Detecting the Star Forming Richness of the Fornax Cluster

It’s often hard to find galaxies when looking out into the night sky with a telescope – because they are so distant, and their mostly diffuse shapes and structures are too faint for the human eye to see them directly. There are some exceptions – for example, the southern Magellanic Clouds, or some nearby galaxies like Andromeda, which can certainly be seen with the naked eye under the right sky conditions - but these are (astronomically speaking) relatively close, in the local neighbourhood.

For us southern hemisphere dwellers, there is however an array of galaxies located in the constellation of Fornax, and with a telescope and CCD imager – a number of these galaxies come to light for even the backyard city-dwelling astrophotographer.

The reason for this is that the Fornax Cluster, located at a distance of about 62 million light-years away, is rich with about 60 visually-identifiable galaxies that come in all shapes and sizes – from barred spirals to giant ellipticals, to merging lenticulars and even irregulars. From our point of view, some of these are face-on in their appearance, others are edge-on, and the rest remain somewhere in between, tilted at an angle.

Due to its relatively close proximity to Earth (as far as galaxy clusters go), along with its rich availability of a variety of galaxies, astrophysicists have favoured this southern jewel in the sky as it provides an excellent laboratory to study the dynamics and evolutions of interacting and merging galaxies.

In particular, radio astronomers have favoured looking upon the Fornax Cluster to study it in longer wavelengths, because one of the brightest radio sources in the sky resides there (the Fornax A Galaxy), as well as the opportunity to study the interacting gases between the galaxies of the cluster.

Now, a new paper (released on the preprint server, arXiv.org) has used the Australian Telescope Compact Array (ATCA) – a collection of six radio telescopes in Narrabri (NSW), which is owned and operated by Australia’s national science agency, CSIRO, to study the dynamics of neutral hydrogen within the cluster.

The international collaboration of scientists who worked on the paper (which was released in February) also includes researchers from several Australian institutes, such as the International Centre of Radio Astronomy Research (ICRAR), the ARC Centre of Excellence for All-Sky Astrophysics in 3D (ASTRO3D), and the CSIRO.

The Fornax Galaxy cluster lies approximately 19 – 20 megaparsecs (Mpc – a distance unit of measurement often used by astronomers when discussing the space between galaxy clusters, where 1 Mpc equates to approximately 3.26 million light-years). Running a quick conversion places the Fornax Cluster about 60 - 65 million light-years away.

The cluster contains about 200 spectroscopically confirmed galaxies, which are centred around two main sub-clusters with the main branch associated around the galaxy known as NGC 1399, and the other associated with another galaxy known as NGC 1316. Another notable galaxy is NGC 1365.

NGC 1316 (also known as Fornax A) is a particularly interesting galaxy, as it is extremely bright in radio wavelengths – being the fourth brightest source in the sky at 1400 MHz. Classed as a Lenticular Galaxy, it features an elliptical overarching structure with scattered dust lanes throughout it.

Scientists now understand that this galaxy is undergoing mergers with other, smaller galaxies that it is consuming, which feeds its supermassive black hole at its heart – an object that measures in at 130 – 150 million solar masses (for comparison, the supermassive black hole at the centre of the Milky Way is roughly 4.3 million solar masses only). This continual consumption of gas is the source of brightness in radio wavelengths.

The main centre of the Fornax Cluster is around NGC 1399 – a large elliptical galaxy, which is roughly 130,000 light-years in diameter. Within this galaxy, an even larger supermassive black hole exists – weighing in at approximately 510 million solar masses.

Interestingly, NGC 1399 is also highly populated with both globular clusters (there’s about 6,000 of them) which astronomers have claimed were ripped away from a nearby galaxy (NGC 1404) through gravitational interactions, as well as 37 planetary nebulae sprinkled across the structure.

One of the most impressive galaxies that amateurs and scientists can observe is the beautiful double-barred spiral structure known as NGC 1365 (the ‘Great Barred Spiral'). The distinct features of this galaxy include its two outstretched long spiral arms that stretch 200,000 light-years in diameter and are almost aligned perpendicular to its central bar structure which crosses over the core region.

Dark dust lanes circle and cross over the central structure of the galaxy, wrapping and creating a whirlpool-like pattern as they lead into the galactic core.

Using X-ray observations from the NuSTAR satellite telescope, astrophysicists measured the mass of the supermassive black hole within NGC 1365 to be approximately two million solar masses, and the rotational velocity of its outer edge to be close to the speed of light (rotational velocities within its radius vary and do not reach these same velocities). The extraordinary results showcase the extreme environments that supermassive black holes present, defying our everyday logic, with the ability to spin at such high velocities.

The cluster also features other smaller, and dimmer galaxies, including several that are continually interacting with each other – as they gravitationally tumble and collide over deep time through mergers.

And with all this activity, comes an important aspect of learning about galaxy evolution – the ability to form stars and continue to regenerate new materials (known as metals) to seed the next population of stars, and so on.

The majority of baryonic matter that makes up the Universe is composed of the lightest element, hydrogen. Most of the hydrogen (along with helium) that we see around us in the Universe was formed as part of the Big Bang (in a process known as Big Bang Nucleosynthesis).

Hydrogen is the main component of what stars are formed from, and as stars live out their lives, they burn and fuse hydrogen into heavier elements like helium, carbon, oxygen, and nitrogen (the metals). Then, when stars explode, these new elements form part of the chemical signature that makes up future generations of stars as they are born (this is also the process in which planets, moons, humans, and more are made).

But hydrogen is the key element, and for stars to form, the conditions need to be just right.

