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

## Abstract

The Galactic Centre 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). Although these clouds have kinematics that can be modelled by a biconical nuclear wind launched from the Galactic Centre, their exact origin is unknown because there has so far been little information on their heavy metal abundances (metallicities). Here we show that FB HVCs have a wide range of metallicities from <20% of solar to ~320% of solar, based on a metallicity survey of twelve FB HVCs. These metallicities challenge the previously accepted tenet that all FB HVCs are launched from the Galactic Centre into the Fermi bubbles with solar or supersolar 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 the circumgalactic medium that the Fermi bubbles expand into, rather than material carried outwards 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 haloes while simultaneously removing gas from their disks.

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## Data availability

The HST/COS datasets for all sources used in this paper are available in MAST at https://doi.org/10.17909/zxzh-4x54 (ref. 66), including HST Program IDs 1533, 7345, 8096, 9410, 11741, 12603, 13448 and 15339, and FUSE Program IDs A108, C149, D157 and P107. The GBT raw datasets are publicly available at the NRAO archive at https://data.nrao.edu; GBT Programs for all sources in this paper with GBT data are GBT14B-299, GBT15B-359, GBT16B-422, GBT17B-015, GBT18A-221, GBT20A-253 and GBT20B-444. Data from GBT20B-444 will be available via the NRAO archive after the proprietary period ends on 9 July 2022. Green Bank 140 ft data used to calculate the H i column density limit for PG 1522+101 can be requested at https://help.nrao.edu/. LAB data used to calculate the H i column density limit for J1509-0702 are publicly available at https://www.astro.uni-bonn.de/hisurvey/AllSky_profiles. Fully reduced data are available from the corresponding author on reasonable request.

## Code availability

The GBTIDL, VPFIT, and CLOUDY software are publicly available. The GBTIDL package for GBT data reduction and analysis can be downloaded from https://gbtidl.nrao.edu/downloads.shtml. The VPFIT package for fitting Voigt profiles to absorption data can be found at https://people.ast.cam.ac.uk/~rfc/. The CLOUDY cloud-modelling code is available at https://gitlab.nublado.org/cloudy/cloudy/-/wikis/DownloadLinks.

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## Acknowledgements

We thank J. Bland-Hawthorn for valuable discussions on ionization from the Seyfert flare event. Support for T.A. through HST programme 15339 was provided by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract number NAS 5-26555. The Green Bank Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc. The GBT data presented in this paper were obtained under Program numbers GBT14B-299, GBT15B-359, GBT16B-422, GBT17B-015, GBT18A-221, GBT20A-253 and GBT20B-444.

## Author information

Authors

### Contributions

T.A. led the investigation, including the sample curation, UV and radio measurements, analysis and writing. A.J.F. led the project conceptualization and management, is the PI of the HST programme that funded the research and contributed directly to the writing of the paper. F.H.C. performed the CLOUDY modelling, contributed directly to the writing of the paper and prepared Extended Data Fig. 7. F.J.L. reduced and analysed the GBT data and contributed directly to the writing of the paper. R.B. provided the 1H1613-097 GBT data and its data reduction. B.P.W. provided the data reduction for the UV data. E.B.J. provided an interpretation of the cloud destruction and survival. T.K. prepared Fig. 2 and Supplementary Fig. 2. All authors reviewed the manuscript and contributed to the editing of the manuscript and discussion of the results’ interpretation and implications.

### Corresponding author

Correspondence to Trisha Ashley.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work

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## Extended data

### Extended Data Fig. 1 Full Metallicity Measurements.

Xi represents the UV-detected ion used for metallicity calculations and v0 UV is the FB HVCs’ LSR velocity centroid for that ion. The ion absorption and H i emission, log $${N}_{{{{{\rm{X}}}}}^{i}}$$. and NHI, respectively, are listed with the H i column errors including beam smearing added in quadrature when available. We also list the ion abundances, [Xi/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 O i solar abundance for M5-ZNG1’s elemental abundance measurement that is updated from that in the literature33,54. For a discussion of the multiple H i 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 Fig. 2 UV and H i 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 direction67. Xi represents the ion used for metallicity calculations. The UV Voigt fit parameters of Xi for each cloud are the LSR velocity centroid, v0 UV, the Doppler broadening parameter (b-value), and the log column density, log $${N}_{{{{{\rm{X}}}}}^{i}}$$. 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, v0 HI, and full-width-half-max, FWHM. For a Gaussian profile, the relation between FWHM and b-value is FWHM=1.665b. The H i log column density, log NHI, is given in the last column. 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 Fig. 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. 1938-4326’s flipped-and-subtracted spectrum is used as a check on the Gaussian H i﻿ column listed in Extended Data Fig. 2 and is discussed in Supplementary Information Section 2.

### Extended Data Fig. 4 UV and H i Fit Parameters for Literature Sight Lines.

This table lists UV and H i fit parameters for the literature FB HVCs, similar to that done for the new sight lines in Extended Data Fig. 2. M5-ZNG1 has a literature H i measurement based on a combination of FUSE profile fitting and curve-of-growth measurements34. J1509-0702 and PG 1522+10 have H i measurements based on their average rms of emission-free channels (rms of 0.055 and 0.032 K, respectively). PKS 2005-489’s log NH i is from Lyman series measurements in the literature33.

### Extended Data Fig. 5 Deconvolution of LS 4825’s H i spectrum.

Left: LS 4825’s and HD 167402’s H i spectra plotted against HD 167402’s low-velocity Milky Way component and LS 4825’s residual H i spectrum after subtracting HD 167402’s low-velocity Milky Way component. We also plot LS4825’s O i λ1302 and C i λ1560 spectrum for comparison. Right: The individual and combined Gaussian fits to LS 4825’s residual negative-velocity components.

### Extended Data Fig. 6 CLOUDY Model Results.

Xi represents the ion used for the metallicity calculations and v0 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 Surveys17,20; 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 nH and log NH, respectively. The depth is the calculated size of the cloud along the line-of-sight. The present-day ionization corrections are given as IC(Xi) Eq. for the equilibrium models, and IC(Xi) Φ(H)8 and IC(Xi) Φ(H)10 for the time-dependent models at two ionizing photon fluxes, log Φ(H)=8 and 10, respectively.

### Extended Data Fig. 7 Time-dependent ionization corrections versus time calculated from CLOUDY models.

The results are given for two ionizing fluxes: log Φ(H)= 8 (solid line) and 10 (dashed line), where Φ(H) has units of photons cm−2 s−1 and where colored shading shows intermediate ionizing fluxes. Each panel shows the ionization correction across the full time interval modeled (0-3.6 Myr), with an inset plot magnifying the flatter part of the curves after the initial flash to emphasize the late-time behavior. The equilibrium results are marked with an ‘x’ at 3.6 Myr in each panel.

## Supplementary information

### Supplementary Information

Supplementary Discussion Sections 1–5, Tables 1 and 2 and Figs. 1 and 2.

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Ashley, T., Fox, A.J., Cashman, F.H. et al. Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo. Nat Astron 6, 968–975 (2022). https://doi.org/10.1038/s41550-022-01720-0

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• DOI: https://doi.org/10.1038/s41550-022-01720-0