paper

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

arXiv:1802.01567 · doi:10.1093/mnras/sty1163

Abstract

We present a sample of 40 AGN in dwarf galaxies at redshifts 2.4. The galaxies are drawn from the \textit{Chandra} COSMOS-Legacy survey as having stellar masses 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 erg s. With 12 sources at , our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid\_1192, at and with erg s. One of the dwarf galaxies has 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 M and typical Eddington ratios . We also study the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to = 0.7. We find that the AGN fraction for M and erg s is 0.4\% for 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.

arXiv comment16 pages, 10 figures, accepted for publication in MNRAS

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