One of the essential aspects of astronomy is that of classification. Whatever their type, celestial objects are mainly categorised according to their basic properties like their size, mass, and brightness. For instance, on the realm of planetary bodies there are objects as small as Ceres and Pluto in our own Solar System as well as exoplanets with two times the size of Jupiter that have been found in orbit around other stars. The latter also exhibit a wide range of masses and sizes from 1/10 to more than a thousand times that of the Sun. When it comes to the specimens of the galactic zoo, these fall under three different categories: spiral, elliptical and irregular galaxies which similarly exhibit a wide range of sizes, with the biggest ones that had been found to date spanning more than 200,000 light-years across—twice that of our own galaxy, the Milky Way. Recently, astronomers were able to shatter this long-held record by announcing the unexpected discovery of a new population of galactic beasts, consisting of gigantic spiral galaxies that are up to four times larger than the Milky Way. In addition to their ‘wow’ factor, these ‘super spirals’ represent a challenge for astronomers in their efforts to determine how, contrary to theoretical models, these monstrous stellar cities can grow to such enormous sizes.
One of the major open questions in cosmology and astrophysics today, is the formation and evolution of galaxies. Despite the great strides that have taken place in the study of galaxies and galactic clusters during the last couple of decades with the help of ground and space-based observatories, the exact physical processes with which the first structures in the primordial Universe formed and later evolved in order to create the large diversity of galaxies that is observable today, remains an actively debated topic among astronomers. According to the standard model of cosmology, minuscule, random density fluctuations in the otherwise uniform distribution of matter in the early Universe were the seeds that gave rise to the great variety of structure that exists on a cosmic scale today. Yet, the specific mechanisms that have driven this process are currently unknown. One hypothesis, known as the ‘top-down scenario’, posits that the hydrogen gas that dominated the early Universe had coalesced to form gigantic gas clouds which eventually collapsed and fragmented under their own gravity into smaller clumps of matter – the protogalaxies. With the passage of cosmic time these primordial, baby galaxies grew bigger, eventually giving rise to the large, fully grown galaxies of all shapes and sizes that we see today. On the other hand, a competing hypothesis which has gained much traction in recent years, known as the ‘bottom-up scenario’, postulates that the first galaxies formed directly from the primordial density fluctuations and through the process of colliding and merging with one another they led to the development of the large-scale structures that characterise the Universe today.
Whatever the exact path was that the Universe chose for the evolution of its constituent stars and galaxies, theoretical predictions have shown that galaxies could only grow to a certain extent, before eventually depleting their gas and dust through star formation. More specifically spiral galaxies, which are named for the spiral arms that extend outward from their center and are rich in young, newly formed blue-white stars as well as huge reservoirs of cold gas and dust, are the places in the Universe where active star formation takes place. Depending on the amount of gas and dust that is available in their spiral arms, spiral galaxies have star-formation rates which range from 1-3 solar masses per year in the case of the Milky Way, to 10 times higher in the case of the so-called ‘starburst’ galaxies like M82. Yet, spiral galaxies can’t hold these rates forever. Eventually their gas and dust runs out, at which point it has been generally hypothesised that their spiral arms slowly fade and star formation comes to a stop. It is thought that this shutting-off of star-formation in spiral galaxies also marks the point where the latter transition towards ellipticals, which are characterised by the presence of aging red stars and are devoid of any gas and dust.
Star formation inside a galaxy can be halted by a host of other processes as well if their reservoirs of cold gas and dust for instance gets expelled from the spiral arms as a result of galaxy collisions, or if it is abruptly heated and compressed by supernovae explosions or from ram-pressure stripping, which results from the galaxy’s movement through the intergalactic medium inside galaxy clusters. These gas-stripping mechanisms are also responsible for limiting the growth of spiral galaxies, leaving galaxy mergers as the dominant process of galaxy evolution with the passage of cosmic time.
