This gigantic image of the Triangulum Galaxy — also known as Messier 33 — is a composite of about 54 different pointings with Hubble’s Advanced Camera for Surveys. With a staggering size of 34 372 times 19 345 pixels, it is the second-largest image ever released by Hubble. It is only dwarfed by the image of the Andromeda Galaxy, released in 2015. The mosaic of the Triangulum Galaxy showcases the central region of the galaxy and its inner spiral arms. Millions of stars, hundreds of star clusters and bright nebulae are visible. This image is too large to be easily displayed at full resolution and is best appreciated using the zoom tool.
To the unaided eye, the Triangulum Galaxy is just a smudge in the night sky. But it’s a smudge that contains about 40 billion stars. It also contains some very active star-forming regions, which have attracted the eyes of astronomers.
The Triangulum has a couple other names: Messier 33 and NGC 598. But Triangulum is the easier name to remember. (It’s also sometimes called the “Pinwheel Galaxy.”) But whatever name you choose to call it, this Hubble image brings it to life.
The NASA/ESA Hubble Space Telescope offers this delightful view of the crowded stellar encampment called Messier 68, a spherical, star-filled region of space known as a globular cluster. Mutual gravitational attraction amongst a cluster’s hundreds of thousands or even millions of stars keeps stellar members in check, allowing globular clusters to hang together for many billions of years. Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars. The chemical elements leave signatures in this light, and the starlight reveals that globular clusters' stars typically contain fewer heavy elements, such as carbon, oxygen and iron, than stars like the Sun. Since successive generations of stars gradually create these elements through nuclear fusion, stars having fewer of them are relics of earlier epochs in the Universe. Indeed, the stars in globular clusters rank among the oldest on record, dating back more than 10 billion years. More than 150 of these objects surround our Milky Way galaxy. On a galactic scale, globular clusters are indeed not all that big. In Messier 68's case, its constituent stars span a volume of space with a diameter of little more than a hundred light-years. The disc of the Milky Way, on the other hand, extends over some 100 000 light-years or more. Messier 68 is located about 33 000 light-years from Earth in the constellation Hydra (The Female Water Snake). French astronomer Charles Messier notched the object as the sixty-eighth entry in his famous catalogue in 1780. Hubble added Messier 68 to its own impressive list of cosmic targets in this image using the Wide Field Camera of Hubble’s Advanced Camera for Surveys. The image, which combines visible and infrared light, has a field of view of approximately 3.4 by 3.4 arcminutes. Credit: Hubble/NASA/ESA
Imagine yourself in a boat on a great ocean, the water stretching to the distant horizon, with the faintest hints of land just beyond that. It’s morning, just before dawn, and a dense fog has settled along the coast. As the chill grips you on your early watch, you catch out of the corner of your eye a lighthouse, feebly flickering through the fog.
Artist's impression of the spiral structure of the Milky Way with two major stellar arms and a bar. Credit: NASA/JPL-Caltech/ESO/R. Hurt
Since the birth of modern astronomy, scientists have sought to determine the full extent of the Milky Way galaxy and learn more about its structure, formation and evolution. According to current theories, it is widely believed that the Milky Way formed shortly after the Big Bang (roughly 13.51 billion years ago). This was the result of the first stars and star clusters coming together, as well as the accretion of gas directly from the Galactic halo.
Artist's impression of the black hole wind at the center of a galaxy. Credit: ESA
During the 1960s, scientists discovered a massive radio source (known as Sagittarius A*) at the center of the Milky Way, which was later revealed to be a Supermassive Black Holes (SMBH). Since then, they have learned that these SMBHs reside at the center of most massive galaxies. The presence of these black holes is also what allows the centers of these galaxies to have a higher than normal luminosity – aka. Active Galactic Nuclei (AGNs).
In the past few years, astronomers have also observed fast molecular outflows emanating from AGNs which left them puzzled. For one, it was a mystery how any particles could survive the heat and energy of a black hole’s outflow. But according to a new study produced by researchers from Northwestern University, these molecules were actually born within the winds themselves. This theory may help explain how stars form in extreme environments.
Artist’s impression of a black hole’s wind sweeping away galactic gas. Credit: ESA
For the sake of their study, Richings developed the first-ever computer code capable of modeling the detailed chemical processes in interstellar gas which are accelerated by a growing SMBH’s radiation. Meanwhile, Claude-André Faucher-Giguère contributed his expertise, having spent his career studying the formation and evolution of galaxies. As Richings explained in a Northwestern press release:
“When a black hole wind sweeps up gas from its host galaxy, the gas is heated to high temperatures, which destroy any existing molecules. By modeling the molecular chemistry in computer simulations of black hole winds, we found that this swept-up gas can subsequently cool and form new molecules.”
