In astronomy we have a problem: we observe far fewer dwarf galaxies than we expect based on computer simulations of the Universe. We call this the dwarf galaxy, missing satellite or substructure problem. Dwarf galaxies are small companions of bigger galaxies, like our own Milky Way. Our computer simulations investigate the evolution of the Universe and are very effective at producing Universes akin to our own.
However, there are some discrepancies, one of which is the dwarf galaxy problem. The simulations predict that a galaxy like our Milky Way should have around 10 times more satellite dwarf galaxies than we actually observe; the same is true for all large galaxies. This is illustrated in Figure 2, which shows a comparison between a simulated galaxy and a real image of the Andromeda galaxy. The simulated galaxy has thousands of companions, whereas only a few can be made out around Andromeda.
So, what are the possible solutions? Flawed physics, limited optical surveys and starless ‘dark galaxies’ are some of the possibilities suggested to account for the discrepancies with models versus observations.
In the first scenario, it could be that the simulations use flawed physics, or do not take enough of the physics into account. However the simulations do such an excellent job at explaining the rest of the Universe this looks unlikely, though it is certainly possible that our laws of physics may need a few ‘tweaks’ down the line. Additionally, many of these simulations are ‘dark matter only’ and so don’t take into account normal atomic matter, but even after the inclusion of these components into the simulation, the problem has largely persisted.
In regards to optical surveys, we may simply not have good enough telescopes to detect the galaxies. The sensitivity of our optical imaging is improving all of the time and allowing us to find fainter and fainter galaxies. Around our own Milky Way a few new satellite galaxies have been found recently by finding regions of dense stars, which are examined in detail to work out if they are outside of the Milky Way. This sounds simple but it is only becoming possible due to the high quality of modern telescopes. It also gets much more dif cult the further away from the Milky Way we look: most of the time we are searching for very faint ‘blobs’.
Simulations predict that a galaxy like our Milky Way should have around 10 times more satellite dwarf galaxies than we actually observe
The third possibility, ‘dark galaxies’ is particularly intriguing - and perhaps not as outlandish as it first appears. All galaxies sit in massive clumps of dark matter, called halos. We can’t see the dark matter directly but we can observe its gravitational effects. Many of the simulations are ‘dark matter only’, so each galaxy is represented by its dark matter halo alone. The dwarfs are then represented by ‘sub-halos’ of dark matter, and each halo has many sub-halos.
Perhaps all of these sub-halos do exist but not all of them have stars for us to observe. What we can look for, however, is neutral hydrogen gas. This gas is extremely abundant in the Universe so there is a high probability that these dark matter halos would contain some. It may not have reached a high enough density to form stars, or stars may have formed and, when one went supernova (exploded upon death), it may have blown out the dwarf galaxy’s gas.
Figure 2a - Via Lactea computer simulation of a Milky Way or Andromeda type galaxy. Thousands of dwarf galaxies can be seen surrounding it. In this image every object is a galaxy.
This neutral hydrogen gas emits radio waves with a wavelength of 21cm and is a ‘forbidden line’, meaning the transition which causes it is very rare and wouldn’t occur under Earth laboratory conditions.
On the other hand, there are such vast quantities of neutral hydrogen in the Universe that, although rare, we can detect a very strong signal using radio telescopes. This means we can observe the Universe in hydrogen gas, and look for big clouds of gas with no stars in order to hunt for the missing dwarf galaxies.
In my own research work I have looked at the dwarf galaxy problem from an observational perspective: I am a missing galaxy hunter, if you will.
M33, or the Triangulum galaxy, is the third largest galaxy in our Local Group (after the Milky Way and Andromeda). It has no confirmed satellite galaxies, despite the fact that simulations predict a galaxy of its size should have around 25.
As part of the AGES (Arecibo Galaxy Environment Survey) we have used the Arecibo telescope, in Puerto Rico, to observe the neutral hydrogen gas in and around M33. Our data cube showing M33 and the surrounding area is shown in Figure 3. One aim of this work was to search for dark dwarf galaxies: those with hydrogen gas but no detected stars.
We detected a total of 32 clouds in the area around M33, eleven of which were previously undetected. Twenty-two of these were discrete clouds meaning they are well separated from the gas disk of M33. The other 10 were all previously discovered clouds that we detect as over- densities in the extended gas disk of M33. We can distinguish this extended gas disk, as the AGES observations of M33 are more sensitive than those of previous surveys.
Figure 2b - An image showing the Andromeda galaxy. The visible dwarf galaxies are circled in red. Compared to the simulated galaxy far fewer dwarf galaxies can be seen.
To ascertain whether the clouds we found were dwarf galaxies or perhaps something else, we analysed their shapes and sizes and their rotational motion, as it is expected that a gravitationally bound galaxy would show ordered rotation. We also looked for stars associated with the clouds, however none were found.
