Astronomers Use Distributed Supercomputer to Reconstruct Ancient Dwarf Galaxy

by johnsmith

Using the 1.5 PetaFLOPS [email protected] distributed supercomputer, astronomers have calculated the original mass and size of a dwarf galaxy that was shredded in a collision with our own Milky Way Galaxy several billions of years ago.

An artist’s impression of a dwarf galaxy. Image credit: Rensselaer Polytechnic Institute.

An artist’s impression of a dwarf galaxy. Image credit: Rensselaer Polytechnic Institute.

A few dozen dwarf galaxies are known to orbit around our Milky Way Galaxy.

Over the course of billions of years, these dwarf galaxies tidally disrupt and stretch around the Milky Way into tidal streams.

The positions and velocities of the stars that make up these streams therefore carry information about the Milky Way’s gravitational field.

As such, dwarf galaxies act as gravitational probes for determining the distribution of gravitating mass in the Milky Way.

In 2006, two teams of astronomers independently discovered a new stellar stream while examining the Sagittarius stream. Due to the lack of a visible progenitor, the stream was named the Orphan Stream.

The southern portion of the stream was later named Chenab before it was discovered that both pieces of the stream — now known as the Orphan-Chenab Stream — resulted from the tidal disruption of the same dwarf galaxy.

“We’ve been running simulations that take this big stream of stars, back it up for a couple of billion years, and see what it looked like before it fell into the Milky Way,” said Professor Heidi Newberg, an astronomer at Rensselaer Polytechnic Institute.

“Now we have a measurement from data, and it’s the first big step toward using the information to find dark matter in the Milky Way.”

To probe the internal structure of the Orphan-Chenab Stream’s dwarf-galaxy progenitor, Professor Newberg and colleagues used the [email protected] distributed supercomputer, a collection of roughly 26,000 volunteered computers connected by the Berkeley Open Infrastructure for Network Computing, operating at 1.5 PetaFLOPS of combined computing power.

“It’s an enormous problem, and we solve it by running tens of thousands of different simulations until we get one that actually matches,” Professor Newberg said.

“And that takes a lot of computer power, which we get with the help of volunteers all over the world who are part of [email protected]

“We’re brute-forcing it, but given how complicated the problem is, I think this method has a lot of merit.”

The astronomers estimate the total mass of the dwarf-galaxy progenitor whose stars today form the Orphan-Chenab Stream as 2*107 times the mass of our Sun.

However, only a little more than 1% of that mass is estimated to be made up of ordinary matter like stars.

The remainder is assumed to be dark matter that exerts gravitational force, but that we cannot see because it does not absorb or give off light.

“Tidal stream stars are the only stars in our Galaxy for which it is possible to know their positions in the past,” Professor Newberg said.

“By looking at the current speeds of stars along a tidal stream, and knowing they all used to be in about the same place and moving at the same speed, we can figure out how much the gravity changes along that stream. And that will tell us where the dark matter is in the Milky Way.”

The team also found that the progenitor of the Orphan-Chenab stream has less mass than the galaxies measured in the outskirts of the Milky Way today.

“The measured progenitor mass is on the low end of previous measurements and, if confirmed, lowers the mass range of ultrafaint dwarf galaxies,” the authors said.

“Our optimization assumes a fixed Milky Way potential, Orphan-Chenab stream’s orbit, and radial profile for the progenitor, ignoring the impact of the Large Magellanic Cloud.”

The study was published in the Astrophysical Journal.


Eric J. Mendelsohn et al. 2022. Estimate of the Mass and Radial Profile of the Orphan-Chenab Stream’s Dwarf-galaxy Progenitor Using [email protected] ApJ 926, 106; doi: 10.3847/1538-4357/ac498a

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