The existence of energetic outflows form SMBHs was first confirmed in 2015, when researchers used the ESA’s Herschel Space Observatory and data from the Japanese/US Suzaku satellite to observe the AGN of a galaxy known as IRAS F11119+3257. Such outflows, they determined, are responsible for draining galaxies of their interstellar gas, which has an arresting effect on the formation of new stars and can lead to “red and dead” elliptical galaxies.
This was followed-up in 2017 with observations that indicated that rapidly moving new stars formed in these outflows, something that astronomers previously thought to be impossible because of the extreme conditions present within them. By theorizing that these particles are actually the product of black hole winds, Richings and Faucher-Giguère have managed to address questions raised by these previous observations.
Artist’s concept of Sagittarius A, the supermassive black hole at the center of our galaxy. Credit: NASA/JPL
Essentially, their theory helps explain predictions made in the past, which appeared contradictory at first glance. On the one hand, it upholds the prediction that black hole winds destroy molecules they collide with. However, it also predicts that new molecules are formed within these winds – including hydrogen, carbon monoxide and water – which can give birth to new stars. As Faucher-Giguère explained:
“This is the first time that the molecule formation process has been simulated in full detail, and in our view, it is a very compelling explanation for the observation that molecules are ubiquitous in supermassive black hole winds, which has been one of the major outstanding problems in the field.”
Richings and Faucher-Giguère look forward to the day when their theory can be confirmed by next-generation missions. They predict that new molecules formed by black hole outflows would be brighter in the infrared wavelength than pre-existing molecules. So when the James Webb Space Telescope takes to space in the Spring of 2019, it will be able to map these outflows in detail using its advance IR instruments.
One of the most exciting things about the current era of astronomy is the way new discoveries are shedding light on decades-old mysteries. But when these discoveries lead to theories that offer symmetry to what were once thought to be incongruous pieces of evidence, that’s when things get especially exciting. Basically, it lets us know that we are moving closer to a greater understanding of our Universe!
Colour composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy’s central black hole. Credit: ESO/NASA/CXC/CfA/WFI/MPIfR/APEX/A.Weiss et al./R.Kraft et al.
In the 1970s, astronomers discovered that a particularly large black hole (Sagittarius A*) existed at the center of our galaxy. In time, they came to understand that similar Supermassive Black Holes (SMBHs) existed in the center of most massive galaxies. The presence of these black holes was also what differentiated galaxies that had particularly luminous cores – aka. Active Galactic Nuclei (AGN) – from those that didn’t.
Since that time, astronomers and cosmologists have pondered what role SMBHs have on galactic evolution, with some venturing that they have a profound impact on star formation. And thanks to a recent study by an international team of astronomers, there is now direct evidence for a correlation between and SMBH and a galaxy’s star formation. In fact, the team demonstrated that a black hole’s mass could determine when star formation in a galaxy will end.
The primary mirror of the Hobby-Eberly Telescope (HET) at McDonald Observatory. The mirror is made up of 91 segments, and has an effective aperture of 9.2 meters. Credit: Marty Harris/McDonald Observatory
For the sake of their study, the team relied on data gathered the Hobby-Eberle Telescope Massive Galaxy Survey in 2015. This systematic survey used the 10m Hobby-Eberly Telescope (HET) at the McDonald Observatory to conduct an optical long-slit spectroscopic survey of over 1000 galaxies. This survey not only provided spectra for these galaxies, but also produced direct mass measurements of the central black holes for 74 of these galaxies.
Using this data, Martín-Navarro and his colleagues found the first observational evidence for a direct correlation between the mass of a galaxy’s central black hole and its history of star formation. While astrophysicists have been operating under this assumption for decades, the proof was missing until now. As Jean Brodie, professor of astronomy and astrophysics at UC Santa Cruz and a coauthor of the paper, said in a UCSC press release:
“We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens. This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.”
Roughly 15 years ago, the correlation between a SMBHs mass and the total mass of a galaxy’s stars was discovered, which led to a major unresolved question in astrophysical circles. While this correlation appeared to be a central feature of galaxies, it was unclear as to what could have caused it. How could the mass of a comparatively small and central black hole be related to the mass of billions of stars distributed throughout a galaxy?
The galaxy NGC 660 – in this and other galaxies, the rate at which new stars are formed appears to be linked to the evolution of the galaxy’s central black hole. Credit: ESA/Hubble/NASA
One possible explanation was that more massive galaxies collected larger amounts of gas, thus resulting in more stars and a more massive central black hole. However, astrophysicists also believed their was a feedback mechanism at work, where growing black holes inhibited the formation of stars in their vicinity. In short, when matter accretes on a central black hole, it sends out a tremendous amount of energy in the form of radiation and particle jets.
