arXiv subjects

Simeon Bird

Publications and source records attributed to Simeon Bird.

At least 91 records · Page 5Linked to original sources

arXiv2022

Improved selection of extremely red quasars with boxy CIV lines in BOSS

Extremely red quasars (ERQs) are an interesting sample of quasars in the Baryon Oscillation Spectroscopic Sample (BOSS) in the redshift range of $2.0 - 3.4$ and have extreme red colours of $i-W3\ge4.6$. Core ERQs have strong CIV emission lines with rest equivalent width of $\ge100$Å. Many core ERQs also have CIV line profiles with peculiar boxy shapes which distinguish them from normal blue quasars. We show, using a combination of kernel density estimation and local outlier factor analyses on a space of the $i-W3$ colour, CIV rest equivalent width and line kurtosis, that core ERQs likely represent a separate population rather than a smooth transition between normal blue quasars and the quasars in the tail of the colour-REW distribution. We apply our analyses to find new criteria for selecting ERQs in this 3D parameter space. Our final selection produces $133$ quasars, which are \emph{three} times more likely to have a visually verified CIV broad absorption line feature than the previous core ERQ sample. We further show that our newly selected sample are extreme objects in the intersection of the WISE AGN catalogue with the MILLIQUAS quasar catalogue in the colour-colour space of ($W1-W2$, $W2-W3$). This paper validates an improved selection method for red quasars which can be applied to future datasets such as the quasar catalogue from the Dark Energy Spectroscopic Instrument (DESI).

astro-ph.GA↗

arXiv2021

Massive Black Hole Mergers with Orbital Information: Predictions from the ASTRID Simulation

We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid. Astrid includes galaxy formation and black hole models recently updated with a MBH seed population between $3\times 10^4M_{\odot}/h$ and $3\times 10^5M_{\odot}/h$ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to $1.5\;\text{ckpc}/h$. We calculate initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above $0.7$ for most MBH pairs before the numerical merger. After approximating unresolved evolution on scales below ${\sim 200\,\text{pc}}$, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ($\sim 500\,\text{Myrs}$) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries ($M_\text{BH}<2M_\text{seed}$). As a result, only $\lesssim20\%$ of seed MBH pairs merge at $z>3$ after considering both unresolved DF evolution and binary hardening. These $z>3$ seed-mass mergers are hosted in a biased population of galaxies with the highest stellar masses of $>10^9\,M_\odot$. With the higher initial eccentricity prediction from Astrid, we estimate an expected merger rate of $0.3-0.7$ per year from the $z>3$ MBH population. This is a factor of $\sim 7$ higher than the prediction using the circular orbit assumption. The LISA events are expected at a similar rate, and comprise $\gtrsim 60\%$ seed-seed mergers, $\sim 30\%$ involving only one seed-mass MBH, and $\sim 10\%$ mergers of non-seed MBHs.

astro-ph.GA↗

arXiv2021

Nucleosynthetic signatures of primordial origin around supermassive black holes

If primordial black holes (PBHs) seeded the supermassive black holes (SMBHs) at the centers of high-redshift quasars, then the gas surrounding these black holes may reveal nucleosynthetic clues to their primordial origins. We present predictions of altered primordial abundances around PBHs massive enough to seed SMBHs at z~6-7.5. We find that if PBHs with initial masses of ~10^5 M$_{\odot}$ are responsible for such SMBHs, they may produce primordial Deuterium and Helium fractions enhanced by >~ 10%, and Lithium abundance depleted by >~10%, at distances of up to ~ a comoving kiloparsec away from the black hole after decoupling. We estimate that ~ 10^8 M$_{\odot}$ of gas is enhanced (or depleted) by at least one percent. Evidence of these modified primordial Deuterium, Helium, and Lithium abundances could still be present if this circum-PBH gas remains unaccreted by the SMBH and in or near the host galaxies of high-redshift quasars. Measuring the abundance anomalies will be challenging, but could offer a novel way to reveal the primordial origin of such SMBH seeds.

