Globular clusters are ancient groups of hundreds of thousands or even millions of stars, gravitationally bound into a single structure about 100-200 light-years across. These objects are thought to be among the oldest stellar structures in the Universe. Globular clusters are favorite targets for amateur astronomers. About 150-180 such clusters are known to exist around our Milky Way Galaxy. To the naked eye they appear as fuzzy-looking stars. Through a small telescope they resolve into glittering snowball-shaped islands of innumerable stars crowded together. The NASA/ESA Hubble Space Telescope is so powerful it can see globular clusters 300 million light-years away. Peering into the heart of the Coma cluster — a huge structure of over a thousand galaxies bound together by gravity — Hubble captured a whopping 22,426 globular clusters.
This Hubble mosaic is of a portion of the Coma cluster of over 1,000 galaxies, located 300 million light-years from Earth. Image credit: NASA / ESA / J. Mack, STScI / J. Madrid, Australian Telescope National Facility.
“Because globular clusters are much smaller than entire galaxies they are a much better tracer of how the fabric of space is distorted by the Coma cluster’s gravity,” said Dr. Juan Madrid of the Australian Telescope National Facility and co-authors.
“In fact, the Coma cluster is one of the first places where observed gravitational anomalies were considered to be indicative of a lot of unseen mass in the Universe — later to be called ‘dark matter’.”
At the distance of the Coma cluster, its globular clusters appear as dots of light even to Hubble’s super-sharp vision.
The survey found 22,426 globular clusters scattered in the space between the galaxies. They have been orphaned from their home galaxy due to galaxy near-collisions inside the traffic-jammed cluster.
Hubble revealed that some globular clusters line up along bridge-like patterns. This is telltale evidence for interactions between galaxies where they gravitationally tug on each other like pulling taffy.
“I first thought about the distribution of globular clusters in Coma when I was examining Hubble images that show the globular clusters extending all the way to the edge of any given photograph of galaxies in the Coma cluster,” Dr. Madrid said.
“I was looking forward to more data from one of the legacy surveys of Hubble that was designed to obtain data of the entire Coma cluster, called the Coma Cluster Treasury Survey.”
“However, halfway through the program, in 2006, Hubble’s Advanced Camera for Surveys (ACS) had an electronics failure.”
Hubble’s incredible sharpness was used to do a comprehensive census of the Coma cluster’s most diminutive members: a whopping 22,426 globular clusters (circled in green). Image credit: NASA / ESA / J. Mack, STScI / J. Madrid, Australian Telescope National Facility.
To fill in the survey gaps, Dr. Madrid and colleagues painstakingly pulled numerous Hubble images of the galaxy cluster taken from different Hubble observing programs. They were able to assemble a mosaic of the central region of the cluster.
They then developed algorithms to sift through the Coma mosaic images that contain at least 100,000 potential sources.
The program used globular clusters’ color (dominated by the glow of aging red stars) and spherical shape to eliminate extraneous objects — mostly background galaxies unassociated with the Coma cluster.
“One of the cool aspects of our research is that it showcases the amazing science that will be possible with NASA’s planned Wide Field Infrared Survey Telescope (WFIRST) that will have a much larger field of view than Hubble,” Dr. Madrid said.
“We will be able to image entire galaxy clusters at once.”
The results are published in the Astrophysical Journal.
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Juan P. Madrid et al. 2018. A Wide-field Map of Intracluster Globular Clusters in Coma. ApJ 867, 144; doi: 10.3847/1538-4357/aae206