If this energy is transferred to gas and dust surrounding the core of the galaxy, stars will be less likely to form in this region since gas and dust need to be cold in order to undergo areas of collapse. For years, feedback of this kind has been included in cosmological simulations to explain the observed star-formation rates in galaxies. According to these same simulations, minus this mechanism, galaxies would form far more stars than have been observed.
However, no direct evidence of this phenomena had previously been available. The first step to obtaining some was to reproduce the stellar formation histories of the 74 target galaxies used for the study. Martín-Navarro and his colleagues did this by subjecting spectra obtained from each of these galaxies to computational techniques that looked for the best combination of stellar populations to fit the data.
In so doing, the team was able to reconstruct the history of star formation within the target galaxies for the past 12.5 billion years. After examining these histories, they noticed some predictable results, but also some rather significant differences. For starters, as predicted, the regions of around the galaxies’ central black holes demonstrated a clear dampening influence on the rate of star formation.
Artist’s concept of the most distant supermassive black hole ever discovered. It is part of a quasar from just 690 million years after the Big Bang. Credit: Robin Dienel/Carnegie Institution for Science
As predicted, there was also a clear correlation between the mass of the central black holes and stellar mass in these galaxies. However, the team also noted that in cases where stellar mass was slightly smaller than expected (relative to the mass of their central black holes), star formation rates were lower. In some other cases, galaxies had larger-than-expected stellar masses (again, relative to their black holes) and their star formation rates were higher.
This correlation was not only more consistent than that observed between black hole mass and stellar mass, it occurred independently of other factors (such as shape or density). As Martín-Navaro explained:
“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes.”
They also noted that this correlation extends into the deep past, where the galaxies with supermassive central black holes have been consistently producing a comparatively low rate of stars for the past 12.5 billion years. This constitutes the first strong evidence for a direct, long-term connection between star formation and the existence of a central black hole in a galaxy.
Close-up of star near a supermassive black hole (artist’s impression). Credit: ESA/Hubble, ESO, M. Kornmesser
Another interesting takeaway from the study was the way it addressed possible correlations between AGN luminosity and star formation. In the past, other researchers have sought to find evidence of a link between the two, but without success. According to Martín-Navarro and his team, this may be because the time scales are incredibly different. Whereas star formation occurs over the course of eons, outbursts from AGNs occur over shorter intervals.
What’s more, AGNs are highly variable and their properties are dependent on a number of factors relating to their black holes – i.e. size, mass, rate of accretion, etc. “We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” said Martin-Navarro.
Looking ahead, the team hopes to conduct further research and determine exactly how central black holes arrest star formation. At present, the possibility that it could be due to radiation or jets of gas heating up surrounding matter are not definitive. As Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, indicated:
“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”
Part of determining how the Universe came to be is knowing what mechanisms were at play and the extent of their roles. With this latest study, astrophysicists and cosmologists can take comfort in the knowledge that they’ve been getting it right – at least in this case!
Artist's Concept of Milky Way vs Galaxy MACS2129-1. Credit: hubblesite.org
Thanks to recent improvements in space-based and ground-based telescopes, astronomers have been able to probe deeper into the Universe than ever before. By looking billions of years back in time, we are able to test our theories about the history of galactic formation and evolution. Unfortunately, studying the very early Universe is a daunting task, and one that is beyond the capabilities of our current instruments.
But by combining the power of the Hubble Space Telescope with a technique known as gravitational lensing, a team of astronomers made the first discovery of a compact galaxy that stopped making stars just a few billion years after the Big Bang. The discovery of such a galaxy existing so early in the Universe is unprecedented and represents a major challenge to \theories of how massive galaxies form and evolve.
Their findings were reported in a study titled “A Massive, Dead Disk Galaxy in the Early Universe“, which appeared in the June 22 issue of the journal Nature. As is indicated in the study, the team relied on data from Hubble which they combined with gravitational lensing – where a massive cluster of galaxies magnifies and stretches images of more distant galaxies beyond them – to study the distant galaxy known as MACS 2129-1.
Image of the Galaxy Cluster MACS J2129-0741, as part of CLASH. Credit: hubblesite.org
What they found was completely unexpected. Given the age of the galaxy – dated to just three billion years after the Big Bang – they expected to see a chaotic ball of stars that were forming due to early galaxies merging. Instead, they noticed that the galaxy, which was disk-shaped (like the Milky Way), was effectively dead – meaning that star formation had already ceased within it.
This was a surprise, seeing as how astronomers did not expect to see this so early in the Universe. What’s more, it was the first time that direct evidence has been obtained that shows how at least some of the earliest “dead” galaxies in the Universe evolved from disk-shaped objects to become the giant elliptical galaxies that we regularly see in the Universe today.
