UNLOCKING GALAXY GROWTH: A TALE OF TWO COMPONENTS – Keerthana Jegatheesan PhD Thesis

Have you ever found yourself wondering how galaxies — those cosmic whirlpools of stars, gas, and dust — form and grow over time? Then you’re in the right place, and your curiosity is well-founded. Galaxies come in a variety of shapes, with their own personalities, just as humans do, and astronomers have caught different processes in the act of shaping these personalities. While we have ideas of how galaxies form, this still remains an open puzzle, due to the diversity that we see in them. When and where does the growth of a galaxy begin? Does it start from the heart and continue to bloom outward? Or does it start from the outside and build up to the heart?

Galactic forensics: the fingerprints of stars

To find out, I use an exciting tool called Integral Field Spectroscopy (I’ll be shortening this to IFS from now), a technique that has been rapidly revolutionising our ability to analyse galaxies over the past two decades. Imagine it as a fancy camera that can capture thousands of pictures of a galaxy all at once. The camera takes each picture in a different color of light (what we like to call wavelength of light). This produces a spectrum for each point in the galaxy, which as we will see in a moment, hold valuable secrets about the stars and gas of the galaxy. We call this set of spectra Data Cubes. The wavelengths I work with are in the optical range, where the light from the stars shine stronger. For instance, younger stars are the hottest and glow like dancing blue flames, while older stars are cooler and shine like the cozy red embers of a fireplace. While a galaxy evolves in time, its stars are also evolving — as they age, they cook heavy ingredients (confusingly, astronomers like to call any elements heavier than helium as “metals”) in their cores — and all this is encoded in the spectra!

The data I use for IFS comes in the form of Integral Field Unit datacubes, or IFU datacubes. Think of it as a 4000-page photo album, where each page is an image of the galaxy at a slightly different wavelength. Sounds like a huge and complex dataset to analyse? You would be right — and this is where the novelty in our group’s approach lies. BUDDI (short for BUlge-Disc decomposition of IFU data) is a software that we developed to untangle all the information in the IFU datacubes.  SDSS-MaNGA is an IFS survey, which now has about 10,000  datacubes for nearby galaxies. Another IFS instrument is MUSE, which provides incredible detail for individual galaxies, both near and far. I use observations from both these instruments in my research. With that, the main goal of BUDDI is to model the light of the galaxy in every image in the IFU cube, and we will see why this is relevant in the context of the questions I wanted to answer.

Galactic anatomy: what’s inside a galaxy?

“Galaxies come in a variety of shapes” – broadly speaking, there are the disc galaxies and the spheroidal galaxies.  If disc galaxies, like the stunning spirals, are spinning flat pizzas, then spheroidal galaxies like the plainer ellipticals are rounder meatballs. If we delve further into these galaxies, we would find more components, such as the central bulge and the extended disc in spirals. However, in ellipticals, things are less straightforward. They could host multiple spheroidal components or include a hidden disc within.

 This is at the core of my research – decomposing a galaxy into its components and studying their individual spectra to understand how they intertwine to form the galaxy. And with that, I open the gates to the BUDDI-MaNGA project, with which we can now peel back the layers of a galaxy within large IFU datacubes.

A DANCE OF BULGES AND DISCS: DECODING THE STAR ORIGINS OF SPIRAL GALAXIES IN BUDDI-MaNGA

For this study, we used BUDDI on a number of galaxies in the SDSS-MaNGA survey, and for the first time, created the largest sample of galaxy bulge and disc spectra – for almost 1500 galaxies. From this, I take an in-depth look at the spiral galaxies, which make up 968 of the sample.  By analysing the light from the bulges and discs, I uncover how these components grow and evolve in time. Once we have the spectra, I use a technique called full spectral fitting to extract a “fossil record” of the stars.  This method lets us trace back how and when stars formed and gathered into bulges and discs since the Big Bang until now. Let me now take you through what I find:

What are we looking at?

 The “mass assembly histories” of bulges and discs in spirals. Let’s begin with Figure 3, where I roughly show how much of the galaxy’s total weight (its stars) is built over time. The lookback time tells us how far back to the Big Bang we’re looking, and Gyr simply refers to a billion years — 13 Gyr is close to the Big Bang, while 0 Gyr is the present day. Since the Big Bang, the galaxy grows through star formation and adds up its mass until the present day, where a galaxy like our own Milky Way can weigh about 3,000 trillion billion Suns.

The “heavier” bulges have assembled their stars very rapidly in a swift star forming episode, creating the bulk of the mass early on in the galaxy’s life. The “lighter” bulges have taken their time, slowly building up the mass in stars, and this occurs even towards the present day.

