Tiny-Scale Atomic Structure and the Cold Neutral Medium

摘要

We consider the tiny-scale atomic structure (TSAS) on scales of tens of astronomical units, which has been detected by 21 cm absorption lines against quasars with VLBI techniques, against pulsars with time variability and against close binary stars in optical interstellar lines. The TSAS is associated with ordinary cold neutral medium (the CNM). Under the conventional interpretation, the thermal pressure of the TSAS gas is extremely high, some 44 times greater than the Galactic hydrostatic pressure at z = 0 and 300 times greater than the standard CNM thermal pressure. This is unacceptable because the TSAS is quiescent, ubiquitous, and appears to reside in all CNM clouds. Moreover, under the conventional interpretation, H2 should be very abundant in the TSAS, thus exacerbating the pressure problem and also leading to very large extinctions.