Old but gold: a huge primordial proto-cluster

Natalia del Coco, a Masters student at the University of São Paolo, Brazil

Today’s guest Post was written from Natalia Del Coco, a Masters student at the University of São Paulo, Brazil. In her research, she is looking for correlations between the physical properties of cluster of galaxies and the cosmic web around then. Besides an astronomer, Natalia is ballerina, shower singer and backpacker.

Title: An extreme proto-cluster of luminous dusty starbursts at zspec = 4.002

Authors: Oteo, Ivison, Dunne, et al.

First Author’s Institution: Institute for Astronomy, University of Edinburgh, Royal Observatory

Status: Published in the Astrophysical Journal, open access version available

The biggest structures know in the Universe are galaxy clusters (GC): 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 actually babies. Right after the Big Bang, when no galaxies, stars or even molecules were formed yet, 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 force, they gradually got closer to each other, growing into galaxies, which congregated further – also because of the gravity – into today’s GC (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 the GC, the proto-clusters. They should be far from us (at high redshifts, z), comprised by dozens of galaxies forming lots of stars, therefore containing a huge amount of dust and gas (the stars’ ingredients). In other words, we should see these proto-clusters as distant aggregations of galaxies that are very bright and very red, and therefore visible only in the submillimeter/millimeter from Earth (which are the color and wavelength detected in earth frame from the gas and dust radiation of distant galaxies when heated by its 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 proto-cluster core with extreme characteristics: super dense, super massive and super old.

Multiple Observations

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

Figure 1: From left to right: wide-field 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 by three star-forming clumps. Figure 1 in the paper.

Getting the Information

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

The easiest way to find the proto-clusters’ redshift is by using the lines emitted by the molecules and atoms of the gas filling the galaxies, such as ¹²CO, H2O and CI. 10 of the 11 galaxies in DRC are at the same redshift of z = 4.002 (the final one didn’t have enough lines for the measurements). Since the expansion of the universe means that more distant objects move faster and have higher redshift, this can be converted to a distance of ~117 billion light years far from us (in luminosity distance) and were formed only 1.512 billion years after the Big Bang. The authors also find that the components are concentrated into an area of 260 kpc × 310 kpc (see parsec). It may seem an enormous area, but for astronomy, it means this is an extreme over-dense region. Knowing that, it is safe to say that at least 10 objects of DRC are members of a proto-cluster 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, SFR). The lower limit value obtained was 6500 solar masses per year, being the highest found until now. Furthermore, the molecular gas and total mass were estimated. For the primer, they utilized the CI emission lines, achieving a minimal value of 6.6 × 1011 times the solar mass. The second one was calculated in three different manners, reaching up to 4.4 × 1013 times the solar mass (for comparison, the estimated mass of the Local Group is ~2 × 1012 times the solar mass). Based on these results and cosmological simulations, the authors concluded that DRC may evolve to a massive GC in today’s Universe, such as the Coma Cluster.

That kind of object is a key for us to know a remote part of the Universe’s history. Moreover, DRC may help us to infer about the unknown parts of the universe – which is huge, since the Dark sector corresponds to 95% of the Cosmos. The GC analysed in this paper is bright and massive, and 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.

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  1. Obviously, I’m missing something. How can an object be 117 billion light years away when the universe is only 13.8 billion years old? What’s the relationship between luminosity distance and actual distance?

    • “Luminosity distance” isn’t necessarily “time of light-travel distance,” but I can’t tell offhand whether it’s truly large due to how it’s calculated or if 11.7 was intended.

      There are other typographical omissions, however, so I’m guessing the decimal point got dropped. (Search for “times the solar mass” in at least two places; whatever numbers were before those phrases are entirely missing, even from the source HTML.)

      • Thank you for the reply. I should have done my homework first, but I checked it out on Wikipedia and eventually concluded what you’ve said, although I still don’t quite understand the usefulness of the measurement. Based on a graph I found in a Wikipedia article, I think that 117 billion is an option, but this is brand new territory for me.

      • Hi! It should in fact read 117 billion light years, since as you correctly point out it’s the luminosity distance, which is different from the light travel time. To respond to Steve (though it sounds like Wikipedia has beaten me to it) the reason for the difference is that the universe is and has been expanding since that light left the distant object. This makes measuring meaningful distances to very distant objects challenging: does “actual distance” mean the distance between us and the object when the light was emitted, or the distance the light has traveled, or the distance light would have to travel to get back to that object now?
        The observable that is easiest to use to measure distance is redshift. The object in this article is at a redshift of z = 4.002, from which you can then calculate other distance measures. So this redshift corresponds to a comoving distance of ~23 billion light years, or a luminosity distance of ~117 billion light years. Luminosity distance is most useful as a definition to work out the luminosity of a source, given the flux that we can measure, as it factors in the redshift and expansion. Comoving distance might be the best “actual distance” to consider. In comoving distance, the observable universe has a radius of ~46.5 billion light years. Hope this makes some sense and helps answer the question a bit!

        p.s. the post has now been updated to include those numbers that were missing in the previous version 🙂

        • Thank you very much for taking the time to send an explanation. I was not aware of all these different “distances” , but your explanation helps a lot.

        • Yes, thanks, Joanna! I knew the comoving radius of the universe was 46 Gly, which is why I was suspicious of the much larger luminosity-distance value here. But since it really is supposed to be 117, as written, then clearly I need to go learn more about luminosity distance. 🙂

  2. I suspect that should be 11.7 billion ly.


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