Galaxy collisions can be described as the cosmic equivalents of train wrecks, often changing the shapes and morphologies of the galaxies that are involved in these violent cosmic events, in spectacular fashion. Yet, contrary to train accidents which often result in significant destruction of rail transportation systems and their associated infrastructure, the stars inside colliding galaxies remain largely unaffected as the latter interact under the force of gravity, often causing them to pass right through each other or even merge, creating new and fascinating galaxy formations in the process. A new study by an international team of astronomers, which was recently published in the online edition of the Astrophysical Journal, adds an important dimension to the study of these cosmic smash-ups by showing that the latter can greatly inhibit star formation in the galaxies involved.
Similar to the study of stellar objects, astronomers can learn more about galactic evolution by observing galaxies of different ages in order to see how these change over time. To that end, scientists have conducted many dozens of deep-field redshift surveys during the last several decades, which have allowed them to measure the distances to millions of galaxies out to the very limits of the observable Universe. Due to the finite nature of the speed of light, it takes a specific amount of time in order for the light emitted by a distant cosmic object to reach observers here on Earth. The farther away an object is the longer it takes for its light to cross the intervening distance, essentially making the night sky the perfect time machine for looking into the Universe’s past: The deeper we probe into the Cosmos, the further we look back in time. This way, by observing galaxies that are located at varying distances from Earth, astronomers can infer the various stages of their evolution throughout cosmic time.
In recent years it has become increasingly obvious that collisions between galaxies have played an important role in determining their shape and structure. The study of these violent cosmic phenomena have proved to be of great value to astronomers, allowing them to gain important insights about the evolution of galaxies and galaxy clusters in the Universe. The Milky Way galaxy is also believed to have grown to its present size by consuming smaller, satellite dwarf galaxies in collisional events throughout its life, while it has been predicted that it will continue to do so, eventually merging with the neighboring Andromeda galaxy in the next few billion years. With the help of highly sophisticated computer simulations, astronomers can watch the dynamical interactions of such collisions—that would normally take millions or even billions of years to play out—unfold in just a matter of minutes. Despite the many theoretical and observational breakthroughs that have occurred in this area, however, the exact mechanisms by which these mergers, as well as the overall evolution of galaxies, take place remain largely unknown, making these some of the most actively researched topics in cosmology and astrophysics today.
By analyzing archival data from NASA’s Hubble, Spitzer, and the European Space Agency’s Herschel Space Observatory, a research team led by Philip Appleton, project scientist for the NASA Herschel Science Center at the California Institute of Technology, studied the properties of the active dwarf elliptical galaxy NGC 3226, which is located 50 million light-years away in the direction of constellation Leo and is caught in a gravitational dance with the neighboring spiral galaxy NGC 3227, forming a pair of interacting galaxies also known as Arp 94. NGC 3226 in particular is thought to be a representative member of the so-called “green valley” galaxies, as defined in astronomers’ galaxy color–magnitude diagram, which categorizes them according to the age and color of their stellar population. Based on this classification, older galaxies where very little star formation is taking place populate the so-called “red-sequence” in the diagram, whereas younger ones which undergo an active star formation populate the “blue cloud,” with the “green valley” representing a transitional, intermediate population of galaxies between the two aforementioned main color classes where star formation is in the process of either gearing up or slowing down.