As Sune Toft – a researcher from the Dark Cosmology Center at the Niels Bohr Institute and the lead author on the study – explained, this may force a rethink of how galaxies evolved in the early Universe:
“This new insight may force us to rethink the whole cosmological context of how galaxies burn out early on and evolve into local elliptical-shaped galaxies, Perhaps we have been blind to the fact that early “dead” galaxies could in fact be disks, simply because we haven’t been able to resolve them.”
In previous studies, it was assumed that distant dead galaxies were similar in structure to the local elliptical galaxies they eventually evolved into. Prior to this study, confirmation of this hypothesis was not possible since current instruments are not powerful enough to see that far into space. But by combining the power of gravitational lensing with Hubble’s high resolution, Toft and his team were able to see this dead galaxy clearly.
Galaxy Cluster MACS J2129-0741 and Lensed Galaxy MACS2129- Credit: hubblesite.org
Combining rotational velocity measurements from the ESO’s Very Large Telescope (VLT) with archival data from the Cluster Lensing And Supernova survey with Hubble (CLASH), they were able to determine the size of the galaxy, mass, and age as well as its (defunct) rate of star formation. Ultimately, they found that the remote galaxy is three times as massive as the Milky Way, though only half its size, and is spinning more than twice as fast.
Why this galaxy stopped forming stars is still unknown, and will require follow-up surveys using more sophisticated instruments. But in the meantime, there are some possible theories. For instance, it could be the result of an active galactic nucleus, where a supermassive black hole at the center of MACS 2129-1 inhibited star formation by heating the galaxy’s gas and expelling it from the galaxy.
Or it may be the result of cold gas being streamed into the galaxy’s center where it was rapidly heated and compressed, thereby preventing it from cooling and forming star-forming clouds. But when it comes to how these types of early, dead galaxies could have led to the elliptical galaxies we see today, Toft and his colleagues think they know the answer. As he explained, it could be through mergers:
“If these galaxies grow through merging with minor companions, and these minor companions come in large numbers and from all sorts of different angles onto the galaxy, this would eventually randomize the orbits of stars in the galaxies. You could also imagine major mergers. This would definitely also destroy the ordered motion of the stars.”
In the coming years, Toft and his team hope to take advantage of the James Webb Telescope (which will be launching in 2018) to search for more early dead galaxies, in the hopes that it can shed light on the unresolved questions this discover raises. And with the ability to probe deeper into space, astronomers anticipate that a great deal more will be revealed about the early Universe.
One of the things I love about astronomy is how it’s rapidly changing and evolving over time. Every day there are new discoveries, and advancements in theories that take us incrementally forward in our understanding of the Universe.
One of the best examples of this is dark matter; mysterious and invisible but a significant part of the Universe and accounting for the vast majority of mass out there.
It was first theorized almost 100 years ago when astronomers surveyed the total mass of distant galaxy clusters and found that the visible mass we can see must be just a fraction of the total material in the clusters. When you add up the stars and gas, galaxies move and rotate in ways that indicate there’s a huge halo of invisible matter surrounding it.
Some of the best evidence came from Vera Rubin and Kent Ford in the 60s and 70s, when they measured the rotational velocity of edge-on spiral galaxies. They estimated that there must be about 6 times as much dark matter as regular matter.
This NASA Hubble Space Telescope image shows the distribution of dark matter in the center of the giant galaxy cluster Abell 1689
Dark matter became a serious mystery in astronomy, and many observers and theorists have spent the last half century trying to work out what it is.
And dark matter hasn’t given up its secrets easily. Originally, astronomers thought it might not actually be invisible mass, but a misunderstanding of how gravity works at the largest scales.
But over the last few decades, techniques have been developed, using the gravity of dark matter itself to measure how it bends light from more distant objects. Astronomers don’t know what dark matter is, but they’re able to use it as a telescope. Now that’s impressive.
They’ve found amazing features in the dark matter web out there, vast walls and filaments defining the largest scale structures in the Universe. Clusters where dark matter and its gas have been separated from each other.
Remember, we are at the cutting edge of this mystery, and you’re watching it unfold in real time. 25 years from now, I’m sure we’ll look back at our quaint attempts to understand dark matter.
One of the most interesting questions I have right now is: could there be dark matter galaxies? Completely invisible to our eyes, but able to interact through gravity?
Dark Matter Distribution in Supercluster Abell 901/902
Of course, in times like this, I like to bring in a ringer. Someone who has dedicated their life to the study of these questions.
And today, I’ve got with my Sarah Pearson, a graduate student in astronomy at Columbia University and the host of “Space with Sarah”. Sarah studies the formation and interactions of dwarf galaxies surrounding the Milky Way to understand how galaxies built up at the earliest times in the Universe and form the large galaxies we see at present day.
Fraser: Sarah, welcome to the Guide to Space.
Sarah: Hi Fraser, thanks.
