Authors: Deborah Lokhorst, Roberto Abraham, Pieter van Dokkum, Nastasha Wijers, Joop Schaye
First Author’s Institution: Department of Astronomy & Astrophysics, University of Toronto
Status: Accepted to ApJ, open access on arXiv
Generally when we look at a picture of a galaxy, we see the bright central region occupied by many stars and brightly glowing gas. This can include shining arms in spiral galaxies or the clumpy splotches of light scattered around irregular galaxies. However, there is much more to a galaxy than what is brightly glowing: every galaxy is surrounded by a thin, cool, and difficult to observe cloud of gas called the circumgalactic medium (CGM). Even farther away there is a yet thinner distribution of gas, called the intergalactic medium (IGM). These structures have such low densities that the gas doesn’t emit enough light to be visible to normal telescopes. Consequently, astronomers only have a vague picture of the geometry, composition, and conditions of these components of the universe. It’s important to understand the CGM and IGM though, because they contain the majority of the baryonic matter (i.e. normal, non-dark matter) in the universe and are crucial for regulating the flow of gas onto galaxies, which allows for things like the formation of stars. Further, the CGM is currently observed mostly with absorption line studies which are restricted to directions in the sky where a background light source, such as a quasar, is available. This means that observations are often quite limited in number, making it difficult to get a comprehensive view of what is happening.
In today’s paper, the authors wrangled two tempestuous creatures: the EAGLE cosmological simulation and the Dragonfly Telephoto Array. EAGLE is a numerical code that creates a simulated chunk of the universe and Dragonfly is a 48-lensed instrument specially designed to observe emission from very dim objects. The idea is that the authors can simulate CGM and IGM around galaxies using EAGLE and predict what their emission should look like. Then, knowing the parameters of the Dragonfly array, they can determine whether this emission should be observable by such an instrument. The authors use the capabilities of a (now in-progress) upgrade to Dragonfly, involving the added capacity to use narrow-band filters).
Modeling emission in EAGLE
The authors are interested in one emission line in particular, the Hα feature. Hα emission is formed by a process called recombination that occurs in the bubbles of ionized hydrogen called H II (pronounced “H 2”) regions that surround young, massive stars. This process occurs so commonly in the gas around young stars that it can be very bright, making it one of the most likely emission lines to be observed.
Information about the state of the gas particles in the simulation, such as their density, temperature, and metallicity, are used to model the gas emission and calculate the surface brightness, or the amount of light per square arcsecond coming from the gas (today’s paper uses units of photons cm-2 s-1 sr-1). The only remaining question: Is the signal bright enough to be seen with the upcoming iteration of Dragonfly?
Does Dragonfly need better eyes?
Extracting the emission of Hα around galaxies within the EAGLE volume, the authors calculate the surface brightness averaged in rings centered on each galaxy, creating radial profiles of the Hα’s glow. Splitting the galaxies into categories based on mass, they find that the inner edge of the CGM (corresponding to the red circle in Figure 2) should be visible for galaxies with stellar masses above about 1010 solar masses. This means that in the search for CGM emission, astronomers shouldn’t need to target rare, outrageously massive galaxies to find a signal. A similar analysis is done by creating a false Dragonfly observation of a single test galaxy, with noise and an instrumental spread function applied to the emission map (Figure 2). The finding is similar: portions of the inner edge of the CGM should be easily visible with only ~10 hour exposures by Dragonfly, without even the need for radial averaging used in the previous test. To push farther into the CGM, however, such averaging would appear to be necessary, since the Hα surface brightness drops as the distance from the galaxy increases.
The IGM has a much lower density than the CGM so should be intrinsically fainter in emission. Using a similar false observation method, the authors isolate an IGM filament from the simulation (Figure 3) and determine if the signal would be observable in a reasonable amount of time by Dragonfly. They find that even the brightest emission of the IGM, coming from dense clumps, reaches only about 1 photon cm-2 s-1 sr-1. With such a low surface brightness, it would take over 1000 hours (almost 6 weeks!) to obtain a signal that outshines the noise. Unfortunately, this means that the Dragonfly instrument upgrade plan would need to incorporate additional lenses to observe the IGM in this way.
Although it is disappointing that the upcoming Dragonfly upgrade likely won’t be able to observe the IGM, the ground it could gain on studies of the CGM are fundamental to studies of galaxies. Compared to the severe limitations on absorption line studies, observations of the CGM in emission may reveal more about its structure, how it is affected by inflows and outflows, and its interaction with the galaxy proper.