High-Velocity Clouds in Nearby Disk Galaxies

Why is it important?

    High-velocity clouds of neutral hydrogen in our Galaxy cover approximately 20% of the sky. These clouds can be explained as arising from three sources:
  1. The warped and flared Galactic disk,
  2. The Magellanic Stream, material that has been tidally torn from the Magellanic Clouds),
  3. A galactic fountain, in which hot gas is created in associations of massive starts, forms superbubbles which break out of the Galactic disk, cools radiatively as it rises upward into the halo, and eventually recombines and returns to the disk ballistically.
    The distance to high-velocity clouds in the Galaxy is very difficult to determine, which means that we can't determine their masses, linear diameters, or densities. This problem that can be avoided by studying high-velocity clouds in external disk galaxies.

What did I do?

I observed 14 nearly face-on disk galaxies with the Arecibo 305 m telescope and found that the neutral hydrogen profiles of 10 of these galaxies had high-velocity wings. The wings can be reproduced by disk galaxy models with a high-velocity cloud component, but not by models with only warped galactic disks. The galaxies without high-velocity clouds lack active star formation, as determined from far-infrared ( IRAS ) and optical (H-alpha) observations. This is consistent with the galactic fountain model in which the young stellar population (responsible for most of the far-infrared and H-alpha emission) produces supernovae that then provide the kinetic energy of the high-velocity clouds.

Very Large Array observations of NGC 5668 confirmed the high-velocity wings detected using the Arecibo 305 m telescope . About 60% of the material in the high-velocity wings is kinematically distinct from the low-velocity gas, located primarily outside the optical disk of the galaxy, and may be infalling material comparable to the Magellanic Stream in our own Galaxy. The total mass of this kinematically distinct neutral hydrogen is about 400 million solar masses. A natural source for the rest of the wing material is halo gas produced by a galactic fountain.

Very Large Array observations of UGC 12732 confirmed the lack of high-velocity wings in this galaxy as suggested by the previous Arecibo observations. There is less than 20 million solar masses of high-velocity H I in this galaxy, consistent with the upper limit of 50 million solar masses inferred from the Arecibo observations. The velocity extent perpendicular to the disk is successfully represented by a constant Gaussian velocity dispersion of 9 km/s. UGC 12732, like the three other galaxies in the survey without high-velocity wings, has a low star formation rate, suggesting a connection between star formation and the presence of high-velocity H I in disk galaxies.

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