AARON SOKASIAN'S SCIENTIFIC PUBLICATIONS
Bio
Aaron Sokasian is an astrophysicist with an academic background in Astrophysics and Computational Astrophysics. He earned his B.S. in Astrophysics from Cornell University and later received his Ph.D. from Harvard University. His Ph.D. thesis focused on "Radiative Transfer In The Early Universe."
List of Academic Publications
The following posts will present a list of Sokasian's publications. Each post will include the title of a specific publication and its abstract. The objective is to provide a comprehensive overview of Sokasian's research work, which spans a range of topics from reionization epochs to microwave anisotropy.
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"Cosmic Reionisation by Stellar Sources: Population III Stars" co-authored with Naoki Yoshida, Tom Abel, Lars Hernquist, and Volker Springel. (abstract) (pdf) (external links: [1] [2])
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"Cosmic Reionisation by Stellar Sources: Population II Stars" co-authored with Tom Abel, Lars Hernquist, and Volker Springel. (abstract) (pdf) (external links: [1] [2])
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"Early Structure Formation and Reionization in a Cosmological Model with a Running Primordial Power Spectrum" co-authored with Naoki Yoshida, Lars Hernquist, and Volker Springel. (abstract) (pdf) (external links: [1] [2])
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"Early Structure Formation and Reionization in a Warm Dark Matter Cosmology" co-authored with Naoki Yoshida, Lars Hernquist, and Volker Springel. (abstract) (pdf) (external links: [1][2])
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"The Nature of the Ionising Background at z=2.5-5" co-authored with Tom Abel and Lars Hernquist. (abstract) (pdf) [1] [2])
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"The Epoch of Helium Reionization" co-authored with Tom Abel and Lars Hernquist. (abstract) (pdf) (external links: [1][2])
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"Simulating Reionization in Numerical Cosmology" co-authored with Tom Abel and Lars Hernquist. (abstract) (pdf) (external links: [1] [2])
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"Contribution of Bright Extragalactic Radio Sources to Microwave Anisotropy" co-authored with Eric Gawiser and George Smoot. (abstract) (pdf) (external links: [1][2])
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"Observing The Reionization Epoch Through 21‐Centimetre Radiation" co-authored with Steven R. Furlanetto and Lars Hernquist. (abstract) (pdf) (external links: [1] [2])
Abstracts
Cosmic Reionisation by Stellar Sources: Population III Stars"
Aaron Sokasian, Naoki Yoshida, Tom Abel, Lars Hernquist, and Volker Springel.
ABSTRACT
We study the reionization of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high-resolution hydrodynamical simulations. By supplementing a one-step radiative transfer code specifically designed for following ionization processes with an adaptive ray-tracing algorithm, we are able to speed up the calculations significantly to the point where handling a vast number of sources becomes technically feasible. This allows us to study how dim low-mass sources, excluded in previous investigations owing to computational limitations, affect the morphological evolution of the reionization process.
Ionizing fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Analysis of numerically converged results for star formation rates and halo mass functions allows us to assess the consequences of not including low-mass objects and enables us to correct for resolution effects. With these corrections, we are able to reduce the effective mass resolution limit for sources to M ∼ 4.0 × 107 h−1 M⊙ in a 10 h−1 Mpc comoving box. Our calculations reveal that the process by which ionized regions in the intergalactic medium (IGM) percolate is complex and is especially sensitive to the inclusion of dim sources. Moreover, we find that, given the same level of cosmic star formation, the number of ionizing photons required to reionize the Universe is significantly overestimated if sources with masses below ∼109 h−1 M⊙ are excluded. This result stems from the fact that low-mass sources preferentially reside in less clumpy environments than their massive counterparts. Consequently, their exclusion has the net effect of concentrating more of the cosmic ionizing radiation in regions which have higher recombination rates.
We present the results of our reionization simulation assuming a range of escape fractions for ionizing photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z ∼ 6 derived from the z = 6.28 Sloan Digital Sky Survey (SDSS) quasar of Becker and coworkers. We find that, given the amplitude and form of the underlying star formation predictions, an escape fraction near f esc = 0.10–0.20 is most consistent with the observational results. In these models, reionization is expected to have occurred between z ∼ 7–8, although the IGM remains fairly opaque until z ≃ 6.