In their study, Appleton’s team reports on the existence of a web of filaments emanating from NGC 3226 with several of them surrounding the neighboring NGC 3227 as well. One of the more pronounced of these structures is a large, 100,000 light-year-wide narrow filament of neutral hydrogen which ends as a curved plume inside a disk of warm hydrogen gas and dusty ring in the central regions of NGC 3226. In addition, the researchers discovered that the part of this filament that lies closer to the center of NGC 3226 also consisted of several blobs of partially ionized hydrogen which are indicative of past star formation activity. Even though Arp 94 was already known from previously ground-based observations to consist of a pair of interacting galaxies that exhibited large-scale, tidally generated structures, the new observations helped to reveal that the latter were more complex than previously thought, indicating that the observed filaments are most probably the remains of a third galaxy that was strewn around and consumed by NGC 3226 sometime in the past, while its debris are currently falling back into NGC 3226 itself. “This suggests a rich dynamical history for the system,” write the researchers in the study. “NGC 3226 shows several huge loops and a narrow optical filament extending from the galaxy to the north-east at a position-angle of approximately 30 degrees. The visible-light imagery presents a complexity that is hard to reconcile with a single tidal interaction between NGC 3227 and NGC 3226. Rather, the structures around NGC 3226 imply that this galaxy is itself the remnant of a recent merger which has launched stellar debris into the joint potential of what was probably a system of at least three constituent galaxies.”
Besides the fascinating complexity of the tidally-generated structures of NGC 3226, additional photometric observations in multiple wavelengths from the ultraviolet to the mid-infrared showed that the galaxy has exhibited very low star formation rates, in the order of approximately 0.04 solar masses per year over the last 100 million years. Furthermore, by analyzing the high-resolution spectrum of NGC 3226 taken in the mid-infrared by the Spitzer space telescope, Appleton’s team discovered that the hydrogen gas in the center of NGC 3226 was much hotter than that typically found inside galaxies. Hydrogen gas within galaxies has a temperature that typically ranges between 10 and 20 K (just a few degrees above absolute zero). This extremely cold environment is highly conducive to stellar formation, allowing atoms to bind together in order to create the molecular clouds that will later collapse to form stars. In contrast, the molecular hydrogen in NGC 3226 was found to be significantly hotter, with temperatures ranging between 144 and 173 K. After examining various possible causes in order to account for the observed heating of the galaxy’s molecular hydrogen, the researchers concluded that it was most likely a result of the tidal accretion of material from the newly discovered narrow filament, as the former falls at supersonic speeds onto the core of NGC 3226. “We can rule out heating of the warm H2 gas disk by either X-rays from the [galaxy’s] low-luminosity active galactic nucleus, or dominant Photo-Dissociation Regions associated with star formation,” write the researchers. “Instead, we suggest that the H2 line luminosity can be explained by shock heating. We favor partially ionized hydrogen gas accretion as the source of the mechanical heating. An accretion rate of 1 solar mass per year would be needed (assuming a 10% efficiency) to balance the observed warm H2 line luminosity, which we show is plausible.”
Instead of feeding material into the supermassive black hole, which is thought to lie in its center as well as resulting in a new round of star formation, the accretion of infalling hot gas in NGC 3226 has the opposite effect, which according to the researchers can also help explain the galaxy’s overall diminished stellar formation rates. “We are discovering that gas does not simply funnel down into the center of a galaxy and feed the supermassive black hole known to be lurking there,” says Appleton. “Rather, it gets hung up in a warm disk, shutting down star formation and probably frustrating the black hole’s growth by being too turbulent at this point in time.” Nevertheless, this state of affairs seems to be temporary, with the galaxy’s star formation rates expected to return to their normal levels in the future as the infalling hot hydrogen eventually cools down. When that happens, NGC 3226 will go back to being a “blue cloud,” active star-forming galaxy again, according to theoretical predictions. “NGC 3226 will continue to evolve and may hatch abundant new stars in the future,” adds Appleton. “We’re learning that the transition from young- to old-looking galaxies is not a one-way, but a two-way street.”
In the meantime, this new study underscores the value of combined space-based observations in deciphering some of the most intriguing astrophysical mysteries relative to the merger of galaxies and the many different ways by which it can influence their evolution. “We have explored the fantastic potential of big data archives from NASA’s Hubble, Spitzer and ESA’s Herschel observatory to pull together a picture of an elliptical galaxy that has undergone huge changes in its recent past due to violent collisions with its neighbors,” says Appleton.
Indeed, these observations are an important reminder that rather than being “static,” “serene,” and unchanging—as has been envisioned for millenia—the Universe is a very lively, dynamic, and ever-evolving place full of exquisite mysteries.