2.2. Basic Source Types
Most of the X-ray sources found in deep surveys are AGN. Considering X-ray, optical, and multiwavelength information, the types of AGN found are
Unobscured AGN. Blue, broad-line AGN are found that do not show signs of obscuration at either X-ray or optical/UV wavelengths. They cover a broad range of redshift (z 0-5), and they comprise a significant fraction of the brightest X-ray sources (e.g., Barger et al. 2003a; Szokoly et al. 2004). At z 1.5 they also comprise a substantial fraction of all X-ray sources with spectroscopic identifications, certainly in part because these objects are the most straightforward to identify spectroscopically.
Obscured AGN with clear optical/UV AGN signatures. Some objects showing X-ray evidence for obscuration have clear AGN signatures in their rest-frame optical/UV spectra. Notably, such AGN can have both type 1 and type 2 optical/UV classifications (e.g., Matt 2002). Most of these objects have z 1.5.
Optically faint X-ray sources. These sources have I > 24-25 and usually cannot be identified spectroscopically (see Figure 4). Many, however, appear to be luminous, obscured AGN at z 1-4 when their X-ray properties, optical photometric properties (including photometric redshifts), and multiwavelength properties are considered (e.g., Alexander et al. 2001; Barger et al. 2003a; Fiore et al. 2003; Mainieri et al. 2004; Treister et al. 2004; Zheng et al. 2004). Thus, many of these objects likely represent an extension of the previous class to higher redshifts and fainter optical magnitudes. Some have no optical detections at all, even in deep images, and are termed extreme X-ray/optical ratio sources (EXOs); most of these are detected in near-infrared images (Koekemoer et al. 2004; Mignoli et al. 2004). EXOs usually can be plausibly explained as z 1.5-5 obscured AGN in dusty or evolved hosts, although a minority may lie at z 7 where the intergalactic medium absorbs essentially all of the observed-frame optical emission.
X-ray bright, optically normal galaxies (XBONGs). XBONGs are early-type galaxies at z 0.05-1 that have X-ray luminosities ( 1041-1043 erg s-1), X-ray spectral shapes, and X-ray-to-optical flux ratios suggesting AGN activity of moderate strength (e.g., Comastri et al. 2004). However, AGN emission lines and non-stellar continua are not apparent in optical spectra. Some XBONGs have X-ray spectra suggesting the presence of obscuration while others do not (e.g., Severgnini et al. 2003). The nature of XBONGs remains somewhat mysterious. Many and perhaps most may just be normal Seyfert galaxies where dilution by host-galaxy light hinders optical detection of the AGN (e.g., Moran et al. 2002; Severgnini et al. 2003), but some have high-quality follow-up observations and appear to be truly remarkable objects (e.g., Comastri et al. 2002). These "true" XBONGs may be (1) AGN with inner radiatively inefficient accretion flows (Yuan & Narayan 2004), (2) AGN that suffer from heavy obscuration covering a large solid angle ( 4 sr), so that optical emission-line and ionizing photons cannot escape the nuclear region (e.g., Matt 2002), or, in some cases, (3) BL Lac-like objects (e.g., Brusa et al. 2003). XBONGs appear related to "optically dull galaxies" (e.g., Elvis et al. 1981) and "elusive AGN" (e.g., Maiolino et al. 2003) studied in the local universe.
Morphological studies show that the AGN from deep X-ray surveys are generally hosted by galaxies with significant bulge components, and they do not show evidence for enhanced merging or interaction activity relative to normal field galaxies (e.g., Grogin et al. 2004). Many of the non broad-line AGN have the rest-frame colors of evolved, bulge-dominated galaxies, and there is little evolution of these colors from z 0-2 (e.g., Barger et al. 2003a; Szokoly et al. 2004). Most AGN from deep surveys are "radio quiet" in the sense that the ratio, R, of their rest-frame 5 GHz and 4400 Å flux densities is R < 10 (e.g., Bauer et al. 2002b).
In addition to AGN, several other types of objects are found in deep extragalactic X-ray surveys. These include
Starburst and normal galaxies. At the faintest measured X-ray flux levels (0.5-2 keV fluxes below 5 × 10-16 erg cm-2 s-1; see Figure 3), a significant and rising (up to 30-40%) fraction of the detected sources appears to be starburst and normal galaxies at z 0.1-1.5 (e.g., Hornschemeier et al. 2003; Bauer et al. 2004). These galaxies are discussed in more detail in Section 3.4.
Groups and clusters of galaxies. Groups and low-luminosity clusters of galaxies at z 0.1-1 are detected as extended, soft X-ray sources in deep surveys (e.g., Bauer et al. 2002a; Giacconi et al. 2002; Rosati et al. 2002a). Their X-ray luminosities (LX 1041.5-1043 erg s-1), basic X-ray spectral properties (kT 3 keV), and sizes appear broadly consistent with those of nearby groups and low-luminosity clusters. The surface density of extended X-ray sources is 100-260 deg-2 at a limiting 0.5-2 keV flux of 3 × 10-16 erg cm-2 s-1; no strong evolution in the X-ray luminosity function of clusters is needed to explain this value.
Galactic stars. Some stars are detected in the high Galactic latitude fields targeted by deep surveys. These are typically of type G, K, and M, and their X-ray emission is attributed to magnetic reconnection flares. The observed X-ray emission constrains the decay of late-type stellar magnetic activity on timescales of 3-11 Gyr (e.g., Feigelson et al. 2004). One of the stars detected in the Lynx field has notably hard X-ray emission, and Stern et al. (2002a) propose this is a binary system where accretion powers the X-ray emission.
Figure 5 shows the source classifications in the Hubble Deep Field-North (HDF-N) and the Hubble Ultradeep Field (UDF). These lie near the centers of the CDF-N (see Figure 2a) and CDF-S, respectively, and thus have the most sensitive X-ray coverage available. Despite the many intensive studies of the HDF-N prior to the acquisition of the Chandra data, several new AGN were identified in the HDF-N based upon their observed X-ray emission (e.g., the three XBONGs; see Brandt et al. 2001a).
Figure 5. HST images of (a) the HDF-N proper and (b) the UDF proper with Chandra sources circled; the circles are much larger than the Chandra source positional errors. The numbers are source redshifts; redshifts followed by a "p" are photometric. Basic source type information for many of the sources is also given. The shown HDF-N and UDF images subtend 5.3 arcmin2 and 9.5 arcmin2, respectively.
Contributions to the total X-ray number counts from a few of the source classes mentioned above are shown in Figure 3. Bauer et al. (2004) provide detailed decompositions of the number counts by source class, luminosity range, and estimated amount of X-ray absorption.