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


Massive galaxies are thought to form in two phases: an initial collapse of gas and giant burst of central star formation, followed by the later accretion of material that builds up their stellar and dark-matter haloes1,2,3,4. The systems of globular clusters within such galaxies are believed to form in a similar manner. The initial central burst forms metal-rich (spectrally red) clusters, whereas more metal-poor (spectrally blue) clusters are brought in by the later accretion of less-massive satellites5,6,7,8,9,10. This formation process is thought to result in the multimodal optical colour distributions that are seen in the globular cluster systems of massive galaxies8,11,12. Here we report optical observations of the massive relic-galaxy candidate NGC 1277—a nearby, un-evolved example of a high-redshift ‘red nugget’ galaxy13,14,15,16,17. We find that the optical colour distribution of the cluster system of NGC 1277 is unimodal and entirely red. This finding is in strong contrast to other galaxies of similar and larger stellar mass, the cluster systems of which always exhibit (and are generally dominated by) blue clusters11. We argue that the colour distribution of the cluster system of NGC 1277 indicates that the galaxy has undergone little (if any) mass accretion 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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Figure 1: Spatial distribution of clusters in the HST/ACS field.
Figure 2: The colour distribution of clusters in NGC 1277 compared with that of the composite cluster system.
Figure 3: The fraction of blue clusters in galaxies of a given stellar mass.
Figure 4: Predictions from the simulated merger histories of NGC 1277.


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We thank C. dalla Vecchia, J. Sanchez Almeida, C. Brook, S. Wellons, M. Fouesneau, A. Vazdekis, B. Dullo, J. Read and G. van de Ven for discussions, and J. Roman and A. Serrano Borlaf for assistance with image alignment. M.A.B. and I.T. acknowledge support from grant AYA2016-77237-C3-1-P from the Spanish Ministry of Economy and Competitiveness (MINECO). R.L. acknowledges funding from a Natural Sciences and Engineering Research Council of Canada PDF award. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and is based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. These observations are associated with programme GO-14215. Support for this work was provided by NASA through grant HST-GO-4215 from the Space Telescope Science Institute, operated by AURA under NASA contract NAS 5-26555 This research has made use of NASA’s Astrophysics Data System and extensive use of Python and Scipy.

Author information




M.A.B. led the data processing and analysis, contributed to the interpretation and HST proposal preparation and produced Figs 1, 2, 3 and Extended Data Figs 2, 3, 4, 5, 6. I.T. contributed to the analysis and the interpretation, produced Extended Data Fig. 1 and lead the HST proposal preparation. R.L. generated and analysed the analytic merger models, produced Fig. 4 and Extended Data Figs 7, 8, and contributed to the analysis, interpretation and HST proposal preparation. M.M. contributed to the analysis, interpretation and HST proposal preparation. All authors contributed to the overall design of this project.

Corresponding author

Correspondence to Michael A. Beasley.

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Reviewer Information Nature thanks R. Abraham, K. Glazebrook and L. Spitler for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Colour-composite HST image of the massive relic galaxy NGC 1277.

The image was constructed from g475W + r625W + z850LP bands. The g475W and z850LP imaging was obtained with the HST programme GO-14215 and the r625W imaging with the programme GO-10546. NGC 1277 is the best example found so far in the nearby Universe with characteristics equivalent to those of the first massive galaxies to form more than 11 Gyr ago. The image is oriented with north pointing up and east to the left. The field of view is 42.2 arcsec × 30.6 arcsec, corresponding to a physical scale of 14.5 kpc × 10.5 kpc at our adopted distance of 73.3 Mpc to the galaxy. The images have been scaled with logarithmic intensity to highlight the various structures in the galaxy. The vast majority of point sources surrounding NGC 1277 are clusters associated with the galaxy. Two example clusters have been marked with ticks.

Extended Data Figure 2 Colour–magnitude diagrams of cluster candidates.

