A population of intermediate-mass black holes in dwarf starburst
  galaxies up to redshift=1.5
  We study a sample of $\sim$50,000 dwarf starburst and late-type galaxies
drawn from the COSMOS survey with the aim of investigating the presence of
nuclear accreting black holes (BHs) as those seed BHs from which supermassive
BHs could grow in the early Universe. We divide the sample into five complete
redshift bins up to $z=1.5$ and perform an X-ray stacking analysis using the
\textit{Chandra} COSMOS-Legacy survey data. After removing the contribution
from X-ray binaries and hot gas to the stacked X-ray emission, we still find an
X-ray excess in the five redshift bins that can be explained by nuclear
accreting BHs. This X-ray excess is more significant for $z<0.5$. At higher
redshifts, these active galactic nuclei could suffer mild obscuration, as
indicated by the analysis of their hardness ratios. The average nuclear X-ray
luminosities in the soft band are in the range 10$^{39}-10^{40}$ erg s$^{-1}$.
Assuming that the sources accrete at $\geq$ 1\% the Eddington rate, their BH
masses would be $\leq$ 10$^{5}$ M$_{\odot}$, thus in the intermediate-mass BH
regime, but their mass would be smaller than the one predicted by the
BH-stellar mass relation. If instead the sources follow the correlation between
BH mass and stellar mass, they would have sub-Eddington accreting rates of
$\sim$ 10$^{-3}$ and BH masses 1-9 $\times$ 10$^{5}$ M$_{\odot}$. We thus
conclude that a population of intermediate-mass BHs exists in dwarf starburst
galaxies, at least up to $z$=1.5, though their detection beyond the local
Universe is challenging due to their low luminosity and mild obscuration unless
deep surveys are employed.
