arXiv subjects

Jordan Mirocha

Publications and source records attributed to Jordan Mirocha.

59 records · Page 4Linked to original sources

arXiv2015

Parametrizations of the global 21-cm signal and parameter estimation from single-dipole experiments

One approach to extracting the global 21-cm signal from total-power measurements at low radio frequencies is to parametrize the different contributions to the data and then fit for these parameters. We examine parametrizations of the 21-cm signal itself, and propose one based on modelling the Lyman-alpha background, IGM temperature and hydrogen ionized fraction using tanh functions. This captures the shape of the signal from a physical modelling code better than an earlier parametrization based on interpolating between maxima and minima of the signal, and imposes a greater level of physical plausibility. This allows less biased constraints on the turning points of the signal, even though these are not explicitly fit for. Biases can also be alleviated by discarding information which is less robustly described by the parametrization, for example by ignoring detailed shape information coming from the covariances between turning points or from the high-frequency parts of the signal, or by marginalizing over the high-frequency parts of the signal by fitting a more complex foreground model. The fits are sufficiently accurate to be usable for experiments gathering 1000 h of data, though in this case it may be important to choose observing windows which do not include the brightest areas of the foregrounds. Our assumption of pointed, single-antenna observations and very broad-band fitting makes these results particularly applicable to experiments such as the Dark Ages Radio Explorer, which would study the global 21-cm signal from the clean environment of a low lunar orbit, taking data from the far side.

astro-ph.CO↗

arXiv2015

Interpreting the Global 21-cm Signal from High Redshifts. II. Parameter Estimation for Models of Galaxy Formation

Following our previous work, which related generic features in the sky-averaged (global) 21-cm signal to properties of the intergalactic medium, we now investigate the prospects for constraining a simple galaxy formation model with current and near-future experiments. Markov-Chain Monte Carlo fits to our synthetic dataset, which includes a realistic galactic foreground, a plausible model for the signal, and noise consistent with 100 hours of integration by an ideal instrument, suggest that a simple four-parameter model that links the production rate of Lyman-$α$, Lyman-continuum, and X-ray photons to the growth rate of dark matter halos can be well-constrained (to $\sim 0.1$ dex in each dimension) so long as all three spectral features expected to occur between $40 \lesssim ν/ \mathrm{MHz} \lesssim 120$ are detected. Several important conclusions follow naturally from this basic numerical result, namely that measurements of the global 21-cm signal can in principle (i) identify the characteristic halo mass threshold for star formation at all redshifts $z \gtrsim 15$, (ii) extend $z \lesssim 4$ upper limits on the normalization of the X-ray luminosity star-formation rate ($L_X$-SFR) relation out to $z \sim 20$, and (iii) provide joint constraints on stellar spectra and the escape fraction of ionizing radiation at $z \sim 12$. Though our approach is general, the importance of a broad-band measurement renders our findings most relevant to the proposed Dark Ages Radio Explorer, which will have a clean view of the global 21-cm signal from $\sim 40-120$ MHz from its vantage point above the radio-quiet, ionosphere-free lunar far-side.

astro-ph.CO↗

arXiv2014

Decoding the X-ray Properties of Pre-Reionization Era Sources

Evolution in the X-ray luminosity -- star formation rate ($L_X$-SFR) relation could provide the first evidence of a top-heavy stellar initial mass function in the early universe, as the abundance of high-mass stars and binary systems are both expected to increase with decreasing metallicity. The sky-averaged (global) 21-cm signal has the potential to test this prediction via constraints on the thermal history of the intergalactic medium, since X-rays can most easily escape galaxies and heat gas on large scales. A significant complication in the interpretation of upcoming 21-cm measurements is the unknown spectrum of accreting black holes at high-$z$, which depends on the mass of accreting objects and poorly constrained processes such as how accretion disk photons are processed by the disk atmosphere and host galaxy interstellar medium. Using a novel approach to solving the cosmological radiative transfer equation (RTE), we show that reasonable changes in the characteristic BH mass affects the amplitude of the 21-cm signal's minimum at the $\sim 10-20$ mK level --- comparable to errors induced by commonly used approximations to the RTE --- while modifications to the intrinsic disk spectrum due to Compton scattering (bound-free absorption) can shift the position of the minimum of the global signal by $Δz \approx 0.5$ ($Δz \approx 2$), and modify its amplitude by up to $\approx 10$ mK ($\approx 50$ mK) for a given accretion history. Such deviations are larger than the uncertainties expected of current global 21-cm signal extraction algorithms, and could easily be confused with evolution in the $L_X$-SFR relation.

astro-ph.GA↗

arXiv2013

Interpreting the Global 21-cm Signal from High Redshifts. I. Model Independent Constraints

The sky-averaged (global) 21-cm signal is a powerful probe of the intergalactic medium (IGM) prior to the completion of reionization. However, it has so far been unclear that even in the best case scenario, in which the signal is accurately extracted from the foregrounds, that it will provide more than crude estimates of when the Universe's first stars and black holes form. In contrast to previous work, which has focused on predicting the 21-cm signatures of the first luminous objects, we investigate an arbitrary realization of the signal, and attempt to translate its features to the physical properties of the IGM. Within a simplified global framework, the 21-cm signal yields quantitative constraints on the Lyman-alpha background intensity, net heat deposition, ionized fraction, and their time derivatives, without invoking models for the astrophysical sources themselves. The 21-cm absorption signal is most easily interpreted, setting strong limits on the heating rate density of the Universe with a measurement of its redshift alone, independent of the ionization history or details of the Lyman-alpha background evolution. In a companion paper we extend these results, focusing on the confidence with which one can infer source emissivities from IGM properties.

astro-ph.CO↗

arXiv2012

Optimized Multi-Frequency Spectra for Applications in Radiative Feedback and Cosmological Reionization

The recent implementation of radiative transfer algorithms in numerous hydrodynamics codes has led to a dramatic improvement in studies of feedback in various astrophysical environments. However, because of methodological limitations and computational expense, the spectra of radiation sources are generally sampled at only a few evenly-spaced discrete emission frequencies. Using one-dimensional radiative transfer calculations, we investigate the discrepancies in gas properties surrounding model stars and accreting black holes that arise solely due to spectral discretization. We find that even in the idealized case of a static and uniform density field, commonly used discretization schemes induce errors in the neutral fraction and temperature by factors of two to three on average, and by over an order of magnitude in certain column density regimes. The consequences are most severe for radiative feedback operating on large scales, dense clumps of gas, and media consisting of multiple chemical species. We have developed a method for optimally constructing discrete spectra, and show that for two test cases of interest, carefully chosen four-bin spectra can eliminate errors associated with frequency resolution to high precision. Applying these findings to a fully three-dimensional radiation-hydrodynamic simulation of the early universe, we find that the HII region around a primordial star is substantially altered in both size and morphology, corroborating the one-dimensional prediction that discrete spectral energy distributions can lead to sizable inaccuracies in the physical properties of a medium, and as a result, the subsequent evolution and observable signatures of objects embedded within it.

astro-ph.CO↗