SMBH-galaxy Coevolution at Cosmic Noon and beyond with GRAVITY+

Daryl Joe Santos 1 , Yixian Cao 1 , Richard Davies 1 , Frank Eisenhauer 1 , Reinhard Genzel 1 , Dieter Lutz 1 , Jinyi Shangguan 1 , Taro Shimizu 1 , Eckhard Sturm 1 , + GRAVITY Collaboration 

  • 1 Max Planck Institute for Extraterrestrial Physics, Garching b. München

Abstract

With the GRAVITY instrument, the beam combiner at the near-infrared (NIR) Very Large Telescope Interferometer (VLTI), the structure of the broad (emission-)line region (BLR) in active galactic nuclei (AGNs) can be spatially resolved. This allows the central black hole (BH) mass to be directly measured through dynamical modelling. With GRAVITY+, the ongoing upgrade to VLTI and GRAVITY, we can now push from the local universe to high redshift and probe black hole growth more precisely than ever before, providing a unique opportunity to examine BH-galaxy coevolution especially at cosmic noon where the star formation and BH accretion histories peaked.

In this talk, I will present our recent efforts in measuring BH masses at high redshift thanks to the advent of GRAVITY+. One of these is the sample of five quasars at cosmic noon observed with GRAVITY+, including the first dynamical mass measurement of a BH at z~2 which was shown to possess a host galaxy that evolved faster than the central BH. Complementary observations with NOEMA/ALMA allow us to examine the five z~2 targets in the dynamical mass – BH mass parameter space. I will also present our first z~4 quasar observed with GRAVITY+, which shows significant amounts of radial flows within the BLR, and discuss how this affects measurements of the black hole mass.

Spectroastrometry with GRAVITY

Fig. 1. An animation showing how the differential phase is related to the photocentre shift of the broad-line region (BLR) emission. The equation on the upper right corner of the figure shows how the differential phase is calculated as a function of line strength, baseline projection, and photocentre offset.

GRAVITY and differential phase

  1. The GRAVITY instrument operates in the K-band (near-infrared) and combines the light of the four unit telescopes (UTs) at the Very Large Telescope Interferometer (VLTI) to conduct interferometric observations.
  2. The differential phase (Δφ) measures the photocentre shift of the broad-line emission on the sky at different wavelength channels with respect to the continuum emission.
  3. A rotating structure normally exhibits an S-shaped differential phase signal that is also centred on the observed central wavelength of the emission line.

GRAVITY+

  1. GRAVITY+ is comprised of a series of upgrades on GRAVITY and VLTI (Fig 2a), which will greatly improve the sensitivity and sky coverage of GRAVITY (Fig 2b).
  2. With GRAVITY+, we expect to extend our work from observing several tens of low-redshift AGNs up to hundreds of faint, high-redshift AGNs, especially at cosmic noon (1 < z < 3), when the star formation and black hole accretion histories have peaked (Madau & Dickinson 2014).
  3. This will allow us to probe the redshift evolution of AGN scaling relations and the overall picture of SMBH-galaxy coevolution across cosmic time.

Fig 2. (a) A schematic diagram showing the different improvements in VLTI/GRAVITY to produce GRAVITY+; (b) A scatterplot showing the BLR differential phase signal in degrees of several quasars that could be observed with GRAVITY as a function of their K-band magnitude. Different colours pertain to different Balmer lines that can be observed with GRAVITY, which operates in the K-band. The sensitivity curves of GRAVITY and GRAVITY+ are shown as grey and blue curves, respectively, while their fringe tracking limits are shown as grey and blue vertical lines, respectively. The plot highlights the improvement in the sensitivity of GRAVITY+, allowing fringe tracking of stars as faint as K ~ 12.5 mag and observations of AGNs as faint as K ~ 18 mag. SDSS J0920, the first z~2 quasar observed with GRAVITY, is highlighted as a red star. Taken from Abuter+22.

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The first z~4 quasar observed with GRAVITY+

  1. The recent implementation of the GRAVITY+ Adaptive Optics (GPAO) over the outdated Multi-Application Curvature Adaptive Optics (MACAO) now provides sharper images of the science targets.
  2. J0529, a z~4 quasar which was found to possess the fastest-growing black hole and is the luminous quasar known to date (Wolf+24), was observed with both GRAVITY-Wide and GPAO (Fig. 5b).
  3. Our preliminary results from modelling the interferometric signal of J0529 show that its BLR has a very asymmetric emission distribution that is dominated by outflowing motion (Fig. 5c).
  4. We were also able to capture the differential phase signals of both the Hβ and Hγ of J0529, which are found to be both asymmetric and almost Gaussian-like, indicative of a non-Keplerian dominated BLR.
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Conclusions

  1. GRAVITY(+), with its unprecedented sensitivity and sky coverage, is now able to peer into the cosmic noon and challenge our understanding of the SMBH-galaxy coevolution scenario through dynamical mass measurements of SMBHs.
  2. The first dynamical mass measurement of a BH in a quasar situated at a lookback time of 11 billion years (z~2) has also revealed its BH to be undermassive and highly accreting, and its host galaxy to have grown much faster than its BH.
  3. Our efforts have increased the number of GRAVITY-Wide-observed quasars at cosmic noon to five, and these targets have been shown to possess smaller BLR sizes than expected from the canonical R-L relation at low redshift. We envision further increasing our sample size in the next few months of GRAVITY-Wide operation.
  4. The recent implementation of GPAO has also greatly expanded the limits of GRAVITY to redshifts beyond cosmic noon, allowing the first observation of a z~4 quasar with GPAO and GRAVITY-Wide.

