Giant magnetized outflows from the centre of the Milky Way

“[19] Yun, M.”

“[23] Kulsrud, R.”

“Note that this result is based on the 9 arcminute resolution SPASS survey (Carretti et al. 2013), so the reduction in beam depolarization may result in even larger fractional polarizations at 2–4 GHz, ∼ 10% (see Section 6.4.2).”

“There is a tentative association of the Fermi bubbles with some features in the S-band Polarization All Sky Survey (S-PASS) radio data (Carretti et al. 2013) and in WMAP polarization maps (Jones et al. 2012).”

“Radio andγ-ray observations of nested giant bubbles indeed provide evidence that stellar feedback has a profound impact onlocalized portions of the CMZ (Sofue & Handa 1984; Su, Slatyer & Finkbeiner 2010; Carretti et al. 2013).”

“They partially overlap two giant radio lobes emanating from the Galactic bulge and permeated by large magnetic fields, up to 1.5 nT (Carretti et al. 2013).”

“…of relativistic gravitation, including the search for gravitational waves (a review is provided in Hobbs & Dai 2017), probing neutron star interiors (such as Demorest et al. 2010), and investigations into the magnetic field structure of our Galaxy (e.g., Han et al. 2018 and Carretti et al. 2013).”

“In combination with their spatial extent, the time since the starburst given here constrains the outflow velocity of the bubbles to be vout = 800–1600 km s−1, which is remarkably consistent with previous estimates from geometric considerations (vout ∼ 1000 km s−1; Carretti et al. 2013).”

“…the gap between the Galactic center and the Fermi bubbles (Su, Slatyer, & Finkbeiner 2010), gamma-ray emitting structures with approximately coincident polarized synchrotron emission (Carretti et al. 2013) that extend to |b|∼50◦, also thought to be caused by previous energetic outflows from the GC.”

“…2014); (2) soft X-ray emission (0.3–1.0keV; Snowden et al. 1997; Kataoka et al. 2013); (3) K-band microwave emission (23–94GHz), known as the so-called WMAP haze (Finkbeiner et al. 2004; Dobler & Finkbeiner 2008); (4) polarized radio emission at 2.3GHz (synchrotron radiation; Carretti et al. 2013).”

“Indeed, there is diffuse, polarized 23 GHz emission evident over most of the solid angle of the bubbles (see Figure 3 of Carretti et al. 2013).”

“…et al. 2011), the Fermi-2nd order acceleration through stochastic scattering by plasma instabilities (Mertsch & Sarkar 2011), directly from the jet (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012; Yang et al. 2013), or from outflows driven by the past star formation (Carretti et al. 2013).”

“…the surrounding ambient magnetic field (Spoelstra 1973), whereas others might be related to large-scale magnetized outflow from the Galactic center (Carretti et al. 2013), related to the so-called “Fermi bubbles” observed in gamma rays (Ackermann et al. 2014) and surrounded by “eROSITA bubbles” in…”

“(2010) and to the two giant radio lobes (Carretti et al. 2013).”

“This picture is consistent with the proposals that the Fermi Bubble outflows are generated by a central region of our Galaxy that has rapid star formation (Bland-Hawthorn & Cohen 2003a; Carretti et al. 2013; Crocker et al. 2015; Lacki 2014).”

“G A ] 2 1 Se 2 whether related to starburst or AGN activity, remains hotly contested (Zubovas et al 2011; Su & Finkbeiner 2012; Carretti et al 2013).”

“Since its discovery, the Fermi bubbles have notably been found to also contain a magnetized radio plasma [135] and have been connected to the previously known microwave “haze” that was re-observed with Planck [136].”

“…direction−Q,−U must be used.5 As the magnetic field lines are parallel to the extension of the filament, a toroidal field along the spur can be ruled out.Carretti et al. (2013), with polarisation data at 2.3 GHz, detected two polarised radio lobes that encompass around 60◦ in the inner Galaxy.”

“…datasets are being analyzed and will be soon available, specifically at 2.3 GHz from the S - band Polarization All Sky Survey (Carretti 2010; Carretti et al. 2013, Carretti et al., in prep., S-PASS), and the C-band All Sky Survey at 5 GHz (Pearson & C-BASS Collaboration 2016; Jones et al.…”

“With the multi-feed Sband receiver which will soon be in operation at the SRT, a survey similar to the S-PASS (S-band Polarization All-Sky Survey; Carretti et al. 2013), conducted in the Southern Hemisphere with Parkes, will become feasible with the SRT. Finally, wide-band mapping of extended…”

“…in hard X-ray emission (0.7-1.5 keV, Bland-Hawthorn & Cohen 2003), soft Xrays emission (0.3-1.0 keV, Kataoka et al. 2013), midinfrared (4.3-21.3 µm, Bland-Hawthorn & Cohen 2003), microwave emission (23-94 GHz, Dobler & Finkbeiner 2008) and polarised radio emission (2.3 GHz, Carretti et al. 2013).”

