arXiv:astro-ph/0207283v1 12 Jul 2002 S P48 uut20 d.F asn&TBD & Jansen F. eds. 2002 August SP-488, ESA Netherlands The ESTEC, rc Symposium Proc. e od:Msin:XEUS Missions: words: Key plane focal con- the of of run-out upgrade after associated package. intervals an year with 5 at sumables spacecraft a replaced mirror in be the flying with will tandem formation to in 4.5 spacecraft, orbit from detector non-Keplerian diameter mirror The the meter. with raising upgrade 10 ISS, mir- the robotic of the a aid through of the foreseen growth fellow is on-orbit in- spacecraft so-called an ror an a present with At ISS, orbit orbit. the earth traveler low with a commensurate in clination injected spacecraft’s. is detector telescope and The mirror laser-locked separate two tele- i.e. comprising X-ray length, spacecraft, focal an meter featuring 50 of developed scope been has concept sion emission. due foreground noise X-ray as galactic well the as to confusion source minimize to seconds, background. cosmologically sky in the Fe-XXV) against sig- emission and sources allows distant X-ray Si-XIII This prominent O-VII, eV. most (e.g. lines the 2 of than detection better nificant resolution spectral a area tive entail: observatory XEUS the of characteristics the till up intergalactic synthesis true metal the of of medium. Characterization evolution epoch. the present of Study groups evolution. – small their i.e. and gravitationally systems, galaxies, first of dominated the matter of Universe. dark formation early bound the the growth, in of of holes Assessment study black – subsequent massive first and the origin, of the post- the: for the as Search formulated – in objectives be Universe science can hot XEUS key of the The of era. evolution Chandra/XMM-Newton the the for of initiative ESA-ISAS study an present at constitutes sion, XEUS oacmoaeteeisrmn eurmnsamis- a requirements instrument these arc accommodate To 5 and 2 between power, resolving angular Its 2. sensi- a combining grasp, spectroscopic effective Its salient 1. two the goals science ambitious these reach To the , > 0m 20 RN ainlIsiuefrSaeRsac,Sorbonnelaan Research, Space for Institute National SRON, -a vligUies Spectroscopy Universe Evolving X-ray 2 NwVsoso h -a nvrei h M-etnadChan and XMM-Newton the in Universe X-ray the of Visions ‘New eo htneeg f2kVwith keV 2 of energy photon a below Abstract oa Bleeker Johan H ESMISSION XEUS THE 1 mis- n ain M´endez Mariano and antb sdfrdtie tde fojcsa eyhigh very at objects of ( studies redshifts detailed for used be cannot sources. to spectroscopic extended high-resolution (up spatially of on bandwidth observations capability and the and area keV), spectroscopic Trans- has Low-Energy larger XMM-Newton and a while High- Spectrometers, the variety Grating a with mission on sources resolution point spectral high of of capability a X- the and of has years Chandra 40 well: other first each very the complement missions in two possible These a astronomy. in been ray Universe, not X-ray had the provid- that of are way vision Newton, clear-focused, new, XMM a ESA’s ing and Chandra spa- operational, nowadays reality. high NASA’s observatories a X-ray of become major promise has two X-rays the The in century, resolution 20th spectral the and tial of end the At oeo h ai omlgclise htne ob re- are: be view to of point need observational that the issues from cosmological solved basic the of Some evolu- Universe. and hot formation the the X-ray of studying that tion in role play unique chal- the can and astronomy this cosmology, and to been Chandra’s has response lenge for X- ESA’s planning of follow-up. future start the XMM-Newton’s typical to is what and the assess developed, astronomy, to and is ray time planned the This be is to this years. mission therefore ten new a next for timescale the of for spectra operational X-ray produce to distance. such able al. at et not objects (Brandt are 6.28, these of we redshifts but to up 2002), allow quasars sky detect the of to pieces us narrow very in surveys deep Newton – – – – – – – ept hi uebcpblte,teetomissions two these capabilities, superb their Despite o n hnwr h ev lmnscreated? elements Universe? heavy the the in were baryons when galaxies? the and of of How centers history the the the was in is what holes What and black evolve, massive of and role form and galaxies stars did or How holes black massive galaxies? first, form form? did structure What large-scale matter? the dark did of How Universe? nature early the the is of What physics the was What be to expected are XMM-Newton and Chandra Both ,38 AUrct h Netherlands The Utrecht, CA 3584 2, z ∼ > ) tpeet h hnr n XMM- and Chandra the present, At 5). .Introduction 1. 1 .Sinecase Science 2. r Era’ dra ∼ < 0 . 63 oebr2001, November 26–30 rscaglrresolution angular arcsec 5 ∼ 15 1 2

