Can the reionization epoch be detected as a global signature in the cosmic background?

12 Pages·0.24 MB·English

by  P. A. Shaver

A&A manuscript no. ASTRONOMY
(will be inserted by hand later) AND
Your thesaurus codes are: ASTROPHYSICS
12.03.1; 12.03.3; 12.04.2, 12.05.1
Can the reionization epoch be detected as a global signature
in the cosmic background?
P.A. Shaver1, R.A. Windhorst2, P. Madau3, and A.G. de Bruyn4
1 European Southern Observatory,Karl-Schwarzschild-Str.2, D-85748 Garching beiMu¨nchen, Germany
2 Arizona State University,Dept.of Physics & Astronomy,Tempe, AZ 85287-1504, U.S.A.
9 3 Space Telescope ScienceInstitute, 3700 San Martin Drive,Baltimore, MD 21218, U.S.A.
9 4 NetherlandsFoundation for Research in Astronomy,Postbus 2, NL-7990 AA Dwingeloo, The Netherlands
9
and Kapteyn Astronomical Institute,Postbus 800, NL-9700 AV Groningen, The Netherlands
1

n
Received 5 October 1998/ Accepted
a
J

2
Abstract. ThereionizationoftheUniverseisexpectedto Loeb 1999).1 We know reionization took place before
2
leave a signal in the form of a sharp step in the spectrum z 5,asevidencedbythelackofhydrogencontinuumab-
≈
1 ofthesky.Ifreionizationoccursat5<z <20,afeature sorptioninthespectraofhigh-redshiftquasars(Schneider
ion
v should appear in the radio sky at 70∼<ν <∼240 MHz due et al.1991)and galaxies(Franx et al.1997).It is unlikely
0 to redshifted Hi21-cmline emission,∼acco∼mpaniedby an- that reionization occurred at z > 50, for in that case the
2
other feature in the optical/near-IR at 0.7 < λ < 2.6µm levelofdegreescaleanisotropyin∼theCMBwouldbelower
3
1 due to hydrogen recombination radiation. ∼The∼expected thanobservedontendegreescales(e.g.Knoxetal.1998).
0 amplitudeiswellabovefundamentaldetectionlimits,and TiltedandneutrinodominatedCDMmodelsgivez 5–
ion
9 the sharpness of the feature may make it distinguishable 8,andopenandΛ-dominatedmodelsgivez 7–30(≃Cen
ion
9 ≃
fromvariationsduetoterrestrial,galacticandextragalac- 1998).
/
h tic foregrounds. Moreover,sincethecharacteristicdistancebetweenthe
p
Becausethisisessentiallyacontinuummeasurementof sources that ionized a rapidly recombining intergalactic
-
o a signal which occurs over the whole sky, relatively small medium(IGM)wasmuchsmallerthantheHubbleradius,
r telescopes may suffice for detection in the radio. In the the transition from Hi to Hii was quite abrupt; the over-
t
s optical/near-IR,a spacetelescope is neededwiththe low- lapping of the Hii regions surrounding the first star clus-
a
est possible background conditions, since the experiment ters and miniquasars was completed on a timescale much
:
v will be severely background-limited. shorter than the Hubble time. The reionizationepoch oc-
i
X curred almost as a “phase transition” of the Universe.
r Key words: cosmology:cosmic microwavebackground– From the observational point of view, this poorly con-
a early Universe – diffuse radiation – observations strained epoch is among the most important unknown
quantities in cosmology.
Varioustechniquesforprobingthe historyofthe tran-
sition from a neutral IGM to one that is almost fully ion-
ized have been proposed in the literature. One way is to
1. Introduction
lookforthe21-cmhyperfinelineofneutralhydrogen,red-
shiftedtofrequenciesintherange70–240MHz(Madauet
The epoch of reionization marked the end of the “dark
al. 1997; Gnedin & Ostriker 1997; see also Swarup 1984;
ages”duringwhichtheever-fadingprimordialbackground
Swarup&Subrahmanyan1987;Scott&Rees1990).Prior
radiation cooled below 3000 K and shifted first into the
tothereionizationepoch,theneutralgasthathadnotyet
infrared and then into the radio. Darkness persisted un-
beenengulfedby anHiiregionmaybe seenas21-cmline
til early structures collapsedand cooled, forming the first
emission if a mechanism existed that decoupled the spin
stars and quasars that lit the universe up again. During
temperature from the CMB temperature. Physical mech-
this epoch the volume filling factor of ionized hydrogen
(Hii) increased rapidly. It is a generic feature of theo- anisms that would produce a 21-cm signature are Lyα
coupling of the spin temperature to the kinetic tempera-
retical models and three-dimensional numerical simula-
tions that reionization occurred at 5 < zion < 20 in most 1 Note,however,thatwhilesimulationsareabletotrackthe
colddarkmatter(CDM)cosmogonies∼(Gned∼in&Ostriker
formation and merging of dark matter halos and the subse-
1997; Haehnelt & Steinmetz 1998; Cen 1998; Haiman & quent baryonic infall, they are much less able to predict the
efficiency and rate of radiation emission from gravitationally
Send offprint requests to: P.A. Shaver([email protected]) collapsed objects.
2 P.A.Shaveret al.: Detection of theReionization Epoch
ture of the gas, preheating by soft X-rays from collapsing theneutralgas.TheevolutionofanexpandingHiiregion
dark matter halos, and preheating by ambient Lyα pho- is governedby the equation
tons (Madau et al. 1997).