astro-ph.CO↗

arXiv2021

Densified pupil spectrograph as high-precision radial velocimetry: From direct measurement of the Universe's expansion history to characterization of nearby habitable planet candidates

The direct measurement of the Universe's expansion history and the search for terrestrial planets in habitable zones around solar-type stars require extremely high-precision radial velocity measures over a decade. This study proposes an approach for enabling high-precision radial velocity measurements from space. The concept presents a combination of a high-dispersion densified pupil spectrograph and a novel telescope line-of-sight monitor. The precision of the radial velocity measurements is determined by combining the spectrophotometric accuracy and the quality of the absorption lines in the recorded spectrum. Therefore, a highly dispersive densified pupil spectrograph proposed to perform stable spectroscopy can be utilized for high-precision radial velocity measures. A concept involving the telescope line-of-sight monitor is developed to minimize the change of the telescope line-of-sight over a decade. This monitor allows the precise measurement of a long-term telescope drift without any significant impact on the Airy disk when the densified pupil spectra are recorded. We analytically derive the uncertainty of the radial velocity measurements, which is caused by the residual offset of the line-of-sights at two epochs. We find that the error could be reduced down to approximately 1 $cm/s$, and the precision will be limited by another factor (e.g., wavelength calibration uncertainty). A combination of the high precision spectrophotometry and the high spectral resolving power could open a new path toward the characterization of nearby non-transiting habitable planet candidates orbiting late-type stars. We present two simple and compact high-dispersed densified pupil spectrograph designs for the cosmology and exoplanet sciences.

astro-ph.IM↗

arXiv2021

The large-scale distribution of ionized metals in IllustrisTNG

We study the intrinsic large-scale distribution and evolution of seven ionized metals in the IllustrisTNG magneto-hydrodynamical cosmological simulation. We focus on the fractions of C\,\textsc{ii}, C\,\textsc{iv}, Mg\,\textsc{ii}, N\,\textsc{v}, Ne\,\textsc{viii}, O\,\textsc{vi}, and Si\,\textsc{iv} in different cosmic web structures (filaments, haloes, and voids) and gas phases (warm-hot intergalactic medium WHIM, hot, diffuse, and condensed gas) from $z=6$ to $z=0$. Our analysis provides a new perspective to the study of the distribution and evolution of baryons across cosmic time while offering new hints in the context of the well-known missing baryons problem. The cosmic web components are here identified using the local comoving dark matter density, which provides a simple but effective way of mapping baryons on large scales. Our results show that C\,\textsc{ii} and Mg\,\textsc{ii} are mostly located in condensed gas inside haloes in high-density and low-temperature star-forming regions ($ρ_{\rm gas}/\barρ_{\rm bar}\gtrsim10^3$, and ${\rm T}\lesssim10^{5}$~K). C\,\textsc{iv} and Si\,\textsc{iv} present similar evolution of their mass fractions in haloes and filaments across cosmic time. In particular, their mass budgets in haloes in condensed phase ($ρ_{\rm gas}/\barρ_{\rm bar}\gtrsim10^3$, and ${\rm T}\lesssim10^{5}$~K) are driven by gas cooling and star formation with a peak at $z\sim2$. Finally, our results confirm that O\,\textsc{vi}, Ne\,\textsc{viii}, and N\,\textsc{v} are good tracers of warm/hot and low-density gas at low redshift ($ρ_{\rm gas}/\barρ_{\rm bar}\lesssim10^3$, and ${\rm T}\gtrsim10^{5}$~K), regions that are likely to contain most of the missing baryons in the local Universe.