There are several missions underway or planned, aimed at tackling some of these questions. ESA’s Planck Sur- veyor Mission (scheduled for launch in 2007) will produce a detailed map of the spectrum of fluctuations of the cos- mic microwave background that will allow us to tightly constrain some of the most fundamental cosmological pa- rameters, and will provide us with a better understanding of the physics that governed the early Universe. Obser- vation with Herschel (2007), NGST (2009), and ALMA (2010) will provide us with information about the forma- tion of the first stars in the Universe, and the formation and early evolution of galaxies. However, in the course of the evolution of the Uni- −5 verse, most of the baryonic matter must have been heated 10 to temperatures in which it will emit X-rays. Part of this

matter, the hottest and denser part of it, is readily de- 3 −3 −6 tectable in clusters of galaxies, but a large fraction of it at 10 much lower densities, the warm-hot intergalactic medium (Cen & Ostriker 1999), has not yet been observed. Massive X−7 50 accreting black holes, which probably played a central role 10 in the formation and evolution of the galaxies that host n (log L > 44.5) [h Mpc ] them, are only observable in X-rays. Hard X-rays can pen- −8 etrate the thick clouds of gas and dust in the centers of 10 young galaxies, and therefore observations in these wave- 0 1 2 3 4 5 6 lengths are needed to distinguish between energy output Redshift from star formation and accretion in these objects. Figure 1. Star formation rate based on UV/optical obser- All of these subjects require very sensitive X-ray in- vations (upper panel; from Steidel et al. (1999), and space struments, with sufficient angular resolution to avoid con- density of X-ray selected AGN with luminosities large than fusion at high redshifts, and high energy resolution to be 3 × 1044 erg s−1 (lower panel; from Miyaji et al. 2000), able to study in detail the physical properties of these plotted as a function of redshift. young objects.

2.1. The first black holes mass of the central black hole and the velocity dispersion of the stars in the bulge of the host galaxy (Ferrarese Deep surveys with XMM-Newton and Chandra are be- & Merrit 2000; Ferrarese & Merrit 2000) demonstrates ginning to show that the fraction of galaxies harboring a a close relation between massive central black holes and > 6 massive (∼10 M⊙) black hole at its center is larger at red- galaxy formation. shifts larger than 1 to 3, than it is in the local universe. Simple cosmological models of hierarchical formation Previous X-ray and infrared surveys show that the star predict the existence of large population of quasars at formation rate and the space density of AGN was a factor higher redshifts. Although original results seemed to in- of ∼ 100 larger at redshifts of 2-5 than it is at present. It dicate that star formation rate (and metal production) is not yet clear what is the reason of this recent decline in peaked at a redshift around 1, new estimations that take the star formation rate and the rate at which black holes into account corrections for intrinsic absorption in quasars at the centers of galaxies accrete matter. However, these at high redshifts indicate that the star formation rate in- results indicate that the universe was much more active creases up to z ∼ 1, and remains more or less constant at redshifts larger than 3 than it is now, and that X-ray above this value (Steidel et al. 1999). Evidence for a popu- emission from those ages is of crucial importance for the lation of high-redshift, highly absorbed, AGN is indicated proper assessment of evolutionary scenarios. by ROSAT results (Fig. 1), which show a more or less The current paradigm of structure formation in the constant AGN density from z ∼ 1 to z ∼ 4 (Miyaji et Universe (Peebles 1974; White & Rees 1978) states that al. 2000). This is consistent with the fact that hard X-ray small-scale objects forms first, and that they afterward observations are much less affected by absorption than op- merge to form larger ones. Within this scenario, it is not tical and UV observations. This means that a large frac- clear whether black holes at the center of active galaxies tion of the high-redshift Universe is obscured. Interalia, it formed in situ, or whether they grew from accretion of should be noted that some models (Haiman & Loeb 1999) smaller black holes as the smaller galaxies merged to form and recent data (Hasinger, private communication), seem larger ones. In any case, the tight correlation between the to indicate that the space density of AGN might decline 3

