Between 300 million and 900 million years ago, our Milky Way galaxy nearly collided with the Sagittarius dwarf galaxy. Data from the ESA’s Gaia mission shows the ongoing effect of this event, with stars moving like ripples on the surface of a pond. The galactic collision is part of an ongoing cannibalization of the dwarf galaxy by the much-larger Milky Way.
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Andromeda Galaxy, also known as Messier 31. Enjoy!
During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.
One of these objects is the famed Andromeda Galaxy, the closest spiral galaxy to the Milky Way which is named for the area of the sky it appears in (in the vicinity of the Andromeda constellation). It is the largest galaxy in the Local Group, and has the distinction of being one of the few objects that is actually getting closer to the Milky Way (and is expected to merge with us in a few billion years!).
Approaching us at roughly 300 kilometers per second, our massive galactic neighbor has been the object of studies of spiral structure, globular and open clusters, interstellar matter, planetary nebulae, supernova remnants, galactic nucleus, companion galaxies, and more for as long as we’ve been peering its way with a telescope. It’s part of our Local Group of galaxies and its two easily visible companions are only part of the eleven others that swarm around it.
One day, this galaxy will collide with our own, much as it is now consuming its neighbor – M32. However, this won’t come to pass for several billions years, so don’t go worrying about the immense gravitational disturbances just yet! And not surprisingly, a giant galaxy like Andromeda doesn’t get to be so big by keeping to itself. How many times now has the Great Andromeda Galaxy consumed another? More than once!
In 1993, the Hubble Space Telescope revealed that M31 has a double nucleus – a ‘leftover’ from another meal! As NASA and the ESA stated about the discovery at the time:
“Each of the two light-peaks contains a few million densely packed stars. The brighter object is the “classic” nucleus as studied from the ground. However, HST reveals that the true center of the galaxy is really the dimmer component. One possible explanation is that the brighter cluster is the leftover remnant of a galaxy cannibalized by M31. Another idea is that the true center of the galaxy has been divided in two by deep dust absorption across the middle, creating the illusion of two peaks. This green-light image was taken with HST’s Wide Field and Planetary Camera (WF/PC), in high resolution mode, on July 6, 1991. The two peaks are separated by 5 light-years. The Hubble image is 40 light-years across.”
Perhaps one of the most fascinating discovery recent years in Messier 31 was made by the orbiting Chandra X-Ray Observatory. The X-ray image below, made with the Chandra X-Ray Astronomy Center’s Advanced CCD Imaging Spectrometer (ACIS), shows the central portion of the Andromeda Galaxy. The Chandra X-ray Observatory is part of NASA’s fleet of “Great Observatories” along with the Hubble Space Telescope.
The blue dot in the center of the image is a “cool” million degree X-ray source where Andromeda’s massive central object, with the mass of 30 million suns, is located, which many astronomers consider to be a supermassive black hole. Most of these are probably due to X-ray binary systems, in which a neutron star (or perhaps a stellar black hole) is in a close orbit around a normal star.”
Over the years our studies have advanced even more with the discovery of an eclipsing binary star in Messier 31. As Ignasi Ribas (et al) put it in a 2005:
“We present the first detailed spectroscopic and photometric analysis of an eclipsing binary in the Andromeda Galaxy (M31). This is a 19.3 mag semidetached system with late O and early B spectral type components. From the light and radial velocity curves we have carried out an accurate determination of the masses and radii of the components. Their effective temperatures have been estimated by modeling the absorption-line spectra. The analysis yields an essentially complete picture of the properties of the system, and hence an accurate distance determination to M31.”
In 2005, we discovered more. At that time, Scott Chapman of Caltech, Rodrigo Ibata of the Observatoire de Strasbourg, and their colleagues conducted detailed studies on the motions and metals of nearly 10,000 stars in Andromeda, which that the galaxy’s stellar halo is “metal-poor.” Essentially, this indicated that the stars lying in the outer bounds of the galaxy are lacking in elements heavier than hydrogen.
