Title: Extragalactic Stellar Tidal Streams: Observations meet Simulation
Authors: Juan Miró-Carretero, Maria A. Gómez-Flechoso, David Martínez-Delgado, Andrew P. Cooper, Santi Roca-Fàbrega, Mohammad Akhlaghi, Annalisa Pillepich, Konrad Kuijken, Denis Erkal, Tobias Buck, Wojciech A. Hellwing, Sownak Bose
First Author’s Institution: Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid; Leiden Observatory, Leiden University
Status: Submitted to A&A [open access]
In the beginning, the Universe was smooth and homogenous. The rapid expansion of spacetime just after the Big Bang due to inflation meant that everything, everywhere, looked pretty much exactly the same. The only differences in the temperature and density of different parts of the Universe were tiny fluctuations, on the order of one part in 100,000, that we can see today by looking at the Cosmic Microwave Background. These miniscule perturbations are the seeds of all structure in the later Universe – every star, galaxy, and galaxy cluster that we observe today grew from a random local overdensity, billions of years ago.
As these galaxies evolved, they grew by merging with the other galaxies around them. The majority of these events would have been minor mergers, in which a host galaxy absorbs a much smaller dwarf galaxy, rather than major mergers between two galaxies of similar size, but even so they would have been dramatic events. The dwarf galaxy is ripped apart by tidal interactions with the host, leaving remnants that we can observe today as faint structures like those in Figure 1, called stellar streams.
Most of the stellar streams discovered so far, have been found within the Milky Way and its neighbors, within about 11 Mpc (36 million light years) from the center of our Galaxy. But now, we’re starting to detect streams even further from home! This will allow us to better compare our theories of how galaxies evolve with our observations, but it requires careful modeling to be able to interpret what we’re looking at. Today’s authors set out to do just that!
Simulating streams
There are many different types of cosmological simulations that can be used to study extragalactic stellar streams. The authors chose three: Illustris TNG50, a large volume simulation; Auriga, a zoom-in simulation; and Copernicus Complexio (COCO), a semi-analytical simulation. Each method has its own advantages and drawbacks. Large volume simulations like Illustris TNG50 have a large sample of galaxies to be studied, but don’t resolve physics on small scales. Zoom-in simulations like Auriga are the opposite, modeling small scales with a higher resolution for only a few galaxies. Semi-analytical simulations like COCO, meanwhile, apply complicated small-scale baryonic physics only after running a much simpler dark matter simulation. This makes them more efficient than the other two types, which model dark matter and baryons simultaneously, but means that they lose some consistency in how the baryonic physics is applied.
The authors took snapshots from each of these three simulations and used them to produce mock Dark Energy Survey (DES) observations of stellar streams, varying the background noise to produce mock observations of streams with differing surface brightnesses. As the amplitude of the background noise decreases, fainter and fainter streams can be detected, as can be seen in Figure 2. They could then compare these mock observations both with each other and with real data!
Minor mergers, major effects
Most of the galaxies the authors studied had at least one detectable stream down to a surface brightness limit of just 32 magnitudes per square arcsecond, but the different simulations behaved differently above this limit – COCO in particular predicts more detectable streams than either Illustris TNG50 or Auriga, which largely agree with each other as well as with real DES observations. This could be due to the different ways in which these simulations implement complex physical processes like cooling and feedback, which move dust and gas around the galaxy and allow it to collapse into stars. Another work the authors referenced found that satellite galaxies in simulations like COCO typically hold together under tidal forces for longer than they do in simulations like IllustrisTNG. If tidal disruption did happen later in COCO than in the other two simulations, that could explain why its streams are less diffuse and easier to see.
It could also explain some of the differences in stream morphology the authors observed between the different simulations and the real DES observations. The IllustrisTNG results agree best with DES, but COCO and Auriga both significantly overestimate the number of streams with umbrella or shell-like morphologies compared to the real observations. However, the morphology that we observe depends strongly on the angle we observe it from as well as on the physics of the merger that produced the stream, so further analysis of this complex problem will have to be left to future work!
Generally, the predictions of the simulations match up with real observations fairly well, so we can be confident of our ability to detect even very faint extragalactic streams. As we get even more data, we’ll be able to reconstruct the histories of faraway galaxies, test our understanding of cosmology, and refine our simulations of the buildup of structure in the Universe. Not bad for a minor merger remnant!
Astrobite edited by Lucie Rowland
Featured image credit: Harrison Keely/Wikimedia Commons/Katherine Lee
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Thank you Catherine for publishing this; I am the first author; just to make a small clarification, the streams in Fig,1 are at distances up to 100 Mpc.
Kind regards, Juan
My bad! Thanks for letting me know, it should be fixed now.
11 MPc is approx 36 million ly, not billion.
Typo! Thanks for catching that!