dV N˙ V
Sofartheemphasishasbeenondetectingfluctuations I 3HV = ion I , (2)
due to individual large Hi concentrations. This is very dt − I n¯H − t¯rec
difficultinthepresenceofastrongfluctuatingforeground (Shapiro & Giroux 1987), where V is the proper volume
I
emission;theHilinesfromindividualconcentrationsmust oftheionizedzone,N˙ isthenumberofionizingphotons
ion
be resolved, and this requires large telescopes with high emitted by the central source per unit time, and H is the
sensitivity and spectral and angular resolution. Alterna- Hubble constant.Acrosstheionizationfrontthedegreeof
tively, if reionization occurred abruptly and the fraction ionizationchangessharplyonadistanceoftheorderofthe
ofneutralhydrogenunderwentadropofafactorof103 in mean free path of an ionizing photon. When t¯ t, the
rec
about a tenth of a Hubble time as predicted (e.g. Gnedin growthoftheHiiregionissloweddownbyrecomb≪inations
&Ostriker1997;Baltzetal.1998),itisconceivablethata in the highly inhomogeneous IGM, and its evolution can
globalsignaturemaybedetectablewithtelescopesofmod- be decoupled from the expansion of the universe. As in
estsizeintheextragalacticbackgroundspectrumatmeter thestaticcase,theionizedbubblewillfillitstime-varying
wavelengths.This isinprinciple amuchsimplermeasure- Stro¨mgren sphere after a few recombination timescales,
ment.Someexistingground-basedtelescopesmaybesuit-
N˙ t¯
able,or,ifterrestrialinterferenceistoosevere,adedicated V = ion rec(1 e−t/t¯rec). (3)
I
spaceprojectmightbeconsidered.Acomplementarytech- n¯H −
nique is to look for a signal in the optical/near-IR back- While the volume that is actually ionized depends on the
groundduetohydrogenrecombinationradiationfromthe luminosity of the central source, the time it takes to pro-
reionization epoch (Baltz et al. 1998). duce an ionization-bounded region is only a function of
The prospects for detecting a global signature of the t¯ .
rec
reionization epoch are considered below. In the following Atearlytimes,thecharacteristicdistancebetweenthe
we shalldenote the present-day Hubble constantas H0 = sourcesthationizedarapidlyrecombiningIGMwasmuch
100h km s−1 Mpc−1. smallerthantheHubbleradius.Thephasetransitionfrom
Hi to Hii was therefore quite abrupt. When the Hii re-
gions surrounding the first star clusters and miniquasars
2. Reionization signature
overlapped, the fraction of neutral hydrogen dropped by
a few orders of magnitude in about a tenth of the Hubble
Numerical N-body/hydrodynamic simulations of struc-
timeatthatepoch,thecontinuumopticaldepthdecreased
ture formation in a strongly clumped IGM have started
suddenlyandthebackgroundionizingintensityunderwent
to provide a picture for the origin of intervening absorp-
adrasticincreaseinaverysmallredshiftinterval(Gnedin
tion systems, one of an interconnected network of sheets
& Ostriker 1997; Baltz et al. 1998). The redshift of this
and filaments, with virialized systems located at their
event is expected to be essentially the same in all direc-
pointsofintersection.Thegasclumpingfactorroseabove
tions. It is this abrupt change in Hi absorptionthat flags
unity whenthe collapsedfractionofbaryonsbecamenon-
negligible, i.e. at z < 20, and grew to a few tens at the epoch of reionization and may be observable at radio
z 8 (Gnedin & Ost∼riker 1997). This made the volume- and optical/near-IRfrequencies, as shown below.
≈
averagedgas recombination time,
3. Detectability of an Hi edge
Ω h2 −1
t¯ =[1.17n¯ α C]−1 =0.08Gyr b The intergalacticmedium priorto the epoch of full reion-
rec H B
(cid:18) 0.02(cid:19)
ization should be detectable in 21-cm line radiation. In
−3
1+z C−1, (1) the absence of a decoupling mechanism, the spin temper-
(cid:18) 9 (cid:19) 10 ature of neutral hydrogen would go to equilibrium with
the CMB, and no emission or absorption relative to the
shorterthanthatforauniformIGMandshorterthanthe CMB would be detected. The collapse and cooling of the
the Hubble time at that epoch. In the above expression, first non-linear structures had a twofold effect: (1) it pre-
n¯H is the mean hydrogen density of the expanding IGM, heated the IGM to temperatures 150 K, above that of
n¯H(0) = 1.7 10−7 (Ωbh2/0.02) cm−3, αB is the recom- the CMB; and (2) through scatte∼ring by Lyα photons it
×
bination coefficient to the excited states of hydrogen,and provided a mechanism to couple the spin temperature to
C ≡hn2Hiii/n¯2Hii is the ionized hydrogen clumping factor. the kinetic temperature of the gas(Madauet al.1997).A
When an isolated point source of ionizing radiation patchwork– due to large-scalestructure and non-uniform
turnson,theionizedvolumeinitiallygrowsinsizeatarate heatingandcoupling–of21-cmlineemissionwouldresult,
fixed by the emission of UV photons, and an ionization a signal which would have disappeared after reionization.
front separating the Hii and Hi regions propagates into A signature (“step”) in the continuum spectrum of the
P.A.Shaveret al.: Detection of theReionization Epoch 3
radio sky, at a frequency of 70 240 MHz for z in
ion 2.77
∼ −
the range 5–20, will therefore flag the reionization epoch.
To illustrate the basic principle of the observationswe
a
propose, consider a neutral IGM with spin temperature 2.75
TS TCMB. In an Einstein-de Sitter universe (Ω0 = 1, K) b
ΩΛ≫= 0), its intergalactic optical depth at 21(1+z)cm T ( c
along the line of sight, 2.73
T Ω h2
τ(z) 10−3h−1 CMB b (1+z)1/2, (4)
≈ (cid:18) T (cid:19)(cid:18) 0.02(cid:19) 2.71
S
90 110 130 150 170 190 210 230 250
will typically be much less than unity. The experiment Frequency MHz)
envisaged consists in its simplest form of two measure-
mentsseparatedinfrequencysuchthatthe oneatshorter
Fig.1. Expected brightness temperature of the cosmic back-
wavelengths detects no line feature because all hydrogen ground in the vicinity of the Hi reionization edge as a func-
has been reionized. The differential antenna temperature tion of observing frequency.Three cases are shown for the Hi
across the “step”, as observed at Earth, will be step, (a): the case from Eq. (5) with zion = 7, ∆z = 1 and
h = 0.7, (b) and (c): revised results provided by N. Gnedin
∆T =(1+z)−1(TS TCMB)(1 e−τ) (private communication) from the simulations by Gnedin &
− −
Ω h2 1+z 1/2 Ostriker (1997) and Baltz et al. (1998) case A respectively,
(0.01 K)h−1 b ion , (5) with ΩM =0.35, ΩΛ =0.65, and h=0.7.
