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

Abstract

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 degree 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, i.e. the tails are more massive than the cluster in its present state. The radial profile of stellar surface density in the tails follows approximately a power law r γ 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 Gyrs. 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 Sgr dwarf galaxy.

Evidence of tidal distortions and mass-loss from the old open cluster NGC 6791

“…To investigate in more detail the two-dimensional distribution of CP stars, we adopted the widely used optimal matched filter technique (see for example Kuhn et al 1996;Rockosi et al 2002;Odenkirchen et al 2001Odenkirchen et al , 2003. It has been shown (Davenport & Sandquist 2010) that this method is robust for identifying low-density stellar populations against a significant background, as in the case of OCs.…”

“…Star density distortions, elongation and tidal tails are expected to be detected also in the radial density profile (see for example Odenkirchen et al 2003;Capuzzo-Dolcetta et al 2005). Therefore as a second step of our analysis, we studied the projected density profile of NGC 6791.…”

Clumpy streams in a smooth dark halo: the case of Palomar 5

“…Then we translated the positions and velocities of the single-mass King model previously described to coincide with those of the cluster barycenter 2.95 Gyr ago, and we integrated the whole cluster 2.95 Gyr forward in time. The current position and velocity of Pal 5 are given by Odenkirchen et al (2003). In particular the position of Pal 5 in the Galaxy is (x, y, z) = (8.2, 0.2, 16.6) kpc; here x, y, and z denote righthanded Galactocentric Cartesian coordinates, where the Sun has coordinates (-8.0, 0.0, 0.0).…”

“…To this aim, we have run several N-body simulations changing both the radial, v r , and tangential, v t , velocities with respect to those of the standard orbit (we recall the reader that, for the standard orbit, v r = −44.3 km s −1 and v t = 90 km s −1 ). In particular, for the tangential velocity, we adopted the extreme values, 80 km s −1 and 110 km s −1 , beyond which the orbit fails to give a good representation of the observations, as shown by Odenkirchen et al (2003), and also two intermediate values similar to those of the standard orbit, v t = 89 km s −1 and v t = 91 km s −1 . For the radial velocity we arbitrarily varied the best fit parameter by ±2 km s −1 , since the error on this value is smaller than the one affecting the tangential velocity (Odenkirchen et al 2003).…”

“…The large spatial extension of Pal 5's tails naturally permitted study of their fine structure. Odenkirchen et al (2003) were the first to show evidence of inhomogeneities in Pal 5's streams, characterized by 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 Appendices A and B are available in electronic form at http://www.aanda.org from the cluster center. The presence of density fluctuations along the stream has been successively confirmed by Grillmair & Dionatos (2006) and Jordi & Grebel (2010), and the nature of these clumps has been long debated.…”