Fraser: Can you talk a little bit about how astronomers map out the distribution of dark matter in the Universe?
Sarah: Yes, definitely. So that is a hard question, as you just explained, we don’t see the dark matter. But one assumption about the Universe we live in is that the light matter or baryonic matter. For example, what you, me and stars consist of, and also galaxies, kind of trace out where the dark matter is located.
So one assumption is that the light matter follows the dark matter. In that way we can actually map out to huge distances, kind of how galaxies and clusters of galaxies are located in our Universe. And we imagine that the dark matter structure is somewhat similar.
Simulation of dark matter. Image credit: NASA
And also recently, very large scale structure simulations of our own Universe have addressed this by kind of starting out with an almost uniform distribution of dark matter in the very early Universe. And what they see is when they let the Universe evolve in time, for example, when the Universe is expanding, you kind of have these dark matter clumps forming into galaxies in all these filaments that you discussed.
You can kind of trace out the location of dark matter by understanding the expansion of space versus gravity that creates the galaxies that we see.
Fraser: And I know in the observations that you see these different distributions of matter and dark matter, it’s not the perfect 1:6 radio that I just mentioned before. You actually see clumping of dark matter that’s sometimes separated from regular matter. So can you actually have whole galaxies that are entirely made of dark matter?
Sarah: Yes, that’s one of the topics I’m super excited about. I work on some of these dark matter only galaxies, and the way you can think about it is that the dark matter is almost uniformly distributed in the early Universe. But some of it is slightly denser than other parts, which collapses down into galaxies. And a lot of those galaxies will actually be a lot smaller than the Milky Way. And because they’re so small, they have a hard time actually holding onto the matter within them.
A bright young star shines Credit: NASA/JPL-Caltech
We think that when star formation turned on in these galaxies, you might actually blow out a lot of the gas that might create more stars, but you won’t blow out the dark matter. That means you could end up with these small tiny galaxies that only have dark matter. They might have some gas, but they’re very hard for us astronomers to find.
Fraser: Well, if they are dark matter, and the dark matter is invisible, how do we find them?
Sarah: Oh, great question. So for example, around our own galaxy Milky Way, it’s hypothesized in our current paradigm of cosmology and the way we think about the Universe, there should actually be thousands of dark matter clumps, these dark matter galaxies, kind of orbiting our own galaxy.
Artist’s impression of dark matter surrounding the Milky Way. (ESO/L. Calçada)
Some of these might be destroyed when they pass through the huge Milky Way disk, that’s one way of destroying them. The smaller ones might be destroyed just by the tides as they orbit around the galaxy. However, we imagine that some of them might survive. Actually they can plough through what we call stellar streams, which are formed when a real galaxy falls into our own Milky Way and tidally stretched out. You should be able to see these density signatures in the stellar stream, and that might indicate what type of dark matter halo that ploughed through them.
Fraser: You hinted at a way that they could form. You’ve got these stars as they’re early forming and blasting themselves apart and the clump of dark matter can’t hold onto them, so that part is gone. Is that the main way these might form, are there other ways you can get these dark galaxies?
Sarah: A different hypothesis is if you have an AGN, an active galactic nuclei within a galaxy from a black hole, you could actually that way blow out a lot of the gas from a galaxy as well. But it’s still not really clear to us astronomers what type of galaxies and if small galaxies would have these active galactic nuclei.
This artist’s impression shows the surroundings of the supermassive black hole at the heart of the active galaxy NGC 3783 in the southern constellation of Centaurus (The Centaur). Credit: ESO/M. Kornmesser
So the best theory right now is that some of them might have attracted a lot of gas initially because they didn’t have a lot of gravity to pull in the gas. But also, because this gas is completely lost. Also from stars exploding, actually, not just from stars turning on initially.
Fraser: And I know that astronomers and physicists are trying to search for dark matter in the Large Hadron Collider, and try to see if they can understand the underlying particle. Does the search that you’re working on give us any sense of that underlying nature of dark matter?
Sarah: Yeah, also a great question, because for example if dark matter is cold. The cold dark matter paradigm is very popular right now. Which states that dark matter might be a very massive weakly interacting particle. When we’re saying warm or cold dark matter, we’re also referring to how fast it’s moving. And depending on what kind of particle dark matter is, that kind of sets the structure for of the early Universe.
So we can start to count, if we have cold dark matter, we would expect to see a certain amount of these cold dark galaxies, where that amount would be different, if we had warm dark matter.
The international Super Cryogenic Dark Matter Search (SuperCDMS) has detected what may be the particle that’s thought to make up dark matter throughout the Universe.
Fraser: That’s really cool, so the observations that you do give the physicists a better idea of what they should be looking for in their particle accelerators, and the two sides can work together. That’s really great.
