Justin Spilker

Galactic Winds, Gravitational Lensing, and the South Pole Telscope Survey

Much of my PhD thesis focused on studying a sample of very dusty, highly star-forming galaxies detected by the South Pole Telescope. Originally built to observe the Cosmic Microwave Background, the SPT also detected many dusty galaxies that are by and large so bright that they're virtually guaranteed to be magnified by a foreground galaxy through gravitational lensing. By virtue of their selection, this sample tends to lie at higher redshift than other samples observed with, e.g., Herschel or SCUBA/SCUBA-2. We also have a complete sample with spectrosopic redshifts and lensing models, which now allows us to pick the "best" sources for any given science question. The SPT sample serves as a very good benchmark sample of bright dusty galaxies spanning all the way to the Epoch of Reionization.

Recently, I've been using this sample of galaxies to detect and measure the structure of galactic winds in the first 1-2 billion years of the history of the universe. This is completely new territory, and I'm really excited by it! We published the first detection in Science, the first time a molecular outflow has been spatially resolved in the early universe. In follow-up work we expanded this to create the largest sample of high-redshift galaxies with detected outflows, and the only galaxies where their structure has been spatially resolved in any substantive detail.

Selection of key and/or recent papers:
Spilker et al. 2022
Spilker et al. 2020b
Spilker et al. 2020a
Spilker et al. 2018 (Science). Press release here.
Marrone, Spilker, et al. 2018 (Nature). Press release here.
Spilker et al. 2016
Spilker et al. 2014

Quenched and Quenching Galaxies

After a decade and thousands of hours of observations, we now have a pretty good idea of which properties are the most important for determining how much molecular gas a star-forming galaxy contains - mostly redshift, star formation rate, and stellar mass. We know a lot less, though, about the gas contents in galaxies that aren't forming stars - they're very faint! I'm working to understand what the quenching process(es) do to galaxies' gas contents as they occur, using two complementary samples at z ~ 0.7, when the universe was about half its current age.

First, I and collaborators have used ALMA to observe the molecular gas in several samples of quiescent galaxies out to about redshift z = 2. In several studies over the past few years, we found that these quiescent galaxies have even less gas than you would have expected - they don't follow the scaling relations for star-forming galaxies that took thousands of hours of observation to determine! Reassuringly, the galaxies are fully consistent with evolving into the gas-poor giant elliptical galaxies we see today.

Second, I'm working with a group of collaborators to do a similar study on post-starburst galaxies at similar redshifts - these are galaxies that are much younger than the older samples we've observed, having stopped forming stars within the last few hundred million years. Called the SQuIGGLE survey, we're working to tease out the connection between gas and quenching as it occurs. So far we've seen that these galaxies tend to have much more gas than we expected.

Selection of key and/or recent papers:
Suess et al. 2022 (SQuIGGLE Survey overview paper)
Setton et al. 2022 (student-led paper)
Bezanson et al. 2022
Spilker et al. 2018
Suess et al. 2017 (student-led paper)

Compact Galaxies: Structural Change and Quenching

One of the neatest things about early passive galaxies is that they were tiny compared to passive giant elliptical galaxies today. Early quiescent galaxies were about 100 times more dense than today's quiescent galaxies, and it's not clear how they formed. One possible population of progenitors are compact star-forming galaxies. Essentially these galaxies have the same structure as early passive galaxies, but are still forming stars. So, all you have to do is get them to stop forming stars (somehow...!) and they'll look very similar to early quiescent galaxies.

To get at this question from an independent angle, I observed a few of these compact star-forming galaxies using the VLA to try to measure the amount of molecular gas they contain. If they're really going to become quiescent, they better not be gas-rich, or they'd continue to form stars over longer timescales and become too massive! I and my collaborators found that the three galaxies we observed are the most gas-poor galaxies ever seen at this redshift, very consistent with the idea that they'll quench on very short timescales. Now having identified this population of galaxies in transition, we're going back and re-observing one galaxy at much higher spatial resolution, using these objects as testbeds for theoretical models of how galaxies quench.

Selection of key and/or recent papers:
Spilker et al. 2019
Spilker et al. 2016

ISM Metal Enrichment: Better Metallicities in the Far-Infrared

There are many different ways to measure the metallicity of galaxies, most of which revolve around observing the brightest lines in visible light. Unfortunately these methods often disagree with each other by up to a factor of 5! This is mostly due to assumptions that the methods have to make about the temperature of the gas and the effects of dust along the line of sight. This project aims to find out if we can do better by using lines in the far-infrared, which are less susceptible to both dust and temperature effects. I'm leading a program to use new and archival data from SOFIA and Herschel to measure the most metallicity-sensitive fine-structure lines in a few nearby low-metallicity dwarf galaxies.

The initial results, from undergraduate-led work, are very promising! We found that the metallicity for a nearby dwarf galaxy agreed with previous determinations to better than 20%! This means that this method could be used for galaxies in the early universe, which tend to be metal-poor like modern-day dwarf galaxies. This has big implications for future far-infrared space missions like SPICA or the Origins Space Telescope, which can measure the necessary lines out to high redshift.

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