Cohen & Blakeslee & Ryzhov, 1998, The Ages and Abundances of a Large Sample of M87 Globular Clusters

# 세줄 요약 #

1. Combining the new (20 GCs; this paper) and existing data (150 GCs; Cohen & Ryzhov, 1997) for the galactic GCs (Globular clusters) and comparing the (U-R) colors and the line indices gave qualitative indications for the ages and abundances of M87 GC system.
2. We find that the M87 GCs span a wide range in metallicity, from very metal-poor to somewhat above solar metallicity.
3. The behavior of these elements (the relative abundances of Na, Mg, and Fe in their spectral features) between the metal-rich and metal-poor M87 GCs is similar to that shown by the galactic GCs and by halo stars in the Galaxy.

 

# 상세 리뷰 #

1. Introduction

• Observation Target: M87 GC(Globular cluster) system
• Abundance analysis: 162 spectra of 150 M87 GCs
• Compare Galactic GCs: U-R & line indices

Fig 1. The galactic core of Messier 87 as seen by the  Hubble Space Telescope with its blue plasma jet clearly visible. [Ref.  https://en.wikipedia.org/wiki/Messier_87#/media/File:Messier_87_Hubble_WikiSky.jpg]

 

2. Method

• Using Lick indices (Burstein et al., 1984)
• Measure the strength of stronger absorption line
  • Mgb, Fe_lamda, NaD, TiO = metallicity compare M87 v.s. galactic GCs
  • Hb = age defendence

Fig 2. comparison of the spectrum of a metal-poor GC in M87 (Strom 910) with that of a metal-rich one (Strom 1010). [Ref. Cohen et al., 1988]

Table 1. Bandpass for absorption indices [Ref. Cohen et al., 1998]

 

3. Result

• The metallicity range of M87 GCs: [Fe/H]z = -0.95 dex
  • similar to Galactic GCs
• The median age of 150 M87 GCs = 13 Gyr 
  • similar to Galactic GCs
• The same trend of M87 & MW?

Fig 3. TiO index is shown as a function of (U-R) color. The upper panel shows the data for the M87 GCs, with objects with QSNR ≥ 40 indicated by the larger circles. In the lower panel the data for the galactic GCs (asterisks, our data; open circles, Burstein et al. 1984) are compared with the median line of the M87 sample. [Ref. Cohen et al., 1998]

Fig 4. NaD index is shown as a function of the Mg b index. The upper panel shows the data for the M87 GCs, with objects with QSNR ≥ 40 are indicated by the larger circles. In the lower panel the data for the galactic GCs (asterisks, our data; open circles, Burstein et al. 1984) is compared with the median line of the M87 sample. [Ref. Cohen et al., 1998]

 

• No obvious color bimodality
  • KMM test shows a marginally bimodal distribution

Fig 5. Histogram of metallicity for 150 M87 GCs is shown (hatched). The solid line represents that of the galactic GC system from Harris (1996). [Ref. Cohen et al., 1998]

 

• The small radial gradient in metallicity vs radius

Fig 6. Mgb, NaD, and Hb indices for the M87 GCs are shown as a function of projected radius. The least-squares fit for each index is indicated by a dashed line, while the median line is shown as a solid line. [Ref. Cohen et al., 1998]

 

* Reference: Cohen, J. G., Blakeslee, J. P., and Ryzhov, A., “The Ages and Abundances of a Large Sample of M87 Globular Clusters”, [The Astrophysical Journal], vol. 496, no. 2, pp. 808–826, 1998. doi:10.1086/305429.