Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Dr. Christine Wilson
We have used Hα and photometric data for two morphologically distinct Local Group galaxies, the spiral M33 and dwarf irregular NGC 6822, to study the distribution of the luminous blue O and B (OB) stars and HII regions in the galaxies as well as to determine whether individual regions of the galaxies are separately and/or collectively in a state of ionization balance. In the case of M33, we have concentrated on the inner 1 kpc region of the galaxy. Using the Hα data, three distinct ionized gas environments (bright, halo and field) defined by the surface brightness of the Hα emission have been identified. We find that ~50% of OB stars are located in the field, so that 1/2 of the lifetime of OB stars must be spent outside recognizable HII regions. This result suggests that if OB stars escape from bright HII regions by destroying their parent molecular clouds, then molecular cloud lifetimes after forming OB stars could be as low as ~5x10⁶ yrs or 1/2 the typical lifetime of OB stars. We show that a possible origin for the large field OB population is that they were born in and subsequently percolated out of the ~10³ giant molecular clouds with masses ≳10₃M⨀ predicted to exist within the inner kpc of the galaxy. Using UBV photometry and stellar ionization models, we predict Hα fiuxes in the bright, halo and field regions and compare them to those observed to find that predicted fluxes are a factor of ~3-7 greater than observed so that the three ionized gas environments, separately as well as collectively, are not in ionization balance. Furthermore, the most substantial loss of ionizing photons appears to he taking place in the field. We find that observed and predicted Hα luminosities are in best agreement when Case A recombination is assumed in the field. Our findings suggest that star formation rates obtained from Hα luminosities must underestimate the true star formation rate within the inner region of M33. A similar analysis of an individual, isolated region with bright and halo Hα emission has produced results that are comparable to those found on the larger scale: the isolated region, as a whole, is also not in ionization balance.
In the case of NGC 6822, four distinct components of the Hα emission (bright, halo, diffuse and field) differentiated by their surface brightnesses have been identified. We find that only 1/4 of the OB stars are found in the combined bright and halo regions, suggesting that OB stars spend roughly 3/4 of their lifetimes outside "classical" HII regions. Molecular cloud lifetimes after forming OB stars could be as low as ~1-3x10⁶ yrs or 1/4 the typical main sequence lifetimes of OB stars if OB stars escape from bright HII regions by destroying their parent clouds. Additionally, the field population of OB stars cannot have originated in and percolated out of existing HII regions. Comparing the observed Hα emission with that predicted from BV photometry and stellar ionizing flux models, we find that although the bright, halo and diffuse regions are probably in a state of ionization balance, the field region, which is producing at least 6 times as much ionizing flux as is observed, is clearly not. The ionization balance results suggest that star formation rates obtained from Hα luminosities must underestimate the true star formation rate in NGC 6822 by about 50%. Comparing the results for NGC 6822 and M33 reveals that the inner kiloparsec region of M33 is in a more serious state of ionization imbalance, perhaps due to its higher surface density of blue stars. Thus the morphological class or surface density of a galaxy may be important factors in how accurately we can determine star formation rates from Hα luminosities.
Finally, we have performed optical spectroscopy of luminous blue stars in both NGC 6822 and M33 using respectively, the 4-m CTIO telescope plus ARGUS spectrograph and the 4-m Mayall telescope at KPNO plus HYDRA spectrograph. Due to the limited S/N ratio of the data, we have been able to classify only ~50% of the stars observed in NGC 6822 and ~40% of the stars observed in M33. Out of the 37 NGC 6822 and 15 M33 stars with classifiable spectra respectively 28 and 11 potential OB stars have been identified. We have used the NGC 6822 spectra to gauge the reliability of the principle stellar ionization model used for the ionization balance calculations in both galaxies.
Patel, Kanan, "The HII Regions and OB Stars of M33 and NGC 6822" (1995). Open Access Dissertations and Theses. Paper 2277.