≈ (cid:18)0.02(cid:19)(cid:18) 9 (cid:19)
where z is the redshift of the transition epoch. Note
ion
thatinauniversewithΩ0 1,∆T increasesmorenearly 3.2. Contamination by galactic and extragalactic
≪
linearly with (1+zion), and the numerical coefficient in foregrounds
Eq.(5) may be largerby up to a factor 3[(1+z )/9]1/2.
ion
Dependingonthecosmology,∆T willincreasetowardlow Thedifficultyposedbythegalacticandextragalacticfore-
frequencies at a rate between ν−1/2 and ν−1. Hence, the grounds is that they can be complex, both in frequency
expectedcosmologicalsignalwillbe anedgeofmorethan and position. The question is whether one can in princi-
0.02 K (for h = 0.5) at 70–240 MHz superimposed on ple extract a 0.02K step from this much stronger varying
∼
the 2.73K cosmic backgroundradiation.Fig. 1 showsthe continuum.
total cosmological signal, comprised of the cosmic back- All-skymapsat150and408MHzarepresentedinLan-
groundradiationwiththereionizationstepsuperimposed. decker&Wielebinski(1970)andHaslametal.(1982).The
regions of lowest brightness temperatures occur in large,
relatively smooth regions away from the galactic plane
3.1. Fundamental sensitivity limits
and galactic loops. This emission is comprised of four
Thefundamentallimitationonthesensitivityachievableis components:galacticsynchrotronemission( 70%at150
∼
set by the intensity of the galactic and extragalactic fore- MHz), galactic thermal (free-free) emission ( 1%), the
∼
grounds. Modern receivers are such that the system tem- integrated emission from extragalactic sources ( 27%),
∼
perature should be largely dominated by these extrater- and the 2.73 K cosmic background itself.
restrial signals. The limiting sensitivity is thus Thegalacticnonthermalsynchrotronemissionisdom-
inantat these low frequencies,andhas beenstudied since
T
∆T sys , (6) the pioneering days of radio astronomy. Its spectrum is
min
∼ √δνt close to a featureless power law, although there are grad-
ual variations in the spectral index with position on the
whereT isthetotalsystemtemperature,δν istheband-
sys
sky and with frequency. The spectral index is smallest in
width,andtis the integrationtime.ForT =150K(the
sys
thecoldestregionsofthesky,awayfromthegalacticplane
coldest regions of the sky at 150 MHz) with a bandwidth
and galacticloops.Variations appear to be mainly due to
of 5 MHz andintegrationtime 24 hours,∆T 0.0002
min
∼ the latter. At 100 MHz the spectral index β of this com-
K. The reionization step predicted by Eq. (5) would be
ponent is about –2.55 in cold sky regions,and it steepens
100σ, independent of telescope size. In a real measure-
∼ to –2.8at1 GHz (Bridle 1967;Sironi1974;Webster 1974;
ment the system temperature would of course be higher
Cane 1979;Lawsonet al.1987;Reich& Reich1988;Ban-
than this fundamental limit of 150K, but clearly sensi-
day & Wolfendale 1991; Platania et al. 1998). In some
tivity is not an issue, and the challenge concerns signal
regions such as the north galactic pole this steepening of
contamination and calibration, as discussed below.
the spectrum appears to be considerably less pronounced
(Bridle 1967).
4 P.A.Shaveret al.: Detection of theReionization Epoch
There may also be a dispersion in the spectral index at 150 MHz, based on observations at 10 MHz. This is
at each position on the sky, due to distinct components consistent with results from integrating the source count
along the line of sight. Some indication of this dispersion data (Willis et al. 1977;Simon 1977;Lawsonet al. 1987).
is given by point-to-point variations of the spectral index Simon(1977)estimatedthatthe totalextragalacticback-
overthe sky (Landecker& Wielebinski 1970;Milogradov- groundshouldturnoverbetween1and2MHzduetosyn-
Turin1974;Lawsonetal.1987;Reich&Reich1988;Ban- chrotron self-absorption in the individual source compo-
day&Wolfendale1990,1991).Itisnotlarge,andappears nents, but individual sources can have complex frequency
tobesmallestatthelowerfrequencies(Lawsonetal.1987; structureatmuchhigherfrequencies.Intheextreme,spec-
Banday&Wolfendale1990).Avalueofσ(β) 0.1isprob- tral indices can range from +2 (sources with sharp syn-
∼
ablyappropriateat100–200MHz.Furtherinformationon chrotron self-absorption turnovers) to –3 (pulsars). Over-
small-scale uniformity can be provided by simultaneous all, the resulting features in the extragalacticbackground
interferometric data on many short baselines using tele- shouldbe smallandslowlyvaryingfunctions offrequency
scopearrayssuchasthe WSRT andGMRT. Therecently due to sourceinhomogeneities andthe averageovermany
completed Westerbork 325 MHz survey of the northern sources.
hemisphere (WENSS, de Bruyn et al. 1998)already gives
This has been simulated using the known spectral
some constraints. At high galactic latitudes, where the
properties of 3C sources as studied by Kellermann et al.
minimum brightness temperature of the diffuse galactic
(1969).Theyclassifiedthespectraofalmostall3Csources
emission is about 20 K at 325 MHz, the variations in the
inthefollowingcategories:S(straight–singlepowerlaw),
brightnesstemperatureofthegalacticforegroundemission
C– (convex – steepening at higher frequencies), C+ (con-
aretypicallylessthanabout0.4K,orabout2%,onscales
cave–flatteningathigherfrequencies),andCpx(complex
ranging from 5–30 arcmin. If spatial intensity variations
–containingmultiplecomponents).Ofthe299extragalac-
areaccompaniedbyspectralvariations(astheywouldbe,
tic sources studied, 42% are S, 52% are C–, 3% are C+,
if caused by low surface brightness Hii regions or super-
and 3% are Cpx. The actual spectral shapes of 37 3C
nova remnants), such measurements can be used to set
sources representing all these classes were measured from
limits on spectral variations.
spectrapublishedinKellermann(1966,1974),Kellermann
The galactic thermal emission at high galactic lati-
et al. (1969), Kellermann & Pauliny-Toth (1969), and
tudesisduetoverydiffuseionizedgas,withtotalemission
Kellermann& Owen (1988).These were then weighted in
measureabout5pccm−6forT =8000K(Reynolds1990).
e accordancewiththefrequencyofoccuranceofthevarious
Morerecentstudiesofthepropertiesofthisgashavebeen
classes, and co-added to give a representative composite
made by Kogut et al. (1996) and de Oliviera-Costa et al.
spectrum of the extragalactic foreground.