“The “jets” could instead be related to stellar clusters in the CMZ (c.f., Carretti et al. 2013); perhaps they are smaller cluster winds within the CMZ wind.”

“The recent discovery of the Fermi bubbles (Su et al. 2010) in γ-ray emission and their apparent counterparts in polarized radio continuum (Carretti et al. 2013) has fueled speculation that there is either a powerful jet from the Galactic Center or there has been a past burst of star formation.”

“…bubbles revealed by the ample observational data from Fermi (Su et al. 2010; Hooper & Slatyer 2013), WMAP (Finkbeiner 2004; Dobler & Finkbeiner 2008), Planck (Planck Collaboration 2013), and S-PASS (Carretti et al. 2013) have offered an unprecedented opportunity to disentangle different mechanisms.”

“This simple two-component model of the diffuse γ-ray emission supports scenarios in which the Fermi bubbles are just outflows of the hot ISM (Yang et al. 2013; Cheng et al. 2011; Dogiel et al. 2011; Chernyshov 2011; Carretti et al. 2013).”

“These largely extended structures have been observed at radio- [162, 269] and micro-wavelengths [191] and up to X-ray [221] and gamma-ray [430] energies.”

“…relevant to understanding the transport of dense material from low to high latitudes in galactic outflows, such as the one observed in the Milky Way (see Bland-Hawthorn & Cohen 2003; Carretti et al. 2013; McClure-Griffiths et al. 2013 for observations, and Crocker 2012 for a theoretical overview).”

“One direction is to use auxiliary data taken at non-CMB frequencies, such as the 408, 1420 and 2300 MHz maps observed by Haslam et al. (1982), Reich (1982) and Carretti et al. (2013) or using Hα data (e.g., Dickinson et al. 2003).”

“More recent polarization surveys, such as the S-band Polarization All Sky Survey (S-PASS) at 2.3 GHz, will provide ≈5000 polarized sources in the Southern sky (Carretti 2011; Carretti et al. 2013)—corresponding to a significant increase in sample size.”

“The low-frequency emission extends westward beyond the FBs in both hemispheres and the spectrum 2.3-23 GHz spectrum becomes softer as we go away from the Galactic center.”

“…2013), Xray properties of the Galactic halo (e.g., Snowden et al. 1997; Bland-Hawthorn & Cohen 2003; Kataoka et al. 2013; Tahara et al. 2015; Kataoka et al. 2015; Miller & Bregman 2016), UV absorption lines (Fox et al. 2015; Bordoloi et al. 2017), and polarized lobes (Carretti et al. 2013).”

“These structures are the radio polarization counterparts of the γRay Fermi Bubbles (Carretti et al. 2013a; Su et al. 2010) emanating from the Galactic Centre.”

“…Survey (S-PASS) to detect a synchrotron bridge connecting the cluster centre to the major outgoing merger shock in the massive cluster Abell 3667 (Carretti et al. 2013b), which demonstrates the ability of S-PASS to detect faint extragalactic synchrotron emission associated with large-scale…”

“Their curved morphologies are not obviously consistent with a collimated jet and both features have been interpreted as being part of a biconical outflow driven by recent star-forming activity in the GC (Carretti et al. 2013).”

“This flux density ratio has considerable scatter and may exhibit a dependence on the luminosity of the 1665-MHz maser emission, similar to 6.7-GHz methanol to OH maser ratios (Caswell 1997) and 6.7-GHz methanol to 12.2-GHz methanol maser ratios (Breen et al. 2011).”

“…unknown 1 Haverkorn (priv. comm.) HartRAO 2.326 20 −83◦ to +13◦ I − Q 25 mK 80 mK 3 Jonas, Baart & Nicolson (1998) S-PASS (Parkes) 2.3 9 <0◦ IQU 0.1 mK unknown 1 Carretti et al. (2013) GEM 4.8–5.2 45 −52◦ to +7◦ QU 0.5 mK unknown 0 Barbosa et al. (2006); Tello et al. (2013) C-BASS 4.5–5.5…”

“Moreover, the recently discovered linearly polarized giant radio lobes emanating from the GC closely correspond to the Fermi bubbles [31].”

“The Fermi bubbles are then just two giant outflows driven by the hot ISM and in particular the light CR content of it, as a number of authors already suggested [46, 50].”