Most of the clouds we detected are very small and irregularly shaped and have no ordered rotational motion
Nonetheless, we did find one particularly interesting cloud that was given the snappy title ‘AGESM33-31’. This cloud is very large and if it is located at the same distance as M33, it is as large as M33 itself (almost 6000 light years across!). AGESM33-31 has a small amount of rotation and it appears to be moving as one cloud.
These features are similar to those that a face-on disk galaxy would have (such as the Milky Way, if we were to have a bird’s eye view of it). Additionally, this cloud has a large hole in the middle, which is very perplexing indeed. What could this ring-cloud be?
AGESM33-31 was previously detected by Thilker, Braun and Walterbos in 2002 as an unresolved blob (they couldn’t make out the hole), and it was listed as a possible ‘dark companion’ to M33 – this is certainly a possibility. If this is the case, then AGESM33-31 could be the remnants of a dark galaxy that has undergone an interaction with another galaxy, carving out a hole in the process as the two collided.
The Arecibo radio telescope in Puerto Rico - a single dish radio telescope with a dish diameter of 305 m.
Another possibility is that the ring-cloud is an extension of the Magellanic stream. This is a hydrogen stream that crosses the sky, originating at the Magellanic clouds (the small and large Magellanic clouds are actually two dwarf galaxies orbiting near our own Milky Way.)
Wright’s cloud, which can be seen at the right of Figure 4, is moving at a similar velocity to the Magellanic stream in this area of the sky and it has been suggested that this cloud is an extension of the stream.
Our ring-cloud is very close to Wright’s cloud and is moving with the same velocity, so this rationale fits with the data. However, the Magellanic stream is very sparse in this area of the sky and it is only consists of many small fragmented blobs of hydrogen. Both the ring-cloud and Wright’s cloud are very large objects that contain a lot of hydrogen, so it is unlikely to be part of the stream.
We also looked into the idea that the hole in the ring-cloud could have been formed by a supernova (the death explosion of a massive star). This would have driven gas out of the local area and caused the hole. However, this too seems unlikely as the hole is about 10 times larger than would be expected if it had been caused by a supernova. All we can say for certain about this cloud is that it is a very interesting object indeed!
Combing the Virgo Cluster
The dark galaxy idea is particularly intriguing - and perhaps not as outlandish as it first appears
We are now trying a very different method to hunt for dwarf galaxies and this involves searching through optical images of the Virgo Cluster – a galaxy cluster containing around 2000 galaxies. The dwarf galaxy problem also applies to the Virgo Cluster and observations show around 10 times fewer dwarf galaxies than is expected from simulations.
Figure 3 - Our M33 data cube. The x and y axes show two spatial dimensions (right ascension and declination) and the z axis shows velocity. Gas from the Milky Way is shown in blue, M33 and associated clouds are shown in red with big features labelled. The red dappling shows the noise in the cube.
Figure 4 - All of the discrete clouds detected. The black outline shows the gas disk of M33. All of the clouds are colour coded according to their velocity relative to M33. Wright’s cloud is shown on the right, AGESM33-31, or the ringcloud, is shown on the left.
To conduct the search, we are using data taken on the Canada France Hawaii Telescope in four colour bands. Having multiple bands is useful as some galaxies are brighter in some bands than others. For example, spiral galaxies have a lot of young, blue stars so are brighter in some bands, whereas Elliptical galaxies have older, red stars, so are easier to see in other bands.
Initially, we just used one band to search for new dwarf galaxies. We used an automatic object detection programme to search for new objects by putting limits on their size and brightness. We found 443 dwarf galaxies, of which 303 were new detections. Figure 5 shows the location of these galaxies within the Virgo cluster.
However, one big challenge when looking at galaxy clusters is determining how to tell apart dwarf galaxies in the cluster from bigger, background galaxies. How do you tell which galaxies appear faint because they really are faint from the ones that appear faint because they’re further away?
Figure 5 - All of the Virgo cluster galaxies are marked in red, and the 303 new dwarf galaxies we have detected are marked in blue. The black ellipses show the extent of the Virgo cluster.
We are currently working on a possible solution to this problem by looking at the colour properties of all of the cluster galaxies to see if this could be used to tell them apart from background galaxies. With accurate colour and brightness measurements for around 1500 galaxies, we hope to explore the differences between dwarf galaxies and other galaxies, while correspondingly discovering many new dwarf galaxies.
Are we close to solving the problem?
New discoveries are being made by astronomers nearly every day. But will this be enough? Will we be able to solve the dwarf galaxy problem through simply finding new dwarfs, or will it require more than that? How accurate are our models and how robust is the physics we put into them? All I can say is that this is a very exciting problem to be working on at the moment, and watch this space (… no pun intended)!
Olivia Keenan completed her Masters degree in Physics with Astronomy at the University of Southampton, England, before moving to Cardiff University In Wales to study for a PhD. She is a researcher in extragalactic astronomy and focuses on dwarf galaxies.