221 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
Mid-Infrared Spectroscopy of Optically Faint Extragalactic 70 micron Sources
We present mid-infrared spectra of sixteen optically faint sources with 70
micron fluxes in the range 19-38mJy. The sample spans a redshift range of
0.35<z<1.9, with most lying between 0.8<z<1.6, and has infrared luminosities of
10^{12} - 10^{13} solar luminosities. Ten of 16 objects show prominent
polycyclic aromatic hydrocarbon (PAH) emission features; four of 16 show weak
PAHs and strong silicate absorption, and two objects have no discernable
spectral features. Compared to samples with 24 micron fluxes >10mJy, the 70\um
sample has steeper IR continua and higher luminosities. The PAH dominated
sources are among the brightest starbursts seen at any redshift, and reside in
a redshift range where other selection methods turn up relatively few sources.
The absorbed sources are at higher redshifts and have higher luminosities than
the PAH dominated sources, and may show weaker luminosity evolution. We
conclude that a 70 micron selection extending to ~20mJy, in combination with
selections at mid-IR and far-IR wavelengths, is necessary to obtain a complete
picture of the evolution of IR-luminous galaxies over 0<z<2.Comment: ApJ accepte
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
[CII] 158 micron Luminosities and Star Formation Rate in Dusty Starbursts and AGN
Results are presented for [CII] 158 micron line fluxes observed with the
Herschel PACS instrument in 112 sources with both starburst and AGN
classifications, of which 102 sources have confident detections. Results are
compared with mid-infrared spectra from the Spitzer Infrared Spectrometer and
with L(IR) from IRAS fluxes; AGN/starburst classifications are determined from
equivalent width of the 6.2 micron PAH feature. It is found that the [CII] line
flux correlates closely with the flux of the 11.3 micron PAH feature
independent of AGN/starburst classification, log [f([CII] 158 micron)/f(11.3
micron PAH)] = -0.22 +- 0.25. It is concluded that [CII] line flux measures the
photodissociation region associated with starbursts in the same fashion as the
PAH feature. A calibration of star formation rate for the starburst component
in any source having [CII] is derived comparing [CII] luminosity L([CII]) to
L(IR) with the result that log SFR = log L([CII)]) - 7.08 +- 0.3, for SFR in
solar masses per year and L([CII]) in solar luminosities. The decreasing ratio
of L([CII]) to L(IR) in more luminous sources (the "[CII] deficit") is shown to
be a consequence of the dominant contribution to L(IR) arising from a luminous
AGN component because the sources with largest L(IR) and smallest
L([CII])/L(IR) are AGN.Comment: Accepted for publication in The Astrophysical Journa
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
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
Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
We present a study of the intrinsic absorption lines in the ultraviolet
spectra of Seyfert 1 galaxies. We find that the fraction of Seyfert 1 galaxies
that show absorption associated with their active nuclei is more than one-half
(10/17), which is much higher than previous estimates (3 - 10%) . There is a
one-to-one correspondence between Seyferts that show intrinsic UV absorption
and X-ray ``warm absorbers''. The intrinsic UV absorption is generally
characterized by high ionization: C IV and N V are seen in all 10 Seyferts with
detected absorption (in addition to Ly-alpha), whereas Si IV is present in only
four of these Seyferts, and Mg II absorption is only detected in NGC 4151. The
absorption lines are blueshifted (or in a few cases at rest) with respect to
the narrow emission lines, indicating that the absorbing gas is undergoing net
radial outflow. At high resolution, the absorption often splits into distinct
kinematic components that show a wide range in widths (20 - 400 km/s FWHM),
indicating macroscopic motions (e.g., radial velocity subcomponents or
turbulence) within a component. The strong absorption components have cores
that are much deeper than the continuum flux levels, indicating that the
regions responsible for these components lie completely outside of the broad
emission-line regions. The covering factor of the absorbing gas in the line of
sight, relative to the total underlying emission, is C > 0.86, on average. The
global covering factor, which is the fraction of emission intercepted by the
absorber averaged over all lines of sight, is C > 0.5.Comment: 56 pages, Latex, includes 4 figures (encapsulated postscript), Fig. 1
has 2 parts and Fig. 2 has 3 parts, to appear in the Astrophysical Journa
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
- …