Our method is also capable of handling the simultaneous reionization of the helium com- ponent in the IGM, allowing us to explore the plausibility of the scenario where sources with harder spectra are primarily responsible for reionization. In this case, we find that if the sources responsible for reionizing hydrogen by z ∼ 8 had spectra similar to active galactic nuclei, then the helium component of the IGM should have been reionized by z ∼ 6. We find that such an early reionization epoch for helium does not necessarily conflict with observational constraints obtained at z ≃ 3, but may be challenged by future observations at higher redshifts.
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608 A. Sokasian et al.
Early Structure Formation and Reionization in a Cosmological Model with a Running Primordial Power Spectrum
Naoki Yoshida, Aaron Sokasian, Lars Hernquist, and Volker Springel
ABSTRACT
We study high-redshift structure formation and reionization in a CDM universe under the assumption that the spectral power index of primordial density fluctuations is a function of length scale. We adopt a particular formulation of the ‘‘ running ’’ spectral index (RSI) model as suggested by the combined analysis of the recent Wilkinson Microwave Anisotropy Probe (WMAP) data and two other large-scale structure observations. We carry out high-resolution cosmological simulations and use them to study the formation of primordial gas clouds where the first stars are likely to form. While early structure forms hierarchically in the RSI model, quite similar to the standard power-law CDM model, the reduced power on small scales causes a considerable delay in the formation epoch of low-mass (106 M) ‘‘ minihalos ’’ compared with the CDM model. The abundance of primordial star-forming gas clouds in such halos also differs by more than an order of magnitude at z > 15 between the two models. The extremely small number of gas clouds in the RSI model indicates that reionization is initiated later than z < 15, generally resulting in a smaller total Thomson optical depth than in the CDM model. By carrying out radiative transfer calculations, we also study reionization by stellar populations formed in galaxies. We show that, in order to reionize the universe by z 7, the escape fraction of ultraviolet photons from galaxies in the RSI model must be as high as 0.6 throughout the redshift range 5 < z < 18 for a stellar population similar to that of the local universe. Even with a top-heavy initial mass function representing an early population of massive stars and/or an extraordinarily high photon emission rate from galaxies, the total optical depth can only be as large as e 0:1 for reasonable models of early star formation. The RSI model is thus in conflict with the large Thomson optical depth inferred by the WMAP satellite.
Observing The Reionization Epoch Through 21‐Centimetre Radiation
Steven R. Furlanetto, Aaron Sokasian, and Lars Hernquist.
ABSTRACT
We study the observability of the reionization epoch through the 21-cm hyperfine transition of neutral hydrogen. We use a high-resolution cosmological simulation (including hydrodynamics) together with a fast radiative transfer algorithm to compute the evolution of 21-cm emission from the intergalactic medium (IGM) in several different models of reionization. We show that the mean brightness temperature of the IGM drops from δTb ∼ 25 mK to ∼10−2 mK during overlap (over a frequency interval #ν ∼ 25 MHz), while the root mean square fluc- tuations on small scales drop abruptly from ⟨δT2b⟩1/2 ∼ 10 mK to ∼10−1 mK at the end of overlap. We show that 21-cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes..
Simulating Reionization in Numerical Cosmology
Aaron Sokasian, Tom Abel, and Lars Hernquist.
ABSTRACT
The incorporation of radiative transfer effects into cosmological hydrodynamical simulations is essential for understanding how the intergalactic medium (IGM) makes the transition from a neutral medium to one that is almost fully ionized. Here, we present an approximate numerical method designed to study in a statistical sense how a cosmological density field is ionized by a set of discrete point sources. A diffuse background radiation field is also computed self-consistently in our procedure. The method requires relatively few time steps and can be employed with simulations having high resolution. We describe the details of the algorithm and provide a description of how the method can be applied to the output from a pre-existing cosmological simulation to study the systematic reionization of a particular ionic species. As a first application, we compute the reionization of He II by quasars in the range 3 <∼ z <∼ 6.
