Globular Cluster and Satellite Orbits: 2008 Status

Dana Casetti-Dinescu - Wesleyan and Yale

The Yale Southern Observatory Team:

Bill van Altena, Terry Girard, Dana Casetti-Dinescu, Kathy Vieira, Carlos Lopez, Elliott Horch, David Herrera, Danillo Castillo

External Collaborators:

Steve Majewski, Young-Wook Lee, Jeff Carlin, Mike Siegel, David Martinez-Delgado, George Wallerstein

SPM's Trademark

PV camera

Photographic plate

Proper Motions, Velocities and Uncertainties

Current formal errors in absolute proper motions of stellar systems:

Ground based: between 0.1 and ~1.5 mas/yr

Space based (HST): between 0.04 and 0.5 mas/yr

Status of Observations

• To date, 53 of ~ 150 GGC have measured absolute proper motions, with formal errors between 0.1 and 2.0 mas/yr; mean value ~ 0.5 mas/yr.

• Of the ones measured, 25 were measured by the Southern Proper-Motion Program (SPM). Only 20 clusters of 53 have two or more determinations.

• HST-based measurements: 2 clusters w.r.t. background galaxies and QSOs, and 2 w.r.t. bulge stars.

• Measured clusters are within 30 kpc from the Galactic center (only NGC 7006 is at ~ 40 kpc).

Milky Way’s Globular Clusters

Status of Observations: Spatial Distribution

SPM limitations for GCs: south of -20o and within 10-15 kpc from the Sun

Status of Observations: Spatial Distribution

The best measurement: M 4 (NGC 6121)

(mas/yr) (mas/yr)

-12.26 (0.54) -18.95 (0.54) Kalirai et al. 2004 - HST, ~12 galaxies

-13.21 (0.35) -19.28 (0.35) Bedin et al. 2003 - HST, 1 QSO

-12.50 (0.36) -19.92 (0.49) Dinescu et al. 1999 - SPM, ~100 Hipparcos stars

σ ∼ 0.5 mas/yr

Status of Observations: Uncertainties

NGC D (kpc)

Δµ(mas/yr)

ΔV (km/s)

5904 (M5) 7.5 4 142

6205 (M13) 7.7 2.5 91

7078 (M15) 10.3 6 293

7089 (M2) 11.5 1.5 82

Uncertain measurements

Kalirai et al. 2004

SPM faint limit

Hipparcos s faint limit for special targets

CMD: Kanatas et al. 1995

M 4: the nearest globular cluster (1.5 kpc)

Clusters with very extended blue horizontal branches (EHBs) appear to have multiple main sequences, subgiant branches.

NGC 2808

NGC 1851

Piotto 2007

Recent Developments from HST Photometry

NGC 2808: EHB-strong

Castellani et al. 2006

NGC 4833: EHB-moderateNGC 1851: bimodal

Melbourne et al. 2000 Walker 1992

From 94 clusters with HB classification (Lee et al. 2007):

14 EHB-strong - 10 measured (excluding M 54, Sgr’s cluster)

7 EHB-moderate - 4 measured

5 Bimodal HB - 3 measured

- There are 26 EHB clusters that make up 53% of the total mass of the GC system.

- 65% of the EHBs (58%, excluding 2 poor measurements*) have orbits measured.

Lee et al. 2007

* M 15 (NGC 7078) and M 5 (NGC 5904)

The Luminosity/Mass Distribution of EHB Globular Clusters

Globular Cluster Results: Velocities

Sample N Π Θ W σΠ σΘ σW

EHB strong 10 31 (39)

63 (36)

-62 (40)

124 112 126

EHB all 17 21 (35)

56 (25)

-75 (28)

143 104 114

EHB all - 2 poor det.* 15 58 (26)

49 (27)

-60 (29)

100 106 114

Other & [Fe/H] < -1.0 29 23 (26)

23 (24)

14 (21)

142 129 114

Average velocities and dispersions (km/s)

* M 15 (NGC 7078) and M 5 (NGC 5904)

EHB clusters :17

Everything else:

29 with [Fe/H] < -1

7 with [Fe/H] > -1.0 (star symbols)

Velocity Structure: Data

The 17 EHB clusters are assigned randomly selected velocities from a halo-like velocity ellipsoid. Averages are always consistent with 0 within 1-sigma uncertainty.

