Too Many Stars, Too Fast?

Title: RUBIES: Evolved Stellar Populations with Extended Formation Histories at z ∼ 7 − 8 in Candidate Massive Galaxies Identified with JWST/NIRSpec

Authors: Bingjie Wang, et al. (RUBIES collaboration)

First Author’s Institution: Department of Astronomy and Astrophysics, The Pennsylvania State University

Status: Preprint on arxiv

When it was launched, scientists were hopeful that JWST would be able to answer hundreds of questions about the early universe. Luckily, it has been able to provide evidence to answer many of these questions. And even more luckily, observations of the early universe with JWST have been able to generate even more questions we don’t know the answer to! 

One of these new conundrums scientists are running into is that it looks like some of these early galaxies have what we call evolved stellar populations. This means that even though these galaxies are only a couple hundred million years old (super young, right?), they already have ~100 million-year old stars, meaning that these galaxies had to make a large amount of stars in only a short span of time. We’re not used to seeing this many stars form this fast–in fact, we expect that star formation should be slow this early in the universe. So how do we reconcile this?

The authors of today’s paper look in more depth at the spectra of three of these such galaxies to figure out if these galaxies have really been forming stars for millions of years already. These three galaxies are located at redshifts of z = 6.7-8.4, which places them just 600 million years after the Big Bang.

Balmer breaks create questions

First, the authors looked to confirm that evolved stellar populations do in fact exist in these galaxies. One foolproof way to do this is to look for a Balmer break in the galaxies’ spectrum. The Balmer Break comes from the atmosphere of older stars, indicating that the galaxy has been forming stars for at least around a 100 million years. And excitingly, the authors confirm that there is in fact a Balmer break in the spectrum of these three galaxies. Unfortunately, the Balmer break only tells you that there are older stars, not necessarily how old they are or how long ago they formed, or how many of them there are.

Figure 1. Balmer break at rest wavelength 3645 observed in the galaxy spectra. The strength of the break (the relative flux difference before and after the break) is 1.71. Figure 2 in the original paper.

Stellar Populations or AGN?

The authors then turn to understanding the finer details of the stellar population, specifically its formation history. The authors found evidence of broad lines in their emission line spectra, signaling that there might be AGN activity in at least two of these galaxies. Broad lines come from Doppler Broadening, where photons emitted by the AGN have a wide distribution of velocities which results in a broad emission line. However, the authors aren’t entirely ready to say conclusively that there are AGN in the spectra. 

In the local, low-redshift universe, we’re pretty good at understanding how these broad lines relate to the AGN because we have hundreds of spectra to test.. But at high-redshift, we don’t have a lot of data. We aren’t sure why supermassive black holes act like that early in the universe, and we don’t want to assume they act the same as the ones we observe in the local universe. So researchers have to rely on testing theoretical models against these high-redshift galaxy observations. 

To test how much of the light in these galaxies is coming from AGN or evolved stellar populations, the authors test three different models. 1. It’s mostly from stars. 2. It’s from a mixture of stars and AGN. and 3. It’s mostly from AGN.  The authors create the model spectra using Prospector, and compare the modeled spectrum to the observed spectrum to see how well they match.

Model Comparisons

Figure 2. (Left) Each model compared to the spectrum for one of the observed galaxies. (Right) Predicted star formation rate as a function of time since Big Bang for each model. As seen, models with higher AGN contribution predict later star formation. Figure 4 in original paper.

The most important thing they are testing here is what these different levels of contributions mean for the formation history of the stellar population. For instance, the model with the maximal stellar contribution means that the stellar population had to form earlier, because there needs to be enough stars to create all the light seen. The minimal stellar model can form stars a bit later because the AGN contributes more to the light seen, so you don’t need as many stars producing a lot of light. However there is no way to verify which model is more accurate at this stage, so the authors turn to theorizing how each model could fit into the evolutionary stages of later galaxies.

Progenitors of massive quiescent galaxies?

To find out how their models fit into the evolution of galaxies, the authors then compare the different suggested star formation histories of the medium and maximal stellar contribution models with the projected histories of massive quiescent galaxies that exist at z ~ 3-5. These massive quiescent galaxies lack an explanation for how they got so massive yet have such slow star formation now. As can be seen in Figure 3, the star formation rate traced across the age of the universe for the massive galaxies (black, blue, green and yellow distributions) matches much better in the maximal stellar contribution model. The authors posit that these galaxies they observe could create a lot of stellar mass in a short time, then have their star formation slowed greatly to become these massive quiescent galaxies. With this explanation, the authors note that these galaxies would be a relatively small percent of all the galaxies in the early universe. 

Figure 3. Comparison of medium (purple) and maximal (red) stellar models with the projected formation histories of a sample of massive quiescent galaxies at z ~ 3-5. Figure 5 in original paper.

Of course, there is still the complication that there’s convincing evidence that there are AGN in this sample of galaxies. Perhaps there is some new physics that creates these broad lines we are yet unfamiliar with. As well, having this many stars form this early is still an uncomfortable thought to many astronomers. The jury is still out on these galaxies and how much star formation they had that early, but future studies looking into the far-infrared can hopefully distinguish between stellar light and AGN and lend more insight into what makes up these galaxies. And hopefully with JWST we can find some more answers while exploring even more questions.

Astrobite edited by Amaya Sinha

Featured image credit: HST 

About Caroline von Raesfeld

I'm a second-year PhD student at Northwestern University. My research explores how we can better understand high-redshift galaxy spectra using observations and modeling. In my free time, I love to read, write, and learn about history.

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