Streams in the Milky Way halo: the case of Palomar 5

» Thursday 25 October 2012

An international team of astronomers, including two scientists from GEPI, has studied the formation
and characteristics of the tidal tails surrounding the globular cluster
Palomar 5 along its orbit in the Milky Way potential, by means of N-body
simulations run on GPUs. This work reproduces, for the first time, the clumpy nature of Palomar 5’streams and unveil the mechanism which determines these inhomogeneities.

At a distance of 23.2 kpc from the Sun and of 16.6 kpc above the Galactic plane, Palomar 5 is a halo globular cluster characterized by very low total mass, a large core radius, and low central concentration. In the past decade, a number of observational studies have shown there are two massive tidal tails emanating from opposite sides of the cluster, made of stars escaped from the system owing to the action of the Galactic tidal field. With an overall detected extension of 22 degrees on the sky, corresponding to a projected spatial length of more than 10 kpc, Pal 5’s tails are so elongated and massive that they contain more stellar mass than the one currently estimated to be in the system itself.

Pal 5’s tail are inhomogeneous, showing evidences of stellar density gaps
(underdense regions) and clumps (overdense regions), particularly visible
in the trailing stream, with the most evident and massive overdensities
found between 100 and 120 arcmin from the cluster center.
The nature of these substructures has long been debated, with several
works proposing that the observed clumps may be due to the gravitational
effect of dark matter subhalos in the outskirts of the Milky Way.

Fig. 1: Map of the surface density of stars that are photometrically concordant with the stellar population of Pal5, as found in SDSS data (from Odenkirchen et al. 2003, AJ, 126, 2385).

By means of direct N-body simulations, run on the GPU cluster "Momentum"
at the Paris Observatory, and simplified numerical models, the team studied the formation and characteristics of the tidal tails around Palomar 5, along its orbit in the Milky Way potential. Unlike previous findings, their work is able to reproduce the substructures observed in the stellar streams of this cluster, without including any lumpiness in the dark matter halo. In
these models, overdensities similar to those observed in Palomar 5 can be reproduced by the epicyclic motion of stars along its tails, i.e. a simple local accumulation of orbits of stars that escaped from the cluster with very similar positions and velocities. In particular, the N-body simulation predicts two prominent density enhancements in the trailing tail of the cluster, the first located very close to the cluster (RA < 230 deg) and the second starting at RA=230.5 deg and ending at RA=232 deg.
The second enhancement shows an underdense region, whose extension is about 0.5deg. These characteristics are found in the observed stream at the same location (see Figs.1-2).

Fig 2 : Trailing tail of Palomar 5, as it results from N-body simulations. Stellar inhomogeneities are evident along the stream (Mastrobuono-Battisti, A., et al. 2012, A&A Letters).

Unlike previous works, these models can reproduce the density contrast
the models reproduce between the clumps and the surrounding tails found in the observed streams, without including any lumpiness in the dark halo, suggesting new upper limits on its granularity.

The location of the clumps along the tails, which depends on the orbital parameters of the cluster, may also help to better constrain Pal5’s orbit in the Milky Way.

This work was carried out under the HPC-EUROPA2 project with the support of the European Commission Capacities Area-Research Infrastructures Initiative. M. Montuori acknowledges financial support of the Observatoire de Paris, through a Temporary position as Associated Astronomer in April 2012.

Reference :
"Clumpy streams in a smooth dark halo: the case of Palomar 5",
Mastrobuono-Battisti, A., Di Matteo, P., Montuori, M., Haywood, M. 2012,
A&ALetters, 546, L7