Ethan J. Schreier

The Great Observatories Origins Deep Survey: Initial Results from Optical and Near-Infrared Imaging

This special issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multiband imaging data obtained with the Hubble Space Telescope and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 arcmin2 in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-Ray Observatory and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory and from the Kitt Peak National Observatory. Deep observations with the Space Infrared Telescope Facility are scheduled. Reduced data from all facilities are being released worldwide within 3-6 months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own Galaxy. This Letter serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.

The Chandra Deep Field-South: Optical Spectroscopy. I.

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field-South (CDFS). A total of 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R - K colors are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R 1044 ergs s-1] at z > 2 (13 sources with unambiguous spectroscopic identification); most X-ray type 1 QSOs are bright, R 24, whereas most X-ray type 2 QSOs have R 24, which may explain the difference with the CDFN results as few spectroscopic redshifts were obtained for R > 24 CDFN X-ray counterparts. There are X-ray type 1 QSOs down to z ~ 0.5, but a strong decrease at z 5) as X-ray counterparts, and their fraction strongly increases with decreasing optical flux, up to 25% for the R ? 24 sample. They cover the whole range of X-ray hardness ratios, comprise objects of various classes (in particular a high fraction of z 1 X-ray absorbed AGNs, but also elliptical and starburst galaxies) and more than half of them should be fairly bright X-ray sources [LX(0.5-10 keV) > 1042 ergs s-1]. Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.

The Chandra Deep Field-South: The 1 Million Second Exposure*

We present the main results from our 940 ks observation of the Chandra Deep Field-South using the source catalog described in an accompanying paper by Giacconi et al. We extend the measurement of source number counts to 5.5 × 10-17 ergs cm-2 s-1 in the soft 0.5-2 keV band and 4.5 × 10-16 ergs cm-2 s-1 in the hard 2-10 keV band. The hard-band log N-log S shows a significant flattening (slope 0.6) below ≈10-14 ergs cm-2 s-1, leaving at most 10%-15% of the X-ray background to be resolved, the main uncertainty lying in the measurement of the total flux of the X-ray background (XRB). On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep log N-log S (slope 1.3) down to 10-15 ergs cm-2 s-1. Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a nonnegligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contributes to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the Very Large Telescope, for one-quarter of the sample to characterize the combined optical and X-ray properties of the Chandra Deep Field-South sample. Different source types are well separated in a parameter space that includes X-ray luminosity, hardness ratio, and R-K color. Type II objects, while redder on average than the field population, have colors that are consistent with being hosted by a range of galaxy types. Type II active galactic nuclei are mostly found at z 1, in contrast with predictions based on active galactic nucleus population synthesis models, thus suggesting a revision of their evolutionary parameters.

A Classic Type 2 QSO

In the Chandra Deep Field-South 1 Ms exposure, we have found, at redshift 3.700 ± 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 σ detection limit of ~3 × 10-19 ergs s-1 cm-2 A-1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ~700-2300 km s-1 with an average of approximately ~1500 km s-1. The emitting gas is metal-rich (Z 2.5-3 Z☉). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with NH 1024 cm-2. An iron Kα line with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~1045 ± 0.5 ergs cm-2 s-1, which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole-powered QSO distribution at high redshift.

Discovery of a periodic pulsating binary X-ray source in Hercules from Uhuru.

We have discovered a new pulsating X-ray source with a 1.24-sec period in the constellation Hercules. Analysis of 5 months of data has shown the existence of periodic variations in the intensity of the source and correlated sinusoidal variations in the period of the 1.24-sec pulsations. As in the case of the pulsating X-ray source Cen X-3, we interpret this effect as due to an occulting binary system, with the intensity changes due to occultation of the X-ray source by its companion and with the sinusoidal variations in the period of the 1.24-sec pulsations due to the Doppler effect. In addition, we have observed a longer-time scale cycle in which the source is bright and pulsing for approximately 9 days during which we can observe the 1.7-day occulting, followed by approximately 27 days during which the source is not detected above background on individual 20-sec scans.

New Results from the X-Ray and Optical Survey of the Chandra Deep Field-South: The 300 Kilosecond Exposure. II.

