van Dokkum et al, 2018, An Enigmatic Population of Luminous Globular Clusters in a Galaxy Lacking Dark Matter

# 세줄 요약 #

• We recently found an ultra-diffuse galaxy (UDG) called NGC1052-DF2 with a half-light radius of Re = 2.2 kpc and little or no dark matter. 
• Their properties are similar to ω Centauri (the brightest and largest globular cluster in the Milky Way), but that the luminosity function of metal-poor globular clusters is not universal and a factor of ∼1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies.
• We infer that a dark matter halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high-redshift galaxies.

 

# 상세 리뷰 #

1. Introduction

• We recently found an ultra-diffuse galaxy (UDG) called NGC1052-DF2
  • half-light radius Re = 2.2kpc
  • little or no dark matter
    • stellar-mass of M_stars ≈ 2 × 10^8 M_sun
    • dark matter halo mass of M_halo < 1.5 × 10^8 M_sun
  • distance: 19.0 ± 1.7 Mpc
• Observation: Combination of Hubble Space Telescope (HST) Imaging & Keck Spectroscopy
• These galaxy properties are similar to ω Centauri 
  
  • Their average size of <r_h>= 6.2 ± 0.5 pc
  • metallicity of [Fe/H] = −1.35 ± 0.12

Fig 1. Keck/LRIS spectra (left and right) and HST images (center) of the 11 clusters associated with NGC1052-DF2. The color images, generated from the V_606 and I_814 data.

Fig 2. . Photometric selection of globular clusters. The top panel shows the color-magnitude relation of all objects in the HST images of NGC1052-DF2. The 11 spectroscopically confirmed objects are marked with yellow and black circles. The bottom panel shows the size-magnitude relation for all objects that satisfy this color criterion.

 

2. Result

• NGC1052-DF2 has two different properties from other UDGs(or other galaxies).
• (1) Luminosity function of GCs peak Mv ≈ -9.1
  • Universal UDGs value Mv ≈ -7.5
• (2) The galaxy has no or very litter dark matter (DM)
  • The total mass of GCs v.s. galaxy ratio (M_gc/M_galaxy)
    
    • Universal UDGs: 3 x 10^-5
    • NGC1052-DF2: 3 x 10^-2 
  • The factor is ~ 1000 higher than universal galaxies.

Fig 3. The luminosity function of the compact objects in NGC1052–DF2. Top left: observed luminosity function, in apparent I_814 mag. The blue line shows the magnitude distribution of objects in blank field 3D-HST/CANDELS imaging that have the same colors and sizes as the GCs. Top right: observed luminosity function, after correcting each bin for the expected number of unrelated objects. Bottom: luminosity function in absolute magnitude, for D = 20 Mpc. The luminosity functions of GCs in the Milky Way and in Coma UDGs are shown in red and blue, respectively.

 

Fig 4. Morphological parameters of the GCs. The top panel shows the circularized half-light radii vs. the absolute magnitude for NGC1052–DF2 (black points with error bars) and the Milky Way (red). The bottom panel shows the ellipticity. Means are indicated with dashed lines.

 

3. Discussion

• metal-poor GC system is not connected with DM halo
• In high-z galaxies, gas-rich central region & no DM halo galaxy is commonly in space.  

 

* Reference: van Dokkum, P., “An Enigmatic Population of Luminous Globular Clusters in a Galaxy Lacking Dark Matter”, The Astrophysical Journal, vol. 856, no. 2, 2018. doi:10.3847/2041-8213/aab60b.