What fraction of Galactic forbidden-velocity H I 21-cm wings (FVWs) are old, radio-faded supernova remnants?

Thousands of supernova remnants (SNRs) are expected to exist in the Milky Way, yet only a few hundred are cataloged, and the oldest remnants — those whose shells have cooled, recombined, and faded from radio and X-ray visibility — are systematically missing from the census. Koo, Kang & Salter (2006) identified one candidate survivor, FVW 190.2+1.1, as a rapidly expanding ($\sim 80\ \mathrm{km\,s^{-1}}$) neutral-hydrogen (H I) shell consistent with a supernova that exploded in the outermost Galaxy roughly $3\times10^{5}$ years ago, and proposed that many of the 87 'forbidden-velocity wings' (FVWs) cataloged by Kang & Koo (2007) are such missing old SNRs. Two decades later the proposal remains untested at population level: only one further FVW has been studied in detail, neither object's SNR nature is confirmed, and the relation between FVWs and the small, fast-moving H I clouds of the Galactic disk and halo is unknown. The problem is to determine the physical nature of the FVW population — resolving the distance, mass, and origin of FVW 190.2+1.1, quantifying the fraction of FVWs that are bona fide old SNR shells, and establishing whether the fast-moving clouds are disrupted SNR-shell fragments — thereby measuring how much of the missing old-SNR population the FVWs account for.

Background

A supernova remnant (SNR) is the expanding shell of shocked interstellar gas left by a stellar explosion. If the canonical Galactic supernova rate of a few per century has held for millions of years, the Milky Way should host thousands of remnants, yet only about 230 were known when Koo, Kang & Salter (2006) wrote, and a modern reanalysis by Ranasinghe & Leahy (2022) still finds the catalog of a few hundred far from complete, with a best-estimate total of roughly 2,400–5,600 SNRs. The deficit is worst for old remnants: after $10^{5}$–$10^{6}$ years the shock slows, the swept-up shell cools and becomes neutral, synchrotron radio emission fades, and the remnant becomes essentially invisible at every wavelength except the 21-cm hyperfine line of neutral atomic hydrogen (H I). The H I 21-cm line is observable only against bright Galactic emission, so an expanding shell is detectable only when its expansion velocity carries its emission beyond the range permitted by Galactic differential rotation — the velocity range that is 'forbidden' for gas in circular orbits (velocities quoted throughout are relative to the local standard of rest, LSR). Systematic searches for such high-velocity H I wings toward cataloged SNRs had found them toward only about two dozen of some two hundred targets (summarized by Koo & Kang 2004), precisely because those searches targeted remnants still bright in radio continuum, i.e., not the oldest ones.

Building on this, Koo & Kang (2004) proposed that small, faint, high-velocity H I features in large-scale Galactic plane surveys could be the missing oldest SNRs. Kang & Koo (2007) cataloged 87 such 'forbidden-velocity wings' (FVWs) — faint, wing-like emission protrusions, typically under $2^{\circ}$ across, extending more than $\sim 20\ \mathrm{km\,s^{-1}}$ beyond the velocities allowed by Galactic rotation in longitude–velocity diagrams — and found that 85% coincide with no known SNR, galaxy, or high-velocity cloud (an H I cloud moving at velocities incompatible with Galactic rotation), so their nature was unknown. Koo, Kang & Salter (2006) then mapped the first candidate, FVW 190.2+1.1 (also designated G190.2+1.1), at high resolution with the Arecibo telescope. It proved to be an approximately $1^{\circ}$-wide shell expanding at $77\pm6\ \mathrm{km\,s^{-1}}$. In this direction (Galactic anticenter), the LSR velocity of gas in circular rotation varies only weakly with distance, so the systemic velocity gives no useful kinematic distance; the distance $d\approx8$ kpc was inferred only by assuming a standard supernova energy of $10^{51}$ erg in a Sedov–Taylor blast-wave relation for radiative shells (the standard self-similar blast-wave solution linking explosion energy, shell radius, and expansion velocity), giving a radius of $\sim88$ pc, a dynamical age of $\sim3.4\times10^{5}$ yr, a total H I mass of $6.5\times10^{3}d_{10}^{2}$ solar masses (where $d_{10}$ is the distance in units of 10 kpc), and a kinetic energy of $5.4\times10^{50}d_{10}^{2}$ erg. The observed mass required a large extrapolation into velocity ranges buried under Galactic background emission, the receding end-cap of the shell was not detected at all, no radio continuum, infrared, optical, or X-ray counterpart exists, and the supernova interpretation rested on eliminating alternatives — a stellar wind would require a star more luminous than the only nearby O star, and the known high-velocity cloud collision in the region produces gas at the wrong (negative) velocities — rather than on positive confirmation.

