Enigmatic extinction: an investigation of the 2175 Å extinction bump in M101, the Pinwheel Galaxy
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Dust is a critical component of the universe, affecting energy flow and the dynamics of star and planetary disk evolution. The light we measure when observing the universe is absorbed and scattered (extinguished) by dust. Studies indicate that active formation of high mass stars modifies the ultraviolet (UV) dust extinction curve, removing the characteristic bump at 2175 Å. For a half century, the source of this bump has not been positively identified. Dust grain models suggest that a leading contender is polycyclic aromatic hydrocarbons. The spiral galaxy M101 is an ideal laboratory, with many HII regions, plus steep metallicity and ionization gradients. The role of environment in this enigmatic "bump-less'' extinction was investigated in M101 with observations from both a sounding rocket mission and the Hubble Space Telescope (HST). The Interstellar Absorption Gradient Experiment Rocket (IMAGER) mission was designed to probe the correlations between dust extinction and environment through photometric observations of the apparent strengths of the 2175 Å bump and UV continuum in M101. IMAGER flew and collected data on November 21, 2012. Although in-flight anomalies caused significant cross-talk, the mission demonstrated the feasibility of simultaneous imaging in three medium-width UV bands. With angular resolution poorer than one arcminute in all bands, and inadequate signal in the field-of-view, these data proved unsuitable for aperture photometry. This analysis method cannot be used with these data to draw conclusions about the correlations between environment and ultraviolet extinction. Spectroscopic observations were performed with the Space Telescope Imaging Spectrograph (STIS) instrument aboard the HST. With additional data from the Spitzer Space Telescope, and radiative transfer and stellar evolution models, the correlation between the bump and the aromatic features was probed across HII regions spanning wide ranges of metallicity and radiation field hardness. A correlation between the strengths of the aromatic features and the 2175 Å feature was not found-- the statistics preclude a definitive statement regarding the origin of the bump. Many models of dust extinction are consistent with the data, implying a need for more observations of the feature and possibly a significant modification of the source of 2175 Å extinction in dust models.
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