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Resubmission of -0000
NOAO Observing Proposal Standard proposal Panel: For oфce use.
Date: September 30, 2003 Category: Active Galaxies
The Hardest X-ray Sources Unveiled
PI: Paul J. Green Status: P Aфl.: Smithsonian Astrophysical Observatory
High Energy, 60 Garden St, Cambridge, MA 02138 USA
Email: pgreen@cfa.harvard.edu Phone: 617-495-7057 FAX: 617-496-7356
CoI: Joe Shields Status: P Aфl.: Ohio University
CoI: Almus Kenter Status: P Aфl.: Smithsonian Astrophysical Observatory
CoI: Christine Jones Status: P Aфl.: Smithsonian Astrophysical Observatory
CoI: Steve Murray Status: P Aфl.: Smithsonian Astrophysical Observatory
CoI: Buell Jannuzi Status: P Aфl.: NOAO
CoI: Michael Brown Status: P Aфl.: NOAO
Chris Kochanek
Bill Forman
Arjun Dey
Brian McNamara
Kate Brand
Abstract of Scientic Justication (will be made publicly available for accepted proposals):
The hard Cosmic X-ray Background (CXRB) has been resolved into point sources by the Chandra
deep eld exposures as predominantly low luminosity objects at z < 1, and the nature of these
AGNs is not well understood. The best X-ray population synthesis models of the CXRB require
these but also a substantial component from high-L X absorbed AGN (Type 2 QSOs). We propose
Gemini spectroscopy of a sample that addresses the 2 primary questions that these populations
pose. We have observed with 126 Chandra pointings a 9 deg 2 region overlapping the NOAO Deep
Wide Field Survey (NDWFS). Our survey selects the heavily absorbed low-L X objects at redshifts
of a few tenths where we can likely detect their AGN spectroscopically even amidst a bright host
galaxy. It also uniformly samples the largest volume to date in the hunt for the elusive Type 2
QSOs.
Summary of observing runs requested for this project
Run Telescope Instrument No. Nights Moon Optimal months Accept. months
1 GEM-N GMOSN 3 dark Mar - May Feb - Jun
2
3
4
5
6
Scheduling constraints and non-usable dates (up to four lines).

NOAO Proposal Page 2 This box blank.
Scientic Justication Be sure to include overall signicance to astronomy. For standard proposals
limit text to one page with gures, captions and references on no more than two additional pages.
We propose Gemini spectroscopy to measure the redshifts and emission-line uxes for a sample of
X-ray sources with particular relevance to understanding the cosmic X-ray background (CXRB)
and black hole growth. The goal is to understand how these objects relate to standard AGN classes,
and hence how to better connect the CXRB sources with our understanding of AGN physics.
The sample consists of a subset of hard X-ray sources in a recent Chandra survey of the NOAO
Deep Wide-Field Survey (NDWFS) Bootes eld. The targets were chosen because they display
particularly large ratios of hard to soft photons in the X-ray spectrum, probably caused by strong
photoelectric absorption of the soft X-rays by gas in front of the X-ray source (Figure 1). Ab-
sorbed sources of this type are believed to contribute a signicant fraction of the CXRB, based on
constraints from source number counts and the background spectrum (e.g., Gilli 2003; Figure 2a).
The Bootes sample we propose here exploits the unusually large contiguous areal coverage of the
Chandra-NDWFS survey, which spans 9 square degrees. The Chandra exposures for this eld
are rather shallow (5 ksec each), and the combination of shallow depth and large area means that
many of the sources that we have identied are relatively bright and nearby. The appearance and
bright optical magnitudes for these counterparts suggest that they have typical redshifts of a few
tenths, which would imply low-to-intermediate X-ray luminosities (typically >
10 42 erg s 1 ). In
contrast, most recent studies of the CXRB source population have relied heavily on small-area
studies employing only a few, very deep Chandra pointings. The small areal coverage means that
relatively few bright examples of low-z hard x-ray sources have been found in these surveys; a
recent compilation by Cowie et al. (2003), which adds in ASCA results, has only about a dozen
objects at z < 0:5 with hard x-ray uxes above the ux limit of our Bootes survey (3 10 14 erg
cm 2 s 1 ).
By working with bright, nearby sources, we want to obtain high signal-to-noise spectra to get at the
question of what kind of AGNs these things are. The alternatives include 1) quasar-like objects with
broad lines, in which case they are prime candidates for harboring broad-absorption-line out ows
that provide the absorption (e.g., Green et al. 2001); 2) Seyfert 2-like objects showing narrow, high-
ionization lines, implicating a probable circumnuclear torus or similar blocking structure; 3) highly
extinguished or intrinsically faint sources in which emission lines from any kind of AGN are not
readily seen. All three categories are found in the existing deep surveys (e.g., Barger et al. 2003),
but the quality of those very faint optical spectra is variable and the properties of the emission lines,
when detected, have not been studied in detail (the spectra are often not ux-calibrated). Objects
in the second category, while expected at high luminosities (Type 2 quasars) are surprisingly rare
(e.g., Norman et al. 2002, Schmidt et al. 2002). Moran et al. (2002) have also noted that
some of the objects in the third category may have their optical AGN signatures swamped by
the circumnuclear galaxy light. The shallow nature of our survey reduces our sensitivity to this
eect: distant objects with large metric apertures will have quasar-like luminosities, while in nearby
sources we can resolve out much of the surrounding galaxy light (see Figure 3).
The larger signicance of the objects we are targeting is that they trace the dominant channel for
the buildup of black holes. As discussed by Cowie et al. (2003), the luminosity dependence of
AGN evolution implies that black hole growth is dominated by intermediate- and low-luminosity
AGNs. The X-ray background results indicate that the majority of these objects should be highly
absorbed (Gilli 2003). With our selection based on X-ray hardness ratios measured in a shallow
survey, we will obtain a quantitative description of the dominant mode of black hole construction
and its associated phenomenology.

