Milky Way's Globular Clusters

[Jamison Lahman]

My question is directly towards figure 2.15 from Galaxies in the Universe, An Introduction by Sparke and Gallagher.

Everything in the figure makes sense to me until the authors make the point, "Clusters with a blue horizontal branch (filled dots) are more concentrated to the center than are those with a red horizontal branch (open circles)."

My understanding is that metal-rich clusters will have a redder horizontal branch and metal-rich clusters are concentrated near the galactic center as shown by the left side. Why then, are the bluer horizontal branches concentrated towards the center?

For context, these are the two paragraphs which follow the figure though there isn't much detail about this.

Thanks,
Jamison

 

[AndyW]

Dear Jamison,

Thank you for your question regarding figure 2.15 from Galaxies in the Universe, An Introduction by Sparke and Gallagher. I understand your confusion about the placement of the blue and red horizontal branches in relation to the galactic center.

Firstly, it is important to note that the authors are referring to the concentration of clusters within the galactic halo, not the overall distribution of clusters within the galaxy. This means that the clusters with a blue horizontal branch (filled dots) are more concentrated towards the center of the halo compared to those with a red horizontal branch (open circles).

This may seem counterintuitive, as you correctly pointed out that metal-rich clusters (which have a redder horizontal branch) are typically found towards the galactic center. However, the authors are specifically referring to the horizontal branch morphology, not the metallicity of the clusters.

Clusters with a blue horizontal branch typically have a larger population of hot, blue stars, which are more massive and have shorter lifetimes compared to the cooler, red stars found in clusters with a red horizontal branch. This means that the blue horizontal branch clusters are younger and have not had as much time to migrate towards the galactic center as the older, red horizontal branch clusters.

I hope this explanation helps to clarify the placement of the blue and red horizontal branches in figure 2.15. Please let me know if you have any further questions.