Ministerio de Ciencia, Innovación y Universidades Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

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A single population of red globular clusters around the massive compact galaxy NGC 1277

Author/s: M. A. Beasley, I. Trujillo, R. Leaman, M. Montes

Reference: 2018 Nature 555 483 | Link

The figure shows the spatial distributions of globular clusters (red points) in our HST field. The positions of NGC 1277 and other galaxies are marked as grey outlines. The blue colours represent the smoothed density distributions of the GCs, where darker blue corresponds to higher spatial densities. The red GCs are shown in the left panel, blue GCs in the right panel. It can be seen that the blue GCs "disappear" in NGC 1277. This is particularly obvious when compared to NGC 1278, which is a similar mass galaxy. NGC 1277 is the only known galaxy of this stellar mass that shows few or no blue globular clusters.
The figure shows the spatial distributions of globular clusters (red points) in our HST field. The positions of NGC 1277 and other galaxies are marked as grey outlines. The blue colours represent the smoothed density distributions of the GCs, where darker blue corresponds to higher spatial densities. The red GCs are shown in the left panel, blue GCs in the right panel. It can be seen that the blue GCs "disappear" in NGC 1277. This is particularly obvious when compared to NGC 1278, which is a similar mass galaxy. NGC 1277 is the only known galaxy of this stellar mass that shows few or no blue globular clusters.

In the current model of galaxy formation, massive galaxies are thought to form in two phases. An initial collapse of gas and giant burst of central star formation at early times builds up the central regions of galaxies, followed by the later accretion of lower-mass galaxies that builds up their stellar and dark-matter haloes. The systems of globular clusters within such galaxies are believed to form in a similar manner. The initial central burst forms metal-rich (with red colours) clusters, whereas more metal-poor (with blue colours) clusters are brought in by the later accretion of less-massive satellites. This formation process is thought to result in the double- or multi-peaked optical colour distributions that are seen in the globular cluster systems of massive galaxies. To test this scenario, we performed optical observations with Hubble Space Telescope of the massive galaxy NGC 1277 in the Perseus galaxy cluster, which is a candidate "relic" galaxy that is a nearby example of the compact cores of galaxies seen at high redshift.  We found that the cluster system of NGC 1277 has few blue clusters and is almost entirely red. This finding is in strong contrast to other galaxies of similar and larger stellar mass whose cluster systems always have substantial numbers of blue clusters. We argue that this lack of blue GCs indicates that the galaxy has accreted few (if any) satellite galaxies after its initial collapse, and use simulations of possible merger histories to show that the stellar mass due to accretion is probably at most ten per cent of the total stellar mass of the galaxy. These results confirm that NGC 1277 is a genuine relic galaxy and demonstrate that blue clusters constitute an accreted population in present-day massive galaxies.

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