1811.12374
The Ultraviolet detection of diffuse gas in galaxy groups
Stocke, et al
A small survey of the UV-absorbing gas in 12 low-z galaxy groups has been conducted using COS on-board HST. Targets were selected from a large, homogeneously-selected sample of groups found in SDSS. A critical selection criterion excluded sight line that pass close (<1.5 viral radii) to a group galaxy, to ensure absorber association with the group as a whole. Deeper galaxy redshift observations are used both to search for closer galaxies and also to characterize these 1e13.5 to 1e14.5 Msun groups, the most massive of which are highly-virialized with numerous ETGs. This sample also includes two spiral-rich groups, not yet fully virialized. At group-centre impact parameters of 0.3-2 Mpc, these S/N=15-30 spectra detected HI absorption in 7 of 12 groups, high (OVI) and low (SiIII) ion metal lines are present in 2/3 of the absorption components. None of the 3 most highly-virialized, ETG-doinated groups are detected in absorption. Covering fractions >~50% are seen at all impact parameters probed, but do not require large filling factors despite an enormous extent. Unlike halo clouds in individual galaxies, group absorbers have radial velocities which are too low to escape the group potential well without doubt. This suggests that these groups are "closed boxes" for galactic evolution in the current epoch. Evidence is presented that the cool and warm group absorbers are not a pervasive intra-group medium (IGrM), requiring a hotter (T~1e6 to 1e7 K) iGrM to be present to close the baryon accounting.
1811.12398
Cosmological filaments in the light of excursion set of saddle points
Fard, et al
The universe in large scales is structured as a network, called cosmic web. Filaments are one of the structural components of this web, which can be introduced as a novel probe to study the formation and evolution of structures and as a probe to study the cosmo models and even to address the missing baryon problem. The aim of this work is to introduce an analytical framework to study the statistics of filaments. Plan to present an approach to obtain the length to mass relation of filaments and the number density of filaments per length and mass. For this objective, model filaments as collapsed objects which have an extension in one direction, accordingly use the ellipsoidal collapse to study the evolution of an overdense region via gravitational instability. In this context, the critical density of filament formation is obtained which will be a crucial parameter for number density count. Also, find that the nonlinear density of filaments in the epoch of formation is almost mass independent and is in order of ~30 [units?]. A fitting function is presented for length-mass relation. For the statistics of filaments propose a novel framework which is named as excursion set of saddle points. In this approach, count the saddle points of the density field Hessian matrix, and relate it to the count of filaments. In addition, address the filament in filament problem with up-crossing approximation.
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