135 research outputs found
Tracing Quasar Narrow-Line Regions Across Redshift: A Library of High S/N Optical Spectra
In a single optical spectrum, the quasar narrow-line region (NLR) reveals low
density, photoionized gas in the host galaxy interstellar medium, while the
immediate vicinity of the central engine generates the accretion disk continuum
and broad emission lines. To isolate these two components, we construct a
library of high S/N optical composite spectra created from the Sloan Digital
Sky Survey (SDSS-DR7). We divide the sample into bins of continuum luminosity
and Hbeta FWHM that are used to construct median composites at different
redshift steps up to 0.75. We measure the luminosities of the narrow-emission
lines [NeV]3427, [NeIII]3870, [OIII]5007, and [OII]3728 with ionization
potentials (IPs) of 97, 40, 35, and 13.6 eV respectively. The high IP lines'
luminosities show no evidence of increase with redshift consistent with no
evolution in the AGN SED or the host galaxy ISM illuminated by the continuum.
In contrast, we find that the [OII] line becomes stronger at higher redshifts,
and we interpret this as a consequence of enhanced star formation contributing
to the [OII] emission in host galaxies at higher redshifts. The SFRs estimated
from the [OII] luminosities show a flatter increase with z than non-AGN
galaxies given our assumed AGN contribution to the [OII] luminosity. Finally,
we confirm an inverse correlation between the strength of the FeII4570 complex
and both the [OIII] EW (though not the luminosity) and the width of the Hbeta
line as known from the eigenvector 1 correlations.Comment: 17 pages, colour figures, accepted for publication in MNRA
Observational Tests of Intergalactic Enrichment Models
We summarize recent results assessing the carbon and silicon abundances of
the intergalactic medium (IGM) using the `pixel optical depth' technique. We
briefly discuss the implications of these results for models of intergalactic
enrichment, focusing on distinguishing `early' z >> 4 enrichment by the first
generations of stars and objects from `late' enrichment by 2 < z < 5 Ly-break
galaxies. We then discuss the comparison of observed QSO spectra to simulated
spectra generated from cosmological simulations that self-consistently include
enrichment, and draw qualitative implications for the general picture of
intergalactic enrichment at z > 2.Comment: 6 pages, to appear in proceedings of IAU 199 conference, "Probing
Galaxies through Quasar Absorption Lines," eds. Williams, Shu, Menar
The blackness of the cosmic microwave background spectrum as a probe of the distance-duality relation
A violation of the reciprocity relation, which induces a violation of the
distance duality relation, reflects itself in a change in the normalisation of
the cosmic microwave spectrum in such a way that its spectrum is grey. We show
that existing observational constraints imply that the reciprocity relation
cannot be violated by more than 0.01% between decoupling and today. We compare
this effect to other sources of violation of the distance duality relations
which induce spectral distortion of the cosmic microwave background spectrum.Comment: 5 pages, 2 figures, references added, minor typos correcte
A Slant on Warped Extra Dimensions
We propose an orbifolded, warped, extra dimension scenario in which the
visible brane is not parallel to the hidden brane. This leads automatically to
Lorentz violation in the visible, four dimensional world. The background
solution to the Einstein equations is a function of a parameter that can be
identified with the amount of 'tilting' of the brane. The cosmological constant
is found to coincide with the classic Randall-Sundrum value to first order in
this tilt. Lorentz violating effects induced in the Standard Model are
considered. We find that the strongest constraint on the tilt comes from
determinations of the electron-proton mass ratio in six quasar spectra (four
optical and two radio). Measurements of a third radio source could improve this
by an order of magnitude.Comment: 13 pages, no figure
A Direct Measurement of the IGM Opacity to HI Ionizing Photons
We present a new method to directly measure the opacity from HI Lyman limit
(LL) absorption k_LL along quasar sightlines by the intergalactic medium (IGM).
The approach analyzes the average (``stacked'') spectrum of an ensemble of
quasars at a common redshift to infer the mean free path (MFP) to ionizing
radiation. We apply this technique to 1800 quasars at z=3.50-4.34 drawn from
the Sloan Digital Sky Survey (SDSS), giving the most precise measurements on
k_LL at any redshift. From z=3.6 to 4.3, the opacity increases steadily as
expected and is well parameterized by MFP = (48.4 +/- 2.1) - (38.0 +/-
5.3)*(z-3.6) h^-1 Mpc (proper distance). The relatively high MFP values
indicate that the incidence of systems which dominate k_LL evolves less
strongly at z>3 than that of the Lya forest. We infer a mean free path three
times higher than some previous estimates, a result which has important
implications for the photo-ionization rate derived from the emissivity of star
forming galaxies and quasars. Finally, our analysis reveals a previously
unreported, systematic bias in the SDSS quasar sample related to the survey's
color targeting criteria. This bias potentially affects all z~3 IGM studies
using the SDSS database.Comment: 7 pages, 4 figures; Accepted to ApJ
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
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