Cosmic Duets II. Growth and accretion in Gaia Multi-Peak dual AGN at Cosmic Noon

As a fastidious and diligent AI researcher, I have meticulously analyzed both provided texts concerning the paper Cosmic Duets II: Growth and accretion in Gaia Multi-Peak dual AGN at Cosmic Noon. The first text provides a detailed observational characterization of the physical properties of Cosmic Duets, while the second text offers methodological details regarding simulation comparisons.

Here is a comprehensive, long, and detailed summary combining both excerpts to accurately reflect the scope and findings of the research:

Comprehensive Research Summary: Cosmic Duets II – Growth and Accretion in Gaia Multi-Peak Dual AGN at Cosmic Noon

This research presents a statistical characterization of the physical properties of Cosmic Duets, defined as dual Active Galactic Nuclei (AGN) residing within the same merger remnant, specifically focusing on systems with projected separations between 1 and 7 kpc. The study leverages the largest currently available homogeneous sample, consisting of 30 spectroscopically confirmed Cosmic Duets selected via the Gaia Multi-Peak technique.

Observational Findings and Characterization:

The primary observational analysis utilized rest-frame UV-to-optical integral-field spectroscopy (primarily VLT/MUSE, VLT/ERIS, and Keck/OSIRIS) to spatially resolve the two AGN components within each system. Through spectral decomposition of the continuum, emission lines, and Fe ii complexes, researchers derived key physical parameters:

• Black Hole Masses (M BH): The derived masses fall in the range of (M BH/M) about 7.4 to 10.1.

• Bolometric Luminosities (L bol): These are found to be in the range of (L bol/erg s-1) about 44.8 to 47.2, which are consistent with those of single AGN at similar redshifts.

Crucially, the study investigated the distributions of black-hole masses and Eddington ratios (lambda Edd) for both components as a function of redshift (z) and luminosity. The observed dual AGN population spans a broad range, but key findings include:

1. Luminosity Contrast: The brighter AGN in the pairs are generally more luminous than the bulk of the single SDSS AGN population, and on average, about 0.5 dex more luminous than their fainter counterparts within the main distribution.

2. Mass Comparison: Despite luminosity differences, the two components often exhibit comparable black-hole masses and strongly overlapping mass distributions.

3. Accretion Asymmetry: Significant deviations of the Eddington-ratio contrast from unity indicate asymmetric accretion onto the two black holes, suggesting that secondary SMBH growth during galaxy mergers is more efficient or symmetric than predicted by current models.

Comparison with Theoretical Models (Simulations):

The observational results were rigorously compared against predictions from state-of-the-art cosmological simulations and semi-analytical models, specifically LGalaxiesBH and Astrid.

• Astrid: Provided a closer match to the observed black-hole mass-ratio distribution, although its selected systems are predominantly at lower redshifts, leading to lower bolometric luminosities for a fixed G-band magnitude.

• LGalaxiesBH: Perfectly reproduced the observed parameter space in terms of overall distributions. However, it predicts more heterogeneous pairs characterized by:

• Higher primary-bolometric luminosity.

• Lower-mass secondary SMBHs.

• Higher Eddington ratios, resulting in an excess of near-Eddington accreting AGN due to its adopted accretion prescription.

The overarching conclusion from the comparison is that observations favor a more balanced distribution of black hole masses and accretion power between the two members of each dual AGN system, whereas current models (like LGalaxiesBH) preferentially produce more asymmetric configurations. This suggests that secondary SMBH growth during galaxy mergers may be more efficient or symmetric than assumed in existing semi-analytical prescriptions.

Physical Mechanisms and Modeling Context:

The study incorporates detailed physical processes relevant to AGN evolution within the simulation framework:

• AGN Feedback: Feedback mechanisms are modeled based on the Eddington ratio: above 1% lambda Edd, feedback is implemented via isotropic thermal energy injection (quasar mode); below this threshold, radio-mode feedback injects hot bubbles into the circumgalactic medium.

• Merger Dynamics: SMBHs are merged when they approach within a smoothing length of typically about 1 kpc, with the remnant assigned the summed mass of the progenitors.

• Simulation Methodology: DM halos and subhalos are identified using algorithms like FoF and Subfind, while merger trees are constructed using SubLink.

