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New Astronomy Reviews 50 (2006) 204–207

On the contribution of (mini)quasars to reionization

Mark Dijkstra

Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, United States

Available online 7 February 2006

Abstract

Recent data from WMAP and SDSS implies that the reionization history of the universe is extended and complex. These discoveriesmotivated models in which the ionizing radiation from accreting black holes, residing in shallow dark matter potential wells, dominatethe ionization of the universe at redshifts z = 10–20. Here we show that (1) contrary to previous beliefs, photoionization heating does notprevent gas from cooling and condensing in small (vcirc � 10 km/s) dark matter potential wells at z J 10; but (2) current models in whichan early generation of accreting small black holes dominate full reionization, saturate the unresolved soft X-ray background at the J 2rlevel.� 2006 Published by Elsevier B.V.

Keywords: Cosmology; Theory; Reionization; Quasars

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2042. Photoionization feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2053. Constraints on the contribution of miniquasars to reionization from the soft X-ray background (SXB) . . . . . . . . . . . . . . . 206

1387-6

doi:10.

E-m

3.1. The soft X-ray background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2063.2. Contribution of MiniQSOs to the SXB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

1. Introduction

The recent discovery of the Gunn–Peterson (GP)troughs in the spectra of z > 6 quasars in the Sloan DigitalSky Survey (SDSS; White et al., 2003; Wyithe and Loeb,2004) suggests that the end of the reionization processoccurs at a redshift near z � 6. On the other hand, the highelectron scattering optical depth, se = 0.17 ± 0.04 (Kogutet al., 2003), measured recently by the Wilkinson Micro-

473/$ - see front matter � 2006 Published by Elsevier B.V.

1016/j.newar.2005.11.037

ail addresses:mark@astro.columbia.edu, dijkstra@unimelb.edu.au.

wave Anisotropy Probe (WMAP) experiment (Spergelet al., 2003) suggests that ionizing sources were abundantat a much higher redshift, z � 15. These data imply thatthe reionization process is extended and complex, and is

probably driven by more than one population of ionizing

sources (see e.g. Haiman, 2003 for a post-WMAP review).The exact nature of these ionizing sources remainsunknown. Natural candidates to account for the onset ofreionization at z � 15 are massive, metal-free Population

III stars that form in the shallow potential wells of the firstcollapsed dark matter halos (Wyithe and Loeb, 2003a; Cen,

M. Dijkstra / New Astronomy Reviews 50 (2006) 204–207 205

2003a; Haiman and Holder, 2003). The most natural alter-native cause for reionization is the ionizing radiation pro-duced by gas accretion onto an early population of blackholes (see Haiman and Loeb, 1998; Wyithe and Loeb,2003c; Bromm and Loeb, 2003; Madau et al., 2004; Ricottiand Ostriker, 2004). Note that throughout these proceed-ings, we use the term ‘miniquasar’ for accreting black holesin the mass range MBH � 10 to 106Mx, while the moremassive accreting black holes are ‘quasars’. Below we out-line some differences between population III stars andminiquasars:

� Luminosity: Population III stars have luminosities,L* � 4 · 1039 (M*/100Mx)3 ergs/s, compared toLedd = 1.5 · 1040 (MBH/100Mx) ergs/s for accretion atthe Eddington rate onto black holes of mass MBH.

� Hardness of spectrum: A typical spectrum of a miniqua-sar is expected to be considerably harder, extending wellinto the X-ray band (e.g. Madau et al., 2004, see Fig. 1).The hardness of the spectra emitted by the first ionizingsources has a profound influence of how reionizationproceeds. Because the cross-section for absorption forUV-photons is larger, they first need to reionize the highdensity regions in which they are produced, before theycan escape into the intergalactic medium (IGM) andstart reionization there. Reionization of these high den-sity regions consumes on average �10 ionizing photonsper hydrogen atom (e.g. Haiman et al., 2001). X-rays, onthe other hand, have smaller cross-section for absorp-tion, which allows them to escape unimpeded from thehigh density regions and immediately start reionizingthe IGM. Combined with the fact that one X-ray pho-ton can cause more than one ionization (the energeticphoto-electron produced can collisionally ionize addi-tional hydrogen atoms, Shull and van Steenberg,

0.01 0.1 1.0 10.0

log

F E

E (keV)

Template Spectra

Pop III star

MiniQSO

E

Fig. 1. Schematic depiction of theoretical template spectra for a popu-lation III star (dashed-line) and a miniquasar (solid-line). Miniquasarsproduce copious X-rays, while population III stars do not. The units onthe vertical axis are arbitrary.

