A brief description of galactic clusters and their detection


The oldest galaxies are observed today as elliptical, and to be found in clusters. These clusters are the remnants of older protoclusters that dominated the landscape of outer space in the universe’s early years, years that witnessed the formation of the first stars and, subsequently, the first galaxies. In  regions of space where the population density of stars is low and the closest cluster farther away from the closest star, more recently formed galaxies may be found. These relatively emptier regions are called ‘general fields’.

A galaxy takes hundreds of millions of years to form fully, and involves processes quite complex; imagine, the simplest among them are nuclear transmutations! At the same time, the phenomenology of the entire sequence – the first steps taken, the interaction of matter and radiation at large scales, the influence of the rest of the universe on the galaxy’s formation itself – can be understood by peering into history through some of Earth’s most powerful telescopes. The farther through their lenses we look, the deeper into the universe’s history we are gazing. And so, looking hard enough, we may observe a protocluster in its formative years, and glean from the spectacle the various forces at play!

That is what two astronomers and their team from the National Astronomical Observatory of Japan (NAOJ) have done. Using the Multi-object Infrared Camera and Spectrograph (MOIRCS – mounted on the Subaru Telescope), Drs. Masao Hayashi and Tadayuki Kodama identified a highly dense and active protocluster 11 billion light-years from Earth (announced September 20, 2012). In other words, the cluster they are looking at exists 11 billion years in our past, at a time when the universe was only 2.75 billion years old! Needless to say, it makes for an excellent laboratory, one that need only be carefully observed  to answer our burning questions.

USS1558-003 (The horizontal and vertical axes show relative distances in right ascensions and declinations in arcminute units with respect to the radio galaxy. North is up, and east is to the left. The black dots are all galaxies selected in this field. Magenta dots show old, passively evolving galaxies. Blue squares represent star-forming galaxies with H-alpha emission lines, while red ones show red-burning galaxies. Large gray circles show the three clumps of galaxies.)

The first among them is why galaxies “choose” to cluster themselves. The protocluster, as usual named inelegantly as USS1558-003, actually consists of three large closely-spaced clusters of galaxies, with an astral density as much as 15 times that of the general fields in the same cosmic period and a star-formation rate equivalent to a whopping 10,000 Suns/year. These numbers effectively leave such clusters peerless in their formative libido, as well as naked in the eyes of infrared telescopes such as the MOIRCS, without with such bristling cosmic laboratories could not have been found.

Because of the higher star-formation rate, a lot of energy is traded between different bits of matter. However, there is an evident problem of plenty: what do the telescopes look for? Surely, they must somehow be able to measure the amount of energy riding on each exchange. However, the frequency of the associated radiation is not confined to any one bracket of the electromagnetic spectrum – even if only thermal or visible radiation is being tracked. What exactly do the telescopes look for, then?

Leave alone the quintillions of kilometers; the answer to this question lies in the angstroms, within the confines of hydrogen atoms. Have a look at the image below.

The galaxies marked by green circles are emitting radiation with a wavelength of 656 nm, also called H-alpha radiation. It falls within what is called the Balmer series of hydrogen’s emission spectrum, named for Johann Balmer, who discovered the formula for the eponymous series of emission-frequencies in 1885. The presence of an H-alpha line in the emitted radiation is an unmistakable sign of the presence of hydrogen: Radiation is emitted at precisely 656 nm (in the form of a photon of that wavelength) when excited electrons in the hydrogen atom drop from the third to the second energy levels.

The Balmer series, and the H-α line, are important tools used in astronomical spectroscopy: It is not just that they indicate the presence of hydrogen, but where and in what quantities, too. Those values throw light on which stages of formation the stars of that galaxy are in, the influence of the reactive region’s neighborhood, and where and how star-formation is initiated. In fact, by detecting and measuring the said properties of hydrogen, Kodama et al have already found that, in a cluster, star-formation begins at the core – just where the density of extant stars is already high – and gradually spreads outward to its periphery. In the present-day, this finding contrasts with the shape and structure of elliptical galaxies, which have a different mass distribution from what an in-to-out star-formation pattern suggests.

These are only the early stages of Kodama’s and his team’s research. As they sum it up,

We are now at the stage when we are using various new instruments to show in detail the internal structures of galaxies in formation so that we can identify the physical mechanisms that control and determine the properties of galaxies.

Not that it needs utterance, but: This is important research. Astronomy and astrophysics are costly affairs when the genre is experimental and the scale quite big, rendering finds such as that of USS1558-003 very providential as well as insightful. We may have just spotted the Higgs boson, we may just have begun on a long journey to find the smallest thing in the universe. However, as it stands, of the largest things in the universe we have very little to say, too.