HI in galaxies traces the fuel for future star formation and reveals the effects of feedback on neutral gas. Using a statistically uniform, HI-selected sample of 565 galaxies from the ALFALFA H-alpha survey, we explore HI properties as a function of
star formation activity. ALFALFA H-alpha provides R-band and H-alpha imaging for a volume-limited subset of the 21-cm ALFALFA survey. We identify eight starbursts based on H-alpha equivalent width and six with enhanced star formation relative to the main sequence. Both starbursts and non-starbursts have similar HI to stellar mass ratios (MHI/M*), which suggests that feedback is not depleting the starbursts HI. Consequently, the starbursts do have shorter HI depletion times (t_dep), implying more efficient HI-to-H2 conversion. While major mergers likely drive this enhanced efficiency in some starbursts, the lowest mass starbursts may experience periodic bursts, consistent with enhanced scatter in t_dep at low M*. Two starbursts appear to be pre-coalescence mergers; their elevated MHI/M* suggest that HI-to-H2 conversion is still ongoing at this stage. By comparing with the GASS sample, we find that t_dep anti-correlates with stellar surface density for disks, while spheroids show no such trend. Among early-type galaxies, t_dep does not correlate with bulge-to-disk ratio; instead, the gas distribution may determine the star formation efficiency. Finally, the weak connection between galaxies specific star formation rates and MHI/M* contrasts with the well-known correlation between MHI/M* and color. We show that dust extinction can explain the HI-color trend, which may arise from the relationship between M*, MHI, and metallicity.
The shape of the OB-star spectral energy distribution is a critical component in many diagnostics of the ISM and galaxy properties. We use single-star HII regions from the LMC to quantitatively examine the ionizing SEDs from widely available CoStar,
TLUSTY, and WM-basic atmosphere grids. We evaluate the stellar atmosphere models by matching the emission-line spectra that they predict from CLOUDY photoionization simulations with those observed from the nebulae. The atmosphere models are able to reproduce the observed optical nebular line ratios, except at the highest energy transitions > 40 eV, assuming that the gas distribution is non-uniform. Overall we find that simulations using WM-basic produce the best agreement with the observed line ratios. The rate of ionizing photons produced by the model SEDs is consistent with the rate derived from the Halpha luminosity for standard, log(g) = 4.0 models adopted from the atmosphere grids. However, there is a systematic offset between the rate of ionizing photons from different atmosphere models that is correlated with the relative hardness of the SEDs. In general WM-basic and TLUSTY atmosphere models predict similar effective temperatures, while CoStar predicts effective temperatures that are cooler by a few thousand degrees. We compare our effective temperatures, which depend on the nebular ionization balance, to conventional photospheric-based calibrations from the literature. We suggest that in the future, spectral type to effective temperature calibrations can be constructed from nebular data.
Some theories of star formation suggest massive stars may only form in clustered environments, which would create a deficit of massive stars in low density environments. Observationally, Massey (2002) finds such a deficit in samples of the field popu
lation in the Small and Large Magellanic Clouds, with an IMF slope of {Gamma} ~ 4. These IMF measurements represent some of the largest known deviations from the standard Salpeter IMF slope of {Gamma}=1.35. Here, we carry out a comprehensive investigation of the mass function above 20 solar masses for the entire field population of the Small Magellanic Cloud, based on data from the Runaways and Isolated O Type Star Spectroscopic Survey of the SMC (RIOTS4). This is a spatially complete census of the entire field OB star population of the SMC obtained with the IMACS multi-object spectrograph and MIKE echelle spectrograph on the Magellan telescopes. Based on Monte-Carlo simulations of the evolved present-day mass function, we find the slope of the field IMF above 20 solar masses is {Gamma}=2.3+/-0.4. We extend our IMF measurement to lower masses using BV photometry from the OGLE II survey. We use a statistical approach to generate a probability distribution for the mass of each star from the OGLE photometry, and we again find {Gamma}=2.3+/-0.6 for stellar masses from 7-20 solar masses. The discovery and removal of ten runaways in our RIOTS4 sample steepens the field IMF slope to {Gamma}=2.8+/-0.5. We discuss the possible effects of binarity and star-formation history on our results, and conclude that the steep field massive star IMF is most likely a real effect.
