WHAM Observations of Hα Emission from High‐Velocity Clouds in the M, A, and C Complexes

Abstract

The first observations of the recently completed Wisconsin H-Alpha Mapper (WHAM) facility include a study of emission lines from high velocity clouds in the M, A, and C complexes, with most of the observations on the M I cloud. We present results including clear detections of Hα emission from all three complexes with intensities ranging from 0.06 to 0.20 R. In every observed direction where there is significant high velocity H I gas seen in 21 cm emission we have found associated ionized hydrogen emitting the Hα line. The velocities of the Hα and the 21 cm emissions are well correlated in every case except one, but the intensities are not correlated. There is some evidence that the ionized gas producing the Hα emission envelopes the 21 cm emitting neutral gas but the Hα "halo", if present, is not large. If the Hα emission arises from the photoionization of the H I clouds, then the implied incident Lyman continuum flux F LC at the location of the clouds ranges from 1.3 to 4.2 × 10 5 photons cm −2 s −1 . If, on the other hand, the ionization is due to a shock arising from the collision of the high-velocity gas with an ambient medium in the halo, then the density of the pre-shocked gas can be constrained. We have also detected the [S II] λ6716 line from the M I cloud and have evidence that the [S II] λ6716 to Hα ratio varies with location on the cloud.

Detection of [N [CSC]ii[/CSC]] λ5755 Emission from Low-Density Ionized Interstellar Gas

“…The observations were made with the WHAM spectrometer, located on Kitt Peak and operated remotely from Madison, Wisconsin Tufte 1997). In 1998 October and 1999 November, long-integration spectra were obtained of a 3.8 Å (200 km s Ϫ1 )-wide spectral interval centered near the LSR velocity of the [N ii] l5755 line.…”

“…This technique is described by and has been used to detect very faint optical emission lines from highvelocity H i clouds. Observations of the fainter, off-source directions are necessary because the spectra are contaminated by very weak, unidentified atmospheric emission lines that are comparable to or brighter than the interstellar emission (N. R. Hausen, R. J. Reynolds, L. M. Haffner, & S. L. Tufte 2001, in preparation;Tufte et al 1998). By subtracting the average of the off-source spectra from the on-source spectrum, we obtain a pure interstellar spectrum of [N ii] l5755 toward l p , .…”

Diffuse Ionized Gas in Spiral Galaxies and the Disk-Halo Interaction

“…The low Hα surface brightness of High Velocity Clouds (HVCs) in the halo of the Galaxy, approximately 1/10 that associated with the WIM (e.g., Tufte et al 1998), implies that the source of the ionization of the WIM must be within the disk. In the solar neighborhood, the H-ionization rate needed to sustain the WIM is about 1/7 that available from stellar Lyman continuum photons.…”

A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies

“…The WHAM instrument is a spectrometer located at the Kitt Peak National Observatory and is remotely operated from Madison, Wisconsin. WHAM consists of a 0.6 m siderostat coupled with a 15 cm dual-etalon Fabry-Perot system (Tufte 1997;Reynolds et al 1998). It produces an optical spectrum integrated over its circular, 1 diameter field of view within a 200 km s À1 wide spectral window, centered on any wavelength between 4800 and 7400 Å , with a resolution of 12 km s À1 .…”