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We combine Hubble Space Telescope (HST) Paschen $beta$ (Pa$beta$) imaging with ground-based, previously published H$alpha$ maps to estimate the attenuation affecting H$alpha$, A(H$alpha$), across the nearby, face-on galaxies NGC 5194 and NGC 6946. We estimate A(H$alpha$) in ~ 2,000 independent 2 ~75 pc diameter apertures in each galaxy, spanning out to a galactocentric radius of almost 10 kpc. In both galaxies, A(H$alpha$) drops with radius, with a bright, high attenuation inner region, though in detail the profiles differ between the two galaxies. Regions with the highest attenuation-corrected H$alpha$ luminosity show the highest attenuation, but the observed H$alpha$ luminosity of a region is not a good predictor of attenuation in our data. Consistent with much previous work, the IR-to-H$alpha$ color does a good job of predicting A(H$alpha$). We calculate the best-fit empirical coefficients for use combining H$alpha$ with 8, 12, 24, 70, or 100 $mu$m to correct for attenuation. These agree well with previous work but we also measure significant scatter around each of these linear relations. The local atomic plus molecular gas column density, N(H), also predicts A(H$alpha$) well. We show that a screen with magnitude ~ 0.2 times the expected for a Milky Way gas-to-dust value does a reasonable job of explaining A(H$alpha$) as a function of N(H). This could be expected if only ~ 40% of gas and dust directly overlap regions of H$alpha$ emission.
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Star formation induced by a spiral shock wave, which in turn is generated by a spiral density wave, produces an azimuthal age gradient across the spiral arm, which has opposite signs on either side of the corotational resonance. An analysis of the spatial separation between young star clusters and nearby HII regions made it possible to determine the position of the corotation radius in the studied galaxies. Fourier analysis of the gas velocity field in the same galaxies independently confirmed the corotation radius estimates obtained by the morphological method presented here.
Observations of ongoing HI accretion in nearby galaxies have only identified about 10% of the needed fuel to sustain star formation in these galaxies. Most of these observations have been conducted using interferometers and may have missed lower column density, diffuse, HI gas that may trace the missing 90% of gas. Such gas may represent the so-called cold flows predicted by current theories of galaxy formation to have never been heated above the virial temperature of the dark matter halo. As a first attempt to identify such cold flows around nearby galaxies and complete the census of HI down to N(HI)~10^18 cm^-2, I used the Robert C. Byrd Green Bank Telescope (GBT) to map the circumgalactic (r < 100-200 kpc) HI environment around NGC 2997 and NGC 6946. The resulting GBT observations cover a four square degree area around each galaxy with a 5-sigma detection limit of N(HI)~10^18 cm^-2 over a 20 km/s linewidth. This project complements absorption line studies, which are well-suited to the regime of lower N(HI). Around NGC 2997, the GBT HI data reveal an extended HI disk and all of its surrounding gas-rich satellite galaxies, but no filamentary features. Furthermore, the HI mass as measured with the GBT is only 7% higher than past interferometric measurements. After correcting for resolution differences, the HI extent of the galaxy is 23% larger at the N(HI)~1.2x10^18 cm^-2 level as measured by the GBT. On the other hand, the HI observations of NGC 6946 reveal a filamentary feature apparently connecting NGC 6946 with its nearest companions. This HI filament has N(HI)~10^18 cm^-2 and a FWHM of 55+-5 km/s and was invisible in past interferometer observations. The properties of this filament are broadly consistent with being a cold flow or debris from a past tidal interaction between NGC 6946 and its satellites.
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