Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The Galaxy’s veil of excited hydrogen


Many of the baryons in our Galaxy probably lie outside the well-known disk and bulge components. Despite a wealth of evidence for the presence of some gas in galactic halos—including absorption line systems in the spectra of quasars, high-velocity neutral hydrogen clouds in our Galaxy halo, line-emitting ionized hydrogen originating from galactic winds in nearby starburst galaxies and the X-ray coronas surrounding the most massive galaxies—accounting for the gas in the halo of any galaxy has been observationally challenging, primarily because of the low density in these expansive regions. The most sensitive measurements come from detecting absorption due to the intervening gas in the spectra of distant objects, such as quasars or distant halo stars, but these have typically been limited to a few lines of sight to sufficiently bright objects. Extensive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present evidence for a newly discovered, widely distributed, neutral, excited hydrogen component of the Galaxy’s halo. It is observed as the slight (0.779 ± 0.006%) absorption of flux near the rest wavelength of Hα in the combined spectra of hundreds of thousands of galaxy spectra and is ubiquitous in high-latitude lines of sight. This observation provides an avenue to tracing, both spatially and kinematically, the majority of the gas in the halo of our Galaxy.

This is a preview of subscription content, access via your institution

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Combined, continuum-normalized, line of sight spectrum in the region of rest frame Hα for >700,000 SDSS galaxy spectra.
Figure 2: Apparent motion of the absorbing gas as a function of longitude measured from the Hα and 6,496Å absorption lines.
Figure 3: Mean Hα absorption map calculated within a wavelength window centred on rest frame Hα that extends ±700kms1 in the SDSS continuum-normalized galaxy spectra.


  1. Bregman, J. N. The search for the missing baryons at low redshift. Ann. Rev. Astron. Astrophys. 45, 221–259 (2007).

    ADS  Article  Google Scholar 

  2. Werk, J. K. et al. The COS-Halos survey: physical conditions and baryonic mass in the low-redshift circumgalactic medium. Astrophys. J. 792, 8–28 (2014).

    ADS  Article  Google Scholar 

  3. Zaritsky, D. & Courtois, H. A dynamics-free lower bound on the mass of our Galaxy. Mon. Not. R. Astronom. Soc. 465, 3724–3728 (2017).

    ADS  Article  Google Scholar 

  4. Wakker, B. P. & van Woerden, H. Distribution and origin of high-velocity clouds. III – Clouds, complexes and populations. Astron. Astrophys. 250, 509–532 (1991).

    ADS  Google Scholar 

  5. Weiner, B. J. & Williams, T. B. Detection of H(alpha) emission from the Magellanic stream: evidence for an extended gaseous galactic halo. Astron. J. 111, 1156–1163 (1996).

    ADS  Article  Google Scholar 

  6. Sembach, K. R. et al. Highly ionized high-velocity gas in the vicinity of the Galaxy. Astrophys. J. Suppl. Ser. 146, 165–208 (2003).

    ADS  Article  Google Scholar 

  7. Collins, J. A., Shull, J. M. & Giroux, M. L. Hubble Space Telescope survey of interstellar high-velocity Si iii. Astrophys. J. 705, 962–977 (2009).

    ADS  Article  Google Scholar 

  8. Kerp, J. et al. A search for soft X-ray emission associated with prominent high-velocity-cloud complexes. Astron. Astrophys. 342, 213–232 (1999).

    ADS  Google Scholar 

  9. Murga, M., Zhu, G., Ménard, B. & Lan, T.-W. Calcium H & K and sodium D absorption induced by the interstellar and circumgalactic media of the Milky Way. Mon. Not. R. Astronom. Soc. 452, 511–519 (2015).

    ADS  Article  Google Scholar 

  10. Gupta, A., Mathur, S., Krongold, Y., Nicastro, F. & Galeazzi, M. A huge reservoir of ionized gas around the Milky Way: accounting for the missing mass? Astrophys. J. Lett. 756, L8 (2012).

    ADS  Article  Google Scholar 

  11. Miller, M. J. & Bregman, J. N. Constraining the Milky Way’s hot gas halo with O vii and O viii emission lines. Astrophys. J. 800, 14–42 (2015).

    ADS  Article  Google Scholar 

  12. Nicastro, F., Senatore, F., Krongold, Y., Mathur, S. & Elvis, M. A distant echo of Milky Way central activity closes the galaxy’s baryon census. Astrophys. J. Lett. 828, L12 (2016).

