Observations of Spatially Resolved Dust Evolution in Nearby Galaxies
Dust plays an important role in shaping the interstellar radiation field and chemistry in the interstellar medium. One key factor that reveals dust evolution is the dust-to-metals ratio (D/M), which describes the fraction of heavy elements contained in dust grains. We investigate the evolution of dust by observing how D/M varies with the local environmental properties of agalaxy at ~kpc scales.
In Chapter 2, we measure D/M in M101, a nearby spiral galaxy with metallicity spanning ~1 dex. We fit the spectral energy distribution of dust with five variants of the modified blackbody dust emission model in which we vary the temperature distribution and how emissivity depends on wavelength. Among them, the model with a single-temperature blackbody modified by a broken power-law emissivity gives the statistically best fit and physically most plausible results. Using these results, we show that the dust-to-gas ratio is proportional to Z^1.7, implying that the D/M is not constant in M101, but decreases as a function of galactocentric radius.
In Chapter 3, we investigate the relationship between the D/M and the local environment at ~2 kpc resolution in five nearby galaxies. We examine the D/M with four prescriptions of CO-to-H2 conversion factor (aCO), and compare the measured D/M-metallicity and D/M-density correlations, both of which are expected to be positive based on trends measured in depletion. We find that the aCO prescriptions based on only metallicity yield too much molecular gas in the center of IC342 to obtain the expected correlations. An aCO prescription with decreased gas mass at high surface density regions is preferred. The measured D/M is roughly constant, spanning 0.40-0.58.
In Chapter 4, we present the D/M measurements in 54 nearby galaxies at four physical scales. We confirm that a prescription that predicts lower aCO in high-surface density regions is preferred. The measured D/M is sensitive to the choice of aCO prescription. The measured D/M has weak but significant correlations with most of the physical quantities examined in the study.