Observing the Velocity Anisotropy of Cluster Galaxies

Observing the Velocity Anisotropy of Cluster Galaxies

Title: Spatial Anisotropy of Galaxy Kinematics in Sloan Digital Sky Survey Galaxy ClustersFirst Author: Skielboe, A.Galaxy clusters are beautifully simple, but also fantastically complicated structures. For many years, astronomers have treated these systems as spherical cows, but simulations and observations have repeatedly shown that clusters exhibit triaxial rather than spherical shapes with nice relaxed dynamics (are virialized). Many cluster mass estimators assume spherically symmetric velocity fields (i.e. you measure the same velocities of cluster galaxies regardless of which side you observe from), but if the shape is anisotropic it’s probable the velocities are as well. This makes it crucial to measure the degree of triaxiality of clusters in observations to constrain its impact on mass estimates.The authors sought to show that velocity anisotropy exists by testing for an azimuthal (angle on the sky) dependence of the projected velocity dispersion. To do this, they used a stacked sample of galaxy clusters from the Sloan Digital Sky Survey (SDSS). Stacking is a common technique of taking many clusters with a similar property (in this case galaxy richness or number which is a proxy for mass) and adding them together to make a composite system with many hundreds more galaxies than any one system alone. This gives much better statistics and makes a result more robust.Because the authors are looking for azimuthal variations, they fit each cluster with an ellipse and stack them with their major axes aligned. They then estimate the projected velocity dispersion for galaxies closer to the stacked minor axis, and a separate velocity dispersion for galaxies closer to the major axis. Because they have a stacked sample with...
Preserving our Dark Skies

Preserving our Dark Skies

I never feel more connected with nature than when I am surrounded by it in raw form. Experiences like camping or hiking while greatly removed from the concrete jungles of civilization make me appreciate the beauty of our environment. My thoughts slow down, and it’s even possible to enjoy the lack of cell service for a while. While the number of people living in cities continues to grow around the world, most of us do not have to travel very far to have these types of experiences. Most of us even recognize the need to protect these shrinking natural environments not just for the enjoyment of escape, but because we depend on them. But there is something disappearing faster than undeveloped environments: our universe. Not literally of course, but from our sight. As cities expand all over the world, and countries develop to compete in the modern economy, we are losing our night sky to the train of progress. As astronomers, this is a serious problem as we have watched the pristine views of space from Mt. Wilson and Kitt Peak slowly fall victim to metropolitan growth over the years, so we need little lecturing about the importance of preserving our night sky. However, to most non-astronomers, seeing fewer stars does not mean much.One of my favorite quotations comes from Neil Degrasse Tyson when he is describing his first view of a truly dark sky. He notes that his first thought is how much it reminds him of the Hayden Planetarium, not the other way around! Many of us never forget that first experience of seeing a truly dark...
Mining for Fossils in our Universe

Mining for Fossils in our Universe

Title: The XMM Cluster Survey: The Stellar Mass Assembly of Fossil GalaxiesFirst Author: Craig D. HarrisonFirst Author Institution: University of MichiganHow do you describe a fossil? Old? Imprinted? Dead? Pristine? A link to the past? As it turns out, the word fossil has been used to describe a distinct population of galaxies and systems of galaxies in our universe. Hierarchical structure formation models indicate that small things build larger things over cosmic time. When groups and clusters of galaxies form, dynamical friction causes the galaxies to lose momentum and kinetic energy, which can ultimately lead galaxies to merge.  This dynamical friction has the greatest effect in high density regions, which causes mergers to create a large brightest cluster galaxy (BCG) at the center of these systems. Observations have identified a population of clusters that have a very large BCG, but not very many bright companions (ie, a magnitude gap exists) and called these Fossil Galaxies in Fossil Systems. The use of word fossil here is perfect because the BCG appears to live in a “dead” or depleted environment, and the entire merger history should be imprinted in its stellar population.So what can we learn from these fossil systems? Because of small samples, much of what we already know has come from simulations. These simulations indicate that fossil systems gain a large fraction of their mass at high redshift and earlier than non-fossil groups. One hypothesis is that growth of fossils was dominated early by in-fall of massive satellites which boosted the size of the fossil galaxy relative to its cousins in non-fossil systems, which evolved slightly more passively. Another...
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