4.2. Galaxies With Strong X-ray Emission (Hidden AGNs)
To date many new X-ray galaxies have been located, using modern X-ray satellites such as Chandra and XMM-Newton. However, there are few X-ray-selected very distant galaxies, or AGN. To my knowledge there is one at a redshift in excess of 5; it is #174 in Barger et al. (2002) at z = 5.186, in the Chandra Deep-Field, North. We will return to this galaxy a bit later. There are, however, a considerable number of QSOs and other clearly noticed AGN at z > 4 (cf. Brandt 2002). Why are we physically interested in X-ray galaxies, anyway? As Barger et al. (2001) affirm, X-ray surveys, especially at hard (2-7keV) energies, provide a direct indication of an AGN, presumably due to an ultra-massive black hole at the galaxy nucleus. At 5 keV, absorption will play less of an obscuring role than seen for some "hidden" AGNs at optical frequencies and soft X-ray energies. Complete samples of hard X-ray energies are now possible with the Chandra X-ray Observatory; the 1" X-ray positions produce robust optical identifications of the counterparts. And about half of the sources can be identified with optically bright and "quiet" galaxies; they are at small redshifts.
With the longest integrations (say, one mega-second integrations) we begin to locate the faint X-ray population. Some of these sources are quite distant, z > 4 (cf. Barger et al. 2002). Their survey of the Chandra Deep-Field, North (equivalent to the HDF(N)) yielded a fair number of more-distant X-ray identifications; Table 1, below, puts them in z = 0.5 bins, and includes both narrow and broad-line (AGN) X-ray sources.
A quick inspection of Table 1 and 2, and Fig. 3 suggests no dramatic physical change in the co-moving density of X-ray emitting galaxies compared to all field galaxies.
|Barger (2002) #||z||R||I||Note|
|174||5.186||24.5||23.1||Ly, optically luminous|
The largest redshift in the securely-identified group we discuss is B174 at z = 5.186. This source is associated with a moderately faint optical identification - a bit too faint to classify morphologically. The near-IR I and z band photometry of this z ~ 5.2 source suggest its intrinsic luminosity may lie between that of luminous QSOs and an * galaxy; the AGN may be partly hidden, as the spectra of B174 does not display a broad component to its strong Ly emission line. The other three X-ray galaxies at z > 2 appear to be residents in normal-luminosity host galaxies, based upon their photometry. The rough field galaxy correlation between I mag and the galaxy redshift can be seen in Fig. 3.
Figure 3. I-band (I814) AB magnitudes versus redshift in the HDF(N), diagram by Mark Dickinson. The star-symbols at z = 0 are Galactic stars; filled circle symbols have spectroscopic redshifts, while the small open circles have photometric redshifts. When spectroscopic redshifts are found, the photometric point is removed. We note that three of the four distant X-ray sources (the large star-symbols) have rather normal (non-AGN?) magnitudes - like the field in general.
With the present generation of X-ray satellites and plausible integration times (Mega-secs), we cannot anticipate a large identification content of X-ray (AGN, or even "star-burst") galaxies beyond z = 5. Eventually I would speculate that some sources with fluxes in the 2-8 keV range below 10-16 erg s-1 cm-2 may yield a few very large redshift objects.