arXiv subjects

Adam Lidz

Publications and source records attributed to Adam Lidz.

At least 91 records · Page 5Linked to original sources

arXiv2009

On the Detectability of the Hydrogen 3-cm Fine Structure Line from the EoR

A soft ultraviolet radiation field, 10.2 eV < E <13.6 eV, that permeates neutral intergalactic gas during the Epoch of Reionization (EoR) excites the 2p (directly) and 2s (indirectly) states of atomic hydrogen. Because the 2s state is metastable, the lifetime of atoms in this level is relatively long, which may cause the 2s state to be overpopulated relative to the 2p state. It has recently been proposed that for this reason, neutral intergalactic atomic hydrogen gas may be detected in absorption in its 3-cm fine-structure line (2s_1/2 -> 2p_3/2) against the Cosmic Microwave Background out to very high redshifts. In particular, the optical depth in the fine-structure line through neutral intergalactic gas surrounding bright quasars during the EoR may reach tau~1e-5. The resulting surface brightness temperature of tens of micro K (in absorption) may be detectable with existing radio telescopes. Motivated by this exciting proposal, we perform a detailed analysis of the transfer of Lyman beta,gamma,delta,... radiation, and re-analyze the detectability of the fine-structure line in neutral intergalactic gas surrounding high-redshift quasars. We find that proper radiative transfer modeling causes the fine-structure absorption signature to be reduced tremendously to tau< 1e-10. We therefore conclude that neutral intergalactic gas during the EoR cannot reveal its presence in the 3-cm fine-structure line to existing radio telescopes.

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arXiv2009

Cosmology from the Highly-Redshifted 21 cm Line

One of the next decade's most exciting prospects is to explore the cosmic "dark ages," during which the first stars in the Universe formed, with the 21 cm line of neutral hydrogen. At z>6, this light redshifts into the low-frequency radio regime and can be observed with new generations of wide-field arrays. These experiments have the potential to observe enormous volumes and to improve both cosmological and astrophysical constraints dramatically. In this white paper, we describe how the next decade will see the first steps toward unlocking this potential, allowing us to answer two key questions about the fundamental physics of our Universe and the intergalactic medium: Does the standard cosmological model describe the Universe during the "dark ages?" How did the intergalactic medium evolve during this important time, ending with the reionization of hydrogen?

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arXiv2009

Astrophysics from the Highly-Redshifted 21 cm Line

The cosmic dark ages and the epoch of reionization, during which the first generations of stars and galaxies formed, are among the most compelling frontiers of extragalactic astrophysics and cosmology. Here we describe an exciting new probe of these eras: the 21 cm line of neutral hydrogen, which will allow us to map the neutral intergalactic medium throughout the era of first galaxy formation. In this white paper, we describe how these studies can answer two key questions about galaxy formation: What were the properties of high-redshift galaxies? How did they affect the Universe around them?

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arXiv2008

HeII Reionization and its Effect on the IGM

Observations of the intergalactic medium (IGM) suggest that quasars reionize HeII in the IGM at z ~ 3. We have run a set of 190 and 430 comoving Mpc simulations of HeII being reionized by quasars to develop an understanding of the nature of HeII reionization and its potential impact on observables. We find that HeII reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~ 12,000 K and with large temperature fluctuations on ~ 50 Mpc scales. Much of the heating occurs far from QSOs by hard photons. We find a temperature-density equation of state of gamma -1 ~ 0.3 during HeII reionization, but with a wide dispersion in this relation having sigma ~ 10^4 K. HeII reionization by the observed population of quasars cannot produce an inverted relation (gamma - 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the HeII Ly-alpha forest. We argue that the heat input due to HeII reionization is unable to cause the observed depression at z = 3.2 in the HI Ly-alpha forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of HeII Lyman-limit systems influence our predictions.