(1997), who show that it has structure correlated with
The resulting extragalactic spectrum in the range 50-
high-latitudedustclouds.Thisgasisopticallythinatfre-
300 MHz is close to a power law of spectral index –2.65
quencies above a few MHz, so its spectrum in the region
(the indexissteeperathigherfrequencies).The deviation
100–200MHz is well-determined, with a spectral index of
from this power law is a slowly and continuously varying
–2.1.
function of frequency, with an index ranging from –2.59
Spectral lines present a related potential contamina-
to –2.66 and no sharp features. The smoothness of this
tion. Galactic radio recombination lines occur every 1-2
spectrum based on just 37 sources (see Fig. 3) is reas-
MHz over the frequency range of interest. In addition to
suring, as vastly more sources will contribute to the real
spontaneousemissionfromionizedhydrogenandotherel-
extragalactic signal, which will be correspondingly more
ementsinthe clouds,thereis alsostimulatedemission(or
featureless. The overall spectrum of the real extragalac-
absorption)againstthenonthermalbackground.Peakline
tic signal will differ from that computed here, as only the
intensities can reach 1K or so, although it turns out that
most luminous sources are representedin this simulation,
thegalacticridgerecombinationlineshappentomakethe
but the importantconclusionis that the spectrum should
transition from emission to absorption at 100–200 MHz,
be smooth and featureless.
andsoareataminimuminthisrange(Payneetal.1989).
Even a line intensity of 1K becomes diluted to 0.002K in Nevertheless it will still be important to identify
a 5 MHz band. In any case, spectral lines can be iden- and characterize the spectral properties of the dominant
tified and removed with observations of higher spectral sources within the main beam. Interferometer observa-
resolution. Observations such as these may provide the tionscanbe madeforthis purpose.Asinglesourcewitha
added bonus of discovering or setting limits on presently flux density of 35 mJy at 150 MHz would produce an an-
unknown spectral lines. tenna temperature increase of 0.01 K for a 45m telescope
The isotropic emission from extragalactic sources has with 50% main beam efficiency. All sources should there-
been estimated both directly and from integrated source fore be identified down to at least this level. The GMRT
counts.Bridle (1967)foundthatthis componentamounts would be the instrument of choice to conduct these high
to about 48K at 150 MHz, with a spectralindex of –2.75. resolution observations. With receivers for 50, 150, 235
Cane(1979)obtainedavaluecorrespondingtoabout32K and 325 MHz and a resolution of better than 1’ (at 50
P.A.Shaveret al.: Detection of theReionization Epoch 5
2.549
2.576
a
x| c x| c b
de a de
n 2.547 n
al I b al I 2.574
r r
ct ct
e e
p p
S S
| |
2.545 2.572
2.543 2.57
90 110 130 150 170 190 210 230 250 90 110 130 150 170 190 210 230 250
Frequency (MHz) Frequency (MHz)
Fig.2.Spectralindexvs.observingfrequency,asdeducedover Fig.3. Spectral index vs. observing frequency as in Fig. 2,
10 MHz intervals from simulated spectra which include the exceptthatinthiscasetheemissionfromextragalacticsources
galactic nonthermal emission with median spectral index β = isincluded,representedbythesimulationdescribedinthetext
-2.55 and a Gaussian dispersion around this value with σ = based on theactual spectra of 37 3C sources. Again, thetotal
0.1, and thegalactic thermalemission with EM = 5 pccm−6 brightness temperature at 150 MHz is 150K.
and Te = 6000K, in addition to the reionization signal (the
constant 2.73K cosmic background has been removed). The
emissionfromextragalacticsourcesisomittedinthiscase.The trapolationcouldbemadewithsufficientreliability.Alter-
total brightness temperature at 150 MHz is 150K. The points natively,after thosecomponents thatarewell-determined
represent the case without a reionization step, and the error havebeenremoved(atleastthe2.73Kcosmicbackground
bars correspond to ±5σ for a 24-hour observation. The lines
and the galactic thermal emission), a best fit (power law
represent the threecases shown in Fig. 1.
or low-order polynomial) could be made and subtracted
ordividedfromtheactualspectrumtorevealthestep,al-
MHz),spectraofallsignificantsourcescanbedetermined though the fit may well introduce artifacts that can mask
in the relevant frequency range. orconfusethesignal.Anaturalapproachistomakethefit
Thus,itmaybepossibletoquantifyalltheforeground at the two ends of the spectrum and interpolate, but this
contaminants sufficiently to model them and possibly re- requires a well-behaved spectrum and assumes that the
move some of them. The fact that the reionization signal signalislocatednearthemiddleofthespectrum.Another
shouldbethesameoverthewholeskyisagreatadvantage possibilityistomeasurethespectralindexpoint-by-point
- searches in different positions on the sky with different along the spectrum, and look for an abrupt jump due to
contamination should give the same result. A range of the step.
different measurements can help greatly in disentangling Figure 2 shows the spectral index measured every 10
theseforegroundsignals.Accuratebroadbandspectraover MHz from simulated spectra containing just the galactic
awidefrequencyrangecanhelptodeterminethespectral thermalandnonthermalemissioninadditiontothereion-
behaviorofthegalacticnonthermalcomponent,andmea- izationsignal.InFig.3theextragalacticsourceshavebeen
surements in adjacent positions can constrain the disper- included. These twoexamples giveanidea ofthe rangeof
sion of the spectral index. High frequency measurements spectral curvature that can be expected. The overall cur-
can be used to accurately determine the galactic thermal vature is substantial, but the reionization signal can still
component. Interferometer observations can be used to be seeninallthreecasesbecauseofits relativesharpness.
identifyregionsdevoidofstrongextragalacticsources,and Another technique that does not involve any fitting
to measure the properties of the most prominent sources procedure(butdoesrequirehigh-precisionmeasurements)
in the regions chosen. is trend analysis – analyzing the spectrum for deviations
The analysis can be done in a number of ways, de- from a smooth trend. Fig. 4 shows the ”trend ratio”, de-
pending on how effectively the different components can fined here as the ratio of the observed brightness tem-
be modeled or removed. If the overall spectrum could be perature at a given frequency to that predicted from an
well determined at frequencies abovethat of the reioniza- extrapolationofthe brightnesstemperaturesmeasuredat
tion step (e.g. ν > 237 MHz, corresponding to z < 5), a the three higher frequency points. The disruption caused
simple extrapolation to lower frequencies could be made by the reionization signal to the otherwise smooth spec-
to find the step. It is unlikely, however, that such an ex- trum is obvious.