According to Chapman, this was surprising since one of the key differences thought to exist between Andromeda and the Milky Way was that the former’s stellar halo was metal-rich and the latter’s was metal-poor. If both galaxies are metal-poor, then they must have had very similar evolutions. As Chapman explained:
“Probably, both galaxies got started within a half billion years of the Big Bang, and over the next three to four billion years, both were building up in the same way by protogalactic fragments containing smaller groups of stars falling into the two dark-matter haloes.”
While no one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centers. In fact, current theories of galactic evolution assume that dark-matter wells acted as a sort of “seed” for today’s galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby.
What’s more, galaxies like Andromeda and the Milky Way have each probably gobbled up about 200 smaller galaxies and protogalactic fragments over the last 12 billion years. Chapman and his colleagues arrived at the conclusion about the metal-poor Andromeda halo by obtaining careful measurements of the speed at which individual stars are coming directly toward or moving directly away from Earth.
This measure is called the radial velocity, and can be determined very accurately with the spectrographs of major instruments such as the 10-meter Keck-II telescope, which was used in the study. Of the approximately 10,000 Andromeda stars for which the researchers have obtained radial velocities, about 1,000 turned out to be stars in the giant stellar halo that extends outward by more than 500,000 light-years.
These stars, because of their lack of metals, are thought to have formed quite early, at a time when the massive dark-matter halo had captured its first protogalactic fragments. The stars that dominate closer to the center of the galaxy, by contrast, are those that formed and merged later, and contain heavier elements due to stellar evolution processes.In addition to being metal-poor, the stars of the halo follow random orbits and are not in rotation.
By contrast, the stars of Andromeda’s visible disk are rotating at speeds upwards of 200 kilometers per second.According to Ibata, the study could lead to new insights on the nature of dark matter. “This is the first time we’ve been able to obtain a panoramic view of the motions of stars in the halo of a galaxy,” says Ibata. “These stars allow us to weigh the dark matter, and determine how it decreases with distance.”
History of Observation:
Andromeda was known as the “Little Cloud” to Persian astronomer Abd-al-Rahman Al-Sufi, who described and depicted it in 964 AD in his Book of Fixed Stars. This wonderful galaxy was also cataloged by Giovanni Batista Hodierna in 1654, Edmund Halley in 1716, by Bullialdus 1664, and again by Charles Messier in 1764.
Like most of the objects he added to the Messier Catalog, he mistook the galaxy initially for a nebulous object. As he wrote of the object in his notes:
“The sky has been very good in the night of August 3 to 4, 1764; and the constellation Andromeda was near the Meridian, I have examined with attention the beautiful nebula in the girdle of Andromeda, which was discovered in 1612 by Simon Marius, and which has been observed since with great care by different astronomers, and at last by M. le Gentil who has given a very ample and detailed description in the volume of the Memoirs of the Academy for 1759, page 453, with a drawing of its appearance. I will not report here what I have written in my Journal: I have employed different instruments for examining that nebula, and above all an excellent Gregorian telescope of 30 pouces focal length, the large mirror having 6 pouces in diameter, and magnifying 104 times these objects: the middle of that nebula appeared rather bright with this instrument, without any appearance of stars; the light went diminishing up to extinguishing; it resembles two cones or pyramids of light, opposed at their bases, of which the axis was in the direction form North-West to South-East; the two points of light or the two summits are about 40 minutes of arc apart; I say about, because of the difficulty to recognize these two extremities. The common base of the two pyramids is 15 minutes: these measures have been made with a Newtonian telescope of 4 feet and a half focal length, equipped with a micrometer of silk wires. With the same instrument I have compared the middle of the summits of the two cones of light with the star Gamma Andromedae of fourth magnitude which is very near to it, and little distant from its parallel. From these observations, I have concluded the right ascension of the middle of this nebula as 7d 26′ 32″, and its declination as 39d 9′ 32″ north. Since fifteen years during which I viewed and observed this nebula, I have not noticed any change in its appearances; having always perceived it in the same shape.”
