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Mapping the dark matter in the tiniest galaxies

The spiral galaxy M81, as seen by the Hubble Space Telescope. Galaxies such as this contain roughly 10 times as much dark matter as ordinary matter. [Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)]

The spiral galaxy M81, as seen by the Hubble Space Telescope. Galaxies such as this contain roughly 10 times as much dark matter as ordinary matter. [Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA)]

Galaxies are beautiful but very messy objects, and it seems the more wonderful the galaxy, the more complicated it is. At their most grand, galaxies are full of stars, clouds of gas, dust…and of course dark matter, the invisible substance that helps to bind everything together gravitationally. In a spiral galaxy like the Milky Way, dark matter comprises about 90% of the total mass, spread out into a vast, roughly spherical region known as a halo surrounding the glowing stars we can see.

And that’s where things can get tough. We know there’s a lot of dark matter in galaxies, since it affects the motion of stars, gas, and dust. Dark matter even affects the path of light through gravitational lensing. However, in the denser regions of galaxies, the precise distribution of dark matter — known as the dark matter profile — is harder to measure. That’s because all the components are more tightly packed close to the galaxy’s center, meaning astronomers have to know a lot about the behavior of the ordinary matter before they can fully extract the dark matter profile. This difficulty has led some researchers to abandon dark matter entirely, despite the overwhelming evidence for its existence in a variety of observations.

The Fornax dwarf spheroidal galaxy, a satellite of the Milky Way. The brightness of this galaxy is sufficiently low that it's very hard to see, much less study. [Credit: ESO/Digital Sky Survey 2]

The Fornax dwarf spheroidal galaxy, a satellite of the Milky Way. The brightness of this galaxy is sufficiently low that it’s very hard to see, much less study. [Credit: ESO/Digital Sky Survey 2]

Another type of galaxy, the dwarf spheroidals (dSphs), could provide another way to measure dark matter profiles. Dwarf spheroidals barely look like galaxies at all: they contain relatively few stars — numbering in the low millions or fewer — and almost no gas or dust. However, dark matter comprises more than 99% of the mass of dwarf spheroidals, the highest fraction known in any object. This excess of dark matter and relative simplicity of structure makes dwarf spheroidals interesting for understanding dark matter profiles, as well as testing theories of galaxy formation.

Unlike the Solar System, galaxies aren’t dominated by the gravity of a single object. Instead, a given star moves under the collective influence of everything else: other stars, gas, dust, and dark matter.[1] The velocity of stars and gas provides a way to map the gravitational field of the whole galaxy, which is created by all its contents; that’s how we can reconstruct the amount of dark matter in a galaxy. Even more, the collective motion of stars reveals the total mass of a dwarf spheroidal, since there’s basically nothing but stars and dark matter.

A new paper by John Jardel and Karl Gebhardt took observational data on five dwarf spheroidal galaxies and attempted to find a dark matter profile that could describe all of them.[2] (That sounds like a small sample, and it is — but dwarf spheroidals are difficult to observe. We only know of about 20 total, though there must be a lot more.)This effort is important because most models of dwarf spheroidals begin by assuming they follow a dark matter profile developed for larger galaxies. By contrast, Jardel and Gebhardt began with a minimum of assumptions to see whether they could find a shared dark matter profile from the data alone.

They found that the individual dark matter profiles didn’t look much alike, but when they were combined statistically a clear pattern emerged: the density of dark matter within the galaxy fell proportionally to distance from the center. That means that twice the distance from the galactic center, the dark matter density falls to half. In mathematical terms, DM_density_dSph
This is similar to what is seen in the central regions of many large galaxies (though read on for more details). They also accounted for the fact that the five dwarf spheroidals are all satellites of the Milky Way and lie within our galaxy’s dark matter halo. That affects the total profile at the point where the dwarf galaxy’s dark matter merges with that of the larger galaxy; the researchers found that point using their model.

However, things aren’t perfect: the fit to the simple model grows worse closer to the center of the galaxy, and there’s a lot of variation between the individual dwarf spheroidals. In a sense, that’s not surprising. No two galaxies are alike, whether dwarf spheroidal or flocculent spiral, so it seems improbable that they should have identical dark matter profiles either. The reasons for the dissimilarities are likely due to how they formed and evolved. Dwarf spheroidals evidently lost much of their ordinary matter since their stars formed, and the processed to make that happen surely affected the dark matter content as well.

Similarly, large galaxies present some challenges to astronomers trying to model their dark matter profiles. Near the center of most galaxies, the profile follows a pattern similar to that found for the dwarf spheroidals, which means if that pattern increases all the way to the center, there should be a huge density spike known as a cusp. However, there are other hints that dark matter could form a region in lower-mass galaxies where density stops increasing so drastically, known as a core. Hence, studying dwarf spheroidals with their lack of gas and dust is a nice way to see whether they can provide an answer to the cusp-vs.-core conundrum. So far, the answer is ambiguous: the observations Jardel and Gebhardt used don’t have enough resolution close to the hearts of each galaxy.

Nevertheless, these results are important and interesting. The researchers didn’t assume their dwarf spheroidals would obey the same laws as their larger relatives, but they found similar laws provided the best fit to the data. Whether the variations from galaxy to galaxy are significant or not remains to be seen; that’s often the case in dealing with objects as messy as galaxies.

Notes

  1. I’m simplifying slightly: only mass within the star’s orbit affects the orbit. A galaxy’s supermassive black hole mostly affects stuff near the very center of the galaxy; dwarf spheroidals don’t seem to have central black holes anyway.
  2. John R. Jardel and Karl Gebhardt, “Variations in a universal dark matter profile for dwarf spheroidals.” Astrophysical Journal Letters 775, L30. DOI: 10.1088/2041-8205/775/1/L30; ArXiv: 1309.2637v1
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