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arXiv2008

CMBPol Mission Concept Study: Reionization Science with the Cosmic Microwave Background

We summarize existing constraints on the epoch of reionization and discuss the observational probes that are sensitive to the process. We focus on the role large scale polarization can play. Polarization probes the integrated optical depth across the entire epoch of reionization. Future missions such as Planck and CMBPol will greatly enhance our knowledge of the reionization history, allowing us to measure the time evolution of the ionization fraction. As large scale polarization probes high redshift activity, it can best constrain models where the Universe was fully or partially ionized at early times. In fact, large scale polarization could be our only probe of the highest redshifts.

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arXiv2008

The morphology of HII regions during reionization

It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature of the ionizing sources, the clumpiness of the gas in the IGM, the degree to which photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.), making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc N-body simulation. Analytic prescriptions are used to incorporate small-scale structures that affect reionization, yet remain unresolved in the N-body simulation. We show that the morphology of the HII regions is most dependent on the global ionization fraction x_i. This is not to say that the bubble morphology is completely independent of all parameters besides x_i. The next most important dependence is that of the nature of the ionizing sources. The rarer the sources, the larger and more spherical the HII regions become. The typical bubble size can vary by as much as a factor of 4 at fixed x_i between different possible source prescriptions. The final relevant factor is the abundance of minihalos or of Lyman-limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which minihalos are photo-evaporated. We find that neither source suppression owing to photo-heating nor gas clumping significantly affect the large-scale structure of the HII regions. We discuss how observations of the 21cm line with MWA and LOFAR can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]

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arXiv2008

Probing Reionization with the 21 cm-Galaxy Cross Power Spectrum

The cross-correlation between high redshift galaxies and 21 cm emission from the high redshift intergalactic medium (IGM) promises to be an excellent probe of the Epoch of Reionization (EoR). On large scales, the 21 cm and galaxy fields are anti-correlated during most of the reionization epoch. However, on scales smaller than the size of the H II regions around detectable galaxies, the two fields become roughly uncorrelated. Consequently, the 21 cm-galaxy cross power spectrum provides a tracer of bubble growth during reionization, with the signal turning over on progressively larger scales as reionization proceeds. The precise turnover scale depends on the minimum host mass of the detectable galaxies, and the galaxy selection technique. Measuring the turnover scale as a function of galaxy luminosity constrains the characteristic bubble size around galaxies of different luminosities. The cross spectrum becomes positive on small scales if ionizing photons fail to escape from low mass galaxies, and these galaxies are detectable longward of the hydrogen ionization edge, because in this case some identifiable galaxies lie outside of ionized regions. LOFAR can potentially measure the 21 cm-galaxy cross spectrum in conjunction with mild extensions to the existing Subaru survey for $z=6.6$ Lyman-alpha emitters, while the MWA is slightly less sensitive for detecting the cross spectrum. A futuristic galaxy survey covering a sizable fraction of the MWA field of view ($\sim 800$ deg$^2$) can probe the scale dependence of the cross spectrum, constraining the filling factor of H II regions at different redshifts during reionization, and providing other valuable constraints on reionization models.

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arXiv2008

Probing the Neutral Fraction of the IGM with GRBs during the Epoch of Reionization

We show that near-infrared observations of the red side of the Ly-alpha line from a single gamma ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's redshift to better than ~0.3. Some GRB sight-lines will encounter more neutral hydrogen than others at fixed x_H owing to the patchiness of reionisation. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on x_H from the z = 6.3 burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Ly-alpha absorbers is the same as measured at lower redshifts, a GRB from an epoch when x_H = 0.5 can be used to detect a partly neutral IGM at 98% confidence level 10% of the time (and, for an observation with three times the sensitivity, 30% of the time).

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arXiv2007

Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array

We forecast the sensitivity with which the Murchison Widefield Array (MWA) can measure the 21 cm power spectrum of cosmic hydrogen, using radiative transfer simulations to model reionization and the 21 cm signal. The MWA is sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}), with foreground contamination precluding measurements on larger scales, and thermal detector noise limiting the small scale sensitivity. This amounts primarily to constraints on two numbers: the amplitude and slope of the 21 cm power spectrum on the scales probed. We find, however, that the redshift evolution in these quantities can yield important information about reionization. Although the power spectrum differs substantially across plausible models, a generic prediction is that the amplitude of the 21 cm power spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM) is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales flattens as the ionization fraction increases and the sizes of the HII regions grow. Considering detection sensitivity, we show that the optimal MWA antenna configuration for power spectrum measurements would pack all 500 antenna tiles as close as possible in a compact core. The MWA is sensitive enough in its optimal configuration to measure redshift evolution in the slope and amplitude of the 21 cm power spectrum. Detecting the characteristic redshift evolution of our models will confirm that observed 21 cm fluctuations originate from the IGM, and not from foregrounds, and provide an indirect constraint on the volume-filling factor of HII regions during reionization. After two years of observations under favorable conditions, the MWA can constrain the filling factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.

