What happens when we stop assuming what can't be assumed?
Astrophotography by GrandpaJunkrat - ----- Radio ISM & H I Structures
Cosmology from my armchair..............................
I do it metric .............................................................. Location:
bastion@protonmail.com The Netherlands
Bortle 7 - 9
Interstellar Medium and High‑Velocity Hydrogen in the Second Galactic Quadrant
This section presents my investigation of neutral hydrogen (H I) along the line of sight toward Cepheus, carried out with the SALSA 2.3‑meter radio telescopes at Onsala Space Observatory. Although SALSA was originally designed for education and outreach, its stable 21‑cm receiver and long‑integration capability make it suitable for detecting diffuse Galactic H I, intermediate‑velocity structures, and high‑velocity components.
The observations reveal a layered and dynamically complex H I environment. Multiple velocity features appear along this sightline, including broad positive‑velocity components, high‑velocity enhancements, and deep negative‑velocity dips. These structures vary significantly with Galactic longitude and latitude, indicating that the line of sight intersects several distinct H I regimes, ranging from outer‑disk gas shaped by the Galactic warp and flare to intermediate‑ and high‑velocity material in the lower halo.
All detected features have been cross‑checked against professional surveys such as EBHIS, LAB, GASS, and HI4PI. The agreement between SALSA data and these large‑scale surveys confirms that the structures are astrophysical in origin rather than instrumental artefacts. This makes the region an ideal target for a focused, one‑time mapping project aimed at understanding the multi‑layered H I architecture behind the Cepheus Flare.
The SALSA Radio Telescopes at Onsala Space Observatory
SALSA (Such A Lovely Small Antenna) consists of two identical 2.3‑meter radio telescopes operated by Onsala Space Observatory, part of Chalmers University of Technology. The system observes in the 1–2 GHz range, including the 21‑cm hydrogen line, and can be operated remotely at any time of day or night. A live webcam provides visual confirmation of telescope movement during observations.
Despite their compact size, the SALSA antennas are fully capable of detecting Galactic H I emission. Long integrations reveal broad, low‑contrast features that correspond to real interstellar structures. The telescopes were originally built to introduce students to radio astronomy by observing the spiral arms of the Milky Way, but later upgrades also enabled tracking of GNSS satellites.
For this project, SALSA is used exclusively for 21‑cm spectroscopy. The aim is not limited to a single velocity component but to chart the full range of H I behaviour along this sightline, including positive‑velocity enhancements, high‑velocity peaks, and negative‑velocity absorption‑like dips. The resulting spectra show a consistent pattern of multi‑layered gas, with features that strengthen, weaken, or disappear depending on position. The correspondence with all major H I surveys confirms the astrophysical nature of these structures and highlights the complexity of the interstellar medium in this part of the sky.