We present results from 300 ks of X-ray observations of the Chandra Deep Field-South. The field of the four combined exposures is now 0.1035 deg2, and we reach a flux limit of 10-16 ergs s-1 cm-2 in the 0.5-2 keV soft band and 10-15 ergs s-1 cm-2 in the 2-10 keV hard band, i.e., a factor of 2 fainter than the previous 120 ks exposure. The total catalog is composed of 197 sources including 22 sources detected only in the hard band, 51 only in the soft band, and 124 detected in both bands. We now have the optical spectra for 86 optical counterparts. The log N-log S relationship of the whole sample confirms the flattening with respect to the ASCA hard counts and the ROSAT soft counts. The average logarithmic slopes of the number counts are ? = 0.66 ? 0.06 and ? = 0.92 ? 0.12 in the soft and hard bands, respectively. Double power-law fits to the differential counts show evidence of further flattening at the very faint end to slopes of 0.5 ? 0.1 and 0.6 ? 0.2 in the soft and hard bands, respectively. We compute the total contribution to the X-ray background (XRB) in the 2-10 keV band, which now amounts to (1.45 ? 0.15) ? 10-11 ergs cm-2 s-1 deg-2 (after the inclusion of the ASCA sources to account for the bright end) to a lower flux limit of 10-15 ergs s-1 cm-2. This corresponds to 60%-90% of the unresolved hard XRB, given the uncertainties on its actual value. We confirm previous findings on the average spectrum of the sources, which is well described by a power law with ? = 1.44 ? 0.03, and the progressive hardening of the sources at lower fluxes. In particular, we find that the average spectral slope of the sources is flatter than the average for fluxes lower than 9 ? 10-15 ergs s-1 cm-2 in the hard band. The hardening of the spectra is consistent with an increasing fraction of absorbed objects (NH > 1022 cm-2) at low fluxes. From 86 redshifts available at present, we find that hard sources have on average lower redshifts (z ? 1) than soft sources. Their typical luminosities and optical spectra show that most of these sources are obscured active galactic nuclei (AGNs), as expected by AGN population synthesis models of the XRB. We are still in the process of finding hard sources that constitute the remaining fraction of the total XRB. Most of the sources detected only in the soft band appear to be optically normal galaxies with luminosities LX 1040-1042 ergs s-1. This population appears to be a mix of normal galaxies, possibly with enhanced star formation, and galaxies with low-level nuclear activity.

The UHURU catalog of X-ray souces.

A catalog of X-ray sources observed with the Uhuru satellite is presented. About 70 days of data have been analyzed for this catalog resulting in 125 sources. Approximately two-thirds of the sources are located within plus or minus 20 deg of the galactic plane. Some of the sources at higher galactic latitudes are identified with known extragalactic objects. Most of the strong sources near the galactic plane are found to be variable.

The third Uhuru catalog of x-ray sources

A new edition of the catalog of X-ray sources observed with Uhuru is presented. About 125 days of data have been analyzed for the 3U catalog, yielding a total of 161 X-ray sources. The distribution of sources is similar to that obtained for earlier editions of this catalog. Location error regions for many of the sources previously listed in the 2U catalog have been significantly reduced in size.

Discovery of the binary nature of SMC X-1 from Uhuru.

Analysis of data spanning a year of observations of the pulsating X-ray source Cen X-3 from Uhuru has revealed the existence of periodic variations in intensity of the source and correlated sinusoidal variations in the period of the 4.8-sec pulsations. We interpret this effect as due to an occulting binary system. The changes in intensity are then due to occultation of the X-ray source by a large massive companion, and the sinusoidal variations in the period of the 4.8-sec pulsations are due to Doppler effect. Physical parameters for the system are derived, and evidence for the existence and nature of an extended atmosphere surrounding the massive occulting object is discussed.

DISCOVERY OF PERIODIC X-RAY PULSATIONS IN CENTAURUS X-3 FROM UHURU.

Large amplitude periodic X ray pulsations from Centaurus X-3, observing abrupt source intensity and pulse rate changes