This image of dwarf galaxy NGC in Canes Venatici reveals a dense stream of stars in its halo outer regions. Astronomers point to this as evidence of NGC merging with an even smaller dwarf galaxy. Researchers have captured many mergers involving massive galaxies, but this is one of the first images to show two dwarfs merging.
Putman is part of a group investigating what happens with small galaxy mergers. What are their properties? How do they differ from larger galaxies? Recent research has revealed a bridge of stars between them that formed as a result of their interaction.
Mergers and acquisitions So what happens inside a bigger merger? This color composite image shows the Hubble Ultra Deep Field. Red circles show the locations of even higher redshift candidates. About 20 to 30 percent of these high-z galaxy candidates are close to foreground galaxies, consistent with the prediction that a significant fraction of galaxies at high redshifts are gravitationally lensed by individual foreground galaxies.
Past encounters Putman says the Andromeda Galaxy M31 may have already had one merger in the past. The galaxy has two supermassive black holes SMBHs at its center, something that would be unusual if not for mergers. But having two supermassive black holes merge?
But standing between us and a SMBH merger is the Final Parsec Problem, that physical barrier that holds two supermassive black holes from merging, instead locking into an orbit lasting billions upon billions of years.
But every large galaxy has come together through a merger — and other, bigger mergers may be just over the horizon. Editor's note: The quote from Salvatore Vitale has been adjusted for clarification. An earlier version of the quote did not specify the two black holes involved were supermassive.
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The South-East side of the galaxy shows a rotation curve that is typical for spiral galaxies. It increases out the galactic center, and then levels off around -3 kpc. We expect the rotation curve of a spiral galaxy to level off because of the presence of dark matter. The North-West side, however, looks very different. It increases quickly, levels off for a little, and then begins to increase again.
These high velocities lie in the same region as star-forming H II regions detected through the imaging. This asymmetry in the rotation curve suggests that the motion of the gas has already been disturbed by the interaction with the other galaxy. This type of behavior has been observed in other galaxy minor mergers similar to this one.
Figure 3: Rotation curve for the more massive galaxy, AMA. On the right is shown the location for each of the data points in the plot on the left, in the boxes. The rotation curve on the left shows the asymmetry caused by the interaction with the other galaxy. Having both the surface brightness profiles and the rotation curves allowed the authors to determine the dark matter distribution in this system of galaxies. The authors estimate there is 10 times as much dark matter mass as there is mass from stars and gas.
This significantly affects how the two galaxies interact. In turn, these data allows models of galactic collisions to be tested against actual data.
Next, the authors plan to model this interaction numerically, using the results of this paper to inform that work. Email Address. Suggest a Paper Topic! Hernandez-Jimenez, M. Pastoriza, I. Rodrigues, A. One of the conclusions astronomers have reached from studying distant galaxies is that collisions and mergers of whole galaxies play a crucial role in determining how galaxies acquired the shapes and sizes we see today.
Only a few of the nearby galaxies are currently involved in collisions, but detailed studies of those tell us what to look for when we seek evidence of mergers in very distant and very faint galaxies.
These in turn give us important clues about the different evolutionary paths galaxies have taken over cosmic time. Figure 1 of this chapter introduction shows a dynamic view of two galaxies that are colliding. The stars themselves in this pair of galaxies will not be affected much by this cataclysmic event.
Since there is a lot of space between the stars, a direct collision between two stars is very unlikely. However, the orbits of many of the stars will be changed as the two galaxies move through each other, and the change in orbits can totally alter the appearance of the interacting galaxies.
A gallery of interesting colliding galaxies is shown in Figure 1. Great rings, huge tendrils of stars and gas, and other complex structures can form in such cosmic collisions. Indeed, these strange shapes are the signposts that astronomers use to identify colliding galaxies.
Figure 1: Gallery of Interacting Galaxies. The hydrogen image shows that the two galaxies are wrapped in a common shroud of gas that is being tugged and stretched by the gravity of the two galaxies. The two appear to have already passed through each other once, causing a shockwave that reformed one into a bright blue ring of star formation, like the ripples from a stone tossed into a pond.
Ford JHU , G. Throughout this book we have emphasized the large distances between objects in space. You might therefore have been surprised to hear about collisions between galaxies. Yet except at the very cores of galaxies we have not worried at all about stars inside a galaxy colliding with each other. The reason is that stars are pitifully small compared to the distances between them. The Sun is about 1. In other words, the Sun is 27 million of its own diameters from its nearest neighbor.
This is typical of stars that are not in the nuclear bulge of a galaxy or inside star clusters. The visible disk of the Milky Way is about , light-years in diameter. We have three satellite galaxies that are just one or two Milky Way diameters away from us and will probably someday collide with us. The closest major spiral is the Andromeda Galaxy M31 , about 2. If the Milky Way were a pancake at one end of a big breakfast table, M31 would be another pancake at the other end of the same table.
Galaxies in rich clusters are even closer together than those in our neighborhood see The Distribution of Galaxies in Space. Thus, the chances of galaxies colliding are far greater than the chances of stars in the disk of a galaxy colliding.
And we should note that the difference between the separation of galaxies and stars also means that when galaxies do collide, their stars almost always pass right by each other like smoke passing through a screen door. The details of galaxy collisions are complex, and the process can take hundreds of millions of years. Thus, collisions are best simulated on a computer Figure 2 , where astronomers can calculate the slow interactions of stars, and clouds of gas and dust, via gravity.
These calculations show that if the collision is slow, the colliding galaxies may coalesce to form a single galaxy. Figure 2: Computer Simulation of a Galaxy Collision. This computer simulation starts with two spiral galaxies merging and ends with a single elliptical galaxy.
The colors show the colors of stars in the system; note the bursts of blue color as copious star formation gets triggered by the interaction.
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