An observational test of the plasma lensing effect using QSOs with and without MgII absorption

TL;DR

The paper tests whether plasma lensing by foreground ionized gas leaves detectable imprints on QSO flux distributions. It assembles a large, multi-wavelength sample of QSOs from DESI DR1 with MgII absorbers and non-absorbers, cross-matched to VLASS radio data and WISE infrared photometry, and constructs control samples matched in redshift and luminosity to isolate propagation effects. The analysis reveals a modest bright-end excess in radio counts for MgII and luminosity-matched controls, which largely diminishes under stringent joint matching, while optical g-band shows a faint-end difference; these patterns suggest dust extinction could contribute and that plasma lensing, if present, is weak in this dataset. Consequently, plasma lensing is unlikely to be the dominant mechanism shaping the observed differences, though the multi-wavelength approach provides a valuable framework for probing intervening plasma and motivates low-frequency follow-up and direct image-based tests to reach a definitive conclusion.

Abstract

Radio wave propagation can be perturbed by compact ionized gas clumps through plasma lensing, which induces frequency dependent magnification and may distort the observed number counts of background sources. The quasar (QSO) number densities are a powerful probe for understanding the effects of intervening material. Absorption lines in QSO spectra reveal the presence of interstellar and intergalactic gas, which can change observed fluxes through dust extinction and plasma lensing. By combining observations from radio (VLASS), infrared (WISE), and optical bands (DESI), we assembled a sample of QSOs: ~4000 sources with MgII absorbers, and ~12, 000 non-absorbers. In the radio band, the MgII sample shows a moderate excess at the bright end of the flux distribution, which is broadly consistent with plasma lensing predications. In the optical, the MgII sample turns over at higher g-band fluxes and exhibits a steeper decline at the faint end than the non-MgII sample. Control samples were constructed by matching in redshift, infrared (W1), and optical (g) luminosities. In these comparisons, the radio excess becomes less prominent, suggesting that the apparent magnification may not be robust evidence for plasma lensing. Nevertheless, a weak contribution cannot be ruled out, especially given residual excess observed at the bright end relative to the non-MgII sample. Dust extinction along the line-of-sight remains a plausible alternative. Regardless of the dominant mechanism, the multi-wavelength differences offer a valuable probe of the physical state of the intervening medium.

Figures (8)

• Figure 1: Redshift distributions of the QSO samples used in this study. The blue histograms in the three panels show sources contains MgII absorption lines; and the red histograms show non-MgII (control) samples. Vertical lines in the left panel present the median redshifts of the initial MgII and non-MgII samples. In the middle and right panels, the shaded histogram represents the initial MgII sample before matching.
• Figure 2: The heat map of redshift vs flux in radio (left), redshift vs infrared Vega-magnitude (middle) and redshift vs optical $g$-band AB-magnitude (right). From top to bottom are non-MgII, MgII, and control-W1 samples respectively. The horizontal red lines in the middle panels show the cut in W1 magnitude (17.93). The colour bars represent the numbers of sources in each bin.
• Figure 3: Histograms of radio flux (top), infrared W1 (middle), and optical $g-$band (bottom) for the three samples. Open circles indicate flux bins with fewer than 25 sources. The analytical curves (cyan) follow a broken power law with indices $a=1.2$, $b=2.3$ (top), $a=1$, $b=2.8$ (middle), $a=0.4$, $b=2.4$ (bottom) respectively. A modest excess at the bright end can be seen in radio band for both the MgII-W1 and control-W1 samples.
• Figure 4: Similar as Fig. \ref{['fig:radio-flux']} for only radio flux using control-W1-g sample. The bright-end excess is less pronounced compared to the MgII-W1 and control-W1 samples.
• Figure 5: Similar as Fig. \ref{['fig:radio-flux']} for z-band flux.