Probing the Balance of AGN and Star-forming Activity in the Local Universe with ChaMP (original) (raw)

2009, The Astrophysical …

The combination of the SDSS and the Chandra Multiwavelength Project (ChaMP) currently offers the largest and most homogeneously selected sample of nearby galaxies for investigating the relation between X-ray nuclear emission, nebular line-emission, black hole masses, and properties of the associated stellar populations. We provide X-ray spectral fits and valid uncertainties for all the galaxies with counts ranging from 2 to 1325 (mean 76, median 19). We present here novel constraints that both X-ray luminosity L X and X-ray spectral energy distribution bring to the galaxy evolutionary sequence H II → Seyfert/Transition Object → LINER → Passive suggested by optical data. In particular, we show that both L X and Γ, the slope of the power-law that best fits the 0.5 -8 keV spectra, are consistent with a clear decline in the accretion power along the sequence, corresponding to a softening of their spectra. This implies that, at z ≈ 0, or at low luminosity AGN levels, there is an anti-correlation between Γ and L/L edd , opposite to the trend exhibited by high z AGN (quasars). The turning point in the Γ − L/L edd LLAGN + quasars relation occurs near Γ ≈ 1.5 and L/L edd ≈ 0.01. Interestingly, this is identical to what stellar mass X-ray binaries exhibit, indicating that we have probably found the first empirical evidence for an intrinsic switch in the accretion mode, from advection-dominated flows to standard (disk/corona) accretion modes in supermassive black hole accretors, similar to what has been seen and proposed to happen in stellar mass black hole systems. The anti-correlation we find between Γ and L/L edd may instead indicate that stronger accretion correlates with greater absorption. Therefore the trend for softer spectra toward more luminous, high redshift, and strongly accreting (L/L edd 0.01) AGN/quasars could simply be the result of strong selection biases reflected in the dearth of type 2 quasar detections.