A new dwarf galaxy has been discovered in the constellation Crater. It is the fourth largest dwarf galaxy orbiting our Milky Way, and it’s only 390,000 light years away. So why wasn’t it discovered before? Because it was hidden in plain sight. 

The galaxy, known as Crater 2, has two characteristics that make it difficult to observe: it’s diffuse and faint. While we can easily detect stars that are part of the galaxy, recognizing that the stars are part of Crater 2 rather than our own galaxy is rather difficult. To discover it, a team of astronomers analyzed data from the VLT survey telescope, finding a statistical fluctuation in the apparent density of stars in the region. It’s the size of this dwarf galaxy that made it statistically stand out. There could be similar dwarf galaxies orbiting the Milky Way that are just waiting to be discovered.

What’s interesting about Crater 2 is that it seems to be part of a cluster of dwarf galaxies. Members of this Crater-Leo cluster seem to be gravitationally coming together. It just goes to show that even though we’re now able to observe some of the farthest reaches of our Universe, there are still things waiting to be discovered in our cosmic back yard.

Paper: G. Torrealba, et al. The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater. arXiv:1601.07178 [astro-ph.GA]

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The distribution of hydrogen in the Milky Way is something we’ve measured to a good degree of precision. Combined with computer models we can start to look at the dynamics of our galaxy. For example, back in 2009 a comparison was made between the observed distribution of atomic hydrogen in the Milky Way and possible effects of dark matter. Basically there are variations or ripples in the galactic hydrogen that don’t match up with the known distribution of visible matter, however computer simulations showed that these ripples could be caused by a localized clump of dark matter. That is, it seemed small satellite galaxy comprised mostly of dark matter is perturbing the gas and dust in our galaxy. While it was an interesting idea, proving that the ripples could be caused by dark matter isn’t the same as demonstrating that they are.  But a new paper published in Astrophysical Journal Letters has strengthened the idea.

If such a dark matter “galaxy x” exists, then we should be able to find it. The problem is that a mostly dark matter dwarf galaxy wouldn’t be particularly bright, and what light it does emit could be dimmed by gas and dust in the way. But the simulations predicted a region where the cluster of dark matter should be, so the team began a search in that region. They used public data from the ESO Public survey VISTA Variables of the Via Lactea (VVV), gathered at infrared wavelengths. Near infrared wavelengths are useful because they are less affected by interstellar gas and dust. When they analyzed the data, the team found four Cepheid variable stars clustered in the same region of the sky near the galactic plane. Cepheid variables are useful because they vary in brightness in a specific way, and we can use that fact to determine their distance. When the team did this they found the stars were all about 294,000 light years away, give or take a bit. It would be very unusual to find four Cepheid variables so close together just by chance, so it is most likely the case that they are part of a previously unknown dwarf galaxy.

In the popular press this new work is generally being presented as the “discovery” of a dark matter galaxy, but that isn’t quite the case. This new work doesn’t conclusively prove a dark matter galaxy. What the work has done is taken an earlier prediction on the existence of a dark matter dwarf galaxy, and found a clustering of stars in the general location predicted by their model. This clustering of variable stars is consistent with their model. Once again it demonstrates the predictive power of dark matter models. It also demonstrates how useful public data can be, since data gathered for one project can be used in several others.

Paper: Sukanya Chakrabarti et al. Tidal imprints of a dark subhalo on the outskirts of the Milky Way. MNRAS 399 (1): L118-L122. (2009)

Paper: Sukanya Chakrabarti et al. Clustered Cepheid Variables 90 kiloparsec from the Galactic Center. Astrophysical Journal Letters 1502: 1358 (2015)

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In an earlier post I wrote about one of the mysteries of dark matter. While dark matter matches most observations very well, it doesn’t do well in the area of dwarf galaxies. In particular, computer simulations predict that there should be many more dwarf galaxies than we observe. This has been taken to mean that either the simulations are somehow flawed, or dark matter isn’t the complete solution we’ve thought. But now new research has found that dark matter simulations might be right after all.

Credit: Yale University

The paper has been published in the Astrophysical Journal, and presents the discovery of seven dim dwarf galaxies near the Pinwheel galaxy (also known as M101). Normally this wouldn’t be that big of a deal, but what makes it impressive is how they were discovered.

The team used a new tool at Keck observatory known as the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS). It is able to observe a patch of sky and measure the distance and radial motion of the stars in its field of view. When the team looked at the results, they found that some stars were clustered not only by distance, but also by motion. In other words, they were in the same region of space and moving in the same way. This means the clusters are gravitationally bound as a group. The number of stars in these groups is sufficient to consider them dwarf galaxies. These dwarf galaxies are so dim that they wouldn’t be noticed by the usual methods.

These were found in a single region of sky. If other regions of sky have similar dwarf galaxies, then this could explain the dark matter mystery. It could turn out that there are lots of dwarf galaxies just as dark matter predicts. They are just dim galaxies, so we haven’t noticed them until now.

Paper: Allison Merritt et al. THE DISCOVERY OF SEVEN EXTREMELY LOW SURFACE BRIGHTNESS GALAXIES IN THE FIELD OF THE NEARBY SPIRAL GALAXY M101. ApJ 787 L37 doi:10.1088/2041-8205/787/2/L37 (2014)

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Most of the predictions of cold dark matter agree very well with observation, which is why it is a dominant part of modern cosmology. But there are aspects of the dark matter model that don’t agree well with observation. Take, for example, the clustering of dwarf galaxies around the Milky Way and Andromeda galaxies.

According to the dark matter model, dwarf galaxies should form within small clumps of dark matter, and they should be randomly distributed around their parent galaxy.  We can test this idea by looking at the distribution of galaxies around our own Milky Way galaxy, and a few close galaxies such as Andromeda. There have been some studies of dwarf galaxies that have shown that their distributions are somewhat random, but a new study published in the Monthly Notices of the Royal Astronomical Society has found that they aren’t really that random after all.

The team analyzed the positions and motions of dwarf galaxies orbiting Andromeda and the Milky way, and found that they tend to orbit within a broadly similar plane around their parent galaxy.  Now it is possible that this similarity of motion happened just by chance, so the team then compared these motions with computer simulations of galaxies to determine the likelihood of such an orbital arrangement.  What they found was that the odds of dwarf galaxies being arranged in this way purely by chance is about 1 in 1000.  That doesn’t put it out of the realm of possibility, but it does make the arrangement seem a bit odd.

So it seems that either the Milky Way and Andromeda just happen to have unusual dwarf galaxies, or there is a mechanism we don’t understand. These are only two galaxies, so it isn’t enough evidence to overturn dark matter, though it does seem that the more data we gather the more unusual the situation becomes.  But this is a good thing.  The more observational data we gather, the better chance we have of either confirming or ruling out dark matter candidates.

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