GLOBULAR CLUSTERS IN THE OUTER GALACTIC HALO: NEW HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS IMAGING OF SIX GLOBULAR CLUSTERS AND THE GALACTIC GLOBULAR CLUSTER AGE–METALLICITY RELATION

By Aaron Dotter, A. Sarajedini, Jay Anderson. 2011

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Color–magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST) Advanced Camera for Surveys F606W, F814W photometry of six globular clusters (GCs) are presented. The six GCs form two loose groupings in Galactocentric distance (RGC): IC 4499, NGC 6426, and Ruprecht 106 at ∼15–20 kpc and NGC 7006, Palomar 15, and Pyxis at ∼40 kpc. The CMDs allow the ages to be estimated from the main-sequence turnoff in every case. In addition, the age of Palomar 5 (RGC ∼ 18 kpc) is estimated using archival HST Wide Field Planetary Camera 2 V, I photometry. The age analysis reveals the following: IC 4499, Ruprecht 106, and Pyxis are 1–2 Gyr younger than inner halo GCs with similar metallicities; NGC 7006 and Palomar 5 are marginally younger than their inner halo counterparts; NGC 6426 and Palomar 15, the two most metal-poor GCs in the sample, are coeval with all the other metal-poor GCs within the uncertainties. Combined with our previous efforts, the current sample provides strong evidence that the Galactic GC age–metallicity relation consists of two distinct branches. One suggests a rapid chemical enrichment in the inner Galaxy while the other suggests prolonged GC formation in the outer halo. The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/47a1e3ebbec04b1208ab9b72da1ba94986c52ae8

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MESA ISOCHRONES AND STELLAR TRACKS (MIST). I. SOLAR-SCALED MODELS (2016, methodology): “Ruprecht 106 is a relatively low-mass (M∼ 104.8 M ) globular cluster with a metallicity of [Fe/H]∼ −1.5 (Kaluzny et al. 1995; Brown et al. 1997; Francois et al. 1997; Dotter et al. 2011; Villanova et al. 2013).”
A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters (2018, methodology): “It is located in the Galactic anticenter direction, at a relatively low Galactic latitude, (l = 19◦, b = 24◦), which results in a significantly reddened CMD. Dotter et al. (2011) used imaging from HST/ACS to derive an age of 13±1.5 Gyr.”
The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, methodology): “Forbes & Bridges (2010) Dotter et al. (2010, 2011) VandenBerg et al. (2013) Mean Name MV log M τ [Fe/H] τ [Fe/H] τ [Fe/H] τ [Fe/H] [mag] [M ] [Gyr] [Gyr] [Gyr] [Gyr] NGC104 −9.42 6.00 13.06 ± 0.90 −0.78 12.75 ± 0.50 −0.70 11.75 ± 1.03 −0.76 12.52 ± 0.49 −0.75 NGC288 −6.75 4.93 10.62 ± 0.51 −1.14…”
The Updated BaSTI Stellar Evolution Models and Isochrones. I. Solar-scaled Calculations (2018, methodology): “Figure 28 shows the cluster optical CMD in the HST /ACS camera photometric system (Dotter et al. 2011), together with isochrones for [Fe/H]=−1.55 –close to the mean value [Fe/H]=−1.47±0.02 determined spectroscopically by Villanova et al. (2013) – t=12.5 Gyr (without atomic diffusion) and t=11.5 Gyr…”
THE SHAPE OF THE INNER MILKY WAY HALO FROM OBSERVATIONS OF THE PAL 5 AND GD–1 STELLAR STREAMS (2016, methodology): “Measurements of the distance to the Pal 5 cluster range from 20.9 kpc to 23.2 kpc (Harris 1996; Vivas & Zinn 2006; Dotter et al. 2011) and we conservatively assume a flat prior on the distance between 20 kpc and 24 kpc.”
THE RELATIONSHIPS AMONG COMPACT STELLAR SYSTEMS: A FRESH VIEW OF ULTRACOMPACT DWARFS (2011, background): “Such distinctions are reminiscent of the inner and outer halo GCs of the Milky Way (e.g., Forbes & Bridges 2010; Dotter et al. 2011), and suggest that there may be generically two populations of UCDs: one associated with massive (and therefore metal-rich) bulge formation, and another with dwarf…”