Sunyaev-Zeldovich Decrements with No Clusters?

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 9 October 2018 Sunyaev-Zeldovich decrements with no clusters? Priyamvada Natarajan & Steinn Sigurdsson Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U. K. 9 October 2018 ABSTRACT Sunyaev-Zeldovich decrements in the temperature of the Cosmic Microwave Background (CMB) are produced by Thomson scattering of the CMB photons by the confined hot electrons in the Intra-Cluster Medium encountered along the line-of-sight. In this paper, we propose an alternate physical process that can produce detectable thermal and kinematic Sunyaev-Zeldovich decrements around quasars or quasar pairs in regions where no assembled, virialized clusters are detected either in the optical or X-ray wave-band. Invoking quasar outflows, we argue that both thermal and kinematic decrements can be produced by the baryons swept up by the mechanical luminosity of quasars. In contrast to the case of decrements produced by the hot electrons confined within the cluster potential, the magnitude of these quasar-outflow induced effects depend primarily on the luminosity of the QSO, and the physical scale of the outflow - which in turn depends both on the mean local density and the density gradient in the vicinity of the quasar. Quasar outflows produce (i) a frequency independent kinematic decrement −4 ∆T/T ∼ 10 ; (ii) a frequency dependent fluctuation in the intensity ∆Jν /Jν ∼ a 20% variation relative to ∆T/T , say at 15 GHz and 30 GHz; (iii) a temperature increment ∆T/T ∼ 10−4 on the opposite side of the QSO; (iv) a linear polarization signal over 20” at ∼ 10−7 level; and therefore (v) patches of signal ∆T/T ∼ 10−4 on degree scales which will contribute to the power and hence affect the CMB multipole calculations in the rising region of the first Doppler peak at l ∼ 60 for CDM-like models as well as defect models. These effects are of interest since they are potentially observable in the context of the next-generation of CMB experiments - MAP & PLANCK. Key words: cosmology: cosmic microwave background – diffuse radiation – galaxies: arXiv:astro-ph/9704237v5 2 Jun 1998 quasars – general 1 INTRODUCTION the millimeter wave-band planned or underway, and there is renewed interest in the radio morphology of high-redshift Sunyaev-Zeldovich decrements in the cosmic microwave QSOs and their hosts. In this paper, we expand on the clas- background are produced by the Thomson scattering of the sical S–Z scenarios (clusters, clusters with peculiar veloci- photons by hot electrons - typically by electrons that are ties, relativistic corrections to the scattering cross-sections confined in the dark matter potential well of a rich cluster of and warm bubbles around QSOs with finite peculiar veloc- galaxies. However, we demonstrate in this work that compa- ity) and include another class of objects that may produce rable temperature decrements can be produced by electrons such decrements. For this class of objects: quasar outflows, that are swept up by the outflows powered by the mechanical it is instructive to point out that the gas need not be very luminosity of quasars. While the properties of the outflow hot in order to produce an observable decrement. In particu- are determined primarily by the energy output of the QSO, lar, when the cooling/expansion time-scales are interestingly the kinematics at late times are determined by the mean long, i. e. if the cooling time is comparable to or exceeds the density and the density gradient in the vicinity of the QSO. time-scale of the high luminosity phase of the QSO, the ef- Since in variants of the standard CDM (cold dark matter) fects of the gas can be observed well after the QSO has dominated cosmological models, quasars form from the col- faded. lapse of high-significance peaks and tend to seed the high This paper is organized as follows: in Section 2 we moti- density regions which subsequently are the sites for the for- vate the role of quasar-driven outflows; in Section 3 the ener- mation of clusters, the physical scale of the outflow depends getic constraints in terms of the implied physical scales out on the ambient density and depth of the local potential well. to which the outflows and their effects are likely to manifest There are at present many new observations of the CMB in themselves are outlined; in Section 4 we discuss the kine- 2 Priyamvada Natarajan & Steinn Sigurdsson matic and the thermal S–Z decrement induced by outflows of scale R: and their other observational implications; and we present ǫ ǫL 1 Mbh 1 fQ − 1 v/c − 2 3 3 3 3 our conclusions and prospects for detection in the final sec- R ∼ 1( ) ( 11 ) ( ) ( ) Mpc. (1) 0.05 10 M⊙ 1 0.01 tion. For our