The incidence of eROSITA X-ray AGN in the local Universe: from dwarf to massive galaxies

Combining deep, wide-area X-ray surveys with multi-wavelength catalogues provides insights into rare, highly-accreting AGN and low-mass galaxies at low redshift, the latter potentially representing local analogues of the first galaxies in the early Universe. We use the four-pass eROSITA All Sky Survey to select the largest catalogue of X-ray AGN in a highly complete sample of low-redshift galaxies, including low-mass (logM*/Msol<10) ones. We probe their distribution of specific accretion rates, lambda SAR, and the cumulative AGN fraction above varying lambda SAR thresholds. Our parent sample consists of 5.35 million galaxies selected from the Legacy Survey DR10 with z-band fluxes brighter than 20 mag and redshifts 0.03<z<0.2. We place particular emphasis on the detailed characterisation of our sample, including estimating unbiased physical galaxy properties and rigorous cleaning and validation of the X-ray aperture photometry and host-galaxy associations. We identify 874 X-ray AGN in low-mass galaxies, most of them newly discovered as X-ray emitters. Thanks to a Bayesian framework that makes use of the X-ray information from all parent sample galaxies, we constrain the specific accretion rate distribution, p(log lambda SAR M*, z), across a wide range of lambda SAR and uncover second-order mass-dependent effects. We detect a break at high lambda SAR, possibly indicating Eddington-limited, self-regulated black hole growth. Integrating p(log lambda SAR M*, z) above lambda SAR>10-3, we find a cumulative AGN fraction of 1% for low-mass galaxies, placing a firm lower limit on the black hole occupation fraction in this regime. Overall, our specific accretion rate distributions, sampling down to the as-of-yet unexplored low-mass regime, highlight a more nuanced, mass-dependent view of AGN growth and accretion history that must be taken into account in future modelling.