A 45-Year-Old Mystery Solved:

The Van Horne Hydrogen Cloud

The Ohio SETI Project used a 50-channel spectral receiver, commonly referred to as the SETI Receiver, between 1975 and 1985. Although the data were originally acquired for SETI purposes, the resulting records retain valuable spectral information on Galactic neutral hydrogen (HI) clouds and other astrophysical phenomena. In 1988, volunteer observer Tom Van Horne examined archival SETI records and identified a narrowband radio source that had been detected in early 1980. Unlike a point source, the emission appeared spatially extended. Its morphology and observing frequency were consistent with emission from neutral hydrogen, suggesting the presence of a diffuse hydrogen cloud. This object became informally known as the Van Horne Hydrogen Cloud and was re-observed multiple times in 1993 as part of Big Ear's LOBES project (Figure 1). Those later observations revealed a more complex system of HI clouds, separated both in frequency and position.

One particularly intriguing aspect was that the Van Horne Hydrogen Cloud appeared in 1993 at a substantially different frequency from that inferred from the original 1980 observations. Because the observed frequency depends on the relative velocity between the observer and the source via the Doppler effect, and because corrections for the motions of the Earth and the Sun should render the intrinsic frequency stable over decades, this discrepancy was puzzling. Large hydrogen clouds, extending over light-year scales, do not undergo rapid or random accelerations toward or away from the observer.

In 2025, our Arecibo Wow! team returned to the original SETI records to reanalyze the cloud signal. The emission was recorded in one or three channels near channel 50 across multiple drift scans in the early 1980s (see Figure 2 for an example). Today, we have access to analytical tools that were unavailable 45 years ago, including all-sky maps of Galactic HI such as the HI4PI. More importantly, we identified a critical correction in the method originally used to compute the observing frequency in the archival records (Méndez, 2025).

One observation was obtained on 27 February 1980, when the second local oscillator frequency at the peak of the detection was 120.397 MHz. Using the corrected frequency reconstruction, this corresponds to a sky frequency of 1420.6576 MHz and a velocity with respect to the local standard of rest of VLSR ≈ −52 km s⁻¹. We then examined the HI distribution at the corresponding coordinates and found a bright emission not only at the expected location but also at a velocity of approximately −48 km s⁻¹, well within the uncertainties of our estimate (Figure 3). In contrast, the original analysis methods yield velocities of −134 km s⁻¹ or −155 km s⁻¹, values inconsistent with any bright Galactic HI structure in this location. Thus, once the corrected frequency reconstruction is applied, the measurements from 1980 and 1993 are entirely consistent.

Hydrogen clouds with anomalous velocities have been known since the 1950s and 1960s. Kuntz and Danly (1996) were the first to systematically classify intermediate-velocity (IV) gas into coherent, named structures. Using data from the Leiden–Dwingeloo Survey, they identified three major complexes in the northern hemisphere: the IV Arch, the IV Spur, and the LLIV Arch (Low-Latitude Intermediate-Velocity Arch). By examining modern all-sky HI surveys, we found that the Van Horne Hydrogen Cloud is one of the many cores of the IV Arch, designated IV21 by Kuntz and Danly in 1996 (Figure 4). It was likely first observed by the Big Ear telescope in 1980, with angular and spectral resolutions surpassing those of other facilities operating at the time.

We also verified the locations and velocities of several other HI clouds observed by the Big Ear telescope. Exercises like these increase our confidence not only in the reliability of the data but also in the robustness of the methodology for reconstructing source locations and velocities from archival observations of the Big Ear. These validated methods are directly relevant to the analysis of other interesting signals — most notably the Wow! Signal — which exhibits a velocity comparable to that of intermediate-velocity clouds, although no definitive astrophysical counterpart has yet been identified.

Figure 2. Example of a page of the SETI archival records from 10 March 1980 showing detections up to 7σ from the negative and positive horns of the Big Ear telescope associated with the Van Horne Hydrogen Cloud. The Ohio SETI Project produced more than 75,000 pages like this between 1975 and 1985.

Figure 3. Neutral hydrogen (HI) velocity profile at the position of the Van Horne Hydrogen Cloud (left panel) and corresponding HI brightness map (right panel). The map highlights the peak region detected by the Big Ear telescope (red box).

Figure 4. Map of the intermediate-velocity (IV) arch at velocities between -60 and -30 km s⁻¹, from Hartmann & Burton (1997). The core IV21 is the source of the Van Horne Hydrogen Cloud.