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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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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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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New observations from ALMA show a galaxy in the making.

ALMA observes the universe at millimeter wavelengths, so it’s particularly good at seeing emissions from cold gasses such as carbon. In this case a team observed carbon gas around a galaxy with a redshift of about z = 7. This means we’re seeing the galaxy from a time when the universe was only about a billion years old. What’s striking about the carbon gas is that it’s off-center from the galaxy. This is likely means the gas is being accreted from the intergalactic medium, while the galaxy itself undergoes a period of rapid star production.

This is the first time we’ve been able to see dynamic behavior in an early galaxy. It’s important because it helps us understand the period known as reionization, when the first stars and galaxies began to illuminate the cosmos.

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On a dusty day, you can’t see the smog. At least that’s how it is for galaxies, which is why the detection of carbon in early galaxies is such a big deal.

Ionized carbon seen in early galaxies. Credit: P. Capak; B. Saxton (NRAO/AUI/NSF), NASA/ESA Hubble

Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have detected spectral line emissions from carbon gas in galaxies with a redshift of z = 5 to z = 6. This means the light we observe comes from a time when the universe was only about a billion years old. These emission lines would typically be obscured by dust in the galaxies, so the fact that they are so bright and clear means that there is little dust within these young galaxies. Carbon also tends to bind to other elements readily, so pure carbon doesn’t tend to stick around for long. And that means these galaxies are still in the early stages of star formation.

What’s particularly striking is the fact that these galaxies already appear “fully formed.” They already have much of the structure seen in older galaxies. This would imply that galactic structures form early on rather than developing gradually over time.

Paper: P. L. Capak, et al. Galaxies at redshifts 5 to 6 with systematically low dust content and high [C II] emission. Nature 522, 455–458 (2015)

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Yesterday I talked about measuring the age of a star, but what about a galaxy. For example, how old is our Milky Way galaxy? Given that stars are being born and dying within a galaxy all the time, is it even possible to talk about the age of a galaxy? It turns out we can determine the age of our galaxy, and uses an elemental trick.

One of the basic ways to look at the age of the Milky Way is to look at globular clusters. These are dense clusters of stars that are distributed in a kind of halo around our galaxy. We know that the stars within a globular cluster form around the same time, and we can determine their age by looking at things such as the percentage of their stars that are red dwarfs, or the temperatures of their white dwarfs.

The red dwarf measure is useful because red dwarfs can last for trillions of years, unlike larger stars which only last a few billion. So if a group of stars form at the same time, the larger stars will die off sooner, while the red dwarfs continue to shine. So the more red dwarfs a globular cluster has, the older it is. The white dwarf method relies on the fact a white dwarf is the remnant of a Sun-like star. Once a white dwarf forms, it has no way to produce new energy, so it gradually cools. The cooler the white dwarfs in a globular cluster, the older it is.

It turns out that the oldest of the globular clusters surrounding our galaxy are about 13 billion years old, which means the Milky Way must be at least that old. But when we look at the oldest red dwarfs in these clusters, we find that they aren’t first generation stars. They contain elements that could only be formed by earlier stars, and that means stars must have lived and died in the Milky Way before these globular clusters formed. So the Milky Way is older that 13 billion years, but how much older?

Line spectra of beryllium as seen in three stars. Credit: ESO

That’s where the elemental trick comes in. In this case, the element beryllium. While most of the elements beyond hydrogen and helium are produced in the cores of stars, not all of them are. Some are produced by the radioactive decay of heavier elements, or in the case of beryllium-9, high energy cosmic rays. Cosmic rays are typically protons or helium nuclei that zip through the galaxy at near-light speeds. When these cosmic rays collide with heavier nuclei drifting through space, they cause the nuclei to break apart into lighter elements. One of these is beryllium. Since cosmic rays are pervasive throughout the early galaxy due to early star production and early supernovae, the amount of beryllium in interstellar space builds up over time. By measuring beryllium levels in an old star, you can then determine how long the galaxy was around before the star formed.

