galaxies, but the properties of the population of low surface brightness galaxies (LSBs) as well. We use a gravitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angular momentum of the initial protogalaxy. The model naturally produces smooth, asymptotically flat rotation curves, exponential surface brightness profiles over many disk scale lengths, the Tully-Fisher relation as a function of surface brightness, the observed distribution of scale lengths and surface brightnesses, and the variation of rotation curve shapes. In the model, low mass and/or high angular momentum halos naturally form low surface brightness disks. Theoretical and numerical calculations suggest galaxy halos should form with a wide range of mass and angular momenta, and thus, the disks which form within these halos should have a wide range of surface brightnesses and scale lengths. We use the formalism to calculate the expected change in the observed luminosity function (LF) and luminosity density as a function of limiting surface brightness. Current measurements of the LF may by off by factors of 2 at L*. [SHORTENED], Comment: 48 pages LaTeX w/ figures. Accepted to the Astrophysical Journal

High resolution gravity plus smoothed particle hydrodynamics simulations are used to study the formation of galaxies within the context of hierarchical structure formation. The simulations have sufficient dynamic range to resolve from ten kpc scale galactic disks up to many Mpc scale filaments. Over this range of scales, we find that hierarchical structure development proceeds through a series of increasingly larger filamentary collapses. The well resolved simulated galaxies contain hundreds to thousands of particles and have varied morphologies covering the entire expected range from disks to tidally distorted objects. The epoch of galaxy formation occurs early, about redshift 2.5 for 10^12 M_sun galaxies. Hierarchical formation naturally produces correlations among the mass, age, morphology, and local density of galaxies which match the trends in the observed morphology--density relation. We also describe a method of spiral galaxy formation in which galactic disks form through the discrete accretion of gas clouds which transport most of the angular momentum to the inner regions. Such a process is characteristic of the somewhat chaotic nature of hierarchical structure formation where simple analytical ideas of spherical collapse appear incongruous., Comment: Submitted to The Astrophysical Journal. Low resolution (100 dpi) figures included. Full resolution figures available from the author or at http://astro.princeton.edu/~summers/

This general talk is geared toward non-specialists and addresses three questions: (1) What types of cosmological simulations are done? (2) What are the relevant problems to address and what results can one expect? (3) How can one tell if the results are reasonable?, Comment: To appear in 1995 ASP Symposium on Clusters, Lensing, and the Future of the Universe. Uuencoded compressed Postscript, 10 pages, includes 2 of 4 figures. Full Postscript text and figures available from http://astro.princeton.edu/~summers/

The nonlinear clustering of dark matter particles in an expanding universe is usually studied by N-body simulations. One can gain some insight into this complex problem if simple relations between physical quantities in the linear and nonlinear regimes can be extracted from the results of N-body simulations. Hamilton et al. (1991) and Nityananda and Padmanabhan (1994) have made an attempt in this direction by relating the mean relative pair velocities to the mean correlation function in a useful manner. We investigate this relation and other closely related issues in detail for the case of six different power spectra: power laws with spectral indexes $n=-2,-1$, cold dark matter (CDM), and hot dark matter models with density parameter $\Omega=1$; CDM including a cosmological constant ($\Lambda$) with $\Omega_{CDM}=0.4$, $\Omega_{\Lambda}=0.6$; and $n=-1$ model with $\Omega=0.1$. We find that: (i) Power law spectra lead to self-similar evolution in an $\Omega=1$ universe. (ii) Stable clustering does not hold in an $\Omega=1$ universe to the extent our simulations can ascertain. (iii) Stable clustering is a better approximation in the case of $\Omega<1$ universe in which structure formation freezes out at some low redshift. (iv) The relation between dimensionless pair velocity and the mean correlation function, $\bar\xi$, is only approximately independent of the shape of the power spectrum. At the nonlinear end, the asymptotic value of the dimensionless pair velocity decreases with increasing small scale power, because the stable clustering assumption is not universally true. (v) The relation between the evolved $\bar\xi$ and the linear regime $\bar\xi$ is also not universal but shows a weak spectrum dependence. We present simple theoretical arguments for these conclusions., Comment: Princeton Univ Obs, in press ApJ (Aug 1, 1996) Figures (revised) also available from ftp://astro.princeton.edu/cen/SCALING

