When we look in the night sky, we can see hundreds of stars. In remote and dark areas we can see a few thousand stars with the naked eye. But imagine a night so bare you could only see a couple dozen stars. Most of them distant and dim. This would be our sky if our solar system existed in an ultra-diffuse galaxy. 

Our galaxy spans 100,000 light years, and contains nearly 400 billion stars. Ultra-diffuse galaxies can have a similar size, but only contain a few billion stars.  They are so fluffy that they raise several questions. Since these galaxies contain so few stars despite their size, they either failed to produce stars during their evolution, or they had most of their stars stripped away by some cosmic event. Then there is the question of how long they can last. With fewer stars, the galaxies aren’t tightly bound by gravity compared to galaxies like the Milky Way.  Recently in an attempt to answer some of these questions, a team of astronomers found an interesting clue.

These diffuse galaxies are also known as low surface brightness galaxies. Because they have fewer stars, they aren’t as bright as other galaxies of similar size. The team looked at 89 of these galaxies in a dense supercluster of galaxies known as the Perseus cluster. Since the Perseus cluster has many large and dense galaxies, we would expect that diffuse galaxies would tend to be torn apart by the gravity of more massive galaxies. But the team found that many of the diffuse galaxies were largely intact. In order to survive the tidal forces of nearby galaxies, they must have much more mass than their stars alone, and that means they likely contain large amounts of dark matter.

If that’s the case, it could also explain why they contain so few stars. These diffuse galaxies could have formed with a mass similar to our Milky Way, but with much less gas and dust, producing much fewer stars. To know for sure we’ll need a better understanding of dark matter, but that’s another story.

Paper: C. Wittmann, et al. A population of faint low surface brightness galaxies in the Perseus cluster core. MNRAS, 470, 1512 (2017)

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The evolution of a galaxy is driven by star formation. Typically a galaxy will enter a period of active star formation in its youth, and then star formation will gradually taper off as the amount of available gas and dust decreases. Since bright blue stars live much shorter lives than small red ones, over time an aging galaxy becomes less active and more populated by red stars. But sometimes this can happen quite quickly, and a recently discovered galaxy demonstrates just how quickly.

The galaxy is known as ZF-COSMOS-20115, and we see it as it was when the universe was just 1.6 billion years old. The galaxy is five times more massive than our current Milky Way, but it’s stars are packed into a region less than a tenth of our galaxy. The galaxy is dominated by red dwarf stars, making it a dense red galaxy. When the universe was less than a billion years old, this galaxy was producing stars at a rate of more than 1,000 per year. Most galaxies at the time were producing about 100 stars per year. This rapid star formation drove gas and dust away from the galaxy, leaving it with few resources to continue producing stars. The bright young stars lived short lives, and then died as supernovae or white dwarfs, leaving the small red dwarfs to continue burning.

Just what triggered such a powerful period of star formation is unclear. It’s also not clear how common such galaxies were in the early universe. Because of their dimness and distance, they are very difficult to observe. This could change when the Webb Space Telescope comes online in 2018. For now, it is clear that some galaxies live very vast, and die very young indeed.

Paper: K. Glazebrook et al. A massive, quiescent galaxy at a redshift of 3.717. Nature. Vol. 544. doi:10.1038/nature21680 (2017)

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Most galaxies like our Milky Way have a supermassive black hole in their core. They can have more mass than millions or billions of Suns. But there are a few spiral galaxies, such as the Triangulum galaxy, that has no supermassive black hole. Given that supermassive black holes form within galaxies, how could a galaxy possibly get rid of one? The answer could be gravitational waves. 

Every now and then two galaxies can collide and merge. For example, the Andromeda galaxy and Milky Way are moving toward each other, and will collide in about 4 billion years. When two galaxies collide, their supermassive black holes can be caught in a spiral toward each other. As they orbit each other, their large masses produce strong gravitational waves, which eventually causes them to merge.

A diagram showing how two merging black holes can be ejected from a galaxy. Credit: NASA, ESA/Hubble, and A. Feild/STScI

We’ve observed the gravitational waves of merging black holes, but the gravitational waves of supermassive black holes would be much stronger. If the two merging black holes are roughly the same size, then the gravitational waves would be generated evenly in a range of directions. But if the black holes have very different sizes, then gravitational waves would be produced more strongly in one direction than others. This means more gravitational energy is being emitted in a particular direction, and as a result the black holes would be kicked in the opposite direction. This could be strong enough to kick a supermassive black hole out of the merged galaxy.

At least that’s been one idea, and new observational evidence seems to support it. A team using the Hubble space telescope have found a supermassive black hole moving away from a galaxy. You can see this as the bright object in the center of image at the top of the page. The black hole is bright because it’s actively consuming gas and dust. Behind the black hole is the fainter and diffuse galaxy 3C186. Given the structure of 3C186, it is most likely the result of two merged galaxies. This is exactly what we’d expect to see when merging black holes are kicked out of a galaxy.

