So long, and thanks for all the jellyfish

Title: Jellyfish galaxies with the IllustrisTNG simulations – When, where, and for how long does ram pressure stripping of cold gas occur?

Authors: Eric Rohr, Annalisa Pillepich, Dylan Nelson, Elad Zinger, Gandhali Joshi, Mohommadreza Ayromlou

First Author’s Institution: Max-Planck-Institut für Astronomie, Heidelberg, Germany

Status: Published in Monthly Notices of the Royal Astronomical Society [open access]

On the outskirts of massive galaxies exist perhaps the most whimsical class of astrophysical objects – smaller satellite galaxies that resemble jellyfish diving headfirst into their massive companions. Befittingly called “jellyfish galaxies” (or if you’re certain there are no jellyologists within earshot, “jellyfish” alone suffices), their distinctive shape results from a process called ram pressure stripping (RPS). As a jellyfish galaxy falls through the gaseous medium surrounding a larger host galaxy, the pressure of the gas creates a drag force that pulls gas out of the jellyfish, leaving it streaming behind. Eventually, the jellyfish loses all of its gas and merges completely with the host galaxy.

Astronomers have observed plenty of jellyfish galaxies across multiple wavelengths, like the example in Figure 1, but studying their evolution is tricky with observations that are only snapshots frozen in time. To answer questions like when, where, and for how long RPS occurs, the authors of today’s paper use simulations to follow the lives of jellyfish through cosmic time. Specifically, they focus on the cold gas, below 10^4.5 kelvin, stripped from jellyfish because the less gravitationally bound hot gas is quickly lost, and because stars form from cold gas, so it plays a major role in the stellar evolution of both the jellyfish and the host galaxy where the gas is deposited.

Fishing for Cosmological Jellyfish in the Zooniverse Sea

The authors used the highest resolution version of the IllustrisTNG simulations, TNG50, which models nearly everything needed to evolve thousands of galaxies from just after the Big Bang to present day within an enormous 50 megaparsec-sided box. Rather than torturing a graduate student with the task of picking jellyfish out of tens of thousands of simulated galaxy images, jellyfish galaxies were identified by citizen scientist volunteers as a part of the Zooniverse Cosmological Jellyfish project. From the jellyfish found by volunteers, the authors cleaned their sample by removing any that had already been disturbed by a galaxy other than its final host and were left with a sample of 1543 jellyfish galaxies.

When, where, how long, and how much?

To identify the cold gas lost via RPS throughout the simulation, the authors tracked gas that became gravitationally unbound from the jellyfish and trailed behind as they fell into their host galaxies. Technically, other physical processes can push gas out of a galaxy, but RPS is the dominant cause of gas loss for infalling galaxies, and all galactic outflow processes are interconnected. For example, RPS can initially cause gas in the center of a jellyfish galaxy to compress, leading to increased star formation related outflows – but because the star formation resulted from RPS, this ejected gas is still attributed to RPS.

The jellyfish were found to lose cold gas on timescales varying from 1-7 billion years, and around half of the jellyfish had lost all of their gas by the end of the simulation, typically those with smaller jellyfish-to-host mass ratios. As a jellyfish galaxy falls into a host galaxy, it will orbit the host a few times, spiralling inwards until it fully dissipates and merges. However, the majority of gas is lost within the first orbit 1-2 billion years after infall began. Figure 3 demonstrates one example of the cold gas a jellyfish lost over time, as well as its distance from the host galaxy.

The early gas loss also means that a lot of cold gas is scattered throughout the host galaxy’s gaseous halo, up to 10¹² solar masses worth, dramatically impacting the host galaxy’s evolution by influencing its gas dynamics and pouring fuel on the fire of star formation. The jellyfish galaxies themselves experience bursts of star formation during the first orbit, enhanced by the gas disruption from RPS. Interestingly, star formation in jellyfish persists for a long time – up until around 98% of the initial cold gas is lost, despite losing so much gas early on.

The Simulation-Observation Whirlpool

Overall, this study found that RPS can act on infalling jellyfish galaxies for billions of years, though most of the cold gas is lost in the first orbit, and gas is deposited at a wide range of radii around host galaxies. The source of cold gas in the circumgalactic medium (a galaxy’s gaseous halo) is currently debated, but this work shows that jellyfish are likely a major source.

The authors note that these results are only confirmed for the jellyfish galaxies within their sample, all of which were created using the specific physics and conditions of the IllustrisTNG galaxy formation model, although it does produce satellite galaxies that have star formation rates and gas content that broadly agree with observations. For now, they can’t claim their findings exactly represent the real jellyfish out there, but they are still beneficial because the relationship between simulations and observations is one big positive feedback whirlpool – simulations informed by observations help to interpret observations to make better simulations. Computational advances over the next decades will result in simulations of jellyfish galaxies that increasingly resemble real ones, as long as we just keep swimming.

Astrobite edited by Samantha Wong

Featured image credit: Ana Maria Slivar, https://www.instagram.com/amslivar.art/