1609.00730
Constrained simulations and excursion sets: understanding the risks and benefits of 'genetically modified' haloes
Porciani
Constrained realizations of Gaussian random fields are used in cosmology to design special initial conditions for numerical sims. Review this approach and its application to density peaks providing several worked-out examples. Then critically discuss the recent proposal to use constrained realizations to modify the linear density field within and around the Lagrangian patches that form DM haloes. The ambitious concept is to forge 'genetically modified' halos with some desired properties after the non-linear evolution. Demonstrate that the original implementation of this method is not exact but approximate because it tacitly assumes that protohaloes sample a set of random points with a fixed mean overdensity. Show that carrying out a full genetic modification is a formidable and daunting task requiring a mathematical understanding of what determines the biased locations of protohaloes in the linear density field. Discuss approximate solutions based on educated guesses regarding the nature of protohaloes. Illustrate how the excursion-set method can be adapted to predict the non-linear evolution of the modified patches and this fine tune the constraints that are necessary to obtain preselected halo properties. This technique allows exploration of the freedom around the original algorithm for genetic modification. Find that the quantity which is most sensitive to changes is the halo mass-accretion rate at the mass scale on which the constraints are set. Finally, discuss constraints based on the protohalo angular momenta.
1609.01041
The new world atlas of artificial night sky brightness
Falchi et al
Artificial lights raise night sky luminance, creating the most visible effect of light pollution-artificial sky glow. Despite the increasing interest among scientists in fields such as ecology, astronomy, health care, and land-use planning, light pollution lacks a current quantification of its magnitude on a global scale. To overcome this, present the world atlas of artificial sky luminance, computed with the light pollution propagation software using new high-resolution satellite data and new precision sky brightness measurements. This atlas shows that more than 80% of the world and more than 99% of the U.S. and European populations live under light-polluted skies. The Milky Way is hidden from more than 1/3 of humanity,, including 60% of Europeans and nearly 80% of North Americans. Moreover, 23% of the world's land surfaces between 75 deg N and 60 deg S, 88% of Europe, and almost half of the United States experience light-polluted nights.
1609.01162
Galaxy and mass assembly (GAMA): detection of low-surface-brightness galaxies from SDSS data
Williams, Baldry et al
Report on a search for new LSBGs using SDSS data within the GAMA equatorial fields. The search method consisted of masking objects detected with SDSS photo, combining grip images weighted to maximize the expected SNR, and smoothing the images. The processed images were then run through a detect algorithm that finds all pixels above a set threshold and groups them based on their proximity to one another. The list of detections was cleaned of contaminants such as diffraction spikes and the faint wings of masked objects. From these, selecting potentially the brightest in terms of total flux, a list of 343 LSBGs was produced having been confirmed using VIKING imaging. The photometry of this sample was refined using the deeper VIKING Z band as the aperture-defining band. Measuring their g-i and J-K colors shows that most are consistent with being at redshifts less than 0.2. The photometry is carried out using an AUTO aperture for each detection giving surface brightnesses of mu_r>=25 mag/arcsec2 and magnitudes of r>19.8 mag. None of these galaxies are bright enough to be within the GAMA main survey limit, but could be part of future deeper surveys to measure the low-mass end of the galaxy stellar mass function.
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