Megan E. Eckart

A Galaxy at z = 6.545 and Constraints on the Epoch of Reionization

Wereportthe discoveryofaLy� -emittinggalaxy atredshiftz ¼ 6:545serendipitouslyidentifiedin thecourseof spectroscopic follow-up of hard X-ray sources on behalf of the Serendipitous Extragalactic X-Ray Source Iden- tification (SEXSI) survey. The line flux of the galaxy, 2:1 ; 10 � 17 ergs cm � 2 s � 1 , is similar to line fluxes probed by narrowband imaging surveys; the 5.2 arcmin 2 surveyed implies a surface density of z � 6: 5L yemitters somewhat higher than that inferred from narrowband surveys. This source marks the sixth Ly� -emitting galaxy identified at z � 6:5, a redshift putatively beyond the epoch of reionization when the damping wings of the neutral hydrogen of the intergalactic medium is capable of severely attenuating Lyemission. By comparing the Ly� luminosity functions at z � 5: 7a ndz � 6:5, we infer that the intergalactic medium may remain largely reionized from the local universe out to z � 6:5. Subject headingg cosmology: observations — early universe — galaxies: formation — galaxies: high-redshift

Position sensitive x-ray spectrophotometer using microwave kinetic inductance detectors

The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. The authors present results on position sensitive x-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays.

THE SERENDIPITOUS EXTRAGALACTIC X-RAY SOURCE IDENTIFICATION PROGRAM. I. CHARACTERISTICS OF THE HARD X-RAY SAMPLE

The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) program is designed to extend greatly the sample of identified extragalactic hard X-ray (2-10 keV) sources at intermediate fluxes (~10-13 to 10-15 ergs cm-2 s-1). SEXSI, which studies sources selected from more than 2 deg2, provides an essential complement to the Chandra Deep Fields, which reach depths of 5 × 10-16 ergs cm-2 s-1 (2-10 keV) but over a total area of less than 0.2 deg2. In this paper we describe the characteristics of the survey and our X-ray data analysis methodology. We present the cumulative flux distribution for the X-ray sample of 1034 hard sources and discuss the distribution of spectral hardness ratios. Our log N- log S in this intermediate flux range connects to those found in the Deep Fields, and by combining the data sets, we constrain the hard X-ray population over the flux range in which the differential number counts change slope and from which the bulk of the 2-10 keV X-ray background arises. We further investigate the log N- log S distribution separately for soft and hard sources in our sample, finding that while a clear change in slope is seen for the softer sample, the hardest sources are well described by a single power law down to the faintest fluxes, consistent with the notion that they lie at lower average redshift.

A Comparison of X-ray and Mid-Infrared Selection of Obscured Active Galactic Nuclei

We compare the relative merits of active galactic nuclei (AGNs) selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated photometric and spectroscopic optical follow-up are drawn from a subset of fields studied as part of the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program. Mid-infrared data in these fields are derived from targeted and archival Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the Infrared Array Camera (IRAC) color–color AGN “wedge” selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGNs by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGNs in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGNs are detected at X-ray energies in the moderately deep ({t_(exp} ≈ 100 ks) SEXSI Chandra data. We hypothesize that IRAC sources with AGN colors that lack X-ray detections are predominantly high-luminosity AGNs that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2–8 keV band 3 times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. The hardness ratio of the wedge-selected stack is consistent with moderate intrinsic obscuration, but is not suggestive of a highly obscured, Compton-thick source population. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray, and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR.

The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy*

We present the catalog of 477 spectra from the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program, a survey designed to probe the dominant contributors to the 2-10 keV cosmic X-ray background. Our survey covers 1 deg2 of sky to 2-10 keV fluxes of 1 × 10-14 ergs cm-2 s-1, and 2 deg2 for fluxes of 3 × 10-14 ergs cm-2 s-1. Our spectra reach to R-band magnitudes of 24 and have produced identifications and redshifts for 438 hard X-ray sources. Typical completeness levels in the 27 Chandra fields studied are 40%-70%. The vast majority of the 2-10 keV selected sample are active galactic nuclei (AGNs) with redshifts between 0.1 and 3; our highest redshift source lies at z = 4.33. We find that few sources at z 1022 cm-2, independent of unobscured luminosity. We classify 168 sources as emission-line galaxies; all are X-ray-luminous (LX > 1041 ergs s-1) objects with optical spectra lacking both high-ionization lines and evidence of a nonstellar continuum. The redshift distribution of these emission-line galaxies peaks at a significantly lower redshift than does that of the sources we spectroscopically identify as AGNs. We conclude that few of these sources, even at the low-luminosity end, can be powered by starburst activity. Stacking spectra for a subset of these sources in a similar redshift range, we detect [Ne V] λ3426 emission, a clear signature of AGN activity, confirming that the majority of these objects are Seyfert 2 galaxies in which the high-ionization lines are diluted by stellar emission. We find a total of 33 objects lacking broad lines in their optical spectra that have quasar X-ray luminosities (LX > 1044 ergs s-1), the largest sample of such objects identified to date. In addition, we explore 17 AGNs associated with galaxy clusters and find that the cluster-member AGN sample has a lower fraction of broad-line AGNs than does the background sample.

Optical and Near-Infrared Spectroscopy of a High-Redshift Hard X-Ray-emitting Spiral Galaxy*

We present optical and near-infrared Keck spectroscopy of CXOHDFN J123635.6+621424 (HDFX 28), a hard X-ray source at a redshift of z = 2.011 in the flanking fields of the Hubble Deep Field North (HDF-N). HDFX 28 is a red source (R-Ks = 4.74) with extended steep-spectrum (α GHz > 0.87 GHz) microjansky radio emission and significant emission (441 μJy) at 15 μm. Accordingly, initial investigations prompted the interpretation that HDFX 28 is powered by star formation. Subsequent Chandra imaging, however, revealed hard (Γ = 0.30) X-ray emission indicative of absorbed active galactic nucleus (AGN) activity, implying that HDFX 28 is an obscured type II AGN. The optical and near-infrared spectra presented herein corroborate this result; the near-infrared emission lines cannot be powered by star formation alone, and the optical emission lines indicate a buried AGN. HDFX 28 is identified with a face-on moderately late-type spiral galaxy. Multiwavelength morphological studies of the HDF-N have heretofore revealed no galaxies with any kind of recognizable spiral structure beyond z > 2. We present a quantitative analysis of the morphology of HDFX 28, and we find the measures of central concentration and asymmetry to be indeed consistent with those expected for a rare high-redshift spiral galaxy.

The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. II. Optical Imaging

The Serendipitous Extragalactic X-ray Source Identification (SEXSI) Program is designed to expand significantly the sample of identified extragalactic hard X-ray sources at intermediate fluxes, 10-13 ergs cm-2 s-1 S2-10 keV < 10-15 ergs cm-2 s-1. SEXSI, which includes sources derived from more than 2 deg2 of Chandra images, provides the largest hard X-ray-selected sample yet studied, offering an essential complement to the Chandra Deep Fields (total area ~0.2 deg2). In this paper we describe R-band optical imaging of the SEXSI fields from the Palomar, MDM, and Keck observatories. We have identified counterparts or derived flux limits for nearly 1000 hard X-ray sources. Using the optical images, we derive accurate source positions. We investigate correlations between optical and X-ray flux, and optical flux and X-ray hardness ratio. We also study the density of optical sources surrounding X-ray counterparts, as well as the properties of optically faint, hard X-ray sources.