Spectroscopic Study of Extended Star Clusters in Dwarf Galaxy NGC 6822

Abstract

We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South 8.1m telescope. Their radial velocities derived from the spectra range from $-61.2\pm 20.4$ km s${}^{-1}$ (for C1) to $-115.34\pm 57.9$ km s${}^{-1}$ (for C4) and, unlike the intermediate age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old ($\ge 8$ Gyr) and metal poor ([Fe/H]$\lesssim -1.5$). NGC 6822 is found to have both metal poor ([Fe/H]$\approx -2.0$) and metal rich ([Fe/H]$\approx -0.9$) star clusters within 15 arcmin (2 kpc) from the center, while only metal poor clusters are observed in the outer halo with $r\ge 20$ arcmin (2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: $M_{N6822}=7.5_{-0.1}^{+4.5}\times 10^9{M}_{\odot }$ and $(M/L{)}_{N6822}=75_{-1}^{+45}(M/L{)}_{\odot }$. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group.

28 pages with 6 figures and 5 tables. Accepted for publication in ApJ

References in corpus (1)

• The life cycle of star cluster in a tidal field

Cited by in corpus (15)

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• Ca II triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities
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• A Search for Intergalactic Globular Clusters in the Local Group
• Kinematic study of planetary nebulae in NGC 6822
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• Euclid: Early Release Observations -- The star cluster systems of the Local Group dwarf galaxies IC 10 and NGC 6822