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Register a new userHoles and high velocity HI in NGC 6946
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Authors
Rense Boomsma
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We are studying the properties of the holes and the high velocity gas in NGC 6946. Here we present some puzzling results.
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Multi-wavelength observations of nearby spiral galaxies have shown that neutral and ionized gas are present up to a few kpc from the disk and that star formation and supernovae probably play an important role in bringing gas into the halo. We have obtained very sensitive HI observations of the face-on galaxy NGC 6946 and of the nearly edge-on starburst galaxy NGC 253. We find high velocity HI clouds in NGC 6946 and extra-planar gas with anomalous velocities in NGC 253. In both galaxies there seems to be a close connection between the star-forming disk and the halo HI. In the outer parts of NGC 6946 there is also evidence for recent gas accretion.
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.
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.
The [CII] fine-structure transition at 158 micron is frequently the brightest far-infrared line in galaxies. Due to its low ionization potential, C+ can trace the ionized, atomic, and molecular phases of the ISM. We present velocity resolved [CII] and [NII] pointed observations from SOFIA/GREAT on ~500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multi-phase origin of [CII] emission over a range of environments. We show that ionized gas makes a negligible contribution to the [CII] emission in these positions using [NII] observations. We spectrally decompose the [CII] emission into components associated with the molecular and atomic phases using existing CO(2-1) and HI data and show that a peak signal-to-noise ratio of 10-15 is necessary for a reliable decomposition. In general, we find that in our pointings greater than or equal to 50% of the [CII] emission arises from the atomic phase, with no strong dependence on star formation rate, metallicity, or galactocentric radius. We do find a difference between pointings in these two galaxies, where locations in NGC 6946 tend to have larger fractions of [CII] emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter [CII] emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the [CII] cooling function and find log(P_th/k)=3.8-4.6 [K cm^-3], a value slightly higher than similar determinations, likely because our observations are biased towards star-forming regions.
We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the 12CO (J=1-0) transition. By combining observations from the Nobeyama Radio Observatory 45-meter single dish telescope and the Combined Array for Research in Millimeter Astronomy (CARMA) interferometer, we are able to obtain high image fidelity and accurate measurements of LCO compared with previous purely interferometric studies. We resolve individual giant molecular clouds (GMCs), measure their luminosities and virial masses, and derive Xco - the conversion factor from CO measurements to H2 masses - within individual clouds. On average, we find that Xco = 1.2 times 10^20 cm-2 / (K km s-1), which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within sim400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.
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