NASA Technical Reports Server (NTRS) 20030019873: The Ionization History of The Intergalactic Medium:

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THE IONIZATION HISTORY OF THE INTERGALACTIC MEDIUM: FINAL REPORT
FOR GRANT NASA ATP NAGS-10232 (FORMERLY NAG5-4236)
Principal Investigator: Piero Madau
1. PROJECT DESCRIPTION suits obtained.
The funded project seeked a unified de- 2. PHYSICAL PROPERTIES OF THE LYMAN
scription of the ionization, physical structure, ALPHA FOREST
and evolution of the intergalactic medium
Avery Meiksin and Mike Norman studied
(IGM) and quasar intervening absorption the origin and physical nature of the Lya for-
systems. We proposed to conduct theoretical
est absorption systems as found in hydrody-
studies of the IGM and QSO absorbers in the namical simulations of the IGM in a stan-
context of current theories of galaxy forma- dard Cold Dark Matter cosmology (Zhang
tion, developing and using numerical and an-
et al. 1998). The structures of the systems
alytical techniques aimed at a detailed mod-
that give rise to the Lya forest span a wide
eling of cosmological radiative transfer, gas range in morphologies. At the cosmological
dynamics, and thermal and ionization evolu-
average density, the characteristic morphol-
tion. The ionization history of the IGM has
ogy is cell-like with underdense regions sepa-
important implications for the metagalactic rated by overdense sheet-like partitions. The
UV background, intergalactic helium absorp- lowest density contours tend to enclose amor-
tion, 21-cm tomography, metal absorption
phous, isolated regions. They found that the
systems, fluctuations in the microwave back-
principal structures of the IGM are in place
ground, and the cosmic rate of structure and
by z = 5, with the evolution in the IGM ab-
star formation.
sorption properties due primarily to the ex-
All the original objectives of our program
pansion of the universe and the changing in-
have been achieved, and the results widely tensity of the photoionizing background radi-
used and quoted by the community. Indeed,
ation field. Virtually all the baryons in the
they remain relevant as the level and com- simulation fragment into structures that we
plexity of research in this area has increased
can identify with discrete absorption lines,
substantially since our proposal was submit-
with at most 5% remaining in a smoothly dis-
ted, due to new discoveries on galaxy forma-
tributed component (the Gunn-Peterson ef-
tion and evolution, a flood of high-quality
fect). Given reasonable assumption on the
data on the distant universe, new theoret-
spectrum of the ionizing UV background, the
ical ideas and direct numerical simulations
simulations were able to reproduce the statis-
of structure formation in hierarchical cluster-
tics of the carbon and silicon measurements
ing theories. The dawning of the epoch of
of the Lya forest at z 3 and the intergalac-
galaxies, the physics of the interplay between tic He II opacity measurements of HST and
early galaxy formation and the intergalactic HUT.
medium, and possible observational probes of
3. THE TEMPERATURE OF THE IGM
the end of the "dark ages" remain some of the
most debated topics in observational cosmol- Numerical N-body/hydrodynamics simula-
ogy. The duration of the program was ex- tions of structure formation in the IGM,
tended beyond the original three years as the within the framework of CDM-dominated
PI took a one-year sabbatical in the UK and cosmologies have provided a definite picture
afterwards moved to the University of Cali- for the origin of the Lya forest, one of an
fornia Santa Cruz. A graduate student, C. interconnected network of sheets and fila-
Porciani, has been funded by this proposal. ments with virialized systems (halos) located
Below is a list of some of the most recent re-
at their points of intersection. Most simula-
2
tionsto dateassumethat the IGM is pho- nation radiation). In principle, this is accom-
toionizedandphotoheatedby a UV radia- plished by solving the equation of radiative
tion backgroundcloseto that inferredfrom transfer for the monochromatic intensity Iv.
quasars(Madau,Haardt,& Rees1999).The However, since Iv is a function of seven vari-
width ofthelines,asmeasuredbytheb pa- ables (t, _, _, v), this isimpractical on current
rameter of a Voigt profile, is set by thermal computers if the intensity needs to be evalu-
broadening, peculiar velocities, and Hubble ated many times over the course of a hydro-
expansion across the filaments. While the dynamical calculation. For this reason, we
first simulations showed good agreement with have been exploring physically motivated ap-
the observed line statistics, more detailed proximations to the transfer equation which
studies at higher numerical resolution have retain the essential physics while being nu-
revealed a serious conflict with the data: the merically economical.
