A Brief discussion of interstellar HI structure with special reference to filaments Gerrit Verschuur Physics Department, University of Memphis

Goals for the talk To demonstrate the essentially filamentary nature of the anomalous velocity HI, in particular. Zoom in one some of the areas within the larger filamentary features. Raise questions about the origin of such structures. Seek directions in which answers may be found.

Data used Leiden-Argentina-Bonn HI survey: LAB survey All-sky 36’ resolution 1.03 km/s bandwidth Side-lobe corrected Kalberla et al. (2005)

All-sky Total HI -450 to +450 km/s

The area of sky under consideration High northern galactic latitudes from b= +30° to +70° Also southern latitude data in same intervals Then focus on some details

Orienting us in galactic space Low-velocity gas

Northern Galactic hemisphere HI data The sequences that follow show HI area maps that reveal HI features changing position as the line-of-sight velocity shifts. Some of these can be understood as flow along filaments but other features show large scale “winds”perhaps. NH

V = -82 km/s Corkscrew

Corkscrew close-up

Filamentary features can be followed through velocity space MI MII Complex C “Twisted” Corkscrew

Two position-velocity plots across the major anomalous- velocity filament

In the northern galactic hemisphere the low-velocity HI is severely disturbed. Its l.o.s. velocity has shifted. MI (no LV gas) MII Longitude-velocity plot Baseline structure “normal” Baseline structure “normal”

Southern galactic hemisphere b = -65° For comparison, this plot shows “normal” low velocity HI emission due to local gas Longitude-velocity plot

SH Southern Galactic hemisphere data

Similarities and differences w.r.t NH data The “moving” shell-like feature in the SH is diametrically opposite the feature in the NH. The velocity vectors are confusing

“Clouds” may be illusions created by enhancements of emission where segments of filaments twist into the line-of-sight

Accounting for velocity gradients Streaming motions

Clouds versus filaments in detail MII Filamentary structures at all velocities

Structure in maps of the “cloud” MII could be interpreted as rotation However:

MII area showing wave patterns MII

“Twisted” in color

The ridge lines of “Twisted” overlain on a continuum radio map

What is lacking All theoretical discussions of diffuse interstelllar HI are based on the “cloud” concept. What is needed is a comprehensive theory to account for high complex, twisted filaments controlled by magnetic fields. Only then can be begin to understand the earliest stages of star formation!

Summary A physics of interstellar filaments is needed that brings in: Plasma phenomena Motions along filaments - currents Plasma instabilities Are these filaments all at the surfaces of old shells? And/or is gas “blowing” along field lines from regions of recent star formation?

The End For now!

The associations between the absence of low- velocity gas and the presence of anomalous velocity HI

Longitude-velocity plot

Implications for distance Anomalous velocity gas must be very local