An H-alpha survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
Abstract
In a series of two papers we present results of a new H-alpha imaging survey, aiming at the detection of extraplanar diffuse ionized gas in halos of late-type spiral galaxies. We have investigated a sample of 74 nearby edge-on spirals, covering the northern and southern hemisphere. In 30 galaxies we detected extraplanar diffuse emission at mean distances of |z|~1-2 kpc. Individual filaments can be traced out to |z|<= 6 kpc in a few cases. We find a good correlation between the FIR flux ratio (S60/S100) and the SFR per unit area (LFIR/D225), based on the detections/non-detections. This is actually valid for starburst, normal and for quiescent galaxies. A minimal SFR per unit area for the lowest S60/S100 values, at which extended emission has been detected, was derived, which amounts to (dE/dt)A25thres = (3.2+-0.5)x10E+40 erg/s/kpc2. There are galaxies where extraplanar emission was detected at smaller values of LFIR/D225, however, only in combination with a significantly enhanced dust temperature. The results corroborate the general view that the gaseous halos are a direct consequence of SF activity in the underlying galactic disk.
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