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 Journa
We report on a V ¼ 11:2 early K dwarf, , that hosts a R p ¼ 0:98AE 0:03 0:01 R J , M p ¼ 0:57 AE 0:06 M J transiting extrasolar planet, XO-2b, with an orbital period of 2:615857 AE 0:000005 days. XO-2 has high metallicity, ½Fe/H ¼ 0:45 AE 0:02, high proper motion, tot ¼ 157 mas yr À1 , and a common proper motion stellar companion with 31 00 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, M ? ¼ 0:98 AE 0:02 M and R ? ¼ 0:97AE 0:02 0:01 R . The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, R per < 4 kpc) well confined to the Galactic disk (Z max $ 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.
We use high resolution simulations to study the formation and distribution of galaxies within a cluster which forms hierarchically. We follow both dark matter and a gas component which is subject to thermal pressure, shocks, and radiative cooling. Galaxy formation is identi ed with the dissipative collapse of the gas into cold, compact knots. We examine two extreme numerical representations of these galaxies during subsequent cluster evolution | one purely gaseous and the other purely stellar. The results are quite sensitive to this choice. Gas-galaxies merge e ciently with a dominant central object which grows to contain more than half of the galactic mass within the cluster. Star-galaxies merge less frequently and produce a mass distribution for cluster members which is quite similar in shape to that for non-cluster galaxies. Thus, simulations in which galaxies remain gaseous appear to su er from an \overmerging" problem, but this problem is much less severe if the gas is allowed to turn into stars.We compare the kinematics of the galaxy population in these two representations to the kinematics of dark halos and of the underlying dark matter distribution. Galaxies in the stellar representation are positively biased (i.e., over-represented in the cluster) both by number and by mass fraction. Both representations predict the galaxies to be more centrally concentrated than the dark matter, whereas the dark halo population is more extended. A modest velocity bias also exists in both representations, with the largest e ect, gal = DM ' 0:7, found for the more massive star-galaxies. Phase { 2 { diagrams show that the galaxy population has a substantial net in ow in the gas representation, while in the stellar case it is roughly in hydrostatic equilibrium. Virial mass estimators can underestimate the true cluster mass by up to a factor of 5 because of these various bias e ects. The discrepancy is largest if only the most massive galaxies are used, re ecting signi cant mass segregation. A binding energy analysis suggests that this segregation is primarily a result of dynamical friction. We discuss the relevance of these results both to real clusters and to the general problem of simulating the formation and clustering of galaxies. The incorporation of a realistic star formation algorithm within future simulations will be the key to further progress. { 3 {
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