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arXiv2007

The Co-Formation of Spheroids and Quasars Traced in their Clustering

We compare observed clustering of quasars and galaxies as a function of redshift, mass, luminosity, & color/morphology, to constrain models of quasar fueling and spheroid-BH co-evolution. High redshift quasars are shown to be drawn from progenitors of local early-type galaxies, with the characteristic quasar luminosity L* reflecting a characteristic mass of 'active' BH/host populations at each redshift. Evolving observed high-z quasar clustering to z=0 predicts a trend of clustering in 'quasar remnants' as a function of stellar mass identical to that observed for early-types. However, quasar clustering does not simply reflect observed early (or late)-type populations; at each redshift, quasars cluster as an 'intermediate' population. Comparing with the age of elliptical stellar populations reveals that this 'intermediate' population represents those ellipticals undergoing or terminating their final significant star formation at each epoch. Assuming that quasar triggering is associated with the formation/termination epoch of ellipticals predicts quasar clustering at all observed redshifts without any model dependence or assumptions about quasar light curves, lifetimes, or accretion rates. This is not true for disks or quasar halos: i.e. quasars do not generically trace star formation, disks, or halo assembly. Quasar clustering at all z is consistent with a constant halo mass ~4x10^{12} M_sun, similar to local 'group scales.' The observations support a scenario in which major mergers trigger quasar activity and dominate bright, high-z quasar populations. We show that measurements of quasar clustering versus luminosity at z~1 can be used to constrain different lower-luminosity AGN fueling mechanisms, and that high-z clustering is sensitive to whether or not systems 'shut down' growth at z>3.

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arXiv2007

The Impact of The IGM on High-Redshift Lyman Alpha Emission Lines

We calculate the impact of the intergalactic medium (IGM) on the observed Lyman alpha lines (hereafter Lya) emitted by galaxies in an ionised IGM at z>4. Our model accounts for gas clumping in the IGM and for the fact that high-redshift galaxies reside in overdense regions, which causes the velocity field of the IGM to depart from the Hubble flow. The observed shape of the Lya line varies widely, with dependence on the intrinsic width and systemic velocity of the line, a galaxies star formation rate and the local extra-galactic UV-background. For large star formation rates and levels of the UV-background, absorption in the IGM does not result in a Lya line that is asymmetric as is common among known high-redshift Lya emitters. For models in which the lines do show the observed strong asymmetry, the IGM typically transmits only 10-30% of the Lya flux. The increase in the ionising background that accompanied the completion of reionisation barely increased the IGM transmission, which suggests that LAEs of comparable luminosity should not appear to be significantly dimmer prior to overlap. In this light, we briefly discuss the potential of Lya emitters as a probe into the epoch of reionisation.

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arXiv2007

Quasar Proximity Zones and Patchy Reionization

Lyman-alpha forest absorption spectra towards quasars at z ~ 6 show regions of enhanced transmission close to their source. Several authors have argued that the apparently small sizes of these regions indicate that quasar ionization fronts at z >~ 6 expand into a largely or partly neutral intergalactic medium (IGM). Assuming that the typical region in the IGM is reionized by z <= 6, as is suggested by Ly-a forest observations, we argue that at {\em least} 50% of the volume of the IGM was reionized before the highest redshift quasars turned on. Further, even if the IGM is as much as 50% neutral at quasar turn-on, the quasars are likely born into large galaxy-generated HII regions. The HII regions during reionization are themselves clustered, and using radiative transfer simulations, we find that long skewers through the IGM towards quasar progenitor halos pass entirely through ionized bubbles, even when the IGM is half neutral. These effects have been neglected in most previous analyses of quasar proximity zones, which assumed a spatially {\em uniform} neutral fraction. We model the subsequent ionization from a quasar, and construct mock Ly-a forest spectra. Our mock absorption spectra are more sensitive to the level of small-scale structure in the IGM than to the volume-averaged neutral fraction, and suggest that existing proximity-zone size measurements are compatible with a fully ionized IGM. However, we mention several improvements in our modeling that are necessary to make more definitive conclusions.