astro-ph.GA↗

arXiv2021

Multi-Fidelity Emulation for the Matter Power Spectrum using Gaussian Processes

We present methods for emulating the matter power spectrum by combining information from cosmological $N$-body simulations at different resolutions. An emulator allows estimation of simulation output by interpolating across the parameter space of a limited number of simulations. We present the first implementation in cosmology of multi-fidelity emulation, where many low-resolution simulations are combined with a few high-resolution simulations to achieve an increased emulation accuracy. The power spectrum's dependence on cosmology is learned from the low-resolution simulations, which are in turn calibrated using high-resolution simulations. We show that our multi-fidelity emulator predicts high-fidelity counterparts to percent-level relative accuracy when using only $3$ high-fidelity simulations and outperforms a single-fidelity emulator that uses $11$ simulations, although we do not attempt to produce a converged emulator with high absolute accuracy. With a fixed number of high-fidelity training simulations, we show that our multi-fidelity emulator is $\simeq 100$ times better than a single-fidelity emulator at $k \leq 2 h \mathrm{Mpc}^{-1}$, and $\simeq 20$ times better at $3 \leq k < 6.4 h \mathrm{Mpc}^{-1}$. Multi-fidelity emulation is fast to train, using only a simple modification to standard Gaussian processes. Our proposed emulator shows a new way to predict non-linear scales by fusing simulations from different fidelities.

astro-ph.CO↗

arXiv2021

AI-assisted super-resolution cosmological simulations II: Halo substructures, velocities and higher order statistics

In this work, we expand and test the capabilities of our recently developed super-resolution (SR) model to generate high-resolution (HR) realizations of the full phase-space matter distribution, including both displacement and velocity, from computationally cheap low-resolution (LR) cosmological N-body simulations. The SR model enhances the simulation resolution by generating 512 times more tracer particles, extending into the deeply non-linear regime where complex structure formation processes take place. We validate the SR model by deploying the model in 10 test simulations of box size 100 Mpc/h, and examine the matter power spectra, bispectra and 2D power spectra in redshift space. We find the generated SR field matches the true HR result at percent level down to scales of k ~ 10 h/Mpc. We also identify and inspect dark matter halos and their substructures. Our SR model generate visually authentic small-scale structures, that cannot be resolved by the LR input, and are in good statistical agreement with the real HR results. The SR model performs satisfactorily on the halo occupation distribution, halo correlations in both real and redshift space, and the pairwise velocity distribution, matching the HR results with comparable scatter, thus demonstrating its potential in making mock halo catalogs. The SR technique can be a powerful and promising tool for modelling small-scale galaxy formation physics in large cosmological volumes.

astro-ph.CO↗

arXiv2021

Damped Lyman-alpha Absorbers from Sloan Digital Sky Survey DR16Q with Gaussian processes

We present a new catalogue of Damped Lyman-$α$ absorbers from SDSS DR16Q, as well as new estimates of their statistical properties. Our estimates are computed with the Gaussian process models presented in Garnett et al. (2017); Ho et al. (2020) with an improved model for marginalising uncertainty in the mean optical depth of each quasar. We compute the column density distribution function (CDDF) at $2 < z < 5$, the line density ($\textrm{d} N/ \textrm{d} X$), and the neutral hydrogen density ($Ω_{\textrm{DLA}}$). Our Gaussian process model provides a posterior probability distribution of the number of DLAs per spectrum, thus allowing unbiased probabilistic predictions of the statistics of DLA populations even with the noisiest data. We measure a non-zero column density distribution function for $N_{\textrm{HI}} < 3 \times 10^{22} \,\textrm{cm}^{-2}$ with $95\%$ confidence limits, and $N_{\textrm{HI}} \lesssim 10^{22} \,\textrm{cm}^{-2}$ for spectra with signal-to-noise ratios $> 4$. Our results for DLA line density and total hydrogen density are consistent with previous measurements. Despite a small bias due to the poorly measured blue edges of the spectra, we demonstrate that our new model can measure the DLA population statistics when the DLA is in the Lyman-$β$ forest region. We verify our results are not sensitive to the signal-to-noise ratios and redshifts of the background quasars although a residual correlation remains for detections from $z_{\textrm{QSO}} < 2.5$, indicating some residual systematics when applying our models on very short spectra, where the SDSS spectral observing window only covers part of the Lyman-$α$ forest.

astro-ph.GA↗

arXiv2021

Impact of a Midband Gravitational Wave Experiment On Detectability of Cosmological Stochastic Gravitational Wave Backgrounds