References

  1. Greene, J. E., Strader, J. and Ho, L.C., 2020, ARAA , 58(1), 257-312
  2. Bentz, M.C. et al 2013, ApJ, 767(2), 149
  3. Abuter, R. et al, Messenger, 189, 17
  4. GRAVITY Collaboration et al. 2024, A&A, 684, A167.
  5. Abuter, R. et al 2024, Nature, 627(8003), 281-285
Email for queries

dsantos(at)mpe(dot)mpg(dot)de

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Observing cosmic noon AGNs with GRAVITY+

J0920: the first z~2 quasar observed with GRAVITY-Wide

  1. The first dynamical SMBH mass measurement of a z~2 quasar, J0920 (Fig. 3b), was made possible with the wide-field, off-axis fringe-tracking mode of GRAVITY-Wide (GRAVITY+ Collab.+22; Abuter+24; Fig. 3a).
  2. We have spatially resolved the BLR of J0920 as shown by its clearly separated red and blue channels after photocentre reconstruction (3-6σ significance). Its differential phase signal also possesses an S-shape, suggesting its BLR is Keplerian-dominated (Fig. 5c)
  3. Our observations reveal J0920 to host an undermassive BH accreting at a super-Eddington rate, and a host galaxy that grew faster than its SMBH (Fig. 3d).

Fig. 3 (a) A cartoon depicting how GRAVITY-Wide works: with the on-axis mode of GRAVITY, the science target has to be within 2 arcseconds of the fringe-tracker star. With GRAVITY-Wide, dual-field observations allow this separation to be extended up to a maximum of 30 arcseconds. The sensitivity of GRAVITY-Wide also allows observations of fainter fringe-tracking stars (from K < 10 to K < 13). (b) The properties of J0920, the first z~2 quasar observed with GRAVITY-Wide, are shown, together with its redshift, total on-source time, and BH mass derived from GRAVITY-Wide observations and single-epoch spectroscopy of CIV and Hα lines. (c) GRAVITY-Wide was able to resolve the BLR of J0920 as depicted by the clear separation of the red and blue channels of the BLR after photocentre reconstruction from its differential phase (3-6σ significance). The differential phase of J0920 shows an S-shape signal that is typical of a Keplerian-dominated BLR. (d) Our GRAVITY-Wide and parallel NOEMA observations of J0920 reveal its super-Eddington accreting but undermassive BH that is more typical of its lower luminosity counterparts. This suggests that its host galaxy grew faster than its SMBH.

Four new z~2 AGNs observed with GRAVITY-Wide

  1. Four new z~2 quasars were observed with GRAVITY-Wide and were analysed this year. One of them shows an asymmetric differential phase signal, which suggests their non-Keplerian nature.
  2. On average, the five z~2 quasars, including J0920, possess smaller BLR sizes than expected from the canonical R-L relation (Bentz+13). However, it is still not clear whether it is the high luminosity or the high redshift of these sources that causes such small BLR sizes.
  3. Potential explanations for these small BLR sizes include the potential role of accretion rates on the BLR sizes (e.g. Du+18), or the assumption that the ionizing spectra of all AGNs is actually false (e.g. GRAVITY Collab.+24).
  4. In the MBH-M* relation, the four new z~2 AGNs lie in the same region as the JWST AGNs, which are found to possess overmassive SMBHs (or undermassive host galaxies). Interestingly, J0920 is an outlier, possessing an undermassive SMBH compared to the other four z~2 GRAVITY-Wide AGNs.

Fig. 4 (a) The flux and differential phase spectra of two of the four new z~2 quasars observed with GRAVITY-Wide. Their differential phase spectra have asymmetric shapes, indicating that their BLRs have significant radial flows and/or asymmetric emission distributions. (b) After measuring the BLR sizes from dynamical modelling of their interferometric signals, the five GRAVITY-Wide z~2 AGNs (including J0920) show smaller BLR sizes than what the canonical R-L relation (Bentz+13) predicts. This coincides with the high-luminosity low-redshift GRAVITY AGNs, which also exhibit smaller BLR sizes (GRAVITY Collab.+24). (c) The five z~2 GRAVITY-Wide AGNs (shown as purple stars) are plotted on the MBH-M* parameter space, with the other high-redshift quasars observed by JWST. Interestingly, all GRAVITY-Wide AGNs except J0920 are located in a similar region as those of JWST AGNs, indicating that we are also finding overmassive BH masses (or undermassive host galaxies) at higher redshift.

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SMBH-galaxy coevolution

Fig. 1. Different panels show different scaling relations related to SMBH-galaxy coevolution. (a) The M-σ relation of early-type, late-type, and AGNs taken from Greene+2020. (b) The canonical R-L relation of AGNs from Bentz+13, which shows the best-fit line to have a slope of ~0.5.

Supermassive black holes (SMBHs)

  1. Supermassive black hole (SMBH) masses are important quantities to reveal clues in the coevolution scenario between the SMBHs and their host galaxies (ex: BH mass-stellar velocity dispersion or M-σ relation; Fig. 1a)
  2. However, accurately measuring SMBH masses requires one to resolve the SMBH's sphere of influence, where the central BH's gravitational potential dominates
  3. The broad-line region (BLR) is within this sphere of influence; thus, understanding the physics of the BLR will provide clues to more accurate SMBH mass measurements
  4. One could estimate the BLR size from single-epoch spectroscopy via the radius-luminosity (R-L) relation. However, its validity at higher redshift and higher luminosity remains an open question.

Key open questions about SMBH-galaxy coevolution

  1. How and when were the local scaling relations built up?
  2. What physical processes drive such coevolution?
  3. Which came first: the SMBH or the host galaxy?
  4. What are the seeds of SMBHs?
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