Okay Sarah, place your bets. What do you think is the most likely candidate for dark matter?
Sarah: I still think this is a hard question, and I’m not sure if the particle physicists yet think we’re helping them. We’re still approaching things from different sides, but we’ll see.
I still think it’s going to be one of those weakly interactive massive particles that we just haven’t detected yet.
Fraser: Thank you so much for joining me on the Guide to Space Sarah, I really appreciate you explaining these dark matter galaxies to us.
Well there you have it. Dark matter is strange, strange stuff. We still don’t know what it is, but we can see how it moves, interacts with matter through its gravity. And we can see how it can form entire galaxies of just dark matter.
A big thanks to Sarah Pearson. If you haven’t already, go and check out her YouTube channel: Space with Sarah. She’s covering big topics, like wondering when the Sun will shut off, how big the Universe is, and how galaxies can collide in an expanding Universe.
The Milky Way glitters above the ALMA array in this image taken from a time lapse sequence during the ESO Ultra HD Expedition.
This article is a guest post by Anna Ho, who is currently doing research on stars in the Milky Way through a one-year Fulbright Scholarship at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.
In the Milky Way, an average of seven new stars are born every year. In the distant galaxy GN20, an astonishing average of 1,850 new stars are born every year. “How,” you might ask, indignant on behalf of our galactic home, “does GN20 manage 1,850 new stars in the time it takes the Milky Way to pull off one?”
To answer this, we would ideally take a detailed look at the stellar nurseries in GN20, and a detailed look at the stellar nurseries in the Milky Way, and see what makes the former so much more productive than the latter.
But GN20 is simply too far away for a detailed look.
This galaxy is so distant that its light took twelve billion years to reach our telescopes. For reference, Earth itself is only 4.5 billion years old and the universe itself is thought to be about 14 billion years old. Since light takes time to travel, looking out across space means looking back across time, so GN20 is not only a distant, but also a very ancient, galaxy. And, until recently, astronomers’ vision of these distant, ancient galaxies has been blurry.
Consider what happens when you try to load a video with a slow Internet connection, or when you download a low-resolution picture and then stretch it. The image is pixelated. What was once a person’s face becomes a few squares: a couple of brown squares for hair, a couple of pink squares for the face. The low-definition picture makes it impossible to see details: the eyes, the nose, the facial expression.
A face has many details and a galaxy has many varied stellar nurseries. But poor resolution, a result simply of the fact that ancient galaxies like GN20 are separated from our telescopes by vast cosmic distances, has forced astronomers to blur together all of this rich information into a single point.
The situation is completely different here at home in the Milky Way. Astronomers have been able to peer deep into stellar nurseries and witness stellar birth in stunning detail. In 2006, the Hubble Space Telescope took this unprecedentedly detailed action shot of stellar birth at the heart of the Orion Nebula, one of the Milky Way’s most famous stellar nurseries:
A detailed close-up of stellar birth. Credit: NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team
There are over 3,000 stars in this image: The glowing dots are newborn stars that have recently emerged from their cocoons. Stellar cocoons are made of gas: thousands of these gas cocoons sit nestled in immense cosmic nurseries, which are rich with gas and dust. The central region of that Hubble image, encased by what looks like a bubble, is so clear and bright because the massive stars within have blown away the dust and gas they were forged from. Majestic stellar nurseries are scattered all over the Milky Way, and astronomers have been very successful at uncloaking them in order to understand how stars are made.
Observing nurseries both here at home and in relatively nearby galaxies has enabled astronomers to make great leaps in understanding stellar birth in general: and, in particular, what makes one nursery, or one star formation region, “better” at building stars than another. The answer seems to be: how much gas there is in a particular region. More gas, faster rate of star birth. This relationship between the density of gas and the rate of stellar birth is called the Kennicutt-Schmidt Law. In 1959, the Dutch astronomer Maarten Schmidt raised the question of how exactly increasing gas density influences star birth, and forty years later, in an illustration of how scientific dialogues can span decades, his American colleague Robert Kennicutt used data from 97 galaxies to answer him.
Understanding the Kennicutt-Schmidt Law is crucial for determining how stars form and even how galaxies evolve. One fundamental question is whether there is one rule that governs all galaxies, or whether one rule governs our galactic neighborhood, but a different rule governs distant galaxies. In particular, a family of distant galaxies known as “starburst galaxies” seems to contain particularly productive nurseries. Dissecting these distant, highly efficient stellar factories would mean probing galaxies as they used to be, back near the beginning of the universe.
Enter GN20. GN20 is one of the brightest, most productive of these starburst galaxies. Previously a pixelated dot in astronomers’ images, GN20 has become an example of a transformation in technological capability.
