A 330 MHz pilot survey of the inner Galactic fourth quadrant (about 337°<l<354°, b≈0.37°) with the Giant Metrewave Radio Telescope (GMRT), by Roy & Pal, reached a noise level of 6–10 mJy per beam — a factor 2–3 worse than planned — and so could not test the prediction, derived from the longitude asymmetry of the Galactic supernova-remnant catalogue, that roughly 23 remnants remain undiscovered in that quadrant. New wide-field radio surveys — the MWA GLEAM survey, the ASKAP EMU/POSSUM surveys, and above all the SARAO MeerKAT Galactic Plane Survey, whose 237 new supernova-remnant candidates are 72% concentrated in the fourth quadrant — have supplied the candidate population needed to test that prediction. The objective is to classify these candidates as non-thermal supernova remnants or thermal H ii regions, to quantify the H ii-region contamination of the sample, and to convert the resulting census, combined with an explicit completeness model, into a verdict on the predicted deficit.

Background

A supernova remnant (SNR) is the expanding shell of shock-heated gas left by a stellar explosion, observable for tens of thousands of years. At radio wavelengths SNR shells emit synchrotron radiation — radiation from relativistic electrons spiraling in amplified magnetic fields — whose flux density falls with frequency as a power law S∝ναS\propto\nu^{\alpha} with spectral index α≈−0.5\alpha\approx-0.5. The main source of confusion in the Galactic plane is the H ii region, a cloud of ionized hydrogen around hot young stars that emits thermal (free–free) radiation with a nearly flat spectrum (α≈0\alpha\approx0) and is bright in mid-infrared dust lines; H ii regions can mimic compact or shell-type SNR morphologies, and at low frequencies their own free–free absorption can even flatten an overlapping non-thermal spectrum. The Giant Metrewave Radio Telescope (GMRT) is a low-frequency radio interferometer in India; the pilot survey discussed here used its 330 MHz band, where sensitivity to faint, steep-spectrum shells is high but Galactic background and confusion are severe. About 300 Galactic SNRs are catalogued (the reference compilation is D. A. Green's online Galactic SNR catalogue), against order-of-magnitude larger estimates of the expected population from the Galactic supernova rate — the long-standing 'missing SNR problem', usually attributed to selection effects against old, faint, large remnants and small-diameter remnants in confused, high-background regions. Within the 2009 version of Green's catalogue, 65 remnants lay in the first Galactic quadrant (0∘<l<20∘0^{\circ}<l<20^{\circ}) but only 42 in the mirrored fourth-quadrant strip (340∘<l<360∘340^{\circ}<l<360^{\circ}). Because the true supernova rate and remnant-detectability timescale cannot plausibly differ between the two quadrants, and because a sensitive 330 MHz survey of the first quadrant with the Very Large Array (VLA) had recently added 35 new remnants there, Roy & Pal argued that observations of the inner fourth quadrant at comparable depth should reveal of order 23 currently missing remnants concentrated in that strip. Their GMRT pilot covered 337∘≲l≲354∘337^{\circ}\lesssim l\lesssim354^{\circ} but attained an rms noise of 6–10 mJy per beam (2–3 times worse than planned), detected the 9 previously known SNRs in the strip, and was therefore too shallow to test the prediction; they also noted that roughly ten H ii regions in the same strip can mimic shell-type SNRs, so a systematic count and classification of thermal sources is a prerequisite for interpreting any discovery statistics. Since then, wide-field radio surveys have transformed the data situation: the Murchison Widefield Array (MWA) GaLactic and Extragalactic All-sky MWA (GLEAM) survey and its GLEAM-X extension imaged the southern plane at 72–231 MHz (20 sub-bands), ideal for separating thermal from non-thermal spectra; the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) and Polarization Sky Survey of the Universe's Magnetism (POSSUM) surveys provide 1 GHz-band imaging and polarization; and the South African Radio Astronomy Observatory (SARAO) MeerKAT Galactic Plane Survey (SMGPS) imaged 251∘≤l≤358∘251^{\circ}\le l\le358^{\circ} at 1.3 GHz with arcsecond resolution, using a mid-infrared H ii-region catalogue as a thermal-exclusion mask.

Problem Statement

Determine whether the predicted missing supernova remnants of the inner Galactic fourth quadrant exist below the surface-brightness and confusion limits of earlier surveys, by completing a classification-and-census program whose single overarching objective is a completeness-corrected verdict on the ~23-remnant deficit. Concretely, an answer must: (1) classify the fourth-quadrant candidate shell population — above all the 237 SARAO MeerKAT Galactic Plane Survey candidates of Anderson et al. (2025), together with the EMU/POSSUM and GLEAM candidate lists in the quadrant — as non-thermal supernova remnants or thermal H ii regions, on the basis of non-thermal radio spectral indices (e.g. from matched GLEAM-X 72–231 MHz and SMGPS 1.3 GHz measurements) and/or linear polarization; (2) quantify the H ii-region contamination of the candidate sample, delivering the systematic thermal-source accounting that the source authors identified as a prerequisite for interpreting discovery statistics; and (3) combine the confirmed census with an explicit selection/completeness model to state, with uncertainties and at stated surface-brightness and angular-size limits, whether the predicted deficit of roughly 23 remnants — derived for the mirrored strip 340∘<l<360∘340^{\circ}<l<360^{\circ} — is confirmed, partially confirmed, or refuted over the longitude range adopted for the census. These three results are jointly necessary: classification without the completeness model cannot test the prediction, and a completeness model without resolved classification of the candidates has no input data.

References

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