The Evolution of Active Galactic Nuclei in Warm Dark Matter Cosmology

Recent measurements of the abundance of active galactic nuclei (AGNs) with low luminosities (L _2-10 <= 10⁴⁴ erg s^-1 in the 2-10 keV energy band) at high redshifts (z >= 4) provide a serious challenge for cold dark matter (CDM) models based on interaction-driven fueling of AGNs. Using a semi-analytic model of galaxy formation we investigate how such observations fit in a warm dark matter (WDM) scenario of galaxy formation, and compare the results with those obtained in the standard CDM scenario with different efficiencies for the stellar feedback. Taking on our previous exploration of galaxy formation in WDM cosmology, we assume as a reference case a spectrum which is suppressed—compared to the standard CDM case—below a cutoff scale ≈0.2 Mpc corresponding (for thermal relic WDM particles) to a mass m_X = 0.75 keV. We run our fiducial semi-analytic model with such a WDM spectrum to derive AGN luminosity functions from z ≈ 6 to the present over a wide range of luminosities (10⁴³ <= L _2-10/erg s^-1 <= 10⁴⁶ in the 2-10 keV X-ray band), to compare with recent observations and with the results in the CDM case. When compared with the standard CDM case, the luminosity distributions we obtain assuming a WDM spectrum are characterized by a similar behavior at low redshift, and by a flatter slope at faint magnitudes for z >= 3, which provide an excellent fit to present observations. We discuss how such a result compares with CDM models with maximized feedback efficiency, and how future deep AGN surveys will allow for a better discrimination between feedback and cosmological effects on the evolution of AGNs in interaction-driven models for AGN fueling.