A great many astronomers would observe the Andromeda Galaxy over the years, each colorfully describing it. However, as we know from history, it would be quite some time before its true nature as an external galaxy would be discovered. Here is where we must give the utmost respect to Sir William Herschel, who knew way ahead of everyone else, that there was something very, very different about Messier’s Object 31!
Although he never publicly published his observing notes on another astronomer’s discoveries, it’s a shame he did not for this is what he had to say:
“.. But when an object is of such a construction, or at such a distance from us, that the highest power of penetration, which hitherto has been applied to it, leaves it undetermined whether it belongs to the class of nebulae or of stars, it may be called ambiguous. As there is, however, a considerable difference in the ambiguity of such objects, I have arranged 71 of them into the following four collections. The first contains seven objects that may be supposed to consist of stars, but where the observations hitherto made, of either their appearance or form, leave it undecided into which class they should be placed. Connoiss. 31 [M31] is: A large nucleus with very extensive nebulous branches, but the nucleus is very gradually joined to them. The stars which are scattered over it appear to be behind it, and seem to lose part of their lustre in the passage of their light through the nebulosity; there are not more of them scattered over the immediate neighborhood. I examined it in the meridian with a mirror of 24 inches in diameter, and saw it in high perfection; but its nature remains mysterious. Its light, instead of appearing resolvable with this aperture, seemed to be more milky. The objects in this collection must at present remain ambiguous.”
Locating Messier 31:
Even under moderately light polluted skies the Great Andromeda Galaxy, located in the Andromeda constellation, can be easily be found with the unaided eye – if you know where to look. Seasoned amateur astronomers can literally point to the sky and show you the location of M31, but perhaps you have never tried to find it. Believe it or not, this is an easy galaxy to spot even under the moonlight.
Simply identify the large diamond-shaped pattern of stars that is the Great Square of Pegasus. The northernmost star is Alpha, and it is here we will begin our hop. Stay with the northern chain of stars and look four finger widths away from Alpha for an easily seen star. The next along the chain is about three more finger widths away. Two more finger widths to the north and you will see a dimmer star that looks like it has something smudgy nearby.
Point your binoculars there, because that’s no cloud – it’s the Andromeda Galaxy! Now aim your binoculars or small telescope its way… Perhaps one of the most outstanding of all galaxies to the novice observer, M31 spans so much sky that it takes up several fields of view in a larger telescope, and even contains its own clusters and nebulae with New General Catalog designations.
If you have a slightly larger telescope, you may also be able to pick up M31’s two companions – M32 and M110. Even with no scope or binoculars, it’s pretty amazing that we can see something – anything! – that is over two million light-years away!
Enjoy this wonderful and mysterious galaxy at any and every opportunity! Even the most modest of optical aids will reveal it for what it is… Another island universe!
And here are ye’ ole’ quick facts. Enjoy!
Object Name: Messier 31
Alternative Designations: M31, NGC 224, Andromeda Galaxy
Object Type: Type Sb Galaxy
Right Ascension: 00 : 42.7 (h:m)
Declination: +41 : 16 (deg:m)
Distance: 2900 (kly)
Visual Brightness: 3.4 (mag)
Apparent Dimension: 178×63 (arc min)
We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.
This week, the 229th Meeting of the American Astronomical Society (AAS) kicked off in Grapevine, Texas. Between Monday and Friday (January 3rd to January 7th), attendees will be hearing presentations by researchers and scientists from several different fields as they share the latest discoveries in astronomy and Earth science.
One of the highlights so far this week was a presentation from NASA’s Chandra X-ray Observatory, which took place on the morning of Wednesday, January 5th. In the course of the presentation, an international research team showed some stunning images of two of the most powerful cosmic forces seen together for the first time – a supermassive black hole and two massive galaxy clusters colliding.
The galaxy clusters are known as Abell 3411 and Abell 3412, which are located about two billion light years from Earth. Both of these clusters are quite massive, each possessing the equivalent of about a quadrillion times the mass of our Sun. Needless to say, the collision of these objects produced quite the shockwave, which included the release of hot gas and energetic particles.
