A Deficit of Dark Matter in Elliptical Galaxies

The 'missing mass problem' is a long-standing issue in astrophysics, being present in galaxies, cluster of galaxies and even at the cosmological scale. Astronomers from Taiwan have used archival data from PN.S and SAURON to study the internal dynamics of seven nearby elliptical galaxies, and report finding a dearth of dark matter. They conclude that the dynamics of these galaxies are well explained by MOdified Newtonian Dynamics (MOND).

In the dark matter halo scenario, one expects that close to the centre of a galaxy ordinary matter dominates dark matter, while at large distances from the centre dark matter becomes dominant. Thus measurement of the dynamics at the outskirts of galaxies is crucial to the dark matter scenario. As an alternative to the dark matter scenario, MOND predicts the dynamics at the outskirts solely from the ordinary, luminous matter of the galaxy.

Elliptical galaxies commonly have their stars concentrated at the centre, and gas content is low. This makes the study of dynamics by stars at large distance from the centre very difficult. Due to their strong emission lines, Planetary Nebulae (PNe) are potentially good probes for studying the kinematics and dynamics at the outskirts of elliptical galaxies. The Planetary Nebulae Spectrograph (PN.S) on the William Herschel Telescope (WHT) can detect and measure the velocity of PNe, and in recent years has been obtaining velocity measurements of several hundreds of PNe in several nearby galaxies (see Figure 1).

The first results of the analysis of the observations were reported by Romanowsky et al. in 2003. They found a 'dearth' of dark matter in three elliptical galaxies and that the data are modelled well by Newtonian dynamics. However, the 'lack of dark matter' in these galaxies can be explained by another view of the 'missing mass problem' — MOND.

The 'missing mass problem' is, in fact, the mismatch between the measured gravitational acceleration and the inferred Newtonian gravitational acceleration produced by the observed luminous matter of the system. The mismatch can be accounted for by the existence of non-luminous matter ('dark matter'), a modified law of inertia or a modified theory of gravity. The latter was proposed in the 80s as a modification of Newton's second law when the acceleration is smaller than a small constant, a₀=1.2x10^-10 m/s².

Milgrom & Sanders (2003) showed that the luminous elliptical galaxies reported by Romanowsky et al. can be well explained by MOND. They pointed out that in MOND the acceleration discrepancy is small in these systems.

With refined stellar and planetary nebula velocity measurements obtained using the SAURON integral-field spectrograph and PN.S on the WHT, the Romanowsky sample could be enlarged to seven elliptical galaxies.

Tian and Ko (2016) find that, again, all galaxies in the new sample have a deficit of dark matter. The data can be fit by adding a singular isothermal dark matter halo but the required amount of dark matter does not dominate the mass in the halos, contrary to expectation. They show, however, that MOND naturally explains the dynamics of these seven galaxies out to six effective radii.

More information:

Milgrom M., Sanders R. H., 2003, "Modified Newtonian Dynamics and the ``Dearth of Dark Matter in Ordinary Elliptical Galaxies", ApJ, 599, L25 [ ADS ].
Romanowsky et al., 2003, "A Dearth of Dark Matter in Ordinary Elliptical Galaxies", Science, 301, 1696 [ ADS ].
Yong Tian, and Chung-Ming Ko, 2016, "Dynamics of elliptical galaxies with planetary nebulae in modified Newtonian dynamics", MNRAS, 462, 1092 [ ADS ].
Planetary Nebula Spectrograph (PN.S) web site.
SAURON web site.