The disrupted ‘tadpole’ galaxy has an elliptical head and a long, straight tail, and is approximately one million light-years long — about 10 times larger than our own Milky Way Galaxy, according to Tel Aviv University astronomer Dr. Noah Brosch and co-authors.
This image shows the HCG 98 group. The core of HCG 98 consists of the two ‘smudges’ at the center of the image. Each is a galaxy much like our own Milky Way Galaxy. The tadpole structure covers the central galaxy pair and was formed when the pair demolished a much smaller galaxy. Image credit: Brosch et al, doi: 10.1093/mnras/sty2717.
The newfound galaxy is part of Hickson Compact Group 98 (HCG 98), a small group of galaxies located about 300 million light-years away from Earth.
“In compact group environments, we believe we can study ‘clean’ examples of galaxy-galaxy interactions, learn how matter is transferred between the members, and how newly accreted matter can modify and influence galaxy growth and development,” Dr. Brosch said.
Dr. Brosch and colleagues observed the HCG 98 group with a 28-inch (71 cm) telescope at the Wise Observatory and confirmed with additional observations with a similar 28-inch at the Polaris Observatory.
They also used archival Sloan Digital Sky Survey images from the IAC Stripe 82 Legacy Project.
“The extragalactic tadpole contains a system of two very close ‘normal’ disk galaxies, each about 40,000 light-years across,” Dr. Brosch said.
“Together with other nearby galaxies, the galaxies form a compact group.”
“What makes this object extraordinary is that the tail alone is almost 500,000 light-years long,” added team member Professor Michael Rich, an astronomer at the University of California, Los Angeles.
“If it were at the distance of the Andromeda Galaxy, which is about 2.5 million light-years from Earth, it would reach a fifth of the way to our own Milky Way.”
The giant ‘tadpole’ was produced by the disruption of a small, previously invisible dwarf galaxy containing mostly stars, the astronomers found.
“When the gravitational force of two visible galaxies pulled on stars in this vulnerable galaxy, the stars closer to the pair formed the ‘head’ of the tadpole,” they said.
“Stars lingering in the victim galaxy formed the ‘tail’.”
The discovery is reported in the Monthly Notices of the Royal Astronomical Society.
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Noah Brosch et al. 2019. Hickson Compact Group 98: a complex merging group with a giant tidal tail and a humongous envelope. MNRAS 482 (2): 2284-2293; doi: 10.1093/mnras/sty2717