This was made all the more impressive thanks to the presence of a supermassive black hole (SMBH) at the center of one of the galaxy clusters. As the team described in their paper – titled “The Case for Electron Re-Acceleration at Galaxy Cluster Shocks” – the galactic collision produced a nebulous outburst of x-rays (shown above), which were produced when hot clouds of gas from one cluster plowed through the hot gas clouds of the other.
Meanwhile, the inflowing gas was accelerated outward into a jet-like stream, thanks to the powerful electromagnetic fields of the SMBH. These particles were accelerated even further when they got swept up by the shock waves produced by the collision of the galactic clusters and their massive gas clouds. These streams were detected thanks to the burst of radio waves they released as a result.
By seeing these two major events happening at the same time in the same place, the research team effectively witnessed a cosmic “double whammy”. As Felipe Andrade-Santos of the Harvard-Smithsonian Center for Astrophysics (CfA), and co-author of the paper, described it in a Chandra press release:
“It’s almost like launching a rocket into low-Earth orbit and then getting shot out of the Solar System by a second rocket blast. These particles are among the most energetic particles observed in the Universe, thanks to the double injection of energy.”
Relying on data obtained from the Chandra X-ray Observatory, the Giant Metrewave Radio Telescope (GMRT) in India, the Karl G. Jansky Very Large Array, the Keck Observatory, and Japan’s Subaru Telescope, the team was able to capture this event in the optical, x-ray, and radio wave wavelengths. This not only led to some stunning images, but shed some light on a long-standing mystery in galaxy research.
In the past, astronomers have detected radio emissions coming from Abell 3411 and Abell 3412 using the GMRT. But the origins of these emissions, which reached for millions of light years, was the subject of speculation and debate. Relying on the data they obtained, the research team was able to determine that they are the result of energetic particles (produced by the clouds of hot gas colliding) being further accelerated by galactic shock waves.
Or as co-author William Dawson, of the Lawrence Livermore National Lab in Livermore, California, put it:
“This result shows that a remarkable combination of powerful events generate these particle acceleration factories, which are the largest and most powerful in the Universe. It is a bit poetic that it took a combination of the world’s biggest observatories to understand this.”
Many interesting finds have been shared since the 229th Meeting of the AAS began – like the hunt for the source of a Fast Radio Burst – and many more are expected before it wraps up at the end of the week. These will include the latest results from the Sloan Digital Sky Survey (SDSS), and new and exciting research on black holes, exoplanets, and other astronomical phenomena.
And be sure to check out this podcast from Chandra as well, which talks about the collision between Abell 3411 and 3412 and the cosmic forces it unleashed.
Further Reading: Chandra X-ray Observatory
We don’t want to scare you, but our own Milky Way is on a collision course with Andromeda, the closest spiral galaxy to our own. At some point during the next few billion years, our galaxy and Andromeda – which also happen to be the two largest galaxies in the Local Group – are going to come together, and with catastrophic consequences.
Stars will be thrown out of the galaxy, others will be destroyed as they crash into the merging supermassive black holes. And the delicate spiral structure of both galaxies will be destroyed as they become a single, giant, elliptical galaxy. But as cataclysmic as this sounds, this sort of process is actually a natural part of galactic evolution.
Astronomers have know about this impending collision for some time. This is based on the direction and speed of our galaxy and Andromeda’s. But more importantly, when astronomers look out into the Universe, they see galaxy collisions happening on a regular basis.
Galaxies are held together by mutual gravity and orbit around a common center. Interactions between galaxies is quite common, especially between giant and satellite galaxies. This is often the result of a galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy’s primary spiral arms.
In other cases, the path of the satellite galaxy may cause it to intersect with the giant galaxy. Collisions may lead to mergers, assuming that neither galaxy has enough momentum to keep going after the collision has taken place. If one of the colliding galaxies is much larger than the other, it will remain largely intact and retain its shape, while the smaller galaxy will be stripped apart and become part of the larger galaxy.
