Date on Master's Thesis/Doctoral Dissertation

8-2025

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Holwerda, Benne

Committee Member

Kielkopf, John

Committee Member

Riedel, Thomas

Committee Member

Dowling, Tim

Author's Keywords

astronomy; disk galaxies; interstellar dust; dust attenuation; dust extinction; dust reddening

Abstract

The presence of dust grains as part of the the interstellar medium (ISM) in galaxies is an all-impactful entity in astronomy due its influence on light, and therefore observations. Unlike a true blackbody, dust absorbs, reflects, reddens, and scatters light within a line of sight. We quantify this phenomena with attenuation: the difference between the observed and true magnitudes of an astronomical object due to dust. Therefore, to map dust attenuation, one needs a known background source of light. In the serendipitous case of partially overlapping –occulting– galaxies, the background galaxy serves as the known light source and the transmission of the background galaxy’s light through the foreground galaxy can be measured using a symmetry argument. The transmission, or lack thereof, of light is then a direct measurement of dust attenuation of the foreground galaxy in the area of overlap. We present a geometrically ideal occulting galaxy pair, VV191: a spiral foreground galaxy, VV191b, partially overlapping a smooth elliptical background galaxy, VV191a. We compare the mapped attenuation in the Johnson V-band to ground-based spectroscopic nebular gas attenuation of VV191b. In doing so, we explore mapped dust attenuation differences based on the reddening of the stellar continuum, the

light from all the stars in the galaxy, and nebular gas, hot ionized gas near younger stars. A key conclusion is that the nebular gas attenuation is twice that of the stellar continuum attenuation through the disk, hence, the birthplace of stars has twice the amount of dust around them than the rest of the stellar disk of the galaxy, agreeing with the literature. However, the difference decreases with galactic radius to parity beyond the Petrosian radius (defined below). There are indications that this difference in attenuations varies strongly on small scales. How much dust scatters and absorbs light depends on the wavelength of the light. This is referred to as the wavelength-dependent “attenuation law.” With VV191, observed in many wavelengths with both the James Webb and Hubble Space Telescopes, we can approximate and map this relation in fine detail in the foreground galaxy, VV191b, using Monte Carlo resampling methods. We report the distributions of multiple dust parameters across the outer disk and in smaller hand-picked regions of specific environments. In the outer spiral arms, we report a predominantly Milky Way-like reddening behavior. We see that relatively dense parts of the disk slightly redden the background light more than its diffuse surroundings in the near-infrared. In the same areas, the relative reddening slope displays grayening behavior from the V band to 0.9 μm. Both effects seem to be consequences of the typical size of dense clouds being unresolved, attributing to a blended signal effect, or “Matroyshka” effect. We suspect molecular clouds below the resolution limit in the ISM play an important role at these scales. A photodissociation region (PDR), a region dominated by the activity of massive stars, is discovered in the far outer disk. Evidence indicates the presence of polycyclic aromatic hydrocarbons adjacent to the PDR.

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