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Searching for the True Colors of M87's Globular Clusters

March 28, 2013

Figure 1. This GMOS image shows the field studied, offset from the center of M87, which is the bright region at the top. Hundreds of the point-like objects are globular clusters. The image is made from observations through the g, r, and i filters, which are mapped to blue, green, and red on this color composite image.

Figure 2.  Abundance distributions, [Z/H], which is measured logarithmically relative to solar values. The fainter GCs (lower line) show a significant bimodal distribution, with both groups having average abundance less than solar values. The brighter GCs (upper line, offset for clarity) are consistent with a single abundance distribution. Open circles show uncertainties of the histogram bins.

Do all globular clusters within a galaxy share a common history? Multiple histories could indicate that different processes drive a galaxy's fundamental development at different times. Juan Carlos Forte (Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina) and colleagues used the Gemini Multi-Object Spectrograph (GMOS) to study over 500 globular clusters in the galaxy M87. Their observations indeed point to such complexity, revealing multiple episodes of star formation.

Globular clusters indirectly reveal the star formation history of their host galaxy, and one of their fundamental properties is the metallicity, or abundance of elements more massive than helium. Many globular clusters (GCs) are old, even to the point of challenging other measurements of the age of the universe, but detailed observations can show structure in the distribution of their observed colors. The outstanding question is whether this is evidence of real non-uniformity in the underlying history and metallicity distribution or merely an artifact of the detailed relationship between metallicity and observed color.

Juan Carlos Forte (Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina) and colleagues observed over 500 GCs of the giant elliptical galaxy M87 (also known as NGC 4486; Figure 1) and conclude that their chemical abundance distribution is genuinely bimodal (Figure 2). The effects are strongest among the fainter GCs studied. One development in this work was to determine the metallicity by considering all observed wavelength bands simultaneously, rather than rely on a simplified relationship based on only two bands. The observations in four optical filters (g, r, i, and z’) were obtained using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. The complete results will be published in the Monthly Notices of the Royal Astronomical Society; advance access and a preprint are available now.