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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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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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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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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Recent observations from the ALMA radio telescope array have found some galaxies are extremely efficient at producing new stars, with some galaxies creating stars at an average rate of 800 per year.

Most galaxies produce stars at a rate proportional to their size. The more stars a galaxy has, the more stars it tends to produce. It’s a relation known as the galactic main sequence. Given the size of a typical galaxy, they tend to produce stars at a rate of about 100 – 200 per year. The reason for this relationship seems to be that the more stars a galaxy has, the more gas and dust is produced as those stars die, which is available for new stars to form. Some galaxies are mostly older red dwarfs, which don’t explode when they die, and these galaxies tend to have a low star production. Other galaxies, known as starburst galaxies, produce stars at a much higher rate. These are typically the result of galactic collisions, such as the galaxy Zw II 96 shown above.

A map of the distribution of CO in the galaxy PACS-867. Credit: ALMA Observatory

Some starburst galaxies produce stars at such a high rate that it raised an interesting question. Do galactic collisions cause tremendous amounts of dust to be formed, or are starburst galaxies producing stars more efficiently than other galaxies? It’s been a difficult question to answer, because cold gas and dust in a galaxy is difficult to observe. It doesn’t emit light in the visible spectrum, and the light it does emit isn’t particularly bright. But the Atacama Large Millimeter/submillimeter Array (ALMA) was designed to observe the faint microwave light from cold gas and dust. Recent observations from ALMA have allowed us to measure the amount of this dust in starburst galaxies.

In a recent article in the Astrophysical Journal, a team looked at the distribution of carbon monoxide (CO) in seven starburst galaxies. Carbon monoxide is one of many molecular components of interstellar gas in a galaxy, but it has the advantage of having a bright emission line making it easier to observe. The level of CO in a region is a good indicator of how much gas and dust there is in the region. The team compared CO levels with the rate of star production as measured from Spitzer and Herschel Observatories and found that even after CO levels were depleted star production remained high. This means the rate of star production in starburst galaxies isn’t simply due to the amount of gas and dust available, but that starburst galaxies are actually more efficient at producing stars.

Just why this is remains unclear. It may be that the formation of stars triggers additional star formation in a kind of feedback loop. What’s clear, however, is that star production can be far more efficient than we thought. This could have played an important role in early galaxies where galactic collisions were more common.

Paper: J. D. Silverman, et al. A higher efficiency of converting gas to stars push galaxies at z ~ 1.6 well above the star-forming main sequence.  The Astrophysical Journal Letters, Volume 812, Number 2 (2015)

This post originally appeared on Forbes.

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In the early twentieth century the rise of astrophotography gave us the ability to determine the brightness and spectra of stars with reasonable accuracy. Astronomers such as Annie Jump Cannon were able to use these measurements to classify stars into types. Combined with the ability to determine stellar distances using parallax, Ejnar Hertzsprung and Henry Norris Russell plotted stars by absolute magnitude (brightness) and color. This Hertzsprung-Russell diagram, as it came to be known, found that most stars lay along a roughly linear region known as the main sequence.

A simple HR diagram.

Stars along the main sequence follow a particular trend, where hotter stars tend to be brighter. That might seem like an obvious thing, but the diagram also showed hot but dim stars (white dwarfs) and cool but bright stars (red giants). What the main sequence really showed is that for most stars the more massive a star the brighter and hotter it tends to be. It was an indication that mass was an important factor in a star’s energy production. It was a bit of a surprise, since if stars shined simply due to gravitational collapse, one would expect age to be the main factor of brightness, not mass. The HR diagram led us to the understanding that nuclear fusion powered stars, aided by the extreme heat and pressure created by a star’s weight.

The galactic main sequence.

A similar relation can be found for galaxies. If you plot galaxies by the estimated number of stars they have and the calculated rate at which stars are forming, then you find that most galaxies lie along a line. It turns out that the more stars a galaxy has, the more stars it tends to create. This makes sense because as large stars die they tend to explode to create the gas and dust necessary for more stars. But we also see other groups. Galaxies with lots of red dwarf stars tend to have little star production, since red dwarf stars don’t explode to make gas and dust available. Starburst galaxies are producing stars at a much higher rate than expected, probably due to a collision with another galaxy.

Just as the stellar main sequence can help us understand how stars evolve over time, the galactic main sequence helps us understand how galaxies form and evolve.

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One of the questions about galaxy formation is whether there is a pattern to the formation of stars. One popular model has been that stars form in the central region of a galaxy first, and then later stars further out form. This model is supported in part by the fact that stars in the nucleus of a galaxy tend to have lower metallicity than stars further away, and therefore tend to be older. Now new research supports this model, at least for the Milky Way.

Horizontal branch stars in the upper left of an HR diagram. Credit: B.J. Mochejska, et al.

The study looked at stars known as blue horizontal-branch stars (BHBs). These are stars at the end of their life, and are fusing helium in a last-ditch effort to counter gravitational collapse. Rather than being a red giant, these stars appear blue. They are bright stars, and so are easier to observe in the central region of our galaxy. Since larger BHBs burn bluer and brighter, their color can be used as an indication of their mass. Since larger stars enter the BHB stage earlier than smaller stars, this is also an indication of their age.

The team compared the color/age of 4700 BHBs from the Sloan Digital Sky Survey and their distance from the center of our galaxy, spanning a radius of about 40,000 light years. They found that the closer a star was to the center, the older it was likely to be. This agrees with the inward-out model of galactic star formation.

Paper: Rafael M. Santucci, et al. Chronography of the Milky Way’s Halo System with Field Blue Horizontal-Branch Stars.  The Astrophysical Journal Letters, Volume 813, Number 1 (2015)

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Astronomers have used a lens bigger than a galaxy to observe the faintest and youngest galaxies ever found. This lens isn’t one we’ve built, but a naturally occurring one, and it’s made entirely of gravity.

The gravitational lens effect. Credit: Andrew Hamilton

One of the properties of gravity is that large masses can cause light near them to change direction, or “bend” due to their gravity. So when a distant object is behind a galaxy or cluster of galaxies, it’s light is bent to gravitationally lens the distant object. Because of this lensing effect, we actually receive more light from the distant object than we normally would. Much like a telescope, a gravitational lens magnifies the light we observe from distant objects.

This latest work used observations from the Hubble space telescope’s Frontier Fields project, which observed distant clusters of galaxies. These clusters magnified more distant dwarf galaxies behind them. What the team observed about 250 dwarf galaxies from a period when the universe was only about 600 – 900 million years old. They found that these early dwarf galaxies played a key role in a process known as reionization, when neutral hydrogen in the universe was ionized by ultraviolet light.

Paper: H. Atek et al. Are Ultra-faint Galaxies at z = 6−8 Responsible for Cosmic Reionization? Combined Constraints from the Hubble Frontier Fields Clusters And Parallels. to appear in the Astrophysical Journal (2015)

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