    ADS  Article  Google Scholar 

  13. Zhang, H., Zaritsky, D., Zhu, G., Ménard, B. & Hogg, D. W. Hydrogen emission from the ionized gaseous halos of low-redshift galaxies. Astrophys. J. 833, 276–286 (2016).

    ADS  Article  Google Scholar 

  14. Alam, S. et al. The eleventh and twelfth data releases of the Sloan Digital Sky Survey: final data from SDSS-III. Astrophys. J. Suppl. Ser. 219, 12–38 (2015).

    ADS  Article  Google Scholar 

  15. Fragione, G. & Loeb, A. Constraining Milky Way mass with hypervelocity stars. Preprint at (2016).

  16. Putman, M. E., Peek, J. E. G. & Joung, M. R. Gaseous galaxy halos. Ann. Rev. Astron. Astrophys. 50, 491–529 (2012).

    ADS  Article  Google Scholar 

  17. Le Borgne, J.-F. et al. STELIB: a library of stellar spectra at R 2000. Astron. Astrophys. 402, 433–442 (2003).

    ADS  Article  Google Scholar 

  18. Dehnen, W. & Binney, J. J. Local stellar kinematics from HIPPARCOS data. Mon. Not. R. Astronom. Soc. 298, 387–394 (1998).

    ADS  Article  Google Scholar 

  19. Zhu, G. et al. The large-scale distribution of cool gas around luminous red galaxies. Mon. Not. R. Astronom. Soc. 439, 3139–3155 (2014).

    ADS  Article  Google Scholar 

  20. Wiese, W. & Fuhr, J. Accurate atomic transition probabilities for hydrogen, helium, and lithium. J. Phys. Chem. Ref. Data 38, 565–719 (2009).

    ADS  Article  Google Scholar 

  21. Barger, K. A., Haffner, L. M. & Bland-Hawthorn, J. Warm ionized gas revealed in the Magellanic Bridge tidal remnant: constraining the baryon content and the escaping ionizing photons around dwarf galaxies. Astrophys. J. 771, 132–151 (2013).

    ADS  Article  Google Scholar 

  22. Fox, A. J. et al. Exploring the origin and fate of the Magellanic Stream with ultraviolet and optical absorption. Astrophys. J. 718, 1046–1061 (2010).

    ADS  Article  Google Scholar 

  23. Haffner, L. M. et al. The Wisconsin Hα Mapper Northern Sky Survey. Astrophys. J. Suppl. Ser. 149, 405–422 (2003).

    ADS  Article  Google Scholar 

  24. Boisse, P., Le Brun, V., Bergeron, J. & Deharveng, J.-M. A. HST spectroscopic study of QSOs with intermediate redshift damped Lyalpha systems. Astron. Astrophys. 333, 841–863 (1998).

    ADS  Google Scholar 

  25. Kaplan, K. F., Prochaska, J. X., Herbert-Fort, S., Ellison, S. L. & Dessauges-Zavadsky, M. H. I. Column densities, metallicities, and dust extinction of metal-strong damped Lyα systems. Publ. Astronom. Soc. Pacif. 122, 619–635 (2010).

    ADS  Article  Google Scholar 

  26. Neeleman, M. et al. The H I content of the Universe over the past 10 Gyrs. Astrophys. J. 818, 113–122 (2016).

    ADS  Article  Google Scholar 

  27. Bolton, A. S. et al. Spectral classification and redshift measurement for the SDSS-III Baryon Oscillation Spectroscopic Survey. Astron. J. 144, 144–163 (2012).

    ADS  Article  Google Scholar 

Download references


D.Z. and H.Z. acknowledge financial support from NASA (the National Aeronautical and Space Administration) ADAP NNX12AE27G, the National Science Foundation AST-1311326 and the University of Arizona. The authors thank the Sloan Digital Sky Survey III (SDSS-III) team for providing a valuable resource to the community. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation and the US Department of Energy Office of Science. The SDSS-III website is available at SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration, including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington and Yale University.

Author information

Authors and Affiliations



Both authors contributed to the final analysis and interpretation of the results. H.Z. led the data analysis. D.Z. provided the initial motivation for the programme.

Corresponding author

Correspondence to Huanian Zhang.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Zhang, H., Zaritsky, D. The Galaxy’s veil of excited hydrogen. Nat Astron 1, 0103 (2017).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI:


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing