Mostly from the optically dark regions between the spiral arms, not from the bright arms themselves.
Here’s why that happens, and how it matches what you’re detecting.
1. What the 21-cm line actually traces
The hydrogen line you’re receiving comes from neutral atomic hydrogen (H I). This phase of hydrogen:
- Exists at relatively low densities
- Is not forming stars efficiently
- Fills large volumes of the Galactic disk
That makes it very different from what produces optical spiral arms.
2. Optical spiral arms are not H I-rich
The bright spiral arms stand out optically because they contain:
- Young, massive stars
- H II regions (ionised hydrogen)
- Dust lanes
- Molecular clouds (H₂)
But:
- Ionised hydrogen does not emit at 21 cm
- Molecular hydrogen does not emit at 21 cm
- Atomic H I is often reduced inside the arms because it’s being converted into H₂ or ionised by UV radiation
So the arms are visually bright but comparatively weak H I emitters.
3. Where most of your detected signal comes from
Your telescope is mainly seeing:
- Diffuse H I clouds
- Inter-arm regions
- Outer envelopes of spiral arms
- The extended Galactic disk beyond the optical edge
This gas is cold, neutral, and abundant — ideal for 21-cm emission.
4. Why dark lanes ≠ empty space
Those dark optical regions are dark because of dust absorption, not because they lack gas. In fact:
- Dust and H I are well mixed
- Many optically dark areas are H I-rich
- The Galaxy’s H I disk extends far beyond the visible Milky Way
So when you sweep across the sky, strong hydrogen detections often correlate with optically dull regions.
5. Velocity tells the real story
If you plot intensity versus radial velocity:
- Narrow components often come from nearby inter-arm gas
- Broader components trace large-scale Galactic rotation
- Spiral arms show up as velocity features, not brightness peaks
This is why 21-cm surveys map spiral structure kinematically, not visually.
6. A useful mental picture
Think of it this way:
- Optical view → highlights where stars are forming now
- 21-cm view → highlights the raw, diffuse fuel reservoir spread everywhere
They are complementary maps of very different phases of the same element.