Figure 2. XEUS 2 simulated spectra of a heavily absorbed starburst/AGN. At low energies the input spectrum is domi- 42 nated by thermal gas from a starburst with kT =3 keV, metallicity 30 % solar, and luminosity L0.5−2keV ∼ 2×10 erg −1 44 −1 s . At higher energies the AGN dominates. The AGN luminosity is L2−10keV ∼ 10 erg s ; there is also a strong 24 −2 Fe line with equivalent width EW =1 keV. The intrinsic absorbing material has a hydrogen column NH = 10 cm . The left panel shows the simulated spectrum for a source at redshift z = 4, whereas the right panel shows the same spectrum at a redshift z = 8. Simulated exposure times and observed fluxes are indicated in each panel (reproduced from The XEUS science case, ESA SP-1238). again beyond z ∼ 4. Typical estimates indicate that highly From the black-hole spin rate history it will be possible to obscured AGN will produce fluxes of < 10−16 erg cm−2 assess the way these black holes formed and grew, given s−1 in the 2–10 keV energy range. Neither Chandra nor that steady accretion would yield high spin rates, whereas XMM-Newton will be able to measure X-ray spectra at black hole mergers would yield smaller spin rates. such low flux levels. In order to sample the high-redshift AGN a telescope with a larger spectroscopic effective area 2.2. Groups and clusters of galaxies at high z is required. If indeed black holes and AGN were formed at redshifts In the standard cosmological scenario of hierarchical struc- larger than 5, a question remains as to whether the inter- ture formation, small-scale structures collapse first, and galactic medium at those redshifts was ionized by AGN or grow into larger aggregates (White & Rees 1978). In a nut- by the first massive stars formed in the early Universe, or shell, dark matter initially accretes into larger and larger whether the AGN could have influenced the formation of halos due to the gravitational amplification of the initial structure. small density fluctuations. Baryonic matter associated to these halos can dissipate energy and cool down by radia- Given its expected sensitivity (see below), XEUS will tion to form stars. be capable of addressing all these issues. XEUS will be able 6−7 to detect AGN with central black holes of 10 M⊙ which While observations of the CMB (e.g., with the Planck emit at luminosities of 1043−44 erg s−1 at redshifts of 20, Surveyor Mission) can provide the spectrum of the initial and to study their X-ray spectra (via the detection of line fluctuations, as well as the global parameters governing emission from Fe Kα) up to redshift of 10. Spectroscopic cosmic evolution, they alone cannot provide information studies of the Fe line (Fig. 2) and variability studies of about the physics of gas cooling and heating processes the X-ray luminosity of the underlying galaxy will provide that take place during galaxy formation. information about the geometry of the accretion flow in As the baryonic matter collapses and cools, much of the vicinity of a black hole, will constrain the geometry the gas will emit soft X-rays. Therefore, it is only through close to the black hole’s event horizon, and will allow us X-ray observations that it is possible to study the hot to measure the mass, and possibly the spin, of the black baryonic matter component within the large scale struc- holes. By observing a large sample of AGN at different ture of the Universe. It is through X-ray observations that redshifts it will be possible to study the evolution of black we have been able to trace the hot plasma bound to the hole mass and spin from the early Universe up till now. gravitational potentials of groups and clusters of galaxies, 4

