The Milky Way (Review): the Mass and Velocity Anisotropy of the Carina, Fornax, Sculptor and Sextans Dwarf Spheroidal Galaxies Author: Ewa L

Home , Sextans Dwarf Spheroidal

The Milky Way (review): The mass and velocity anisotropy of the Carina, Fornax, Sculptor and Sextans dwarf spheroidal galaxies Author: Ewa L. Lokas

Maarten Breddels March 16, 2009

In this article the author infers the mass and velocity anisotropy for dwarf spheroidal galaxies Carina, Fornax, Sculptor and Sextans. Using a large data set with radial velocities (Walker et al., 2009) the author claims to have determined the mass with unprecedented precision. In the letter the term accuracy is used as synonym to precision, but an accurate mass means that the inferred mass is close to the true value. This may be hard to believe due to the simplification made. The method to infer both the mass and the velocity anisotropy seems not to have a large mass-velocity anisotropy degeneracy (see Fig. 4). This may be explained by the assumed mass density profile, which is assumed to follow the light distribution. In addition to solving the Jeans equation (2nd moment), the 4th moment (kurtosis) is also used (Merrifield and Kent, 1990). This does not however seem to have a significant effect on the precision of the determination of the two parameters, although it changes the velocity anisotropy and mass for Sculptor and Sextans slightly. Apparently, the biggest difference between the masses obtained in this letter compared to for instance the results from Walker et al. (2007) and Battaglia et al. (2008) seems to be the interloper removal method. Interlopers can con- tribute to the velocity dispersion and can therefore increase the inferred mass of the galaxy. One source of interlopers may come from our Milky Way halo. At larger radius, the density of stars of the dwarf galaxy becomes low that the Milky Way contamination can be significant. According to Klimentowski et al. (2008) the tidal tails of dwarf galaxies point in the direction of the Milky Way, i.e. along the line of sight. This second source of interlopers from the dwarf galaxy itself can also increase the velocity dispersion. This has the same effect as the Milky Way contamination, it increases the inferred mass of the dwarf galaxy. Since the tidal tails are directed towards and away from the line of sight, the tidal effect does not show up in photometry. To remove the interlopers from the sample, the authors use a similar method as used on galaxy clusters (den Hartog and Katgert, 1996), where samples are removed from the total set based on velocities. The circular velocity and the

1 velocity for a star falling freely into the galaxy potential is calculated. If a star at radius r exceeds this velocity, it is removed. This procedure removes the most unbound stars from the galaxies, and thus remove the interloper originating from the tidal tails. Using the kurtosis to lift the mass velocity-anisotropy looked promising in Lokas (2002). However, in this letter the degeneracy was already removed by assuming mass (dark and baryonic) follows light. In the case of this letter, adding the kurtosis to the fitting procedure did not seem to have a large effect on the precision on determining the two parameters. By restricting the unknown dark matter distribution to follow the light distribution, it does not seem fair to claim an accurate mass determination. It would be interesting to see what the masses would be using the method in the earlier method of Lokas (Lokas, 2002). Besides the method to fit the mass and velocity-anisotropy, the interloper removal method seems to have a large effect on the total mass of the galaxy. In the case of Sculptor, it can mean that the estimated mass is at least one order of magnitude smaller if the stars from the tidal tails are removed. Since the chemical composition of these stars are not likely to be different from the rest of the stellar population, abundance information will not help. Since precise distances to these stars will not be available in the near future to get a better 3d picture of these galaxies and their tidal tails, we have to trust (or not) the models and simulations.

References

G. Battaglia, A. Helmi, E. Tolstoy, M. Irwin, V. Hill, and P. Jablonka. The Kinematic Status and Mass Content of the Sculptor Dwarf Spheroidal Galaxy. ApJ, 681:L13–L16, July 2008. doi: 10.1086/590179. R. den Hartog and P. Katgert. On the dynamics of the cores of galaxy clusters. MNRAS, 279:349–388, March 1996. J. Klimentowski, E. L. Lokas, S. Kazantzidis, L. Mayer, G. A. Mamon, and F. Prada. Tidal evolution of a disky dwarf galaxy in the Milky Way potential: the formation of a dwarf spheroidal. ArXiv e-prints, March 2008.

E. L.Lokas. Dark matter distribution in dwarf spheroidal galaxies. MNRAS, 333:697–708, July 2002. doi: 10.1046/j.1365-8711.2002.05457.x. M. R. Merriﬁeld and S. M. Kent. Fourth moments and the dynamics of spherical systems. AJ, 99:1548–1557, May 1990. doi: 10.1086/115438. M. G. Walker, M. Mateo, E. W. Olszewski, O. Y. Gnedin, X. Wang, B. Sen, and M. Woodroofe. Velocity Dispersion Proﬁles of Seven Dwarf Spheroidal Galaxies. ApJ, 667:L53–L56, September 2007. doi: 10.1086/521998. M. G. Walker, M. Mateo, and E. W. Olszewski. Stellar Velocities in the Ca- rina, Fornax, Sculptor, and Sextans dSph Galaxies: Data From the Magel- lan/MMFS Survey. AJ, 137:3100–3108, February 2009. doi: 10.1088/0004- 6256/137/2/3100.