A Large Systematic Search for Close Supermassive Binary and Rapidly Recoiling Black Holes -- IV. Ultraviolet spectroscopy

Jessie C. Runnoe, Michael Eracleous, Tamara Bogdanović, Jules Halpern, Steinn Sigurðsson

First listed 2025-01-17 | Last updated 2025-01-17

Abstract

We present Hubble Space Telescope ultraviolet (UV) of 13 quasars at z<0.7, along with contemporaneous optical spectra from ground-based telescopes. The targets were selected to have broad H-beta emission lines with substantial velocity offsets relative to the rest frame of their host galaxy. By analogy to single-line spectroscopic binary stars, these objects have been regarded as supermassive black hole binary (SBHB) candidates where the offset emission lines may be caused by bulk orbital motion. The best alternative explanation is that the H-beta line profile is the result of non-axisymmetric emission from a disk-like broad-line region associated with a single supermassive black hole. We use the broad UV line profiles to discriminate between these two scenarios. We describe our methodology for isolating the broad optical and UV line profiles and the criteria we apply for comparing them. Of the 13 SBHB candidates, three have strong evidence in support of the SBHB hypothesis, five have tentative support, one is disfavored, and four have such severely absorbed UV line profiles that the results are inconclusive.

Short digest

HST UV spectra (COS/STIS) for 13 z<0.7 quasars with large H-beta velocity offsets are compared to contemporaneous optical Balmer profiles to test supermassive black hole binary (SBHB) versus single-BH disk-like BLR scenarios. The authors isolate broad components and ask whether UV resonance lines (e.g., Lyα, C IV) share the same velocity offset as Balmer lines set by [O III]. They find three objects with strong SBHB support, five tentative, one disfavored, and four inconclusive due to severely absorbed UV profiles. This cross-wavelength line-profile test tightens SBHB vetting by demanding consistent Doppler shifts across independent broad lines.

Key figures to inspect

• Figure 1: Scan the side-by-side UV/optical coverage to see whether Lyα/C IV are narrower and single-peaked compared to broad Balmer lines—behavior expected from a disk wind rather than bulk orbital motion; note heavy UV absorption cases (e.g., J153636) and geocoronal Lyα contamination flagged for J13417.
• Figure 2 (isolated profiles): Use the velocity-zero tied to [O III] to check if UV line centroids share the same offset as Hβ/Hα; matched offsets bolster the SBHB interpretation, while mismatches argue for a single BH with a non-axisymmetric disk-like BLR.
• Figure 2 (bottom panels, Lyα vs Hα overlay and blue residual): Inspect residual structure after scaling Hα to Lyα; large asymmetric residuals imply different emitting regions/transfer effects (disk wind) rather than a single bulk Doppler shift.
• Figure 2 (N V and masked regions): Verify N V λλ1238,1242 on the red side of Lyα and identify masked gray regions where absorption complicates the test—these illustrate why four sources are classified as inconclusive.