6 P.A.Shaveret al.: Detection of theReionization Epoch
Attractiveasthe useofasmalltelescope forthis mea-
1.0002
surement may be, however, there are significant difficul-
ties.Thegainofthetelescope,itsbeamshapeandits(dis-
tant) sidelobes are frequency-dependent, due to match-
1.0001
ing, feed illumination, feed support scattering and edge-
a
o refractioneffects. The beamsize increases with decreasing
ati c
R frequency, so the regions contributing to the galactic and
nd 1.0000 extragalactic emission are different at different frequen-
Tre b cies. This will further complicate the shape of the over-
all spectrum, although the effect should vary slowly and
0.9999
smoothly with frequency, particularly in the directions of
lowest galactic emission. The distant sidelobes may also
introduce(varying)signalsfromregionsofstrongergalac-
0.9998 tic emission.
90 110 130 150 170 190 210 230 250
Thelargebeamofasmalltelescopewouldaverageover
Frequency (MHz)
theeffectsofmanyextragalacticsources,andmoresources
would be included atlower frequencies where the beam is
Fig.4. The “trend ratio” as described in the text, computed
larger.Averagingovermany sourcescouldconceivably be
for the curves used in Fig. 3, plotted against observing fre-
quency. anadvantage,aslongasasmallnumberofsourcesdidnot
dominate, but with a larger telescope and smaller beam
it would be easier to observe in regions devoid of strong
extragalactic sources. Finally, if a strong source such as
Clearly, both the magnitude and sharpness of the Cas A is to be used as a calibrator (see below), a fairly
reionization step are critical parameters. As mentioned largeantennaisrequiredforthesignalfromthissourceto
at the beginning of Sect. 3, the magnitude of the step dominate over the surrounding galactic emission.
will be greater in a low density Universe. The sharpness A larger parabolic antenna would solve some of these
dependsontheclumpinessoftheUniverseatthereioniza- problems, but not all. Some sort of phased array with
tion epoch and the formationrate of the ionizing sources, beam-forming circuitry which is still sensitive to emission
andmay wellbe greaterthan indicatedby curves (b) and onthelargestscalesmayhaveseveraladvantages.Itwould
(c).Simulationsoftheevolutionofstructurecangivesome bepossibletousescaledbeamsatallfrequenciesandcon-
indication of these parameters, but ultimately they have trol sidelobes, so that exactly the same area of sky would
to be determined from observations. be sampled at all frequencies. New arrays of this type,
In summary,itappearsthatthe reionizationstepmay such as the THousand Element Array (THEA) in The
bedetectableinprinciple,eveninthepresenceofcontam- Netherlands and the Square Kilometer Array (SKA) are
ination due to the galactic and extragalactic foregrounds. presently under consideration (van Ardenne 1998).
How easily it can be detected, and which method of anal- If terrestrialcomplications such as radio-frequencyin-
ysis is best, will depend on how well the different con- terferencewereconsideredinsurmountable,onemightcon-
taminatingcomponentscanbemeasured,modeledandre- sider a small satellite project. Then there would be only
moved.Forthispurpose,newandaccuratestudiesofthese the galactic and extragalactic foreground contamination
foreground contaminants will be very important. The re- to contend with, as well as calibration(although solarra-
maininglimitationsarethenpurelytechnical,andpresent diation may also be a significant problem). The antenna
aninterestingchallenge.Someofthepracticalitiesaredis- size would obviously be limited, but even a deployable
◦
cussed below. 10m antenna,giving a 10 beam at 180MHz, may be ad-
equate. At these frequencies the precision of the antenna
surface would not be an issue. While space projects are
3.3. Observational prospects
not inexpensive, a space radiotelescope such as this may
find noveluses, as it would be exploring a new domain in
3.3.1. Telescope requirements
observational phase space.
As (1) the signal is present over the entire sky, (2) the
system temperature is dominated by the galactic and ex-
3.3.2. Calibration options
tragalactic nonthermal emission, and (3) this is a broad-
band (rather than line) observation, this experiment can To be specific we will discuss three different calibration
inprinciplebedoneusingalreadyexistingradiotelescopes strategies,andmentionspecificproblemsforeachofthem
ofmoderateaperture.Themainrequirementistoachieve and how they might be solved. Calibrating the frequency
highly accurate relative intensity measurements over a responseofthetelescope/receivercombinationisprobably
considerable frequency range (70–240 MHz). the crucial issue. This has to be done to better than a
P.A.Shaveret al.: Detection of theReionization Epoch 7
few partsin 105 overa wide frequency range.Possibilities stablecontaminatingeffects(galaxy,discretesources,side-
(which could be combined) include internal loads, or the lobesetc)willtofirstandperhapssecondorderbe identi-
use of astronomical sources - a very strong ’featureless’ cal and will cancel, leaving only time-dependent concerns
continuumsourcesuchasCasA,orthemoon.Wediscuss -overallstabilityandradio-frequencyinterference.Atrue
each of these in turn. differencing experiment such as this may be very attrac-
tive in coping with the contribution to the system tem-
a) Internal loads
perature coming in via distant sidelobes. It will be hard
For precise calibration over many frequency channels,
to limit this contribution to less than 50 K at the low
internal broad-band noise sources of the highest quality
frequencies of this experiment. This componentmay have
would undoubtedly be the best option. They would have
significant frequency structure which could be difficult to
tobeextremelystableandveryaccuratelycalibratedasa
model. This may be a great advantage of using the moon
function of frequency. The properties of available internal
compared to an experiment using Cas A.
calibration sources will have to be examined carefully to
The brightness temperature of the moon is about 220
determine whether this is an option – until now, calibra-
K in this frequency range (Keihm & Langseth 1975); it
tion of this accuracy and stability has not been required
varies slowly with lunar phase. This is close to the mini-
at these frequencies, so this is uncharted territory. Cali-
mumskytemperatureat150MHz,soifthemoonfillsthe
bration with an internal load must also take care of any
beamitwillnotincreasetheantennasignalbymuch,ifat
frequency dependence of the load-coupling into the signal
all. The fact that the moon completely blocks the emis-
path.
sion behind it (galactic and extragalactic) can be very
useful,andcouldhelpsolvethe problemofthe frequency-
b) Using Cas A
dependent beamsize. The moon is not a very bright cali-
The alternative is to use astronomical calibration
bration source so it takes a large telescope to avoid dilut-
sources.Thishastheadvantagethattheastronomicalsig-
ing the signal too much. For the moon to fill the beam,
nalandthecalibrationsignalbothtraversethesamepath
we require a telescope of about 200–400 m diameter at
through the telescope and receiver system and therefore
the frequencies that we are considering. Currently only
sharesimilargain-frequencyeffects.CasAisthestrongest
Arecibo would be able to provide a sufficiently narrow
sourceintheskyat150MHz,withafluxdensityof13,000
beam. A specially designed low-frequency phased-array,
Jy. In order for this source to produce an antenna tem-
such as those mentioned in Sect. 3.3.1, may be required.
