Dwarf Galaxies with Optical Signatures of Active Massive Black Holes (1308.0328v2)

Abstract: We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ~25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad H-alpha emission. For these broad-line AGN candidates, we estimate BH masses using standard virial techniques and find a range of 10^5 < M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad H-alpha in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad H-alpha is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is Mg = -18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds.

• The paper significantly extends the known sample of BH-hosting dwarf galaxies by identifying 151 candidates from 25,974 emission-line galaxies.
• It employs narrow emission-line ratio diagnostics to classify 35 AGN and 101 composite galaxies as active BH hosts.
• BH mass estimates ranging from 10⁵ to 10⁶ M⊙ provide new insights into BH feedback in low-mass galaxies.

Overview of "Dwarf Galaxies with Optical Signatures of Active Massive Black Holes"

This paper presents an extensive paper on the presence of active massive black holes (BHs) in dwarf galaxies, significantly advancing our understanding of BH activity in less massive galactic systems. Utilizing the Sloan Digital Sky Survey (SDSS) Data Release 8 and stellar masses from the NASA-Sloan Atlas, the researchers meticulously analyze a sample of 25,974 emission-line galaxies, identifying 151 dwarf galaxies displaying optical signatures indicative of active BHs. This effort increases, by more than an order of magnitude, the known number of active BH host galaxies within the stellar mass range of $10^{8.5} \lesssim M_{\star} \lesssim 10^{9.5}$ M$_\odot$.

Methodology and Key Findings

The paper employs a systematic approach to identify BH activity within dwarf galaxies by analyzing narrow emission-line ratios, specifically using the /H$\alpha$ versus /H$\beta$ diagnostic diagram. This method classifies the galaxy spectra into AGN-dominated, composite, and star-forming types based on their emission-line signatures. Through this analysis, the authors identify:

• 136 galaxies exhibiting photoionization signatures characteristic of BH accretion: 35 AGN, 101 composites.
• A small subset of galaxies displaying broad H$\alpha$ emission, suggesting dense gas processes in the vicinity of massive BHs potentially within a broad-line region (BLR).

The virial mass estimation technique is applied to the broad-line AGN candidates, resulting in BH mass estimates ranging from $10^5$ to $10^6$ M$_\odot$, with a median of approximately $2 \times 10^5$ M$_\odot$.

Implications

This paper provides critical evidence for the formation and presence of massive BHs in dwarf galaxies, contravening the prevailing assumption that smaller galaxies are unlikely hosts for such massive objects. The findings offer significant insights into the feedback mechanisms of BHs in low-mass galaxies and inform the ongoing discourse on the origins of the first supermassive BH seeds in the early universe. Moreover, the BH masses identified suggest that massive BHs can exist in dwarf galaxies, with potential implications for understanding galactic evolution and BH feedback across different mass regimes.

Future Directions

Given the detection challenges, the authors acknowledge that the current methods are only sensitive to actively accreting BHs, predominantly observable through their narrow-line photoionization signatures. This raises the necessity to explore multi-wavelength detection strategies, including radio and X-ray observations, to capture a more comprehensive BH population, particularly those obscured or in non-active states. Additionally, further studies are needed to refine BH mass estimates and explore potential correlations with galaxy host properties such as morphology, size, and environment.

Conclusion

By substantially increasing the known sample of dwarf galaxies hosting active massive BHs, this paper provides a valuable dataset for unraveling the complex interplay between BH activity and host galaxy evolution, particularly in the low-mass regime. The research sets a foundation for future investigations aimed at understanding the demographic distribution of BHs in dwarf galaxies and their role across cosmic time.