Such collisions are relatively common, and Andromeda is believed to have collided with at least one other galaxy in the past. Several dwarf galaxies (such as the Sagittarius Dwarf Spheroidal Galaxy) are currently colliding with the Milky Way and merging with it.
However, the word collision is a bit of a misnomer, since the extremely tenuous distribution of matter in galaxies means that actual collisions between stars or planets is extremely unlikely.
Andromeda–Milky Way Collision:
In 1929, Edwin Hubble revealed observational evidence which showed that distant galaxies were moving away from the Milky Way. This led him to create Hubble’s Law, which states that a galaxy’s distance and velocity can be determined by measuring its redshift – i.e. a phenomena where an object’s light is shifted toward the red end of the spectrum when it is moving away.
However, spectrographic measurements performed on the light coming from Andromeda showed that its light was shifted towards the blue end of the spectrum (aka. blueshift). This indicated that unlike most galaxies that have been observed since the early 20th century, Andromeda is moving towards us.
In 2012, researchers determined that a collision between the Milky Way and the Andromeda Galaxy was sure to happen, based on Hubble data that tracked the motions of Andromeda from 2002 to 2010. Based on measurements of its blueshift, it is estimated that Andromeda is approaching our galaxy at a rate of about 110 km/second (68 mi/s).
At this rate, it will likely collide with the Milky Way in around 4 billion years. These studies also suggest that M33, the Triangulum Galaxy – the third largest and brightest galaxy of the Local Group – will participate in this event as well. In all likelihood, it will end up in orbit around the Milky Way and Andromeda, then collide with the merger remnant at a later date.
In a galaxy collision, large galaxies absorb smaller galaxies entirely, tearing them apart and incorporating their stars. But when the galaxies are similar in size – like the Milky Way and Andromeda – the close encounter destroys the spiral structure entirely. The two groups of stars eventually become a giant elliptical galaxy with no discernible spiral structure.
Such interactions can also trigger a small amount of star formation. When the galaxies collide, it causes vast clouds of hydrogen to collect and become compressed, which can trigger a series of gravitational collapses. A galaxy collision also causes a galaxy to age prematurely, since much of its gas is converted into stars.
After this period of rampant star formation, galaxies run out of fuel. The youngest hottest stars detonate as supernovae, and all that’s left are the older, cooler red stars with much longer lives. This is why giant elliptical galaxies, the results of galaxy collisions, have so many old red stars and very little active star formation.
Despite the Andromeda Galaxy containing about 1 trillion stars and the Milky Way containing about 300 billion, the chance of even two stars colliding is negligible because of the huge distances between them. However, both galaxies contain central supermassive black holes, which will converge near the center of the newly-formed galaxy.
This black hole merger will cause orbital energy to be transferred to stars, which will be moved to higher orbits over the course of millions of years. When the two black holes come within a light year of one another, they will emit gravitational waves that will radiate further orbital energy, until they merge completely.
Gas taken up by the combined black hole could create a luminous quasar or an active nucleus to form at the center of the galaxy. And last, the effects of a black hole merger could also kick stars out of the larger galaxy, resulting in hypervelocity rogue stars that could even carry their planets with them.
Today, it is understood that galactic collisions are a common feature in our Universe. Astronomy now frequently simulate them on computers, which realistically simulate the physics involved – including gravitational forces, gas dissipation phenomena, star formation, and feedback.
And be sure to check out this video of the impending galactic collision, courtesy of NASA:
We have written many articles about galaxies for Universe Today. Here’s What is Galactic Cannibalism?, Watch Out! Galactic Collisions Could Snuff Out Star Formation, New Hubble Release: Dramatic Galaxy Collision, A Virtual Galactic Smash-Up!, It’s Inevitable: Milky Way, Andromeda Galaxy Heading for Collision, A Cosmic Collision: Our Best View Yet of Two Distant Galaxies Merging, and Determining the Galaxy Collision Rate.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.
