Galaxies
In the 10th century the Persian astronomer Al-Sufi observed and described “a small cloud” we now know was the Andromeda Galaxy. 听In 1785, William Herschel mapped the plane of our galaxy, the Milky Way. In the 1920s, astronomers debated whether the Milky Way was the universe and the clouds or nebulae they observed were听gaseous HII regions within it听or “island universes” of their own. Edwin Hubble used the new 100-inch telescope on Mt. Wilson听to solve the puzzle. Taking long photographic exposures, he resolved stars in the nebulae and concluded that they were “island universes”. Examining the spectra of their light, he discovered that they were also moving away from us and that the universe was expanding.
Today, theoretical astrophysicists study galaxies on all scales and from all perspectives:听they use the cosmic microwave background radiation, weak lensing, and galaxy surveys to constrain the initial fluctuations that gave rise to galaxies today; they model the first generation of galaxies and supermassive black holes in the听era of re-ionization;听they simulate the dynamical evolution of galaxies over cosmic history as they interact in clusters and are transformed from spirals into ellipticals in mergers.
The challenge today is how to turn what has been a descriptive science into one that makes testable predictions, by understanding the physics of galaxies. The main听tool in galaxy research today is large scale simulations, and the problems revolve around how to model the complicated physics that arise when you have gas in galactic systems.
Gas turns into stars and stars back-react on their environment. This feedback isn’t well understood.听The most obvious form of feedback is a supernovae but stars at all stages of their life听听push on gas and heat it, and the combination. In simulations, you听can incorporate the physics if you put in some rules and those subscriptions have a huge effect on the number of stars that form in your model galaxies. 听How do you turn this into a predictive science?听
Among the questions听researchers studying galaxies are asking are, for galaxies of different types, how does gas get into the galaxy? How does it turn into stars and how does it leave the galaxy? Does the enormous amount of light that stars produce affect the gas in a substantial way?听If so, how? Does it heat the gas, or do their solar winds shock the gas? How does a stellar mass change with time? How does the metalicity of gas change of over time?听How does the amount of gas in the halo change over time?听Scientists check those things observationally, and then they can compare their simulations and see if they鈥檙e really modelling the physics. How do magnetic fields 听effect the efficiency of feedback? How does a star forming galaxy die? Why do supermassive black holes always develop at the centre of disk galaxies? What鈥檚 the mechanism?听How does a six million solar mass black hole form?
We don鈥檛 know yet.
Evolution of a Galaxy听
Prof. Norm Murray and the FIRE Collaboration are producing simulations to address questions in galaxy formation with unprecedented predictive power.
This is a mathematically and computationally accurate simulation of the formation and evolution of a Milky Way-type galaxy over the life-span of the universe.听In the model, young stars are shown as blue,听and old stars are red.
Antonini,听Fabio; Murray,听Norman; Mikkola,听Seppo
The Astrophysical Journal, Volume 781, Issue 1, article id. 45,听13听pp. (2014)
arxiv:1308.3674v2
Proga,听Daniel; Jiang,听Yan-Fei; Davis,听Shane听W.; Stone,听James听M.; Smith,听Daniel
The Astrophysical Journal, Volume 780, Issue 1, article id. 51,听7听pp. (2014)
arxiv:1311.1540v1
Antonini, Fabio and Merritt, David
The Astrophysical Journal Letters, Volume 763, Issue 1, article id. L10, 6 pp. (2013).
arxiv:1211.4594
Martin, Crystal L. et al. (Murray, Norman)
The Astrophysical Journal, Volume 770, Issue 1, article id. 41, 19 pp. (2013).
arxiv:1304.6405
Planck Collaboration et al. (Bond, J. R. and Martin, P. G.)
Astronomy & Astrophysics, Volume 554, id.A139, 15 pp.
Hopkins, Philip F., Kere拧, Du拧an and Murray, Norman
Monthly Notices of the Royal Astronomical Society, Volume 432, Issue 4, p.2639-2646 (2013)
arxiv:1301.4500v2
Antonini,听Fabio; Murray,听Norman; Mikkola,听Seppo
The Astrophysical Journal, Volume 781, Issue 1, article id. 45,听13听pp. (2014)
arxiv:1308.3674v2
Proga,听Daniel; Jiang,听Yan-Fei; Davis,听Shane听W.; Stone,听James听M.; Smith,听Daniel
The Astrophysical Journal, Volume 780, Issue 1, article id. 51,听7听pp. (2014)
arxiv:1311.1540v1
Antonini, Fabio and Merritt, David
The Astrophysical Journal Letters, Volume 763, Issue 1, article id. L10, 6 pp. (2013).
arxiv:1211.4594
Martin, Crystal L. et al. (Murray, Norman)
The Astrophysical Journal, Volume 770, Issue 1, article id. 41, 19 pp. (2013).
arxiv:1304.6405
Planck Collaboration et al. (Bond, J. R. and Martin, P. G.)
Astronomy & Astrophysics, Volume 554, id.A139, 15 pp.
Hopkins, Philip F., Kere拧, Du拧an and Murray, Norman
Monthly Notices of the Royal Astronomical Society, Volume 432, Issue 4, p.2639-2646 (2013)
arxiv:1301.4500v2