Figure 3. Simulated spectrum of a small group with kT=1.5 Figure 4. Simulated XEUS-2 absorption-line spectrum of keV and a bolometric luminosity of 1043 erg s−1 at a a small group of galaxies in front of an AGN with an ob- redhsift of z=2 (reproduced from The XEUS science case, served 0.5–2 keV flux of 10−12 erg cm−2 s−1. The expo- ESA SP-1238). sure time is 100 ksec. The group is assumed to be at z =1, to have a core radius of 50 Kpc, and 0.1–2.4 keV luminos- ity of 10−42 erg s−1 (reproduced from The XEUS science the largest mass aggregates in the Universe. These studies case, ESA SP-1238). have provided some of the most fundamental constraints to-date used to test cosmological models. X-ray studies of clusters of galaxies have been used to measure the ratio of back is likely to affect cluster formation and evolution. dark matter to baryonic matter at different length scales, Central cooling in the early dense groups and cluster cores which reflects the way large objects are formed through can have an important effect on the evolution of these sys- the hierarchical merging of smaller units, but also has al- tems. Additionally, by tracing the outer parts of clusters lowed us to trace the abundance of iron and other heavy and studying the larger scale structures, such as filaments, element in intergalactic space, which seems to imply a we expect to observe the growth of clusters of galaxies by much more violent star burst history than previously as- accretion of intergalactic matter. sumed. However, because of the limitations of XMM-Newton Deep X-ray observations will test the hierarchical for- mation scenario from merging activity at high z and the and Chandra, so far these studies could only be carried z out for objects in the local Universe; similar studies at evolution of sub-clustering with . For instance, the col- large redshifts, which could provide evidence for evolution lision of two sub-clusters should be manifest in tempera- ture maps, through heated gas between the sub-clusters from the early Universe up till now, require much higher sensitivity than current missions can provide. The large before the collision as well as steep temperature gradi- effective area and high angular resolution of XEUS will ents (changes by factor of 2–3 over 200 kpc) at the shocks formed during the collision. The velocities of the gas can enable these studies to be extended to redshifts of z ∼> 2, to study groups and clusters of galaxies at the epoch range from 200–2000 km/s. when these massive objects first emerged (Venemans et al. It is important to notice that the history of the gas 2002), presumably when the star formation rate, and the and galaxy formation are deeply interconnected. During production of heavier elements, peaked. galaxy formation, the temperature, density, and chemical These observations will provide a better understand- composition of the gas are fundamental in determining ing of the role of feedback from the cluster galaxies to the the fate of the collapsing gas: it is critical whether the gas physical and chemical state of the gas at high z. For in- can cool or not. X-ray studies are the most direct way to stance, the evolution of the intra-cluster medium is not obtain information on the physical state of the gas, which purely governed by gravitational effects. Galaxies are in- ultimately controls the overall history of galaxy formation. jecting metals and energy, probably at early epochs via XEUS will have the angular resolution and sensitivity supernova driven winds (Madau, et al. 2001); this feed- to study the dynamics of the gas in those systems in detail, 5

