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Most people have just ignored them—the galaxies so quiet or faint that they were deemed silent some two decades ago. It seems that a closer look is worth the trouble, though: some of these same galaxies are surprising researchers by giving off clear and measurable radiation.  

It’s a big, big world 

From our perspective on Earth, our galaxy the Milky Way is huge, with somewhere in the range of 100-400 billion stars. In the grand scheme of things, this massive group is just one drop in a much, much larger sea. Current estimates place the number of galaxies in the observable universe—so the part that we can currently see—at up to 2 trillion.  

Some galaxies, like ours, are pretty normal; the energy they put out can be explained by the stars within their confines. Others are a little more complicated. 

Galaxies can emit various kinds of electromagnetic radiation, including radio, x-rays, ultraviolet light, and microwaves. Since these all occur at different frequencies, tuning into the right wavelength can fill in details of what’s actually happening. That means that if we aren’t watching the right channel, we aren’t seeing the whole picture. 

Radio galaxies emit most of their energy in radio waves—not from stars, like in the Milky Way—so we need to use radio telescopes to see them. The evidence gained so far suggests that radio emissions are coming from plasma jets near the core of each galaxy’s supermassive black hole. From their central spot in the neighbourhood, these jets shoot plasma far and wide, even beyond the boundaries of the galaxy, at nearly the speed of light. 

These systems can actually tell us a lot about galaxies. But because they are so very far away, research has focused on the loudest ones, the galaxies that give off the strongest radio waves, even though these make up only about ten percent of the thousands known.   

 ‘It was a big surprise to detect radio emissions from galaxies that have been labelled radio silent,’ says Anne Lähteenmäki, Professor at Aalto University, who started studying these distant galaxies in 2012. ‘Most studies have concentrated on the bright galaxies instead.’ 

Quiet yet powerful 

Anne got curious about fainter signals in her work at Finland’s Metsähovi Radio Observatory. She wanted to take a closer look at the quieter galaxies, the less obvious choices, as a way to study how these gravitationally bound systems evolve. It turns out that young, radio-quiet galaxies provide the perfect context, because they just haven’t had the chance to grow up.  

‘I got interested in their evolution and the various evolutionary lines galaxies could take,’ says Anne. ‘Researchers have always assumed that these galaxies, which are very young, cannot launch plasma jets.’  

Yet all of sudden, evidence showed that they can. It also showed that the masses of central black holes of young galaxies are lower than in brighter sources—roughly a difference of a million of our suns compared to a billion. 

So the question is, how do young sources become the really bright, powerful ones?  

Anne’s research has delivered another surprise in the quest for an answer. Not only can so-called radio-silent galaxies produce active jets, they’re spitting out an unexpected kind of energy, gamma rays, typically seen in more mature versions.  

‘You need a lot of energy to create gamma rays. Normally we only see them from elliptical galaxies, which are more evolved, which means these young spiral galaxies are actually really powerful, as well,’ Anne says.  

If Anne’s work highlights one thing, it’s that dedicated eyes and ears, and perhaps a curious mind, will help us understand the universe just a little bit more. The galaxies classified as radio-silent got their labels in one limited all-sky study; her research has shown us why even the quiet ones need attention. After all, with new technologies and the universe as a playground, there are bound to be further developments—and more surprises.