2018MNRAS.478.2576M

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 x 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 L_0.5-10 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 L_0.5-10 keV_ ∼ 10⁴⁴ erg s^–1. One of the dwarf galaxies has M_* = 6.6 x 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 x 10⁹ M_☉ and L_X ∼ 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.