How galaxies form
* Have you read the preceding page on galaxies?
Galaxies form when vast “halos” or spheres of unseen dark matter pull ordinary matter in the form of gas towards itself by means of its gravitational pull. Stars form. Towards the end of their lives, these stars explode triggering another generation of stars to form from any remaining gas and dust. In this way the central heart (the “bulge”) of the Milky Way and its spiral arms (the “disk”) most likely formed.
A second halo of more diffuse stars also surrounds the bulge and disk. Many of these stars are probably interlopers from long-destroyed dwarf galaxies. As a dwarf galaxy orbits the Milky Way, it feels the gravitational pull of the big galaxy, which gets stronger as the satellite gets closer to the larger galaxy[1]. The stars, gas, dust and dark matter on the side of the galaxy closest to the Milky Way experiences a slightly greater force of attraction than the matter on the far side. As a result the dwarf gets stretched along the line between it and the larger galaxy which causes tidal forces strong enough strong enough to actually pull stars from the body of the dwarf. Once removed, the stars stay in the grip of the Milky Way’s gravity and continue along a path slightly offset from the satellite’s own orbit. Over time the slight offset causes the debris to steadily spread, becoming more diffuse and moving away from the satellite to form long filaments or tails called stellar streams, and the area surveyed by the Sloan[2] is the perfect dig site for the discovery of these galactic fossils.
The fraction of interloper stars initially born in other galaxies would comprise only 1% or less of the Milky Way’s hundreds of billions of stars, but Sloan’s map gave astronomers potentially almost one million interloper stars to examine for evidence of long dead galaxies. Galactic archaeologists looking at stars the right distance in the galactic halo located star streams by homing in on areas that were denser with stars than their surroundings and took the shape of tails. Aided by an understanding of how dark matter halos form hierarchically and the forces of tidal physics, enabled astronomers to predict the sizes and spreads of the stellar streams that resulted when dwarf galaxies got swallowed up during the Milky Way’s formation, and from there computer simulations gave galactic astronomers some idea what the tails would look like.
In 2003, astronomers observed giant tails emanating from the Milky Way’s closest known satellite, the Sagittarius dwarf galaxy. The tails are so large that they entirely encircle our own galaxy. From the length of these tails, Sagittarius appears to have been losing stars in this fashion for some two to three billion years. The known stellar streams are probably just a fraction of those that exist, since many more streams should be out there, but are two faint to see.
Techniques of detection[3]
Method 1: stars with common orbits
One way to look for the leftovers of stars from now defunct galaxies from a period when hundreds of small galaxies accreted to form the Milky Way involves looking for stars with common orbits. Long after the stars have become too scattered to recognise from their positions, we can take advantage of their motion to identify those which were once part of the same satellite galaxy and learn how they once joined the Milky Way. In this regard, the Gaia satellite launched in December 2013 will spend the next four years measuring the distances, positions and motions for more than a billion stars
Method 2: stars’ chemical compositions
Another means by which one can trace stars is by virtue of their chemical composition. Hydrogen and helium are still by far the dominant components of the universe. Other elements are less than one-thousandth as abundant as hydrogen. However, since each generation of stars polluted the interstellar gas with materials made during its lifetime, later generations of stars contained more and more heavy elements.
By measuring a star’s metallicity – the relative abundance of (say) carbon or iron – we can have a good idea of how pristine the gas from which it formed was. Since we think that galaxies were assembled by the merging of different clumps, it seems likely that these different populations of stars came from different events. One model suggests that the halo and bulge formed from the collapse of the initial cloud. Later, about 10 billion years ago, there was another accretion event – the collision of a satellite galaxy – which formed the thin disk, which has been growing ever since as gas is accreted at the edges. In other words, stars forming later within a galaxy generally contain more heavy elements than those forming earlier because the material that constitutes them has already been enriched with the remains of previous generations of stars.
When the sun came into being about 4.5 billion years ago, this metallic enrichment process had been going on for billions of years in our galactic neighbourhood, but even so, the sun contains roughly 71 percent hydrogen, 27 percent helium and just 2 percent metals. Its composition mirrors that of the cloud that made the solar system, so the rocky planets, including Earth, formed from only that tiny amount of elemental construction material. This is why stars older than the sun have even fewer metals and, correspondingly, less chance of making rocky, Earth-like planets. In other words, even if we are not the only technological civilization in the galaxy, we must be one of the first [3.5].