“…in the GC, such as an active galactic nucleus (AGN)like outburst (e.g., Guo et al. 2012; Yang et al. 2012) or from nuclear starburst activity (e.g., Lacki 2014) in the past with an energy release of 1055–56 erg over 10Myr ago (Su et al. 2010; Crocker & Aharonian 2011; Carretti et al. 2013).”

“They point towards a magnetic field larger than 10 μG at 7 kpc from the Galactic plane (Carretti et al. 2013).”

“…Fermi second-order accelera- tion through stochastic scattering by plasma instabilities (Mertsch & Sarkar 2011); 3) directly carried by the jet (Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012, 2013; Barkov & Bosch-Ramon 2014) or winds driven by the past star formation (Carretti et al. 2013).”

“3 GHz is also available in the vicinity of the Fermi bubbles [10].”

“However, based on observations of the radio lobes’ morphology, Carretti et al. (2013) concluded that they originate from a starburst event in the 200 pc diameter region around the GC rather than from the supermassive black hole.”

“Thus we can make a comparison between the statistics of |∇P| of the S-PASS observations and the MHD turbulence simulations of Burkhart et al. (2012), since the emission in both arises from turbulent fluctuations of ne and B along the line of sight.”

“[97] have connected these γ-ray features with spurs and loops of polarized radio emission seen in the S-Band All Sky Survey (S-PASS) data.”

“Remarkably, structures coincident or similar to the FBs can be seen at other wavelengths, including the (total intensity) microwave haze found in WMAP (Finkbeiner 2004) and, most recently, in Planck data (Planck Collaboration 2013); the polarized microwave structures reported by Jones et al. (2012); the largescale, bi…”

“…X-ray emission, polarized radio emission, microwave emission, and UV absorption (Bland-Hawthorn & Cohen 2003; Su et al. 2010; Dobler et al. 2010; Ackermann et al. 2014; Dobler & Finkbeiner 2008; Carretti et al. 2013; Fox et al. 2015; Bordoloi et al. 2017a; Savage et al. 2017; Karim et al. 2018).”

“The above estimate indicates that the polarized microwave bubbles are ≳ 25 Myr older (Carretti et al. 2013, supplements).”

“Evidence has come from across the electromagnetic spectrum, including extended soft X-ray emission (Snowden et al. 1997; Sofue 2000; Bland-Hawthorn & Cohen 2003), diffuse microwave excess from WMAP (Finkbeiner 2004; Dobler et al. 2010) and polarized radio continuum (Carretti et al. 2013).”

“CR electrons accelerated in association with star formation have already been shown to arise from the Galactic Center (Carretti et al. 2013) and parallel studies of gamma rays and radio emission have revealed an excess at GeV energies within the Milky Way (Orlando & Strong 2013).”

“A dramatic effect of these energy sources might be the ∼ 10 deg sized Fermi bubbles that extend perpendicular to the Galactic disk far into the Galaxy’s halo (e.g. Su et al. 2010; Crocker et al. 2011; Carretti et al. 2013).”

“Signals from roughly the Fermi bubble regions were also observed in themicrowave band byWMAP [2] and, recently, in the radio-wave band [3].”

“The Fermi bubbles (FBs) and concentric structures at their feet in the Galactic Plane are also seen in total-intensity or polarized radio and microwave emission (Carretti et al. 2013), their inner parts are close to the X-ray bulge (see Fig.”

“A similar analysis has been carried out by Carretti et al. (2013).”

“In particular, the 2.3 GHz polarisation observations (Carretti et al. 2013) suggest the outflow feeding electrons into the radio lobes has a vertical speed of ∼ 1100 km/s giving an advective timescale over the ∼8 kpc extent of the radio lobes (marginally larger than the Bubbles) of only 7 × 106 yr.”

“We expect the full C-BASS survey to have a significantly reduced uncertainty in its calibration, and in conjunction with other modern surveys such as S-PASS (Carretti et al. 2013) we should be able to constrain the spectral index and possible curvature (steepening/flattening) of the spectrum even more accurately.”

“…wavelengths, including hard X-rays (Bland-Hawthorn & Cohen 2003), soft X-rays (Kataoka et al. 2013), γ-ray (Su et al. 2010), K-band microwaves (Dobler & Finkbeiner 2008) as the so-called Wilkinson Microwave Anisotropy Probe (WMAP) haze, and polarized radio waves at 2.3 GHz (Carretti et al. 2013).”

“Other starburst explanations are given by Crocker et al. (2015) and Carretti et al. (2013).”

“The structure in the hot Milky Way CGM is also observed in the form of the Fermi ( M. Su et al. 2010 ) , Parkes ( E. Carretti et al. 2013 ) and eROSITA ( P. Predehl et al. 2020 ) bubbles.”

