Day 1473

Tuesday.

1810.00002
The missing light of the Hubble Ultra Deep Field
Borlaff, et al

The HUDF is the deepest region ever observed with the HST.  With the main objective of unveiling the nature of galaxies up to z~7-8, the observing and reduction strategy have focused on the properties of small and unresolved objects, rather than the outskirts of the largest objects, which are usually over-subtracted.  Aim to create a new set of WFC3/IR mosaics of the HUDF using novel techniques to preserved the properties of the low surface brightness regions.  Created BYSS: a pipeline that optimists the estimate and modeling of low-level systematic effects to obtain a robust background subtraction.  Improved 4 key points in the reduction: 1) creation of new absolute sky flat fields, 2) extended persistence models, 3) dedicated sky background subtraction and 4) robust co-adding.  The new mosaics successfully recover the low surface brightness structure removed on the previous HUDF published reductions.  The amount of light recovered with a mean surface brightness dimmer than mu=26 mag arcsec^{-2} is equivalent to a m=19 mag source when compared to the XDF and a m=20 mag compared to the HUDF12.  Present a set of techniques to reduce ultra-deep images (mu>32.5 mag arcsec^{-2}, 3 sigma in 10x10 arcsec boxes), that successfully allow to detect the low surface brightness structure of extended sources on ultra deep surveys.  The developed procedures are applicable to HST, JWST, EUCLID and many other space and ground-based observatories.  Make the final ABYSS WFC3/IR HUDF mosaics publicly available at www.iac.es/proyecto/abyss/.


1810.00843
Nearly all the sky is covered by Lyman-alpha emission around high redshift galaxies
Wisotzki, et al

Galaxies are surrounded by large reservoirs of gas, mostly H, fed by inflows from the IGM and by outflows due to galactic winds.  Absorption-line measurements along the sightlines to bright and rare background quasars indicate that this circumgalactic medium pervades far beyond the extent of galaxies, but very little is known about the spatial distribution of this gas.  A new window into circumgalactic environments was recently opened with the discovery of ubiquitous extended Ly-alpha emission from H around high-redshift galaxies, facilitated by the extraordinary sensitivity of the MUSE instrument at the ESO VLT.  Due to the faintness of this emission, such measurements were previously limited to especially favorable systems or to massive statistical averaging.  Here, demonstrate that low surface brightness Ly-alpha emission surrounding faint galaxies at 3<z<6 adds up to a projected sky coverage of nearly 100%.  The corresponding rate of incidence (the mean number of Ly-alpha emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high column density absorbers frequently detected in the spectra of distant quasars.  This similarity suggests that most circumgalactic atomic H at these redshifts has now been detected also in emission.