Dr. Kristian Finlator

Profile picture of Kristian Finlator



I am interested in a variety of aspects of galaxy evolution and the relationship between galaxies and their environments.  Most of my first-author work draws on comparisons between predictions from cosmological hydrodynamical simulations and observations.  My most recent simulations include self-consistent continuum ionizing radiation transport, allowing them to model cosmological reionization, the growth of the extragalactic ultraviolet background (UVB), and spatial fluctuations in the UVB’s intensity and spectral hardness.

Galaxies and Galactic Outflows: I am interested in the impact of gas flows (both inflows and outflows) on the slope and evolution of the mass-metallicity relation as well as third-parameter dependencies such as star formation rate and gas fraction.  I have also studied the observable signatures of outflows on ensemble statistics such as the luminosity function and the luminosity – halo mass relationship.

Reionization: I use Gadget-3 simulations that incorporate an on-the-fly continuum radiation transport solver to model reionization, the impact of the early UVB on galaxies and the IGM, and the signature of the UVB’s slope and spatial fluctuations on metal absorbers.

Previous students

  • Asger Grønnow, Master’s, 2015, Dark Cosmology Center
  • Gavin Mathes – Lyman Alpha Emitters and Reionization

Current students

  • Caitlin Doughty – high-redshift metal absorbers and the UVB
  • Rogelio Ochoa – Lyman Alpha Emitters and Reionization


ASTR105G (The Planets); ASTR110G (Introduction to Astronomy); ASTR616 (Galaxy Formation and Evolution); ASTR625 (Cosmology)

Recent Publications

The Faint Host Galaxies of C IV Absorbers at z > 5: Triply-ionized carbon (CIV) is relatively easy to see in absorption against background quasars, but little is known about the galaxies that put it there. This paper started as an attempt to use numerical simulations to predict what sort of galaxies lurked near clouds of gas that were rich in CIV (“strong CIV absorbers”) when the Universe was less than about one and a half billion years old. I had been wanting to write this paper for years, but kept having the problem that the model could not produce the right number of such clouds. For this project, I finally verified that the updated Technicolor Dawn framework produces the correct number of stars and metals and a fairly accurate reionization history, but even so there still was not enough CIV. The only viable explanation is that high-energy light is missing from the model. I showed that the problem can be largely resolved under the assumption (the so-called “density-bounded” scenario) that most ionizing photons escape galaxies through optically-thin barriers that let out the far-UV light but block the near-UV light. Within this scenario, galaxies are expected to be tends to hundreds of times more abundant near strong CIV absorbers. The galaxies are generally too faint for the Hubble Space Telescope to be able to spot at very early times, but ground-based integral field unit spectroscopy and, soon, the James Webb Space Telescope will be able to test the predictions in great detail.

Reionization in Technicolor: In our current data-rich era, there are many complementary observational probes of the progress of hydrogen reionization including galaxies, the circumgalactic medium, the intergalactic medium, and the cosmic microwave background. Whereas most theoretical studies treat only one of these observables in detail at a time, this study asks how all of these observations “talk to” one another when considered within the context of a single, unifying theoretical framework. For example, does matching one observational inference force the model to violate another? At the moment, the answer seems to be yes! Simulations that reproduce the observationally-inferred amplitude of the ionizing background (that is, the number density of intergalactic photons with energies near the hydrogen ionizing threshold) tend to underproduce the intergalactic opacity in the Lyman-alpha transition even though these ought to be measurements of the same thing. Likewise, they underproduce the observed number of intergalactic clouds that are rich in triply-ionized carbon, the so-called strong CIV absorbers. There are other mysteries as well. In short, although it’s generally believed that galaxies dominated hydrogen reionization, there are observational challenges to the hypothesis.

The minimum halo mass for star formation at z = 6-8: The bath of ultraviolet photons that galaxies and quasars create heats the intergalactic medium.  This heat chokes off gas accretion into dark matter halos that are less massive than a certain threshold.  Intuitively, this occurs because heated gas simply has too much energy to be confined by shallow gravitational wells of low-mass halos.  We know this happens, but we do not know what the mass threshold is.  Locally, it may play a role in determining which of the many hundreds of satellite dark matter halos that presumably orbit the Milky Way host stars: Those that were never massive enough to accrete gas never formed stars.  Analogously, in the high-redshift Universe, the threshold must manifest as a turnover in the observed rest-frame UV luminosity function.  By comparing recent observations of the UV luminosity function with our simulations, we were able to infer that the threshold mass at z=6 (when the Universe was only a billion years old) was likely below three billion solar masses.  This constraint already rules out some models for early galaxy formation.

The Reionization of Carbon: As the UVB grew during the first few billion years, it famously stripped the electrons from the intergalactic medium’s (IGM) hydrogen and helium atoms.  Unfortunately, it is not possible to use hydrogen absorbers to trace hydrogen reionization because even teeny amounts of neutral hydrogen lead to a great deal of absorption.  Instead, observers have thought about using singly- and triply-ionized carbon (CII and CIV) absorbers to map out reionization because a growing UVB should ionize more of the carbon.  Simple enough, but in reality the abundance of carbon absorbers depends also on the gas temperature and metallicity evolution.  In this work, I used simulations to show that the evolving abundance of CII and CIV absorbers is impacted by both metallicity and UVB evolution.  These effects cancel in the case of CII but co-add in the case of CIV, leading to strong CIV evolution but weak CII evolution.

The soft, fluctuating UVB at z ˜ 6 as traced by C IV, Si IV, and C II: Even at redshifts where the UVB’s normalization (as quantified by its intensity at 1 Rydberg, or the hydrogen ionization potential) is well-constrained by the Lyman-alpha forest, its spectral slope is unconstrained without ancillary measurements.  The slope encodes information regarding the nature of the ionizing population: A harder UVB would imply an origin in massive stars or AGN.  Fortunately, the UVB slope is constrained by metal absorbers because a harder UVB yields higher ratios of CIV/CII and SiIV/CII.  In this paper, we post-processed simulations with three trial UVBs and compared the resulting statistics of metal absorbers to observations.  We found that an AGN-only UVB model is inconsistent with observations at z = 5.5-6, while a softer galaxies+QSOs model (where AGN contribute ~1% of the UVB) yields much better agreement.  This argues against recent suggestions that faint AGN could have dominated the UVB at z > 4.

All Publications