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

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.