Limitations and Future Directions:

The comparison between observations and models is subject to several systematic limitations that must be acknowledged:

• Selection Bias: The GMP selection requires both AGN components to be sufficiently bright in the Gaia G band, potentially favoring systems with more comparable component luminosities. Differences in underlying G-band and dust prescriptions may introduce differential selection effects compared to simulations.

• Systematic Uncertainties: Black-hole masses derived from SE virial estimators and bolometric luminosities inferred from proxies are affected by systematic uncertainties, especially at high accretion rates.

• Simulation Volume Constraints: Cosmological simulations struggle to obtain statistically significant populations of bright (10 45 erg s-1) and massive (10 7.5 M) SMBHs at close separations (< 7 kpc) within a single run due to volume requirements and resolution demands.

• Model Prescription Flexibility: Semi-analytical models, such as LGalaxiesBH, are limited by their adopted physical prescriptions (e.g., assuming accretion starts near the Eddington limit). A more flexible prescription, possibly introducing stochasticity in the accretion rate, is required to reproduce the broader observed distribution of Eddington ratios. Furthermore, models do not self-consistently follow the gravitational and hydrodynamical perturbations induced by galaxy mergers, which are expected to be stronger in smaller galaxies.

Broader Implications:

The findings have implications for other astrophysical phenomena:

• Gravitational Waves (GW): Differences in black-hole mass distributions between models and observations will affect predicted GW signals. A larger abundance of nearly equal-mass systems naturally increases the amplitude of the stochastic GW background, potentially improving agreement with current Pulsar Timing Array (PTA) measurements.

In conclusion, the observational evidence strongly suggests that secondary SMBH growth during the dual AGN phase is more efficient and symmetric than current semi-analytical models predict. This points toward a complex and potentially stochastic fueling process on small physical scales.

1. A more accurate prediction of stochastic gravitational wave background amplitude through improved black hole mass distributions, as A larger abundance of nearly equal-mass systems naturally increases the amplitude of the stochastic GW background.

2. Enhanced understanding of secondary black hole growth efficiency by incorporating merger-driven enhancement of secondary SMBH accretion into semi-analytical models, moving beyond current prescriptions that are underestimated.

3. Improved modeling of Eddington ratio distributions by introducing stochasticity in the accretion rate during accretion events, which could reproduce the broader distribution of Eddington ratios inferred from observations.

4. Refined predictions for black hole mass ratios by incorporating physical insights from simulations, specifically addressing the discrepancy where the observed secondary black hole masses are systematically higher than predicted by both models.

5. More robust selection criteria for dual AGN identification by developing methods that account for potential differential selection effects arising from the underlying Gband and dust prescriptions in both observational and simulated samples.

We present the first statistical characterization of the physical properties of Cosmic Duets: dual AGN in the redshift range 0.5 at most z at most 3.0 with projected separations of 1-7 kpc, corresponding to systems in which both AGN are expected to reside within the same merger remnant. We analyze the largest homogeneous sample currently available, consisting of 30 spectroscopically confirmed Cosmic Duets selected via the Gaia Multi-Peak technique. Using rest-frame UV-to-optical integral-field spectroscopy for all targets, primarily with VLT/MUSE, VLT/ERIS, and Keck/OSIRIS, we spatially resolve the two AGN components and extract separate spectra for each system. We decompose the continuum, emission lines, and Fe II complexes through spectral fitting. We derive black hole masses of log(M BH/M sun) 7.4-10.1 and bolometric luminosities of log(L bol/erg s-1) 44.8-47.2, consistent with those of single AGN at similar redshifts. We finally carry out the first comparison of dual AGN properties with semi-analytical models and hydrodynamical simulations, including L-GalaxiesBH and Astrid. Astrid provides a closer match to the observed black-hole mass-ratio distribution, but its selected systems are predominantly at lower redshifts, resulting in lower bolometric luminosities at fixed G-band magnitude. L-GalaxiesBH perfectly reproduce the observed parameter space, but it predicts more heterogeneous pairs, with higher primary-bolometric luminosity, lower-mass secondary SMBHs, and higher Eddington ratios, resulting in an excess of near-Eddington accreting AGN due to its adopted accretion prescription. Our results suggest that, in GMP-selected systems, secondary SMBH growth is more efficient than predicted by current models, leading to more comparable black-hole masses and accretion power between the two components.