Giant magnetized outflows from the centre of the Milky Way

“[19] Yun, M.”

“[23] Kulsrud, R.”

“Other continuum Corresponding author: Dhanesh Krishnarao krishnarao@astro.wisc.edu observations, e.g. polarized synchrotron radiation at radio wavelengths (Carretti et al. 2013), have been interpreted in the context of these gamma-ray results.”

“For example, there is evidence that the GC experienced episodic starbursts a few Myr ago, producing more than 100 SNe during the entire starburst (q.v. Tamblyn & Rieke 1993; Sjouwerman et al. 1998; Simpson et al. 1999; Hartmann 1995; Ozernoy 1996; Carretti et al. 2013).”

“In addition, polarized giant radio lobes emanating from the GC seem to be closely connected to the Fermi bubbles (Carretti et al. 2013).”

“…12, we calculate the polarization curvature in the direction that maximizes the directional derivative for the 2.3 GHz (top, S-band Polarization All Sky Survey; Carretti 2010; Carretti et al. 2013) and 4.8 GHz (bottom, Sino-German λ6 cm survey; Sun et al. 2011) data used by Sun et al. (2014).”

“The computed spectra nicely match the spectral index of −0.5 observed at a few tens of GHz, with a hardening below a few GHz; this can explain why no radio counterpart of the microwave haze has been observed (see, however Carretti et al. 2013).”

“This was later confirmed with the discovery of the Fermi bubbles, the bipolar lobes extending vertically up to 8 kpc, observed in the γ-ray bands (Su et al. 2010), which also have radio counterparts (Carretti et al. 2013).”

“…gradient to the E- and B-modes (Zaldarriaga & Seljak 1997) of diffuse synchrotron emission in the S-band Polarization All Sky Survey (Carretti 2010; Carretti et al. 2013), and found that the two had similar properties, and provide complementary information on observed magnetoionic turbulence.”

“There are arguments, indeed, in favour of plasma outflow from the Galactic central region (see Crocker & Aharonian 2011; Carretti et al. 2013).”

“…mechanism as either hadronic (Crocker & Aharonian 2011; Fujita et al. 2013) or leptonic (Yang et al. 2013); the underlying engine as a starburst (Carretti et al. 2013; Lacki 2014; Sarkar et al. 2015), a jet from the the central massive black hole (Cheng et al. 2011; Guo & Mathews 2012; Zubovas…”

“As shown below, this scenario can produce the partly ordered magnetic-field structure similar to that found in S-PASS.”

“As shown later, significant contribution to the CR flux comes from sources that are within few kilo-parsecs and hence the details of the GMF near the Galactic center, such as Fermi bubble (e.g. Carretti et al. 2013), would not affect our results and have not been considered in the present analysis.”

“The magnetized ridges in the inner Galaxy ( ∣ l ∣ ≲ 60°) are spatially correlated with Galactic structures: the ridges associated spatially with the Fermi bubbles appear to emanate from the Central Molecular Zone 8,20,21 , while the ridges in the outer region appear to originate in the disk a few kiloparsecs from the…”

“In the other, a nuclear starburst drives a powerful wind from the inner region of the Galaxy that inflates the bubbles (Crocker and Aharonian 2010; Crocker 2012; Carretti et al. 2013; Lacki 2013).”

“The associated multi-wavelength observations in microwaves (WMAP haze) [5,6], X-ray [7,8] and polarized radio waves [9] provide comprehensive information for studying the physical origin of the bubbles.”

“The FBs show counterparts in other bands: in Xrays, a . keV shell (Bland-Hawthorn & Cohen 2003; Keshet & Gurwich 2017b, henceforth KG17b); in microwaves, the so-called microwave haze (Finkbeiner 2004); and possibly in the radio, an extended polarization signal (Carretti et al. 2013).”

“Structures roughly coincident with the gamma-ray bubbles are known in X-rays (Snowden et al., 1997; Bland-Hawthorn & Cohen, 2003), microwave (Finkbeiner, 2004; Dobler & Finkbeiner, 2008; Ade et al., 2013), and polarized radio (Carretti et al., 2013).”

“The very large column density of the magneto-ionic medium and the dynamical complexity of the Galactic centre also makes the polarised signal in large areas near the Galactic centre highly depolarised (Carretti et al. 2013).”

“These structures may either relate to wind driven by star formation in the Galactic center (GC) (Crocker & Aharonian 2011; Crocker et al. 2014, 2015; Carretti et al. 2013; Sarkar et al. 2015), or relate to the past activities of supermassive black hole residing in the GC — Sgr A* (Guo & Mathews…”

“Finally, the radio polarisation data are from the S-band Polarization All Sky Survey (S-PASS; Carretti et al. (2013); Carretti et al., in preparation).”

“Within the hot wind the pressure is many times larger than the typical ISM (Bland-Hawthorn & Cohen 2003; Carretti et al. 2013); this is expected to force clouds into a purely cold phase (Field et al. 1969; Gatto et al. 2015).”

“Carretti et al. (2013) estimated the magnetic energy of the GRLs with the leptonic scenario for the gamma-ray intensity as U B , lobe ∼ 10 55 erg.”

“…directly from the Galactic center, observed in emission in hard and soft X-rays, in absorption at UV wavelengths, in polarized radio emission, and in γ rays, referred to as the Fermi Bubble ( Snowden et al. 1997; Su et al. 2010; Carretti et al. 2013; Bordoloi et al. 2017; Ashley et al. 2022 ) .”

