Discrete clouds of neutral gas between the galaxies M31 and M33


Spiral galaxies must acquire gas to maintain their observed level of star formation beyond the next few billion years1. A source of this material may be the gas that resides between galaxies, but our understanding of the state and distribution of this gas is incomplete2. Radio observations3 of the Local Group of galaxies have revealed hydrogen gas extending from the disk of the galaxy M31 at least halfway to M33. This feature has been interpreted to be the neutral component of a condensing intergalactic filament4, which would be able to fuel star formation in M31 and M33, but simulations suggest that such a feature could also result from an interaction between both galaxies within the past few billion years (ref. 5). Here we report radio observations showing that about 50 per cent of this gas is composed of clouds, with the rest distributed in an extended, diffuse component. The clouds have velocities comparable to those of M31 and M33, and have properties suggesting that they are unrelated to other Local Group objects. We conclude that the clouds are likely to be transient condensations of gas embedded in an intergalactic filament and are therefore a potential source of fuel for future star formation in M31 and M33.

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Figure 1: H i column density maps.
Figure 2: A position–velocity plot of the H i clouds and selected Local Group objects.
Figure 3: A plot of baryonic mass versus the equivalent circular rotation speed.


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D.J.P. is an Adjunct Assistant Astronomer at the National Radio Astronomy Observatory; F.J.L. is an Adjunct Professor in the Department of Physics, West Virginia University. S.A.W. acknowledges the student observing support (GSSP11-012) provided by the NRAO for this project. The NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. D.J.P. acknowledges support from NSF CAREER grant AST-1149491. S.S.M. acknowledges support in part by NSF grant AST-0908370.

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All authors assisted in the development and writing of this work. S.A.W. is the Principal Investigator, and was responsible for data collection, processing and analysis. D.J.P. and F.J.L. assisted in the experimental design, data collection and analysis. S.S.M. provided the techniques of estimating the baryonic masses. E.J.S. provided necessary insights on potential interactions with M31 and ways of estimating circular rotation speeds. All authors aided in the interpretation of the results and provided comments for revisions to this work.

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Correspondence to Spencer A. Wolfe.

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Supplementary Figure

This file contains Supplementary Figure 1, which presents channels maps, as a single figure, to allow the reader to see the HI clouds' evolution through the data cube. The text points out the velocity ranges of each HI cloud and points out specific features, when apparent. (PDF 1495 kb)

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Wolfe, S., Pisano, D., Lockman, F. et al. Discrete clouds of neutral gas between the galaxies M31 and M33. Nature 497, 224–226 (2013). https://doi.org/10.1038/nature12082

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