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Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo

The Galactic Center is surrounded by two giant plasma lobes known as the Fermi Bubbles, extending ~10 kpc both above and below the Galactic plane. Spectroscopic observations of Fermi Bubble directions at radio, ultraviolet, and optical wavelengths have detected multi-phase gas clouds thought to be embedded within the bubbles referred to as Fermi Bubble high-velocity clouds (FB HVCs). While these clouds have kinematics that can be modeled by a biconical nuclear wind launched from the Galactic center, their exact origin is unknown because, until now, there has been little information on their heavy-metal abundance (metallicity). Here we show that FB HVCs have a wide range of metallicities from<20% solar to ~320% solar. This result is based on the first metallicity survey of FB HVCs. These metallicities challenge the previously accepted tenet that all FB HVCs are launched from the Galactic center into the Fermi Bubbles with solar or super-solar metallicities. Instead, we suggest that FB HVCs originate in both the Milky Way's disk and halo. As such, some of these clouds may characterize circumgalactic medium that the Fermi Bubbles expand into, rather than material carried outward by the nuclear wind, changing the canonical picture of FB HVCs. More broadly, these results reveal that nuclear outflows from spiral galaxies can operate by sweeping up gas in their halos while simultaneously removing gas from their disks.

ionized the Magellanic Stream [5][6][7] . However, star formation in the Galactic center cannot be ruled out as a partial contributor to the origin and growth of the bubbles 8,9 . Their large angular extent (±55°in latitude and ±20°in longitude [10][11][12] ) allows us to resolve a nuclear outflow and study its effect on the baryons of a large spiral galaxy in unparalleled detail.
The bubbles have been detected across the electromagnetic spectrum in gamma-rays, microwaves, polarized radio emission, and X-rays 4,[13][14][15] . Spectroscopic studies in UV metal line absorption, and atomic hydrogen and molecular emission have revealed populations of cool gas clouds embedded within the Bubbles [16][17][18][19][20][21][22][23][24] . These Fermi Bubble high-velocity clouds (FB HVCs) are identified by their projected location towards the gamma-ray-defined bubbles and their velocities, which cannot be accounted for by Galactic rotation (typically exceeding absolute velocities of ∼90 km s −1 ). FB HVCs are thought to be associated with the bubbles because their UV-absorption covering fraction (80%) far exceeds that of HVCs directly outside of the Fermi Bubbles that are similarly not co-rotating with the Milky Way disk (28%) [17] .
Previously, all FB HVCs have been assumed to originate in the disk of the Milky Way, becoming entrained in the Fermi Bubbles and traveling to higher Galactic latitudes 16,17,20 . FB HVCs detected in H I radio surveys are close to the Milky Way disk's central H I cavity 25 (|b| < 10°) and have kinematics that can be successfully modeled with a biconical outflow.
As such, they are thought to originate in the disk of the Milky Way. However, UV-detected FB HVCs seen at Galactic latitudes up to 55°(a projected distance of ∼10 kpc from the disk) are not so clearly connected to the Milky Way disk and have never had their origin confirmed. Simulations have shown that it is difficult to accelerate a cool cloud in a galactic outflow without destroying it [26][27][28][29] , as the cloud has to survive the shock and shear of the outflow. Instead, some of the high-latitude UV-detected FB HVCs may be halo clouds that pre-existed the formation of the bubbles.
Metallicity measurements (heavy metal abundances) from H I and UV data can directly constrain the origin of the FB HVCs, because clouds driven into the bubbles from the Galactic disk are expected to show solar (or super-solar) metallicities, whereas halo clouds should have sub-solar metallicities [30][31][32] . We have conducted the first metallicity survey of FB HVCs by combining measurements of three newly-detected FB HVCs with measurements of nine clouds categorized as FB HVCs in the literature 17,18,20,33 . We calculate new ionization corrections and metallicities for the literature sample in all but one case (see the Methods section for more details). The three newly-detected FB HVCs each have UV spectra from the Cosmic Origins Spectrograph on the Hubble Space Telescope (HST/COS) as well as H I maps and deep single-pointing H I spectra from the Green Bank Telescope (GBT). In shown to sporadically deplete (up to 1 dex in the dustiest clouds) 34 , sulfur is not typically thought to strongly deplete onto dust 35 . We therefore use these two ions for metallicity measurements. We have selected our sample to contain all known FB HVCs with detected O I or S II absorption and associated H I detections. Another measurement included in our sample is a FB HVC toward J1938-4326, which has a C II λ1334 detection and an associated  [17] . In Figure 2 we plot the O I or C II absorption profiles and H I single-pointing emission profile for the three newly-detected FB HVCs in our sample.
