ESA: Gaia Spots Stars Flying Between Galaxies

[bystander]

Gaia Spots Stars Flying Between Galaxies
ESA | Space Science | Science & Technology | Gaia | 2018 Oct 02

Sprinting stars in the Milky Way -- Credit: ESA (artist's impression and composition); Marchetti et al. 2018 (star positions and trajectories); NASA/ESA/Hubble (background galaxies);

---

A team of astronomers using the latest set of data from ESA’s Gaia mission to look for high-velocity stars being kicked out of the Milky Way were surprised to find stars instead sprinting inwards – perhaps from another galaxy.

In April, ESA’s stellar surveyor Gaia released an unprecedented catalogue of more than one billion stars. Astronomers across the world have been working ceaselessly over the past few months to explore this extraordinary dataset, scrutinising the properties and motions of stars in our Galaxy and beyond with never before achieved precision, giving rise to a multitude of new and intriguing studies.

The Milky Way contains over a hundred billion stars. Most are located in a disc with a dense, bulging centre, at the middle of which is a supermassive black hole. The rest are spread out in a much larger spherical halo.

Stars circle around the Milky Way at hundreds of kilometres per second, and their motions contain a wealth of information about the past history of the Galaxy. The fastest class of stars in our Galaxy are called hypervelocity stars, which are thought to start their life near the Galactic centre to be later flung towards the edge of the Milky Way via interactions with the black hole.

Only a small number of hypervelocity stars have ever been discovered, and Gaia’s recently published second data release provides a unique opportunity to look for more of them. ...

Gaia Spots Stars Flying Between Galaxies
Royal Astronomical Society | 2018 Oct 02

Gaia DR2 in 6D: Searching for the Fastest Stars in the Galaxy ~ T. Marchetti, E.M. Rossi, A.G.A. Brown

• Monthly Notices of the RAS (online 20 Sep 2018) DOI: 10.1093/mnras/sty2592
  arXiv.org > astro-ph > arXiv:1804.10607 > 27 Apr 2018 (v1), 19 Sep 2018 (v2)

[neufer]

<<The positions and reconstructed orbits of 20 high-velocity stars, represented on top of an artistic view of our Galaxy, the Milky Way. These stars were identified using data from the second release of ESA’s Gaia mission. The seven stars shown in red are sprinting away from the Galaxy and could be travelling fast enough to eventually escape its gravity. Surprisingly, the study revealed also thirteen stars, shown in orange, that are racing towards the Milky Way: these could be stars from another galaxy, zooming right through our own. Credit: ESA (artist’s impression and composition); Marchetti et al. 2018 (star positions and trajectories); NASA / ESA / Hubble (background galaxies); CC BY-SA 3.0 IGO>>

• 1) Note that the 13 stars in orange are all very close to the plane of the Milky Way; stars from other galaxIES should be coming in from all directions like Oort Cloud comets.
  
  2) Also, Oort Cloud comets represent a minuscule fraction of the Oort Cloud with virtually no angular momentum; stars from another galaxy would also represent a minuscule fraction of "free range" stars that have virtually no angular momentum.
  
  3) With galactic stars having a near constant orbital velocity V_c out to some distance R_max
  escape velocity at around distance R_☉ is about:
  
  V_e = V_c SQRT[2 + 2 ln(R_max/R_☉)]; this could be many times larger than V_c.

Hence, all 13 stars in orange are probably returning Milky Way stars that couldn't achieve escape velocity.

[BDanielMayfield]

Two more paragraphs from the RAS article:

It is possible that these intergalactic interlopers come from the Large Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further afield. If that is the case, they carry the imprint of their site of origin, and studying them at much closer distances than their parent galaxy could provide unprecedented information on the nature of stars in another galaxy – similar in a way to studying Martian material brought to our planet by meteorites.

....

An alternative explanation is that the newly identified sprinting stars could be native to our Galaxy’s halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history. Additional information about the age and composition of the stars could help the astronomers clarify their origin.

Even at hyper velocity, aren't the distances between galaxies so great that only stars from relatively close members of our Local Group could have had enough time to reach the Milky Way?

Bruce

[neufer]

Even at hyper velocity, aren't the distances between galaxies so great that only stars from relatively close members of our Local Group could have had enough time to reach the Milky Way?

• Indeed

These stars have velocities <700 km/s while the (local) Milky Way escape velocity is >500km/s.

Interloper stars probably have intergalactic velocities of only ~0.001 c
which would restrict them to the Local Group.