“…have revealed many interesting large-scale structures related to the Galactic nuclear wind, i.e., the GC lobes in radio (e.g., Sofue & Handa 1984; Carretti et al. 2013), IR (e.g., Bland-Hawthorn & Cohen 2003), X-ray (e.g., Predehl et al. 2020), γ-ray (i.e., the Fermi bubbles in Su et al. 2010)…”

“This region is also characterized by the WMAP haze in K-band microwaves (Finkbeiner 2004; Dobler & Finkbeiner 2008) and by polarized synchrotron radiation at 2.3 GHz (Carretti et al. 2013), in both case showing biconical structures that can arise from an active Galactic Center.”

“It contains known structures that extend across several to tens of kiloparsecs, such as the Fermi bubbles ( M. Su et al. 2010 ) , polarized radio lobes ( E. Carretti et al. 2013 ) , and the eROSITA bubbles ( P. Predehl et al. 2020 ) .”

“The advantage of performing polarization surveys at frequencies higher than 2 GHz to reduce the effects of Faraday depolarization has been demonstrated by the S-PASS data (Carretti et al. 2013; Krachmalnicoff et al. 2018).”

“Outflows are also described in Carretti et al. (2013), and even if the size of the region they consider is an order of magnitude smaller than ours we still consider this option a possibility.”

“They could form due to occasional formation of jets due to the central black hole, or from stellar activity in this region (Crocker et al., 2011; Carretti et al., 2013).”

“74,75 Further evidence for the nuclear wind is provided by high-velocity clouds (HVCs) seen in 21-cm emission 58,61,76 and UVabsorption 77 – 83 in sightlines passing through the Fermi Bubbles.”

“The bubbles emit radiation from gamma rays (Ackermann et al. 2014), through X-rays and the mid-IR (Bland-Hawthorn & Cohen 2003; Predehl et al. 2020), to microwaves and radio waves (Finkbeiner 2004; Carretti et al. 2013).”

“11) show flat spectrum regions across the Haze area at 23–30GHz, and an evident steepening at low frequencies, in agreement with Carretti et al. (2013).”

“This activity may be leptonic (Cheng et al. 2011; Crocker et al. 2011; Mertsch & Sarkar 2011; Guo & Mathews 2012; Guo et al. 2012; Yang et al. 2012; Carretti et al. 2013; Lacki 2014) or hadronic (Crocker & Aharonian 2011; Crocker et al. 2011; Lacki 2014; Crocker et al. 2015).”

“…may have formed during a previous star formation episode, as well as the two young, massive Arches and Quintuplet clusters (e.g. Longmore et al. 2014), and the CMZ also resides at the root of the Fermi bubbles, which are possibly star formation-driven (e.g. Su et al. 2010; Carretti et al. 2013).”

“Radio emission with Parkes-64m has been detected by Carretti et al. (2013).”

“The GCL may have a connection to large-scale polarized radio continuum emission (Carretti et al. 2013).”

“While no measurement yet exists of the outflowing plasma velocity within the putative tunnel, we argue that it must be at least as large as the velocity of the wind emanating from the overall Galactic center region, ≳ 1000 km s − 1 (Carretti et al. 2013; Fox et al. 2015; Fujita 2023).”

“Therefore, an instrument with an extended source sensitivity of ≲ 10 − 4 MeV cm − 2 s − 1 sr − 1 in the energy range between 10 MeV and 100 MeV, such as newASTROGAM (Berge et al. 2025) or other proposed instruments, is required to study the DGE variations over the entire sky.”

“…than that reported for polarization at intermediate and high Galactic latitudes, but both share morphology that may be associated with the Galactic Haze and Fermi bubbles/eROSITA lobes (Dobler et al. 2010; Su et al. 2010; Carretti et al. 2013; Krachmalnicoff et al. 2018; Pre-dehl et al. 2020).”

“for the bubble edges comes from a comparison of their morphology as seen in gamma-rays [7] and in radio [10].”

“This is consistent with the linearly polarized intensity S-PASS maps (Carretti et al. (2013)) that show high level of polarized emission in features approximately aligned with the jet propagation direction.”

“This region is visible across the electromagnetic spectrum and is commonly called the Fermi Bubbles ( FBs; e.g., J. Bland-Hawthorn & M. Cohen 2003; B. A. Keeney et al. 2006; M. Su et al. 2010; E. Carretti et al. 2013 ) .”

“Adopting a representative field of B ∼ 5 μ G, motivated by magnetic-field estimates for the radio lobes ( E. Carretti et al. 2013 ) , the several-hundred-GeV to a few-TeV electrons contributing to the LAT-band emission through inverse-Compton scattering on the cosmic microwave background have…”

“The magnetic field strength is considered to be in the range µ = ÷ B 10 30 G ( E. Carretti et al. 2013 ) .”

“…solution to the lack of a defined collimated structure, however, is that the FRAMEx targets may be exhibiting radio morphology similar to that of the bipolar symmetric radio bubbles observed around the Galactic Center ( extending 140 pc × 430 pc; E. Carretti et al. 2013; I. Heyw-ood et al. 2019 ) .”

“…as the Galactic Center Lobe (GCL; Sofue & Handa 1984), the microwave haze (Finkbeiner 2004; Planck Collaboration et al. 2013), the polarized lobes (Carretti et al. 2013), the Fermi bubbles (Su et al. 2010), the radio bubbles (Heywood et al. 2019), the X-ray chimneys (Ponti et al. 2019) and the…”

“Figure 1 shows the γ -ray (red) FB edges (dot-dashed orange curves, KG17), and the polarized synchrotron (green) putative 4 lobe-edges (double-dot dashed green curves, Carretti et al., 2013).”

“A further motivation for such an origin for the bubble edges comes from a comparison of their morphology as seen in γ-rays [6] and in radio [9].”

“In particular, the effect of convective transfer (Galactic wind) may be essential in the Galaxy as it follows from observations (see e.g. Carretti et al. 2013) as well as from theoretical treatments (Bloemen et al. 1993 and Breitschwerdt et al. 2002).”