These levels were measured in a 2004 paper in Astronomy and Astrophysics, and the result was that the galaxy is about 13.6 billion years old. What’s interesting is that the age of the universe is just shy of 13.8 billion years, which means that our Milky Way must have been one of the early galaxies, forming just after the “dark ages” of the universe, when the first stars were just beginning to shine.

Paper: L. Pasquini, et al. Beryllium in turnoff stars of NGC 6397: Early Galaxy spallation, cosmochronology and cluster formation. A&A 426, 651-657 (2004)

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In a broad sense, galaxies can be grouped into three types: spirals, ellipticals and irregulars. Of these, ellipticals and spirals are by far the most common. Most of the galaxies we see around us are spiral galaxies, but throughout the universe ellipticals are more common, making up about 60% of all galaxies. The categorization of galaxies was first made by Edwin Hubble in 1926, and became known as Hubble’s tuning fork. It was so-named because the variety of galaxies could be laid out along a scale of ellipticals which then branched into two types of spirals. When Hubble proposed this scheme, some suggested that it indicated the nature of galactic evolution, where round, elliptical galaxies gradually changed into flat, spiral galaxies. Hubble himself was cautious not to assume too much about the classification scheme. Still, it does raise an interesting question about these types of galaxies. Why do some become spirals and others not?

Hubble’s tuning fork. Credit: GalaxyZoo

When we look at elliptical and spiral galaxies, we notice some pretty clear differences (beyond their apparent shapes). Spiral galaxies tend to be brighter, with lots of young blue stars. Spirals also have lots of gas and dust available to make new stars. It’s this propensity of dust in our own Milky Way that creates things like the zone of avoidance so bothersome to astronomers. Ellipticals, on the other hand, contain mostly old, red stars. They also tend to be largely devoid of the gas and dust necessary for star production.

This would seem to imply an evolution of galaxies moving from spiral to elliptical. In this model a young galaxy develops a spiral structure through density waves in the galactic cloud, which encourages the production of stars. The largest and brightest stars live short lives ending in supernovae, which feeds the production of new stars for a while. But eventually the available gas and dust is consumed by star production, and only the small red stars (which will continue to shine for a trillion years or more) remain.

While this seems a reasonable model, it turns out to be a bit too simplistic. When we look at the metallicity of stars (that is, the amount of elements other than hydrogen and helium they contain) we find that stars in elliptical galaxies can have metallicities three times higher than that of spiral galaxies. Metallicity is a rough indicator of the age of a star, since the first generation of stars would be almost entirely hydrogen and helium, while subsequent generations formed from the remnants of earlier stars have increasingly higher metallicity. If the difference between elliptical and spiral galaxies were simply that ellipticals are older, they must have formed earlier in the universe, and thus their red stars should have lower metallicity, not higher.

Another clue come from the fact of where we find different types of galaxies. Elliptical galaxies tend to be seen in large superclusters of galaxies, and in dense galaxy clusters. Spirals tend to be found in less clustered regions, and rarely exist in dense clusters. In the early universe, the first clusters of galaxies would tend to form larger and tighter clumps due to gravity and dark matter. More dense clusters would tend to have more galactic collisions early on.

Credit: NASA/ESA/A. Feild (STScI)

So the current model is that ellipticals did begin as small spiral galaxies, in early galactic clusters, but frequent collisions stirred them up, causing them to enter a time of rapid star production known as a dusty starburst period. This caused them to consume their gas and dust early on. Subsequent collisions disrupted any remaining spiral structure, and without gas and dust the density wave effect seen in modern spirals couldn’t occur. The dusty starburst period would explain why ellipticals have high metallicity, and this model also explains why we see them in the large superclusters that formed in the early period of the universe.

The spiral galaxies we see today could be younger galaxies, but they could also be older galaxies that weren’t supercharged by early collisions. Their star production could thus occur at a more sedate pace.

As we observe galaxies at increasing redshifts (and thus farther back into the history of the universe) we see things such as small spirals in early clusters, so the model seems to work well. So it seems that Hubble’s quest to understand galaxy evolution was on the right track, it was just the opposite of what his tuning fork implied.

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