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Stars typically form where there is a large buildup of gas and dust, known as a stellar nursery. But new observations show that star formation can also be triggered by a supermassive black hole in the center of a galaxy. 

Most galaxies contain a supermassive black hole within them. These black holes can have masses of millions or even billions of Suns. Their immense gravity can not only rip stars apart, they can generate powerful outflows of material (or jets) as they are active. These outflows are a rich source of gas and dust that can trigger the formation of new stars.

While it has been speculated that stars could form from these outflows, the process is difficult to observe. But recently a team of astronomers observed a pair of colliding galaxies about 600 million light years away. Because of their collision, their supermassive black hole is extremely active, and producing a large outflow of material. Using sensitive spectroscopic measurements, the team was able to detect stars forming within the outflow. These stars were hotter and brighter than stars usually found in stellar nurseries, and their observed speeds were consistent with that of the outflow.

This discovery helps us understand the evolution of both stars and galaxies. Since supermassive black holes can trigger the formation of large, bright stars, these stars could provide the heavy elements necessary for the formation of Sun-like stars and metal-rich planets. It provides another avenue for star production within a young galaxy.

Paper: R. Maiolino, et al. Star Formation in a Galactic Outflow. Nature (2017)

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Young galaxies are often surrounded by a halo of hydrogen gas. Over time this gas can be pulled inward, where it can feed star production in the galaxy. While we’ve known these halos existed, it has been difficult to determine their size. But new research from the Atacama Large Millimeter/submillimeter Array (ALMA) has found that some galactic halos are surprisingly large. 

Light from the galaxy (green) is widely separated from the quasar (red), which indicates a large super-halo around the galaxy. Credit: ALMA (ESO/NAOJ/NRAO), M. Neeleman & J. Xavier Prochaska; Keck Observatory

These young galaxies are so distant that their light travels for 12 billion years to reach us. They are so distant that we can’t observe the halos directly. In fact, until this latest research we couldn’t even observe the galaxies because they are so faint. The hydrogen gas halos were detected by looking at the light of even more distant quasars. Quasars are intense sources of light, powered by the immense gravity of supermassive black holes. As the light travels to Earth, it passes through material such as these hydrogen halos. The hydrogen absorbs certain wavelengths of the light, giving it a unique spectral signature.

While the quasar light clearly showed the presence of hydrogen, we couldn’t determine the size of the halo without knowing where the center of the galaxy was. It was generally thought that the galaxy would be seen fairly close to the quasar, since the galactic halo was probably rather small. But ALMA showed the quasar wasn’t next to the galaxy, meaning the halo is quite large.

ALMA found the galaxy by observing the glow of light from ionized carbon. This kind of ionized carbon is found in the dusty star-forming regions of a galaxy. They observed two galaxies, widely separated from quasars. By observing this light the ALMA team also found the galaxies were rotating, and that they were producing stars at a moderately high rate. The rate of rotation indicates that these are young spiral galaxies, similar to our own Milky Way. The spacing of these galaxies from the quasar indicates that the halo extends at least 60,000 light years from one galaxy, and 137,000 light years from the other.

Observation of these extended super-halos tells us that at least some young galaxies had large regions of surrounding material. This material would later fuel the growth of these galaxies as well as star production within the galaxy.

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At the heart of most galaxies lies a supermassive black hole. How such black holes came to be is a matter of some debate. Did black holes form first, and galaxies later formed around them (bottom up model), or did galaxies form first, and only later did their cores collapse into a black hole (top down model). To answer this question we need to have a good understanding of when these black holes started to form. A new ultra-deep x-ray image is helping to answer these questions.

A deep field image is one that has a clear view of very distant objects. The most famous deep field is the Hubble Ultra Deep Field (HUDF), which showed us just how many galaxies there are in the cosmos. The HUDF was in the visible and infrared, but there are others such as the ALMA Deep Field, which was taken at microwave wavelengths, which gave us a view of distant gas and dust. Now the Chandra X-ray Observatory has taken an x-ray deep field, which gives us a view of distant black holes.

Black holes don’t emit light themselves, but the gas and dust near a black hole can become superheated by the gravitational squeezing of the black hole. Such “active” black holes can emit huge jets of plasma that give off intense x-rays. By studying these x-ray emissions, we can determine things such as the size and rate of growth of the black hole. This new deep field image gathered light from supermassive black holes when the Universe was about 2 billion years old. Since the region observed was the same as the Hubble Deep Field, the team could match x-ray black holes to galaxies in the Hubble deep field, and get an idea of the size and evolution of the black holes and their galaxies. What they found was that the “seeds” for these supermassive black holes were likely on the order of 10,000 to 100,000 times more massive than our Sun. This would tend to support the bottom up model where black holes formed first. If the top down model was correct, we would assume the seeds would be smaller, on the order of 100 t0 1,000 solar masses.

