What you’re seeing is real, and it surprises a lot of people the first time they map the Galaxy in the 21-cm line.
In short: the Galactic centre is not poor in hydrogen — it’s poor in neutral atomic hydrogen that can emit the 21-cm line.
Here are the main reasons.
1. Most hydrogen there is molecular (H₂), not atomic (H I)
The central few kiloparsecs of the Milky Way are extremely dense and turbulent. Under those conditions:
- Atomic hydrogen readily converts into molecular hydrogen
- H₂ does not emit at 21 cm
- The gas instead shows up strongly in CO and other molecular lines
So the hydrogen hasn’t vanished — it’s just in the wrong phase for your receiver.
2. Strong continuum background absorbs the H I line
The Galactic centre is one of the brightest radio continuum regions in the sky:
- Synchrotron emission
- H II regions
- Sgr A* and surrounding complexes
Cold H I in front of this bright background produces H I self-absorption, which:
- Cancels out emission
- Can even produce apparent dips at the line frequency
To a single-dish or SDR system, this can look like “missing hydrogen”.
3. High temperatures weaken the 21-cm contrast
The 21-cm line depends on the spin temperature of the hydrogen. In the Galactic centre:
- Gas is hotter
- The line-to-continuum contrast is reduced
- Emission spreads over very wide velocity ranges (hundreds of km/s)
This smears the signal out, lowering peak brightness in any one channel.
4. Extreme velocity dispersion spreads the signal thin
Near the centre:
- Orbital speeds are very high
- Gas motions are chaotic (bars, shocks, inflow)
So instead of a sharp line, you get a very broad, shallow profile, which is easy to miss unless you integrate deeply.
5. Geometry works against us
We are observing:
- Through multiple spiral arms
- Through dust, ionised gas, and molecular clouds
- With velocity crowding near ℓ ≈ 0°
Many different distances map to nearly the same radial velocity, washing out structure.
How to confirm this observationally
If you want to prove to yourself that the hydrogen is really there:
- Compare your H I spectrum with CO (J=1→0) surveys
- Look for H I absorption against strong continuum sources
- Integrate over very wide velocity ranges (±250 km/s)
Professional surveys show that the Galactic centre is actually one of the most gas-rich regions in the Galaxy — just not in atomic form.