Steven R. Furlanetto

Intergalactic magnetic fields from quasar outflows

Outflows from quasars inevitably pollute the intergalactic medium (IGM) with magnetic fields. The short-lived activity of a quasar leaves behind an expanding magnetized bubble in the IGM. We model the expansion of the remnant quasar bubbles and calculate their distribution as a function of size and magnetic field strength at different redshifts. We generically find that by a redshift z ~ 3, about 5%-20% of the IGM volume is filled by magnetic fields with an energy density 10% of the mean thermal energy density of a photoionized IGM (at ~104 K). As massive galaxies and X-ray clusters condense out of the magnetized IGM, the adiabatic compression of the magnetic field could result in the field strength observed in these systems without a need for further dynamo amplification. The intergalactic magnetic field could also provide a nonthermal contribution to the pressure of the photoionized gas that may account for the claimed discrepancy between the simulated and observed Doppler width distributions of the Ly? forest.

Metal absorption lines as probes of the intergalactic medium prior to the reionization epoch

Winds from star-forming galaxies provide the most promising explanation for the enrichment of the intergalactic medium with heavy elements. Theoretical and observational arguments indicate that the pollution may have occurred at z 6; however, direct observational tests of such a scenario are needed. We model starburst winds in the high-redshift universe and find that the fraction of space filled by enriched material varies strongly with the assumed star formation efficiency f* and the fraction of supernova energy powering each wind fesc. We show that metals carried by these winds can be seen in absorption against bright background sources, such as quasars or gamma-ray bursts, in narrow lines with characteristic equivalent widths ~0.5 A W 5 A. We argue that a substantial fraction of the metals in high-redshift winds are likely to reside in low-ionization states (C II, O I, Si II, and Fe II), but higher ionization states (C IV and Si IV) could also provide useful probes of the winds. The number of such lines can constrain both f* and fesc. Statistics of metal absorption lines can also be used to identify whether H2 is an efficient coolant in the early universe and to study the initial mass function of stars at high redshifts.

Observing the reionization epoch through 21-centimetre radiation

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 fluctuations on small scales drop abruptly from � δT 2 � 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.

Constraining the Collisional Nature of the Dark Matter through Observations of Gravitational Wakes

We propose to use gravitational wakes as a direct observational probe of the collisional nature of the dark matter. We calculate analytically the structure of a wake generated by the motion of a galaxy in the core of an X-ray cluster for dark matter in the highly collisional and collisionless limits. Although the difference between these limits is most likely too subtle to be detected with weak lensing observations, we show that the two cases can be distinguished through detailed X-ray observations. We also discuss the sizes of subhalos in these limits. Preliminary X-ray data on the motion of NGC 1404 through the Fornax group disfavors fluidlike dark matter but does not exclude scenarios in which the dark matter is weakly collisional.

Identifying Gamma-Ray Burst Remnants through Positron Annihilation Radiation

We model the annihilation of relic positrons produced in a gamma-ray burst (GRB) after its afterglow has faded. We find that the annihilation signal from at least one GRB remnant in the Milky Way should be observable with future space missions such as the International Gamma-Ray Astrophysical Laboratory and the Energetic X-ray Imaging Survey Telescope, provided that the gas surrounding the GRB source has the typical density of the interstellar medium, 1 cm-3. Three fortunate circumstances conspire to make the signal observable. First, unlike positrons in a standard supernova, the GRB positrons initially travel at a relativistic speed and remain ahead of any nonrelativistic ejecta until the ejecta become rarefied and the annihilation time becomes long. Second, the GRB remnant remains sufficiently hot (T 5 × 105 K) for a strong annihilation line to form without significant smearing by three-photon decay of positronium. Third, the annihilation signal persists over a time longer than the average period between GRB events in the Milky Way.

Emission of Positron Annihilation Line Radiation by Clusters of Galaxies

Clusters of galaxies are enriched with positrons from jets of active galactic nuclei (AGNs) or from the interaction of cosmic rays with the intracluster gas. We follow the cooling of these positrons and show that their eventual annihilation with cluster electrons yields a narrow annihilation line. Unlike annihilation in the interstellar medium of galaxies, the line produced in clusters is not smeared by three-photon decay of positronium because positronium formation is suppressed at the high (1 keV) temperature of the cluster electrons. We show that if AGN jets are composed of e+e- pairs, then the annihilation line from rich clusters within a distance of 100 Mpc might be detectable with future space missions, such as the International Gamma-Ray Astrophysical Laboratory or the Energetic X-Ray Imaging Survey Telescope.