Tidal Streams Probe Dark Matter Forces

One of the biggest mysteries in cosmology today is dark matter, which is believed to contribute about five times more to the energy density of the Universe than the ordinary matter from which stars and planets form.  Dark matter is seen on scales ranging from individual galaxies to galaxy clusters and horizon-size fluctuations in the cosmic microwave background.  Although we can detect dark matter through its gravitational influence on visible matter and radiation, an important question to ask is whether dark matter experiences any other forces in addition to gravity.  Since we cannot test dark matter in the laboratory, we must come up with new ways to look for dark matter forces on astrophysical scales. In a recently published paper in Physical Review Letters, Prof. Marc Kamionkowski and I proposed one such test, based on the appearance of the tidal streams of the Sagittarius dwarf, a nearby satellite galaxy of our own Milky Way [1].

Why should we look for a dark matter force in the first place?  An attractive dark matter force might help to explain several observed discrepancies in the standard picture of the growth of large-scale structure [2].  Such a force might also be a consequence of an interaction between the dark matter and an almost massless scalar field [3].  A scalar field of this kind has been proposed as the source of the dark energy responsible for the observed acceleration in the Universe's expansion [4].  String theory conjectures the existence of many massless scalar fields, and natural scenarios can be constructed in which these fields mediate new forces of roughly gravitational strength between dark matter particles [5].  Given the uncertainty surrounding dark matter and dark energy, and the importance of understanding them, it is worth pursuing new approaches that might yield clues to their true nature.

Finding a dark matter force would be very important, but how exactly would we do it?  At first it might seem easy to detect a dark matter force of gravitational strength, as gravity is the
dominant force responsible for the growth of large-scale structure. However, most probes of dark matter like galactic rotation curves and gravitational lensing are only sensitive to the density distribution of dark matter, as it is the density that sources gravity.  In equilibrium systems, dark matter density profiles can be maintained in the presence of a dark matter force by changing the velocity of dark matter particles, an effect that is very difficult to measure directly.  To reveal a dark matter force, we must look to systems far from equilibrium such as tidally disrupting galaxies being ripped apart before our very eyes.

Just as the Sun and Moon raise tides on the Earth's oceans, a large galaxy like our Milky Way can raise tides on smaller satellite galaxies orbiting it.  However, these tides can be powerful enough not just to temporarily raise the sea level but to rip stars free from the satellite galaxy entirely.  Stars that fall towards the Galactic center orbit the Galaxy faster than the satellite itself, as the inner planets like Mercury orbit the Sun faster than the Earth does.  Conversely, stars pulled from the side of the satellite further from the Galactic center orbit more slowly and trail behind the satellite.  As they spread out, these stars form leading and trailing tidal streams respectively.  They wrap entirely around the Galaxy, and if you had good enough vision you could see them forming a band across the sky as the Milky Way disk does on a clear night.  There is a delicate balance between the number of stars in the leading and trailing tidal streams, and the key point of our paper is that a dark matter force could tip this balance making the ratio of leading to trailing stars an incredibly sensitive probe of a dark matter force.  An attractive dark matter force would pull the dark matter in the satellite galaxy closer to the Milky Way, implying that the stars would be left behind and pushed out the side of the satellite furthest from the Milky Way.  We would therefore expect a lower ratio of leading to trailing stars for an attractive dark matter force.

Nature has generously provided us with a nearby tidally disrupting satellite galaxy almost ideal for testing this effect.  The Sagittarius dwarf galaxy is only about 80,000 light-years away, and its extensive leading and trailing tidal streams have been observed by the Two Micron All Sky Survey (2MASS) [6].  Simulations based on this data have provided estimates for the gravitational potential of the Milky Way, and the mass and orbit of the Sagittarius dwarf [7].  We have performed a new series of simulations based upon these models, but including a dark matter force between the dark halos of the Sagittarius dwarf and Milky Way.  Our simulations reveal that an attractive inverse-square-law dark matter force greater than 10% the strength of gravity would strongly suppress the leading tidal stream.  As this is not observed, models that propose the existence of such a force are strongly challenged.  While more work remains to be done to fit simulations of the Sagittarius dwarf to the very detailed observations, our work has opened the door on a promising new way to search for dark matter forces.

[1] Kesden, M. & Kamionkowski, M., Phys. Rev. Lett. 97, 131303 (2006)
[2] Nusser, A., Gubser, S. S., & Peebles, P. J. E., Phys. Rev. D, 71, 083505 (2005)
[3] Frieman, J. A. & Gradwohl, B. A., Phys. Rev. Lett. 67, 2926 (1991)
[4] Farrar, G. & Peebles, P. J. E., Astrophys. J. 604, 1 (2004)
[5] Gubser, S. S. & Peebles, P. J. E., Phys. Rev. D 70, 123511 (2004)
[6] Majewski, S. R. et al., Astrophys. J. 599, 1082 (2003)
[7] Law, D. R., Johnston, K. V., & Majewski, S. R., Astrophys. J. 619, 807 (2005)