A Very Sensitive 21 centimeter Survey for Galactic High-Velocity H I
F. J. Lockman, E. M. Murphy, S. Petty-Powell, V. J. Urick;
ApJS, 2002, 140, 331
ABSTRACT:Very sensitive H I 21 cm observations have been made in 860 directions at δ>=-43deg in search of weak, Galactic, high-velocity H I emission lines at
moderate and high Galactic latitudes.
One-third of the observations were made
toward extragalactic objects that are visible at optical and UV
wavelengths.
The median rms noise in the survey spectra is 3.4 mK, resulting in a median 4
σ detection level of
NHi=8×1017 cm-2 averaged over the 21' beam of the
telescope.
High-velocity H I emission is detected in 37% of the directions; about half of the lines
could not have been detected in previous surveys.
The median FWHM of
detected lines is 30.3 km s-1.
High-velocity H I lines are seen down
to the sensitivity limit of the survey, implying that there are likely
lines at still lower values of NHi.
The weakest
lines have a kinematics and distribution on the sky similar to that of the
strong lines and thus do not appear to be a new population.
Most of the emission
originates from objects which are extended over several degrees; only a few appear
to be compact sources.
At least 75%, and possibly as many as 90%, of the
lines are associated with one of the major high-velocity complexes.
With
the increased sensitivity of this survey, the Magellanic Stream is seen to
extend at least 10° to higher Galactic latitude than previously thought
and to be more extended in longitude as well.
Wright's Cloud near M33 has an
extended low-NHi component in the direction of the
Magellanic Stream.
The bright H I features which have dominated most surveys may
be mere clumps within larger structures, and not independent
objects.
Although there are many lines with low column density, their numbers do not
increase as rapidly as N-1Hi, so most of
the H I mass in the high-velocity cloud phenomenon likely resides in the
more prominent clouds.
KEYWORDS: galaxy: halo, galaxy: structure, ism: clouds, surveys
PERSOKEY:h_i, 21 cm, hvc, ,
CODE: lockman2002