CHAMPSDSSA - CHAMP/SDSS Nearby Low-Luminosity AGN Catalog

Overview

The combination of the Sloan Digital Sky Survey (SDSS) and the Chandra Multiwavelength Project (ChaMP; Green et al. 2004, ApJS, 150, 43) currently offers the largest and most homogeneously selected sample of nearby galaxies for investigating the relations between X-ray nuclear emission, nebular line emission, black hole masses, and the properties of the associated stellar populations. The authors provide X-ray spectral fits and valid uncertainties for all the galaxies with counts ranging from 2 to 1325 (mean 76, median 19). They present in their paper novel constraints that both X-ray luminosity L_X and X-ray spectral energy distribution bring to the galaxy evolutionary sequence HII -> Seyfert/Transition Object -> LINER -> Passive suggested by optical data. In particular, the authors show that both L_X and Gamma, 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 Gamma and L/L_Edd, opposite to the trend which is exhibited by high-z AGN (quasars). The turning point in the Gamma - L/L_Edd LLAGN + quasars relation occurs near Gamma ~ 1.5 and L/L_Edd ~ 0.01. Interestingly, this is identical to what stellar mass X-ray binaries exhibit, indicating that the authors 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 the authors find between Gamma 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) AGNs/quasars could simply be the result of strong selection biases reflected in the dearth of type 2 quasar detections.

The cross-match of all ChaMP sky regions imaged by Chandra/ACIS with the SDSS DR4 spectroscopic footprint results in a parent sample of 15,955 galaxies on or near a chip and a subset of 199 sources that are X-ray detected. Among those, only 107 sources have an off-axis angle (OAA) Theta <0.2 degrees and avoid ccd=8 due to high serial readout noise; these 107 objects comprise the main sample that the authors employ for this study and that are listed in this table.

The authors performed direct spectral fits to the X-ray counts distribution using the full instrument calibration, known redshift, and Galactic 21-cm column nH_Gal. Source spectra were extracted from circular regions with radii corresponding to energy encircled fractions of ~90%, while the background region encompasses a 20 arcsec annulus, centered on the source, with separation 4 arcsecs, from the source region. Any nearby sources were excised, from both the source and the background regions. The spectral fitting was done via yaxx ('Yet Another X-ray eXtractor': Aldcroft 2006, BAAS, 38, 376), an automated script that employs the CIAO Sherpa tool. Each spectrum was fitted in the range 0.5 - 8 keV by two different models: (1) a single power law plus absorption fixed at the Galactic 21-cm value (model 'PL'), and (2) a fixed power law of photon index Gamma = 1.9 plus intrinsic absorption of column nH (model 'PLfix'). For the nine objects with more than 200 counts, the authors employed a third model in which both the slope of the power law and the intrinsic absorption were free to vary (model 'PL_abs').

Catalog Bibcode

2009ApJ...705.1336C

References

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.,
    Anderson S.F.
   <Astrophys. J., 705, 1336-1355 (2009)>
   =2009ApJ...705.1336C    (SIMBAD/NED BibCode)

1Provenance

This table was created by the HEASARC in January 2012 based on CDS Catalog J/ApJ/705/1336/ file table1.dat.

Parameters

SDSS_ID
The SDSS unique object identifier, Object ID.

SDSS_RA
The Right Ascension of the SDSS object in the selected equinox. This was given in J2000.0 equatorial coordinates in decimal degrees to a precision of 10^-6 degrees in the original table.

SDSS_Dec
The Declination of the SDSS object in the selected equinox. This was given in J2000.0 equatorial coordinates in decimal degrees to a precision of 10^-6 degrees in the original table.

Redshift
The spectroscopic redshift of the SDSS object.

Name
The ChaMP X-ray source identification, e., 'CXOMP JHHMMSS.s+DDMMSS' based on the truncated J2000.0 equatorial coordinates.

RA
The Right Ascension of the ChaMP X-ray source in the selected equinox. This was created by the HEASARC from the ChaMP source name and thus is precise only to 0.1 seconds of time and is truncated. More accurate positions for these sources are given in the ChaMP Point Source Catalog (the HEASARC Browse CHAMPPSC table).

Dec
The Declination of the ChaMP X-ray source in the selected equinox. This was created by the HEASARC from the ChaMP source name and thus is precise only to 1 arcsecond and is truncated. More accurate positions for these sources are given in the ChaMP Point Source Catalog (the HEASARC Browse CHAMPPSC table).

LII
The Galactic Longitude of the ChaMP X-ray source.

BII
The Galactic Latitude of the ChaMP X-ray source.

Off_Axis
The Chandra off-axis angle Theta of the X-ray source from the nominal direction of the pointing, in arcminutes. All sources have Theta values less than 0.2 degress (12 arcminutes).

Counts
The net 0.5 - 8 keV X-ray source counts.

Exposure
The vignetting-corrected exposure time for the X-ray source, in seconds (s).

Target_Flag
This flag parameter is set to 'I' to indicates that the X-ray source was the intended Chandra PI target, ele it is left blank.

NH_Gal
The Galactic Hydrogen column density nH_Gal in the direction of the Chandra aimpoint, in H atoms cm^-2, taken from Dickey and Lockman (1990, ARAA, 28, 215).

NH_Limit
This flag is set to '<' if the corresponding best-fit Hydrogen column density is the 90% confidence level upper limit rather than a value.

NH
The best-fit intrinsic column density for the X-ray source as obtained by yaxx, in H atoms cm^-2 (see Overview above for more details on the X-ray spectral fitting procedure).

NH_Pos_Err
The upper (positive) 1-sigma uncertainty in the corresponding parameter value, in the same units as the latter.

NH_Neg_Err
The lower (negative) 1-sigma uncertainty in the corresponding parameter value, in the same units as the latter.

Spectral_Index
The best-fit X-ray spectral power-law index Gamma for the X-ray source as obtained by yaxx, (see Overview above for more details on the X-ray spectral fitting procedure).

Spectral_Index_Pos_Err
The upper (positive) 1-sigma uncertainty in the corresponding parameter value, in the same units as the latter.

Spectral_Index_Neg_Err
The lower (negative) 1-sigma uncertainty in the corresponding parameter value, in the same units as the latter.

Flux
The X-ray source flux in the 0.5 - 8 keV band, in erg cm^-2 s^-1.

Log_Lx
The logarithm of the X-ray luminosity in the 0.5 - 8 keV band, in erg/s.

Opt_Class
The optical spectral classification coded as follows:

      H II = HII galaxy
         S = Seyfert
         L = LINER (narrow-lined Low Ionization Nuclear Emission Regions)
         T = Transition Object (properties border on the definition of a
             starburst galaxy and an AGN)
         P = Passive galaxy (lacking optical emission line activity)
  no-class = Not classified

Class
The HEASARC Browse object classification, based on the value of the opt_class parameter.

Contact Person

Questions regarding the CHAMPSDSSA database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:25:39 EDT