1985), we need [1 X-ray photon per baryon to (par-tially) reionize the IGM.1 Because, X-rays start (partial)reionization in the low-density IGM, after which theionized regions slowly expand into the higher densityregions, this scenario is referred to as the ‘outside-in’scenario. The reverse is true for reionization by UV pho-tons, and is the ‘inside-out’ scenario.

� Negative feedback: Various negative feedback processesare operating which may cause population III star for-mation to be self-limiting, while miniquasars suffer lessfrom these processes. Photo-dissociation of H2 mole-cules by the radiation produced by the first populationIII star in a given dark matter halo, can terminate sub-sequent star formation in the same halo. Additionally,the ionizing radiation produced by a single star can heatand unbind all the gas in dark matter halos withM [ 106Mx (Kitayama et al., 2004). Stars in the massrange 10Mx < M* < 260Mx will go supernova (Hegerand Woosley, 2002, note that the quoted mass range isonly for valid for non-rotating stars). Apart from theirdisruptive effect, these supernovae inject metals intoIGM which terminates further formation of metal-freestars in surrounding regions. All these processes collab-orate to limit the time span of the population III epoch.The miniquasars epoch, on the other hand, is notexpected to be self-limiting (as will be shown in Section2). Because black holes do not have finite lifetimes, newgas may be supplied to the black hole during subsequentmergers of dark matter halos, which can initiate anotherminiquasar phase. In fact, copious gas accretion ontoblack holes at high redshifts (z � 6) is required to havea 109Mx black hole in place at a redshift of z � 6.

The previous discussion suggests that miniquasars maycontribute to the total ionizing background at z � 6. Infact, it has been suggested that they may make the domi-

nant contribution to the ionizing background (e.g. Madauet al., 2004; Ricotti and Ostriker, 2004). In Section 2 weinvestigate to what degree the presence of a hard ionizingbackground, produced by accretion onto black holes, cansuppress gas cooling and collapse in dark matter halos withvcirc J 10 km s�1. In Section 3 we show that the present-day soft X-ray background can put constraints on modelsusing miniquasars to reionize the universe. In Section 4 wesummarize our conclusions.

2. Photoionization feedback

Photoionization heating by an external ionizing radiationfield can significantly suppress the cooling and collapse ofbaryons into dark matter halos at z [ 3. This effect is stron-ger when the spectrum of the ionizing background is harder

1 Note that X-rays cannot fully reionize the IGM, because as the ionizedfraction of hydrogen atoms in the IGM approaches one, X-rays are morelikely to scatter off electrons than to photoionize the remaining neutralhydrogen atoms.

2 4 6 8 10 12

20

40

60

80No Self Shielding

With Self Shielding

Fig. 2. The plot shows v1/2 as a function of redshift. v1/2 denotes the circularvelocity associated with that mass scale at which half of the baryons cancollapse compared to the amount that would have collapsed if the baryonswere pressureless. This figure is taken from Dijkstra et al. (2004).

206 M. Dijkstra / New Astronomy Reviews 50 (2006) 204–207

(e.g. Thoul andWeinberg, 1996), suggesting that a hard ion-izing background, produced by an ensemble of miniquasars,may strongly limit subsequent structure formation. At red-shifts z J 10, however, this photoionization feedback ismuch weaker for numerous reasons (Dijkstra et al., 2004).

� Dwarf galaxy sized objects can self-shield more effec-tively against the ionizing background.

� Collisional cooling processes are more efficient.� The amplitude of the ionizing background is lower athigh z than at low z.