models predict median values for the b dis- Norman, Paschos & Abel (1998) developed
tribution of about 20 kms -t at z -- 3, com- a hydrid method that decomposes the radia-
pared with the observed median of 30 km s-1 . tion field into point source and diffuse com-
One possible way to restore the agreement ponents. The point source radiation field is
with the observations is to increase the tem- attenuated along radial rays, while the dif-
perature of the IGM, adding to the thermal fuse radiation field iscomputed by solving the
broadening. We pointed out (Madau & Ef- zeroth and first radiation moment equations
stathiou 1999) that Compton heating of in- field with variable Eddington factor closure.
tergalactic gas by the hard X-ray background In the limit of small (compared to the hori-
(XRB) may help to resolve this discrepancy zon) volumes, the two moment equations can
in underdense regions. The rate of gain in be simplified and combined into a single non-
thermal energy by Compton scattering will linear elliptic equation for the diffuse field.
dominate over the energy input from hydro- The elliptic equation can be solved using a
gen photoionization if the XRB energy den- multigrid algorithm.
sity is ,-_ 0.2x/(e) times higher than the en- In Abel, Norman, & Madau (1999) a nu-
ergy density of the UV background at a given merical method that solves the radiative
epoch, where x is the hydrogen neutral frac- transfer around point sources within a three
tion in units of 10-6 and (e) is the mean dimensional cartesian grid was developed.
X-ray photon energy in units of rne c2. The The method is energy conserving indepen-
numerical integration of the time-dependent dently of resolution: this ensures the correct
rate equations showed, however, that in pop- propagation speeds of ionization fronts. We
ular models for the redshift evolution of the described the details of the algorithm, and
extragalactic background radiation this effect computed as first numerical application the
was not large enough to match the observa- ionized region surrounding a miniquasar in a
tions. Perhaps a more promising mechanism cosmological density field at z = 7.
that could increase the line-width involves ra-
diative transfer effects during the (late) reion- 5. RADIO SIGNATURES OF HI AT HIGH
REDSHIFT
ization of helium. As discussed below, a ma-
jor effort has been devoted to develop a radia- The emission of 21-cm radiation from a
tive transfer code for treating thermal effects neutral intergalactic medium (IGM) at high
during the inhomogeneous reionization. redshifts has been discussed in a paper (Tozzi
et al. 2000) in connection with the thermal
4. SIMULATING INHOMOGENEOUS
and ionization history of the universe. The
REIONIZATION
physical mechanisms that make such radia-
A key requirement for numerical simula- tion detectable against the cosmic microwave
tions of inhomogeneous reionization is an ef- background include Lyman-alpha coupling of
ficient algorithm for computing the radiation the hydrogen spin temperature to the kinetic
field Jr(Z) in three spatial dimensions given temperature of the gas and preheating of
an arbitrary distribution of absorbers and the IGM by the first generation of stars and
sources, both point and diffuse (e.g., recombi- quasars. Three different signatures were in-
vestigatedin detail: (a) thefluctuationsin mation in CDM cosmologies the smallest
theredshifted21-cmemissioninducedbythe nonlinear objects are the numerous small
gasdensityinhomogeneitietshat developat halos that condense with virial tempera-
earlytimesincolddarkmatter(CDM)dom- tures between the cosmological Jean temper-
inatedcosmologies(;b)thesharpabsorption ature and _ 104 K. Such "minihalos" are
featurein theradioskyduetotherapidrise not yet fully resolved nor is the process
of the Lyman-alphacontinuumbackground of their photoevaporation captured in large-
that marksthe birth ofthefirst UV sources
scale three-dimensional cosmological simula-
in the universe;and(c) the21-cmemission tions. In Haiman et al. (2001) we have used
andabsorptionshellsthat aregeneratedon semi-analytic methods combined with three-
severalMpc scalesaroundthe first bright dimensional numerical simulations of individ-
quasars.Futureradioobservationwsith the ual photoevaporating minihalos to (1) quan-
Giant MetrewaveRadioTelescopeandthe tify the importance of high-redshift miniha-
SquareKilometerArray mayshedlight oil los as sinks of ionizing photons; and (2) as-
the powerspectrumof densityfluctuations sess whether by extrapolating to early times
atz > 7, and map the end of the "dark age", the known population of galaxies and quasars
i.e. the transition from a neutral universe to a a sufficient number of UV photons are pro-
ionized one populated with radiation sources. duced for hydrogen reionization. If reion-
ization occurs at sufficiently high redshifts
6. PHOTON CONSUMPTION IN MINIHALOS (Zr _ 20), the intergalactic medium is
DURING REIONIZATION heated to _ 104K and most minihalos never
The process of reionization and its im- form. On the other hand, if Zr _< 20 as it
pact on several key cosmological issues (from appears more likely, then a significant frac-
the role it plays in allowing collapsed ob- tion (_> 10%) of all baryons have already col-
jects to cool and make stars, to determin- lapsed into minihalos, and are subsequently
ing the small-scale structure in the temper- removed froul the halos by photoevaporation
ature fluctuations of the CMB) has recently as the ionizing background flux builds up.
