Spectroscopic AGN survey at $z$ $\sim$ 2 with NTT/SOFI for GRAVITY+ observations

D. J. D. Santos, T. Shimizu, R. Davies, Y. Cao, J. Dexter, P. T. de Zeeuw, F. Eisenhauer, N. M. Förster-Schreiber, H. Feuchtgruber, R. Genzel, S. Gillessen, L. Kuhn, D. Lutz, T. Ott, S. Rabien, J. Shangguan, E. Sturm, L. J. Tacconi

First listed 2025-03-04 | Last updated 2025-03-04

Abstract

With the advent of GRAVITY+, the upgrade to the beam combiner GRAVITY at the Very Large Telescope Interferometer (VLTI), fainter and higher redshift active galactic nuclei (AGNs) are becoming observable, opening an unprecedented opportunity to further our understanding of the cosmic coevolution of supermassive black holes and their host galaxies. To identify an initial sample of high-redshift type~1 AGNs that can be observed with GRAVITY+, we have obtained spectroscopic data with NTT/SOFI of the most promising candidates. Our goal is to measure their broad line region (BLR) fluxes and assess their physical geometries by analysing the spectral profiles of their Balmer lines. We present 29 $z$ $\sim$ 2 targets with strong H$α$ emission in the $K$-band. Their line profiles are strongly non-Gaussian, with a narrow core and broad wings. This can be explained as a combination of rotation and turbulence contributing to the total profile or two physically distinct inner and outer regions. We find small H$α$ virial factors, which we attribute to the low full-width-half-maximum (FWHM)/$σ$ ratios of their non-Gaussian profiles, noting that this can lead to discrepancies in black hole masses derived from scaling relations. We also find two targets that show tentative evidence of BLRs dominated by radial motions. Lastly, we estimate the expected differential phase signals that will be seen with GRAVITY+, which will provide guidance for the observing strategy that will be adopted.

Short digest

NTT/SOFI K-band spectroscopy vets 29 z≈2 type 1 quasars for GRAVITY+, measuring Hα fluxes and Balmer-line shapes to preview BLR kinematics. The Balmer profiles are strongly non-Gaussian with narrow cores plus broad wings, consistent with rotation plus turbulence or a two-zone BLR; two objects show tentative radial-motion dominance. Low FWHM/σ ratios yield small Hα virial factors, cautioning that single-epoch mass scalings can under-estimate MBH for these profiles. Predicted differential-phase signals guide VLTI/GRAVITY+ strategies to secure dynamical BLR masses at cosmic noon.

Key figures to inspect

• Figure 1 (line decompositions across Hα and Hβ): Inspect Fe II blends, [O III], and [N II] features and the narrow-core/broad-wing structure in objects like QBQS J051411.75-190139.4 and SDSS J220245.60-024407.1; this shows why single-Gaussian fits fail and motivates the BLR modeling used for GRAVITY+ predictions.
• Figure 2 (FWHM/σ vs FWHM): Most points lie below the Gaussian limit (2.35), aligning better with rotationally broadened Lorentzians; this plot visually links line shape to kinematics and explains the small Hα virial factors derived.
• Figure 3 (L(Hα)/L(Hβ) vs L(Hβ)): Use the Balmer luminosity ratio to check internal consistency of single vs double-Gaussian fits and to gauge reddening/geometry effects; note the removal of the two exceptions where Hβ could not match Hα’s line-shape model.
• Figure 4 (Lbol vs MBH with Eddington-ratio tracks): See where the sample sits relative to Suh+2020 and WISSH quasars and how Woo+2015 σ-based masses position many sources at moderate to high λEdd, underscoring their suitability for GRAVITY+ phase signals.