92 research outputs found
Joint Bayesian Estimation of Quasar Continua and the Lyman-Alpha Forest Flux Probability Distribution Function
We present a new Bayesian algorithm making use of Markov Chain Monte Carlo
sampling that allows us to simultaneously estimate the unknown continuum level
of each quasar in an ensemble of high-resolution spectra, as well as their
common probability distribution function (PDF) for the transmitted Ly
forest flux. This fully automated PDF regulated continuum fitting method models
the unknown quasar continuum with a linear Principal Component Analysis (PCA)
basis, with the PCA coefficients treated as nuisance parameters. The method
allows one to estimate parameters governing the thermal state of the
intergalactic medium (IGM), such as the slope of the temperature-density
relation , while marginalizing out continuum uncertainties in a fully
Bayesian way. Using realistic mock quasar spectra created from a simplified
semi-numerical model of the IGM, we show that this method recovers the
underlying quasar continua to a precision of and at
and , respectively. Given the number of principal component spectra,
this is comparable to the underlying accuracy of the PCA model itself. Most
importantly, we show that we can achieve a nearly unbiased estimate of the
slope of the IGM temperature-density relation with a precision of
at , at , for an ensemble of ten mock
high-resolution quasar spectra. Applying this method to real quasar spectra and
comparing to a more realistic IGM model from hydrodynamical simulations would
enable precise measurements of the thermal and cosmological parameters
governing the IGM, albeit with somewhat larger uncertainties given the
increased flexibility of the model.Comment: 21 pages (+ Appendix), accepted at Ap
The Opacity of the Intergalactic Medium Measured Along Quasar Sightlines at
We publicly release a new sample of medium resolution quasar spectra at
observed with the Echellette Spectrograph and
Imager (ESI) on the Keck telescope. This quasar sample represents an ideal
laboratory to study the intergalactic medium (IGM) during the end stages of the
epoch of reionization, and constrain the timing and morphology of the phase
transition. For a subset of of our highest signal-to-noise ratio spectra
(S/N, per pixel), we present a new measurement of
the Lyman- (Ly) forest opacity spanning the redshift range
. We carefully eliminate spectral regions that could
be causing biases in our measurements due to additional transmitted flux in the
proximity zone of the quasars, or extra absorption caused by strong intervening
absorption systems along the line of sight. We compare the observed evolution
of the IGM opacity with redshift to predictions from a hydrodynamical
simulation with uniform ultraviolet background (UVB) radiation, as well as two
semi-numerical patchy reionization models, one with a fluctuating UVB and
another with a fluctuating temperature field. Our measurements show a steep
rise in opacity at and an increased scatter and thus support the
picture of a spatially inhomogeneous reionization process, consistent with
previous work. However, we measure significantly higher optical depths at
than previous studies, which reduces the contrast
between the highest opacity Gunn-Peterson troughs and the average opacity trend
of the IGM, which may relieve some of the previously noted tension between
these measurements and reionization models.Comment: accepted for publication at Ap
The Circular Velocity Curve of the Milky Way from to kpc
We measure the circular velocity curve of the Milky Way with
the highest precision to date across Galactocentric distances of kpc. Our analysis draws on the -dimensional phase-space coordinates of
luminous red-giant stars, for which we previously determined
precise parallaxes using a data-driven model that combines spectral data from
APOGEE with photometric information from WISE, 2MASS, and Gaia. We derive the
circular velocity curve with the Jeans equation assuming an axisymmetric
gravitational potential. At the location of the Sun we determine the circular
velocity with its formal uncertainty to be with systematic uncertainties at the
level. We find that the velocity curve is gently but significantly declining at
, with a systematic uncertainty of
, beyond the inner kpc. We exclude the inner
kpc from our analysis due to the presence of the Galactic bar, which
strongly influences the kinematic structure and requires modeling in a
non-axisymmetric potential. Combining our results with external measurements of
the mass distribution for the baryonic components of the Milky Way from other
studies, we estimate the Galaxy's dark halo mass within the virial radius to be
and a local dark matter
density of .Comment: Accepted for publication in ApJ. All data can be downloaded here:
https://doi.org/10.5281/zenodo.146805
Unravelling 13 Billion Years of Cosmic History with Spectroscopic Studies: From the Milky Way to the Epoch of Reionization
In this thesis we trace the cosmic history from the Epoch of Reionization to the local universe by means of several spectroscopic studies. In the first part, we analyze quasar spectra at and measure the redshift evolution of the opacity of the intergalactic medium (IGM) within the Ly as well as the Ly forest to set new constraints on the timing and morphology of the reionization epoch.
