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

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Context . Combining deep, wide-area X-ray surveys with multi-wavelength catalogues provides insights into rare, highly-accreting active galactic nuclei (AGN) and low-mass galaxies at low redshift, the latter potentially representing local analogues of the first galaxies in the early Universe. Aims . We use eROSITA, aboard the Spectrum Roentgen Gamma satellite (SRG), and its four-pass All Sky Survey (eRASS:4), to select the largest catalogue of X-ray AGN in a highly complete sample of low-redshift galaxies, including low-mass (log M ∗ /M ⊙ ≤ 10) ones. We probe their distribution of specific accretion rates, λ SAR ∝ L X /M ∗ , and the cumulative AGN fraction above varying λ SAR thresholds. Methods . Our parent sample consists of ∼5.35 million galaxies selected from the tenth Data Release of the Legacy Imaging Survey with z -band fluxes brighter than 20 mag and redshifts of 0.03 < z < 0.2 (∼17% of which are spectroscopic, with the rest being good-quality photometric redshifts). We place particular emphasis on the detailed characterisation of our sample, including: (i) estimating unbiased physical galaxy properties through SED fitting; (ii) rigorous cleaning and validation of the X-ray aperture photometry and associations with optical host galaxy counterparts; and (iii) building a stellar mass- and luminosity- complete sample. Results . We identify 874 X-ray AGN in low-mass galaxies, most of them newly discovered as X-ray emitters, with some reaching 2–10 keV luminosities above 10 43 erg s −1 . 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 λ SAR | M ∗ , z ), across a wide range of λ SAR and uncover second-order mass-dependent effects. We detect a break at high λ SAR , possibly indicating Eddington-limited, self-regulated black hole growth. Integrating p (log λ SAR | M ∗ , z ) above λ 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. We also observe a suppression in the efficiency of fuelling AGN beyond λ SAR ≥ 10 −2 at both low and high masses, in comparison to those located in galaxies with log M ∗ /M ⊙ ∼ 10 − 10.5. Conclusions . 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.