In about 4 billion years, scientists estimate that the Andromeda and the Milky Way galaxies are expected to collide, based on data from the Hubble Space Telescope. And when they merge, they will give rise to a super-galaxy that some are already calling Milkomeda or Milkdromeda (I know, awful isn’t it?) While this may sound like a cataclysmic event, these sorts of galactic collisions are quite common on a cosmic timescale.
As an international group of researchers from Japan and California have found, galactic “hookups” were quite common during the early universe. Using data from the Hubble Space Telescope and the Subaru Telescope at in Mauna Kea, Hawaii, they have discovered that 1.2 billion years after the Big Bang, galactic clumps grew to become large galaxies by merging. As part of the Hubble Space Telescope (HST) “Cosmic Evolution Survey (COSMOS)”, this information could tell us a great about the formation of the early universe.
Even though the spacecraft has exhausted its supply of liquid helium coolant necessary to observe the infrared energy of the distant Universe, data collected by ESA’s Herschel space observatory are still helping unravel cosmic mysteries — such as how early elliptical galaxies grew so large so quickly, filling up with stars and then, rather suddenly, shutting down star formation altogether.
Now, using information initially gathered by Herschel and then investigating closer with several other space- and ground-based observatories, researchers have found a “missing link” in the evolution of early ellipticals: an enormous star-sparking merging of two massive galaxies, caught in the act when the Universe was but 3 billion years old.
It’s been a long-standing cosmological conundrum: how did massive galaxies form in the early Universe? Observations of distant large elliptical galaxies full of old red stars (and few bright, young ones) existing when the Universe was only a few billion years old just doesn’t line up with how such galaxies were once thought to form — namely, through the gradual accumulation of many smaller dwarf galaxies.
But such a process would take time — much longer than a few billion years. So another suggestion is that massive elliptical galaxies could have been formed by the collision and merging of large galaxies, each full of gas, dust, and new stars… and that the merger would spark a frenzied formation of even more stars.
Investigation of a bright region first found by Herschel, named HXMM01, has identified such a merger of two galaxies, 11 billion light-years distant.
The enormous galaxies are linked by a bridge of gas and each has a stellar mass of about 100 billion Suns — and they are spawning new stars at the incredible rate of about 2,000 a year.
“We’re looking at a younger phase in the life of these galaxies — an adolescent burst of activity that won’t last very long,” said Hai Fu of the University of California at Irvine, lead author of a new study describing the results.
Hidden behind vast clouds of cosmic dust, it took the heat-seeking eyes of Herschel to even spot HXMM01.
“These merging galaxies are bursting with new stars and completely hidden by dust,” said co-author Asantha Cooray, also of the University of California at Irvine. “Without Herschel’s far-infrared detectors, we wouldn’t have been able to see through the dust to the action taking place behind.”
Herschel first spotted the colliding duo in images taken with longer-wavelength infrared light, as shown in the image above on the left side. Follow-up observations from many other telescopes helped determine the extreme degree of star-formation taking place in the merger, as well as its incredible mass.
The image at right shows a close-up view, with the merging galaxies circled. The red data are from the Smithsonian Astrophysical Observatory’s Submillimeter Array atop Mauna Kea, Hawaii, and show dust-enshrouded regions of star formation. The green data, taken by the National Radio Astronomy Observatory’s Very Large Array, near Socorro, N.M., show carbon monoxide gas in the galaxies. In addition, the blue shows starlight.
Although the galaxies in HXMM01 are producing thousands more new stars each year than our own Milky Way does, such a high star-formation rate is not sustainable. The gas reservoir contained in the system will be quickly exhausted, quenching further star formation and leading to an aging population of low-mass, cool, red stars — effectively “switching off” star formation, like what’s been witnessed in other early ellipticals.
Dr. Fu and his team estimate that it will take about 200 million years to convert all the gas into stars, with the merging process completed within a billion years. The final product will be a massive red and dead elliptical galaxy of about 400 billion solar masses.
The study is published in the May 22 online issue of Nature.
Main image credit: ESA/NASA/JPL-Caltech/UC Irvine/STScI/Keck/NRAO/SAO