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SYFY WIRE Bad Astronomy

Across the Universe, it's the normal galaxies doing all the star-making work

By Phil Plait
130 hours of MeerKAT observations in radio wavelengths yielded an image of tens of thousands of normal galaxies at distances of billion of light years. Credit: SARAO; NRAO/AUI/NSF

Sometimes, there are simple statements that can lead to profound conclusions.

For example: Stars exist.

It doesn't get more simple than that, a noun and a verb. And it's obviously true; we can see stars in the night sky, and one very bright one in the day sky. Stars exist. We also know that stars die. We've seen that happen many times, or we’ve seen what comes after.

We've also seen stars being born. You can look to practically any large cloud of gas and dust in the sky to see that, too. We also know the Sun is about 4.6 billion years old. So a little longer ago than that, a nebula brought forth our nearest star. Some stars are even older, which implies that stars were born long ago, too.

If you're an astronomer, that brings up an obvious question: Is the rate stars are being born now the same as the rate when the Universe was young? Or were more stars made back then? Fewer?

Here's another profound statement: Light takes time to move. If you see an object a light year away, you’re seeing it as it was when it emitted that light a year ago. Exploiting this, we can look to very distant galaxies to see what they looked like when they were young. From there, we can investigate how quickly they are forming stars.

The problem with that is that at very large distances we tend to see the brightest objects, the superstar galaxies blasting out radiation due to so many stars being born (and other factors). Faint ones, galaxies that don't produce as many stars, dim with distance and fade to invisibility.

Until now. Behold!

130 hours of MeerKAT observations in radio wavelengths yielded an image of tens of thousands of normal galaxies at distances of billion of light years. Credit: SARAO; NRAO/AUI/NSF

I know, it's just a bunch of dots. But take care here: Each one of those dots is a mind-crushingly distant galaxy, seen as it was billions of years ago. And not only that, but these are relatively normal galaxies, ones that are just routinely making stars, not too unlike our Milky Way.

The image was taken using MeerKAT — an array of 64 radio telescope dishes in South Africa. Each dish is 13.5 meters across, which is small for a radio telescope… but 64 of them together provides a lot of aperture. They're spread out over 8 kilometers, so when their signals are combined it's like having the resolution of a telescope 8 km wide! That means it can see very fine details on the sky, which was critical for this observation.

The image shows a region of the southern sky a little over 1° across, or the area of roughly 5 full Moons. They specifically chose a location in the sky where there were no bright sources of radio light, so they could see extremely faint objects. The observations took just under 130 hours to complete… and in that small patch of sky they found tens of thousands of galaxies.

The brightest sources in the image are likely active galaxies, ones with supermassive black holes in their centers actively and eagerly gorging themselves on material like gas and dust falling into them. That material heats up before it takes The Final Plunge, and gets so hot it glows fiercely. We can see active galaxies clear across the observable Universe.

But that’s only a fraction of the objects seen here. The vast majority are pretty much normal galaxies, ones like our own galaxy, just out there doing their usual galactic stuff: Making stars. Some galaxies make stars at a phenomenal rate, dozens of solar masses per year*. But this image shows galaxies much more laconic about star birth, ones down to about 5 solar masses per year. While that’s still fairly busy — the Milky Way makes about 1 solar mass of stars per year — it's much more like your average galaxy in the local Universe than those blockbuster ones cranking out stars.

The nearby spiral galaxy NGC 1232 is producing stars at a relatively modest rate. If we could see the dots in the MeerKAT image up close, many would look much like this. Credit: ESO

That's the beauty of observing at these wavelengths: For normal galaxies, far and away the majority of the radio emission comes from gas churning out stars. There's no 'noise' from other sources adding their light and confusion to the observations. And the more stars the gas makes, the brighter the radio emission, so if you can measure the radio waves coming from these galaxies, that gives you a direct handle on the rate of star birth there. That's how astronomers could figure out that these are pretty much normal galaxies like we see in the local Universe.

Still, there are so many of them that it means they dominated the star-making business when the Universe was young! The brightest galaxies are the easiest to see, and demand attention, but in the end they are the outliers. It’s the average citizen galaxy doing the bulk of the work back then. This is the first time any telescope has been sensitive enough to see them like this.

Interestingly, looking at the numbers that come out of the observation, the astronomers conclude that the rate of star formation 8 – 11 billion years ago is higher than expected. Either galaxies were more efficient at making stars than was previously thought, or there was more gas available to them to make stars. Either way, it's a puzzle to figure out.

Oh, and hey, this image? It's just the beginning. First of all, it was done during the MeerKAT commissioning phase after it was completed, and was designed to be used to calibrate the complex system as well as get cool science. So the array can produce a lot more stuff than this. Also, MeerKAT itself is just the precursor to SKA, the Square Kilometer Array — which refers to the actual total combined dish area of all the components! So it'll be huge and sprawling, and incredibly sensitive as well as high-resolution. It'll eat images like this for lunch, so that will be very exciting when it comes online (scheduled for 2027, though likely a bit later).

What we will discover about the early Universe then? And what new questions will we have?

* This is an average, so it's not like one star like the Sun pops out every year. Also, stars have different masses, so maybe you get 10 stars with 1/10th the Sun's mass, and sometimes, more rarely, one with a dozen or more solar masses. Like I said, it's an average so think of it more like millions of stars of all different kinds over a million years and you'll get a better idea of this number.