purposes typical swept up regions are of the order of 0.1–1 Mpc or so; fQ is the local baryon fraction in units of 0.01, and a density contrast of ∼ 103 over the mean 2 QUASAR-DRIVEN OUTFLOWS cosmological density at redshift ∼ 3 is required for the numbers to be consistent, this is a plausible over-density for the The ubiquity and high luminosity of quasars out to high highest significance peaks that will seed the massive clus- redshifts suggests that they might have played an important ters observed at lower redshifts. The total energy budget role in the subsequent formation of structure (Efstathiou & ET ∼ ǫ ǫLMbh; and the restrictions on the available param- Rees 1988; Rees 1993; Silk & Rees 1998). Babul & White eter space in terms of the mass that needs to be swept up (1991) suggest that the ionization produced by UV pho- and the bulk velocity required to produce a kinematic S–Z tons from the quasar might inhibit galaxy formation in their decrement are dictated by the following relations, vicinity due to the ‘proximity’ effect. Thus quasar activity is 2 likely to be one of the important non-gravitational processes ET ∼ Msweepvsweep ; R ∼ tQSO vsweep ; (2) to have influenced structure formation at redshifts z > 2. ∆T vsweep ∼ τ ∼ Msweep vsweep , (3) We propose quasar-driven outflows as a possible phys- T c ical mechanism for concentrating baryons. Such a scenario where tQSO is the quasar lifetime and τ the optical depth. has been explored previously in the context of explosion in- Beam dilution imposes a minimum Msweep assuming the line duced structure formation models (Ostriker & Cowie 1981; of sight length through the gas is comparable to its trans- Daly 1987) as well as in theoretical models of the Inter- verse scale, which is likely for the resulting outflow geometry Stellar Medium (McKee & Ostriker 1977). Mechanically studied here. Scaling to typical Abell cluster parameters, the powered outflows can push baryons around active quasars mean electron thermal speeds are βT = vT /c ∼ 0.1. Since in shells cleaning out regions of Rtoday ∼> 1Mpc. We exam- the kinematic effect is first order in βK = v/c, we require 2 −2 ine the effectiveness of this mechanism in the production of βK ≈ βT ∼ 10 . This is consistent with speeds required massive clumps of baryons that can in turn produce a signal for the mass of swept up gas which would produce the re- in the CMB. The observational evidence for these outflows quired optical depths over angular scales of a few arc min- are seen in a subset of radio-quiet QSOs - the broad ab- utes. It is these mass and velocity requirements which drive sorption line systems (Hazard et. al 1984; Weymann et. al us to postulate ultra–massive central black holes to power 1991) in which speeds v/c ∼ (0.01 – 0.1) are inferred dur- the outflows. Note that τ ∼ neR whereas the mass swept 3 ing a wind-phase. These ‘hyper-active’ quasars can create up Msweep ∼ neR . All the numbers quoted in this work −1 both regions that are baryon-rich as well as voids via their have been computed for H0 = 50 kms Mpc; Ω0 = 1.0 energetic outflows. This powered outflow model is compati- and Λ = 0. As illustrated by eqns. 2 and 3, the best sources ble with both scenarios of quasar population evolution: the for large energy effects are the massive BHs that power the single generation of long-lived quasars as well as the mul- QSOs. We also note here that according to some models of tiple generation model wherein populations fade and flare unified schemes for AGNs, jets and efficient energy release (Haehnelt & Rees 1994). Superwinds of the kind detected occurs only if the QSO is accreting close to or at super- by Heckman et al. (1996) from galaxies can also in principle Eddington rates, possibly via the Blandford-Znajek mech- produce S–Z decrements, but they are typically an order of anism for rotating BHs. This might preferentially happen magnitude weaker than the QSO outflows proposed here. at high redshift, where a higher fraction of high luminosity QSOs are observed, consistent with a picture wherein the most massive BHs form rapidly at high redshift and are fu- eled by the baryons that are present in these high-density 3 ENERGETICS OF THE MECHANISM regions. We do not necessarily need fully assembled, virialized dark halos for this to happen, we only need an efficient The total energy available to power a quasar can be directly way to funnel the baryons already there to the bottom of estimated from the mass accreted by a black hole of mass the deepest potential wells. Mbh (modulo an efficiency factor, ǫ). In general, the total energy available is 10-20% of the rest mass of the central black hole (ǫ ∼ 0.1 − 0.2).

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