σΠ = 138 km/s

σΘ = 104 km/s

σW = 111 km/s

Velocity Structure: Randomly Assigned Velocities

Prieto and Gnedin 2007

All clusters within 30 kpc from the main galaxy’s center.

Same as above, and separated by original host system.

• Single episode cluster formation at z=4; designed to model the metal-poor cluster system.

• Uncertainties: 10% in distance, 0.4 mas/yr in each proper-motion component, and 1 km/s in radial velocity.

Velocity Structure: Prieto & Gnedin Model

Integrals of Motion

Helmi and de Zeeuw 2006

EHB clusters

Everything else +

Sgr clusters

Integrals of Motion

Helmi and de Zeeuw 2006

EHB clusters

Everything else +

Sgr clusters

Milky Way’s Globular Clusters: Summary of the Kinematical Data

The EHB cluster system (also comprising the most massive clusters) shows mean velocity components significantly different from 0, as well as correlations between velocity components. This is determined from the data comprising 58% of the clusters in this system. The non-EHB, metal-poor clusters do not show these velocity trends.

This result is best interpreted if the EHB system is comprised of clusters formed in a few satellite galaxies that were accreted by our Galaxy and some of the original phase-space structure is preserved in their overall kinematics. (Note: most EHBs are within 20 kpc from the Galactic center.)

Milky Way’s Current Satellite Galaxies

The Stromlo Missing Satellite Survey: 23 known satellites within 250 kpc

The Shape and Size of Satellite’s Orbits: How Have Satellite Galaxies Formed?

1) Is the orbit distribution of satellites consistent with predictions from ΛCDM simulations?

2) Are their orbits highly eccentric and thus their internal velocity dispersion can be explained solely via tidal effects?

3) Are their orbital angular momenta consistent with the spatial planar alignments? (eg Kroupa et al. 2005).

• Absolute proper-motion measurements exist for 7 satellites out of some 20 known, with formal errors between 0.04 and 0.25 mas/yr; 5 of these have 2 or more measurements: ground-based and HST-based. SPM contributed with the measurement of Sgr.

• Ground-based: typically use many galaxies + a few QSOs, ~20 to 50 years time baselines.

• HST-based: typically include 2-3 QSOs and 2-3 years time baselines. Except for the MCs, the HST measurements sample few (2-3) rather small areas in a galaxy.

Status of Observations

Galaxy µα cos δ

(mas/yr)

µδ

(mas/yr)

eccentricity

Reference

LMC

1.9 σ

1.68 (0.16) 0.34 (0.16) 0.48 (0.09) Van der Marel et al. 2003

2.03 (0.08) 0.44 (0.05) 0.74 (0.06) K2006a (ACS)

SMC

1.8 σ

0.92 (0.20) -0.69 (0.20) 0.21 (0.13) Irwin et al. 1999

1.16 (0.18) -1.17 (0.18) 0.53 (0.14) K2006b (ACS)

Sculptor

2.5 σ

0.72 (0.22) -0.06 (0.25) 0.35 (0.11) Schweitzer et al. 1995

0.09 (0.13) 0.02 (0.13) 0.30 (0.06) Piatek et al. 2006, w. CTI correction

UMi

2.9 σ

0.06 (0.08) 0.07 (0.10) 0.34 (0.02) Schweitzer et al. 1997

-0.50 (0.17) 0.22 (0.16) 0.40 (0.10) Piatek et al. 2005 w. CTI correction

Fornax

1.6 σ

0.59 (0.16) -0.15 (0.16) 0.27 (0.12) Dinescu et al. 2004

0.49 (0.13) -0.59 (0.13) 0.52 (0.12) Piatek et al. 2002, before CTI correction

0.47 (0.05) -0.36 (0.04) 0.12 (0.06) Piatek et al. 2007, w. CTI correction

GB

HST

Largest discrepancy

¢ Piatek et al. 2002 (before CTI correction)

=> ecc = 0.52 (0.12)

¢ Dinescu et al. 2004

=> ecc = 0.27 (0.12)

¢ Piatek et al. 2006

=> ecc = 0.12 (0.06)

Fornax - 140 kpc

Milky Way’s Dwarf Satellites: Summary

The recent HST results appear to determine larger size proper motions than ground-based determinations. While this may not necessarily be incorrect, it underlines the challenges in these type of measurements, and the fact that these measurements are not yet secure.