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arXiv2006

The Cross-Correlation of High-Redshift 21 cm and Galaxy Surveys

We study the detectability of the cross-correlation between 21 cm emission from the intergalactic medium and the galaxy distribution during (and before) reionization. We show that first-generation 21 cm experiments, such as the Mileura Widefield Array (MWA), can measure the cross-correlation to a precision of several percent on scales k~0.1/Mpc if combined with a deep galaxy survey detecting all galaxies with m>10^10 Msol over the entire ~800 square degree field of view of the MWA. LOFAR can attain even better limits with galaxy surveys covering its ~50 square degree field of view. The errors on the cross-power spectrum scale with the square root of the overlap volume, so even reasonably modest surveys of several square degrees should yield a positive detection with either instrument. In addition to the obvious scientific value, the cross-correlation has four key advantages over the 21 cm signal alone: (1) its signal-to-noise exceeds that of the 21 cm power spectrum by a factor of several, allowing it to probe smaller spatial scales and perhaps to detect inhomogeneous reionization more efficiently; (2) it allows a cleaner division of the redshift-space distortions (although only if the galaxy redshifts are known precisely); (3) by correlating with the high-redshift galaxy population, the cosmological nature of the 21 cm fluctuations can be determined unambiguously; and (4) the required level of foreground cleaning for the 21 cm signal is vastly reduced.

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arXiv2006

Higher Order Contributions to the 21 cm Power Spectrum

We consider the contribution of 3rd and 4th order terms to the power spectrum of 21 cm brightness temperature fluctuations during the epoch of reionization, which arise because the 21 cm brightness temperature involves a product of the hydrogenic neutral fraction and the gas density. The 3rd order terms vanish for Gaussian random fields, and have been previously neglected or ignored. We measure these higher order terms from radiative transfer simulations and estimate them using cosmological perturbation theory. In our simulated models, the higher order terms are significant: neglecting them leads to a >~100% error in 21 cm power spectrum predictions on scales of k >~ 1 Mpc^{-1} when the neutral fraction is ~0.5. The higher order terms have a simple physical interpretation. On small scales they are produced by gravitational mode coupling. Small scale structure grows more readily in large-scale overdense regions, but the same regions tend to be ionized and hence do not contribute to the 21 cm signal. This acts to suppress the influence of non-linear density fluctuations and the small-scale amplitude of the 21 cm power spectrum. These results modify earlier intuition that the 21 cm power spectrum simply traces the density power spectrum on scales smaller than that of a typical bubble, and imply that small scale measurements contain more information about the nature of the ionizing sources than previously believed. On large scales, higher order moments are not directly related to gravity. They are non-zero because over-dense regions tend to ionize first and are important in magnitude at late times owing to the large fluctuations in the neutral fraction. (Abridged)

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arXiv2006

The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum

We explore the impact of spatial fluctuations in the intergalactic medium temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We develop a semianalytic model to examine temperature fluctuations resulting from inhomogeneous HI and incomplete HeII reionizations. Detection of these fluctuations might provide insight into the reionization histories of hydrogen and helium. Furthermore, these fluctuations, neglected in previous analyses, could bias constraints on cosmological parameters from the Lyman-alpha forest. We find that the temperature fluctuations resulting from inhomogeneous HI reionization are likely to be very small, with an rms amplitude of < 5%, $σ_{T_0}/ < 0.05$. More important are the temperature fluctuations that arise from incomplete HeII reionization, which might plausibly be as large as 50%, $σ_{T_0}/ ~ 0.5$. In practice, however, these temperature fluctuations have only a small effect on flux power spectrum predictions. The smallness of the effect is possibly due to density fluctuations dominating over temperature fluctuations on the scales probed by current measurements. On the largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the flux power spectrum may be as large as ~10% in extreme models. The effect is larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening. Our results suggest that the omission of temperature fluctuations effects from previous analyses does not significantly bias constraints on cosmological parameters.

