Another important component of galaxy formation and evolution is supernovae and young stars. Supernovae eject enormous amounts of energy into the surrounding gas. Depending on circumstances, this process can either stimulate or inhibit star formation (McKee & Ostriker 1977; Lada 1985; Wolf & Durisen 1987; Spitzer 1990). In a cold, dense cloud, it can stimulate star formation by increasing the pressure at the periphery and thus causing collapse in the interior. On the other hand, close to the supernova the shock heats the gas and evaporates clouds (McKee & Cowie 1975). This eventually turns off star formation. Supernovae also enrich the gas with metals, which drastically affects cooling rates and the conditions for star formation. Much of the energy of supernovae may contribute to the velocity dispersion of cold clouds (McKee FEEDBACK MECHANISMS REDUZEM A REFLETÂNCIA DA LUZ SOLAR E COM O GELO A DESAPARECER CADA VEZ FICA MAIS QUENTINHO EXCEPTUANDO O DIA DE HOJE QUE TÁ FRIO COMO O CARVALHO VOLTA PRA CIMA Ó FRENTE POLAR...Mechanisms of feedback in the visual system. by Adam Thomas Eggebrecht Feedback is an ubiquitous feature of neural systems though there is little consensus on the roles of mechanisms involved with feedback. We set up an in vivopreparation to study and characterize an accessible and isolated feedback loop within the visual system of the leopard frog, Rana pipiens. We recorded extracellularly within the nucleus isthmi, a nucleus providing direct topographic feedback to the optic tectum, a nucleus that receives the vast majority of retinal output. The optic tectum and nucleus isthmi of the amphibian are homologous structures to the superior colliculus and parabigeminal nucleus in mammals, respectively. We formulated a novel threshold for detecting neuronal spikes within a low signal-to-noise environment, as exists in the nucleus isthmi due to its high density of small neuronal cell bodies. Combining this threshold with a recently developed spike sorting procedure enabled us to extract simultaneous recordings from up to 7 neurons at a time from a single extracellular electrode. We then stimulated the frog using computer driven dynamic spatiotemporal visual stimuli to characterize the responses of the nucleus isthmi neurons. We found that the responses display surprisingly long time courses to simple visual stimuli. Furthermore, we found that when stimulated with complex contextual stimuli the response of the nucleus isthmi is quite counter-intuitive. When a stimulus is presented outside of the classical receptive field along with a stimulus within the receptive field, the response is actually higher than the response to just a stimulus within the classical receptive field. Finally, we compared the responses of all of the simultaneously recorded neurons and, together with data from in vitro experiments within the nucleus isthmi, conclude that the nucleus isthmi of the frog is composed of just one electrophysiological population of cells In this paper, we study the formation and evolution of galaxies in the context of large-scale structure. We focus our attention on the role of supernova feedback and hydrodynamics in combination with gravitational dynamics in determining the evolution of some of the most important observational properties of galaxies, including luminosities and colours, and the extent of their dependence on mass and environment. The formation and evolution of galaxies and the dynamics of the intergalactic medium are complicated processes that cannot be modelled purely from first principles, and that are not yet completely understood. One of the main challenges in modelling is the fact that a variety of effects over an exceptionally wide range of scales all come into play: cosmological fluctuations on scales larger than tens of Mpc are important because these scales define the large-scale structure of the galaxy distribution - positions and sizes of clusters, superclusters, filaments and voids. Many parameters of galaxies (like the morphological type, rate of collisions and merging) are known to depend on the position of a galaxy within the structures. Conditions for galaxy formation are different for galaxies inside groups and in the field, which brings in a scale of 1 Mpc. Although the dark matter still dominates gravitationally on scales above 100 kpc, the baryonic component becomes more and more important as we descend from the Mpc scale. The dissipation of energy by the hot gas definitely is an important factor on scales below 100 kpc. For the formation ofthe luminous component of galaxies, scales below 10 kpc are essential

Hydrodynamical simulations of galaxy formation: effects of supernova feedback

Monthly Notices of the Royal Astronomical Society, Jan 1997

G. Yepes

R. Kates

A. Khokhlov

A. Klypin

In this paper we numerically simulate some of the most critical physical processes in galaxy formation: the supernova feedback loop, in conjunction with gas dynamic processes and gravitational condensations, plays a crucial role in determining how the observable properties of galaxies arise within the context of a model for large-scale structure. Our treatment incorporates a multiphase model of the interstellar medium and includes the effects of cooling, heating and metal enrichment by supernovae, and evaporation of cold clouds. Star formation occurs inside cold gas clouds produced via thermal instability. In the present work, we simulate galaxy formation within the framework of a standard biased cold dark matter (CDM) model for a variety of parameters and for several resolutions. In our picture, supernova feedback regulates the evolution of the gas components and star formation. The efficiency of cold cloud evaporation by supernovae strongly influences star formation rates. This feedback results in a steady rate of star formation in ‘large’ galaxies [mass larger than (2–3) × 1011 M⊙ within 100 kpc radius] at a level of 1–10 M⊙ per year for z <3 (Ho=50 km s−1 Mpc−1). Supernova feedback has an even stronger effect on the evolution of ‘dwarf’ galaxies. Most of the dwarf galaxies in our models have a small fraction of stars and extremely low luminosities: MR> −15 for parent dark halo masses Mtot < (2–3) × 1010 M⊙ within a 50-kpc radius. The observational properties (colours, luminosities) of galaxies identified in the simulations are computed using a stellar population synthesis model. In the case of both large and small galaxies, the distribution of luminous matter (stars) is strongly biased with respect to the dark matter. For a range of parameter values and resolutions we find an approximate biasing measure of the form Álum = (Ádm/133)1.7, for overdensities exceeding about 1000. Deviations from this relation depend strongly on the environment. For halo masses exceeding 2 × 1010 M⊙, the dependence of the absolute visual magnitude MV on the total mass can be approximated as MV= −18.5 − 4 log (Mtot/1011 Mȩ), with a scatter of less than 1/2 magnitude.