Astronomers map globular clusters in the NGC 5018 galaxy group

Using the VLT Survey Telescope (VST), astronomers from Italy and Germany have observed a group of galaxies designated NGC 5018. Results of the observational campaign, published Dec. 23 on the pre-print server arXiv, yield important insights into the globular cluster system of this group.

Globular clusters (GCs) are collections of tightly bound stars orbiting galaxies. Astronomers perceive them as natural laboratories enabling studies on the evolution of stars and galaxies. In particular, GCs could help researchers to better understand the formation, history and evolution of early-type galaxies, as the origin of GCs seems to be closely linked to periods of intense star formation. Moreover, studying GCs can deliver crucial insights into the interaction events of the host galaxy and, more broadly, the galaxy group.

The NGC 5018 galaxy group is named after its brightest member, NGC 5018, which is a massive elliptical galaxy at a distance of about 132.5 million light years in the constellation of Virgo. The other bright galaxies in the group are the edge-on spiral NGC 5022, the face-on spiral NGC 5006 and two lenticulars named MCG-03-34-013 and PGC 140148.

Previous studies have found that the GCs detected in the galaxy NGC 5018 can be divided into two sub-populations: a small population of young GCs, between several hundred million and six billion years old, and a larger population of older GCs.

Now, a team of astronomers led by Pratik Lonare of the Abruzzo Astronomical Observatory in Teramo, Italy, has decided to further investigate the GC system of NGC 5018 and also the entire group.

“In this work, we use the deep, multi-passband, wide-field imaging of the NGC 5018 galaxy group collected with the VLT Survey Telescope (VST) as part of the VST Elliptical GAlaxy Survey (VEGAS) and conduct a systematic study of its GC system,” the researchers wrote.

VEGAS imaging data allowed Lonare’s team to identify globular cluster candidates across the field of the NGC 5018 galaxy group, which revealed the presence of an intra-group GC system. The 2D distribution map shows an overdensity of GC candidates in NGC 5018, while no significant GC overdensities are observed in the other galaxies of the group.

In general, the radial density profile of GC candidates in NGC 5018 follows the surface brightness profile of the galaxy. The color profile of these GC candidates in this galaxy shows a major component with a peak at about 0.75 mag.

The observations found that the observed intra-group GC population aligns along the five bright galaxies and surrounds the NGC 5018 group itself. This observed geometry aligns with intra-group light (IGL) detected in the group, but extended to larger group-centric distances.

The analysis of the color profile of intra-group GC candidates suggests the presence of blue and red GC components with peaks at about 0.45 and 0.80 mag, respectively. The blue GC component is found to be more extended compared to the red GC component with respect to NGC 5018.

“This, combined with previous results on the IGL of the system, might suggest that a part of the blue GC population in the intra-group space originally belonged to NGC 5018, which was dispersed due to tidal interactions with the neighboring galaxies,” the authors of the paper conclude.

The astronomers add that the estimated number of GC candidates for the whole NGC 5018 group is about 4,000, while for the NGC 5018 itself it is estimated to be 485.

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