Imaging and spectroscopy of the multiphase halo of NGC 4631

We have performed narrowband imaging of NGC 4631 in the [O III] 5007 and H alpha lines, and long-slit spectroscopy using the Low-Resolution Imaging Sp ectrograph at the W. M. Keck Observatory. H alpha and [O III] lambda 5007 a re detected far from the disk, with the highest [O III]/H alpha ratios coin cident with the soft X-ray emission. Spectroscopy reveals that the ionizati on is higher than that of the Milky Way diffuse ionized gas (DIG), with an average ratio [O III]/H beta similar to1, rising to 6 in one region. We use the Balmer decrement to probe extinction in the halo. Extinction appears a s high as 5 kpc above the disk. Distinct smaller scale variations of [O III ], H beta, and [S II] indicate that we are viewing multiple zones with dist inct ionization conditions along the line of sight, in some cases separated by dust. This suggests that the gas has multiple phases and that the ioniz ation may be much higher in some zones. A model using photoionized clouds r eproduces the average trends in the line ratios. The inferred densities and pressures are high for halo gas but consistent with observations of other phases that may be colocated with this gas. A combination of high- and low- ionization gas is best for explaining the majority of the fluctuations. The high-ionization phase could be in shocks, but the velocity resolution is n ot adequate to prove or reject this possibility. Gas at 6 x 10(5) K discove red by ROSAT by Wang and colleagues in 1995 could be a source of radiativel y cooling gas. We present a new model of optical line emission from cooling clouds and compare it to the data. We show that it can produce part or all of the observed [O III] and fluctuations, may generate part or all of the EDIG gas itself, and may contribute to the photoionization of the upper hal o. Our most important conclusions are that the halo is complex and multicom ponent and that average line-of-sight line ratios alone cannot be used to d etermine the ionization conditions.