received much theoretical attention, and has This process requires a significant budget of
been studied by several authors using semi- ionizing photons, about 10-20 of them per
analytic models and three-dimensional nu- background hydrogen atom. This exceeds the
merical simulations. In general, these works production by a straightforward extrapola-
aim to follow the time evolution of the fill- tion back in time of known quasar and galaxy
ing factor of ionized (H II ) regions, based on populations by a factor of up to -_ 10and _ 3,
some input prescriptions for the emissivity respectively (Haiman et al. 2001).
and spectra of the ionizing sources.
7. THE EARLIEST LUMINOUS SOURCES
More recent works have focused on the in-
creased rate of recombinations in a clumpy Prior to complete reionization at redshift
medium relative to a homogeneous one. zr, sources of ultraviolet radiation will be
These studies have left significant uncertain- seen behind a large column of intervening
ties on the details of how reionization pro- gas that is still neutral. In this case, be-
ceeds in an inhomogeneous medium. Since cause of scattering off the line-of-sight due
the ionizing sources are embedded in dense to the diffuse neutral IGM, the spectrum of
regions, one might expect that these dense a source at Zero > Zr should show the red
regions are ionized first, before the radiation damping wing of the Gunn-Peterson absorp-
escapes to ionize the low-density IGM. Alter- tion trough at wavelengths longer than the
natively, most of the radiation might escape local Lya resonance, )%bs > An(1 + Zero),
from the local, dense regions along low col- where ,k,_= c/v,_ = 1216/_. At Zero2 6, this
umn density lines of sight. In this case, the characteristic feature extends for more than
underdense 'voids' are ionized first, with the 1500 kms -1 to the red of the resonance, and
ionization of the denser filaments and halos may significantly suppress the Lya emission
lagging behind. line in the spectra of the first generation of
At the earliest epochs of structure for- objects in the universe. Measuring the shape
of the absorption profile of the damping wing known to be already in place at z -- 3. Mas-
could provide a determination of the density sive stars will deposit both radiative and me-
of the neutral IGM near the source. chanical energy into the interstellar medium
In Madau & Rees (2000) we focused on the of protogalaxies. A complex network of 'feed-
width of the red damping wing - related to back' mechanisms is likely at work in these
the expected strength of the Ly(_ emission systems, as the gas in shallow potential is
line - in the spectra of very distant QSOs more easily blown away, thereby quenching
as a flag of the observation of the IGM be- star formation. To start addressing some
fore reionization. We assessed, in particu- of these issues and simulate the process of
lar, the impact of the photoionized, Mpc-size blow-away, we have developed (Mori, Fer-
regions which will surround individual lumi- rara, & Madau 2002) a 3D Eulerian code
nous sources of Lyman continuum radiation that solves the hydrodynamic equations for
on the transmission of photons redward of the a perfect fluid in Cartesian geometry. To
Lyc_ resonance, and shown that the damp- deal with very different length scales in our
ing wing of the Gunn-Peterson trough may simulation we have adopted a 'nested grid
nearly completely disappear because of the method' with six levels of fixed Cartesian
lack of neutral hydrogen in the vicinity of grids. The scheme has a wide dynamic range
a bright QSO. The effect of this local pho- in the space dimension. The results of our
toionization is to greatly reduce the scatter- simulations show that SN-driven pregalac-
ing opacity between the redshift of the quasar tic outflows may be an efficient mechanism
and the boundary of its H II region. The de- for spreading metals around. Furthermore,
tection of a strong Ly(_ emission line in the as the blastwaves produced by supernova ex-
spectra of bright QSOs shining for _> 107yr plosions reheat the surrounding intergalactic
cannot then be used, by itself, as a constraint gas and enrich it with newly formed heavy
on the reionization epoch. The first signs elements, they appear to inhibit the forma-
of an object radiating prior to the transition tion of surrounding low-mass galaxies due to
from a neutral to an ionized universe may be 'baryonic stripping' (Scannapieco, Ferrara, &
best searched for in the spectra of luminous Madau 2002).
sources with a small escape fraction of Lyman
continuum photons into the IGM, or sources
with a short duty cycle (like, e.g., gamma-ray
bursts).
