A Glimpse into Santa’s Workshop: Galaxy Edition

Title: Massive galaxy formation caught in action at z~5 with JWST

Authors: Shuowen Jin, Nikolaj B. Sillassen, Georgios E. Magdis, et al.

First Author’s Institution: Cosmic Dawn Center (DAWN) and 2 DTU Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark

Status: Submitted to A&A Letters [open access]

If you were an astronomer looking at the universe in the earliest days of its existence, you wouldn’t find galaxies like the Milky Way or Andromeda. In fact, the galaxies you’d see probably wouldn’t look anything like our familiar galactic neighborhood, but instead would look like tadpoles – small and blobby, with no beautiful spiral arms or any particularly coherent structure. If you were lucky enough, you might even see groups of these cosmic tadpoles, seemingly stationary in the starry ocean. You wouldn’t know it, but these groups were actually engaged in millennia-long, deadly dances. Over time, due to the unstoppable force of gravity, these small galaxies will merge with each other continuously, growing in size until they become the grand behemoths we know today. 
The idea that massive galaxies are built from the mergers of many smaller galaxies is what astronomers call hierarchical clustering. For many years, we’ve only been able to paint a picture of this behavior in simulations, since these low mass, high redshift galaxies are extremely faint and their light is shifted so far into the infrared that mighty space telescopes built to see primarily in the optical and near-IR, like Hubble (HST), can’t see them. Though we can’t physically go back in time to hunt for these galactic building blocks, we do have an alternate time machine that can help us look at our universe in its infancy and find observational evidence for hierarchical clustering at play – JWST!

Breaking the Mold

Since JWST finally launched last holiday season, it has been giving us loads of presents every week, which are helping astronomers understand how galaxies grow and evolve. The authors of today’s paper looked at a catalog of objects imaged by JWST and found a concentrated field of galaxies – the highest overdensity of galaxies at redshift z>2. This system is a group of 6 galaxies, all with redshifts between 5<z<5.4 that the authors call CGG-z5. They looked at both JWST and Hubble images of this field and ran a source extraction algorithm on them to tell how bright each of the galaxies are in different wavelengths. The HST+JWST combination lets them access 14 different filters, giving them a broad spectrum over which they can examine this galaxy group. After creating these photometric catalogs of sources, they used them to run a spectral energy distribution (SED) fitting code. SEDs tell us how much light an object emits over different wavelengths, and by looking at features like the shape of the resulting spectrum or the presence of strong emission or absorption lines, the authors calculated properties like the mass, redshift, star formation rate, and age of each constituent galaxy. 

The authors found that this galaxy group is invisible in filters that let through visible and near-IR light with wavelengths shorter than ~800 angstroms, making these Lyman Break Galaxies (LBGs). At these higher energy wavelengths, almost all the energy emitted by these galaxies gets absorbed by surrounding gas, so there is a “break” in their spectrum. The most massive galaxy in the group has a stellar mass of around 109.8 Msun, with its siblings being around 4 times less massive. Figure 1 shows two images of CGG-z5, each made from different combinations of filters. Looking at the left image, we see that two of the group’s components, labeled a and d in the right image, are masquerading as a single galaxy but in reality seem to be made up of three and two separate components, respectively. The authors are not certain as to what these are – this could indicate that we are witnessing ongoing mergers, or that these are clumps with active ongoing star-formation, or perhaps this system has some low-mass satellite galaxies in the mix that can only be resolved with high resolution imaging.

Left: An image of 6 galaxies, all looking like they're lined up down the center. This is a composite image from the F115W, F150W, and F200W filters, listed on the top left of the image. Right: The same image of 6 galaxies, but made from different filters: the F277W, F356W, and F410M filters.
Figure 1: Left: A JWST color RGB image of the galaxy group CGG-z5 using filters centered on 115, 150 and 200 microns for the blue, green, and red, respectively. Right: The same JWST color image but made from different filters: 277, 356, and 410 microns for blue, green, and red, respectively. The 6 different galaxies in the group are labeled with letters. Notice that in the left figure, components a and d resolve into more than one blob, indicating the presence of another feature like satellite galaxy or starbursting region. Figure 1 in the paper.

Cosmic Coalescence

Since images from JWST and Hubble are merely snapshots frozen in time, and galaxy evolution takes millions of times longer than the average human lifetime, the authors turn to their trusty simulations to predict what the future of this galaxy group might look like. The paths it can take vary – all 6 of these galaxies might merge together, forming one big galaxy, or they could form the core of a massive galaxy cluster, or just merely be 6 galaxies that happen to arrange themselves in an interesting pattern in our line of sight. In order to gain a glimpse of the future, the authors use their cosmic crystal ball – the hydrodynamical EAGLE simulation – to find structures similar to CGG-z5 and trace out their evolution over time. They found 14 systems, each of which has central masses similar to CGG-z5 and has more than 3 galaxies within 3 proper kiloparsecs.

Figure 2 shows the simulated galaxy group that resembles CGG-z5 the closest, with 7 member galaxies. By running the simulation forward in time, they found that the members of this galaxy group all merge together by z=4 and form one giant galaxy with a stellar mass of around 1010.7 Msun – a bit less massive than our Milky Way, but not by much. By z=1, it grows six times more massive than the Milky Way, to a stellar mass of 1011.5 Msun. The other 13 simulated galaxy systems all share the same fate, merging into a single galaxy around z~3-4 and growing to a >10^11 Msun galaxy by z=1. From this, the authors propose that their real-life galaxy group CGG-z5 will also share the same fate and merge together into one large galaxy, dubbing it a “proto-massive galaxy”.

Four panels showing the evolution of a simulated galaxy group from redshift z=6 (top left panel), to z=5 (top right), z=4 (bottom left) and z=3 (bottom right). Each panel goes from -30 to 30 pkpc on the x and y axes and each show the mass of the dark matter halo of the group (from 11.6 to 12.1 to 12.4 to 12.5 solar masses from z=6 to z=3). There is a legend for each panel that shows the stellar mass of different components, marked by differently colored and shaped markers.
Figure 2: The simulated system that most closely resembled CGG-z5. These four blocks show how this system looks at different redshifts during the simulation, showing how the stellar mass of its constituents evolves over time from z=6 to z=3. We see that by z=5, the system of galaxies has coalesced into one, massive galaxy. Figure 4 in the paper.

These simulations can also help explain why systems like CGG-z5 are so rare in our current galaxy census. The merging timescale for these proto-massive galaxies are very short in cosmic time – about 400 Myr. That leaves a pretty small window for these kinds of structures to exist before they coalesce around z=4. Beyond z=5, JWST might not have deep enough imaging capabilities to detect lower mass, fainter galaxies coming together in these groups, especially if they are lower density. All this makes CGG-z5 an even more intriguing and unique observational example of hierarchical clustering at play. As JWST keeps on gifting us new data, hopefully we’ll find evidence of other systems like CGG-z5 and figure out how the universe builds the foundations of galaxies like the one we call home.

Astrobite edited by Jana Steuer

Featured image credit: NASA, ESA, STScI, Julianne Dalcanton (Center for Computational Astrophysics / Flatiron Inst. and University of Washington); Image Processing: Joseph DePasquale (STScI)

About Katya Gozman

Hi! I’m a third year PhD candidate at the University of Michigan. I’m originally from the Northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM, as well as work at our on-campus observatory and planetarium.

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