Starts With A Bang #103 - Active galaxies and the universe

When the general public speaks about dark matter, most of the discussion centers on the topic of whether dark matter exists or not: on whether it's real. But in astrophysical circles, we're overwhelmingly confident of dark matter's existence, as we've indirectly detected it through an enormous number of independent lines of evidence. Instead, the conversation normally focuses on how to identify and characterize it, with a particular interest in what sorts of measurements can help reveal its underlying nature. We can learn a whole lot about dark matter's nature simply by observing the luminous signatures of the normal matter within galaxies. Matter follows a path dictated by gravity, and so if you can find a signature within a galaxy that will lead to very different observables depending on the shape, profile, and clumpiness of a dark matter halo, that can help teach you which models of dark matter remain viable and which ones are disfavored by the observations. While simulations can help us in many ways, the key is data: in novel places and in the most precise laboratories of all. That's why it's so exciting that we've just discovered our very first globular cluster stellar stream in an ultra-diffuse galaxy. Not only is it our first extragalactic globular cluster stellar stream, but it happens to be located in a galaxy that's exceptionally rich in dark matter and poor in normal matter: an ideal laboratory for probing the gravitational nature and distribution of dark matter. Joining us on this episode of the podcast is its discoverer: PhD candidate Julie Kiel Holm of the Niels Bohr Institute at the University of Copenhagen. You won't want to miss this one! (This image shows an ultra-diffuse galaxy as imaged with the Hubble Space Telescope, and, in an inset box, a globular cluster and a curved stellar stream associated with it. This marks the very first discovery of a globular cluster stellar stream outside of the Milky Way galaxy. Credit: J.K. Holm et al., Nature, 2026)

Here in our Universe, there are all sorts of places where stars are actively forming right now. Our galaxy is going through a relatively quiet phase at present as far as star-formation is concerned, but that rate was much higher in the past: perhaps 30 (or more) times higher than what it is today. If we want to understand what our galaxy was like earlier on, we have two ways to go about it: look farther away, which has the disadvantage that we can't really observe those places in such great detail, or look to the galactic center, which has the highest star-formation rate of anyplace in the Milky Way. The central molecular zone, in particular, is where the greatest amount of activity is: just two degrees across and half a degree high, centered on our supermassive black hole some 26,000 light-years away. This region was obscure to humanity for nearly all of cosmic history: blocked by all of the intervening neutral matter, and by cosmic dust in particular, that optical light simply can't penetrate. But in the 20th and 21st centuries, with the advent of multiwavelength astronomy, the secrets of the galactic center are finally being revealed as never before. I'm so pleased to get to speak with Prof. Cara Battersby of the University of Connecticut for this wonderful episode of the Starts With A Bang podcast. From space telescopes to technosignatures and a whole lot more that you might not have considered, the central molecular zone, or CMZ for short, is one of the most scientifically interesting regions in all of known space. Come find out what makes it so fascinating on this all-new episode! This multiwavelength view of the Milky Way's galactic center showcases the central molecular zone: the most active star-forming region in the entire galaxy. In purple, hot, massive stars illuminate clouds of neutral hydrogen, with cold, dusty molecular gas at just 20-30 K illuminated in orange. The cyan and colored points arise from stars and from complex molecules, such as polycyclic aromatic hydrocarbons. This whole region spans only around 2 degrees across. (Credit: NRAO/AUI/NSF; Adam Ginsburg and John Bally (Univ of Colorado - Boulder), Farhad Yusef-Zadeh (Northwestern), Bolocam Galactic Plane Survey team; GLIMPSE II team)

Whenever you look at the Universe in a more powerful way than you ever have before, either in a new set of wavelengths, at higher sensitivity, on wider-field scales, or with novel capabilities, you open yourself up to a remarkable possibility. In addition to the "obvious" science gains that you'll achieve from finding more examples and revealing greater details about the types of objects and events you already know about, you also give the Universe an opportunity to surprise you: by finding things, thanks to the new power of your novel observatory, that you never expected would be out there. This isn't necessarily restricted to flagship missions that "brute force" their way past the limits of our prior frontiers, but can sometimes, for certain specific use cases, be accomplished by much lower-cost missions that specialize in one particular set of applications: what I call "finesse missions" rather than flagship missions. In the far-ultraviolet part of the spectrum, there's very little that we can do for astronomy from down here on Earth; we have to go to space to access that portion of the spectrum. But other than NASA's Hubble Space Telescope, far-ultraviolet telescopes have been few and far between. Thanks to Principal Investigator of the ASPERA mission, Dr. Carlos Vargas of the University of Arizona, that's all about to change. In this fascinating conversation, we talk about the science that's awaiting us in the far-ultraviolet, including in one of the most poorly studied regions of our own galaxy: the circumgalactic medium. It's an important place, as it's quite likely where the majority of the baryons, or the components of normal matter in the Universe, are actually located. Have a listen to Carlos and I talking about the far-ultraviolet, the ASPERA mission, and more on this edition of the Starts With A Bang podcast! (This image shows a mock-up of the ASPERA mission, slated to launch in just a few months, in space, where it can probe the far-ultraviolet emissions in the outer halo of our own galaxy and a whole lot more. Credit: NASA/ASPERA/C. Vargas/University of Arizona)