Early Structure Formation and Reionization in a Warm Dark Matter Cosmology
Naoki Yoshida, Aaron Sokasian, Lars Hernquist, and Volker Springel
ABSTRACT
We study first structure formation in Lambda-dominated universes using large cosmological N-body/SPH simulations. We consider a standard LCDM model and a LWDM model in which the mass of the dark matter particles is taken to be m_X=10 keV. The linear power spectrum for the LWDM model has a characteristic cut-off at a wavenumber k=200 /Mpc, suppressing the formation of low mass (< 10^6 Msun) nonlinear objects early on. The absence of low mass halos in the WDM model makes the formation of primordial gas clouds with molecular hydrogen very inefficient at high redshifts. The first star-forming gas clouds form at z~21 in the WDM model, considerably later than in the CDM counterpart, and the abundance of these gas clouds differs by an order of magnitude between the two models. We carry out radiative transfer calculations by embedding massive Population III stars in the gas clouds. We show that the volume fraction of ionized gas rises up close to 100% by z=18 in the CDM case, whereas that of the WDM model remains extremely small at a level of a few percent. Thus the WDM model with m_X=10 keV is strongly inconsistent with the observed high optical depth by the WMAP satellite.
Contribution of Bright Extragalactic Radio Sources to Microwave Anisotropy
Aaron Sokasian, Eric Gawiser, and George Smoot
ABSTRACT
We estimate the contribution of extragalactic radio sources to fluctuations in sky temperature over the range of frequencies (10-300 GHz) used for cosmic microwave background (CMB) anisotropy measurements. CMB anisotropy observations at high resolution and low frequencies are especially sensitive to this foreground. Our catalog of 2207 bright radio sources includes 758 sources with flux measurements at 90 GHz. We develop a method to extrapolate the source spectra and predict skymaps of extragalactic radio sources at instrument resolutions of 10′-10° FWHM. Our results indicate that the brightest radio sources will dominate microwave anisotropy for a wide range of resolutions and frequencies. We predict the location and flux of the brightest radio sources at each frequency, making it straightforward to develop a template for masking the pixels containing them. This masking should be sufficient to protect most CMB anisotropy observations from unacceptable radio source confusion.
The Epoch of Helium Reionization
Aaron Sokasian, Tom Abel, and Lars Hernquist
ABSTRACT
We study the reionization of He II by quasars using a numerical approach that combines 3D radiative transfer calculations with cosmological hydrodynamical simulations. Sources producing the ionizing radiation are selected according to an empirical quasar luminosity function, and are assigned luminosities according to their intrinsic masses. The free parameters associated with this procedure are (1) a universal source lifetime, (2) a minimum mass cut-off, (3) a minimum luminosity cut-off, (4) a solid angle specifying the extent to which radiation is beamed, and (5) a tail-end spectral index for the radiative energy distribution of the sources. We present models in which these parameters are varied, and examine characteristics of the resultant reionization process that distinguish the various cases. In addition, we extract artificial spectra from the simulations and quantify statistical properties of the spectral features in each model.
We find that the most important factor affecting the evolution of He II reionization is the cumulative number of ionizing photons that are produced by the sources. Comparisons between He II opacities measured observationally and those obtained by our analysis reveal that the available ranges in plausible values for the parameters provide enough leeway to provide a satisfactory match. However, one property common to all our calculations is that the epoch of He II reionization must have occurred at a redshift in the range 3 & z & 4. If so, future observational programmes will be able to directly trace the details of the ionization history of helium and to probe the low-density phase of the intergalactic medium during this phase of the evolution of the Universe.
