A Complex Luminosity Function for the Anomalous Globular Clusters in NGC1052-DF2 and NGC1052-DF4

TL;DR

This study analyzes the globular cluster luminosity functions in the ultra-diffuse galaxies NGC1052-DF2 and NGC1052-DF4, revealing a complex luminosity distribution with a bright peak and a subpopulation at typical luminosities, challenging previous assumptions about their GC systems.

Contribution

It provides a detailed analysis of the combined globular cluster luminosity function in DF2 and DF4, showing a complex structure and a total GC count that does not correlate with halo mass.

Findings

The GCLF peak remains at M_V ≈ -9 mag.

A subpopulation of GCs at M_V ≈ -7.5 mag was identified.

Total GC count is approximately 37, independent of halo mass.

Abstract

NGC1052-DF2 and NGC1052-DF4 are ultra-diffuse galaxies (UDGs) that were found to have extremely low velocity dispersions, indicating that they have little or no dark matter. Both galaxies host anomalously luminous globular cluster (GC) systems, with a peak magnitude of their GC luminosity function (GCLF) that is $\sim1.5$ magnitudes brighter than the near-universal value of $M_V \approx -7.5$. Here we present an analysis of the joint GCLF of the two galaxies, making use of new HST photometry and Keck spectroscopy, and a recently improved distance measurement. We apply a homogeneous photometric selection method to the combined GC sample of DF2 and DF4. The new analysis shows that the peak of the combined GC luminosity function remains at $M_V \approx -9$ mag. In addition, we find a subpopulation of less luminous GCs at $M_V \approx -7.5$ mag, where the near-universal GCLF peak is located. The number of GCs in the magnitude range of $-5$ to $-8$ is $7.1_{-4.34}^{+7.33}$ in DF2 and $8.6_{-4.83}^{+7.74}$ in DF4, similar to that expected from other galaxies of the same luminosity. The total GC number between $M_V$ of $-5$ to $-11$ is $18.5_{-4.42}^{+8.99}$ for DF2 and $18.6_{-4.92}^{+9.37}$ for DF4, calculated from the background-subtracted GCLF. The updated total number of GCs in both galaxies is $37^{+11.08}_{-6.54}$. The number of GCs do not scale with the halo mass in either DF2 or DF4, suggesting that $N_{GC}$ is not directly determined by the merging of halos.