This paper examines several methods of tracing galaxies in N-body simulations and their effects on the derived galaxy statistics, especially measurements of velocity bias. Using two simulations with identical initial conditions, one following dark matter only and the other following dark matter and baryons, both collisionless and collisional methods of tracing galaxies are compared to one another and against a set of idealized criteria. None of the collisionless methods proves satisfactory, including an elaborate scheme developed here to circumvent previously known problems. The main problem is that galactic overdensities are both secularly and impulsively disrupted while orbiting in cluster potentials. With dissipation, the baryonic tracers have much higher density contrasts and much smaller cross sections, allowing them to remain distinct within the cluster potential. The question remains whether the incomplete physical model introduces systematic biases. Statistical measures determined from simulations can vary significantly based solely on the galaxy tracing method utilized. The two point correlation function differs most on sub-cluster scales with generally good agreement on larger scales. Pairwise velocity dispersions show less uniformity on all scales addressed here. All tracing methods show a velocity bias to varying degrees, but the predictions are not firm: either the tracing method is not robust or the statistical significance has not been demonstrated. Though theoretical arguments suggest that a mild velocity bias should exist, simulation results are not yet conclusive., Comment: ApJ, in press, 23 pages, plain TeX, 8 of 13 figures included, all PostScript figures (4.8 MB) available via anonymous ftp from ftp://astro.princeton.edu/summers/tracers . Also available as POPe-616 on http://astro.princeton.edu/~library/prep.html

The formation of low surface brightness galaxies is an unavoidable prediction of any hierarchical clustering scenario. In these models, low surface brightness galaxies form at late times from small initial overdensities, and make up most of the faint end of the galaxy luminosity function. Because there are tremendous observational biases against finding low surface brightness galaxies, the observed faint end of the galaxy luminosity function may easily fall short of predictions, if hierarchical structure formation is correct. We calculate the number density and mass density in collapsed objects as a function of baryonic surface density and redshift, and show that the mass in recently formed low surface brightness galaxies can be comparable to the mass bound into ``normal'' high surface brightness galaxies. Because of their low gas surface densities, these galaxies are easily ionized by the UV background and are not expected to appear in HI surveys. Low surface brightness galaxies (LSBs) are not a special case of galaxy formation and are perhaps better viewed as a continuance of the Hubble sequence., Comment: 31 pgs, LaTex using "aasms.sty", 2 additional pgs of figures on ftp at ftp://ociw.edu/jd/ as lsbformationfig*.ps.Z, submitted to ApJ

The formation of galaxies in hierarchical cosmogonies is studied using high resolution N-body plus SPH hydrodynamics simulations. The collapse of structure is followed self-consistently from Mpc scale filamentary structures to kpc scale galactic objects. The characteristics and formation processes of the galaxy like objects are studied in detail, along with the aggregation into a poor cluster. Related studies consider the effects of modelling star formation, the reliability of tracing galaxies in simulations, and tests of SPH methods. This submission serves first to notify that the full text and figures of my thesis are available in compressed PostScript form via anonymous ftp from astro.princeton.edu in the directory /summers/thesis (122 files, 19 MB compressed, 65 MB uncompressed). See the README file first. Second, this submission contains the title page, abstract, table of contents, introductory chapter, summary chapter, and references for my thesis. Those who are curious about the work may scan these pages to identify which chapters may be interesting to get via ftp., Comment: plain TeX, PhD thesis, UC Berkeley 1993

We investigate the formation of galaxies and larger structure with a simulation modeling two gravitationally coupled fluids representing dark matter and baryons. The baryon gas dynamics are calculated with a smoothed particle hydrodynamics (SPH) method, and the physics modeled includes thermal pressure, shock heating, and radiative cooling. We simulate a 16 Mpc periodic cube with 64(exp 3) particles in each fluid and 10% baryon mass fraction. We confirm, for the first time experimentally, disk formation as a natural consequence of hierarchical clustering in a large-scale cosmological environment. The majority of isolated galaxies exhibit centrifugally supported disks. A power-law relation between cold baryonic mass and maximum rotation velocity is found, M varies as nu(sub rot)(exp alpha) with alpha = 2.5 after correcting for differential numerical resolution. Both the spatial and velocity distributions of the simulated galaxies are biased with respect to the dark matter. A counts-in-cells analysis indicates that an unphysical degree of merging in the central cluster is likely responsible for the antibias signal in the correlation function. A robust, scale-dependent velocity bias is measured. The ratio of galaxy to dark matter pairwise velocity dispersions on a scale of 1 Mpc is 0.7. The amplitude is only mildly dependent on redshift or mass cutoff and scales with separation as r(exp 0.2). The degree to which these results depend on numerical parameters is discussed. Mass resolution plays a key role in controlling the resulting fraction of cold, dense baryons. The mass fraction associated with galaxies decreases by a factor of approximately greater than 3 when the mass per particle is increased by a factor 8. Photoionization and energy input from supernova will have to be included to determine more carefully the fraction of highly dissipated material and the characteristics of the stellar component of galaxies.

Using the method of smoothed particle hydrodynamics, we have modeled the formation of a compact group of galaxies with sufficient resolution to trace galaxies. Radiative cooling allows the baryons to dissipate their thermal energy and collapse to overdensities characteristic of real galaxies. With their cross section greatly reduced, these galaxy tracers remain distinct during cluster formation while their dark matter halos merge. In addition, the number density, the mass distribution function, and even the morphology of these objects are similar to those of observed galaxies. A viable population of galaxy tracers can be unambiguously defined.