Cartography of the Local Cosmos
Astronomers map out the local universe in a way that is both intuitive and fascinating, marking the birth of the new science of “Cosmography”.
The Top 12 of 2012
What were astronomers reading and talking about in their research last year? Check out figures from the top 12 most-cited astronomy papers from 2012 (so far) and find out what researchers were up to and why!
Gravitational Lensing in the Canary Islands
I recently attended a two-week crash course in the “Astrophysical Applications of Gravitational Lensing”. In this post, I overview a few of the ways astronomers employ lensing to study the Universe, from extrasolar planets to distant quasars and large-scale structure.
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...
Mixing up gas in the wake of a strangled satellite
Most simulations to date have implied that satellite galaxies traveling through galaxy clusters are stripped of gas for future star formation in a process known as “strangulation”. In contrast, the authors of this paper suggest that satellite galaxies may not be as cut off as some might think: instead, their simulations show that the cooler, stripped gas from the corona will mix with the surrounding intra-cluster medium and remain near the original galaxy as a potential new source of star-forming fuel.
Strands in the Cosmic Web
Throughout much of the 20th century, it was an open question in astronomy as to what the universe looked like on the largest observable scales. Were galaxies and galaxy clusters distributed uniformly throughout space, or was there a pattern? Thanks to galaxy surveys we know that, on large scales, the matter distribution of the universe is clumpy instead of smooth. Through these surveys we observe directly the distribution of luminous matter like stars, gas, and galaxies. However, luminous matter comprises only a small fraction of the matter in the universe (17%), the rest is dark matter which interacts via gravity but does not absorb and emit electromagnetic radiation like normal matter. Theoretical simulations of dark matter cosmologies firmly predict that there is a dark matter backbone to the cosmic web, with filaments of dark matter stretching between clusters of galaxies, though has not yet been a robust detection of a dark matter filament, until now.