Anna Frebel, assistant professor of physics at the Massachusetts Institute of Technology, has spent many years researching the ultrafaint stars in the dwarf galaxies that orbit the Milky Way as well as stars in our galaxy’s halo. We know these stars are very old because of the proportions of chemicals they contain – basically hydrogen, helium and a small amount of lithium. In other words, they are metal-poor, having been born in the universe’s infancy. Their composition can be viewed through a telescope with a spectrograph which splits sunlight into a rainbow spectrum of different wavelengths. Recall that a similar process is used by scientists when measuring the age of the universe.
The process is depicted in the following graphic:
Galaxies form when vast “halos” or spheres of unseen dark matter pull ordinary matter in the form of gas towards itself by means of its gravitational pull. Stars form. Towards the end of their lives, these stars explode triggering another generation of stars to form from any remaining gas and dust. In this way the central heart (the “bulge”) of the Milky Way and its spiral arms (the “disk”) most likely formed.
A second halo of more diffuse stars also surrounds the bulge and disk. Many of these stars are probably interlopers from long-destroyed dwarf galaxies. As a dwarf galaxy orbits the Milky Way, it feels the gravitational pull of the big galaxy, which gets stronger as the satellite gets closer to the larger galaxy[1]. The stars, gas, dust and dark matter on the side of the galaxy closest to the Milky Way experiences a slightly greater force of attraction than the matter on the far side. As a result the dwarf gets stretched along the line between it and the larger galaxy which causes tidal forces strong enough strong enough to actually pull stars from the body of the dwarf. Once removed, the stars stay in the grip of the Milky Way’s gravity and continue along a path slightly offset from the satellite’s own orbit. Over time the slight offset causes the debris to steadily spread, becoming more diffuse and moving away from the satellite to form long filaments or tails called stellar streams, and the area surveyed by the Sloan[2] is the perfect dig site for the discovery of these galactic fossils.
The fraction of interloper stars initially born in other galaxies would comprise only 1% or less of the Milky Way’s hundreds of billions of stars, but Sloan’s map gave astronomers potentially almost one million interloper stars to examine for evidence of long dead galaxies. Galactic archaeologists looking at stars the right distance in the galactic halo located star streams by homing in on areas that were denser with stars than their surroundings and took the shape of tails. Aided by an understanding of how dark matter halos form hierarchically and the forces of tidal physics, enabled astronomers to predict the sizes and spreads of the stellar streams that resulted when dwarf galaxies got swallowed up during the Milky Way’s formation, and from there computer simulations gave galactic astronomers some idea what the tails would look like.
In 2003, astronomers observed giant tails emanating from the Milky Way’s closest known satellite, the Sagittarius dwarf galaxy. The tails are so large that they entirely encircle our own galaxy. From the length of these tails, Sagittarius appears to have been losing stars in this fashion for some two to three billion years. The known stellar streams are probably just a fraction of those that exist, since many more streams should be out there, but are two faint to see.
Techniques of detection[3]
Method 1: stars with common orbits
One way to look for the leftovers of stars from now defunct galaxies from a period when hundreds of small galaxies accreted to form the Milky Way involves looking for stars with common orbits. Long after the stars have become too scattered to recognise from their positions, we can take advantage of their motion to identify those which were once part of the same satellite galaxy and learn how they once joined the Milky Way. In this regard, the Gaia satellite launched in December 2013 will spend the next four years measuring the distances, positions and motions for more than a billion stars
Method 2: stars’ chemical compositions
Another means by which one can trace stars is by virtue of their chemical composition. Hydrogen and helium are still by far the dominant components of the universe. Other elements are less than one-thousandth as abundant as hydrogen. However, since each generation of stars polluted the interstellar gas with materials made during its lifetime, later generations of stars contained more and more heavy elements.
By measuring a star’s metallicity – the relative abundance of (say) carbon or iron – we can have a good idea of how pristine the gas from which it formed was. Since we think that galaxies were assembled by the merging of different clumps, it seems likely that these different populations of stars came from different events. One model suggests that the halo and bulge formed from the collapse of the initial cloud. Later, about 10 billion years ago, there was another accretion event – the collision of a satellite galaxy – which formed the thin disk, which has been growing ever since as gas is accreted at the edges. In other words, stars forming later within a galaxy generally contain more heavy elements than those forming earlier because the material that constitutes them has already been enriched with the remains of previous generations of stars.
When the sun came into being about 4.5 billion years ago, this metallic enrichment process had been going on for billions of years in our galactic neighbourhood, but even so, the sun contains roughly 71 percent hydrogen, 27 percent helium and just 2 percent metals. Its composition mirrors that of the cloud that made the solar system, so the rocky planets, including Earth, formed from only that tiny amount of elemental construction material. This is why stars older than the sun have even fewer metals and, correspondingly, less chance of making rocky, Earth-like planets. In other words, even if we are not the only technological civilization in the galaxy, we must be one of the first [3.5].
