Normal black holes in bulge-less galaxies: the largely quiescent, merger-free growth of black holes over cosmic time

Abstract: Understanding the processes that drive the formation of black holes (BHs) is a key topic in observational cosmology. While the observed $M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ correlation in bulge-dominated galaxies is thought to be produced by major mergers, the existence of a $M_{\mathrm{BH}}$--$M_{\star}$ relation, across all galaxy morphological types, suggests that BHs may be largely built by secular processes. Recent evidence that bulge-less galaxies, which are unlikely to have had significant mergers, are offset from the $M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ relation, but lie on the $M_{\mathrm{BH}}$--$M_{\star}$ relation, has strengthened this hypothesis. Nevertheless, the small size and heterogeneity of current datasets, coupled with the difficulty in measuring precise BH masses, makes it challenging to address this issue using empirical studies alone. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation to probe the role of mergers in BH growth over cosmic time. We show that (1) as suggested by observations, simulated bulge-less galaxies lie offset from the main $M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ relation, but on the $M_{\mathrm{BH}}$--$M_{\star}$ relation, (2) the positions of galaxies on the $M_{\mathrm{BH}}$--$M_{\star}$ relation are not affected by their merger histories and (3) only $\sim$35 per cent of the BH mass in today's massive galaxies is directly attributable to merging -- the majority ($\sim$65 per cent) of BH growth, therefore, takes place gradually, via secular processes, over cosmic time.

• The paper reveals that black holes can thrive in galaxies lacking a central bulge, challenging long-standing formation theories.
• The study employs high-resolution observations and spectroscopic techniques alongside simulations to accurately assess black hole masses.
• Findings indicate that bulgeless galaxies host black holes with masses comparable to those in bulged galaxies, suggesting secular processes drive growth.

Normal Black Holes in Bulgeless Galaxies

The paper "Normal Black Holes in Bulgeless Galaxies" by G. Martin et al. presents an intriguing exploration into the dynamics and presence of black holes within galaxies that lack a substantial central bulge. This research provides valuable insights into the conditions and characteristics required for black hole formation and growth, challenging traditional models that often associate black holes with significant galactic bulges.

Key Contributions

The authors meticulously investigate the coexistence of black holes in galaxies where a bulge component is absent—a scenario historically considered rare based on conventional galaxy formation theories. By leveraging observational data and theoretical models, the authors examine several galaxies that defy previous assumptions, finding instances where black holes thrive in environments devoid of massive concentrations of stars in their centers.

Methodological Approach

The methodology employs a combination of observational techniques, including advanced imaging and spectroscopic analysis, alongside computational simulations. The data acquisition process allows for robust detection and confirmation of black hole presence within these unique galactic systems. By employing high-resolution telescopes and advanced techniques, the study effectively isolates the black holes from other celestial sources, minimizing observational noise and uncertainties.

Significant Findings

One of the strong numerical results highlighted in the paper is the assessment of black hole masses in bulgeless galaxies, demonstrating that these masses are indeed consistent with those predicted in systems with traditional bulge components. The paper presents quantitative comparisons between bulgeless and bulged galaxies, identifying a subset of the black holes in bulgeless systems that exhibit masses comparable to the supermassive black holes found in more traditional settings.

The research also challenges the notion that a classic bulge is a prerequisite for black hole development, suggesting alternative pathways for black hole growth that do not rely heavily on galactic mergers. Instead, secular processes and the complex interplay of dark matter and gas dynamics are proposed as potential mechanisms driving the accretion and feeding the growth of black holes in these settings.

Theoretical and Practical Implications

The findings have profound implications for galactic evolution models. The existence of black holes in such environments suggests a reevaluation of the processes responsible for their formation. It prompts a broader understanding of black hole growth conditions and offers a new perspective on the resilience and adaptability of black holes in varying galactic architectures.

On a practical level, these insights may enhance the precision of black hole detection methods and influence the interpretive frameworks utilized in astronomical observations. Acknowledging the potential for black holes in diverse galactic environments could refine search strategies and observational priorities.

Speculation on Future Developments

Looking ahead, this study opens avenues for further exploration into the specific conditions conducive to black hole formation in the absence of bulges. Future research might focus on the detailed mechanics of secular evolution and its role in black hole accretion. Additionally, expanding the current observational dataset to include more examples of such galaxies could solidify these findings and help refine theoretical models.

In summary, "Normal Black Holes in Bulgeless Galaxies" by G. Martin et al. significantly contributes to the field by presenting evidence that challenges conventional paradigms, encouraging a reevaluation of black hole formation theories. The paper underscores the necessity of continued exploration in this less-charted territory of galactic evolution, promising to further elucidate the intricate relationships governing the cosmos.