New Milky Way Observations, Impact on Old and New Theories
As a result of recent multi-wave length surveys with large star counts, there is now comprehensive data for the Milky Way. I thought the new papers that discuss this data would be interesting to discuss, as any theory must explain the observations. The observations show clear evidence of structures that indicates there are fundamental mechanisms involved, to form the structures, rather than random mergers.
This thread starts with a brief outline of the formation of the galaxy using the old abrupt cloud collapse theory (see below for details) that has been discussed before in the forum along with some known observation vs theory problem for the abrupt cloud collapse model.
The galactic formation model that has been mentioned a few times in the forum is the “Collapse Model”, which is also called the ELS model, named after the model founders Eggen, Lynden-Bell, and Sandage. The ELS model was developed when there was limited data concerning the Milky Way, as noted below and subsequent papers, new data shows the ELS model, is fundamental flawed. As the ELS model is a dark matter based model the data in question also fundamentally challenges the existence of dark matter. The dark matter theory has bounds, as to how the hypothesized dark matter can affect real matter. (See Dark Matter Puzzle thread in Q&A for additional details.)
Excerpt from “An Introduction to Astrophysics” 2nd Editon, published 2007 by Bradley Carrol & Dale Ostlie, Page 1016, Ch #26 Galactic Evolution
There work (my comment the work of Eggen, Lynden-Bell, and Sandage) was based on observed correlations between the metallicity of stars in the solar neighbourhood, and their (my comment: the stars in question) orbital eccentricity and orbit angular momentum. Eggen, Lynden-Bell, and Sandage noted that most metal-poor stars tend to have the highest eccentricities, the largest w components of their peculiar moments, and the lowest angular momenta about the rotational axis of the Galaxy. On the other hand, metal-rich stars tend to exist in nearly circular orbits and are confined to regions near the plane of the Galaxy......To explain the kinematic and chemical properties of the stars in the solar neighbourhood ELS proposed a model where the Milky Way formed from the sudden collapse (roughly 200 million years) of a large prototype galaxy.
The oldest halo stars formed early in the collapse process while still on nearly radial trajectories, resulting in their highly elliptical orbits above and below the Galactic plane. As a further consequence of their rapid formation, the model predicts that the halo stars are naturally very metal-poor (Population II) since the interstellar medium had not yet had time to become enriched by the by-products of stellar nucleoysnthesis. ...
Problems with ELS Model (Same source but paraphrased for length.)
1. Roughly half of the stars in the Milky Way’s halo have retrograde orbits, so the net angular momentum of the halo is 0 km/m^2. On the other hand stars in the inner halo appear to have a small net rotation velocity.
2. A second problem with the ELS model is an age variance between the global clusters and halos stars of approximately 2 billion years which would indicate the collapse took an order of magnitude longer the 200 million years predicted. The ELS model also does not explain the difference in age of the narrow region of the Milky Way’s galactic disk which is roughly 8 billion years old and the thick portion of the disk which is 10 billion years old.
3. A third age difficulty is that clusters near the Galactic core are generally the most metal rich and the oldest, while clusters in the outer halo exhibit a wider range in metallicity and tend to be younger.