The first thing that needs to be present is large, vast clouds of cold hydrogen gas. The reason it has to be cold is that hot gas is energetic, and the more energy the gas particles have, the less they are going to want to clump together.

But cold gas, along with dust, can form clumps of bigger gas, which then attract even more gas particles, to form bigger clumps and the process continues until enough gas pools together to generate pressures and temperatures in which nuclear fusion can kick start. That’s how stars are born.

Within spiral galaxies, there are plenty of these large gas and dust reserves, which is known as the interstellar medium – mostly consisting of hydrogen and helium, with traces of everything else (which were produced by older stars which have since detonated in supernovae and spread their metals).

As galaxies age, they start to use up these gas reserves and so they start to lose their ability to form stars, and only the older, more low-mass stars remain behind (these tend to be redder in colour as well).

Another way that galaxies can lose their star-forming gas reserves is through mergers with other galaxies – a process that breaks down spiral structures into bigger, elliptical halo-like galaxies over the course of billions of years.

A key way to study the cold hydrogen gas reserves in the Milky Way, or even out at galaxy clusters like the Fornax Cluster is to detect the cold, neutral hydrogen (known as ‘HI’) signature using radio telescopes.

The process in which neutral hydrogen generates a photon of radio wavelengths comes down to a very basic hydrogen atom, which contains a single proton and electron. Normally, in what is known as the ‘rest state’, the electron’s spin axis is pointing in the opposite direction from the proton’s spin axis (think of it as two north poles, pointing in opposite directions).

Everyone now and then, the electron gets a jolt from a photon which turns the spin axis so both north poles are pointing in the same direction – this is known as the ‘excited state’. Eventually, the electron will jettison the photon energy it absorbed earlier, as a radio wave of 21cm wavelength, and return back to its rest state.

By observing these photons at 21cm wavelengths, astrophysicists can therefore map out the large gas reserves of HI across galaxies, and galaxy clusters and determine how the interactions between galaxies either strips, or tidally disrupt these reserves, and as such, provide modelling to the historical evolution of galaxies involved.

Prior to this recent paper, the Fornax Cluster has been studied extensively with the CSIRO Parkes radio telescope (known as Murriyang) – a single 64m dish radio telescope, which is equipped to observe the 21cm wavelength in detail.

These studies, ranging from 1996 through to 2005 analysed the HI around the Fornax Cluster, finding several galaxies presenting with the neutral hydrogen signature and telling a story about which galaxies were interacting with each other. In the more recent studies, scientists confirmed that the central region of the Fornax Cluster was being depleted of HI through these interactions and mergers.

To achieve a higher resolution of what was happening in the cluster, in 2005, the CSIRO ATCA was used to study 28 individual galaxies within the Fornax Cluster to determine both HI distribution and dynamics between the interacting structures.

Now, in this most recent paper, the ATCA was once again utilised by the science teams (in the 750B configuration where the largest baseline between any two antennas is 4.5 km) for a period between December 2013 through to January 2014, and total observation of 336 hours. The data was collected across a bandwidth of 64 MHz, which was centred on 1396 MHz over the observation period.

The results from this study indicate that HI has been detected in 16 galaxies within the Fornax Cluster, including three new galaxies where the gas was detected. As part of these detections, and partly due to the improved resolution of ATCA antennas since the last surveys, some of these HI regions were found to be offset from the optical centres of galaxies, showing asymmetries and even HI tails. These non-symmetrical features could be due to the environmental interactions of the cluster’s member galaxies.

By studying which of these galaxies were deficient in HI vs. those which were not, and this relation to surrounding galaxies, the team will be able to analyse these environmental interactions before a large portion of the important, cold, neutral hydrogen gas is stripped away.

From this, astronomers were also able to determine that the galaxies in the Fornax Cluster that were observed in HI, had a much lower star formation rate when compared to non-cluster galaxies at a fixed stellar mass as well.

What this study contributes towards is further knowledge into how large clusters of galaxies interact with each other, merging and colliding in events that trigger start formation or throw huge amounts of materials towards feeding, central supermassive black holes.

When we observe the large elliptical galaxies, which appear redder in colour due to their lack of new stars forming and older stars ageing, we are looking at the end process of billions of years of merger events between smaller galaxies, in which precious star-forming cold hydrogen gas, HI, as been stripped, removed, utilised, and converted. Without this sufficient material, and with further mergers, galaxies start to age and grow in size.

Eventually, our own Galaxy – the Milky Way – will enter this stage of evolution when it merges with the Andromeda Galaxy. As both galaxies collide, they will trigger off a burst of star formation that will use up a lot of the free HI in both galaxies, and eventually, settle down into a larger, elliptical galaxy in the distant future.

A similar scenario will play out at the Fornax Cluster – one that we are already seeing through visual observations, and studies of interacting HI, as outlined in this recent paper.

Australia’s prime location in the southern hemisphere gives us an advantage, combined with our excellent radio astronomy infrastructure (such as ATCA, ASKAP, MWA, Parkes telescope and within the decade, the Square Kilometre Array) to help start to resolve the questions of galactic evolution and when galaxies stop giving birth to stars.

And it’s because of these types of studies that scientists are able to work towards resolving some of the biggest questions in astronomy and cosmology – such as how galaxies start to form, defining the large-scale structure of the Universe, and what types of galaxies would be best to support life as we know it.

We acknowledge Kamilaroi people as the traditional owners of the land where ATCA resides, and the Wajarri Yamatji people as the traditional owners of the Murchison Radio-astronomy Observatory site, where ASKAP resides.

Galaxy Merger video credit: Hubble Space Telescope/NASA.