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

We present a sample of 40 AGN in dwarf galaxies at redshifts $z\lesssim$ 2.4. The galaxies are drawn from the $\mathit{\text{Chandra}}$ COSMOS-Legacy survey as having stellar masses ${10}^7\le M_{\ast }\le 3\times {10}^9$ 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 $L_{0.5-10\mathrm{k}\mathrm{e}\mathrm{V}}\sim {10}^{39}-{10}^{44}$ 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 $L_{0.5-10\mathrm{k}\mathrm{e}\mathrm{V}}\sim {10}^{44}$ erg s${}^{-1}$. One of the dwarf galaxies has $M_{\ast }=6.6\times {10}^7$ 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 $>1\mathrm{\%}$. 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}$ergs$^{-1}$is$\sim$0.4\mathrm{\%}for$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.