� The ionizing background turns on only after the dwarfgalaxy has already grown to substantially higheroverdensities.

The decreasing importance of photoionization feedbackwith redshift is quantified in Fig. 2. Self-shielding is notincorporated in the filled circles and filled triangles withthe errorbars. The filled triangle at z = 10 is does includeself shielding. The spectrum of the ionizing backgroundwas assumed to be of the form FE � E�1. Fig. 2 shows thatobjects with vcirc as low as 10–15 km/s can collapse atz J 10. We conclude that gas cooling and collapse is notsuppressed in the presence of a hard ionizing backgroundin these dark matter halos.

3. Constraints on the contribution of miniquasars to

reionization from the soft X-ray background (SXB)

3.1. The soft X-ray background

The observed soft (E = 0.5–2.0 keV) X-ray backgroundis 94þ6

�7% accounted for by discrete z < 4 sources (Moretti

et al., 2003; Barger et al., 2002, 2003). Deeper observations(with future X-ray telescopes such as Generation-X) willmost likely reveal fainter z < 4 sources such as:

� high redshift clusters (Wu and Xue, 2001)� the WHIM (Dave et al., 2001)� Thompson scattered X-rays (Sołtan, 2003)

Therefore: At most 6þ7�6% of the total SXB can come

from high z (z J 6) accreting black holes. We will showthis can put significant constraints on models in whichaccreting black holes dominate reionization.

3.2. Contribution of MiniQSOs to the SXB

In Dijkstra et al. (2004) (hereafter DHL04), we calcu-lated the contribution miniquasars would make to theSXB, if they provide the dominant contribution to the totalionizing background. For this, we assumed a spectrum forthe ionizing background as a whole. We emphasize that thisbackground is produced by the entire ensemble of miniqua-sars present at that epoch, and may consist of accretingblack holes with a wide range of masses. Here we focuson the template spectrum shown in Fig. 1, which is basedon the template spectrum described by Madau et al.(2004). Given the shape of the spectrum of the ionizingbackground, we normalize it to produce reionization atz = zQ

nc ¼4pc

Z Emax

13:6 eV

dEF E

E¼ gnH; ð1Þ

where nH = Xbh2qcrit(1 � YHe)/mp = 2.05 · 10�7 (1 + zQ)

3

cm�3 is the number density of H-atoms at z = zQ and g isthe number of ionizations per hydrogen atom that are re-quired to achieve reionization. As mentioned in Section1, full reionization must be done by the UV photons, whichcan only escape into the IGM after they cleared out thehigh density regions from their neutral hydrogen. Our cal-culations therefore assume g = 10. We point out that ourresults depend linearly on g. Once the spectral shape andnormalization are fixed, the (unobscured part of the) spec-trum is redshifted to z = 0, after which we calculated thecontribution to the soft X-ray background. Results are gi-ven below at zQ = 6, 10 and 20 for g = 10 and FE as shownin Fig. 3

Allowed by SXB

6 ± 6%

Redshift of full reionization

Percentage of SXB (%)

zQ = 6

44 ± 14 zQ = 10 30 ± 11 zQ = 20 19 ± 5

The main results is that models in which miniquasars

dominate full reionization saturate the unresolved softX-ray background at by J 2r. This result depends moststrongly on assumed spectrum and the assumed value of

> 20% of SXB

< 6% of SXB

α )

η#

of io

nizi

ng p

hoto

ns p

er H

ato

m (

)

Spectral Slope (1

Fig. 3. Constraints on g, and the power-law a, based on the intensity ofthe present-day SXB. The quasars are assumed to form at zQ = 10 andhave a power-law spectrum, FE � E�a for E > 13.6 eV. Models that lie onthe left/right of the solid-line/dotted-line contribute [6%/J 20% to thesoft X-ray background, respectively. This Figure is taken from DHL04.

M. Dijkstra / New Astronomy Reviews 50 (2006) 204–207 207

g. We found our results to be very similar when other tem-plate spectra for the ionizing background are taken (e.g.The template spectra for quasars given by Elvis et al.(1994) or Sazonov et al., 2004). For any power-law spec-trum, FE � E�a, the SXB can constrain a and g, which isshown in Fig. 3.

