Title: No evidence for gamma-ray emission from the Sagittarius dwarf spheroidal galaxy
Authors: Christopher Eckner, Silvia Manconi, Francesca Calore
First Author’s Institution: University of Nova Gorica, Slovenia and LAPTh/LAPP, CNRS, France
Status: Published in Physical Review D [open access]
This bite was written and published as part of Astrobites’s new partnership with the American Physical Society (APS). As part of this partnership, we cover selected articles from the Physical Review Journals, APS’s premier publications covering all aspects of physics. For more coverage as part of this partnership, see our other PRJ posts.
In today’s paper, the authors make use of the far end of the electromagnetic spectrum – gamma rays. These are the shortest-wavelength and highest-energy photons in existence, which makes them really difficult to work with. Traditional telescope setups (photons from a specific area being gathered and focused into a detector by mirrors) really don’t work with gamma rays because gamma rays pass right through the mirrors. Instead, gamma ray telescopes have to make use of more innovative techniques. For example, the telescope used for today’s analysis (called Fermi) measures gamma rays indirectly, by measuring the electron–positron pairs the gamma rays generate when they hit a thin foil. By stacking multiple layers of foil, and measuring where the electron-positron pairs are in each successive layer, you can get a pretty good idea of where the gamma-ray photons came from. The photons then hit a calorimeter, which measures how much energy they have (this is analogous to a spectrometer in an optical telescope).
A Complex Sauce
Unfortunately, the complex detectors aren’t the only hurdle gamma-ray astronomers have to leap to get a good measurement of the gamma rays being emitted by a source. There are many large and complex sources of gamma rays in the sky, including things like the Moon and the Sun, the Milky Way, and a wide variety of smaller bright sources, as well as a diffuse background. Gamma rays emitted by these sources tend to follow complex trajectories on their way to the Earth, because they can be bumped around and redirected from their original trajectories by cosmic rays in intervening gas. When studying gamma rays, astronomers have to figure out whether the emission they’re seeing is from the source they’re trying to see or any of these other potential sources. Much like separating just one ingredient out of a complicated soup, this is very difficult! Typically, astronomers manage this by modeling all the different sources simultaneously using complex analysis software.
Some Big Bubbles
Despite the difficulties of gamma-ray astronomy, gamma-ray telescopes like Fermi have already taught us a lot we didn’t know about the sky. One of Fermi’s biggest results was the discovery of the Fermi Bubbles in 2010 – enormous lobes of gamma-ray emission extending out of the Milky Way. These structures span about 50 000 light years, and we still don’t have a particularly clear picture of what caused them (some astrobites have gone into potential explanations!). The Fermi Bubbles contain several regions of enhanced emission, whose origins are also a mystery.
Today’s paper explores one of those regions in particular, known as the Cocoon region in the south (shown along with the rest of the Fermi Bubbles in Figure 1). A previous paper had suggested that this region of increased emission comes from a background dwarf galaxy. In today’s paper, the authors argue that that may not be the case.
A Galactic Ingredient?
The Sagittarius Dwarf Spheroidal Galaxy, the galaxy which some astronomers believe is responsible for this region of extra gamma-ray emission, is pretty special – it’s one of the Milky Way’s nearest and most massive satellite galaxies. We’ve also definitely observed gamma rays coming from other galaxies, so it’s not unreasonable that this galaxy would emit some. Today’s authors don’t argue that it’s impossible for the Sagittarius dwarf galaxy to emit gamma rays, just that the ones we’re seeing here can be better explained by other sources. They use two strategies to do so – a state-of-the art code which models different sources of gamma-ray emission, and a statistical trick that enables differentiation between gamma rays emitted from point sources and diffuse sources.
Getting the skyFACTs
The first, modeling-based strategy uses skyFACT, a code pipeline designed to determine the origins of each gamma ray making up the sky. The skyFACT software generates a model for each potential source of gamma-ray emission (based on other measurements of those sources, such as the gas in the Milky Way, and known physical relations between those things and gamma rays), propagates the modeled gamma rays emitted from those sources through modeled fields of cosmic rays, and then accounts for uncertainties in how the telescope would receive those cosmic rays.
Using this strategy, the authors determined that the extra emission seen coinciding with the Sagittarius dwarf galaxy is totally explicable by other gamma ray sources (such as the Milky Way galaxy or the Fermi bubbles themselves). They argue that the main things missing from the previous analysis were the accurate modeling of the Milky Way and the intervening cosmic rays – once these are added in, the enhanced emission seen over the Fermi Bubbles could just be a part of the Fermi bubbles themselves. Models of all the background sources, as well as potential emission from the Sagittarius dwarf galaxy, are shown in Figure 2. The best-fitting model doesn’t require any contribution at all from the Sagittarius dwarf galaxy to explain the gamma ray data.
Sauce Stats
Secondly, the authors checked the statistics of the gamma rays observed in the region of enhanced emission. Because gamma rays are so high-energy, they’re pretty rare, and gamma-ray detectors see them as individual photons. This means that the statistical distribution of the individual photons can be measured. For smooth, diffuse sources of gamma rays, the gamma rays should follow Poisson (counting) statistics. For point sources, or diffuse sources with a lot of structure, however, the gamma rays are not Poissonian. Gamma rays from galaxies are mostly from millisecond pulsars and structured gas, so they should fall into this latter case. The authors find that there isn’t any evidence for non-Poissonian statistics in the gamma rays from this region, further casting doubt on their potential galactic origin.
A Possible Hint of Flavor
Neither of these strategies rule out there being any emission at all from the Sagittarius dwarf galaxy – they’re just arguing that at Fermi’s current level of sensitivity (after accounting for all of the other sources of gamma-ray emission), the Sagittarius dwarf galaxy is not being definitively seen. With this in mind, they approach the problem from the other end: is it possible that the galaxy is bright in gamma rays, but they just look too similar to other gamma ray sources to be picked out? The authors find that the answer is yes, but the gamma rays would have to be fainter than those measured in the previous paper, and mostly from a very diffuse population of millisecond pulsars. This scenario isn’t impossible, but it is a very specific constraint.
Ultimately, this paper underlines that gamma ray astronomy is very difficult! Not only has it unveiled whole new structures (such as the Fermi Bubbles) that we still don’t fully understand, our modeling and analysis of what we observe in the first place has to be done extremely carefully. There are all sorts of nuanced effects that must be taken into account. This makes gamma-ray astronomy daunting, but also very exciting – there remain a lot of questions to be asked and answered!
Astrobite edited by Lucas Brown
Featured image credit: NASA/Hubble Advanced Camera for Surveys
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