This new data doesn’t completely rule out the top down model, but it is consistent with other evidence that supports the bottom up model. Right now it looks like black holes formed early in the Universe, and this triggered the formation of galaxies around them.

Paper: Fabio Vito, et al. The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion. MNRAS 463 (1): 348-374. doi: 10.1093/mnras/stw1998 (2016)

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As we’ve recently seen, the cosmos is much larger than we’ve thought, with more than 2 trillion galaxies in the observable universe. Actually observing many of the most distant and faint galaxies is a real challenge, but more of them are being detected thanks to a trick that relies on relativity. 

The more distant a galaxy, the more dim it can appear. This is due in part to the fact that the apparent brightness of an object decreases with the square of its distance (known as the inverse square law). For galaxies, this effect is even more dramatic due to cosmic expansion, which further dims objects billions of light years away. Because of this dimming, small dwarf galaxies can be difficult to observe. This is a problem because in the nearby universe dwarf galaxies are the most numerous, so we could be missing a lot of galaxies when we look across great distances.

But it turns out that relativity can help, thanks to an effect known as gravitational lensing. The path of starlight can be deflected by the gravity of a nearby mass, as Arthur Eddington first demonstrated in 1919. This means that light from a distant galaxy can be deflected and focused if a closer galaxy is between us and it. Through gravitational lensing, the distant galaxy can appear brighter than it would otherwise, just as a glass lens can magnify and brighten a distant star.

Recently, a team used this method to observe faint dwarf galaxies at a redshift between z = 1 and z = 3.  We see these galaxies as they were when the Universe was 2 to 6 billion years old, which is a period of peak star formation. They found that dwarf galaxies were most abundant at the greatest redshifts, and thus the earliest period. Since most of the stars in these early dwarf galaxies were hot and bright, they flooded the Universe with ultraviolet light, driving the reionization period of the early Universe.

When the James Webb telescope launches in 2018, we should have an even better view of these dim and distant galaxies. Until then, gravitational lensing will help us explore this critical period of galaxy formation.

Paper: Anahita Alavi, et al. The Evolution Of The Faint End Of The UV Luminosity Function During The Peak Epoch Of Star Formation (1<z<3). The Astrophysical Journal, Volume 832, Number 1 (2016)  arXiv:1606.00469

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Suppose you picked up a single grain of sand and held it at arms length. The sand grain would cover just a tiny patch of sky. Now imagine you found the darkest patch of night sky you could find and looked long and hard at an area no bigger than that single grain of sand. In 2004 the Hubble space telescope did just that, observing a tiny patch of dark sky for a total of 55 hours. The image it produced is known as the Hubble Ultra Deep Field. It found more than 10,000 galaxies that were present when the Universe was just 400 – 800 million years old. 

It was a bit of a shock to find so many galaxies in such a small region. If the HUDF is typical, then there must be about 100 billion galaxies in the observable cosmos. Each galaxy would have about 100 billion stars on average, and stars typically have roughly 10 planets, and hundreds or thousands of asteroids and comets. A single image of sky no larger than a grain of sand showed us the Universe was larger than we’d ever imagined. But it turns out our estimate was wrong by a factor of 10.

New research has looked at the positions and redshifts of galaxies in deep field surveys, and created a 3D map of the distribution of young galaxies. This included many faint galaxies that can be particularly difficult to observe. The map was then compared to computer simulations matching the galaxies by distance and brightness. The simulations indicated that there must be far more galaxies we can’t see than the ones we can. Most galaxies are simply too dim and too distant to be seen with our current telescopes. The vast sea of galaxies seen in the Hubble Ultra Deep Field is just a glimpse of what’s really out there. Rather than a Universe filled with 100 billion galaxies, there are likely 2 trillion galaxies in the observable Universe. That’s about 200 galaxies for every man, woman, and child on Earth.

Imagine if you gave names to 200 galaxies in the cosmos, and so did everyone else on the planet. There would still be billions of nameless galaxies out there. The Universe is vast indeed.

Paper: M. Huertas-Company, et al. Mass assembly and morphological transformations since z ∼ 3 from CANDELS. MNRAS 462 (4): 4495-4516 (2016)
DOI: 10.1093/mnras/stw1866

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The Hubble Ultra Deep Field is a particularly dark patch of sky about the size of a grain of sand held at arm’s length. It was first imaged by the Hubble telescope in 2004, and was found to contain about 10,000 galaxies in its field of view. Other observations of this patch of sky have been made over the years, so that we now have images ranging from infrared to ultraviolet. The galaxies of the HUDF are among the most distant ever observed, stretching back to the earliest age of galaxies. Since Hubble can only detect light within the roughly visible range, its view of the galaxy formation period is limited. But now the ALMA telescope array has captured the HUDF, and it’s given us a new view of these distant galaxies. 