We make forecasts for the impact a future "midband" space-based gravitational wave experiment, most sensitive to $10^{-2}- 10$ Hz, could have on potential detections of cosmological stochastic gravitational wave backgrounds (SGWBs). Specific proposed midband experiments considered are TianGo, B-DECIGO and AEDGE. We propose a combined power-law integrated sensitivity (CPLS) curve combining GW experiments over different frequency bands, which shows the midband improves sensitivity to SGWBs by up to two orders of magnitude at $10^{-2} - 10$ Hz. We consider GW emission from cosmic strings and phase transitions as benchmark examples of cosmological SGWBs. We explicitly model various astrophysical SGWB sources, most importantly from unresolved black hole mergers. Using Markov Chain Monte Carlo, we demonstrated that midband experiments can, when combined with LIGO A+ and LISA, significantly improve sensitivities to cosmological SGWBs and better separate them from astrophysical SGWBs. In particular, we forecast that a midband experiment improves sensitivity to cosmic string tension $Gμ$ by up to a factor of $10$, driven by improved component separation from astrophysical sources. For phase transitions, a midband experiment can detect signals peaking at $0.1 - 1$ Hz, which for our fiducial model corresponds to early Universe temperatures of $T_*\sim 10^4 - 10^6$ GeV, generally beyond the reach of LIGO and LISA. The midband closes an energy gap and better captures characteristic spectral shape information. It thus substantially improves measurement of the properties of phase transitions at lower energies of $T_* \sim O(10^3)$ GeV, potentially relevant to new physics at the electroweak scale, whereas in this energy range LISA alone will detect an excess but not effectively measure the phase transition parameters. Our modelling code and chains are publicly available.

gr-qc↗

arXiv2021

AI-assisted super-resolution cosmological simulations

Cosmological simulations of galaxy formation are limited by finite computational resources. We draw from the ongoing rapid advances in Artificial Intelligence (specifically Deep Learning) to address this problem. Neural networks have been developed to learn from high-resolution (HR) image data, and then make accurate super-resolution (SR) versions of different low-resolution (LR) images. We apply such techniques to LR cosmological N-body simulations, generating SR versions. Specifically, we are able to enhance the simulation resolution by generating 512 times more particles and predicting their displacements from the initial positions. Therefore our results can be viewed as new simulation realizations themselves rather than projections, e.g., to their density fields. Furthermore, the generation process is stochastic, enabling us to sample the small-scale modes conditioning on the large-scale environment. Our model learns from only 16 pairs of small-volume LR-HR simulations, and is then able to generate SR simulations that successfully reproduce the HR matter power spectrum to percent level up to $16\,h^{-1}\mathrm{Mpc}$, and the HR halo mass function to within $10 \%$ down to $10^{11} \, M_\odot$. We successfully deploy the model in a box 1000 times larger than the training simulation box, showing that high-resolution mock surveys can be generated rapidly. We conclude that AI assistance has the potential to revolutionize modeling of small-scale galaxy formation physics in large cosmological volumes.

astro-ph.CO↗

arXiv2021

Dynamical Friction Modeling of Massive Black Holes in Cosmological Simulations and Effects on Merger Rate Predictions

In this work we establish and test methods for implementing dynamical friction for massive black hole pairs that form in large volume cosmological hydrodynamical simulations which include galaxy formation and black hole growth. We verify our models and parameters both for individual black hole dynamics and for the black hole population in cosmological volumes. Using our model of dynamical friction (DF) from collisionless particles, black holes can effectively sink close to the galaxy center, provided that the black hole's dynamical mass is at least twice that of the lowest mass resolution particles in the simulation. Gas drag also plays a role in assisting the black holes' orbital decay, but it is typically less effective than that from collisionless particles, especially after the first billion years of the black hole's evolution. DF from gas becomes less than $1\%$ of DF from collisionless particles for BH masses $> 10^{7}$ M$_{\odot}$. Using our best DF model, we calculate the merger rate down to $z=1.1$ using an $L_{\rm box}=35$ Mpc$/h$ simulation box. We predict $\sim 2$ mergers per year for $z>1.1$ peaking at $z\sim 2$. These merger rates are within the range obtained in previous work using similar-resolution hydro-dynamical simulations. We show that the rate is enhanced by factor of $\sim 2$ when DF is taken into account in the simulations compared to the no-DF run. This is due to $>40\%$ more black holes reaching the center of their host halo when DF is added.

