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تسجيل مستخدم جديدIntermediate-mass black holes in dwarf galaxies out to redshift $sim$ 2.4 in the Chandra COSMOS Legacy Survey
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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 $10^{7}leq M_{*}leq3 times 10^{9}$ 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 $L_mathrm{0.5-10 keV} sim10^{39} - 10^{44}$ 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 $L_mathrm{0.5-10 keV} sim 10^{44}$ erg s$^{-1}$. One of the dwarf galaxies has $M_mathrm{*} = 6.6 times 10^{7}$ 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 $10^{9}< M_{*}leq3 times 10^{9}$ M$_{odot}$ and $L_mathrm{X} sim 10^{41}-10^{42}$ erg s$^{-1}$ is $sim$0.4% 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.
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