148 research outputs found
Spectrophotometry of Michigan-Tololo quasars
Emission-line quasar characteristics are confirmed for 80% of the objects observed, including at least four new quasars with spectral features indicative of supernova-like outflow. Approximately 73% of the redshifts predicted from the discovery plates are found accurate with a mean error in z of 0.03, and a large range of z (from about 0.1 to 3.16) is represented in the sample. The observed redshift distribution for quasars is marginally consistent with a constant co-moving quasar density above z approximately 2.0. The shape of the redshift distribution may be used as an isotropy probe with a cosmic time resolution of a few times one-hundred million years in the early universe; therefore, continued surveys of this sort are important even if accurate magnitudes are not determined
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
Dust emission from a parsec-scale structure in the Seyfert 1 nucleus of NGC 4151
We report mid-IR interferometric measurements with \sim 10 mas resolution,
which resolve the warm (T = 285 +25 / -50 K) thermal emission at the center of
NGC 4151. Using pairs of VLT 8.2 m telescopes with MIDI and by comparing the
data to a Gaussian model, we determined the diameter of the dust emission
region, albeit only along one position angle, to be 2.0 +/- 0.4 pc (FWHM). This
is the first size and temperature estimate for the nuclear warm dust
distribution in a Seyfert 1 galaxy. The parameters found are comparable to
those in Seyfert 2 galaxies, thus providing direct support for the unified
model. Using simple analytic temperature distributions, we find that the
mid-infrared emission is probably not the smooth continuation of the hot
nuclear source that is marginally resolved with K band interferometry. We also
detected weak excess emission around 10.5 micron in our shorter baseline
observation, possibly indicating that silicate emission is extended to the
parsec scale.Comment: 5 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
CASSIS: The Cornell Atlas of Spitzer/Infrared Spectrograph Sources. II. High-resolution observations
The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope observed about 15,000 objects during the cryogenic mission lifetime. Observations provided low-resolution (R~60-127) spectra over ~5-38um and high-resolution (R~600) spectra over ~10-37um. The Cornell Atlas of Spitzer/IRS Sources (CASSIS) was created to provide publishable quality spectra to the community. Low-resolution spectra have been available in CASSIS since 2011, and we present here the addition of the high-resolution spectra. The high-resolution observations represent approximately one third of all staring observations performed with the IRS instrument. While low-resolution observations are adapted to faint objects and/or broad spectral features (e.g., dust continuum, molecular bands), high-resolution observations allow more accurate measurements of narrow features (e.g., ionic emission lines) as well as a better sampling of the spectral profile of various features. Given the narrow aperture of the two high-resolution modules, cosmic ray hits and spurious features usually plague the spectra. Our pipeline is designed to minimize these effects through various improvements. A super sampled point-spread function was created in order to enable the optimal extraction in addition to the full aperture extraction. The pipeline selects the best extraction method based on the spatial extent of the object. For unresolved sources, the optimal extraction provides a significant improvement in signal-to-noise ratio over a full aperture extraction. We have developed several techniques for optimal extraction, including a differential method that eliminates low-level rogue pixels (even when no dedicated background observation was performed). The updated CASSIS repository now includes all the spectra ever taken by the IRS, with the exception of mapping observations
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
The distribution of silicate strength in Spitzer spectra of AGNs and ULIRGs
A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS)
on Spitzer is analyzed to study the distribution of the strength of the 9.7
micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and
Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate
features in emission and Seyfert 1s equally by emission or weak absorption.
Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are
characterized by strong silicate absorption (mean apparent optical depth about
1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron
are similar for the most luminous quasars and ULIRGs and are almost 10^5 times
more luminous than the least luminous AGN in the sample. The distributions of
spectral characteristics and luminosities are compared to those of optically
faint infrared sources at z~2 being discovered by the IRS, which are also
characterized by strong silicate absorption. It is found that local ULIRGs are
a similar population, although they have lower luminosities and somewhat
stronger absorption compared to the high redshift sources.Comment: Accepted for publication on ApJ
Moderate Resolution Spectroscopy For The Space Infrared Telescope Facility (SIRTF)
A conceptual design for an infrared spectrometer capable of both low resolution (λ/Δ-λ = 50; 2.5-200 microns) and moderate resolution (1000; 4-200 microns) and moderate resolution (1000; 4-200 microns) has been developed. This facility instrument will permit the spectroscopic study in the infrared of objects ranging from within the solar system to distant galaxies. The spectroscopic capability provided by this instrument for SIRTF will give astronomers orders of magnitude greater sensitivity for the study of faint objects than had been previously available. The low resolution mode will enable detailed studies of the continuum radiation. The moderate resolution mode of the instrument will permit studies of a wide range of problems, from the infrared spectral signatures of small outer solar system bodies such as Pluto and the satellites of the giant planets, to investigations of more luminous active galaxies and QS0s at substantially greater distances. A simple design concept has been developed for the spectrometer which supports the science investigation with practical cryogenic engineering. Operational flexibility is preserved with a minimum number of mechanisms. The five modules share a common aperture, and all gratings share a single scan mechanism. High reliability is achieved through use of flight-proven hardware concepts and redundancy. The design controls the heat load into the SIRTF cryogen, with all heat sources other than the detectors operating at 7K and isolated from the 4K cold station. Two-dimensional area detector arrays are used in the 2.5-120μm bands to simultaneously monitor adjacent regions in extended objects and to measure the background near point sources
CASSIS: The Cornell Atlas of Spitzer/IRS Sources
We present the spectral atlas of sources observed in low resolution with the
Infrared Spectrograph on board the Spitzer Space Telescope. More than 11,000
distinct sources were extracted using a dedicated algorithm based on the SMART
software with an optimal extraction (AdOpt package). These correspond to all
13,000 low resolution observations of fixed objects (both single source and
cluster observations). The pipeline includes image cleaning, individual
exposure combination, and background subtraction. A particular attention is
given to bad pixel and outlier rejection at the image and spectra levels. Most
sources are spatially unresolved so that optimal extraction reaches the highest
possible signal-to-noise ratio. For all sources, an alternative extraction is
also provided that accounts for all of the source flux within the aperture.
CASSIS provides publishable quality spectra through an online database together
with several important diagnostics, such as the source spatial extent and a
quantitative measure of detection level. Ancillary data such as available
spectroscopic redshifts are also provided. The database interface will
eventually provide various ways to interact with the spectra, such as
on-the-fly measurements of spectral features or comparisons among spectra.Comment: Accepted for publication in ApJ Supplement Serie
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