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Cosmic Reionization by Stellar Sources: Population II Stars
Aaron Sokasian, Tom Abel, Lars Hernquist and Volker Springel
ABSTRACT
We study the reionization of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high-resolution hydrodynamical simulations. By supplementing a one-step radiative transfer code specifically designed for following ionization processes with an adaptive ray-tracing algorithm, we are able to speed up the calculations significantly to the point where handling a vast number of sources becomes technically feasible. This allows us to study how dim low-mass sources, excluded in previous investigations owing to computational limitations, affect the morphological evolution of the reionization process. Ionizing fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Analysis of numerically converged results for star formation rates and halo mass functions allows us to assess the consequences of not including low-mass objects and enables us to correct for resolution effects. With these corrections, we are able to reduce the effective mass resolution limit for sources to M ∼ 4.0 × 107 h−1 M⊙ in a 10 h−1 Mpc comoving box. Our calculations reveal that the process by which ionized regions in the intergalactic medium (IGM) percolate is complex and is especially sensitive to the inclusion of dim sources. Moreover, we find that, given the same level of cosmic star formation, the number of ionizing photons required to reionize the Universe is significantly overestimated if sources with masses below ∼109 h−1 M⊙ are excluded. This result stems from the fact that low-mass sources preferentially reside in less clumpy environments than their massive counterparts. Consequently, their exclusion has the net effect of concentrating more of the cosmic ionizing radiation in regions which have higher recombination rates. We present the results of our reionization simulation assuming a range of escape fractions for ionizing photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z ∼ 6 derived from the z = 6.28 Sloan Digital Sky Survey (SDSS) quasar of Becker and coworkers. We find that, given the amplitude and form of the underlying star formation predictions, an escape fraction near f esc = 0.10–0.20 is most consistent with the observational results. In these models, reionization is expected to have occurred between z ∼ 7–8, although the IGM remains fairly opaque until z ≃ 6.
Our method is also capable of handling the simultaneous reionization of the helium com- ponent in the IGM, allowing us to explore the plausibility of the scenario where sources with harder spectra are primarily responsible for reionization. In this case, we find that if the sources responsible for reionizing hydrogen by z ∼ 8 had spectra similar to active galactic nuclei, then the helium component of the IGM should have been reionized by z ∼ 6. We find that such an early reionization epoch for helium does not necessarily conflict with observational constraints obtained at z ≃ 3, but may be challenged by future observations at higher redshifts. The recent WMAP measurements of the electron scattering optical depth (τe = 0.17 ± 0.04 according to the ‘model independent’ analysis of Kogut et al.) appear to be inconsistent with the relatively late onset of reionization by the normal Population II type stars that we consider. In order to simultaneously match the observations from the z = 6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modelled here, we require a boosting factor for the number of ionizing photons released in the fesc = 0.20 model which evolves from unity at z = 6 to 50 by z ∼ 18. Such a steep enhancement in the stellar production rate of ionizing photons would be consistent with an IMF that becomes more and more top heavy with increasing redshift.
The Nature of the Ionizing Background at z ≈ 2.5–5
Aaron Sokasian, Tom Abel and Lars Hernquist
ABSTRACT
Using radiative transfer calculations and cosmological simulations of structure formation, we study constraints that can be placed on the nature of the cosmic ultraviolet (UV) background in the redshift interval 2.5 ≲z≲ 5. Our approach makes use of observational estimates of the opacities of hydrogen and singly ionized helium in the intergalactic medium during this epoch. In particular, we model the reionization of He II by sources of hard ultraviolet radiation, i.e. quasars, and infer values for our parametrization of this population from observational estimates of the opacity of the He II Lyman-α forest. Next, we estimate the photoionization rate of H I from these sources and find that their contribution to the ionizing background is insufficient to account for the measured opacity of the H I Lyman-α forest at a redshift z∼ 3. This motivates us to include a soft, stellar component to the ionizing background to boost the hydrogen photoionization rate, but which has a negligible impact on the He II opacity.
In order to simultaneously match observational estimates of the H I and He II opacities, we find that galaxies and quasars must contribute approximately equally to the ionizing background in H I at z≃ 3. Moreover, our analysis requires the stellar component to rise for z > 3 to compensate for the declining contribution from bright quasars at higher redshift. This inference is consistent with some observational and theoretical estimates of the evolution of the cosmic star formation rate. The increasing dominance of the stellar component towards high redshift leads to a progressive softening of the UV background, as suggested by observations of metal line absorption. In the absence of additional sources of ionizing radiation, such as mini-quasars or weak active galactic nuclei, our results, extrapolated to z > 5, suggest that hydrogen reionization at z∼ 6 mostly probably occurred through the action of stellar radiation