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arXiv2005

Have We Detected Patchy Reionization in Quasar Spectra?

The Lyman-alpha forest at z >~ 5.5 shows strong scatter in the mean transmission even when smoothed over very large spatial scales, >~ 50 Mpc/h. This has been interpreted as a signature of strongly fluctuating radiation fields, or patchy reionization. To test this claim, we calculate the scatter arising solely from density fluctuations, with a uniform ionizing background, using analytic arguments and simulations. This scatter alone is comparable to that observed. It rises steeply with redshift and is of order unity by z ~ 6, even on ~ 50 Mpc/h scales. This arises because: i) at z ~ 6, transmission spectra, which are sensitive mainly to rare voids, are highly biased (with a linear bias factor b >~ 4-5) tracers of underlying density fluctuations, and ii) projected power from small-scale transverse modes is aliased to long wavelength line-of-sight modes. Inferring patchy reionization from quasar spectra is therefore subtle and requires much more detailed modeling. Similarly, we expect order unity transmission fluctuations in the z ~ 3 HeII Lyman-alpha forest from density fluctuations alone, on the scales over which these measurements are typically made.

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arXiv2005

The Luminosity Dependence of Quasar Clustering

We investigate the luminosity dependence of quasar clustering, inspired by numerical simulations of galaxy mergers that incorporate black hole growth. These simulations have motivated a new interpretation of the quasar luminosity function. In this picture, the bright end of the quasar luminosity function consists of quasars radiating nearly at their peak luminosities, while the faint end consists mainly of very similar sources, but at dimmer phases in their evolution. We combine this model with the statistics of dark matter halos that host quasar activity. We find that, since bright and faint quasars are mostly similar sources seen in different evolutionary stages, a broad range in quasar luminosities corresponds to only a narrow range in the masses of quasar host halos. On average, bright and faint quasars reside in similar host halos. Consequently, we argue that quasar clustering should depend only weakly on luminosity. This prediction is in qualitative agreement with recent measurements of the luminosity dependence of the quasar correlation function (Croom et al. 2005) and the galaxy-quasar cross-correlation function (Adelberger & Steidel 2005). Future precision clustering measurements from SDSS and 2dF, spanning a large range in luminosity, should provide a strong test of our model.

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arXiv2005

Tightening Constraints from the Lyman Alpha Forest with the Flux Probability Distribution Function

The analysis of the Lyman-alpha forest of absorption lines in quasar spectra has emerged as a potentially powerful technique to constrain the linear matter power spectrum. In most previous work, the amplitude of the ionizing background was fixed by calibrating simulations to match the observed mean transmitted flux in the Lyman-alpha forest. This procedure is undesirable in principle as it requires the estimation of the unabsorbed quasar continuum level, a difficult undertaking subject to various sources of systematic error and bias. We suggest an alternative approach based on measuring the one-point probability distribution function (pdf) of the fluctuations in the flux about the mean, relative to the mean, i.e. the pdf of delta_f = (f - )/ . This statistic, while sensitive to the amplitude of the ionizing background, has the virtue that its measurement does not require an estimate of the unabsorbed continuum level. We present a measurement of the pdf of delta_f from seven Keck HIRES spectra, spanning a redshift range of z = 2.2 - 4.4. To illustrate that our method is useful, we compare our measurements of the pdf of delta_f, and measurements of the flux power spectrum from Croft et al. (2002) at z = 2.72, with cosmological simulations. From this comparison, we obtain constraints on the mean transmission in the Lyman-alpha forest, the slope of the temperature-density relation, as well as the amplitude and slope of the mass power spectrum. Our methodology will be useful for obtaining more precise constraints with larger data samples from the Sloan Digital Sky Survey (SDSS).

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