NOAO Proposal Page 3 This box blank.
Figure 1: Hard sources detected in our 9deg 2 Chandra mosaic. The subsample we have chosen for
study are all those with Hardness > 0:1, corresponding to an intrinsic absorbing column of at least
NH = 10 22 atoms cm 2 , similar to columns of the absorbed AGN that dominate the CXRB.
Figure 2: LEFT: Hard band logN-logS with an X-ray population synthesis of absorbed AGN that
reproduces the observed CXRB spectrum and normalization (Gilli et al. 2003). RIGHT: Most such
models fail without the addition of luminous hard sources, the optically elusive Type-2 quasars.
REFERENCES
Barger, A. J., et al. 2003, AJ, 126, 632 Moran, E. C., et al. 2002, ApJ, 579, L71
Cowie, L. L., et al. 2003, ApJ, 584, L57 Norman, C., et al. 2002, ApJ, 571, 218
Gilli, R. 2003, astro-ph/0303115 Schmidt, G., et al. 2002, ApJ, 578, L99
Green, P. J., et al. 2001, ApJ, 558, 109

NOAO Proposal Page 4 This box blank.
Figure 3: K-band nder charts for our hard sample. Units indicate arcsec, so nders are 30 00 on
a side. Magnitudes, colors, and morphologies are consistent with most of these hard X-ray sources
having nearby galaxy hosts, while some are likely to be luminous obscured QSOs at z <
1.

NOAO Proposal Page 5 This box blank.
Experimental Design Describe your overall observational program. How will these observations
contribute toward the accomplishment of the goals outlined in the science justication? If you've requested
long-term status, justify why this is necessary for successful completion of the science. (limit text to one
page)
This year with 126 contiguous pointings of 5 ksec each, we obtained the largest Chandra image to
date - a 9 deg 2 region towards Bootes chosen to coincide with the NOAO Deep-Wide eld survey
(NDWFS). The NDWFS is unique in its wide-eld coverage and availability of deep multiwave-
length imaging (including VLA and SIRTF). Our Chandra survey is designed to sample volumes
appropriate for statistical investigations of clusters and their environments, and AGN phenomena
in relation to galaxy host properties and their evolution.
By running wavelet detection in 3 bands (soft S - 0.5-2 keV, hard H - 2-7 keV, and broad - 0.5-7 keV),
we detect 3400 high condence sources (P < 10 6 ) down to f X (2-8 keV)= 10 14 erg cm 2 s 1 ,
consistent with number counts expected from previous surveys. From this list we culled all sources
detected in the H band (2-7 keV), that are bright (3 the ux limit) with S
N > 2, and an X-ray
spectral hardness ratio H S
H+S > 0:1. This hardness is equivalent to a typical AGN spectrum absorbed
by a column of at least NH = 10 22 atoms cm 2 , similar to columns of the absorbed AGN that
dominate the CXRB.
Matching to the deep (R 26) NDWF imaging means that we sample the full celestial range of
f x =f opt in a fair-sized census, so matching can be unbiased by f x =f opt . The deep BW , R, I imaging
also premits detailed environmental information with high quality photometric redshifts. On the
other hand, high optical source densities ( 100/arcmin 2 ) mean that our matching must be very
careful. The Chandra PSF centroid error px varies both as a function of Chandra o-axis angle
and source strength. We match hard Chandra sources to existing NDFWS catalogs by requiring
optical/X-ray source positions to agree within 3 px or 1: 00 5, whichever is larger; the minimum radius
is set by systematic uncertainties. All matches are inspected by eye. A full 90% of our bright, hard
sources are matched in the NDWFS catalogue.
In the nearby Universe, our sample probes the obscured AGN proposed as the most important
constituent of the CXRB. At the lowest denitively AGN luminosities (LX > 10 42 ), our Chan-
dra/NOAO survey combination is highly complete to z = 0:1, where an L E/S0 host has R = 16.
These AGN are exactly those that appear at higher redshifts to have no AGN component (Moran
et al. 2002). Gemini spectroscopy aords adequate sensitivity to the AGN optical spectroscopic
signature buried in the host light, and the longslit spectra allow reconstruction of how the spectrum
would appear at higher redshifts where the host lls the aperture.
Another elusive and hotly debated component of the CXRB are the Type 2 QSOs. Our X-ray ux
limit samples Type 2 QSOs (LX > 10 44 ) to z = 1, where even an L E/S0 galaxy host has R < 22:2
(H ф =70 km s 1 Mpc 1
,
= 0:7,
and
M = 0:3.). Thus we are complete for the elusive Type 2
quasars in the largest volume available (33 million Mpc 3 ) from any extant modern X-ray dataset.
Proprietary Period: 18 months

