Intermediate-mass black holes in dwarf galaxies out to redshift $\sim$ 2.4 in the Chandra COSMOS Legacy Survey (1802.01567v2)

Abstract: We present a sample of 40 AGN in dwarf galaxies at redshifts $z \lesssim$ 2.4. The galaxies are drawn from the \textit{Chandra} COSMOS-Legacy survey as having stellar masses $10^{7}\leq M_{}\leq3 \times 10^{9}$ M${\odot}$. Most of the dwarf galaxies are star-forming. After removing the contribution from star formation to the X-ray emission, the AGN luminosities of the 40 dwarf galaxies are in the range $L\mathrm{0.5-10 keV} \sim10^{39} - 10^{44}$ erg s$^{-1}$. With 12 sources at $z > 0.5$, our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid_1192, at $z = 2.39$ and with $L_\mathrm{0.5-10 keV} \sim 10^{44}$ erg s$^{-1}$. One of the dwarf galaxies has $M_\mathrm{} = 6.6 \times 10^{7}$ M${\odot}$ and is the least massive galaxy found so far to host an AGN. All the AGN are of type 2 and consistent with hosting intermediate-mass black holes (BHs) with masses $\sim 10^{4} - 10^{5}$ M${\odot}$ and typical Eddington ratios $> 1\%$. We also study the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to $z$ = 0.7. We find that the AGN fraction for $10^{9}< M_{*}\leq3 \times 10^{9}$ M${\odot}$ and $L\mathrm{X} \sim 10^{41}-10^{42}$ erg s$^{-1}$ is $\sim$0.4\% for $z \leq$ 0.3 and that it decreases with X-ray luminosity and decreasing stellar mass. Unlike massive galaxies, the AGN fraction seems to decrease with redshift, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. Mindful of potential caveats, the results seem to favor a direct collapse formation mechanism for the seed BHs in the early Universe.

• The paper presents the discovery of 40 active galactic nuclei (AGN) likely hosting intermediate-mass black holes (IMBHs) in dwarf galaxies at redshifts up to 2.4 using Chandra COSMOS-Legacy Survey data.
• Key findings include a broad range of X-ray luminosities ($10^{39}$ to $10^{44}$ erg s$^{-1}$), black hole mass estimates between $10^4$ and $7 imes 10^5$ solar masses, and the identification of 12 sources at $z > 0.5$, representing the highest-redshift IMBH dataset in dwarf galaxies.
• The study's results, particularly the decrease in AGN fraction with redshift in dwarf galaxies, support the direct collapse mechanism for seed black hole formation in the early universe.

The Discovery of Intermediate-Mass Black Holes in High-Redshift Dwarf Galaxies

Mezcua et al.'s paper represents a significant investigation into the presence of intermediate-mass black holes (IMBHs) in dwarf galaxies out to redshift $z \sim 2.4$. This work leverages data from the Chandra COSMOS-Legacy Survey, expanding the exploration of active galactic nuclei (AGN) to include low-mass galaxies at significant cosmological distances. The sample comprises 40 AGN residing in dwarf galaxies with stellar masses ranging between $10^{7}$ and $3 \times 10^{9}$ M$_{\odot}$. The paper marks a significant extension in redshift for such research, providing insights into the nature and growth of IMBHs in the early universe.

Sample and Methodology

The research utilized a sample of 40 AGN selected based on their X-ray properties detected by the Chandra X-ray Observatory from the COSMOS field. In determining the nature of these AGN, the contribution of X-ray binaries (XRBs) and hot interstellar medium (ISM) gas had to be carefully disentangled from the AGN emission. The paper derived the AGN X-ray luminosity after systematically removing contamination from these other sources of X-ray emission. Additionally, stellar masses were determined through spectral energy distribution (SED) fitting, while the hardness ratio of X-ray spectra offered a means to gauge the level of obscuration in these AGN.

Key Findings and Numerical Results

Among the findings, the paper identifies a broad range of X-ray luminosities from $L_\mathrm{0.5-10 keV} \sim 10^{39}$ to $10^{44}$ erg s$^{-1}$ across the sample. Notably, the number of discovered AGN includes 12 sources at $z > 0.5$, establishing the highest-redshift dataset for IMBHs in dwarf galaxies. Black hole mass estimates extracted from scaling relations yield a range from $10^{4}$ M$_{\odot}$ to $7 \times 10^{5}$ M$_{\odot}$, reinforcing the classification as IMBHs. Most of these dwarf galaxies, characterized by star-forming activity, are consistent with hosting IMBHs based on their X-ray properties and Eddington ratios, suggesting super-Eddington or near-Eddington accretion for many sources.

Implications for Black Hole Seed Formation

From a theoretical perspective, the paper provides potential implications for understanding the early universe's seed black holes. The evolution of the AGN fraction with stellar mass, X-ray luminosity, and redshift suggests a unique behavior compared to more massive galaxies. Unlike the typical trend seen with supermassive black holes, the AGN fraction in dwarf galaxies appears to decrease with redshift, a result that supports the hypothesis of a direct collapse mechanism for seed BH formation. This pattern is suggestive of a growth modality in which initial seed BHs could form from phenomena such as collapsing dense gas within protogalaxies, rather than from stellar remnants.

Future Directions and Developments

While this research extends our understanding of IMBH activity in high-redshift galaxies, future work should aim toward deeper surveys capable of detecting even lower-luminosity AGN in dwarf galaxies to refine BH occupation models. Upcoming missions with greater sensitivity and resolution like the Lynx X-ray Observatory could unravel the detailed evolutionary history of seed BHs, thus enhancing our understanding of SMBH formation in the early universe.

Overall, Mezcua et al.'s work stands as a critical contribution to the astrophysical community's efforts in exploring the complex processes that govern black hole growth and galaxy evolution on cosmological scales. The impact of their findings broadens the scope of AGN research, especially concerning intermediate-mass black holes within dwarf galaxies at significant cosmic epochs.