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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/galaxy. 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.

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.

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/galaxy. 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.

I have never come across the term "gastrophysics" in astrophysics. I am aware it has been used to describe the physics of cookery.

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/galaxy. 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.

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".

I have never come across the term "gastrophysics" in astrophysics. I am aware it has been used to describe the physics of cookery.

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/galaxy. 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.

I have never come across the term "gastrophysics" in astrophysics. I am aware it has been used to describe the physics of cookery.