We find that four FB HVCs in our sample have low metallicities of ≤ 21% solar (one of which is a lower limit), five have metallicities between ≥ 30% and ≥ 59% solar, and three have near solar or supersolar metallicities of ≥ 85% solar (see Table 1). The low metallicities (∼ 20% solar) are indicative of Galactic halo clouds 30,31 and are similar to that measured for the X-ray component of the Fermi Bubbles using Suzaku X-ray data 36  solar. These results challenge the previously accepted picture that all FB HVCs were driven out from the Galactic center with solar or even super-solar metallicity.
One potential explanation for the sub-solar metallicities observed in many of the FB HVCs is that the clouds originate in the Milky Way disk with solar metallicity and then become diluted to subsolar metallicities via metal mixing as they travel through the Fermi Bubbles to higher latitudes. However, as we discuss below, we find that metal mixing cannot explain FB HVC metallicities of Z 20% solar. Mixing can occur when cold gas clouds traveling through a hot ambient medium leave a wake of stripped gas behind them.
The stripped gas then mixes with the ambient gas reducing its temperature enough to allow for condensation of the hot medium onto the stripped cold cloudlets, resulting in a cloud with a metallicity between the cold cloudlets and hot plasma 39,40  We suggest an alternative explanation. FB HVCs have a large range in metallicities because they have two origins: the Milky Way's disk and its halo. In this picture, one population of FB HVCs is composed of the low-latitude FB HVCs that originate in the disk of the Milky Way and are accelerated away from the Galactic center by the same mechanism that created the Fermi Bubbles. These cool gas clouds could be accelerated by the hot gas flow and eventually become eroded as their mass transfers from a cool to hot phase 29,45 . The remnants of many of these clouds likely become too dispersed, ionized, and hot to be seen in the UV as they move to higher latitudes. The second population of FB HVCs is composed of pre-existing halo clouds that are shocked and accelerated as the Fermi Bubbles expand into the halo. As such, they represent the medium into which the Fermi Bubbles expand, rather than material carried out by the Milky Way's nuclear outflow.
The division between the disk and halo clouds does not occur at a single latitude, as evidenced by the FB HVC toward M5-ZNG1 (high metallicity at a high latitude of 47°).
This outlier suggests that the outflow is complex and that some disk clouds may be able to survive to higher latitudes in the bubbles. While it is not clear how these clouds survive the shock and shear of the Fermi Bubbles, a range of ideas have been suggested from internal magnetic fields suppressing the destruction of the clouds to radiative cooling increasing a cloud's lifetime in a hot outflow [26][27][28][29] . In any case, the UV-detected FB HVCs reported in this paper have a large range of metallicities from < 20% solar to 320% solar, a result that requires a revision to the canonical picture of FB HVCs and indicates that biconical outflow models need to be refined to account for the clouds' dual origins.
Our new results on the metallicity and origin of FB HVCs reveal the properties of a nuclear outflow in more detail than is possible in any other spiral galaxy. As such, the Fermi Bubbles serve as an important analog for extragalactic nuclear outflows, which can typically only be studied with one-dimensional down-the-barrel pointings. In general, galactic outflows and outward-moving shocks are thought to prevent inflowing cool gas clouds from reaching the disk of a galaxy, thus quenching the star-formation process 46    No H I emission associated with J1853-4158's FB HVC was detected at the sensitivity of the data. J1919-2958's H I column limits were computed using the velocity range indicated by the grey shaded region after flipping the spectrum around its peak emission and subtracting the resulting spectrum from the original, as discussed in detail in the Methods section. J1938-4326's Gaussian fit to the FB HVC H I emission is indicated by a red line. The references give the source for the UV and H I measurements (TW=This Work). The Galactic latitude and longitude are given as l and b, respectively. The type of each background source is listed as quasar or the stellar spectral type. X i represents the ion used for metallicity calculations. The UV Voigt fit local standard of rest (LSR) velocity centroid is v 0 UV . Z represents the corrected elemental abundances in linear form (for further details on Z calculations, see the Methods Section and Extended Data Fig. 1). J1919-2958's metallicity is represented by a range due to stray radiation effects (see the Methods section for details). LS 4825 has four detected clouds, listed separately. The M5-ZNG1's measurement applies to two absorption components that are blended in H I. We treat them as a single cloud because only an average metallicity measurement is possible. PKS 2005-489's metallicity measurement is a literature measurement made by comparing UV ion and Lyman measurements to other Milky Way HVCs 33 . J1938-4326's metallicity limit includes an estimate for dust depletion and is a range due to multiple H I measurements, as explained in detail in Supplementary Information Section 1. have not had their metallicities previously calculated and many of the metallicity calculations in the literature do not include an ionization correction, an important correction for these clouds that are exposed to the Galactic, extragalactic, and Seyfert flare ionizing radiation fields, in addition to cosmic rays. Table 1

