Einstein’s theory of general relativity predicts the existence of gravitational waves, and if it’s good enough for Einstein, it’s good enough for LISA, the Laser Interferometer Space Antenna mission scheduled for launch in 2015. LISA will search the universe for gravitational waves, a coup if detected since they have until now remained in the domain of theory. And if the spacecraft finds its target, chances are it will be picking up gravitational waves from the collision of supermassive black holes that occur when galaxies merge.
All of which is germane to new work by a team led by Stelios Kazantzidis (University of Chicago). Kazantzidis is working on galaxy collisions, simulating them to identify what leads to the mergers of such black holes. After all, if supermassive black hole collisions are numerous, the chances of LISA detecting their gravitational waves go up. The team is using supercomputers to simulate an intricate gravitational dance. Most stars in such galaxy collisions fly past each other, but their gravity fields play havoc with the galaxies’ journey through space, causing them to come back together again and again.
Over the course of a billion years, both galaxies have lost speed and energy. Their gases migrate to their respective cores, increasing the density and stability of the cores. The new work shows that what happens next depends upon the amount of gas in the respective galaxies, for two gas-poor galaxies may not necessarily complete the merger. Gas-rich galaxies, on the other hand, invariably produce a supermassive black hole merger. “We found,” says Kazantzidis, “that gas is essential in driving the co-evolution of galaxies and supermassive black holes.”
That’s good news for LISA. “At very early times in the universe,” Kazantzidis adds, “there was a lot of gas in the galaxies, and as the Universe evolves, the gas is consumed by star formation. And large amounts of gas mean more colliding galaxies and merging supermassive black holes.” If he’s right, LISA should have plenty to do, and our understanding of gravity’s propagation should be materially advanced.
The paper is Mayer, Kazantzidis, Madau et al., “Multi-scale simulations of merging galaxies with supermassive black holes,” published in The Astrophysical Journal 641, Number 2, Part 1, pp. 647-664 and also available here.
The Contribution of Halo White Dwarf Binaries to the LISA Signal
Authors: Ashley J. Ruiter (1), Krzysztof Belczynski (2,1), Matthew Benacquista (3), Kelly Holley-Bockelmann (4) ((1) NMSU, (2) LANL, (3) CGWA at UTB, (4) Vanderbilt)
(Submitted on 5 Dec 2007)
Abstract: Galactic double white dwarfs were postulated as a source of confusion limited noise for the Laser Interferometer Space Antenna, a space based gravitational wave observatory. The Galactic population consists of a relatively well studied disk population, anda mostly unknown, but potentially large halo population. It has been argued that the halo population may produce a signal that is much stronger (factor of ~5 in spectral amplitude) than the disk population. However, this surprising result was not based on an actual calculation of a halo white dwarf population but was derived on (i) the assumption that one can extrapolate the halo population properties from those of the disk population and (ii) the postulated (unrealistically) high number of white dwarfs in the halo. We perform the first calculation of a halo white dwarf population using population synthesis models. Our comparison with the signal arising from double white dwarfs in the Galactic disk clearly shows that it is impossible for the double white dwarf halo signal to exceed that of the disk. Using microlensing results to give an upper limit on the content of white dwarfs in the halo (~30% baryonic mass in white dwarfs), our predicted halo signal is a factor of ~10 lower than the disk signal. Even in the implausible case where all of the baryonic halo mass is found in white dwarfs, the halo signal does not become comparable to that of the disk, and thus would still have a negligible effect on the detection of other LISA sources.
Comments: 5 pages, 1 figure, submitted to ApJ Letters
Subjects: Astrophysics (astro-ph)
Report number: LA-UR-07-7963
Cite as: arXiv:0712.0847v1 [astro-ph]
Submission history
From: Ashley Ruiter [view email]
[v1] Wed, 5 Dec 2007 23:13:06 GMT (167kb,D)
http://arxiv.org/abs/0712.0847