We performed similar analyses for cases in which theuniverse was partially ionized by X-rays. We properlyaccounted for secondary ionizations, and used g � 1 (seeDHL04 for a more detailed motivation of this choice). Inthese partial reionization scenarios (Ricotti and Ostriker,2004), the SXB is saturated by more than 20% when theuniverse is partially reionized more than �50% by number(DHL04). The analysis here sketched ignores helium. Weshow in DHL04 that a proper treatment of helium willnot change our results by more than 15%.

4. Conclusions

We demonstrated that gas cooling and collapse is notsuppressed in the presence of a hard ionizing backgroundgenerated by a population of accreting black holes. Currentmodels in which an early generation of accreting blackholes dominate full reionization, however, saturate theunresolved SXB at the J 2r level. Models in which accret-ing black holes only partially ionize the IGM up to �50%

are still consistent with the soft X-ray background. Theconstraints the SXB can put depend most strongly on theassumed spectrum of the ionizing background and linearlyon the number of required photons per hydrogen atom toproduce full reionization, g. An improved determination ofthe unresolved SXB will enable tighter constraints on theabundance of miniquasars at redshifts z > 6, and their con-tribution to reionization.

Acknowledgement

I thank my advisor, Zoltan Haiman, for many usefuldiscussions.

References

Barger, A.J., Cowie, L.L., Brandt, W.N., Capak, P., Garmire, G.P.,Hornschemeier, A.E., Steffen, A.T., Wehner, E.H., 2002. AJ 124, 1839.

Barger, A.J. et al., 2003. AJ 126, 632.Bromm, V., Loeb, A., 2003. ApJ 596, 34.Cen, R., 2003a. ApJ 591, 12.Dave, R. et al., 2001. ApJ 552, 473.Dijkstra, M., Haiman, Z., Loeb, A., 2004. ApJ 613, 646.Dijkstra, M., Haiman, Z., Rees, M.J., Weinberg, D.H., 2004. ApJ 601,

666.Elvis, M. et al., 1994. ApJS 95, 1.Haiman, Z., 2003. In: Ho, L.C. (Ed.), To Appear in Carnegie Observa-

tories Astrophysics Series, vol. 1: Coevolution of Black Holes andGalaxies. Cambridge University Press, Cambridge. Available from:<astro-ph/0304131>.

Haiman, Z., Abel, T., Madau, P., 2001. ApJ 551, 599.Haiman, Z., Holder, G.P., 2003. ApJ 595, 1.Haiman, Z., Loeb, A., 1998. ApJ 503, 505.Heger, A., Woosley, S.E., 2002. ApJ 567, 532.Kitayama, T., Yoshida, N., Susa, H., Umemura, M., 2004. ApJ 613, 631.Kogut, A. et al., 2003. ApJS 148, 161.Madau, P., Rees, M.J., Volonteri, M., Haardt, F., Oh, S.P., 2004. ApJ

604, 484.Moretti, A., Campana, S., Lazzati, D., Tagliaferri, G., 2003. ApJ 588, 696

(M03).Ricotti, M., Ostriker, J.P., 2004. MNRAS 352, 547.Sazonov, S.Y., Ostriker, J.P., Sunyaev, R.A., 2004. MNRAS 347, 144

(SA04).Shull, J.M., van Steenberg, M.E., 1985. ApJ 298, 268.Sołtan, A.M., 2003. A&A 408, 39.Spergel, D.N. et al., 2003. ApJS 148, 175.Thoul, A.A., Weinberg, D.H., 1996. ApJ 465, 608.White, R.L., Becker, R.H., Fan, X., Strauss, M.A., 2003. AJ 126, 1.Wu, X., Xue, Y., 2001. ApJ 560, 544.Wyithe, J.S.B., Loeb, A., 2003a. ApJ 586, 693.Wyithe, J.S.B., Loeb, A., 2003c. ApJ 595, 614.Wyithe, J.S.B., Loeb, A., 2004. Nature 427, 815. Available from: <astro-

ph/0401188>.