2000). This issue is strongly related to the star formation history, the possible variations of the initial stellar mass function with environmental conditions, and the circula- tion of matter between the different phases of the Uni- verse. This issue will be addressed in part by constraining the history of star formation rate using NGST, Herschel, and ALMA. However, the X-rays provide the best possi- ble way of studying the history of heavy element produc- tion. Clusters of galaxies are the largest closed systems where the chemical enrichment process can be studied in detail. X-ray observations of lines emitted by the hot intra- cluster medium can be used to determine abundances up to much higher redshifts than possible via optical obser- vations of normal galaxies (Fig. 4), with the advantage Figure 5. Simulated XEUS-2 spectra of a cluster of galax- that X-ray observations provide direct measurement of the ies at z =0.5 having a bolometric luminosity of 1.6 × 1044 abundances, without having to rely on the indirect indi- erg s−1. The upper and lower spectra represent simulated cators used in the optical to estimate the element abun- measurements from within 2 core radii, and an annulus be- dances in galaxies. tween 0.9 and 1 core radius, respectively (reproduced from Abundance gradients in clusters and groups of galaxies The XEUS science case, ESA SP-1238). can be used to directly assess the chemical enrichment his- tory of the intergalactic medium, much better than would be possible using individual galaxies. Abundances in very as well as the spectral resolution to measure the mass poor groups can be measured to any redshift using X- motion from emission line spectroscopy. ray absorption line spectroscopy as long as a bright back- But the hot plasma in clusters of galaxies is probably ground source can be found. In the case of Gamma-ray a small fraction of all the baryonic matter in the Universe. burst afterglows this is also true for the host galaxy and all Numerical simulations predict (Dav´eet al. 2001) that 30– intervening systems. XEUS will measure the abundances 40% of the baryons are in the warm-hot intergalactic of all astrophysically abundant elements, down to the pho- medium (Cen & Ostriker 1999), and therefore their ra- ton detection limit. Since XEUS’ energy resolution will be diation is too faint to be detected. However, absorption much better than the equivalent width of the strongest features against a bright source in the background (e.g., emission lines over a large range of temperatures, it will gamma-ray bursts may in principle be detected at redshift be possible to obtain information on the heavy element of 10 or more) can make them visible (Fig. 3), as is well enrichment history of the intracluster medium of a qual- known from the classical Lyman-α forest spectroscopy. If ity now only reached for our Galaxy. This result will have the temperature of this matter is > 105 K, the correspond- direct implications on our understanding of the evolution ing absorption features lie in the X-ray range, the “X-ray of cluster galaxies. forest” (Hellsten et al. 1998; Perna & Loeb 1998). XEUS will make it possible to trace the evolution of Recent hydrodynamic cosmological simulations (Hell- the intra-cluster medium abundances back to at least z ∼ sten et al. 1998) show that the strongest absorption fea- 2, and down to poor clusters, therefore constraining the tures from the intergalactic medium (IGM) should be pro- epoch of production and ejection of the heavy elements, duced by O-VII and O-VIII; for an IGM with a metallic- and unveiling the interplay between the dynamical, in par- ity 10% solar, the absorbers that may be detectable given ticular the effect of mergers, and chemical history of clus- Chandra and XMM-Newton sensitivities have tempera- ters. By comparing the spatial distribution of heavy ele- . . tures in excess of 105 5 − 106 5 K and overdensities δ ∼> 100 ments and galaxies up to high redshifts, XEUS will pro- (Chen et al. 2002). To be able to sample the IGM over vide a strong constraint on ejection process, wind or ram a larger range of temperatures and overdensities, and to pressure stripping. be able to detect other elements besides oxygen, a mission Furthermore, using XEUS it will be possible to probe with the characteristics of XEUS is needed (Chen et al. abundances in the intergalactic medium, and estimate the 2002). metal production efficiency of field and cluster galaxies. It will also be possible to independently constrain the star 2.3. Heavy elements enrichment history formation history, since the overall cluster metal content is a fossilized integral record of the past star formation, and One of the fundamental issues in astrophysics today is to precisely measure the abundance relative to Fe back to how heavy elements formed, and what is the evolution of z ∼ 2, which can be used to assess the relative importance the metallicity of the intergalactic medium (Ferrara et al. of type I and type II supernovae and their past rates. This 6

Table 1. XEUS 1/2 design specifications MSC1/2 rotates in steps of 45 XEUS-1 XEUS-2

Mirror sector integration Energy range 0.05 – 30 keV by robotic arm 2 2 Effective area @ 1 keV 6 m 30 m 2 MSC1 Effective area @ 8 keV 3 m Angular resolution (HEW) 5” 2” Mirror sector with RGS & Balance SS folded baffles and ERA GF Specifications that do not change in XEUS-2 are not indicated.

