1. Pete Kuzma PhD student, Research School of Astronomy and Astrophysics
Beyond the Tidal Radius: Outer Envelope Structure In Massive Globular Clusters
Pete Kuzma
PhD student, Research School of Astronomy and Astrophysics

2. Simulation of a tidally disrupted dwarf galaxy (K. Johnston 2014)

3. Field Of Streams
V. Belokurov, SDSS
The Sagittarius stream is the real-world example of dwarf galaxy accretion in the Milky Way. Several globular clusters such as Terzan 8, M54 and Arp 2 are linked to Sagittarius.

4. M31 halo structure from the Pan Andromeda Archeological Survey (PAndAS) (McConnachie et al. 2009, Mackey et al. 2010, 2016)

5. Searching For Debris
We have targeted GCs that show anomalies that suggest an extra-Galactic origin to search for signs of a stream or tidal debris that might provide insight into their origins and the build up of the Galactic halo.

6. The Targets NGC 7089 (M2) NGC 1851 NGC 5824 Distance: 11.5 kpc
MV: -9.0 mag
NGC 1851
Distance: 12.1 kpc
MV: -8.3 mag
NGC 5824
Distance: 32.1 kpc
MV: -8.9 mag
Internal iron dispersion (e.g., Yong et al. 2014)
Multiple stellar populations (e.g., Milone et al. 2015)
Possible extra tidal structure (Grillmair et al. 1995)
Internal Iron dispersion (e.g., Carretta et al. 2010)
Multiple sub-giant branch populations (e.g., Milone et al. 2008)
Known to possess an envelope (Olzewski et al. 2009)
Internal iron dispersion (e.g., Da Costa et al. 2012)
Hinted to have extra tidal stars (Grillmair et al. 1995)
Possible connection to the Cetus Polar Stream (Newberg et al. 2009)

7. CMD of M2 calibrated to SDSS photometry. Left: MegaCam, entire 0.8 x 0.8 degree field of view. right: DECam, all stars within 40 arcmin.
Isochrone weighting scheme. Left: MegaCam, right: DECam
(Dotter et al. 2008)

8. M2 2D surface density map (background subtracted) smoothed with a Gaussian kernal of width 36’’ x 36’’ – We have uncovered an envelope that extends to a distance of ~60 arcmin (~210 pc), almost 5 times the size of the tidal radius (12.5 arcmin). It contains ~1.6 % of the total mass of the system. (Kuzma et al. 2016)

9. Azimuthally averaged radial profile of M2, outer parts follow a profile fit a power law with index ϒ = -2.2 ± 0.2.

10. 2D Distribution – NGC1851
Left : CMD within 60 arcmin. Right: 2D background subtracted distribution of NGC We trace the envelope to ~60 arcmin (~211 pc, tidal radius: 6.7 arcmin, distance from Sun: 12.1 kpc from Harris 1995).

11. 2D Distribution – NGC5824
Left : CMD within 40 arcmin. Right: 2D background subtracted distribution of NGC We trace the envelope to at least ~20 arcmin (~185 pc, tidal radius: 5.7 arcmin, distance from sun: 32.1 kpc from Harris 1995).

12. What exactly are we looking at here?
Is the cluster/envelope system a product of natural dynamical evolution of globular clusters? Is the cluster/envelope system the last vestige of accreted dwarf galaxies of which these clusters were the nucleus?

13. Cluster dynamical evolution?
Globular clusters can suffer dynamical shocks as a result of passage through the disk of the Milky Way.
Modeling shows that disk shocks leads to tidal tails (e.g. Kupper et al. 2010a), which we do not see.
Modeling also shows that a envelope can form, but probably it does not extend far beyond the cluster boundary (e.g., Renaud et al. 2011).
Kupper et al. (2010a) noted that beyond the cluster boundary, density profiles decline with a power law of index ϒ = -4 to -5.
Palomar 5 and its tidal tails.
Odenkirchen et al. 2001

14. Dwarf galaxy accretion?
Simulations performed by Bekki and Yong (2012) have demonstrated that a diffuse envelope can indeed form around the compact nucleus of a dwarf galaxy after the original host has been largely stripped away by tidal forces.
M2, NGC 1851 and NGC 5824 have very similar properties to other clusters suggested to be dwarf galaxy nuclei (ω Cen, M54).
The apparent radii of the envelopes is similar to the half-light radii of many typical dwarf galaxies in the Local Group (McConnachie 2012).

15. Concluding Comments
M2, NGC 1851 and NGC 5824 have been targeted for wide-field photometry due to their anomalous properties.
All show apparent large extended envelopes of roughly the same size, which do not show narrow 2-arm symmetric structure expected with tidal tails.

16. Concluding Comments
We are potentially seeing the remains of disrupted dwarf galaxies that these clusters were originally embedded.
Additional studies required to verify the frequency of extended envelopes around massive globular clusters.

17. Extra slides

18. Radial Profiles – N1851 and N5824
Early results!
Early analysis!

19. Arrangement of MegaCam observations (left) and DECam observations (right) of M2.

20. Completeness for M2 based on each camera.

21. Cleaning of unwanted detections from Source Extractor.

22. M2 MegaCam 2D Distribution. Inner 10 arcmin removed.

23. Pipeline overview Run Source Extractor on the g- and i- band images.
Catalogs cleaned through internal Source Extractor flags, and difference in magnitude measured through different aperture sizes. Cross matched by Stilts.
Photometry is calibrated and extinction corrected across all fields to create a uniform photometric catalogue.
Artificial star tests were performed to check the possibility of any over densities detected could be related to varying completeness levels across the fields.
Presenting DECam data for M2