Astrophysicist Ignacio Trujillo explores the faintest galaxies to unravel the true nature of the universe’s missing mass.
Ignacio Trujillo (IAC)
Ignacio Trujillo Cabrera is an astrophysicist and researcher at the Instituto de Astrofísica de Canarias (IAC), where he leads studies on the formation and evolution of galaxies. His research focuses on low surface brightness structures, such as ultra-diffuse galaxies, and the investigation of dark matter. He has participated in international projects, including observations with the Hubble Space Telescope and the Gran Telescopio Canarias. His work has contributed to studies suggesting that dark matter might experience forces beyond gravity. Trujillo is an active member of the International Astronomical Union and has published numerous scientific articles in peer-reviewed journals.
How did your research interest in ultra-diffuse galaxies begin? What was known about them when you started?
One of the things that the study of the universe has brought us is the discovery of a missing mass problem, which we now know as the dark matter problem. Although the idea is nearly a century old, essentially what we observe is that astrophysical objects — stars within galaxies, galaxies within clusters — move at much higher speeds than we would expect if only the matter we can see existed. Most astrophysicists believe this is not a failure in the laws of gravity, but rather evidence of the existence of a substance that we do not directly observe. We call it dark matter because it doesn’t emit light, or if it does, it’s so faint we are unable to detect it. Today, this remains one of the great unresolved problems in physics: what is this substance?
“Dark matter remains one of the great unresolved problems in physics: what is this substance?”
Decades have been invested in particle accelerator experiments and space observations, but no particle explaining this missing mass has yet been detected. There’s even a pessimistic approach, called “the nightmare scenario,” in which this particle does not interact with ordinary matter at all. In that case, the only way to detect it would be through its gravitational effects.
And that’s where the study of ultra-diffuse galaxies comes in?
Exactly. If we want to extract properties of dark matter, we have to look for galaxies with very few stars but a lot of dark matter. The working hypothesis is that the distribution of stars is not determined by the stars themselves, but by the halo of dark matter surrounding them. In the galaxies we usually see in images — spirals, for example — the mass is dominated by stars, gas, and dust. Dark matter is present but it doesn’t dominate the internal gravitational behavior. It only clearly manifests in the outer regions of galaxies, or in systems with few stars, where its gravitational influence is predominant. To study dark matter, paradoxically, we focus on objects with very few stars. For example, the Milky Way has between 100 and 200 billion stars. In contrast, the galaxies we’re looking for have up to 100,000 times fewer stars, these are galaxies with less than 1 million stars. That’s very little in astronomical terms. They’re objects so faint that, with enough dedication, you could almost count the stars one by one.
Within the group of ultra-diffuse galaxies there’s a steep variation in sizes; some have 100,000 stars and others several million. Does that help you understand how dark matter behaves, or does it make it more complicated?
There are two important aspects. On the one hand, we look for galaxies with few stars relative to their amount of dark matter, that is, for every kilo of stars there are a hundred kilos of dark matter. The fewer stars, the better, because the proportion of dark matter is higher. But at the same time, that makes them much harder to detect. Also, the more stars a galaxy has, the more phenomena like supernovae or stellar winds occur, and that alters the environment. We seek galaxies with few stars because that ensures their structure hasn’t been altered by internal processes. The fewer stars, the fewer interferences and the more “purity,” so to speak. Ultimately, what we’re looking for are luminous points that indicate how dark matter is distributed. And the fainter these galaxies are, the better.
We want to characterize how stars are distributed in the faintest galaxies we can find, and what we’re finding, still with few objects, is that the distribution in galaxies with as few stars as possible is very consistent. This is very important, because it’s telling us that dark matter is not what we thought it was.
“We want to characterize how stars are distributed in the faintest galaxies we can find”
To be compatible with the laws of gravity, the star distribution we observe in those galaxies suggests that dark matter must have some kind of interaction, either with itself or with the stars. If that result were confirmed, we would be opening the possibility that not only does dark matter exist, but also a new type of dark force. We would suddenly understand that the universe is much more complex. Just as nuclear forces were discovered in the early 20th century, humanity could now discover in the 21st century that there are new types of interactions.
Progress is happening then on two fronts. First, there’s the improvement of instruments, but also better understanding of what dark matter is.
The way we usually work from a scientific point of view is not so much about confirming something, but about refuting or rejecting an idea. Normally you start building with the simplest hypotheses, but at some point reality tells you, “no, there’s a greater degree of complexity here.” We know that ordinary matter has a level of complexity such that a person cannot be described as a collection of hydrogen atoms, with all their interactions, their different families of particles, and so on. Our approach to dark matter right now is at the level of considering that everything is “dark hydrogen atoms,” so to speak. We are now at the point of understanding that it has to be something more complex than that.
