Caught: A ghost galaxy that may have hit ours long ago | Science News for Students

Caught: A ghost galaxy that may have hit ours long ago

The star system Antlia 2 turned up not far from where astronomers predicted the culprit should be
Jul 17, 2019 — 6:45 am EST
an illustration of the galaxy Antila 2

Antlia 2, a dim galaxy that orbits the Milky Way, appears at the right in this illustration. It’s a bit bigger but has fewer stars than the Large Magellanic Cloud (left), which also orbits our Milky Way.

V. Belokurov/Univ. of Cambridge/CCA, based on the images by Marcus and Gail Davies and Robert Gendler

The Milky Way survived a galactic hit and run. It took place millions of years ago. But astronomers may have finally found the culprit. Their suspect is a dwarf galaxy called Antlia 2.

That a galaxy likely hit our galaxy is not new. Sukanya Chakrabarti and Leo Blitz suggested it 10 years ago. Back then, both astrophysicists were working at the University of California, Berkeley. They pointed to ripples in the outer edges of the Milky Way as evidence of a collision with some dwarf galaxy. A dwarf galaxy is far smaller than a massive spiral one, like our Milky Way.

The collision probably shook the Milky Way’s gas like a pebble dropped into a pond, the scientists said. They predicted how massive the galaxy that hit the Milky Way had to be. They even estimated roughly where it should be.

Still, astronomers couldn’t find it. None of the small galaxies known to orbit the Milky Way fit the bill.

Then last year, other astronomers discovered Antlia 2. They found this dwarf using the Gaia space telescope. Antlia 2 doesn’t have many visible stars. That’s why the astronomers called it a hidden giant. Intrigued, Chakrabarti looked at the data for this galaxy late last year. And she thought it might just be the missing culprit. Antlia 2’s location, she says, is “stupidly close” to where she and Blitz predicted that the dwarf galaxy should be today.

Antlia 2’s mass also is close to what the surviving remnant of the colliding galaxy’s mass would be, she estimates. Chakrabarti now works at the Rochester Institute of Technology in New York. The collision might even explain why Antlia 2 has so few stars, she says. Its encounter with the Milky Way could have stripped many of them away.

Chakrabarti reported the finding June 12. at the American Astronomical Society meeting, in St. Louis, Mo. Chakrabarti also posted a study about the findings on

She and her colleagues, however, are not yet positive Antlia 2 is to blame. To make sure, they have predicted where the culprit’s stars should be. They can check it in the next set of Gaia data. Those data are due out in 2020 or 2021.

“If this is what’s observed a year from now,” she says, “I’d say it’s indisputable, really, that Antlia 2 is the dwarf galaxy that we predicted.”

Power Words

(more about Power Words)

arXiv     A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

colleague     Someone who works with another; a co-worker or team member.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

Milky Way     The galaxy in which Earth’s solar system resides.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

remnant     Something that is leftover — from another piece of something, from another time or even some features from an earlier species.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.


Meeting:​ ​​ S. Chakrabarti. Antlia 2: The dark dwarf galaxy that crashed into the Milky Way. American Astronomical Society meeting, St. Louis, June 12, 2019.

Journal:​ S. Chakrabarti et al. Antlia 2’s role in driving the ripples in the outer gas disk of the galaxy. arXiv:1906:04203. Posted June 12, 2019.

Journal:​ G. Torrealba et al. The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2. arXiv:1811.04082. Posted November 9, 2018.

Journal:​ S. Chakrabarti and L. Blitz. Tidal imprints of a dark subhalo on the outskirts of the Milky Way. Monthly Notices of the Royal Astronomical Society. Vol. 399, October 2009, p. L118. doi: 10.1111/j.1745-3933.2009.00735.x.