Searching for hidden AGN in nearby star-forming galaxies with Chandra

Aims. We searched for X-ray signatures of AGN in a sample of star-forming, relatively early-type, nearby spiral galaxies, from the optical spectroscopic sample of Ho et al. The tight correlations between the masses of central supermassive black holes and bulge mass or velocity dispersion suggest that such galaxies are likely to host black holes in their centres. The aim is to look for X-ray signs of activity in these gas-rich environments. Methods. We identified Chandra ACIS-S images of the X-ray counterparts of 9 sources from the optical sample. We isolated 10 individual X-ray sources closest to the optical position and calculated their X-ray luminosity, L-X, and X-ray colours. Using H alpha luminosities, we also calculated star formation rates. For four sources with a high number of net counts in the X-ray band, we extracted X-ray spectra to which we fitted standard spectral models. We modelled Fe K alpha emission by Gaussians, and assessed the significance of adding such a component to a fit by means of a calibration of the standard F-test. For the rest of the sources, we estimated values for the intrinsic hydrogen column density, N-H(int), and the power-law photon index, Gamma, which can reproduce the observed soft and hard X-ray colours. Results. All spectral fits include a power law, a diffuse hot gas, a Galactic absorption and an intrinsic absorption component. For the nuclear sources of NGC 2782 and NGC 3310, a Fe K alpha emission-line component is included with high significance. At the same time the power-law component is flat (index Gamma similar to 0 and 1.3). Both sources have high star-formation rates, with the rate for NGC 2782 being the highest in our sample. Conclusions. The detection of Fe K alpha emission coming from a central, isolated source in NGC 2782 and NGC 3310, points towards a hidden AGN in two out of nine relatively early-type, star-forming spirals observed both with Palomar and Chandra. Larger samples need to be explored, but this evidence suggests that the presence of an AGN is likely in massive, disturbed starbursts. The contribution of X-ray emission from these AGN is small: For the central source, the bolometric luminosity is a fraction of a few x 10(-5) of the Eddington luminosity. The X-ray flux of the central source is between 10 and 20% of the total galactic flux in the 0.2-10 keV band.