Authors: J.M. Diego, S.P. Willner, J.M. Palencia, R.A. Windhorst
First Author’s Institution: Instituto de Física de Cantabria Avda. Los Castros s/n. 39005 Santander, Spain
Status: Published on Arxiv [open access]
“There he lay, a vast red-golden dragon, fast asleep; a thrumming came from his jaws and nostrils, and wisps of smoke, but his fires were low in slumber. Beneath him, under all his limbs and his huge coiled tail, and about him on all sides stretching away across the unseen floors, lay countless piles of precious things, gold wrought and unwrought, gems and jewels, and silver red-stained in the ruddy light.”
-J.R.R. Tolkein, The Hobbit
Anyone who knows anything about dragons will tell you about their love of shiny things. The celestial “dragon” we are discussing today is no different, but their hoard comprises something invaluable to cosmologists: standard candle stars. Today’s authors will determine if it is possible to pickpocket these important stars and make off with an incredibly powerful cosmological tool from the dragon’s clutches!
The Ladder into the Dragon’s Den
A good chunk of doing cosmology comes down to measuring distances, in order to determine the size and shape of our universe and how it changes over time. This is quite a bit harder than it might seem at first: how do you determine the distance to a pinprick of light in the sky? Astronomers and Cosmologists have developed what is called the cosmic distance ladder to answer that question. This ladder allows us to determine the distance to close astronomical objects, and then use those measurements to build up to find the distances to objects at various different scales.
The first rung on the ladder is called parallax. The next time you’re driving in a car, take a look outside the passenger window. Notice how things closer seem to be moving faster than those farther away? If you know how quickly you are travelling and do some geometry, you can work out the distance to these objects based on how fast they appear to move. We can do this here on Earth: by observing the same star twice at opposite ends of the year (and hence opposite points in Earth’s orbit) and looking at how far it moved in the sky, we can determine the distance to it. However, like mountains that are super far away from us in the car which don’t seem to move at all, super distant stars or galaxies will move imperceptibly and therefore we can’t measure a distance to them.
This is where the second rung of the ladder comes into play. As has come up in many a previous astrobite, a standard candle is an astronomical object for which we know the intrinsic brightness ahead of time (this is called the absolute magnitude) and can compare it to what we observe on Earth (called the apparent magnitude), the dimmer the object is than its absolute magnitude, the further it must be. However, there is an issue: how would we ever know the intrinsic brightness of an object? We can’t go visit them! This is why the cosmic distance ladder needs to be a ladder. If you can observe a standard candle close enough that you can determine its distance via parallax, then you can work out what its intrinsic brightness must be, because we know how much dimmer objects get with distance. Then, you can observe a different standard candle and work out the distance to it because you’ve determined the absolute magnitude with parallax and voila: you’ve taken a step up the distance ladder! The most iconic standard candle is the Cepheid variable star, which we know obeys a strict relationship between brightness and how quickly they pulse.
Key for this paper are two more types of standard candles. The first is called the “tip of the red giant branch” stars or TRGB stars. On a Hertzsprung-Russell (HR) diagram, which is a plot of the brightness versus color of stars, there is a sharp bend in the region of the plot that encapsulates red giant stars. Thanks to our understanding of nuclear and stellar physics, we have a pretty good idea of where that tip should occur. So, it can be used as a standard candle: just make an HR diagram of the galaxy, look for the brightest red giant and compare it to what we know it should be at the tip of the red giant branch. Finally, the cool new method on the block is doing the same trick as before, but with stars that lie in the asymptotic giant branch (AGB) of the HR diagram, which all have very similar luminosities in the infrared.
These three types of stars: Cepheid, TRGB, and AGB, called non-explosive standard candles (NESCs)), are the gems from the vast hoard of other stars the authors care about in today’s paper. However, if one wants to look even farther out into the universe, eventually these stars are too dim to see; what if we could find a way to make NESCs brighter?