We find evidence for an extended reionization process down to , and, while the observed scatter in the Ly forest optical depth can be well reproduced by current state-of-the-art simulations including spatial fluctuations in the temperature field or the ultraviolet background, we find a strong mismatch between simulations and observations in the Ly forest opacity, suggesting an inversion of the thermal state of the post-reionization IGM.
We also measure the sizes of the quasars' proximity zones, which are regions of enhanced ionization in the vicinity of the quasars, ionized by their own radiation. We find a dependency of the proximity zone sizes to the quasars' lifetime, which presents a novel method to estimate the lifetime of individual quasars, providing unprecedented constraints on the formation and growth of supermassive black holes in the early universe. We discover three quasars with very short lifetimes, i.e. ~yr, that pose significant challenges to all current black hole formation theories.
In the second part of this thesis we explore the structure and dynamics of the Milky Way. We develop a new data-driven model to determine precise parallaxes by combining multi-band photometry and spectroscopy to make global kinematic maps of our Galaxy from luminous red giant stars with only parallax uncertainties. Our map extends to Galactocentric distances of ~kpc, well beyond the reach of parallax estimates by the \textit{Gaia} mission.
Making use of these new spectrophotometric parallaxes, we determine the most precise measurement to date of the circular velocity curve of the Milky Way over a wide range of Galactocentric distances. Based on Jeans modeling in an axisymmetric gravitational potential we find that the velocity curve is gently but significantly declining
Evidence for episodic black hole growth of reionization-era quasars observed with Magellan/FIRE
Spectrophotometric parallaxes with linear models: Accurate distances for luminous red-giant stars
With contemporary infrared spectroscopic surveys like APOGEE, red-giant stars
can be observed to distances and extinctions at which Gaia parallaxes are not
highly informative. Yet the combination of effective temperature, surface
gravity, composition, and age - all accessible through spectroscopy -
determines a giant's luminosity. Therefore spectroscopy plus photometry should
enable precise spectrophotometric distance estimates. Here we use the
APOGEE-Gaia-2MASS-WISE overlap to train a data-driven model to predict
parallaxes for red-giant branch stars with (more luminous
than the red clump). We employ (the exponentiation of) a linear function of
APOGEE spectral pixel intensities and multi-band photometry to predict parallax
spectrophotometrically. The model training involves no logarithms or inverses
of the Gaia parallaxes, and needs no cut on the Gaia parallax signal-to-noise
ratio. It includes an L1 regularization to zero out the contributions of
uninformative pixels. The training is performed with leave-out subsamples such
that no star's astrometry is used even indirectly in its spectrophotometric
parallax estimate. The model implicitly performs a reddening and extinction
correction in its parallax prediction, without any explicit dust model. We
assign to each star in the sample a new spectrophotometric parallax estimate;
these parallaxes have uncertainties of a few to 15 percent, depending on data
quality, which is more precise than the Gaia parallax for the vast majority of
targets, and certainly any stars more than a few kpc distance. We obtain
10-percent distance estimates out to heliocentric distances of kpc, and
make global maps of the Milky Way's disk.Comment: Submitted to ApJ, comments are welcome. All data can be downloaded
here: https://doi.org/10.5281/zenodo.146805
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