astro-ph.GA↗

arXiv2021

Effect of Separate Initial Conditions on the Lyman-$α$ Forest in Simulations

Using a set of high resolution simulations, we quantify the effect of species specific initial transfer functions on probes of the IGM via the Lyman-$α$ forest. We focus on redshifts $2-6$, after H~{\sc i} reionization. We explore the effect of these initial conditions on measures of the thermal state of the low density IGM: the curvature, Doppler width cutoff, and Doppler width distribution. We also examine the matter and flux power spectrum, and potential consequences for constraints on warm dark matter models. We find that the curvature statistic is at most affected at the $\approx2\%$ level at $z=6$. The Doppler width cutoff parameters are affected by $\approx5\%$ for the intercept, and $\approx8\%$ for the fit slope, though this is subdominant to sample variation. The Doppler width distribution shows a $\approx30\%$ effect at $z=3$, however the distribution is not fully converged with simulation box size and resolution. The flux power spectrum is at most affected by $\approx5\%$ at high redshift and small scales. We discuss numerical convergence with simulation parameters.

astro-ph.CO↗

arXiv2020

Automated Measurement of Quasar Redshift with a Gaussian Process

We develop an automated technique to measure quasar redshifts in the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey (SDSS). Our technique is an extension of an earlier Gaussian process method for detecting damped Lyman-alpha absorbers (DLAs) in quasar spectra with known redshifts. We apply this technique to a subsample of SDSS DR12 with BAL quasars removed and redshift larger than 2.15. We show that we are broadly competitive to existing quasar redshift estimators, disagreeing with the PCA redshift by more than 0.5 in only 0.38% of spectra. Our method produces a probabilistic density function for the quasar redshift, allowing quasar redshift uncertainty to be propagated to downstream users. We apply this method to detecting DLAs, accounting in a Bayesian fashion for redshift uncertainty. Compared to our earlier method with a known quasar redshift, we have a moderate decrease in our ability to detect DLAs, predominantly in the noisiest spectra. The area under curve drops from 0.96 to 0.91. Our code is publicly available.

astro-ph.GA↗

arXiv2020

Cosmic Filaments from Cosmic Strings

Cosmic strings are generically predicted in many extensions of the Standard Model of particle physics. We propose a new avenue for detecting cosmic strings through their effect on the filamentary structure in the cosmic web. Using cosmological simulations of the density wake from a cosmic string, we examine a variety of filament structure probes. We show that the largest effect of the cosmic string is an overdensity in the filament distribution around the string wake. The signal from the overdensity is stronger at higher redshift, and more robust with a wider field. We analyze the spatial distribution of filaments from a publicly available catalog of filaments built from SDSS galaxies. With existing data, we find no evidence for the presence of a cosmic string wake with string tension parameter $Gμ$ above $5\times 10^{-6}$. However, we project WFIRST will be able to detect a signal from such a wake at the $99\%$ confidence level at redshift $z=2$, with significantly higher confidence and the possibility of probing lower tensions ($Gμ\sim 10^{-6}$), at $z=10$. The sensitivity of this method is not competitive with constraints derived from the CMB. However, it provides an independent discovery channel at low redshift, which could be a smoking-gun in scenarios where the CMB bound can be weakened.