perature well in excess of the local galactic emission (Cas
An interesting aspect of a moon calibration experi-
A sits on the galactic ridge), a telescope of 25m diame-
ment is that the detection of the signal could in principle
ter or more would be required. The use of such a source
bedoneinterferometrically.Becausethe moonoccultsthe
may introduce a further complication: although its over-
background signal it introduces spatial structure in the
all power-law spectrum is remarkably straight down to
globaledgesignal.Dependingonthefrequencyandthein-
a few MHz (Baars et al. 1977), many small components
tensity of the galactic background,the 220K mooncould
with a range of spectral indices are superimposed, mod-
be a colder or warmer spot in the sky. But the intensity
ifying the detailed overall spectrum to an extent yet to
of this negative or positive sourcecontains the frequency-
bedetermined(Anderson&Rudnick1996,andreferences
dependent global background temperature.
therein). Strong extragalactic sources such as Cygnus A
Of course, the motion of the moon also introduces
have more complex, curved spectra (Baars et al. 1977),
smearing effects, which would probably be advantageous.
and so may be less suitable as calibrators.
If the ’edge’ signal is universal, as expected, we need not
Probably the best existing telescope to begin with is
worry that the moon moves by approximately its diame-
the GMRT. Cas A would produce an antenna tempera-
ter every hour. Both the sought-after signal and the side-
ture of about 4300 K at 150 MHz with the 45m anten-
lobes will be averaged. Since we need to integrate over
nas, totally dominating the sky and receiver noise. The
many (tens of) hours by tracking the moon we are effec-
GMRT has receiversoverawide rangeoffrequencies,but
tively sensitive only to a large-scale signal. The possibil-
not contiguous over the desired range from 70–240 MHz.
ity that emission from the moon, which includes a time-
Total power measurements, accompanied by autocorrela-
variable contribution from scattered solar radiation, may
tion spectra of the input signals to identify data affected
haveweakfrequencystructurewillhavetobeinvestigated.
by narrow band interference, are required. Digital spec-
Alternatively(especiallyifsmallertelescopesareused)
trometerswithatleast20-40MHz bandwidthandseveral
even the sun might conceivably serve for calibration pur-
thousand spectral channels would be needed.
poses. Although the solar spectrum has considerable fre-
c) Using the moon quencystructureduringbursts,the quietsunis lesslikely
AnadvantageofthemoonoverCasAisthatitmoves, tohavepermanentspectralfeaturesat70–240MHz,since
andatruedifferencingexperimentcouldthereforebedone we are looking well into the “quiet” spectrum of the so-
atexactlythesameposition(s)inthesky,andinthesame larchromospheresuperimposedontheRayleigh-Jeanstail
alt-az position(s), but of course not at the same time. All of its 6000 K atmosphere. Several issues will have to be
8 P.A.Shaveret al.: Detection of theReionization Epoch
further studiedbefore decidingonthe optimalcalibration showed that intergalactic absorption brightens some lines
strategy. while it dims others. At z z , the optical depth in the
ion
≥
Lyman series is very high. All Lyman series lines except
Lyα are absorbed immediately and redistributed. This
3.3.3. Terrestrial interference and complications
makes Lyα and the Balmer series significantly brighter,
Radio-frequency interference (RFI) is of course a major becausetheyreceivetheenergyoftheLymanseries.Once
issue for observations at the frequencies of interest. The reionization occurs, the Lyman series becomes optically
WorldRadioConferencehadallocatedonlyoneprotected thinandthebrightnessofLyαandtheBalmerseriesdrops
radio astronomy band in this regime, at 150–153 MHz, considerably, producing sharp features in the background
which itself is already severely affected by interference spectrum.
(Spoelstra1998).Thisregionoftheradiospectrumissub- If reionizationoccursabruptly,sucha sharpedge may
ject to local, regional and world-wide interference. Some be observableas a globalsignaloverthe entire sky due to
of these are stationary sources of interference such as TV Lyαemissionatz 5–20(λ 0.7–2.6µm).Thiscanbeseen
stations, many other in the 100–200 MHz regime are of a fromthe models o≃fBaltz et≃al.(1998)showninFig.5.As
mobilenaturesuchasaeronauticalandmilitarytransmit- we look redward, crossing Lyα around 1µm during the
ters and beacons. Fortunately, many of these sources are reionization event, the brightness sudde∼nly increases due
strongly time-variable and confined to relatively narrow to the absorptionand redistribution of the higher Lyman
bands. The 88–108 MHz band is allocated to FM radio serieslines.Haimanetal.(1997)calculatedasimilareffect
andmaybe completelyinaccessibleforradioobservations inwhichLyαisbrightenedbyradiativeexcitations,butit
ofthe requiredaccuracy.This means thatif the Hi’edge’ appears that recombination gives a significantly stronger
signal is at redshift greater than 10 (i.e. at frequencies signal. Baltz et al. (1998) and Longair (1995) estimate
less than 130 MHz) there may not be a reliable spectral thestrengthoftheedgetobeintherangeJ (Ly) 0.3
ν
baseline left. 3 10−23 erg/cm2/s/Hz/sr(these estimates are pr≃obabl−y
It is interesting to consider the role that could be un×certain by > 0.5 dex).
played by interferometers in this experiment. Such arrays ∼
Further recombinationfeatures are also present in the
can, with many independent monitors of the total power,
spectrainFig.5.Thebroadhumps(mostnoticeablythat
provide an excellent discriminator (filter) for local or ex-
blueward of Lyα) are due to blends of lines from the Ly-
ternalRFI.BecauseRFIgenerallydoesnotcomefromthe
man and Balmer series (Baltz et al. 1998;see also Stancil
directionofthesignalitsdelayandfringeratewillalsobe
et al. 1998 for other possible transitions with lower prob-
different. This then provides a way of detecting and pos-
ability). The positions of these humps are of course fixed
sibly eliminating RFI, so using telescopes that are part
relativetothesharpfeatures.Theirshapesaredetermined
of an interferometric array may yield powerful diagnostic
bytheevolutionofthereionization/recombinationhistory.
capabilities for weak RFI.
