Astrophys. J., 705, 1336-1355 (2009/November-2)
Probing the balance of AGN and star-forming activity in the local universe with ChaMP.
CONSTANTIN A., GREEN P., ALDCROFT T., KIM D.-W., HAGGARD D., BARKHOUSE W. and ANDERSON S.F.
Abstract (from CDS):
The combination of the Sloan Digital Sky Survey (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 LX 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 LX 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 active galactic nucleus (AGN) levels, there is an anticorrelation between Γ and L/Ledd, opposite to the trend exhibited by high z AGN (quasars). The turning point in the Γ-L/Ledd LLAGN + quasars relation occurs near Γ ~ 1.5 and L/Ledd~ 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 anticorrelation we find between Γ and L/Ledd 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/Ledd≳ 0.01) AGNs/quasars could simply be the result of strong selection biases reflected in the dearth of type 2 quasar detections.
galaxies: active - galaxies: evolution - galaxies: nuclei - surveys - X-rays: galaxies
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<Available at CDS (J/ApJ/705/1336): table1.dat>
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