Probing Gravity in the Low Acceleration Regime with Globular Clusters

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1 Probing Gravity in the Low Acceleration Regime with Globular Clusters By Riccardo Scarpa Instituto de astrofisica de Canarias Gran Telescopio Canarias

2 The idea for this study sparked from the desire of differentiate MOND from dark matter. What we need is to detect MOND effects in a structure that does not contain dark matter

3 There is one astrophysical option: Globular Clusters GCs are believed to be free from large amount of Dark Matter for various reasons: dark matter, being non-collisional, does not form compact structures. Luminous and dynamical masses of GCs are in good agreement. Tidal stripping should effectively remove any possible darkmatter halo surrounding GCs. At most, GCs are filled by the dark matter of the galactic halo. Dynamically negligible for such small structures.

4 Far enough from GC center we probe MOND The black circle Indicates the radius where the internal acceleration of gravity is a0. Stars surrounded by a red circle are cluster members

5 What should we expect in GC below a0? Look at pressure supported structure: Elliptical galaxies velocity dispersion profile is flat at large radii NGC 1600 Pu et al A&A in press (Astro-ph ) NGC 4895

6 ω Centauri pilot study Our UVES data (black diamonds) cm s Radial velocities from Meylan et al and Meylan & Mayor 1986 (crosses) Proper motion data for thousands of stars from van Leeuwen et al Scarpa, Marconi, & Gilmozzi 2003

7 All globular clusters behave the same way MOND or Tidal Heating? Scarpa et al. 2007, the messenger 128, 41

8 ω Centauri revisited: Scarpa and Falomo 2010 (Astro-ph ) New radial velocity data from Sollima et al. 2009, MNRAS 396, 2183 Inner sample 628 stars (red dots). Outer sample 318 stars (blue dots) with 98 stars within 20<r<45 pc. Data from Scarpa et al 2003 (black dots): 75 stars within 20<r<30 pc. Almost double the data for r>r0

9 ω Centauri revisited: agreement with MOND improved. The flattenig occurs at 32±3 pc, in better agreement with MOND Prediction: Assuming M/L=1 a = a0 at r = 22 arcmin = 36 pc Scarpa and Falomo 2010 A&A in press. (Astro-ph ) a=1.2x10-8

10 Moderately distant globular clusters: NGC 1851 and 1904 Scarpa et al. 2010, A&A submitted New data collected at ESO VLT with FLAMES. Spectral resolution R~ NGC 1851 located at 16.7 kpc from Milky Way NGC 1904 located at 18.8 kpc from Milky Way Twice as distant than clusters studied so far. Tidal stress one order of magnitude weaker..

11 NGC stars selected for observations 184 stars with radial velocity accuracy better than 1.2 km/s MOND radius at 4.1 arcmin (14.5 pc) assuming M/L=1

12 NGC 1851 velocity dispersion profile The cluster is NOT rotating (max rotation 0.8 km/s consistent with 0) Flattening at r= 12.5±2.5 pc, asymptotic dispersion 4±0.5 km/s For M/L=1, acceleration is a0 at 14.5 pc

13 NGC stars selected for observations 146 stars with radial velocity accuracy better than 1.0 km/s MOND radius 3.1 arcmin (11.7 pc) assuming M/L=1

14 NGC 1904 velocity dispersion profile The cluster is NOT rotating, max rotation in the outer region of 0.4 km/s consistent with 0. Flattening at r= 12±2 pc, asymptotic dispersion 2.25±0.4 km/s For M/L=1, acceleration is a0 at 11.7 pc

15 Summary of results Name RMW Kpc Mass rtidal r0 104 Msun pc pc rflat pc km/s a at rflat 10 8 cm/s2 NGC ±2 4.0± ±0.4 NGC1904 (M79) ±2 2.2± ±0.3 NGC5139 (ωcen) ±3 7.0± ±0.4 NGC6171 (M107) ±2 2.7± ±0.6 NGC6341 (M92) ±2 3.1± ±0.6 NGC7078 (M15) ±2 3.2± ±0.4 NGC7099 (M30) ±2 2.2± ± Columns meaning. 2) Distance from Milky Way center. 3) Cluster mass from luminosity assuming M/L=1 in solar units. 4) Radius where the acceleration is a0= cm/s2. 5) Cluster tidal radius from Harris ) Radius where velocity dispersion flattens out. 7) Asyntotic velocity dispersion. 8) Acceleration at rflat.

16 Summary of results All profiles combined together to increase signal strength: Distances in units of the MOND radius r0, dispersion in units of the dispersion at r0. In this units according to what observed in elliptical galaxies the dispersion should be constant and equal to 1. Best fit of 16 points at r/r0>1.2 Constant dispersion hypothesis: χ2= 12.0 for 15 d.o.f., 67% probability, fully acceptable. Keplerian falloff hypotesis: χ2= 26.9 for 15 d.o.f. Rejected at 97% confidence level.

17 Acceleration where flattening occurs Indipendent from cluster distance from Milky Way center Indipendent from cluster total mass First conclusion: A strong similarity exists between High surface brightness elliptical galaxies and globular cluster

18 LSB galaxies have flat velocity dispersion profiles Carina Wilkinson et al. 2006

19 Another example of LSB Fornax Mateo 1997

20 Leo II dwarf spheroidal D=233 kpc, M=2.7x107 up to 0.4 kpc, M/L~37±10 Koch et al 2007 ApJ 134, 566

21 What should we expect for LSB globular cluster: Dark Matter: Nothing like that Or MOND: Exactly the same

22 NGC 288: a low-concentration cluster At 11.6 kpc from Milky Way Round, no tidal deformations Central µ~20 mag/asec2 126 velocities with accuracy better that 1 km/s Only 2 non-members

23 NGC 288: a < a0 everywhere Surface brightness profile from Pryor et al The solid line is a Plummer profile with total mass Msolar and M/L=1 This corresponds to the total luminosity of NGC288

24 NGC 288: flat dispersion profile

25 Conclusions: Eight clusters studied, in all cases velocity dispersion profile different from what Newtonian dynamics would suggest. Seven are highly concentrated and behave like HSB elliptical galaxies One is poorly concentrated, and behaves like LSB elliptical galaxies. Evidences mounting against Newtonian dynamics below a0

26 One important implication for MOND All these clusters experience a TOTAL gravitational acceleration above a0 and therefore no MOND effects should be detected, against observations. This is in formal disagreement with the usual formulation of MOND.

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