Date of Award

8-2026

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Aerospace, Physics, and Space Sciences

First Advisor

Eric Perlman

Second Advisor

Jay Kovats

Third Advisor

Donald C. Warren

Fourth Advisor

Donald Platt

Abstract

The giant elliptical galaxy NGC 6251 is a well-studied active radio galaxy that hosts one of the largest known relativistic jet systems, with a projected size of approximately 3 Mpc. In addition to its large-scale radio structure, the galaxy hosts a prominent circumnuclear dust feature surrounding the active nucleus, previously interpreted as a warped dust disk. This thesis focuses on the inner morphology and dust structure of NGC 6251 using archival Hubble Space Telescope observations obtained with the WFPC2 and NICMOS instruments.

The central structure of the galaxy was investigated across multiple wavelengths through image calibration, PSF modeling and subtraction, isophote fitting, Nuker profile analysis, and color and extinction mapping. The parameters of the Nuker model fit to NGC 6251 are reported as follows: an inner brightness profile slope of gamma = 0.39, an outer profile slope of beta = 2.17, and a break sharpness of alpha  = 1.74. These values, along with an absolute magnitude of MV = -22.24, indicate NGC 6251 is more consistent with a cuspy core galaxy than a pure power-law galaxy. Isophote fitting shows significant variation in ellipticity within the inner 2” (~1 kpc) of the galaxy. Additionally, the position angle varies within the inner 1” (~500 pc) before stabilizing in the outer regions. These distortions are consistent with a core influenced by AGN activity and the prominent dust structure. The color maps reveal strong reddening along the circumnuclear dust lane, a bluer PSF-dominated core, and a visible optical extension that aligns with the previously reported UV feature. This feature aligns with the southern portion of the circumnuclear dust disk and is likely the result of scattered AGN emission, with the possibility that a small star-forming region also contributes to the observed extended emission.   This study reports an average visible extinction value of AV = 0.5 mag in the disk that surrounds the core of the galaxy. The extinction structure and asymmetric morphology support the interpretation of a warped dust disk viewed close to edge-on. Additionally, derived dust and gas masses of MDust = 9.3 × 103 solar masses and MGas = 1.2 × 106  solar masses, calculated from the hydrogen gas column density of   NH = 10 × 1020 atoms  cm-2,  suggest that the central region contains a substantial material reservoir that is capable of contributing to long-term AGN fueling.

The results presented in this work provide further insight into the relationship between AGN activity, circumnuclear dust geometry, and extended optical/UV emission in radio-loud active galaxies.

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