MSC2

mirror plates and their integration into modules, and the

Integrated mirror sector integration of the individual modules in space into the fi- (12 petals) nal configuration ensuring the required angular resolution of 2 to 5 arcsec. Alternative mission profiles, e.g. Ariane 5 Figure 6. Schematic figure showing XEUS mirror growth launch of the complete XEUS in low-Earth orbit, are also (reproduced from The XEUS telescope, ESA SP-1253). being studied in case the necessary ISS infrastructure is not available The instrument payload model presently comprises a is a strong constraint on the initial mass function, and also Wide-Field Imager (WFI), and two Narrow-Field Cam- has far reaching consequences on our understanding of the eras (NFC), one of them optimized for soft X-rays and thermal history of the inter-cluster medium. the other for hard X-rays. The WFI is based on pn-CCD technology, with an energy resolution of 50 eV at 1 keV. 3. The mission The WFI CCD covers 5′ × 5′, and has a count rate capa- < ∼ −1 The key characteristics of XEUS are large X-ray mirror bility with low pile-up (∼5% of up to 1000 count s aperture, high spectral resolution over wide-band energy within the PSF (HEW). The NFC is based on Supercon- ducting Tunnel Junctions (the soft-X camera) and Transi- range, and good angular resolution (see Table 1). With an area larger than 20m2 and an energy resolution of 2 tion Edge Sensors (the hard-X camera) technologies, will cover 30′′ ×30′′, and will allow for a time resolution of less eV below 2 keV, the XEUS final configuration (see below) than 5 microseconds. The energy resolution will be better will be able to significantly detect the most prominent X- ray emission lines of O-VII, SI-XIII and Fe-XXV against than 2 eV at 1 keV for the soft NFC, and 2 eV and 5 eV, at 1 keV and 8 keV, respectively for the hard NFC. The the sky background and source continuum, while an an- high energy resolution required seems feasible, given that gular resolution of 2–5 arcsec will help minimize source confusion and will reduce the background due to galactic at present a single pixel micro-calorimeter can achieve an energy resolution of 3.9 eV at 5.9 keV, with a thermal foreground X-ray emission. XEUS will consist of two separate spacecrafts, the mir- response of 100 microseconds. ror and the detector spacecraft, respectively, injected in When XEUS grows to its final configuration, the de- low-Earth orbit with an inclination equal to that of the tector spacecraft will be replaced with a new generation ISS (fellow-traveler orbit). The mirror and detector space- of instrument technology. XEUS expected lifetime is 25 crafts will fly in formation, yielding a 50-m focal length. years or more. The detector spacecraft will track the focus of the X-ray The main characteristics of XEUS are indicated in Ta- telescopes with a precision of ±1 mm per degree of free- ble 1. Given those specifications, XEUS will be able to × −18 −2 −1 dom. Because the detector spacecraft will be flying in a detect a source at a flux of 4 10 erg cm s in non-Keplerian orbit, it will need active orbit control. the 0.5–2 keV energy range in a 100 ks exposure, and it will be capable of measuring spectra down to a flux of XEUS is a two-step mission that will grow in space. −17 −2 −1 XEUS-1, which will be launched by an Ariane 5 rocket, 10 erg cm s . In order to achieve these fluxes, the < − will have a primary mirror of 4.5-m in diameter comprising Half-Energy Width of the PSF must be ∼2 5 arcsec. two concentric rings with so-called petal structure, each petal consisting of a set of heavily stacked thin mirror 4. Present status and technology developments plates with a Wolter I type geometry. After five years in space, the mirror spacecraft will dock with the ISS, where Several studies relating to XEUS enabling technologies are the European Robotic Arm will be used to add additional currently in progress or have been planned for the near fu- mirror segments around the central core, so the XEUS ture. An 18-month system level study will be carried out mirror will grow to its final size, 10-m in diameter (Fig. by the ESA directorate for Science and the directorate for 6). The construction of the mirrors poses several techno- Manned Space and Microgravity, with technical support logical challenges, among them the manufacturing of the from ISAS. This activity will kick off in April 2002 and 7

will address several basic feasibility issues concerning tele- scope configuration, orbit, ISS interfacing and flight im- plementation scenarios. More specific studies, targeted at formation flying (station keeping), robotic assembly tech- niques, optical/IR straylight suppression filters, and 50- milli-Kelvin closed cycle coolers are now being addressed in the ESA Technology Research Programme (TRP). Un- der the TRP, two X-ray sensor study contracts, one re- lated to the Wide-Field active pixel detector array and one dealing with a squid-read-out X-ray bolometer/TES array, will be started at the beginning of 2002, based on quite promising single-pixel results obtained in various Eu- ropean (space) research institutes. The major development item for the coming years is obviously the technology, metrology, assembly, and active alignment of the X-ray mirror petal. Starting from the XMM replica technology, several alternative routes for manufacturing are now being pursued, including the ap- plication of new, lightweight, materials like slumped glass and SiC. Since the mirror technology is at the very heart of the XEUS feasibility, it has been designated as a core technology development in the ESA science programme, implying major development funding in the period 2002- 2004. These efforts will be part of the so-called Core Tech- nology Programme, which was recently established within ESA science for strategic technology investments. Acknowledgements We like to acknowledge the members of the XEUS Steering committee and XEUS Astrophysics working group who have put together the science and mission case as described in this overview paper.

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