How do the most massive galaxies in the Universe form, grow, and die? What kinds of galactic environments are more likely to produce massive galaxies? And, what role do dusty galaxies play in this evolutionary picture? These are the questions I seek to answer.
I am an observational astronomer, which means that I use data from ground and space-based telescopes across the electromagnetic spectrum to answer these questions.
Missing Massive Galaxies
Most of what we know about the Universe is based on visible stellar light from the nearby and distant cosmos. However, observations over the last decade tell us that half of all cosmic starlight is absorbed and reprocessed by dust, which means that the cosmic history generated by visible-light telescopes is incomplete.
The consequences of our biases are apparent in our most modern cosmological simulations: they struggle to produce sufficient populations of massive galaxies in the first 2 billion years of the Universe. These galaxies reach masses equivalent to or greater than our own Milky Way — but 10 billion years sooner.
Such rapid stellar growth produces an overabundance of dust that obscures starlight and makes the galaxies undetectable with visible-light telescopes, but bright like beacons in the infrared and millimeter spectrum. Since their surprising discovery, astronomers have been racing to understand how dusty, star-forming galaxies fit into our understanding of galaxy formation and evolution in the early Universe. Learn more about dusty, star forming galaxies.
Using observational data from across the literature, I built a numerical model that estimates the population growth and evolution of massive dusty, star-forming galaxies (DSFGs) across cosmic time. I derive a dust-obscured stellar mass function that extends beyond the knee of visible-light based stellar mass function. And, I show that, to first order, massive DSFGs are ideal candidate ancestors to massive quiescent galaxy populations seen at later times. This work was submitted to ApJ in Fall 2022.
Galaxy Protoclusters
Some of the most massive galaxies observed today are found in structures called galaxy clusters: groups of hundreds to thousands of galaxies, all gravitationally bound to one another. Astronomers believe galaxies in clusters formed the majority of their stars in the first few billion years of the cosmos (which is much quicker than most isolated galaxies). However, we are still searching for these extreme proto-cluster environments in the early Universe. And, it turns out, a significant portion of them may be hidden, obscured by the dust they produce through extremely rapid and excessive star formation. Learn more about galaxy cluster evolution.
In the last few years, we’ve discovered a handful of dusty galaxy protoclusters with similar characteristics as those predicted for the ancestors of the massive, quenched galaxies dominating local galaxy clusters. In fact, some of these dusty protoclusters have properties so extreme that they are not seen in several cosmological simulations, meaning that we still have a long way to go before we can understand how the most massive galaxies in the Universe form so quickly. See Long et al. 2020, ApJ, 898, 133L for more details.
This work was also featured in Astrobites, as well as the January 2021 paper publication of Scientific American (email for a copy).
Black Hole & Galaxy Co-Evolution
Most if not all galaxies are now believed to host a supermassive black hole at their centers. When growing and actively accreting nearby gas and dust, the supermassive black holes are extremely luminous and thus dubbed active galactic nuclei (AGN). Some AGN may be triggered by the same processes that trigger extreme star formation in a galaxy, and some AGN may play a role in quenching star formation within its host galaxy. The full picture is still unclear, though we have some ideas. Learn more about this gravitational love story.
Using observations across the electromagnetic spectrum, from X-ray (e.g. Chandra) to sub-millimeter (e.g. Herschel) wavelengths, I found no statistical evidence that AGN suppress star-formation in their host dusty, star-forming galaxies during Cosmic Noon. Furthermore, I demonstrated that bright AGN emission does not significantly contaminate observations in the far-infrared (lambda > 30 um). See Brown et al. 2019, ApJ, 871, 87B for more details.
I am also a part of a team of astrophysicists that focus on understanding how galaxy mergers may or may not trigger the majority of AGN. See e.g. Lambrides et al. 2021, ApJ, in press for more details.