Did CANDELS find the first stars in the Universe?

Did that title grab your attention? Spoiler alert: the answer is no (probably), but the journey to that answer was interesting! Read on!

In today's blog post, I'm reporting on a paper I wrote with the help of many other members of the CANDELS team which has just been published in the Astrophysical Journal. The title of the paper is "The Evolution of Galaxy Rest-Frame Ultraviolet Colors from z = 8 to 4" (and you can find it here). There's a lot in the title, so let's break it down: the word "evolution" at the beginning means that we're studying how galaxies change (i.e. evolve) with time. In this particular paper, we're focusing on a time period in the early universe, from around 0.5 to 1.5 billion years after the Big Bang. The universe is now 13.8 billion years old, so this is early on indeed, but as I'll show, things change quickly even in this short time period. The "from z = 8 to 4" denotes this time period; astronomers use a term called "redshift" to denote distance (see these earlier blog posts, #1 and #2, for more details), so a redshift (or "z") of 4 corresponds to 1.5 billion years after the Big Bang, and a redshift of 8 is about 0.5 billion years (or 500 million years) after the Big Bang.

The meat of the title is in the middle: "rest-frame ultraviolet colors"; this means we are looking at the colors in the ultraviolet, which is just bluer than the eye can see. However, due to the expansion of the universe, at these high redshifts this ultraviolet light is shifted to the near-infrared. Thus, we add the qualifier "rest-frame". What this means is that even though the light we observe is in the near-infrared, we have a good idea of the redshift of a given galaxy, so we can figure out what the intrinsic ultraviolet (or UV) color is (you may have seen this referred to as "k-correcting", which is a similar concept).

So, why is this interesting? The UV light tells us a lot about ongoing star-formation. This is because the UV light from galaxies is dominated by massive stars (many times more massive than the Sun). Massive stars are very hot, so they are very bright in the ultraviolet, and they look blue (by comparison, the Sun appears yellowish, and lower mass stars look red). But, massive stars are very short-lived, some exploding as supernovae after only 10 million years. So, if you look at a galaxy and it is bright in the UV and also blue in color, you immediately know that massive stars are still around, so it must be actively forming a lot of stars!  Cool, right?

The story doesn't end here. A number of studies predict that when we look at galaxies very early on, they should appear even bluer than galaxies observed at later times. This is because in the Big Bang the only elements to form were hydrogen and helium (with trace amounts of lithium). If you make a star with just those elements, it will be very hot; this is because gas needs to cool to very low temperatures to condense and form "normal" stars like those we see in today's universe. However, to cool down, gas typically relies on atoms heavier than helium (which we astronomers call "metals") to radiate away energy. If you don't have those elements, then stars do form, but we think that they end up being much more massive (maybe even more than 100 times the mass of the Sun); these massive stars are extremely hot, so they are very blue, much bluer than any star with a normal allotment of metals.

Figure 1: The ultraviolet colors of galaxies at different redshifts (the top vertical axis shows the amount of time in billions of years since a redshift of 20; add 0.2 to change it to since the Big Bang). The red circles show the average of all galaxies, while the blue circle shows the average colors of faint galaxies at a redshift of 7.

So, we were motivated to look through the CANDELS data for distant galaxies, and measure their colors to see how blue they looked. Sounds simple, right?  I'll skip over a lot of the details (though you can learn how we found the galaxies here), but our main results are shown in Figure 1. This plot shows the colors of galaxies at each redshift we study.  The red points show the average colors of all galaxies, but we'll focus on the blue point first, since it represents the average color of faint galaxies only (which tend to be bluer). The gray line and arrow shows the color we would expect if these galaxies had stars with very little metals; if our galaxies had very little metals, we would expect them to be below this line. What we see is that even when we look at the faintest galaxies, while they do look very blue, they are not so blue that we think they host stars with very low metallicities. In fact, taking into account the error bar on this point, these galaxies have colors consistent with the local galaxy NGC 1705 (shown as the cyan bar); while NGC 1705 is blue, it has been studied very intensely, and it doesn't have any metal-free stars.

What this tells us is that even though we're looking less than one billion years after the Big Bang, galaxies have already been enriched by metals a significant amount. This means that star-formation must have already been occurring for a while, so that when we have the capability to look to yet higher redshifts (which we will later this decade with the James Webb Space Telescope), there should be plenty of galaxies burning bright for us to see!

There is one last point to make, this time looking at the red points, which are the average colors of all galaxies at a given redshift.  We see that they start out fairly blue at z=7, and get progressively redder. We think that this is due to the increased presence of cosmic dust grains (composite particles made up primarily of carbon and silicon) in these galaxies. Dust has the funny effect of making the colors of a galaxy look redder, since they absorb and scatter blue light more efficiently than red light, hence the phrase "dust reddening". What we're witnessing here is the build-up of vast reservoirs of dust in these galaxies, in only one billion years of cosmic time! Although dust may sound boring, it has a dramatic effect on the colors of galaxies, as you can see. Also, all of this UV light absorbed by the dust has to go somewhere, and its energy actually gets re-emitted in the far-infrared (much redder than your eye can see). In fact, when we look at galaxies closer to home, many of them are brighter in the far-infrared than in the UV, implying that much of their UV light has been absorbed by dust. What our results here tell us is that while this is likely not an issue at a redshift of 7, it becomes progressively stronger as you move to lower redshift. So, if you want to study galaxies at high redshift, much like the Alamo you must "remember the dust!"