In December 2014, an international team of astronomers led by Dr. Jacqueline Hodge of the National Radio Astronomy Observatory in the USA, and comprising astronomers from Germany, the United Kingdom, France, and Austria, were able to construct an unprecedentedly detailed picture of the stellar nurseries in GN20. Their results were published earlier this year.
The key is a technique called interferometry: observing one object with many telescopes, and combining the information from all the telescopes to construct one detailed image. Dr. Hodge’s team used some of the most sophisticated interferometers in the world: the Karl G. Jansky Very Large Array (VLA) in the New Mexico desert, and the Plateau de Bure Interferometer (PdBI) at 2550 meters (8370 feet) above sea level in the French Alps.
With data from these interferometers as well as the Hubble Space Telescope, they turned what used to be one dot into the following composite image:
GN20 in unprecedented detail (false color image). The 10 kpc (10,000 parsec) scale corresponds to 32,600 light-years. Image credit: Jacqueline Hodge et al. 2015
This is a false color image, and each color stands for a different component of the galaxy. Blue is ultraviolet light, captured by the Hubble Space Telescope. Green is cold molecular gas, imaged by the VLA. And red is warm dust, heated by the star formation it is shrouding, detected by the PdBI.
Unbundling one pixel into many enabled the team to determine that the nurseries in a starburst galaxy like GN20 are fundamentally different from those in a “normal” galaxy like the Milky Way. Given the same amount of gas, GN20 can churn out orders of magnitude more stars than the Milky Way can. It doesn’t simply have more raw material: it is more efficient at fashioning stars out of it.
Some of the 66 radio antennas of ALMA, which can be linked to act like a much larger telescope. Image credit: ALMA (ESO/NAOJ/NRAO)/B. Tafreshi (twanight.org)
This kind of study is currently unique to the extreme case of GN20. However, it will be more common with the new generation of interferometers, such as the Atacama Large Millimeter/submillimeter Array (ALMA).
Located 5000 meters (16000 feet) high up in the Chilean Andes, ALMA is poised to transform astronomers’ understanding of stellar birth. State-of-the-art telescopes are enabling astronomers to do the kind of detailed science with distant galaxies – ancient galaxies from the early universe – that was once thought to be possible only for our local neighborhood. This is crucial in the scientific quest for universal physical laws, as astronomers are able to test their theories beyond our neighborhood, out across space and back through time.
An illustration that shows the powerful winds driven by a supermassive black hole at the centre of a galaxy. The schematic figure in the inset depicts the innermost regions of the galaxy where a black hole accretes, that is, consumes, at a very high rate the surrounding matter (light grey) in the form of a disc (darker grey). At the same time, part of that matter is cast away through powerful winds. (Credits: XMM-Newton and NuSTAR Missions; NASA/JPL-Caltech;Insert:ESA)
The combined observations from two generations of X-Ray space telescopes have now revealed a more complete picture of the nature of high-speed winds expelled from super-massive black holes. Scientist analyzing the observations discovered that the winds linked to these black holes can travel in all directions and not just a narrow beam as previously thought. The black holes reside at the center of active galaxies and quasars and are surrounded by accretion discs of matter. Such broad expansive winds have the potential to effect star formation throughout the host galaxy or quasar. The discovery will lead to revisions in the theories and models that more accurately explain the evolution of quasars and galaxies.
This plot of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s (ESA’s) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (Credit: NASA/JPL-Caltech/Keele Univ.;XMM-Newton and NuSTAR Missions, [Ref])The observations were by the XMM-Newton and NuSTAR x-ray space telescopes of the quasar PDS 456. The observations were combined into the graphic, above. PDS 456 is a bright quasar residing in the constellation Serpens Cauda (near Ophiuchus). The data graph shows both a peak and a trough in the otherwise nominal x-ray emission profile as shown by the NuSTAR data (pink). The peak represents X-Ray emissions directed towards us (i.e.our telescopes) while the trough is X-Ray absorption that indicates that the expulsion of winds from the super-massive black hole is in many directions – effectively a spherical shell. The absorption feature caused by iron in the high speed wind is the new discovery.
X-Rays are the signature of the most energetic events in the Cosmos but also are produced from some of the most docile bodies – comets. The leading edge of a comet such as Rosetta’s P67 generates X-Ray emissions from the interaction of energetic solar ions capturing electrons from neutral particles in the comet’s coma (gas cloud). The observations of a super-massive black hole in a quasar billions of light years away involve the generation of x-rays on a far greater scale, by winds that evidently has influence on a galactic scale.
A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. (Illustration Credit: ESA/XMM-Newton)
The study of star forming regions and the evolution of galaxies has focused on the effects of shock waves from supernova events that occur throughout the lifetime of a galaxy. Such shock waves trigger the collapse of gas clouds and formation of new stars. This new discovery by the combined efforts of two space telescope teams provides astrophysicists new insight into how star and galaxy formation takes place. Super-massive blackholes, at least early in the formation of a galaxy, can influence star formation everywhere.
