Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared (k P 5 m) continuum of stellar populations is dominated by the composite blackbody curve and peaks at approximately 1.6 m, the ultraviolet to mid-infrared continuum of active galactic nuclei (AGNs) is dominated by a power law. Consequently, with a sufficient wavelength baseline, one can easily distinguish AGNs from stellar populations. Mirroring the tendency of AGNs to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGNs tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. On the basis of this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGNs, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1 galaxies. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGNs and find evidence for a large population of optically obscured active galaxies.

We measured the K-band luminosity function using a complete sample of 4192 morphologically-typed 2MASS galaxies with µ Ks = 20 mag/arcsec 2 isophotal magnitudes 7 < K 20 < 11.25 mag spread over 2.12 str. Early-type (T ≤ −0.5) and late-type (T > −0.5) galaxies have similarly shaped luminosity functions, α e = −0.92 ± 0.10 and α l = −0.87 ± 0.09. The early-type galaxies are brighter, M * e = −23.53 ± 0.06 mag compared to M * l = −22.98 ± 0.06 mag, but less numerous, n * e = (0.45 ± 0.06) × 10 −2 h 3 Mpc −3 compared to n * l = (1.01 ± 0.13) × 10 −2 h 3 Mpc −3 for H 0 = 100h km s −1 Mpc −1 , such that the late-type galaxies slightly dominate the K-band luminosity density, j late /j early = 1.17 ± 0.12. Our morphological classifications are internally consistent, consistent with previous classifications and lead to luminosity functions unaffected by the estimated uncertainties in the classifications. These luminosity functions accurately predict the K-band number counts and redshift distributions for K < ∼ 18 mag, beyond which the results depend on galaxy evolution and merger histories.

The Infrared Array Camera (IRAC) is one of three focal plane instruments in the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 µm. Two nearly adjacent 5.2×5.2 arcmin fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 µm; 4.5 and 8 µm). All four detector arrays in the camera are 256×256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

We present the first results from a 500 ks Chandra ACIS-I observation of M87. At soft energies (0.5Y1.0 keV), we detect filamentary structures associated with the eastern and southwestern X-ray and radio arms. Many filaments are spatially resolved with widths of $300 pc. This filamentary structure is particularly striking in the eastern arm, where we suggest the filaments are outer edges of a series of plasma-filled, buoyant bubbles whose ages differ by $6 ; 10 6 yr. These X-ray structures may be influenced by magnetic filamentation. At hard energies (3.5Y7.5 keV), we detect a nearly circular ring of outer radius 2.8 0 (13 kpc), which provides an unambiguous signature of a weak shock, driven by an outburst from the supermassive black hole (SMBH ). The density rise in the shock is shock / 0 % 1:3 (Mach number, M % 1:2). The observed spectral hardening in the ring corresponds to a temperature rise T shock /T 0 % 1:2, or M % 1:2, in agreement with the Mach number derived independently from the gas density. Thus, for the first time, we detect gas temperature and density jumps associated with a classical shock in the atmosphere around a SMBH. We also detect two additional surface brightness edges and pressure enhancements at radii of $0.6 0 and $1 0 . The $0.6 0 feature may be overpressurized thermal gas surrounding the relativistic plasma in the radio cocoon, the ''piston,'' produced by the current episode of AGN activity. The overpressurized gas is surrounded by a cool gas shell. The $1 0 feature may be an additional weak shock from a secondary outburst. In an earlier episode, the piston was responsible for driving the 2.8 0 shock.