Wanting to shed more light to the processes that drive galactic evolution, a research team led by Patrick Ogle, a professor of astrophysics at the California Institute of Technology, went through the archival data of the NASA/IPAC Extragalactic Database, or NED for short, with the goal of identifying the most massive and luminous galaxies that are located more than 1 billion light-years away. Funded and operated by NASA and Caltech’s Infrared Processing and Analysis Center, or IPAC, respectively, NED is a large online database which consists of multi-wavelength data for more than 200 million extragalactic objects that have been gathered by a series of different projects and space missions, like NASA’s IRAS and GALEX satellites as well as the Sloan Digital Sky Survey and the 2-Micron All-Sky Survey among others.
By far, some of the most massive and luminous galaxies known to date are ellipticals. Even though they are composed of old red stars and have long stopped forming new ones, elliptical galaxies can nevertheless grow to supergiant status, like the famous M87 in the constellation Virgo which has a diameter of approximately one million light-years—10 times greater than that of the Milky Way. Ogle and colleagues expected to only find similar supergiants like M87 in a sample of nearly 800,000 galaxies that they extracted from the NED database for their research. Yet, to their surprise, their search revealed a total of 53 spiral galaxies with sizes and luminosities that by far exceeded that of spirals like our own. Dubbed ‘super spirals’ by the researchers, these newly found behemoths which lie between 1.2 and 3.5 billion light-years away, were found to be eight and fourteen times brighter than the Milky Way while also exhibiting a star formation rate that was up to 30 times higher. Most impressively, the diameter of the biggest one among them is a whopping 440,000 light-years across, making it by far the biggest known spiral galaxy to date. “We have found a previously unrecognized class of spiral galaxies that are as luminous and massive as the biggest, brightest [elliptical] galaxies we know of,” says Ogle. “It’s as if we have just discovered a new land animal stomping around that is the size of an elephant but had shockingly gone unnoticed by zoologists.”
One of the things that stands out regarding these newly found super spiral galaxies, is the strange fact that four of them appear to have two galactic nuclei, making them look like two egg yolks in a frying pan, similar to what has also been observed in recent years in a handful of much smaller spirals. Could this be evidence of an ongoing galaxy merger? If so, that would run counter to the view galactic mergers as cosmic train wrecks where participating galaxies are generally severely deformed, often resulting to spiral galaxies being stripped off their spiral arms and settling into their new life as ellipticals. Nevertheless, according to Ogle’s team, this new population of super spirals may be the missing link between the massive members of both galaxy types. “Super spirals display a range of morphologies, from flocculent to grand-design spiral patterns”, write the researchers in their study which was published at The Astrophysical Journal. “At least 9 super spirals have prominent stellar bars visible in the SDSS images. There are morphological peculiarities in several cases, including one-arm spirals, multi-arm spirals, rings, and asymmetric spiral structure. These types of features may indicate past or ongoing galaxy mergers or collisions … We suggest that super spirals may be the progenitors of red and dead lenticular galaxies of similar mass”.
Whatever the case may be, the finding of just 53 such galaxies out of a total of 800,000 suggests that super spirals are extremely rare specimens in the wider population of the galactic zoo. Nevertheless, theoretical models of galactic evolution will need to be revised in order to account for the presence of even such a small sample, since it was thought that spiral galaxies in general could not grow to such sizes. Furthermore, the discovery of spiral galaxies with two apparently distinct cores could shed more light to the dynamics of galactic mergers, which remains a dominant process in the Universe today. “Super spirals could fundamentally change our understanding of the formation and evolution of the most massive galaxies,” says Ogle. “We have much to learn from these newly identified, galactic leviathans.”
In this regard, the discovery by Ogle’s team is just the start. With more than 200 million extragalactic objects listed in its database, NED is a treasure trove of data for astronomers to analyse further, that could be full of more such fascinating surprises in the future. “Remarkably, the finding of super spiral galaxies came out of purely analyzing the contents of the NASA/IPAC Extragalactic Database, thus reaping the benefits of the careful, systematic merging of data from many sources on the same galaxies,” says George Helou, Executive Director for IPAC and member of Ogle’s team. “NED is surely holding many more such nuggets of information, and it is up to us scientists to ask the right questions to bring them out.”
If such exciting discoveries are the product of past astronomical surveys and space missions, one can’t help but be excited for what the future may bring, when the next-generation of ground- and space-based observatories will come online within the next five years.