I just came across a New York Times article talking about a newly found Low Surface Brightness (LSB) galaxy (also called “ultra-diffuse galaxies” or “dark galaxies”). The new galaxy, J0613+52, was accidentally discovered using the Green Bank Observatory. To be sure, I verified the story via a Space.com article, a page on the Green Bank Observatory’s website, and a slideshow on the American Astronomical Society’s website. What makes this galaxy unique from other LSB galaxies is that there is not a single star visible in the ENTIRE galaxy, meaning this could just be an ENORMOUS, two billion solar masses cloud of hydrogen gas spiraling around in space, which would further suggest that this could be a “primordial galaxy”, or one that formed just after the Big Bang, before the stellar medium was enriched with heavier elements. So far, the galaxy has only been imaged in radio waves, but more studies are to follow. The researchers were careful to note that it is possible there are stars in the galaxy, but we are just unable to observe them through the thick clouds of hydrogen.

So, this led me to my question: How can we even “see” J0613+52? While I am aware that clouds of hydrogen gas are OFTEN imaged in astronomy, I had always presumed the way this stuff was imaged was either by:

A) The gas is illuminated by nearby objects or objects behind the gas cloud. As I understand, atoms can absorb a photon, which excites the atom, but when the atom returns to its ground state, it emits the same wavelength of light, which is then visible to our detectors.

— OR — by:

B) Its gravitational effects (perturbations) on surrounding objects or, light that comes close to the area, a.k.a. lensing.

The problem, or at least what appears to be a problem is that there are no known galaxies within 112 Mpc, or over 365 million light years from the diffuse cloud. Further, according to the articles, nearby galaxies could trigger stellar formation, but this galaxy is just so far away from anything else. If it is a primordial galaxy, would this not also imply that it has remained distant from other galaxies throughout its entire lifespan, because at some point if it were within the gravitational effects of another galaxy, some stars would have formed?

Now, to be clear, this doesn’t mean that the hydrogen gas cloud is not absorbing and emitting ANY light, as space is awash in light from distant objects, its distance to other galaxies would simply mean it receives a lot less light. Also, considering they got the mass of the cloud, I presume this means some gravitational effect of some kind was measured regarding the galaxy? So, have I answered my own question, is it just minuscule light from other astronomical bodies that are illuminating the galaxy, or did I get something wrong?

I considered the possibility that some of the hydrogen is radioactive, so maybe that is what we are seeing. However, after working with ChatGPT and a half-life calculator, I am convinced this is not the case. According to ChatGPT, there are (approximately?/exactly?) 2,379,999,999,999,999,761,365,273,050,982,686,751,747,396,855,793,718,681,448,271,052,800 (2.38×10^66) hydrogen atoms in 2 billion solar masses of hydrogen gas. Tritium has the longest half-life of any non-stable hydrogen isotope at 12.32 years. Then, according to [Wikipedia, the first galaxies started to form 200 million to 500 million years after the Big Bang, and the age of the Universe is about 13.7 billion years.

I took this info and used this OmniCalculator for half-life. For the initial quantity, I put the 2.38×10^66 number. I understand that only a small percentage of the original primordial hydrogen would have been tritium, but as we shall see, this does not matter. Next, I put the half-life time as 4,496.8 days. Finally, I (under)estimated the total time in the calculator at 11.2 billion years. While I believe this would be pretty late for a “primordial galaxy” (around 2.5 billion years ABB), and I technically think the time should start when hydrogen formed after the Big Bang, as we'll see, none of this matters. Given these inputs, the number of remaining radioactive hydrogen atoms is 0. In fact, according to the calculator, even if 100% of the hydrogen gas is tritium, there would be ZERO tritium left over after only 3,341 years. So I am convinced radioactivity is NOT the answer.

So, to be clear, the question is “How do we observe J0613+52?” Again, I understand we are using radio astronomy, but where are these radio waves coming from that our detectors are picking up? What is emitting them? I appreciate any help anyone could give.

-------------------------SORT OF UNRELATED AND UNIMPORTANT SIDE QUESTION------------------------

I don't think this deserves its own question, but if you believe it does, let me know and I will make a new question for this. The NYT article linked above contains the following sentence:

Many astrophysicists attribute this discrepancy to their inability to model complex, messy phenomena like shock waves and magnetic fields — so-called gastrophysics — that prevail when atoms get close together.

I have gone through the first two pages of Google when searching for “gastrophysics”, and without exception, every single one of them says this refers to an interdisciplinary approach to gastronomy and cooking. Can anyone find anything talking about gastrophysics in relation to astronomy?


1 Answer 1


The low surface brightness survey at the GBT is looking for H(I) emission, i.e. emission from neutral hydrogen atoms (for example see O'Neil 2023).

The most obvious signature they use is the 21 cm hydrogen line, which arises from "hyperfine" transitions in atoms where the proton and electron spins are aligned, "flipping" to become anti aligned. This happens spontaneously via a magnetic dipole transition with a very long half life (about 10 million years). This means you need a lot of hydrogen to see it, but it produces a spectral line that is very narrow and hence easy to pick out in any spectrum.

The line is fairly easy to thermally "excite" since the difference in energy levels corresponds to just 6 $\mu$eV - so the excited state is plentiful in essentially all atomic hydrogen gas with temperature $>1 $K. It turns out that actually there is usually three times as much hydrogen in the excited state, simply because there are actually three combinations of quantum numbers that can describe the aligned state but only one combination to describe the anti-aligned "ground state".

Because the transition has a long lifetime, it is also very hard to absorb 21 cm photons. That means a cloud of hydrogen is essentially transparent to its own 21 cm emission and that the amount of 21 cm emission can be used directly to estimate the number of hydrogen atoms and hence baryonic mass of a cloud of neutral hydrogen. The fact that the line is sharply defined in frequency means that Doppler shifts can be readily measured and, in this case, used to measure the rotation of the cloud, estimate it's total mass from gravitational considerations and hence infer the amount of dark matter.

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    $\begingroup$ I didn't have the knowledge to answer the primary question. i do have the google fu to answer the side question. References to that word are easy to find if one gives google a helping hand. Here are four such references: 1, 2, 3, 4. There are several others. $\endgroup$ Commented Jan 28 at 13:02
  • $\begingroup$ For a 21 cm radio image of an ordinary galaxy, see nrao.edu/pr/2001/m33gas $\endgroup$
    – John Doty
    Commented Jan 29 at 1:03
  • $\begingroup$ baryonic mass of a cloud/galaxy - For a galaxy with stars, it wouldn't be counting mass of hydrogen inside stars, right? Because they'd be plasma, not atoms with electrons. Are we just talking about "dark galaxies" here, or is most of the hydrogen in a typical galaxy not in stars? (Or you meant to write something else, or I'm missing something...) $\endgroup$ Commented Jan 29 at 18:52
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    $\begingroup$ @PeterCordes indeed. Only the baryonic mass in the form of neutral hydrogen. $\endgroup$
    – ProfRob
    Commented Jan 29 at 18:57

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