NOAO Proposal Page 6 This box blank.
Use of Other Facilities Describe how the proposed observations complement data from non-NOAO
facilities. For each of these other facilities, indicate the nature of the observations (yours or those of others),
and describe the importance of the observations proposed here in the context of the entire program.
The Chandra Bootes raster imaging this past Spring 2003 required more than 180 hours of Chandra
time over about 2 weeks, and represents a major legacy observation from Chandra.
Once the MMT/Hectospec combination is available, we plan to use its 1deg multiber capability for
a redshift survey of our full X-ray sample of 3300 objects. We expect the the poor sky subtraction
with bers to limit those spectra to R <
21:5. In any case, bers do not allow for host/nuclear
separation.
The NOAO co-Is are PIs on the NOAO Deep Wide Field Survey project, an NOAO legacy that
provides the optical multicolor imaging foundation of our proposal.
Previous Use of NOAO Facilities List allocations of telescope time on facilities available through
NOAO to the PI during the past 2 years, together with the current status of the data (cite publications
where appropriate). Mark with an asterisk those allocations of time related to the current proposal. Please
include original proposal semesters and ID numbers when available.
The PI observed with the WIYN/Hydra in April 2001 2001, and supervised observing and reduction
of numerous NOAO 4m/MOSAIC runs for followup of optically bright, primarily soft Chandra X-
ray sources. The bulk of these data are published in Green et al. 2003 (ApJS, in press) and
calibrated images are posted publicly on the web at http://hea-www.harvard.edu/CHAMP/

NOAO Proposal Page 7 This box blank.
Observing Run Details for Run 1: GEM-NQ/GMOSN
Technical Description Describe the observations to be made during this observing run. Justify the
specic telescope, the number of nights, the instrument, and the lunar phase. List objects, coordinates, and
magnitudes (or surface brightness, if appropriate) in the Target Tables section below (required for WIYN-2hr,
WIYN-SYN, YALO, and Gemini runs).
We have selected 85 hard X-ray targets, and we are now completing counterpart matching to existing
NOAO Deep Wide Field Survey photometric catalogs. We have so far matched sources that lie
within the coverage of the ONIS IR data - 35 sources in about 4.5 deg 2 , i.e., 41% of the sources.
We nd K-band counterparts for 90%, indicating an excellent match to the X-ray ux limit. This
representative matched sample has a mag range 19 < R < 22:5. Since the redshift, object type, and
admixture of nuclear/stellar ux is unknown a prior for each object, we assume a survey approach
geared to detect at 3 an unresolved H emission line of ux 1 10 17 erg cm 2 s 1 , above a
continuum level calculated from the observed total R mag. This assumed line ux is that expected
from a low-luminosity AGN based on our limiting sample X-ray ux of 3 10 14 (Koratkar et al.
1995, ApJ, 400, 132). For a typical R = 22 object, this requires 45min on source, which we thus
assume for all objects. This strategy is well-dened, and simple for the Queue Observer.
Representative mags and exposure (+overhead) times for 2 objects whose total magnitudes bracket
the sample are shown in the Target Table. This Our `snapshot survey' approach may be rened in
PhaseII by incorporating photometric redshifts. While we expect 75 hard X-ray sources matched
to counterparts 19 < R < 22:5, we request 3 nights of Gemini/GMOS time for a random subsample
of 30% (24 objects) for a fair probe of the nature of these targets. A detailed target table will be
provided in PhaseII, depending on the amount of awarded time. Since these targets are all in the
same Bootes region, classical mode is also a viable option, if the TAC prefers.
Principal Contact: Paul J. Green
Instrument Resources
Filters: GG455 G0305
Dispersers: R150 G5306
Focal Plane Units: Longslit 0.75 arcsec
R.A. range of principal targets (hours): 14 to 15
Dec. range of principal targets (degrees): 34 to 36
Target Table for Run 1: GEM-N/GMOSN
Obj
ID Object ф Epoch Mag.
Obs.
time
WFS
stars
IQ
%
SB
%
WV
%
CC
% Comment
1001 XB1 14:28:27.48 +34:51:22.2 J2000 22.23 70 PPO 70 50 any 70 typical fnt
1002 XB2 14:29:50.65 +35:08:43.0 J2000 19.06 70 PpO 70 50 any 70 typical brt
NOAO observing proposal L A T E X macros v2.9.