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Using archival Chandra observations of 19 LINERs, we explore the X-ray properties of their inner kiloparsec to determine the origin of their nuclear X-ray emission, to investigate the presence of an AGN, and to identify the power source of the optical emission lines. The relative numbers of LINER types in our sample are similar to those in optical spectroscopic surveys. We find that diffuse, thermal emission is very common and is concentrated within the central few hundred parsecs. The average spectra of the hot gas in spiral and elliptical galaxies are very similar to those of normal galaxies. They can be fitted with a thermal plasma (kT ~ 0.5 keV) plus a power-law (photon index of 1.3-1.5) model. There are on average three detected point sources in their inner kiloparsec with 1037 ergs s-1 < L0.5-10 keV < 1040 ergs s-1. The average cumulative luminosity functions for sources in spiral and elliptical galaxies are identical to those of normal galaxies. In the innermost circle of 2.''5 radius in each galaxy we find an AGN in 12 of the 19 galaxies. The AGNs contribute a median of 60% of the 0.5-10 keV luminosity of the central 2.''5 region, and they have luminosities of 1037-1039 ergs s-1 (Eddington ratios of 10-8 to 10-5). The ionizing luminosity of the AGNs is not enough to power the observed optical emission lines in this particular sample. Thus, we suggest that the lines are powered either by the mechanical interaction of an AGN jet (or wind) with the circumnuclear gas or by stellar processes, such as photoionization by post-AGB stars from an old or intermediate-age population (in most cases) or by young stars (in a few cases).

We report on observations of a type II quasar at redshift z = 3.288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-Ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXO J084837.9+445352 (hereafter CXO 52) shows an unusually hard X-ray spectrum from which we infer an absorbing column density NH = (4.8 +- 2.1) x 1023 cm-2 (90% confidence) and an implied unabsorbed 2-10 keV rest-frame luminosity of L2-10 = 3.3 x 1044 ergs s-1, well within the quasar regime. Hubble Space Telescope imaging shows CXO 52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO 52 shows high-ionization emission lines with velocity widths ~1000 km s-1 and flux ratios similar to a Seyfert 2 galaxy or radio galaxy. The latter are the only class of high-redshift type II luminous active galactic nuclei that have been extensively studied to date. Unlike radio galaxies, however, CXO 52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f4.8 GHz < 40 mJy. High-redshift type II quasars, expected from unification models of active galaxies and long thought necessary to explain the X-ray background, are poorly constrained observationally, with few such systems known. We discuss recent observations of similar type II quasars and detail search techniques for such systems, namely, (1) X-ray selection, (2) radio selection, (3) multicolor imaging selection, and (4) narrowband imaging selection. Such studies are likely to begin identifying luminous, high-redshift type II systems in large numbers. We discuss the prospects for these studies and their implications for our understanding of the X-ray background.

We use observations of three galaxies hosting low-ionization nuclear emission-line regions (LINERs; NGC 404, NGC 4736, and NGC 4579) with the Chandra X-Ray Observatory to study their power sources. We find very diverse properties within this small group: NGC 404 has an X-ray faint nucleus with a soft, thermal spectrum; NGC 4736 harbors a plethora of discrete X-ray sources in and around its nucleus; and NGC 4579 has a dominant nuclear point source embedded in a very extended, diffuse nebulosity. From their multiwavelength properties we conclude the following about the power sources in these LINERs: the nucleus of NGC 404 is the site of a weak, compact starburst, whose X-ray emission is due to gas heated by stellar winds and supernovae; NGC 4736 appears to be in a recent or aging starburst phase, where the X-ray emission is dominated by a dense cluster of X-ray binaries; and NGC 4579 is powered by accretion onto a supermassive black hole. We detect 39 discrete sources in NGC 4736 and 21 in NGC 4579, most with LX > 1037 ergs s-1. One source in the disk of NGC 4579 appears to be an ultraluminous X-ray binary with LX(2-10 keV) = 9 x 1039 ergs s-1, but it could also be a background quasar. The most luminous discrete sources have simple power-law spectra, which along with their luminosities suggest that these are X-ray binaries accreting near or above the Eddington rate for a neutron star. By comparing the luminosity functions of discrete X-ray sources in these and other galaxies, we find a potential connection between the age of the stellar population and the slope of the cumulative X-ray luminosity function: galaxies with primarily old stellar populations have steeper X-ray luminosity functions than starburst galaxies. We suggest that this difference results from the contribution of high-mass X-ray binaries from the young stellar population to the upper end of the luminosity function.

We analyze new optical spectra of IRAS 20181-2244 that confirm its classification as a I Zw 1 object (or narrow-line Seyfert 1 galaxy), one of 20 such objects detected in the ROSAT/IRASall-sky survey. ASCAX-ray data on IRAS 20181-2244 support this classification since they are fitted by a simple power-law spectrum of ? = 2.37 ± 0.09 that is typical of this class but steeper than that of ordinary Seyfert 1 or Seyfert 2 galaxies. Rapid X-ray variability is also observed in IRAS 20181-2244, ruling out a hidden/scattered QSO interpretation for this object. Its 0.5-10 keV luminosity is 5.5 × 1044ergs s-1, and its variability signifies an efficiency of at least 4% for conversion of gravitational energy to X-rays. We discuss the important differences between the I Zw 1 class and the "type 2 QSO," or high-luminosity Seyfert 2 galaxy. While I Zw 1 objects of QSO-like luminosity are ubiquitous in infrared, optical, and soft X-ray selected samples, type 2 QSOs are rare, and there is a dearth of evidence for even a single one in X-rays. It is not clear how this imbalance relates to unified models of AGNs.