From left to right, the plots show all point sources in the field, NGC 1278 clusters, raw (not background-corrected) NGC 1277 clusters and an example background field. Also shown are the photometric uncertainties (1σ; blue crosses) based on our artificial point-source test, and the 100% completeness limit (red dashed lines; see Methods).

Extended Data Figure 3 Surface-density profile of NGC 1277 clusters.

The figure shows the raw (blue circles) and completeness-corrected (orange stars) counts. Uncertainties are 16th–84th percentiles. Within the range 3–10 kpc, the clusters closely follow the light distribution of the galaxy. This is typical of red clusters. The background is determined from the Sérsic function fits (which include a background term; green line) to the cluster data and defines the radial extent of the cluster system, which ends at 11 kpc (about 10Re). The stellar-mass density profile15 (red line) has arbitrary normalization. GCs, globular clusters.

Extended Data Figure 4 Colour distribution of clusters in NGC 1277 compared with that for the companion galaxy in projection NGC 1278.

The figure shows that NGC 1278 (blue) has a strong peak of blue clusters that is not seen in NGC 1277 (orange). The expected background contamination of NGC 1278 clusters and of intracluster clusters to the cluster system of NGC 1277, with which we have corrected the colour distribution of NGC 1277, is also shown (green). The predominantly blue colours of the contaminating clusters is expected because they correspond to the outskirts of NGC 1278 (see Fig. 1). Solid curves are two-Gaussian solutions from the GMM code28; dashed curves are individual Gaussian components from GMM. A single Gaussian fit to the NGC 1277 clusters with GMM gives 〈g475Wz850LP〉 = 1.22 ± 0.03, with a FWHM of 0.28 ± 0.02.

Extended Data Figure 5 Distribution of the blue cluster fraction from Monte Carlo simulations.

The vertical dashed lines indicate the medians of the distributions and the vertical dotted lines show the 16th and 84th percentiles.

Extended Data Figure 6 Colour distributions of NGC 1277 and NGC 1278 in three magnitude bins.

The bin values for the NGC 1277 clusters represent the medians of the bin values from Monte Carlo simulations. Uncertainties are the 16th–84th percentiles of the distributions.

Extended Data Figure 7 Accretion models for the build-up of the NGC 1277 cluster system.

Each point is a single merger history for NGC 1277, characterized by the total fraction of accreted stellar mass versus the number ratio of in situ to accreted clusters for that realization. The observed ratio for NGC 1277 is indicated by the cyan dashed line (Nblue/Nred = 0.21) and is characteristic of galaxies with merger histories leading to facc ≈ 10% (orange). The cyan arrow indicates that the in situ fraction is a lower limit. Galaxies with comparable stellar mass to NGC 1277 in the ACS VCS tend to have equal numbers of blue and red clusters, which is more common for accretion histories of facc ≈ 50%–90% (green). The extremely rare (about 0.02%) high-mass-accretion realizations, which produce the observed blue/red fraction in NGC 1277, are shown in magenta. Contours represent 10%, 25%, 50%, 75%, 90% and 99% of the maximum of the two-dimensional distribution.

Extended Data Figure 8 Predictions of galaxy properties from accretion models.

From left to right, the black lines show the accretion fraction, the number of mergers, the maximum mass ratio of the merger and the final ratio of stellar to dark-matter mass for all of the merger realizations. The magenta distributions (top) show the high-facc merger histories that satisfy the constraint on the colour ratio of the clusters (which happens rarely; see, for example, Extended Data Fig. 7). The cyan distributions (bottom) show the low-facc merger histories that satisfy the observed colour ratio of the clusters and the SN constraints. The high-accretion-fraction models (magenta) also predict a dark-matter-halo mass for NGC 1277 that is larger than the observational constraints17 (indicated by the arrow in the bottom right panel). The vertical dashed lines represent the median of the distributions and the vertical dotted lines show the 16th and 84th percentiles.

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Beasley, M., Trujillo, I., Leaman, R. et al. A single population of red globular clusters around the massive compact galaxy NGC 1277. Nature 555, 483–486 (2018). https://doi.org/10.1038/nature25756

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