Black Hole Jets Determine a Galaxy’s Fate

They Pack a Huge Punch

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An artist’s conception of a black hole with a bipolar jet streaming away from it. These jets could carry away heavy atoms of iron and other elements.

I haven’t talked about black holes lately, so let’s take a look at the latest in what these suckers are doing. Astronomers have been watching the jets streaming out from the neighborhood of supermassive black holes (in the centers of galaxies) and found out that these high-speed streams are packed with heavy atoms of material that somehow don’t get swallowed up into the black hole. Instead, these jets act as cosmic recycling barges, sending matter and energy out to space.  If the jets are strong enough, and have enough influence, they can actually influences where and when a galaxy forms its stars.

So, what kind of heavy atoms are we talking about here?  Most black hole jets only carry electrons, which are quite l0w in mass. It turns out that astronomers using the European Space Agency’s XMM-Newton mission and a compact array radio telescope in Australia have found evidence of some unusual stuff — like iron and nickel — racing away from the black holes inside these superheated jets. They studied the black hole called 4U1630-47, which is a small black hole perhaps only a few times the mass of the Sun. Even though it’s small, the effects it has can be scaled up to larger black holes like the ones that hide in the cores of galaxies.

Jets carrying iron atoms away from a black hole must be very strong jets indeed. That’s because it takes a lot more energy to move iron than it does other, lighter particles such as electrons. The jets strong enough to carry iron and other heavy atoms are moving quite fast and are very powerful. When they smash into matter in space, they could annihilate it, thus destroying the material that a galaxy needs to create stars (and planets).

Black holes are implicated in the formation and ongoing evolution of galaxies, and this study shows one way in which they influence their surroundings.  I wonder what we’ll learn next about these pesky guys?

Snapshots of a Galaxy

The Milky Way’s Formative Years

This composite image shows examples of galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. The galaxies are arranged according to time. Those on the left reside nearby; those at far right existed when the cosmos was about 2 billion years old. The bluish glow from young stars dominates the color of the galaxies on the right. The galaxies at left are redder from the glow of older stellar populations. Courtesy STScI.
This composite image shows examples of galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years.  Courtesy STScI.

How do you see a galaxy in all its stages of formation?  You can’t watch it in real time because galaxies take millions or billions of years to fully assemble themselves. They form by assimilating each other, colliding, and interacting.

The only way to see a galaxy like the Milky Way form (for example) is to find galaxies at all stages of formation and take snapshots. Eventually, you get a series of views of how our own galaxy would have looked as it formed. (For what it’s worth, the Milky Way is still ingesting other dwarf galaxies, and it will likely gobble up the Magellanic Clouds and collide with the Andromeda Galaxy in the far future, so it’s really a work in progress.)

So, astronomers did just that: they found a lot of galaxies at different stages of their evolution, and put together this timeline of how our galaxy looked throughout its history. The galaxies are arranged according to time. Those on the left are nearby to our galaxy. Those at the far right existed when the cosmos was about 2 billion years old. The bluish glow from young stars dominates the color of the galaxies on the right. The galaxies at left are redder from the glow of older stellar populations.

To create this timeline,  astronomers traced 400 galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. They used Hubble Space Telescope and its sharp eye to peer out across the universe. The result taught astronomers a lot about how our galaxy built itself up. It turns out that our galaxy built up most of its stars (about 90 percent) between 11 and 7 billion years ago. It also means that our galaxy began forming only about a billion or two years after the Big Bang. All those billions of years ago, the Milky Way was likely a faint, blue, low-mass object. It had a lot of gas, which it used to create many new stars. When star formation began in earnest, it likely had a bluish color. The blue colors of the Milky Way ancestors are a signpost of rapid star formation. At the peak of star birth, when the universe was about 4 billion years old, the Milky Way-like galaxies in this timeline were pumping out about 15 stars a year, and our Milky Way would likely have been that active, too. By comparison, our galaxy today is creating only an average of one star a year.

Things will change in the future, when our galaxy collides with Andromeda. The gravitational interactions will fuel huge outbursts of star formation, and the night sky from our planet (if it’s still around), would be spectacular!