Anna Frebel, assistant professor of physics at the Massachusetts Institute of Technology, has spent many years researching the ultrafaint stars in the dwarf galaxies that orbit the Milky Way as well as stars in our galaxy’s halo. We know these stars are very old because of the proportions of chemicals they contain – basically hydrogen, helium and a small amount of lithium. In other words, they are metal-poor, having been born in the universe’s infancy. Their composition can be viewed through a telescope with a spectrograph which splits sunlight into a rainbow spectrum of different wavelengths. Recall that a similar process is used by scientists when measuring the age of the universe.
The process is depicted in the following graphic:
The black vertical bars are known as absorption lines which correspond to the abundances of different chemical elements in the outer shell of the star. The high resolution spectroscopy used is so precise as to reveal even how many atoms of each chemical element a star contains. The thinner the absorption line, the less of that particular element exists in the star.
Frebel’s research over the last several years has demonstrated that both halo stars and dim dwarf galaxy stars have very weak absorption lines corresponding to heavy elements such as iron. This suggests that the ancient halo stars are chemically similar to dwarf galaxy stars because they were once part of dwarf galaxies too. Over time the Milky Way has “ingested” these nearby dwarf galaxies, stealing their stars and growing larger all the while, a process which continues to the present day. Right now the Milky Way is eating up the Sagittarius dwarf elliptical galaxy bit by bit as the latter moves around our galaxy, and with every turn, stars are torn away from Sagittarius and absorbed into our galaxy’s halo[4].
Method 3: the influence of gravity on gas flows
Another method of determining how enrichment proceeds is by studying gas flows that are governed in part by the gravitational influence of the galaxy’s dark matter halo. In this manner, a star’s chemical composition and gravitation suggest that galaxies of roughly the same mass that are accreted and destroyed at roughly the same time should contribute stars with the same distribution of chemical elements. Hence the overall distribution of chemical composition of stars around the Milky Way could allow us to discern what fraction come from similar mass galaxies at similar times, if not exactly the same galaxy.
The fossils of galaxies subsumed into the Milky Way therefore teach us not only about the Milky Way’s history, but also about the histories of all the smaller galaxies it includes, and provide a convenient window on the early universe and the first steps of galaxy formation, impossible to access by other means.
[1] This material is drawn from Kathryn V Johnston’s article, “Fossil Hunting in the Milky Way”, Scientific American, December 2014, 38 at 40.
[2] The Sloan Digital Sky Survey (SDSS), referred to on the previous page.
[3] Kathryn Johnston, Ibid.
[3.5] John Gribbin, "Alone in the Milky Way - Why we are probably the only intelligent life in the galaxy", Scientific American, September 2015, 86-91, at 88.
[4] Source: Frebel, “Four Starry Nights”, Scientific American, December 2012, 53.
Frebel’s research over the last several years has demonstrated that both halo stars and dim dwarf galaxy stars have very weak absorption lines corresponding to heavy elements such as iron. This suggests that the ancient halo stars are chemically similar to dwarf galaxy stars because they were once part of dwarf galaxies too. Over time the Milky Way has “ingested” these nearby dwarf galaxies, stealing their stars and growing larger all the while, a process which continues to the present day. Right now the Milky Way is eating up the Sagittarius dwarf elliptical galaxy bit by bit as the latter moves around our galaxy, and with every turn, stars are torn away from Sagittarius and absorbed into our galaxy’s halo[4].
Method 3: the influence of gravity on gas flows
Another method of determining how enrichment proceeds is by studying gas flows that are governed in part by the gravitational influence of the galaxy’s dark matter halo. In this manner, a star’s chemical composition and gravitation suggest that galaxies of roughly the same mass that are accreted and destroyed at roughly the same time should contribute stars with the same distribution of chemical elements. Hence the overall distribution of chemical composition of stars around the Milky Way could allow us to discern what fraction come from similar mass galaxies at similar times, if not exactly the same galaxy.
The fossils of galaxies subsumed into the Milky Way therefore teach us not only about the Milky Way’s history, but also about the histories of all the smaller galaxies it includes, and provide a convenient window on the early universe and the first steps of galaxy formation, impossible to access by other means.
[1] This material is drawn from Kathryn V Johnston’s article, “Fossil Hunting in the Milky Way”, Scientific American, December 2014, 38 at 40.
[2] The Sloan Digital Sky Survey (SDSS), referred to on the previous page.
[3] Kathryn Johnston, Ibid.
[3.5] John Gribbin, "Alone in the Milky Way - Why we are probably the only intelligent life in the galaxy", Scientific American, September 2015, 86-91, at 88.
[4] Source: Frebel, “Four Starry Nights”, Scientific American, December 2012, 53.