Old but Gold: A Huge Primordial Protocluster

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Title: An Extreme Protocluster of Luminous Dusty Starbursts in the Early Universe
Authors: I. Oteo et al.
First Author’s Institution: University of Edinburgh, UK
Status: Published in ApJ

The biggest structures known in the universe are galaxy clusters: they are made of hundreds or even thousands of galaxies, lots of gas, and a huge amount of dark matter. But a long time ago, these giants were babies. Right after the Big Bang, when no galaxies, stars or even molecules had yet formed, the universe was extremely homogeneous, although it had density fluctuations with relative amplitude of ~10^-5. During the cosmos’ expansion, the regions that initially were slightly heavier became increasingly heavier, because mass attracts mass. Then, clumps of gas turned into stars. Due to their mutual gravitational attraction, these stars gradually moved closer to each other, growing into galaxies that congregated further — also because of the gravity — into today’s galaxy clusters (for a deeper understanding, read this).

Since looking at distant astronomical objects means to look into the past, we may be able to see the progenitors of galaxy clusters, known as protoclusters. They should be far from us (at high redshifts, z), composed of dozens of galaxies forming lots of stars, and they should therefore contain a huge amount of dust and gas (the stars’ ingredients). From our perspective on Earth, we should view these protoclusters as distant aggregations of galaxies that are very bright and very red, visible particularly in submillimeter/millimeter wavelengths (the wavelength of the emission perceived from the gas and dust of distant galaxies that has been heated by the galaxy’s stars). To observe these systems may teach us about how the universe and its structures evolve through the cosmic time. In today’s paper, the authors report the discovery of a protocluster core with extreme characteristics: it is super dense, super massive, and super old.

Multiple Observations

In the attempt to find ideal protoclusters, the authors looked for sources in the H-ATLAS survey. They chose the reddest protocluster-like system and baptized it Dusty Red Core (DRC).
To find out more about DRC, they made many different observations:

• Atacama Pathfinder Experiment (APEX) telescope, in the wide-field (left panel in Figure 1);
• Atacama Large Millimeter Array (ALMA), to detect emission lines, make a deep continuum observation of DRC and look at its brightest component (DRC-1) in detail (central and right panels in Figure 1, respectively);
• Very Large Array (VLA), for a continuum map;
• Very Large Telescope (VLT), for broadband spectroscopy and imaging;
• Gemini Observatory, for broadband imaging;
• Spitzer Space Telescope, for imaging in two bands;
• Australia Telescope Compact Array, to detect ¹²CO emission lines and for continuum observations.

Figure 1: From left to right: wide-field view of DRC (the A region) from APEX; DRC galaxies observed with ultra-deep ALMA continuum; high-resolution ALMA imaging shows details of DRC-1, composed of three star-forming clumps. [Oteo et al. 2018]

Getting the Information

All this data leads to several new findings about DRC. Firstly, the continuum and imaging observations show that DRC is actually composed of 11 bright, dusty galaxies instead of a single object as first thought. Its brightest component, DRC-1, is formed by three bright clumps.

The easiest way to find the protocluster’s redshift is by using the lines emitted by the molecules and atoms of the gas filling the galaxies, such as ¹²CO, H₂O and CI. Ten of the 11 galaxies in DRC are at the same redshift of z = 4.002 (the final one didn’t have enough lines to measure the redshift). Since the expansion of the universe means that more distant objects move faster and have higher redshift, this can be converted to a luminosity distance of ~117 billion light-years, meaning they were formed only 1.51 billion years after the Big Bang. The authors also find that the components are concentrated into an area of 0.85 × 1.0 million light-years. This may seem like an enormous area — but for astronomy, this qualifies as an extremely overdense region. Knowing that, it is safe to say that at least ten objects of DRC are members of a protocluster core in the initial evolutionary states of the universe.

The continuum observations are also useful to calculate how much gas and dust mass is converted into stars per year (the star formation rate, or SFR). The lower limit obtained is 6,500 solar masses per year, which is the highest star formation rate ever found for such a distant protocluster. Furthermore, the protocluster’s molecular gas mass and total mass were estimated. The gas mass — estimated using the CI emission lines — is found to be at least 6.6 × 10¹¹ solar masses. The total mass of the protocluster was calculated in three different ways, with an outcome of as much as 4.4 × 10¹³ solar masses (for comparison, the estimated mass of the Local Group is ~2 × 10¹² solar masses). Based on these results and cosmological simulations, the authors concluded that DRC may evolve to a massive galaxy cluster in today’s universe, like the Coma Cluster.

Protoclusters like DRC are a key for us to understand a remote part of the universe’s history. Moreover, DRC may help us to infer information about the unknown part of the universe — which is huge, since the dark sector corresponds to 95% of the cosmos. The protocluster analyzed in this paper is bright and massive, and it was measured with accuracy by modern telescopes, despite its enormous distance from us. Those measurements may be used as parameters to test different cosmological theories, thus helping us to understand the universe’s big picture.

About the author, Natalia Del Coco:

Today’s guest post was written by Natalia Del Coco, a masters student at the University of São Paulo, Brazil. In her research, she looks for correlations between the physical properties of clusters of galaxies and the cosmic web around them. Besides being an astronomer, Natalia is also a ballerina, a shower singer, and a backpacker.