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Intermediate-mass black holes in dwarf galaxies out to redshift ∼2.4 in the Chandra COSMOS-Legacy Survey

Abstract : We present a sample of 40 active galactic nucleus (AGN) in dwarf galaxies at redshifts z ≲ 2.4. The galaxies are drawn from the Chandra COSMOS-Legacy survey as having stellar masses 107 ≤ M* ≤ 3 × 109 M. 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 L0.5-10 keV ∼ 1039-1044 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 L0.5-10 keV ∼ 1044 erg s-1. One of the dwarf galaxies has M* = 6.6 × 107 M 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 ∼104-105 M and typical Eddington ratios >1 per cent. 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 109 < M* ≤ 3 × 109 M and LX ∼ 1041-1042 erg s-1 is ∼0.4 per cent for z ≤ 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 favour a direct collapse formation mechanism for the seed BHs in the early Universe.
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https://hal-insu.archives-ouvertes.fr/insu-03747683
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Submitted on : Monday, August 8, 2022 - 3:47:02 PM
Last modification on : Thursday, November 24, 2022 - 5:02:58 PM
Long-term archiving on: : Wednesday, November 9, 2022 - 6:47:47 PM

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M. Mezcua, F. Civano, S. Marchesi, H. Suh, G. Fabbiano, et al.. Intermediate-mass black holes in dwarf galaxies out to redshift ∼2.4 in the Chandra COSMOS-Legacy Survey. Monthly Notices of the Royal Astronomical Society, 2018, 478, pp.2576-2591. ⟨10.1093/mnras/sty1163⟩. ⟨insu-03747683⟩

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