8. EARLY METAL ENRICHMENT OF THE IGM
Understanding the origin of the chemical
elements, following the increase in their abun-
dances with cosmic time, and uncovering
the processes responsible for distributing the
products of stellar nucleosynthesis over very
large distances are all key aspects of the evo-
lution of gaseous matter in the universe. One
of the major discoveries with Keck concern-
ing the IGM has been the identification of
metal absorption lines associated with many
of the Lya forest systems at z -- 3 - 3.5.
From a theoretical perspective, it is un-
known whether the existence of heavy ele-
ments in the low-density IGM points to an
early (z > 6) enrichment epoch by low-mass
subgalactic systems (Madau, Ferrara, & Rees
2001), or israther due to late pollution by the
population of star-forming galaxies galaxies
PUBLICATIONS FUNDED BY THE PROGRAM
• Scannapieco, E., Ferrara, A., & Madau, P. 2002, "Early Enrichment of the Intergalactic
Medium and its Feedback on Galaxy Formation", ApJ, 574, 590
• Mori, M., Ferrara, A., & Madau, P. 2002, "Early Metal Enrichment by Pregalactic Out-
flows: II. Simulations of Blow-Away", ApJ, 571, 40
• Metcalf, R. B., & Madau, P. 2001, "Compound Gravitational Lensing as a Probe of Dark
Matter Substructure within Galaxy Halos", ApJ, 563, 9
• Oh, S. P., Nollett, K. M., Madau, P., & Wasserburg, G. J. 2001, "Did Very Massive Stars
Pre-Enrich and Reionize the Universe?", ApJ, 562, L1
• Madau, P., Ferrara, A., & Rees, M. J. 2001, "Early Metal Enrichment of the Intergalactic
Medium by Pregalactic Outflows", ApJ, 555, 92
• Madau, P., & Rees, M. J. 2001, "Massive Black Holes as Population III Remnants", ApJ,
551, L27
• Haiman, Z., Abel, T., & Madau, P. 2001, "Photon Consumption in Minihalos during
Cosmological Reionization", ApJ, 551,599
• Haehnelt, M., Madau, P., Kudritzki, R., & Haardt, F. 2001, "An Ionizing UV Background
Dominated by Massive Stars", ApJ, 549, L151
• Keeton, C. R., & Madau, P. 2001, "Lensing Constraints on the Cores of Massive Dark
Matter Halos", ApJ, 549, L25
• Porciani, C., & Madau, P. 2001, "On the Association of Gamma-ray Bursts with Massive
Stars: Implications for Number Counts and Lensing Statistics", ApJ, 548, 522
• Madau, P., & Rees, M. J. 2000, "The Earliest Luminous Sources and the Damping Wing
of the Gunn-Peterson Trough", ApJ, 542, L69
• Porciani, C., & Madau, P. 2000, "Gravitational Lensing of Distant Supernovae in Cold
Dark Matter Universes", ApJ, 532, 679
• Tozzi, P., Madau, P., Meiksin, A., & Rees, M. J. 2000, "Radio Signatures of H I at High
Redshift: Mapping the End of the Dark Age", ApJ, 528, 597
• Abel, T., Norman, M. L., & Madau, P. 1999, "Photon Conserving Radiative Transfer
around Point Sources in Multi-Dimensional Nmnerical Cosmology", ApJ, 523, 66
• Madau, P., & Efstathiou, G. 1999, "Compton Heating of the Intergalactic Medium by the
Hard X-ray Background", ApJ, 517, L9
• Haiman, Z., Madau, P., & Loeb, A. 1999, "Constraints from the Hubble Deep Field on
High Redshift Quasar Models", ApJ, 514, 535
• Madau, P., Haardt, F., & Rees, M. J. 1999, "Radiative Transfer in a Clumpy Universe:
III. The Nature of Cosmological Ionizing Sources", ApJ,
• Zhang, Y., Meiksin, A., Anninos, P., & Norman, M. L. 1998, "Physical Properties of the
Lyman-alpha Forest in a Cold Dark Matter Comology", ApJ, 495, 63
• Madau, P., Della Valle, M., & Panagia, N. 1998, "Oil tile Evolution of the Cosmic Super-
nova Rates", MNRAS, 297, L17
• Madau, P. 1998, "What Keeps the Universe Ionized at z = 57", in Molecular Hydrogen
in the Early Universe, ed. E. Corbelli, D. Galli, & F. Palla (Mem. S.A.It.), 471
• Madau, P. 1999, "Starlight in the Universe", Physica Scripta, 85, 156
• Norman, M. L., Paschos, P. & Abel, T. 1998, "Simulating Inhomogeneous Reionization",
in Molecular Hydrogen in the Early Universe, ed. E. Corbelli, D. Galli, & F. Palla (Mere.
S.h.It.), 142