The Extended Tails of Palomar 5: A 10° Arc of Globular Cluster Tidal Debris

Using wide-field photometric data from the Sloan Digital Sky Survey (SDSS), we recently showed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted. Its tidal tails were initially detected in a 2.°5 wide band along the celestial equator. A new analysis of SDSS data for a larger field now reveals that the tails of Pal 5 have a much larger spatial extent and can be traced over an arc of 10° on the sky, corresponding to a projected length of 4 kpc at the distance of the cluster. The tail that trails behind the Galactic motion of the cluster fades into the field at an angular distance of 6.°5 from the cluster center but shows a pronounced density maximum between 2° and 4° from the center. The leading tail, of length 3.°5, extends down to the border of the available field and thus presumably continues beyond it. The projected width of these tails is small and almost constant (FWHM ∼ 120 pc), which implies that they form a dynamically cold and hence long-lived structure. The number of former cluster stars found in the tails adds up to about 1.2 times the number of stars in the cluster, that is, the tails are more massive than the cluster in its present state. The radial profile of stellar surface density in the tails approximately follows a power law rγ with -1.5 ≤ γ ≤ -1.2. The stream of debris from Pal 5 is significantly curved, which demonstrates its acceleration by the Galactic potential. The stream sets tight constraints on the geometry of the cluster's Galactic orbit. We conclude that the cluster is presently near the apocenter but has repeatedly undergone disk crossings in the inner part of the Galaxy leading to strong tidal shocks. Using the spatial offset between the tails and the cluster's orbit, we estimate the mean drift rate of the tidal debris and thus the mean mass-loss rate of the cluster. Our results suggest that the observed debris originates mostly from mass loss within the last 2 Gyr. The cluster is likely to be destroyed after the next disk crossing, which will happen in about 100 Myr. There is strong evidence against the suggestion that Pal 5 might be associated with the Sagittarius dwarf galaxy.