Observations and Data Reduction for Three New Sight Lines
The three new targets were selected from the UV-bright QSO Survey (UVQS) catalog 50  we are unable to confirm that there are two absorption features contributing to the FB HVC.
Therefore, we treat the absorption and emission associated with the FB HVC as one cloud.
The GBT single-pointing H I spectra were calibrated using standard techniques 51  The H I column density for J1919-2958 and 1938-4326 was calculated using the standard relation: where where ∆v is the channel width in km s −1 , and n is the number of H I spectrum channels in

Beam-Smearing Analysis
We

Metallicity Measurements
We calculate the metallicities from the ion abundances [X i /H] as: where X is the element under study, X i is the observed ion of that element, IC is the ionization correction, N X

Equilibrium Models
We first model the time-independent ICs for an ionization equilibrium state in CLOUDY.
Physically these models describe the steady-state ionization conditions of clouds existing in the halo before a Seyfert flash event. These models are used as input parameters for the time-dependent models and are later used to confirm whether the time-dependent models strongly affect the present-day ionization corrections.
We adopt a plane-parallel geometry with a slab of uniform density illuminated on both sides by a position-dependent combined Galactic and extragalactic radiation field [56][57][58] . The field has a normalization scaled to a distance of 10  Si II absorption associated with J1938-4326's FB HVC was not detected. We instead use the Si III/C II ratio assuming that both ions have the same abundance relative to solar. Once a value for log n H was determined for each FB HVC, the model was run again for that specific value in order to obtain exact model results.

Non-Equilibrium Time-dependent Models
There is the strong evidence that a ∼3. an Eddington fraction f E = 0.3 [6] . We run our models for the range 8 < log Φ(H) <

Corresponding author
Correspondence to Trisha Ashley.  Figure 1: Full Metallicity Measurements. X i represents the UV-detected ion used for metallicity calculations and v 0UV is the FB HVCs' LSR velocity centroid for that ion. The ion absorption and HI emission, log N X i . and N HI , respectively, are listed with the HI column errors including beam smearing added in quadrature when available. We also list the ion abundances, [X i /H]. The ionization correction, IC, calculations are discussed throughout the Methods section. [X/H] is the corrected gas-phase elemental abundance and does not account for dust depletion. We include an OI solar abundance for M5-ZNG1's elemental abundance measurement that is updated from that in the literature 33,54 . For a discussion of the multiple HI measurements for J1919-2958, see the Methods Section. For a discussion of dust depletion and the multiple H I measurements for J1938-4326, see Supplementary Information Section 1.  Extended Data Figure 2: UV and HI Fit Parameters for New Sight Lines. The second column represents the allowed velocity range of gas co-rotating with the Milky Way disk in each quasar's direction 66 . X i represents the ion used for metallicity calculations. The UV Voigt fit parameters of X i for each cloud are the LSR velocity centroid, v 0 UV , the Doppler broadening parameter (b-value), and the log column density, log(N X i ). The UV velocity centroid errors include the 7.5 km s −1 COS zero-point uncertainty. The H I Gaussian fit parameters are the LSR velocity centroid, v 0 HI , full-width-half-max, FWHM, and log column density, log N H I . For a Gaussian profile, the relation between FWHM and b-value is FWHM=1.665b. J1853-4158's H I column was obtained using the spectrum's rms, as described in the Methods Section. J1919-2958's H I column was obtained by through the "flip-and-subtract" method (described in the Methods Section) using two velocity ranges for integration, which encompass all potential emission associated with the FB HVC (upper column limit) and emission least affected by stray radiation (lower column limit). J1938-4326's H I column was obtained using a Gaussian fit to emission. Second and third measurements of the H I column were made by integrating over the C II absorber's FWHM and then using Equation 2; see Supplementary Information Section 1 for more details.

