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

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 $10^7\le M_{\ast }\le 3\times 10^9$ 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 $L_{0.5-10keV}\sim 10^{39}-10^{44}$ 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 $L_{0.5-10keV}\sim 10^{44}$ 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 $>1\%$. 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 $10^9<M_{\ast }\le 3\times 10^9$ M${}_{\odot }$ and $L_{\mathrm{X}}\sim 10^{41}-10^{42}$ erg s${}^{-1}$ is $\sim$0.4\% for $z\le$ 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.

16 pages, 10 figures, accepted for publication in MNRAS