In the figure 4a, we see the same downsizing trend clearly in the discs, except the manner in which they assemble their stars in general is slower than in the bulges. 

The MaNGA survey provides us with a wealth of galaxies, and so I can look at different types of spirals. We’ve already seen the early Sa types, and I also study Sb, Sc, and Sd types, and everything in between — the intermediates. However, for the purposes of this article, I’d like to show the late Sd types next (these have a less prominent centre and very loose spiral arms).

The downsizing still prevails for the bulges! The assembly time is much longer than for the Sa bulges, there is a preference for slow and steady growth for both the heavier and lighter bulges. Downsizing falls apart in the discs — the curves are overlapping with each other, and we can’t say for certain which came first. This is one of our main results, showing the transition in star formation stories across the different types of spiral galaxies. With further analysis that I don’ t show here, I further confirm a clear inside-out growth pattern for most spirals, with the most massive galaxies showing this trend more strongly. On the other hand, I find that Sc and Sd spirals have no preference in which direction they grow.

What does it mean that a galaxy has grown from the inside-out or the outside-in?

The pattern of growth is strongly linked to the galaxy’s past, and what kinds of processes led to the formation of its components. For one, when a huge gas cloud collapses rapidly, it tends to form a concentrated bulge with stars whizzing around in random orbits. The disc then slowly builds from the left-over gas after it cools — which explains the inside-out formation pathway. On the other hand, a galaxy can start as a pure disc, and over time, becomes disturbed due to internal processes. It then pushes gas towards the centre, where it slowly builds up what we call a “pseudo-bulge”, where the stars are not as chaotic, and move about in an orderly rotating fashion.  This explains the outside-in formation pathway.  

Why does it matter?

Understanding how galaxies form and assemble their stars, how the different components communicate and grow, helps us piece together the Universe’s stellar history, bit by bit. While the inside-out growth mode is quite common, I find this is not universal. Spiral galaxies are more than their seemingly simple “spinning pizza” pictures might suggest — they’re diverse and are shaped by a variety of processes over cosmic time. In my research, by studying bulges and discs separately, we’re constraining these factors in a statistical way for the first time using integral field spectroscopy. And while I answer a few questions here, there’s always more to explore and dig into.

THE MANY FACES OF ELLIPTICAL GALAXIES: UNVEILING THE STRUCTURAL COMPLEXITY AND STELLAR HISTORIES

Spirals are all well and beautiful — however, in another project, I turn my focus to the often overlooked “plain” elliptical galaxies. Using a similar approach to the BUDDI-MaNGA project, I now explore the multiple components that these apparent simple present-day ellipticals might have. Three galaxies observed with the MUSE instrument formed the basis for this, and I use BUDDI again to peel away the potential layers of these galaxies.

How many components did we find?

I find that each galaxy was best described with two components: a compact inner core and a more “puffy” outer region.

How did they form?

From the mass assembly plots again (Elliptical A, B, C), we find a clear difference in how each component assembled its stars in time. The inner component formed first, when the Universe was still young, over 10 billion years ago. It likely formed from intense processes like when a massive cloud of gas collapsed swiftly, and left behind a small, dense core of old stars. The outer component seems to have grown more slowly, and likely built up through less intense mergers (when galaxies close to each other combine with each other) over time. This would simply add stars from the smaller galaxies to the outskirts.

What kinds of stars do they have?

I find that most stars in both components are quite old, over 8 billion years. However, the stellar populations we find are clearly different for them both, which shows us that they are truly different components with their own histories. In the stellar population figure, the inner components of the three ellipticals lie on the upper-right: a region of very old, very metal-rich stars. The high metallicity tells us that this component formed really fast, allowing its stars to rapidly age and evolve, and cook the heavier elements in them. On the other hand, looking at the outer components, they are more spread out on the figure — but the consistent difference is that this component is relatively slightly younger than the inner component, and have strikingly different metallicities. This result tells us that the outer component probably formed by gobbling up stars from the surrounding smaller galaxies (with their own stellar populations). This two-step process is called the two-phase scenario in galaxy formation models.

Why does this matter?

This pilot study offers a new, original way to understand elliptical galaxy structure by breaking them down into their individual components. It helps us associate each component to different phases of the galaxy’s formation in different stages of its life. The combination of the state-of-the-art MUSE integral field spectroscopy concept with the novel BUDDI analysis technique allows us to dig deeper than previous studies, and has strong prospects for continuing down this road with other diverse elliptical galaxy types.

Together with the BUDDI-MaNGA project, this research of four years has helped build a framework for both statistical as well as deep individual analyses of galaxies in the era of integral field spectroscopy, specifically in the context of peeling back the layers of different galaxy types and understanding how they all are intricately connected in the fabric of the Universe.