Even now, experts on the field can’t agree on what it is made of: axions, primordial black holes, a new uncharacterized subatomic particle…
Such a broad range of possibilities it’s simply a manifestation of our ignorance. One of the reasons we go after these types of objects is that they are a collection of galaxies so extreme that we didn’t even know they existed until just 15 years ago. So all the hypotheses developed during the 20th century to explain dark matter didn’t take this type of object into account. And that’s wonderful because there was no ad hoc prejudice to explain them. Confronting a new type of galaxy it’s going to give us many clues about what dark matter could be.
Moving into the improvement of telescopes, how are new instruments installed at IAC like the Two-meter Twin Telescope (TTT) or the Transient Survey Telescope (TST) helping you?
The telescope that’s helping us the most is the TST, the 1-meter telescope with a wide field. That telescope allows us to study nearby galaxies in a way that’s three times more efficient due to its large area. Three times more efficient than, for example, the Gran Telescopio Canarias (GTC).
NGC 2403 (TST)
Keep in mind that the TST covers the equivalent of 16 full moons at once, whereas the GTC covers a small region of the moon, perhaps 10% of its area. To cover the same field with the 10-meter telescope, which is 100 times faster at collecting photons than the 1-meter telescope, we’d need three times more time to cover that field. All of this, combined with other technical issues. But basically, the TST allows us to study nearby galaxies in a much cleaner and more detailed way than traditional telescopes. The TST is truly revolutionary for the study of nearby galaxies.
How about Light Bridges’ observation model?
The working format of Light Bridges is very dynamic. It allows us to carry out observations that, through traditional time allocation committees, would be impossible. The traditional committees we astrophysicists work with are usually very rigid and conservative: they favor proposals they know will produce results. With Light Bridges I can investigate very “crazy” things, so to speak, over a length of time that no traditional panel would ever allow.
“It allows us to carry out observations that, through traditional time allocation committees, would be impossible”
For example, we’re dedicating 100 hours to study a region of the sky, in order to obtain the deepest and most continuous image of space anyone has ever made. And it probably can’t be done any other way, unless you have your own telescope. The working model and the type of telescope really give us the chance to do things no one has done before.
So I understand you’re finding new candidates for ultra-diffuse galaxies that weren’t in previous surveys like SDSS, PanSTARRS…
Correct, all of that is now completely surpassed. Right now, we are detecting, in the Coma cluster, a series of galaxies that have never been seen before. We’re conducting a unique 100-hour observation. 100 hours is a crazy amount of time. To put it in simple terms, it means you arrive to work Monday morning, press the camera button, and finish the photo on Friday afternoon. Obviously, we don’t do it that way — it’s split across many months.
Coma cluster (TST)
With just 20 hours of observation, a series of very faint and extended galaxies appeared. So far we’ve found four candidates without having done a detailed exploration yet. We don’t yet know what they are or their exact significance, but we’re on it.
And what’s the next step?
What we usually do is observe them again in more detail until we can better characterize them. Although we’ve accumulated about 70 hours, it’s a task that requires a huge investment of time and processing. Even though the observation is equivalent to five days of photography, reducing and processing that data takes us about a month of computation on powerful computers. We’re pushing current technological capabilities to the limit to detect these objects.
And once they’re detected and characterized?
Once they’re detected, we request time on large telescopes like the GTC, or even space telescopes like the Webb, to observe them with higher resolution. From small telescopes, we detect that there’s something there, but to see fine details we need larger instruments.
The idea is to consolidate a catalog of ultra-diffuse galaxies that everyone can consult.
What role could the Canary observatories play in complementing sky cartographies like LSST?
LSST will do a deep sky survey, but it mainly covers the southern hemisphere. From the Teide and Roque de los Muchachos observatories we could do something similar in the northern hemisphere with the right instruments. For example, instead of just one TST, we could have a network of TST-type telescopes. With ten, we’d go ten times faster. With 100, we could do in one hour what now takes us 100. They’re modest telescopes but easy to build and install. Light Bridges has done it in no time.
In a study published a few years ago, you said that, to properly characterize ultra-diffuse galaxies, it wasn’t enough to measure the concentration of light, that it was more useful to observe the gas concentration. Have you made progresses in that direction?
That’s a good question because it connects with another of our research lines: studying the edge of galaxies. Until now we’ve mostly talked about theory, but for example, one image we’re using a lot is the comparison between how a galaxy looks in a traditional survey versus how it looks with three hours of observation with the TST.
NGC 598 (TST)
In traditional surveys you can barely see the galaxy’s periphery, whereas with the TST you can define that edge very clearly. We’re investigating how star formation propagates from the core outward: the core forms first, and then new stars form outward. We’re seeing that galaxies are growing in size at a rate of about 4,000 to 5,000 km/h. And these types of telescopes are fantastic for studying that growth in detail.
Last question. What are you focusing on right now?
After accumulating 300 or 400 hours of data, we’re focusing on developing the best possible combination and calibration tool. We want to move from the raw image to a scientific image of the highest quality. That’s what we’re working on now. Starting in the coming months, we hope to begin publishing the results systematically.
Obviously, I don’t do this alone: behind me there’s a large team working, especially researchers doing their PhDs.