The question connects to a second population. High-resolution H I surveys reveal small ($\sim10$ pc), fast-moving atomic clouds in the Galactic halo and at forbidden velocities in the disk (Lockman 2002; Stil et al. 2006), and the GALFA-H I compact cloud catalog of Saul et al. (2012) (GALFA-H I is the Arecibo Galactic H I survey) lists nearly two thousand such clouds, apparently a mix of clouds co-rotating with the disk and clouds that do not. These clouds resemble FVWs in appearing at forbidden velocities and in having smaller masses at higher peculiar velocities, and Koo et al. (2006) conjectured that they might be fragments of even older, fully disrupted SNR shells. Subsequent work sharpened rather than settled the problem: Kang, Koo & Salter (2012) showed a second FVW, 172.8+1.5, is a $55\ \mathrm{km\,s^{-1}}$ expanding shell within the H II region complex G173+1.5 at a well-determined distance of 1.8 kpc, with X-ray emission inside the shell — the strongest individual case, though still described as 'most likely' a SNR; Park et al. (2016) demonstrated that the impact of a compact high-velocity cloud on the disk can itself produce a supershell at forbidden velocities, a concrete non-supernova channel; and Koo, Kim, Park & Ostriker (2020), in a review of radiative SNRs, included G190.2+1.1 and G172.8+1.5 among seven fast-expanding atomic shells whose momenta are consistent with ordinary $10^{51}$ erg supernovae, but stated explicitly that the SNR nature of these two FVW objects 'has not been confirmed'. Most recently, Li, Giacinti & Liu (2026) detected extended GeV gamma-ray emission near G172.8+1.5 whose SNR-versus-H II-region identity they note is still debated. Thus, twenty years after the discovery, neither the prototype FVW nor the population question has been resolved.

Problem Statement

Determine the physical nature of the forbidden-velocity wing (FVW) population of the Galactic H I 21-cm line — specifically, what fraction of the 87 cataloged FVWs are the fast-expanding atomic shells of old, radio-faded supernova remnants — and thereby measure how much of the missing old-supernova-remnant population the FVWs account for. The problem, as posed by the source work and still unanswered, comprises three linked parts. (1) Prototype object: establish or refute the supernova-remnant interpretation of FVW 190.2+1.1 (G190.2+1.1) by determining its distance despite the near-uselessness of kinematic distances in this anticenter direction, its total H I mass including the velocity range hidden under Galactic background emission, and its expansion energetics, and by quantitatively testing the stellar-wind and high-velocity-cloud-impact alternatives rather than eliminating them by plausibility alone. (2) Population: from systematic, high-sensitivity, high-angular-resolution H I 21-cm observations of a statistically meaningful subsample of the FVW catalog, determine which sources are fast-expanding shells with parameters (expansion velocity, mass, kinetic energy, ambient density) consistent with old supernova remnants, identify which are better explained by other mechanisms, and derive the fraction of FVWs that are bona fide old SNRs, with uncertainties and a defensible characterization of the selection function. (3) Connection to the compact-cloud population: establish whether the small, fast-moving H I clouds of the Galactic disk and halo are related to FVWs — in particular whether they are disrupted fragments of old supernova-remnant shells. A complete answer states, with uncertainties, the FVW-to-old-SNR fraction, resolves the origin of FVW 190.2+1.1 itself, and says whether the fast-moving clouds are (or are not) old SNR shell fragments.

References

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