Ionosphericeffectsarealsoimportantatthesefrequen-
cies.Theionospherenotonlyrefractsthesignalbyavary- 4.1. Fundamental sensitivity limits
ingamount(easily1-2arcminutes)butthereisalsorefrac-
tivefocussingandscintillation.Thesescalewithfrequency The sky brightness at these wavelengths is dominated
inaquadraticway.However,theionospherechangesdaily, by the zodiacal light. The Diffuse Galactic Light (DGL),
so one can always select the best conditions. The effects and the Extragalactic Background Light (EBL) also con-
would be of greatest concern for the observations of the tribute,butatalowerlevel.These“foreground”emissions
discrete calibration sources, and should not be important determine the fundamental sensitivity limits for this ob-
for low-resolution observations of the frequency depen- servation.
dence of large areas of sky. Thedominantforegroundsignalisthezodiacal“back-
ground”, which at high galactic and ecliptic latitudes is
well determined by HST in the WFPC2 B, V, I bands
4. Detectability of a Lyman reionization edge
at 23.9, 23.00, and 22.45 arcsec−2, respectively (Wind-
∼
In this section we discuss the possibility of detecting the horst et al. 1992, 1994b, 1998), and >20.9 and >19.6
reionizationedgeviatheLymanlines.Wefirstdiscussthe mag arcsec−2 in the Near-Infrared Cam∼era/Multi O∼bject
nature of this optical/IR signal, next its observability in Spectrograph (NICMOS) near-IR J and H bands, respec-
general,andthenweconcludebydiscussinganexperiment tively (Thompson et al. 1999; Windhorst & Wadding-
with existing equipment. ton 1999). The latter are upper limits to the IR zodi-
Therateofhydrogenrecombinationspeaksatz z , acal sky, since there is some remaining uncertainty in
ion
≃
andthis is the sourceofthe backgroundwe seek.Because the global NICMOS dark-current. At 0.55µm, the zodi-
wewanttoobserveaspectralsignatureintheall-skyback- acal sky as measured from low earth orbit corresponds to
ground, we are only concerned with how energy is redis- J 6.8 10−19erg/cm2/s/Hz/sr, and at 1µm it corre-
ν
tributedamongdifferentspectrallines.Baltzetal.(1998) spo∼nds to×3.1 10−18erg/cm2/s/Hz/sr.
×
P.A.Shaveret al.: Detection of theReionization Epoch 9
sumption.Inanycase,whileforegrounddustcoulddimin-
ish the observed amplitude of any reionization signal, it
would not significantly affect its global spectral signature
(c.f., Seaton 1979).
Taking the above foreground emissions into account,
thehorizontallinesinFig.5showthesensitivityexpected
from a one-day integration with HST/STIS (Space Tele-
scope Imaging Spectrograph), HST/WFC3 (Wide Field
Camera3),NGST(theNextGenerationSpaceTelescope),
andalongintegrationonasimilarspacecraftat3AUfrom
the sun where the zodiacal emission would be reduced by
twoordersofmagnitude.Thereionizationsignaliswithin
reach, at least for the deep space mission.
We note that inground-baseddirectCCD-imaging lo-
cal sky-removal to a precision of 10−3 is obtained rou-
∼
tinely, and that it has been obtained to a precision bet-
ter than 10−4 when using very careful calibrations of
∼
the kind we propose below (e.g. Tyson 1988).There is no
fundamental reasonwhy a CCD spectrographoutside the
Earth’s atmosphere should not be able to achieve these
limits in spectroscopic mode, although this has yet to be
Fig.5. Predicted spectra of the Lyman reionization edge demonstrated.
from Baltz et al. (1998). Models A–D were generated from The sensitivity requirement may be eased by taking
CDM simulations including radiative transfer, star-formation,
advantage of the considerable spectral structure present
feedback, evolution and chemistry of the gas, and assume
in the curves shown in Fig. 5. Rather than looking only
ΩM = 0.35, ΩΛ = 0.65, and h = 0.7. The sequence A–D has
for the Lyα and Hα steps, it may be possible to search
decreasing values of Ωb (0.055-0.03), simulation box-size (3–2
h−1 Mpc), mass resolution (107.0–106.0 M⊙), and number of forthe overallpatterninthese curvesbycross-correlating
the observed spectrum with a theoretical template (e.g.
particles. The amplitude prediction in the models is probably
uncertain by >∼ 0.5 dex. The most noticeable hydrogen edges, Baltzetal.1998),asisoftendoneindeterminingredshifts
LyαandHα,areindicated(formodelDonly).Thehumpblue- of elliptical galaxies. This could ease the sensitivity re-
wardofLyαiscausedbyhigheratomictransitions.Thedotted quirementbyaconsiderablefactor(whichdepends onthe
curvesindicatethein-principle3-σ limitsexpectedfromaday actual structure in the reionization signal). In this case,
ofintegrationwithHST/STIS,HST/WFC3,NGST,andalong however,the observable would only be a cross-correlation
integration on a similar spacecraft at 3 AUfrom thesun.The peak rather than the actual spectral features, and such a
uncertainty in these limits are about ∼0.3 dex.
techniquewouldrequireexcellentflat-fieldingandcalibra-
tion over a wide wavelengthrange.
At high galactic latitudes and in regions of low N ,
the DGL contributes less than 26.0 mag arcsec−2 Hat 4.2. Contamination by galactic and extragalactic
UV–optical wavelengths, or <6 10−20erg/cm2/s/Hz/sr foregrounds
(Henry 1991,O’Connell et a∼l. 19×92). Any spectralfeatures or variationsin the foregroundcon-
The EBL surface brightness is estimated to be taminants couldconceivablymaskthe reionizationsignal,
I>24–25 mag/arcsec2, since the galaxy counts are now butitislikelythatsuchcontaminationcanbe adequately
k∼nown to permanently converge with a sub-critical mag- dealt with. Deep ground-based imaging of each field ob-
nitude slope α <0.4 for B>25 mag (Metcalfe et al. served would identify any large-scale low-surface bright-
1995) and for I>∼24 mag (W∼illiams et al. 1996). The ness objects or structures to < 29–30 mag arcsec−2 (e.g.
