Space

Andromeda's "halo" already bumping into Milky Way's ahead of collision

Andromeda's "halo" already bum...
A composite image, showing how Andromeda's halo would look in the night sky were it visible to the naked eye
A composite image, showing how Andromeda's halo would look in the night sky were it visible to the naked eye
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A composite image, showing how Andromeda's halo would look in the night sky were it visible to the naked eye
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A composite image, showing how Andromeda's halo would look in the night sky were it visible to the naked eye
An illustration of Andromeda's gas halo – the team studied this gas using the light from 43 background quasars, marked here as dots
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An illustration of Andromeda's gas halo – the team studied this gas using the light from 43 background quasars, marked here as dots

There’s more to galaxies than meets the eye, and now astronomers have mapped out a huge invisible region of the galaxy next door. Observations by the Hubble Space Telescope have detailed the size and structure of the gas halo of Andromeda – and found that it’s already bumping up against that of the Milky Way, in advance of a cosmic collision.

Galaxies are usually seen as flat disks of stars, but that’s not the full story. Surrounding most galaxies is a huge spherical envelope of gas and plasma that extends for thousands or even millions of light-years. The trouble is that these structures are very hard to see, because the molecules making it up are so diffuse and don’t give off much radiation.

To study these halos, astronomers instead analyze the way light filters through from much more distant objects. Quasars make a good source since they’re so bright, but for most galaxies only one or two fall in the right line of sight.

But for Andromeda, which is far closer to us than other galaxies, astronomers were able to use 43 quasars scattered around the halo. The team used Hubble’s Cosmic Origins Spectrograph (COS) to study the ultraviolet light of these quasars, and how it’s absorbed by the gas in Andromeda’s halo.

An illustration of Andromeda's gas halo – the team studied this gas using the light from 43 background quasars, marked here as dots
An illustration of Andromeda's gas halo – the team studied this gas using the light from 43 background quasars, marked here as dots

The team found that the halo has a layered structure, with one “shell” of gas nested inside another. The signatures of large amounts of heavy elements were also found floating around out there – a smoking gun for supernova explosions.

“We find the inner shell that extends to about a half million light-years is far more complex and dynamic,” says Nicolas Lehner, lead researcher on the study. “The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”

The team also found that the outer layers reach much further than previously known. The halo extends 1.3 million light-years from Andromeda, and in some directions it stretches as far as 2 million light-years.

The intriguing thing about that figure is that it means Andromeda’s halo is already beginning to press up against that of the Milky Way. The two galaxies are expected to collide and merge in about four billion years’ time, and it looks like that process is already in the (very, very) early stages.

Since Andromeda is very similar to the Milky Way, this kind of work could help us better understand our home galaxy – after all, it’s tricky to study the Milky Way’s halo from our position inside it.

The research was published in the Astrophysical Journal.

Source: Hubble

6 comments
Username
If the universe is universally expanding why aren't the galaxies moving further apart? (even if they themselves are expanding)
solas
@Username You are at the wrong scale in space-time: two local galaxies can still collide, just like two local stars can randomly collide from time to time. At least right now, the picture is a bit dark a few trillion years out...
Fritz
We bumping into theirs too!
Brett Pendley
If the outer layer is bumping the milky way and extends two millions years does that mean the outer layer is going to hit us soon or is it still going to be years and years before any part of Andromeda hits us?
Brett Pendley
If it is bumping the milky way already is the outer halo or any part of Andromeda going to be hitting us soon and is it going to still hit in four billion years or is it going to happen sooner because i am a bit worried about it bumping into the milky way already and being two millions light years in length or longer makes me think its going to hit us soon
Maxim Bestnameever
That is like two ants sitting on a branch of the highest tree in the woods looking up to the sky "this is obviously the biggest glow-worm you can imagine, cause the only thing which we know can glow is a glow-worm". We are sitting here on a very small rock in the universe, have never set a foot on another planet. Our modern scientific knowledge of Astronomy is like 60-70 years old, but here we go, blabbering about thousands of light years, composition of stars and halos of galaxies. I know, physics is the new religion for many, they prefer to ignore, that as long as there is no explanation to how a universe can be infinite, there is some huge and very critical bug in of understanding of it. Compared to its current state, even creationism offers a more logical explanation.