Extended Data Figures
Extended Data Figure 3: Flipped-and-subtracted GBT H I spectrum for J1919-2958 and 1938-4326. The blue shaded spectrum represents the original H I spectrum at a resolution of ∼1 km s −1 . The maximum velocity range used to calculate the H I column densities is shaded in grey. The black line is the flipped-and-subtracted spectra smoothed to ∼2 km s −1 in the integrated velocity ranges including an additional 7 channels on each side of those velocity ranges. 2 Extended Data Figure 6: X i represents the ion used for the metallicity calculations and v 0 UV is the UV velocity centroid of each component. The logarithmic Si III to Si II ion ratio, [ Si III /Si II, is calculated using the column densities measured in previous Fermi Bubble UV Surveys 17,20 ; for J1938-4326 we use the [Si III /C II] ratio. U is the ionization parameter, equal to the ratio of the ionizing photon density to the gas density. The log hydrogen number density and column density of the clouds are given as log n H and log N H , respectively. The depth is the calculated size of the cloud along the line-of-sight. The present day ionization corrections are given as IC(X i ) Eq. for the equilibrium models, and IC(X i ) Φ(H) 8  Dust depletion does not strongly affect oxygen measurements and is largely thought to not strongly affect sulfur measurements 34,35 . However, dust depletion can be significant for carbon, which is used to measure the metallicity for J1938-4326. Without taking into account dust depletion affects, the log of J1938-4326's gas-phase elemental abundance is 0.28% solar (see Extended Data Fig. 1). In the densest clouds, carbon levels can be depleted by up to −0.58 dex 34,68 . That would raise the metallicity of the FB HVC towards J1938-4326 to 1% solar (or −1.98 dex in log space), a resolutely low metallicity.

Sight line X i UV Voigt Fit Parameters H I Integration H I Gaussian Fit log N HI
With such a low metallicity, we also explore the possibility that the H i spectrum towards J1938-4326 may have emission from multiple clouds contributing to the H i Gaussian fit shown in Figure 2, which may not all be associated with the FB HVC towards J1938-4326.
To do this, we integrate the H i emission over the velocity range of the C ii absorption's FWHM and centered on the C ii absorption's v 0 UV (−127 to −87 km s −1 ). This results in a log N HI = 18.20. Assuming the same IC as listed in Extended Data Fig. 1 and a dust depletion of −0.58 dex, the resulting metallicity would still only reach 2.7% solar. If we instead assume that none of the H i detected in this velocity range is associated with the FB HVC and use Equation 2 to estimate an upper limit on the H i column density, then we measure log N HI ≤ 17.62; assuming the same IC and dust depletion, the metallicity is raised to only 10% solar. Therefore, the data indicate that this FB HVC has a low metallicity even when dust depletion is taken into account. In Table 1 we include J1938-4326's metallicity as a range between that which is calculated using the Gaussian fit to the H i and that which is calculated using Equation 2; both limits contain a dust depletion level of −0.58 dex. We also include the various H i column measurements in Extended Data Fig. 1 and 2. 2 Check of J1938-4326's HI Gaussian fit To verify the Gaussian-fit H i column density measurement of J1938-4326 in Extended Data

Effects of the XX-polarization on GBT maps
All XX-polarization data has been discarded in the GBT H i emission maps presented in Figure 1 due to spurious features at levels of tens of mK produced by frequency switching.
Typically these features can be ignored due to their low levels; however, H i emission of the FB HVCs in this work also appear on the level of 10-20 mK (see Supplementary Fig. 1), making it possible to misinterpret these spurious features as emission. The YY polarization only suffers minor spurious waves that are not likely to be misinterpreted as emission. An example of the difference between the YY-only and XX-YY combined spectra from the GBT maps is shown in Supplementary Fig. 1. In the single-pointing H I data near the emission velocity regions these spurious features were not as pronounced; therefore, we left the XXpolarization in the single-pointing data to obtain the better signal-to-noise for the column density measurements presented in Extended Data Fig. 2  tend to be near zero for log N HI >18.5 [17] . We therefore test the robustness of these metallicity measurements by changing the saturated log N Si III measurement by 1 dex and recalculating the metallicities using the ionization equilibrium models. We show the model parameters and results in Supplementary Table 2. The metallicity measurements for 1H1613-097 and J1919-2958's upper H I column density limit from a 1 dex increase in Si III columns are approximately within the metallicity errors listed in Extended Data Fig. 1. This 1 dex increase shows that the metallicity results are not strongly dependent on Si III saturation for high H I columns. Conversely, J1919-2958's lower H I column density limit (log N HI =18.06) does have a significant change in IC of about −0.41 dex when the Si III measurement increases by 1 dex. We note, however, that J1919-2958's lower limit H I column model results for the increase in 1 dex give an unrealistic cloud depth of ∼6 kpc. Additionally, the CLOUDY model with log N Si III =13.71 has predicted ion columns significantly closer to the measured columns, with an average absolute relative error on the linear ion columns of 2.2, whereas the model with log N Si III =14.71 has an average absolute relative error of 25. We therefore remove the metallicity measurements' lower limits in Table 1 and Extended Data Fig. 1 show a significant change in metallicity; however, the model produces unrealistic cloud depths and ion columns that have high average absolute relative errors on the ion column densities.