A Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei (1805.01467v1)

Abstract: Nearly every massive galaxy harbors a supermassive black hole (SMBH) in its nucleus. SMBH masses are millions to billions $M_{\odot}$, and they correlate with properties of spheroids of their host galaxies. While the SMBH growth channels, mergers and gas accretion, are well established, their origin remains uncertain: they could have either emerged from massive "seeds" ($10^5-10^6 M_{\odot}$) formed by direct collapse of gas clouds in the early Universe or from smaller ($100 M_{\odot}$) black holes, end-products of first stars. The latter channel would leave behind numerous intermediate mass black holes (IMBHs, $10^2-10^5 M_{\odot}$). Although many IMBH candidates have been identified, none is accepted as definitive, thus their very existence is still debated. Using data mining in wide-field sky surveys and applying dedicated analysis to archival and follow-up optical spectra, we identified a sample of 305 IMBH candidates having masses $3\times10^4<M_{\mathrm{BH}}<2\times10^5 M_{\odot}$, which reside in galaxy centers and are accreting gas that creates characteristic signatures of a type-I active galactic nucleus (AGN). We confirmed the AGN nature of ten sources (including five previously known objects which validate our method) by detecting the X-ray emission from their accretion discs, thus defining the first bona fide sample of IMBHs in galactic nuclei. All IMBH host galaxies possess small bulges and sit on the low-mass extension of the $M_{\mathrm{BH}}-M_{\mathrm{bulge}}$ scaling relation suggesting that they must have experienced very few if any major mergers over their lifetime. The very existence of nuclear IMBHs supports the stellar mass seed scenario of the massive black hole formation.

• The paper identifies 305 IMBH candidates and confirms the AGN nature of ten sources through broad-line diagnostics and X-ray emissions.
• It employs an automated, non-parametric spectral analysis of nearly one million SDSS DR7 spectra to detect faint AGN signatures.
• Findings support stellar seed models for SMBH formation and highlight distinct evolutionary paths for galaxies with minimal major mergers.

Insights into Intermediate Mass Black Holes through Low Luminosity Active Galactic Nuclei

The research conducted by Chilingarian et al. investigates the elusive population of intermediate mass black holes (IMBHs) and their identification as low-luminosity active galactic nuclei (AGN) through a meticulous data mining approach across wide-field sky surveys. Historically, the existence of IMBHs, with masses ranging from $10^2$ to $10^5 \, M_{\odot}$, remains under debate due to the lack of robust and universally accepted confirmations, despite the identification of numerous candidates. These IMBHs are envisaged as critical elements in the comprehension of supermassive black holes (SMBHs) formation theories and the evolution of galaxy structures.

Methodology and Candidate Identification

The authors employed an automated workflow that analyzed approximately one million spectra from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). The key was to identify spectral signatures indicative of AGN activity—specifically, broad-line components in Balmer lines triggered by the accretion processes of black holes. By using a non-parametric emission line profile model to exhaustively analyze these lines, the team could detect potential IMBH candidates with high sensitivity.

From this approach, the researchers isolated 305 candidates that fell within the IMBH mass range. Among these, they confirmed the AGN nature of ten sources through the detection of X-ray emissions from accretion discs, including five objects previously identified in literature.

Results and Implications

The paper underlines the vital role of IMBHs in galaxy dynamics and the mass growth history of central black holes. The findings support the stellar mass seed scenario for the formation of SMBHs, suggesting that many SMBHs could have originated from smaller stellar mass black holes. The presence of nuclear IMBHs bolsters this hypothesis, providing evidence for the continuation of mass assembly processes into the lower mass regime effectively extending the $M_{\mathrm{BH}}$-$M_{\mathrm{bulge}}$ relation observed for more massive systems.

Furthermore, the existence of IMBHs points towards unique evolutionary pathways for host galaxies, characterized by minimal major mergers, thereby preserving their small bulges. This observation has profound implications for our understanding of bulge and black hole co-evolutionary dynamics across cosmic timescales.

Challenges and Future Directions

The limitations of this paper highlight areas for future exploration, such as the need for deeper spectroscopic and X-ray surveys to enhance the detection of less luminous IMBHs. The advent of upcoming missions with capabilities to perform more sensitive and extensive sky surveys could increase the sample size and reduce uncertainties associated with IMBH mass estimates.

Going forward, a multi-wavelength observational strategy that integrates optical, near-infrared, and X-ray data could provide a more holistic view of these objects. Additionally, improvements in theoretical models concerning accretion physics and BH-galaxy co-evolution could provide further insights into the role of IMBHs within the larger framework of galactic dynamics.

In conclusion, the work presented makes significant strides toward identifying and understanding IMBHs and reinforces the intricate linkages between these smaller black holes and the broader astrophysical structures within which they reside, inviting further investigation into this enigmatic facet of cosmic architecture.