Randall Dannen, PhD @ UNLV

Hi, I'm Randall Dannen, a Las Vegas native and astrophysicist at the University of Nevada, Las Vegas (UNLV) where I completed my PhD and now work as a postdoctoral scholar under in Daniel Proga's research group since 2015 in the Department of Physics & Astronomy. I started my academic journey in 2009, initially working with Ken Nagamine's group on early galaxy formation using the SPH code GADGET-3. I transitioned to Dr. Proga's group and earned my bachelor's degrees in mathematics and physics in 2016. My research centers on the physics of active galactic nuclei (AGN) the brightest and most powerful engines in the universe powered by supermassive black holes at galaxy centers. I have focused on how these systems drive multiphase gas outflows, also referred to as winds, driven by thermal instability or spectral lines. In particular I have examined how radiation force from spectral line opacity depends on the gas's ionization state, showing that overionization due to ionizing radiation can weaken this force and produce variable, periodic, and clumpy winds.

My main interest is on modeling active galactic nueclei (AGN), the most powerful engines in the universe. This engine at the heart of nearly every galaxy turns converts gravitational energy from the material infalling and turns it into radiation across the all bands and kinetic energy in the form of outflows.

I am trying to model these outflows, describing how gas leaves the AGN and surrounding accretion disk. To do so, I have built the computational machinery to couple the effects of radiation and hydrodynamics using the photoionization code XSTAR and GRMHD code and adaptive mesh refinement (AMR) framework Athena++ . This effort established that thermal instability and line-driving physics could be a mechanisms for multiphase, clumpy AGN winds that we know must exist due to the absorption profiles in the observed spectra.

My recent efforts are focused on modeling observed spectra from AGN. By combining a modified analytic wind model with the Monte-Carlo radiative transfer code SIROCCO , I have been able to produce absorption profiles due to UV lines with similar properties what are called Extremely High Velocity Outflows (EHVOs) in the UV part of the spectrum.

• Dyda, S., Dannen, R.C., Kallman, T.R., Davis, S.W., and Proga, D. (2025)
       Flux-averaged force multipliers
       Monthly Notices of the Royal Astronomical Society, 550, 4. ADS
• Dannen, R.C., Proga D., Hidalgo P. R., Smith K., Ritchie A., Flores L., Kalet M. (2026)
       Wind Acceleration as a Driver of Detached Blueshifted Absorption in Quasar Disk Winds
       The Astrophysical Journal Letters, 1007. ADS
• Dyda, S., Dannen, R.C., Kallman, T.R., Davis, S.W., and Proga, D. (2025)
       Oscillatory Line-driven Winds: The Role of Atmospheric Stratification
       Monthly Notices of the Royal Astronomical Society, 540, 2612. ADS
• Key, J.A., Proga, D., Dannen, R., Vivier, S., & Waters, T. (2025)
       POscillatory Line-driven Winds: The Role of Atmospheric Stratification
       The Astrophysical Journal, 987, 144. ADS
• Smith, K., Proga, D., Dannen, R., Dyda, S., & Waters, T. (2024)
       Position-dependent Radiation Fields near Accretion Disks.
       The Astrophysical Journal, 970, 150. ADS
• Dannen, R., Proga, D., Waters, T., & Dyda, S. (2024)
      On the Transition from Efficient to Inefficient Line Driving in Irradiated Flows
       The Astrophysical Journal, 961, 221. ADS
• Waters, T., Proga, D., Dannen, R., & Dyda, S. (2022)
       Dynamical Thermal Instability in Highly Supersonic Outflows
       The Astrophysical Journal, 931, 134. ADS
• Ganguly, S., Proga, D., Waters, T., Dannen, R. C., Dyda, S., Giustini, M., Kallman, T., Raymond, J., Miller, J., & Rodriguez Hidalgo, P. (2021)
       On Synthetic Absorption Line Profiles of Thermally Driven Winds from Active Galactic Nuclei
       The Astrophysical Journal, 914, 114. ADS
• Waters, T., Proga, D., & Dannen, R. (2021)
       Multiphase AGN Winds from X-Ray-irradiated Disk Atmospheres
       The Astrophysical Journal, 914, 62. ADS
• Dannen, R. C., Proga, D., Waters, T., & Dyda, S. (2020)
       Clumpy AGN Outflows due to Thermal Instability
       The Astrophysical Journal Letters, 893, L34. ADS
• Dannen, R. C., Proga, D., Kallman, T. R., & Waters, T. (2019)
       Photoionization Calculations of the Radiation Force Due To Spectral Lines in AGNs
       The Astrophysical Journal, 882, 99. ADS
• Dyda, S., Dannen, R., Waters, T., & Proga, D. (2017)
       Irradiation of astrophysical objects - SED and flux effects on thermally driven winds
       Monthly Notices of the Royal Astronomical Society, 467(4), 4161–4173. ADS
• Waters, T., Proga, D., Dannen, R., & Kallman, T. R. (2017)
       Synthetic absorption lines for a clumpy medium: a spectral signature for cloud acceleration in AGN?
       Monthly Notices of the Royal Astronomical Society, 467(3), 3160–3171. ADS