242 research outputs found
Limits on the Number of Close Optical Quasar Pairs
A new search has been conducted for close pairs of quasars with identical spectra, including both emission line objects and blue stellar objects. Survey plates covering 3.9 deg^2 were selected for image quality, full image widths being 0."8 to l."2. Although 200 to 400 quasar spectral images should have been examined, no candidate pairs with separations < 4" were found. Eight such pairs from 4" to 10" were found, but none were subsequently confirmed as lensed quasars. The selection bias is derived and applied to these limits. It is concluded that the absence of close pairs expected from gravitational lensing models cannot be explained by observational selection effects
Redshifts from Spitzer Spectra for Optically Faint, Radio Selected Infrared Sources
Spectra have been obtained with the Infrared Spectrograph on the Spitzer
Space Telescope for 18 optically faint sources (R > 23.9,mag) having f(nu)
(24um) > 1.0,mJy and having radio detections at 20 cm to a limit of 115
microJy. Sources are within the Spitzer First Look Survey. Redshifts are
determined for 14 sources from strong silicate absorption features (12 sources)
or strong PAH emission features (2 sources), with median redshift of 2.1.
Results confirm that optically faint sources of ~1 mJy at 24um are typically at
redshifts z ~ 2, verifying the high efficiency in selecting high redshift
sources based on extreme infrared to optical flux ratio, and indicate that 24um
sources which also have radio counterparts are not systematically different
than samples chosen only by their infrared to optical flux ratios. Using the
parameter q = log[f(nu)(24um)/f(nu)(20 cm)] 17 of the 18 sources observed have
values of 0<q<1, in the range expected for starburst-powered sources, but only
a few of these show strong PAH emission as expected from starbursts, with the
remainder showing absorbed or power-law spectra consistent with an AGN
luminosity source. This confirms previous indications that optically faint
Spitzer sources with f(nu)(24um) > 1.0mJy are predominately AGN and represent
the upper end of the luminosity function of dusty sources at z ~ 2. Based on
the characteristics of the sources observed so far, we predict that the nature
of sources selected at 24um will change for f(nu)(24um) < 0.5 mJy to sources
dominated primarily by starbursts.Comment: Accepted ApJ 20 February 2006, v638 2 issue, 10pages including 3
figure
Limits on the Number of Close Optical Quasar Pairs
A new search has been conducted for close pairs of quasars with identical spectra, including both emission line objects and blue stellar objects. Survey plates covering 3.9 deg^2 were selected for image quality, full image widths being 0."8 to l."2. Although 200 to 400 quasar spectral images should have been examined, no candidate pairs with separations < 4" were found. Eight such pairs from 4" to 10" were found, but none were subsequently confirmed as lensed quasars. The selection bias is derived and applied to these limits. It is concluded that the absence of close pairs expected from gravitational lensing models cannot be explained by observational selection effects
Obscuration in extremely luminous quasars
The spectral energy distributions and infrared (IR) spectra of a sample of
obscured AGNs selected in the mid-IR are modeled with recent clumpy torus
models to investigate the nature of the sources, the properties of the
obscuring matter, and dependencies on luminosity. The sample contains 21
obscured AGNs at z=1.3-3 discovered in the largest Spitzer surveys (SWIRE,
NDWFS, & FLS) by means of their extremely red IR to optical colors. All sources
show the 9.7micron silicate feature in absorption and have extreme mid-IR
luminosities (L(6micron)~10^46 erg/s). The IR SEDs and spectra of 12 sources
are well reproduced with a simple torus model, while the remaining 9 sources
require foreground extinction from a cold dust component to reproduce both the
depth of the silicate feature and the near-IR emission from hot dust. The
best-fit torus models show a broad range of inclinations, with no preference
for the edge-on torus expected in obscured AGNs. Based on the unobscured QSO
mid-IR luminosity function, and on a color-selected sample of obscured and
unobscured IR sources, we estimate the surface densities of obscured and
unobscured QSOs at L(6micron)>10^12 Lsun, and z=1.3-3.0 to be about 17-22
deg^-2, and 11.7 deg^-2, respectively. Overall we find that ~35-41% of luminous
QSOs are unobscured, 37-40% are obscured by the torus, and 23-25% are obscured
by a cold absorber detached from the torus. These fractions constrain the torus
half opening angle to be ~67 deg. This value is significantly larger than found
