Doubling NIRSpec/IFS capability to calibrate the single epoch black hole mass relation at high redshift

Eleonora Parlanti, Bartolomeo Trefoloni, Stefano Carniani, Francesco D'Eugenio, Michele Perna, Giulia Tozzi, Hannah Übler, Giacomo Venturi, Sandra Zamora

First listed 2025-12-16 | Last updated 2025-12-16

Abstract

The recent discovery of a large population of overmassive black holes (BHs) in the early Universe challenges the validity of the BH-host galaxy coevolution framework. However, the reliability of the estimated BH masses (M$_{BH}$) is being questioned, as these are typically derived using single-epoch (SE) relations calibrated locally. Calibrating SE relations at high redshift would therefore enable more accurate M$_{BH}$ estimates and help identify potential biases. In this work, we release a data-reduction technique for JWST/NIRSpec IFU observations that doubles the effective wavelength coverage, enabling detection of otherwise inaccessible emission. Whenever adjacent dispersers are required, observers should carefully evaluate the tradeoff between integrating longer in the bluer configuration alone versus distributing the exposure time across two dispersers. We apply this pipeline to a sample of 5 quasars at z~2 with M$_{BH}$ independently measured through reverberation mapping (RM). This enables a joint analysis of both H$β$ and H$α$; the latter lying beyond the nominal wavelength range. We assess the reliability of the most widely adopted SE calibrations, finding that H$β$ yields the closest agreement with RM-based M$_{BH}$ estimates, whereas H$α$-based estimators exhibit a larger scatter. For the least massive BH in our sample ($M_{BH,RM}$~$10^{7.5}M_\odot$), which is accreting at a rate close to the Eddington limit ($λ_{Edd}=0.8$), all SE calibrators overestimate M$_{\rm BH, RM}$ by one order of magnitude. This may indicate a systematic overestimation of M$_{BH}$ for highly accreting BHs at high redshift. Finally, we provide the first high-redshift SE calibration based on H$α$ and H$β$. Although a larger sample is needed to reduce the uncertainties, our calibration can already be applied to the newly discovered BH population in the early Universe.

Short digest

Presents a NIRSpec/IFU reduction that exploits the red signal on NRS2 for medium‑resolution gratings, effectively doubling the usable wavelength coverage and recovering Hα beyond the nominal range; the authors also note the exposure‑time trade‑off when using adjacent dispersers. Applied to five z≈2 quasars with reverberation‑mapped MBH, the extended cubes enable joint Hβ–Hα measurements at R≈1000. Hβ‑based single‑epoch masses best match RM, while Hα recipes show ≈0.5 dex larger scatter; for the least‑massive, high‑λEdd object (MBH,RM≈10^7.5 M⊙, λEdd=0.8) all SE estimators overpredict by ~1 dex, suggesting a bias at high accretion. The work delivers the first high‑z SE calibrations from Hβ and Hα for the newfound early‑BH population, with uncertainties still limited by sample size.

Key figures to inspect

• Figure 1 (RM332 G140M/F100LP): Verify that emission falls on NRS2 beyond the nominal cutoff and that Hα is detected alongside the [O III]–Hβ complex; use the insets to gauge S/N and how far past the red limit the recovery extends.
• Figure 2 (P330‑E calibrator): Check flux consistency of the extended G140M/F100LP response across cycles and field positions; this validates the throughput stability needed for reliable Hα fluxes in the extended region.
• Figure 3 (wavelength‑dependent corrections): Inspect the G140M/F100LP and G235M/F170LP correction curves used to stitch the extended coverage; the smoothness and rise at long wavelengths indicate where the extended tail remains photometrically trustworthy.
• Figure 4 (flux ratio test): Compare nominal‑range fluxes to those from the extended filter to quantify systematics; look for departures from unity toward the red end that set the error budget for SE mass scaling based on Hα.