“They are observed as hardspectrum gamma-ray structures (Su et al. 2010; Dobler et al. 2010) which coincide with radio lobes (Carretti et al. 2013).”

“We interpret this as a ‘scrambling’ of the smooth synchrotron emission from the Galaxy in the background by Faraday screens associated with each object (e.g., Carretti et al. 2013b).”

“Associated components are visible in broad-band emission across the electromagnetic spectrum (Dobler & Finkbeiner 2008; Kataoka et al. 2013; Carretti et al. 2013).”

“…to the associated X-ray features including the North Polar Spur (NPS) (Totani 2006; Fujita et al. 2013; Kataoka et al. 2013, 2015; Ackermann et al. 2014; Mou et al. 2014; Fang and Jiang 2014; Carretti et al. 2013; Crocker et al. 2015; Inoue et al. 2015; Tahara et al. 2015; Sarkar et al. 2015).”

“…2014), their counterparts in X-rays (Bland-Hawthorn & Cohen 2003; Miller & Bregman 2016), microwaves (Finkbeiner 2004), and polarized radio emission (Carretti et al. 2013), the presence of smaller-scale (≈400 pc) radio lobes within the Bubbles (Heywood et al. 2019), and the tentative detection of…”

“These values of magnetic field are less than the estimated equipartition values of ∼6 μG inside the bubbles (Carretti et al. 2013), and ∼15 μG of the bubble walls (Jones et al. 2012).”

“The speed of the hot phase of the Milky Way’s wind has been estimated to be upwards of 1000 km/s (Carretti et al. 2013; McClureGriffiths et al. 2013), while these are the upper estimates, this study is relevant to the hot phase in galactic winds as well as applicable to the general study of the…”

“Even within the PH, XE can be Faraday-thick if both emission and rotation occur within the same volume, resulting in different amounts of Faraday Rotation for emission produced at different distances (Burn 1966; Gardner & Whiteoak 1966; Sokoloff et al. 1998; Carretti et al. 2013).”

“This may be owing to the broader region Carretti et al. (2013) studied or the difference of the background estimation.”

“…this paper may be applied to blast waves launched by any rapid energy release such as may occur in the centre of the Galaxy or any other galaxy, leading possibly to the formation of the Galactic Fermi bubbles (Carretti et al 2013) or the SNR-like shocks observed in Centaurus A (Croston et al 2009).”

“We have also explored similarities between structures in our own Galaxy and Cen A.”

“Recently, the existence of outflows or winds in the Galactic center region are extensively discussed (Everett et al. 2008; Crocker et al. 2011), and it might be closely linked to the formation mechanisms of the Fermi bubbles via magnetic activity (Carretti et al. 2013).”

“Recent radio observations of the two giant, linearly polarized radio lobes with the Parkes telescope (Carretti et al. 2013) also show evidence of large scale manifestations of activity at the Galactic Center.”

“Compelling evidence also exists for outflows at still larger (kiloparsec) scales (Su et al. 2010; Carretti et al. 2013; Di Teodoro et al. 2018, 2020; Predehl et al. 2020), but the physical relation between the outflows on different scales, e.g., whether they were produced by the same mechanism,…”

“…Southern sky that was not covered by RMs from the catalogue by Taylor et al. S-PASS/ATCA builds on S-PASS, the S-band Polarization All-Sky Survey, which mapped the sky below a declination of 0◦ with the Parkes radio telescope at a frequency of 2.3 GHz (Carretti 2010; Carretti et al. 2013, 2019).”

“Estimations of the magnetic field strength in the FB ranges from several µG up to 15 µG (see Strong et al. 2010; Jones et al. 2012; Carretti et al. 2013).”

“We note, however, that as follows from Equation (11), it is problematic to reproduce the gamma-ray spectrum in the leptonic model if we accept the magnetic field strength in the bubbles to be 15 μG, as derived by Carretti et al. (2013).”

“…the total energy content of the diffuse∼108 K gas that might occupy the central few hundred parsecs of the Galaxy (Koyama et al. 2009; Koyama 2011, and references therein), and it is substantially less than te magnetic energy content of the overlying Fermi Bubbles (see below Carretti et al. 2013).”

“Specifically, the coarse-grained edge 1 extends farther to the west at high latitudes; interestingly, the linearly polarized lobes (Carretti et al. 2013) show a similar but much stronger tendency.”

“A description of SPASS is given in Carretti et al. (2013b) and Carretti (2010); here we report a summary of the main details.”

“Possible counterparts to the Bubbles have also been claimed at other frequencies – notably in polarized radio (Carretti et al. 2013) and X-ray data (Su, Slatyer & Finkbeiner 2010; Tahara et al. 2015; Kataoka et al. 2015). c© 2016 RAS ar X iv :1 60 3.”