Disclosing Submillimeter Galaxy Formation: Mergers or Secular Evolution?

John: Welcome to our seminar on Topics in High-Redshift Galaxy Evolution. Today's lecture is on the paper 'Disclosing Submillimeter Galaxy Formation: Mergers or Secular Evolution?' by Chan, Ao, and Tan from the Purple Mountain Observatory in China. For a long time, the field has leaned heavily on the idea that the most intense starbursts at cosmic noon were triggered by major galaxy mergers. This paper uses new JWST data to challenge that long-standing paradigm, suggesting internal processes might be more important. John: Yes, Noah? Noah: Hi Professor. So, is the core argument that the previous consensus, largely based on Hubble data, might have been skewed by observational limitations? John: Precisely. The central question is what drives the formation of these submillimeter galaxies, or SMGs. They are incredibly dusty and responsible for a huge fraction of star formation when the universe was only a few billion years old. The prevailing theory compared them to local ultraluminous infrared galaxies, which are almost all merger-driven. Early Hubble observations seemed to confirm this, finding merger rates of fifty to sixty percent. But SMGs are so dust-obscured that seeing their true stellar morphology in the rest-frame optical was difficult. John: This is where the James Webb Space Telescope comes in. Its sensitivity in the near-infrared allows us to peer through the dust and see the underlying structure of these galaxies much more clearly. The main contribution of this paper is leveraging that capability to perform a detailed morphological analysis, aiming to settle the debate between mergers and what's known as secular evolution. Noah: So what exactly falls under the umbrella of 'secular evolution' in this context? Is it just any non-merger process? John: Good question. In this high-redshift context, 'secular evolution' primarily refers to internal processes that can rearrange mass and fuel star formation. The main candidates are violent disk instabilities, where massive, gas-rich disks fragment into giant star-forming clumps that can then migrate to the center to build a bulge. Another is the steady accretion of cold gas from the cosmic web, which can sustain star formation without a disruptive merger. The paper's key finding is that these secular processes appear to be the dominant mechanism. Their analysis points to a merger fraction of only about twenty-four percent, a stark contrast to the earlier Hubble estimates. Noah: That's a significant drop. How did they arrive at that number? What was their methodology? John: Their approach was multi-faceted. They started with a sample of 125 ALMA-detected SMGs in the COSMOS field, which they then analyzed using deep JWST NIRCam images. Their analysis had two main prongs. The first was parametric modeling, where they fit the galaxy light profiles using a double Sersic model. This lets them quantitatively separate the galaxy into a central bulge and an outer disk component, and measure properties like the bulge-to-total light ratio and the Sersic index, which describes the concentration of the bulge. Noah: Why a double Sersic fit? Is a single component not sufficient? John: A single component can give you a general sense of the galaxy's concentration, but it can't disentangle the structures. A disk galaxy with a prominent bulge will look very different from a pure elliptical galaxy, and a single Sersic fit might struggle to capture that. By modeling the bulge and disk separately, you can directly test hypotheses about how each component is forming. For example, merger-built classical bulges tend to have high Sersic indices, while secularly-built pseudo-bulges have low, disk-like indices. John: The second prong was non-parametric. They used tools like the Gini-M20 classification scheme, which measures light concentration and spatial distribution without assuming any model. Disturbed, merging systems occupy a distinct region in this parameter space. Finally, they also searched for stellar bars, which are unambiguous signposts of secular evolution in disk galaxies. Combining all these methods gives a more robust picture than relying on just one. Noah: Okay, that makes sense. But the most intriguing result you mentioned was this large population of 'unclassified' bulges. What does that imply? John: Right, this is perhaps the paper's most significant finding. After classifying bulges based on the standard local criteria, they found that nearly half their sample—forty-eight percent—didn't fit. These galaxies had very prominent bulges, with high bulge-to-total light ratios, but their bulges were not very concentrated, showing low Sersic indices, similar to disks. This directly challenges the neat dichotomy we see in the local universe between classical, merger-built bulges and secularly-formed pseudo-bulges. Noah: So what could they be? Are they a completely new type of structure, or maybe an intermediate phase? John: It suggests a formation path that's common at high redshift but rare today. The authors propose these could be bulges in the process of being built through violent disk instabilities. The clumps migrate inward, depositing stars and gas, creating a luminous central component that hasn't yet settled into a dense, dynamically hot spheroid. This reinforces the idea that the physics of galaxy formation in the gas-rich early universe is fundamentally different. It shifts our focus from external triggers like mergers to the internal dynamics of the galaxies themselves. John: The key takeaway here is that JWST is showing us that many of these extreme starburst galaxies at cosmic noon are not the chaotic merger events we long thought they were. Instead, they appear to be massive, gas-rich disks undergoing intense, but internally-driven, periods of growth and star formation. The universe at that epoch was fully capable of building massive galaxies through these more secular pathways, reshaping our models of galaxy evolution. John: Thanks for listening. If you have any further questions, ask our AI assistant or drop a comment.