astro-ph.CO↗

arXiv2020

Detecting Multiple DLAs per Spectrum in SDSS DR12 with Gaussian Processes

We present a revised version of our automated technique using Gaussian processes (GPs) to detect Damped Lyman-$α$ absorbers (DLAs) along quasar (QSO) sightlines. The main improvement is to allow our Gaussian process pipeline to detect multiple DLAs along a single sightline. Our DLA detections are regularised by an improved model for the absorption from the Lyman-$α$ forest which improves performance at high redshift. We also introduce a model for unresolved sub-DLAs which reduces mis-classifications of absorbers without detectable damping wings. We compare our results to those of two different large-scale DLA catalogues and provide a catalogue of the processed results of our Gaussian process pipeline using 158 825 Lyman-$α$ spectra from SDSS data release 12. We present updated estimates for the statistical properties of DLAs, including the column density distribution function (CDDF), line density ($dN/dX$), and neutral hydrogen density ($Ω_{\textrm{DLA}}$).

astro-ph.CO↗

arXiv2020

Inhomogeneous HeII reionization in Hydrodynamic Simulations

The reionization of the second electron of helium shapes the physical state of intergalactic gas at redshifts between 2 < z < 5. Because performing full in situ radiative transfer in hydrodynamic simulations is computationally expensive for large volumes, the physics of HeII reionization is often approximated by a uniform UV background model that does not capture the patchiness of reionization. We have devised a model that implements the effects of HeII reionization using semi-analytic calculations of the thermal state of intergalactic gas-- a way to bypass a full radiative transfer simulation while still realizing the physics of HeII reionization that affects observables such as the Lyman alpha forest. Here we present a publicly-available code that flexibly models inhomogeneous HeII reionization in simulations at a negligible computational cost. Because many of the parameters of HeII reionization are uncertain, our model is customizable from a set of free parameters. We show results from this code in MP-Gadget, where this model is implemented. We demonstrate the resulting temperature evolution and temperature-density relation of intergalactic gas-- consistent with recent measurements and previous radiative transfer simulations. We show that the impact of HeII reionization gives rise to subtle signatures in the one-dimensional statistics of the Lyman alpha forest at the level of several per cent, in agreement with previous findings. The flexible nature of these simulations are ideal for studies of HeII reionization and future observations of the HeII Lyman alpha forest.

astro-ph.CO↗

arXiv2020

More accurate simulations with separate initial conditions for baryons and dark matter

We revisit techniques for performing cosmological simulations with both baryons and cold dark matter when each fluid has different initial conditions, as is the case at the end of the radiation era. Most simulations do not reproduce the linear prediction for the difference between the cold dark matter and baryon perturbations. We show that this is due to the common use of offset regular grids when setting up the particle initial conditions. The correct behaviour can be obtained without any loss of simulation resolution by using a Lagrangian glass for the baryon particles. We further show that the difference between cold dark matter and baryons may affect predictions for the Lyman-alpha forest flux power spectrum at the 5% level, potentially impacting current cosmological constraints.

astro-ph.CO↗

arXiv2020

Massive neutrinos and degeneracies in Lyman-alpha forest simulations

Using a suite of hydrodynamical simulations with cold dark matter, baryons, and neutrinos, we present a detailed study of the effect of massive neutrinos on the 1-D and 3-D flux power spectra of the Lyman-$α$ (Ly$α$) forest. The presence of massive neutrinos in cosmology induces a scale- and time-dependent suppression of structure formation that is strongest on small scales. Measuring this suppression is a key method for inferring neutrino masses from cosmological data, and is one of the main goals of ongoing and future surveys like eBOSS, DES, LSST, Euclid or DESI. The clustering in the Ly$α$ forest traces the quasi-linear power at late times and on small scales. In combination with observations of the cosmic microwave background, the forest therefore provides some of the tightest constraints on the sum of the neutrino masses. However there is a well-known degeneracy between $Σm_ν$ and the amplitude of perturbations in the linear matter power spectrum. We study the corresponding degeneracy in the 1-D flux power spectrum of the Ly$α$ forest, and for the first time also study this degeneracy in the 3-D flux power spectrum. We show that the non-linear effects of massive neutrinos on the Ly$α$ forest, beyond the effect of linear power amplitude suppression, are negligible, and this degeneracy persists in the Ly$α$ forest observables to a high precision. We discuss the implications of this degeneracy for choosing parametrisations of the Ly$α$ forest for cosmological analysis.

astro-ph.CO↗