best estimate of∼the extragalactic background is about Tyson 1988) that can be exc∼luded afterwards. The ob-
1.7 10−20erg/cm2/s/Hz/sr or 27.3 mag arcsec−2 at served sky background spectrum can then be corrected
0.5×5µm, and 6.7 10−20erg/cm2∼/s/Hz/sr or 25.1 mag for the diffuse galacticandextragalacticforegroundemis-
arcsec−2 at 1µm×(Dwek et al. 1998; Hauser et∼al. 1998) sions as follows:
It is assumed here that the high-redshift intergalactic (a) The zodiacal ( solar) spectrum is the main con-
≃
medium is not obscured by dust and neutral clouds at tributor to the variations in the spectral baseline, and is
redshifts somewhatlowerthanz .Comparisonsofradio mostly featureless over the wavelength range of interest,
ion
andopticalsamplesofhigh-redshiftquasars(Shaveretal. exceptforperhapsthesolarCafeatureat0.89µm.Theso-
1996, 1999) and the discovery of Lyα emitters at z 5–6 larspectrumisknowntoveryhighaccuracy.Thezodiacal
≃
(Dey et al. 1998, Weymann et al. 1998) support this as- dustparticleswithsizes<1µmdonotsignificantlychange
10 P.A.Shaveret al.: Detection of theReionization Epoch
the high frequency part of the zodiacal spectrum around havelargerIRdetectorswithmuchbetternoisecharacter-
1µm, but they slightly modify the global solar spectral istics than HST/NICMOS, making possible a substantial
gradientdue tonon-greyscattering.Suchchangesarenot gaininsensitivityforthisexperiment.TheNGSTwillop-
expected to be sudden with wavelength. A slightly red- erate over the range λ =0.5–10µm, and will have a large
dened solar spectrum can be subtracted from the final spectroscopicmultiplexingcapabilityprovidedbyaninte-
observedsky spectrum after appropriatescaling to fit the gral field or multi-object spectrograph with some 40,000
HST broad-band zodiacal sky-measurements. spectralpixels,Onewouldsearchfortheisotropicstep-like
(b) Possible high spatial frequency structures in the spectral features in the collapsed 1-dimensional spectrum
galacticDGL atverylowsurfacebrightnesslevelsmaybe of the sky.
of some concern, but only at a level of >27 mag/arcsec2, Alternatively,aninfraredspacetelescope dedicatedto
as has been seen in deep ground-based imaging at studying the CIBR should be able to detect the Lyman
low galactic latitudes (Szomoru & Guhathakurta 1998; edge signals. Two versions, EGBIRT and DESIRE, have
Guhathakurta & Tyson 1989). At high galactic latitudes, alreadybeenproposed(Mather&Beichman1996).These
iftheobservedfieldscanbeselectedtohaveEB−V <0.01 wouldobserveat3AUfromthesun,reducingthezodiacal
and/orlowNH,theDGLandvariationsthereinsho∼uldbe foreground by two orders of magnitude.
very small, as shown by deep ground-based images (e.g.,
The sensitivities attainable with such space-based fa-
Tyson 1988) and deep HST images such as the Hubble
cilitiesareshowninFig.5.Asindicatedabove,thelevelof
Deep Field (Williams et al. 1996).Fields near potentially
the reionizationsignal can be reached,but it may require
contaminating stars or galactic cirrus would have to be
the “deep-space”missionwithits muchlowerbackground
avoided. Since the DGL likely bears the spectral signa-
and long dedicated integrations. In the meantime, an ex-
tures of early-type stars in our Galaxy, we do not expect
ploratoryexperimentcanbedonewithHST,asdescribed
a large spectral feature around 1µm from the combined
below.
stellar population that causes the reflected DGL. In any
case,suchasignalcanbesubtractedfromtheresidualsky
spectrum at the B 27 mag arcsec−2 level by subtracting 4.3.2. An experiment with the HST
≃
a scaled OB star template, as for the zodiacal spectrum.
In view of the large uncertainties, both in the predicted
(c) Finally, the integratedbackgroundfrom faint fore-
amplitude of the reionization signal and in the attainable
ground galaxies (EBL) should be featureless, given their
sensitivities, it is worth considering preliminary experi-
large redshift range (cf. Driver et al. 1998). Therefore no
ments using the currently existing Space Telescope Imag-
global template for the EBL from faint galaxies at z < 5
shouldhavetobesubtractedfromthecollapsedspectr∼um ingSpectrograph(STIS).Suchexperimentswouldalsobe
exploringnewobservationaldomains,andunexpecteddis-
to find a Ly-edge.
coveries may result. For example, in addition to the hy-
drogen recombination signal shown in Fig, 5, there may
4.3. Observational prospects also be a broader and possibly stronger spectral feature
due to the continuum emission from the ionizing sources
4.3.1. Observing the Lyman edge with space-based
nearz as showninfig.11ofGnedin& Ostriker(1997).
ion
instruments
Such an experiment would in any case provide important
experience in making observations of this kind.
Anear-infraredinstrumentinspacewouldbeessentialfor
this experiment, because of the low sky background (in The STIS has a wide variety of long slits, permitting
space the zodiacal sky in the H-band is 6–7 mag darker a spectral image of the sky to be obtained. To obtain
than from the ground (Thompson et al. 1999; Windhorst sufficient surface brightness sensitivity, a long observa-
& Waddington 1999) and complete absence of terrestrial tion with the widest available STIS long slit of 52” 2.0”
×
OHlines.Wenotethatarecentdeeplong-slitsearchwith should be made. One would use the longest-wavelength
Keck provided quite useful upper limits to the ionizing low-resolution STIS grating G750L to make a deep spec-
backgroundJ at levels 2 10−21 erg/cm2/s/Hz/sr,but trum of the sky. The observation would be limited by the
ν0
at lower redshifts (2.7<z<4×.1;Bunker et al. 1998).In our STIS CCD to < 1.03µm, or z < 7.5 for Ly-α.
caseweareconcerned∼wi∼th redshiftsz>5,andsowe must This HST o∼bservationcould∼be done inparallelmode,
do the experiment from space to avo∼id the strong OH- at no extra cost in primary HST observing time. As
forest. the primary exposures would most likely be taken with
Ideally, one would do this experiment on a faster tele- WFPC2 and therefore be dithered, the STIS exposures
scopewithalargerapertureandlowerskythanHST,such would also be dithered along the slit, resulting in better
as the NGST, to be launched sometime after 2007. Im- spectral flatfields. Severalindependent fields could be ob-
proved infrared cameras with spectroscopic multiplexing servedinordertoidentifyandremove“spurious”features
capability, such as those under consideration for WFC3 fromforegroundobjects.Oneortwofieldsshouldbetaken
andbeingdesignedforNGST,arealsoessential.Theywill at lower ecliptic latitudes – or higher zodiacal foreground