My Research
My main research interests involve exploring the effects of active galactic nuclei (AGN) feedback on intergalactic scales. In particular I explore ways that AGN feedback may affect the Lyman-alpha forest. Constraining AGN feedback is a multi-scale problem so developing accurate models is complex. Exploring the life cycle of supermassive black holes from the accretion disk level (~sub-pc scales) out to intergalactic scales (~Mpc scales) is necessary to fully understand the complexity of these objects. I outline a couple of my projects below. For a full list of my publications check out my ORCiD.
The Effects of AGN Feedback on the Flux Power Spectrum
Out of the various Lyman-alpha forest statistics we can analyze, arguably the one containing the most information is the 1D transmitted flux power spectrum (P1D). At small k the P1D provides information about flux fluctuations on large scales revealing details about the overall ionization state of hydrogen gas in the intergalactic medium (IGM). At large k the P1D informs us about small physical scales revealing information about the thermal state/history of the IGM and the temperature of individual absorbers that make up the forest. AGN feedback has been seen to have varying effects on the P1D. Massive heating from AGN can suppress the P1D on all scales through the ionization of neutral hydrogen. This heating may also change the thermal state of the IGM as a whole or broaden individual absorption line features suppressing the high k end of the P1D even further. From simulations, these effects appear to dominate at lower redshifts (z<1), however some AGN feedback models have effects that occur at even higher redshifts (z>2). This implies that galactic feedback may play a larger role in setting large-scale structure than we initially thought.
Figure 4 from Tillman et al. 2025: The Lyman-alpha forest 1D transmitted flux power spectrum ratios for higher redshifts with shaded regions representing current observational precision. The red shaded region represents the error from the eBOSS derived P1D. The blue shaded region represents the error from the DESI (early release) derived P1D. The orange shaded region represents the error from P1D derived from a combination of KODIAQ, SQUAD, and XQ-100 surveys.
Feedback Effects from the CAMELS Project
The CAMELS project (Cosmology and Astrophysics with MachinE Learning Simulations) provides thousands of simulations with varying feedback and cosmological models. This allows for an immersive study of how individual aspects of different feedback models may affect the predicted observables. From this work it is clear that different AGN feedback models have varying impacts on the temperature and density of gas on large scales. AGN jet feedback as modeled in the Simba simulations has the largest affect on the gas in the IGM and thus the Lyman-alpha forest. Also apparent from this work is that the interaction between stellar and AGN feedback is quite complex. Understanding how these different models can impact each other will be vital for constraining our understanding of galactic feedback as a whole.
Figure 1 from Tillman et al. 2023b: Simba temperature and column density projections of a single absorber slice as defined in this study (525 kpc/h. The top plots display the projected mean temperature weighted by mass. The bottom plots display the integrated HI column density of the slice. We show projections for varying AGN momentum flux and AGN jet speed. The left plots are the lowest value for the parameter, the right plots are for the highest value, and the middle plots are the fiducial results. The projections help visualize the effect of varying the AGN feedback parameters.