Peering into the Dragon’s Hoard
That’s enough talking about ladders, I believe I promised you a dragon? The dragon in question is actually a galaxy. The so-called Dragon arc is so special because between it and us lies the galaxy cluster A370. The huge mass of this galaxy cluster bends the light coming from the Dragon in a process called gravitational lensing. In the same way that a magnifying glass can bend light to make smaller things easier to see, this gravitational lensing can make faint stars much easier to see. In the case of the Dragon, certain stars can have their brightnesses magnified up to 5000 times! This proposes an amazing opportunity for cosmologists: what if these NESCs can be magnified? Typically, NESCs can only be seen out to a distance of only about 50 megaparsecs, or a cosmological redshift of about 0.01. The Dragon sits at a redshift of 0.725 or about 100 times further away than this limit. Could the standard candle jewels of the Dragon’s hoard be magnified enough to be visible by state-of-the-art telescopes like JWST?
Well, when studying the lensed stars seen in the Dragon, one can see individual stars that are 10,000 to 100,000 times brighter than the Sun before being magnified, which is comparable to how bright NESCs can be, making them detectable with the JWST in just a few hours! The authors want to investigate if investing more JWST time into observing this system could yield the furthest-ever measurement of a NESC: offering a sanity check on our other standard candle methods as well as yielding an independent measure of the Hubble Constant.
Sifting through Piles of Treasure
To determine what a NESC might look like when magnified by the gravitational lensing seen here, we need an idea of what the general population of stars looks like. To do so, the authors start with the population of stars of the Large Magellanic Cloud (LMC), a nearby dwarf galaxy that has been thoroughly studied. They then redshift the light of these stars until they are the color that they would appear in the Dragon, and dim them by the amount as to be expected due to their distance.
Now that we’ve simulated the stars in the dragon’s hoard, what are the odds that once is magnified enough to be detected? This is a bit of a tricky question. The main issue is that the magnification comes in two steps: a macrolensing magnification and microlensing magnification. The macrolensing is the magnification that comes from the overall mass of the A370 cluster. This means that essentially every light ray reaching us from the cluster gets a magnification of about 100. Additionally, light rays coming from the dragon could happen to line up with stars in the intracluster medium of A370, which magnifies the light even further; we call this a microlensing event. However, the star that is doing this lensing will only maintain this perfect alignment for a few days before the star’s velocity moves it out of position.
Thankfully, since the Dragon arc is quite large, these microlensing events are quite common. To determine exactly how common, the authors run a ray tracing simulation, studying which rays of light end up being gravitationally lensed and which don’t. The results of their simulation are shown in Figure 3. Notice that the yellow curve, which represents the probability of magnification for regions that are strongly lensed by the galaxy cluster, has a very sharp peak. This is great news and means that, for stars sitting close to regions that are strongly lensed, we have a good chance of knowing exactly what the magnification is, which is key for using them as standard candles.
With the expected distribution of stars in hand and an understanding of what we expect the gravitational lensing to do, all that remains to do is apply that lensing to those stars and see what happens. Giving each star the appropriate magnification, the authors find around 1100 NESCs that would be detectable by JWST, see Figure 4. This is great news, but which kind should we specifically be looking for with our (extremely in demand) JWST telescope time? There’s pros and cons to each kind of NESC. Observing a Cepheid is great because a single Cepheid is enough to get a distance measurement, meanwhile for TRGB and AGB stars one needs enough measurements to create the plot required to see the “tip” as mentioned above.
However, for a Cepheid, one needs to measure their brightness multiple times to determine how quickly they are pulsing, which could easily be contaminated by a stray microlensing event. This issue could be overcome if one observes multiple Cepheids nearby, allowing one to assume their magnifications are consistent. This opens the door to using Cepheids as a standard candle at a distance one hundred times further than ever before! With some more data, an independent measure of the Hubble constant is coming soon!
Return with the Elixir
The authors conclude that using this method, we may be able to reliably measure Cepheids and other NESCs out to a staggering redshift of z ~ 1, or 100 times further than we can typically! Of course, we do need to get lucky and find galaxies that are being strongly lensed, but thankfully several other examples other than the Dragon are known and currently being studied, such as the Warhol or Spock galaxy (seriously, who is giving these galaxies these great names?). Regardless of which galaxy we find them in, these lensed NESCs offer an amazing new cosmological tool that can validate and strengthen the accuracy of our distance ladder. We just have to find the spots on the sky labeled: “Here there be dragons.”
Featured Image Credit: Dragon by Hokusai, Wikimedia Commons
Edited by Samantha Wong
Lovely article !