1. J. Cohen, Concepcion, Feb 2006
Part 1 – the very young GCs in M31,
already published: ApJL (Nov 2005)
Part 2 – integrated light photometry of Galactic GCs (ready for publication summer 2006 ?)

2. Globular clusters in M31 with LGSAO Judy Cohen, Keith Matthews and Brian Cameron

3. M31 Globular Sample Slide
Globular cluster B232 in M31, Keck LGS AO, Aug 10, 2005, K' filter, 1,650 second total exposure, image size is approximately 70
milliarcseconds in diameter. The field shown here is approximately five arcseconds on a side.
Credit: J. Cohen, K. Matthews, B. Cameron (Caltech) +
Keck LGS-AO team.

4. HST Comparison HST Keck-LGSAO
Each field is approximately 10 arcseconds on a side. A best attempt was made to use a similar contrast stretch for each image.

5. HST Comparison
HST
Keck LGSAO

7. Age Distribution of Milky Way GCs from De Angelo et al (2005), most are 12 Gyr old, with a few as young as 9 Gyr (most of these are associated with the Sgr dwarf galaxy), and none younger

10. LGSAO Images of the 6 youngest M31 GCs with suitable tip/tilt stars

12. M31 B223, 25x1200 sec, K’ filter, calibrated via WIRC/P200 onto 2MASS Ks point source catalog. K 23.3 (isolated star) easily detected.

13. M31 GC B232 CMD: very rough F814W (arbitrary zero point) from 400 sec WFPC2/HST image vs. K’ from Keck LGS/AO. HST frame lacks spatial res. to match Keck and limits the number of stars that can be used. K’ of brightest apparent cluster members matches expected level of RGB tip.

14. Summary
It appears that most of the very young GCs in M31 are spurious – they are asterisms.
It may be that some/all of the M31 GCs with ages ~5 Gyr are OK.
Beware of this problem in more distant spirals. Contamination of GC samples with asterisms will be much worse than in M31.
LGSAO works, and opens up much more of the high galactic latitude sky. Our images have core FWHM of ~70 mas.
Paper in ApJL (Nov 2005): “To Be or Not To Be: Very Young GCs in M31, Cohen, Matthews and Cameron

15. Integrated light photometry of galactic GCs J. Cohen, S. Hsieh, S
Integrated light photometry of galactic GCs J.Cohen, S.Hsieh, S.Metchev, G.Djorgovski
ACMM 1978, JK photometry for 33 galactic GCs
Data never published, Marc Aaronson killed in tragic accident
2MASS allows us to do many more GCs in JHK
Mosaic 2MASS images, mask out stars too bright to be in the GC outside the core, find the sky level.
Create surface brightness profiles, fit King profiles (rtidal from optical profile)
Match onto V profile (Harris on line, mostly from Trager, King & Djorgovski 1995)
Adopt [Fe/H], E(B-V) from Harris on-line database
We can do about 100 Galactic GCs in JHK(2MASS)

16. V-K (2MASS) dereddened (ALL PLOTS ARE PRELIMINARY !)

17. V-K(2MASS/2006 and ACMM/1978) (both dereddened)

18. V-K(2MASS + SSP Models)

19. J-K(2MASS) (dereddened)

20. J-K (2MASS/2006 and ACMM/1978) (dereddened)

21. J-K/2MASS + SSP Models

22. Comparison ACMM/1978 vs. 2MASS/2006, observed colors

23. Current Status 2MASS V-K and J-K
V-K: ACMM/1978 & 2MASS agree ~OK, models ~ OK.
J-K: a small systematic trend, may be related to uncertainties in the transformations between the 2MASS and 1978 photometric systems.
For J-K, the SSP models don’t fit metal-poor end well.
Total range of J-K is smaller, details matter.
ACMM did extremely well – smaller scatter than current preliminary 2MASS results.
Our colors for core collapsed clusters require more work, they are not being handled properly at present.
Hopefully additional work will reduce the scatter somewhat for the GCs with the lowest E(B-V) and with reasonable SNR.