In order for stars to form, you need the right ingredients to make it happen: gravity, mass, time, and of course the right type of matter in the form of baryons. Shortly after the Big Bang, the Universe had plenty of them, but they were all very simple: protons, deuterons, helium-3 and helium-4 nuclei, and a tiny bit of lithium-7. These nuclei, made out of protons and neutrons, were all that the Universe gave us to work with prior to the formation of stars. But, in clumps of material small and large, from individual star clusters to enormous galaxy cluster scales and everything in between, these baryons went to work and, across the Universe over the past 13.8 billion years, created sextillions of stars within the observable Universe. In a very real way, the story of how we ourselves came to be is the story of how baryons evolved, matured, and wound up deducing our own cosmic history. Yet so many mysteries about that process still remain, from the baryon life cycles within galaxies to identifying the right conditions to trigger star-formation in the vastly different environments where they formed throughout cosmic history. Yet, it's by examining so many different environments where stars do and don't form, in galaxies of all different masses and metallicities, and in the modern, nearby Universe as well as far back in ancient cosmic history, that we can begin to piece the full story together. Here to guide us through this topic, and starring in this episode of the Starts With A Bang podcast, I'm so pleased to welcome newly-minted Dr. Ava Polzin to the show. Ava got her Ph.D. from the University of Chicago and is about to begin a postdoc at the University of Toronto, and has a whirlwind tour of information in store for us all on this episode of the podcast. Plus, at the end, there are a few bonuses she was eager to highlight, and you can find the links to two fabulous papers she's written for the good of the world below. Make sure you tune in; this is one episode you won't want to miss! Astronomy as a Field: https://arxiv.org/abs/2312.04041 Picture an Astronomer: https://arxiv.org/abs/2512.24465 (This image shows a composite of Hubble and JWST data for gas-rich spiral galaxy NGC 7496, which highlights not only the starlight you're used to seeing in optical images, but heated dust that shines brilliantly in JWST imagery, showcasing the locations of near-future episodes of star-formation yet to come. Credit: NASA / ESA / CSA / Judy Schmidt)

One of the most foundational questions we know how to ask in astronomy is simply this: given a cloud of gas of a given mass, what types of stars will form? How many stars of a given mass will you wind up with, and what factors does that depend on? The answer to this question, if we can give an answer, is known as the "initial mass function," and is generally very difficult to measure except in the most nearby of places: within our own Milky Way. It's possible that every time we form stars, we have a different initial mass function to reckon with. It's possible that in a different environment, perhaps with less dust, fewer heavy elements, or earlier in the Universe (when the background temperature was hotter), things behaved very differently from how they do in the here-and-now. Yet because of the extreme difference in brightness between high-mass and low-mass stars, we can only measure both high and low mass stars together nearby. It's as though we're only measuring the tip of the cosmic iceberg as far as stars go, where we're compelled to use what we know to draw conclusions about the rest of the Universe. In a very exciting new development, University of Missouri professor Charles Steinhardt, along with his undergraduate students Carter Meyerhoff and Alexander Luening, just put out a paper (link here: arxiv.org/abs/2603.23594) that could wind up revolutionizing what we think about star-formation across the Universe. Astronomers have long considered a top-heavy mass function as a possibility, but early on, perhaps "bottom-light" is a better answer. Have a listen and a good think for yourself in this truly remarkable episode of the Starts With A Bang podcast! (This image shows the Eagle Nebula, Messier 16, in a three-color composite that closely approximates the colors a very sensitive human eye would be able to see. Although the gas and dust makes prominent features, those are transient; they will be blown away in only a few million years. Although the new stars inside have formed across all different masses, the majority of the new starlight is dominated by massive, bright, blue, short-lived stars. Credit: ESO.)