for FIR selected samples of AGN at lower luminosity (~46 deg), supporting the
receding torus scenario. A far-IR component is observed in 8 objects. The
estimated far-IR luminosities associated with this component all exceed
3.3x10^12 Lsun, implying SFRs of 600-3000 Msun/yr. (Abridged)Comment: ApJ accepte
Mid-Infrared Spectra of Classical AGN Observed with the Spitzer Space Telescope
Full low resolution (65<R<130) and high resolution (R~600) spectra between 5
microns and 37 microns obtained with the Infrared Spectrograph (IRS) on the
Spitzer Space Telescope are presented for eight classical active galactic
nuclei (AGN) which have been extensively studied previously. Spectra of these
AGN are presented as comparison standards for the many objects, including
sources at high redshift, which are being observed spectroscopically in the
mid-infrared for the first time using the IRS. The AGN are NGC4151, Markarian
3, I Zwicky 1, NGC 1275, Centaurus A, NGC 7469, Markarian 231, and NGC 3079.
These sources are used to demonstrate the range of infrared spectra encountered
in objects which have widely different classification criteria at other
wavelengths but which unquestionably contain AGN. Overall spectral
characteristics - including continuum shape, nebular emission lines, silicate
absorption and emission features, and PAH emission features - are considered to
understand how spectral classifications based on mid-infrared spectra relate to
those previously derived from optical spectra. The AGN are also compared to the
same parameters for starburst galaxies such as NGC 7714 and the compact, low
metallicity starburst SBS 0335-052 previously observed with the IRS. Results
confirm the much lower strengths of PAH emission features in AGN, but there are
no spectral parameters in this sample which unambiguously distinguish AGN and
starbursts based only on the slopes of the continuous spectra.Comment: Accepted by Ap
HST Morphologies of z~2 Dust Obscured Galaxies I: Power-law Sources
We present high spatial resolution optical and near-infrared imaging obtained
using the ACS, WFPC2 and NICMOS cameras aboard the Hubble Space Telescope of 31
24um--bright z~2 Dust Obscured Galaxies (DOGs) identified in the Bootes Field
of the NOAO Deep Wide-Field Survey. Although this subset of DOGs have mid-IR
spectral energy distributions dominated by a power-law component suggestive of
an AGN, all but one of the galaxies are spatially extended and not dominated by
an unresolved component at rest-frame UV or optical wavelengths. The observed
V-H and I-H colors of the extended components are 0.2-3 magnitudes redder than
normal star-forming galaxies. All but 1 have axial ratios >0.3, making it
unlikely that DOGs are composed of an edge-on star-forming disk. We model the
spatially extended component of the surface brightness distributions of the
DOGs with a Sersic profile and find effective radii of 1-6 kpc. This sample of
DOGs is smaller than most sub-millimeter galaxies (SMGs), but larger than
quiescent high-redshift galaxies. Non-parametric measures (Gini and M20) of DOG
morphologies suggest that these galaxies are more dynamically relaxed than
local ULIRGs. We estimate lower limits to the stellar masses of DOGs based on
the rest-frame optical photometry and find that these range from ~10^(9-11)
M_sun. If major mergers are the progenitors of DOGs, then these observations
suggest that DOGs may represent a post-merger evolutionary stage.Comment: 23 pages, 9 figures, 6 tables, accepted to ApJ; lower limits on
stellar mass revised upwards by factor of (1+z
HST Morphologies of z ~ 2 Dust-Obscured Galaxies II: Bump Sources
We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared
galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured
galaxies, or DOGs) which have a local maximum in their spectral energy
distribution (SED) at rest-frame 1.6um associated with stellar emission. These
sources, which we call "bump DOGs", have star-formation rates of 400-4000
Msun/yr and have redshifts derived from mid-IR spectra which show strong
polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going
star-formation. Using a uniform morphological analysis, we look for
quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected
ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are
more typical of obscured active galactic nuclei than starbursts),
sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from
the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than
power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc)
and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/-
0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with
expectations from simulations of major mergers in which merging systems during
the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7.
Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally
peaked, single-object morphologies rather than diffuse and irregular
morphologies. This distinction in morphologies may imply that less obscured
ULIRGs sample the merger near the end of the peak star-formation rate period.
Alternatively, it may indicate that the intense star-formation in these
less-obscured ULIRGs is not the result of a recent major merger.Comment: Accepted to ApJ; 22 pages, 8 Figures, 7 Table
First mid-infrared spectrum of a faint high-z galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope
The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer
Space Telescope allows for the first time the measurement of mid-infrared
spectra from 14 to 38 microns of faint high-z galaxies. This unique capability
is demonstrated with observations of sources having 16 micron fluxes of 3.6 mJy
(CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is
illustrated which determines the redshift by fitting emission and absorption
features characteristic of nearby galaxies to the spectrum of an unknown
source. For CFRS 14.1157, the measured redshift is z = 1.00+/-0.20 in agreement
with the published result of z = 1.15. The spectrum is dominated by emission
from an AGN, similar to the nucleus of NGC 1068, rather than a typical
starburst with strong PAH emission like M82. Such spectra will be crucial in
characterizing the nature of newly discovered distant galaxies, which are too
faint for optical follow-up.Comment: Accepted in ApJ Sup. Spitzer Special Issue, 4 pages, 5 figure
Long term variability of the Broad Emission Line profiles in AGN
Results of a long-term monitoring ( years) of the broad line and
continuum fluxes of three Active Galactic Nuclei (AGN), 3C 390.3, NGC 4151, and
NGC 5548, are presented. We analyze the H and H profile
variations during the monitoring period and study different details (as bumps,
absorption bands) which can indicate structural changes in the Broad Line
Region (BLR). The BLR dimensions are estimated using the time lags between the
continuum and the broad lines flux variations. We find that in the case of 3C
390.3 and NGC 5548 a disk geometry can explain both the broad line profiles and
their flux variations, while the BLR of NGC 4151 seems more complex and is
probably composed of two or three kinematically different regions.Comment: 10 pages, 9 figures, New Astronomy Reviews (Proceeding of 7th
SCSLSA), in pres
The Spitzer Infrared Nearby Galaxies Survey: A High-Resolution Spectroscopy Anthology
High resolution mid-infrared spectra are presented for 155 nuclear and
extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS).
The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared
emission lines are also provided, along with upper limits in key lines for
infrared-faint targets. The SINGS sample shows a wide range in the ratio of
[SIII]18.71um/[SIII]33.48um, but the average ratio of the ensemble indicates a
typical interstellar electron density of 300-400 cm^{-3} on ~23"x15" scales and
500-600 cm^{-3} using ~11"x9" apertures, independent of whether the region
probed is a star-forming nuclear, a star-forming extranuclear, or an AGN
environment. Evidence is provided that variations in gas-phase metallicity play
an important role in driving variations in radiation field hardness, as
indicated by [NeIII]15.56um/[NeII]12.81um, for regions powered by star
formation. Conversely, the radiation hardness for galaxy nuclei powered by
accretion around a massive black hole is independent of metal abundance.
Furthermore, for metal-rich environments AGN are distinguishable from
star-forming regions by significantly larger [NeIII]15.56um/[NeII]12.81um
ratios. Finally, [FeII]25.99um/[NeII]12.81um versus [SiII]34.82um/[SIII]33.48um
also provides an empirical method for discerning AGN from normal star-forming
sources. However, similar to [NeIII]15.56um/[NeII]12.81um, these mid-infrared
line ratios lose their AGN/star-formation diagnostic powers for very low
metallicity star-forming systems with hard radiation fields.Comment: Accepted for publication in Ap
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