The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design – optics in the foreground, 10 meter truss and detectors at back – images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)
Both the ESA built XMM-Newton and the NuSTAR X-Ray space telescope, a SMEX class NASA mission, use grazing incidence optics, not glass (refraction) or mirrors (reflection) as in conventional visible light telescopes. The incidence angle of the X-rays must be very shallow and consequently the optics are extended out on a 10 meter (33 foot) truss in the case of NuSTAR and over a rigid frame on the XMM-Newton.
Diagram of one of three x-ray telescopes of the XMM-Newton design. Only a few of the grazing angle concentric mirrors are shown. Inset: a simplified illustration of how a Wolter telescope works. (Credits: Wikimedia, ESA) [click to enlarge]
The spectral ranges of the XMM-Newton and NuSTAR Telescopes. (Credits: NASA, ESA)
The ESA built XMM-Newton was launched in 1999, an older generation design that used a rigid frame and structure. All the fairing volume and lift capability of the Ariane 5 launch vehicle was needed to put the Newton in orbit. The latest X-Ray telescope – NuSTAR – benefits from tens years of technological advances. The detectors are more efficient and faster and the rigid frame was replaced with a compact truss which required all of 30 minutes to deploy. Consequently, NuSTAR was launched on a Pegasus rocket piggybacked on a L-1011, a significantly smaller and less expensive launch system.
So now these observations are effectively delivered to the theorists and modelers. The data is like a new ingredient in the batter from which a galaxy and stars are formed. The models of galaxy and star formation will improve and will more accurately describe how quasars, with their active super-massive black-holes, transition into more quiescent galaxies such as our own Milky Way.
This image combines NASA/ESA Hubble Space Telescope observations with data from the Chandra X-ray Observatory. As well as the electric blue ram pressure stripping streaks seen emanating from ESO 137-001, a giant gas stream can be seen extending towards the bottom of the frame, only visible in the X-ray part of the spectrum.
Credit:
NASA, ESA, CXC
Some galaxies shine with a red ghostly glow. Once these galaxies stop forming new stars, they can only host long-lived stars with low masses and red optical colors. Astronomers often call these ghostly galaxies “red and dead.” But the basics behind why some form so quickly is still a mystery.
“It is one of the major tasks of modern astronomy to find out how and why galaxies in clusters evolve from blue to red over a very short period of time,” said lead author Michele Fumagalli from Durham University in a news release. “Catching a galaxy right when it switches from one to the other allows us to investigate how this happens.”
And that’s exactly what Fumagalli and colleagues did.
The team used ESO’s Multi Unit Spectroscopic Explorer (MUSE) instrument mounted on the 8-meter Very Large Telescope. With this instrument, astronomers collect 90,000 spectra every time they look at an object, allowing them to gain a detailed map of the object’s motion through space.
The location of the distant galaxy ESO 137-001. Credit: ESO / IAU / Sky & Telescope
The target, ESO 137-001, is a spiral galaxy 200 million light-years away in the constellation better known as the Southern Triangle. But more importantly, it’s currently hurtling toward the Norma Cluster and embarking on a grand galactic collision.
ESO 137-001 is being stripped of most of its gas due to a process called ram-pressure stripping. As the galaxy falls into the galaxy cluster, it feels a headwind, much as a runner feels a wind on even the stillest day. At times this can compress the gas enough to spark star formation, but if it’s too intense then the gas is stripped away, leaving a galaxy that’s empty of the material needed to form new stars.
So the galaxy is in the midst of a brilliant transformation, changing from a blue gas-rich galaxy to a red gas-poor galaxy.
The observations show that the outskirts of the galaxy are already completely devoid of gas. Here the stars and matter are more thinly spread, and gravity has a relatively week hold over the gas. So it’s easier to push the gas away.
In fact, dragging behind the galaxy are 200,000 light-year-long streams of gas that have already been lost, making the galaxy look like a jellyfish trailing its tentacles through space. In these streamers, the gas is turbulent enough to compress small pockets of gas and therefore actually ignite star formation.
The center of the galaxy, however, is not yet devoid of gas because the gravitational pull is strong enough to hold out much longer. But it will only take time until all of the galactic gas is swept away, leaving ESO 137-001 red and dead.
Surprisingly the new MUSE observations show the gas trailing behind continues to rotate in the same way that the galaxy does. Furthermore, the rotation of stars at the center of the galaxy remains unhindered by the great fall.
Astronomers remain unsure why as this is only a snapshot of one galactic crash, but soon MUSE and other instruments will pry more out of the cosmic shadows.
The results will be published in the journal Monthly Notices of the Royal Astronomical Society and are available online.
