Publications by Harley Katz


Magnetogenesis at Cosmic Dawn: Tracing the Origins of Cosmic Magnetic Fields

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HARLEY Katz, S Martin-Alvarez, JULIEN Devriendt, A Slyz, T Kimm

Despite their ubiquity, the origin of cosmic magnetic fields remains unknown. Various mechanisms have been proposed for their existence including primordial fields generated by inflation, or amplification and injection by compact astrophysical objects. Separating the potential impact of each magnetogenesis scenario on the magnitude and orientation of the magnetic field and their impact on gas dynamics may give insight into the physics that magnetised our Universe. In this work, we demonstrate that because the induction equation and solenoidal constraint are linear with $B$, the contribution from different sources of magnetic field can be separated in cosmological magnetohydrodynamics simulations and their evolution and influence on the gas dynamics can be tracked. We present a suite of simulations where the primordial field strength is varied to determine the contributions of the primordial and supernovae-injected magnetic fields to the total magnetic energy as a function of time and spatial location. We find that, for our specific model, the supernova-injected fields rarely penetrate far from haloes, despite often dominating the total magnetic energy in the simulations. The magnetic energy density from the supernova-injected field scales with density with a power-law slope steeper than 4/3 and often dominates the total magnetic energy inside of haloes. However, the star formation rates in our simulations are not affected by the presence of magnetic fields, for the ranges of primordial field strengths examined. These simulations represent a first demonstration of the magnetic field tracer algorithm which we suggest will be an important tool for future cosmological MHD simulations.


Stellar feedback and the energy budget of late-type Galaxies: Missing baryons and core creation

Monthly Notices of the Royal Astronomical Society Oxford University Press 480 (2018) 4287–4301-

H Katz, H Desmond, S McGaugh, F Lelli, A Di Cintio, J Schombert, C Brook

In a ΛCDM cosmology, galaxy formation is a globally inefficient process: it is often the case that far fewer baryons are observed in galaxy discs than expected from the cosmic baryon fraction. The location of these ‘missing baryons’ is unclear. By fitting halo profiles to the rotation curves of galaxies in the SPARC data set, we measure the ‘missing baryon’ mass for individual late-type systems. Assuming that haloes initially accrete the cosmological baryon fraction, we show that the maximum energy available from supernovae is typically not enough to completely eject these ‘missing baryons’ from a halo, but it is often sufficient to heat them to the virial temperature. The energy available from supernovae has the same scaling with galaxy mass as the energy needed to heat or eject the ‘missing baryons’, indicating that the coupling efficiency of the feedback to the ISM may be constant with galaxy virial mass. We further find that the energy available from supernova feedback is always enough to convert a primordial cusp into a core and has magnitude consistent with what is required to heat the ‘missing baryons’ to the virial temperature. Taking a census of the baryon content of galaxies with 109 < Mvir/M⊙ < 1012 reveals that ∼86 per cent of baryons are likely to be in a hot phase surrounding the galaxies and possibly observable in the X-ray, ∼7 per cent are in the form of cold gas, and ∼7 per cent are in stars.


The tight empirical relation between dark matter halo mass and flat rotation velocity for late-type galaxies

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 483 (2018) L98–L103-

H Desmond, H Katz, F Lelli, S McGaugh

We present a new empirical relation between galaxy dark matter halo mass (Mhalo) and the velocity along the flat portion of the rotation curve (Vflat), derived from 120 late-type galaxies from the SPARC data base. The orthogonal scatter in this relation is comparable to the observed scatter in the baryonic Tully–Fisher relation (BTFR), indicating a tight coupling between total halo mass and galaxy kinematics at r ≪ Rvir. The small vertical scatter in the relation makes it an extremely competitive estimator of total halo mass. We demonstrate that this conclusion holds true for different priors on M*/L[3.6μ] that give a tight BTFR, but requires that the halo density profile follow DC14 rather than NFW. We provide additional relations between Mhalo and other velocity definitions at smaller galactic radii (i.e. V2.2, Veff, and Vmax) which can be useful for estimating halo masses from kinematic surveys, providing an alternative to abundance matching. Furthermore, we constrain the dark matter analogue of the radial acceleration relation and also find its scatter to be small, demonstrating the fine balance between baryons and dark matter in their contribution to galaxy kinematics.


A Census of the LyC Photons that Form the UV Background During Reionization

Monthly Notices of the Royal Astronomical Society Oxford University Press 478 (2018) 4986–5005-

H Katz, T Kimm, M Haehnelt, D Sijacki, J Rosdahl, J Blaizot

We present a new, on-the-fly photon flux and absorption tracer algorithm designed to directly measure the contribution of different source populations to the metagalactic ultraviolet (UV) background and to the ionization fraction of gas in the Universe. We use a suite of multifrequency radiation hydrodynamics simulations that are carefully calibrated to reproduce a realistic reionization history and galaxy properties at z ⩾ 6, to disentangle the contribution of photons emitted by different mass haloes and by stars with different metallicities and ages to the UV background during reionization. While at very early cosmic times low-mass, metal-poor haloes provide most of the Lyman continuum photons, their contribution decreases steadily with time. At z = 6 it is the photons emitted by massive systems ( Mhalo/M⊙>1010h−1 ) and by the metal enriched stars (10−3 < Z/Z⊙ < 10−1.5) that provide the largest contribution to the ionising UV background. We demonstrate that there are large variations in the escape fraction depending on the source, with the escape fraction being highest ( ∼45--60percent ) for photons emitted by the oldest stars that penetrate into the intergalactic medium via low opacity channels carved by the ionising photons and supernova from younger stars. Before H II regions begin to overlap, the photoionization rate strongly fluctuates between different, isolated H II bubbles, depending on the embedded ionising source, which we suggest may result in spatial variations in the properties of dwarf galaxies.


Tracing the sources of reionization in cosmological radiation hydrodynamics simulations

Monthly Notices of the Royal Astronomical Society Oxford University Press 483 (2018) 1029–1041-

H Katz, T Kimm, D Sijacki, J Rosdahl, J Blaizot

We use the photon flux and absorption tracer algorithm presented in Katz et al. 2018, to characterise the contribution of haloes of different mass and stars of different age and metallicity to the reionization of the Universe. We employ a suite of cosmological multifrequency radiation hydrodynamics AMR simulations that are carefully calibrated to reproduce a realistic reionization history and galaxy properties at z ≥ 6. In our simulations, haloes with mass 109M⊙h−1 < M < 1010M⊙h−1, stars with metallicity 10−3Z⊙ < Z < 10−1.5Z⊙, and stars with age 3 Myr < t < 10 Myr dominate reionization by both mass and volume. We show that the sources that reionize most of the volume of the Universe by z = 6 are not necessarily the same sources that dominate the meta-galactic UV background at the same redshift. We further show that in our simulations, the contribution of each type of source to reionization is not uniform across different gas phases. The IGM, CGM, filaments, ISM, and rarefied supernova heated gas have all been photoionized by different classes of sources. Collisional ionisation contributes at both the lowest and highest densities. In the early stages of the formation of individual HII bubbles, reionization proceeds with the formation of concentric shells of gas ionised by different classes of sources, leading to large temperature variations as a function of galacto-centric radius. The temperature structure of individual HII bubbles may thus give insight into the star formation history of the galaxies acting as the first ionising sources. Our explorative simulations highlight how the complex nature of reionization can be better understood by using our photon tracer algorithm.