Small-scale H i Channel Map Structure Is Cold: Evidence from Na i Absorption at High Galactic Latitudes

The spatial distribution of neutral hydrogen (H i) emission is a powerful probe of interstellar medium physics. The small-scale structure in H i channel maps is often assumed to probe the velocity field rather than real density structures. In this work we directly test this assumption, using high-re...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2019-11, Vol.886 (1), p.L13
Main Authors: Peek, J. E. G., Clark, S. E.
Format: Article
Language:English
Subjects:
Absorption
Columnar structure
Density
Emission
Hydrogen
Interstellar absorption
Interstellar atomic gas
Interstellar magnetic fields
Interstellar matter
Interstellar medium
Quasar absorption line spectroscopy
Quasars
Sky surveys (astronomy)
Spatial distribution
Velocity
Velocity distribution
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Summary:The spatial distribution of neutral hydrogen (H i) emission is a powerful probe of interstellar medium physics. The small-scale structure in H i channel maps is often assumed to probe the velocity field rather than real density structures. In this work we directly test this assumption, using high-resolution GALFA-H i observations and 50,985 quasar spectra from the Sloan Digital Sky Survey. We measure the equivalent widths of interstellar Na i D1 and Na i D2 absorption, and robustly conclude that together they depend nearly four times as strongly on the column density of small-scale structure in H i than on either the large-scale H i structure or the total H i column. This is inconsistent with the hypothesis that small-scale channel map structure is driven by velocity crowding. Instead, the data favor the interpretation that this emission structure predominantly originates in cold, dense interstellar material, consistent with a clumpy cold neutral medium.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/ab53de