ALMA is a radio telescope capable of viewing the sky at millimeter and submillimeter wavelengths (just below infrared), which is the type of wavelength that cold gas and dust emit. Since these wavelengths are longer than infrared, ALMA can also see more distant galaxies than Hubble, since the light of distant galaxies are greatly redshifted. The ALMA survey was able to see galaxies rich in carbon monoxide (CO), which is commonly found in large molecular clouds that can trigger star formation (seen in yellow in the image above). Since cold CO gas emits a specific spectral fingerprint, ALMA could also determine the redshift and thus the distance of these star-forming galaxies. They found that more distant galaxies had higher CO levels than less distant ones, which could have triggered the peak period of star formation that occurred when the Universe was about 4 billion years old.

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Most galaxies are dominated by dark matter. Our own galaxy is about 85% dark matter by mass. But recently a galaxy was discovered that is almost entirely dark matter. Known as Dragonfly 44, less than 0.01% of its mass is regular matter. 

Dark matter is highly mysterious due to the fact that we haven’t detected it directly. Since dark matter doesn’t interact with light, we can only observe its gravitational effect on light and the motions of stars. The elusive nature of dark matter has led some to propose other ideas such as modified gravity, but these don’t agree well with the Universe we observe. Given that dark matter is so mysterious, how do we know Dragonfly 44 is a dark matter galaxy?

To begin with the galaxy is very diffuse. It is only slightly smaller than the Milky Way in size, but has about 1% the number of stars. By itself that might not mean much, but Dragonfly 44 is part of a cluster of galaxies known as the Coma cluster. Over time such a diffuse galaxy would be torn apart by other nearby galaxies unless it had a large mass to hold it together. This is supported by the speeds of the stars within the galaxy. Stars within a galaxy generally orbit the center of a galaxy similar to the way planets orbit a star. The more mass a galaxy has, the faster the stars tend to orbit. By looking at the distribution of stellar speeds within a galaxy, we can get a measure of how much mass a galaxy has. Finally, Dragonfly 44 has about 100 globular clusters orbiting it. These are small but dense clusters of stars that generally form a kind of halo around larger galaxies. Typically, the larger the galaxy the more globular clusters it will have. All of this supports the idea that Dragonfly 44 has a mass about equal to that of our Milky Way. Given the low number of visible stars, that means 99.99% of its mass must be dark matter.

The spectra of Dragonfly 44. Credit: P. van Dokkum, A. Romanowsky, J. Brodie.

These kinds of Ultra Diffuse Galaxies (UDGs) could help us better understand dark matter. While galaxies such as the Milky Way are mostly composed of dark matter, their central regions where stars are most dense are dominated by regular matter. As a result it’s difficult to study the effects of dark matter in detail. With diffuse dark matter galaxies we can see the effects of dark matter more clearly. If dark matter does interact with itself to produce small amounts of light, as some have proposed, it might be visible within UDGs.

Paper: Pieter van Dokkum, et al. A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44. The Astrophysical Journal Letters, Volume 828, Number 1 (2016) arXiv:1606.06291 [astro-ph.GA]

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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 thing about records is they keep getting broken. Take, for example, the record for the most distant known galaxy. In October of 2013 the most distant galaxy had a redshift of z = 7.51. In February of 2015 we found one at z = 7.73, then one at z=8.86 in late 2015. Now there’s a new record at z=11.1. 

The distance of far galaxies is typically given by their redshift value z, where the greater the redshift the higher the number. Because of Hubble’s law describing the expansion of the universe, the greater a galaxy’s redshift the greater its distance.  The reason we don’t simply give a distance is because the whole notion of “distance” in an expanding universe is a bit fuzzy. The light from this latest galaxy (known as GN-z11) traveled for 13.3 billion years to reach us. The galaxy was 2.6 billion light years away when the light began its journey, and the galaxy is “now” 32 billion light years away.

This new galaxy compared to others. Credit: NASA, ESA, AND A. Feild (STSCI)

The universe was only about 400 million years old when this galaxy was as we see it, which puts the galaxy close to the “dark ages” of the early universe. It is probably one of the early galaxies of the universe.  GN-z11 is quite small, with only about 1% the mass of our own Milky Way, and only about 4% the size of our galaxy. Yet the small galaxy is producing stars at 25 times the rate of our current Milky Way. It’s that intense rate of star production that makes the galaxy bright enough for us to observe. This may be indication that the young galaxy is forming around a supermassive black hole, which would imply that black holes formed before galaxies.

Paper: P. A. Oesch, et al. A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy.  arXiv:1603.00461 [astro-ph.GA] (2016)

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