Paper day! (🧵)

Through the last couple years, I have spent a lot of time looking really hard at the galaxies in the Hubble Ultra Deep Field, trying to find galaxies in there which are emitting ionizing "Lyman Continuum" (what the streetwise know as LyC) into intergalacitic Space. Today, the article hit the preprint-server ArXiv, signifying that we think it is ready to be presented to the world...


The Hubble Ultra Deep Field is a little, seemingly empty, patch of the sky in the Southern Hemisphere. Astronomers first pointed the Hubble Telescope at it for a full 11 days back in 2003, and just let the light trickle in. Since then, a number of follow-up campaigns have added infrared and ultraviolet light to the original observations. The result is stunning!

In the image above, every light source except a couple handfuls of stars, are distant galaxies, containing millions if not billions of stars. The galaxies we were looking for were some of the smallest and faintest in the field. Very much a needle-in-haystack situation.

So why would we want to do this?
Until around one billion years after the Big Bang, the Universe was largely made up of hydrogen atoms. But the first stars emitted highly energetic ultraviolet light, called "Lyman Continuum", or LyC, which can knock the electron off a Hydrogen atom and send it reeling through empty Space, turning the neutral atom into an ion. And they did. Over a period of a few hundred million years, the vast majority of atoms in the Universe were ionized.

The problem is that in this process, the UV light gets absorbed by the atom that it ionizes. And the first galaxies were full of Hydrogen atoms they could ionize and get absorbed by, way before they would make it out of the galaxy they were born in. We know that quite a lot of this light did actually get out, but we don't know how, or which galaxies were better at letting the LyC light escape....

A lot of researchers, including myself, are trying to figure out how this all fits together. Which galaxies the LyC photons escape from, and which mechanisms allow them to. To do so, we need to observe these LyC-leaking galaxies, preferably as many as possible.

But observing them is expensive. It requires the best telescopes in the world, such as the Hubble Space Telescope, and it requires quite a lot of hours. So you do not want to come unprepared. To make sure we get the most out of our telescope, people often look at some other properties of a given galaxy which might be easier to observe, and which we know are good indications that they are probably leaking LyC light. This is called "preselection" and is a good way to ensure that you get the smallest possible number of duds when pointing your telescope at them.

But what if this preselection actually throws away some perfectly good LyC emitters just because they don't look like we think they ought to look? What if we are not getting the full picture, because we are rejecting them before we even get to the point of testing if they are emitting anything? Do we get a wrong picture of how this whole business works, and is it a problem?
That is the question we have been trying to answer in this project...

So we tried to go about the problem the opposite way: What if, instead of pre-selecting galaxies that look the way we think they should, and then pointing the telescope at them - what if we instead looked at everything that possibly could be escaping LyC, and then went through it all and checked if indeed it was escaping LyC? That would be a lot of work, for sure, but we would not be limiting ourselves by way of our own prejudice.

The Hubble Ultra Deep Field was the perfect place to do this.

The HUDF already has a ton of data; and even though it only fills a fraction of the size of the Moon on the sky, it literally contains tens of thousands of galaxies.

Now all we needed to do was to check each single one of them and see if it could possibly be a LyC leaker. That sounds easy, right?

We basically looked at what we could find in Ultraviolet light - which is much less than you can find in normal visible light - and then, for each of these UV sources, we worked our way backwards to see if it looked like it was LyC emitting galaxy, knowing that the vast majority would not be. Out of the 5000 galaxies we looked at from the start, we expected that maybe 1-5 would be leakers. We ended up with 7, but a couple of them were a bit shaky, so the estimate turned out to be pretty good.

Now, as y'all may remember, the original questions was, would we find any of these leaking galaxies that did not look like we usually expect them to? The answer was yes! a few of them were indeed completely unlike the typical picture we have of leaking galaxies.

This means that yes, it does indeed look like we, when we try to be frugal and effective with telescope resources, actually do miss a part of the picture. Now the next question is: How wrong are we? Does it completely overturn Science as we know it (probably not)...?
Is it completely inconsequential and unimportant (also probably not)...?

Those questions are for future research projects to find out. For now, we have shown that there seems to be stuff out there that does not behave the way we thought it did.

By the way, here's what four of them look like after 11 days of exposure with Hubble Space Telescope. The light from all these galaxies was sent out long before our own Solar System had even started to form! Even with the world's most powerful telescope, these galaxies are only small, fuzzy blobs. It's pretty cool that we can still learn so much about them.

/End 🧵

@thriveth 🎼 We ought to thank Princess @Luna for making the #night so fascinating (and so she doesn’t snap and attempt to put Equestria into permanent darkness again) 🦄

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