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Register a new userLyman-Werner Escape Fractions from the First Galaxies
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Direct collapse black holes forming in pristine, atomically-cooling haloes at $z approx 10-20$ may act as the seeds of supermassive black holes (BH) at high redshifts. In order to create a massive BH seed, the host halo needs to be prevented from forming stars. H$_2$ therefore needs to be irradiated by a large flux of Lyman-Werner (LW) UV photons in order to suppress H$_2$ cooling. A key uncertainty in this scenario is the escape fraction of LW radiation from first galaxies, the dominant source of UV photons at this epoch. To better constrain this escape fraction, we have performed radiation-hydrodynamical simulations of the growth of HII regions and their associated photodissociation regions in the first galaxies using the ZEUS-MP code. We find that the LW escape fraction crucially depends on the propagation of the ionisation front (I-front). For an R-type I-front overrunning the halo, the LW escape fraction is always larger than 95%. If the halo recombines later from the outside--in, due to a softened and weakened spectrum, the LW escape fraction in the rest-frame of the halo (the near-field) drops to zero. A detailed and careful analysis is required to analyse slowly moving, D-type I-fronts, where the escape fraction depends on the microphysics and can be as small as 3% in the near-field and 61% in the far-field or as large as 100% in both the near-field and the far-field.
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Population III stars can regulate star formation in the primordial Universe in several ways. They can ionize nearby halos, and even if their ionizing photons are trapped by their own halos, their Lyman-Werner (LW) photons can still escape and destroy H$_2$ in other halos, preventing them from cooling and forming stars. LW escape fractions are thus a key parameter in cosmological simulations of early reionization and star formation but have not yet been parametrized for realistic halos by halo or stellar mass. To do so, we perform radiation hydrodynamical simulations of LW UV escape from 9--120 M$_{odot}$ Pop III stars in $10^5$ to $10^7$ M$_{odot}$ halos with ZEUS-MP. We find that photons in the LW lines (i.e. those responsible for destroying H$_{2}$ in nearby systems) have escape fractions ranging from 0% to 85%. No LW photons escape the most massive halo in our sample, even from the most massive star. Escape fractions for photons elsewhere in the 11.18--13.6~eV energy range, which can be redshifted into the LW lines at cosmological distances, are generally much higher, being above 60% for all but the least massive stars in the most massive halos. We find that shielding of H$_2$ by neutral hydrogen, which has been neglected in most studies to date, produces escape fractions that are up to a factor of three smaller than those predicted by H$_2$ self-shielding alone.
Lyman-alpha (Ly{alpha}) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Ly{alpha} radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Ly{alpha} emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of $M_{rm star} approx 5 times 10^8 {rm M}_odot$. We find that many of the interesting features of the Ly{alpha} line can be understood in terms of the galaxys star formation history. The time variability, spatial morphology, and anisotropy of Ly{alpha} properties are consistent with current observations. For example, the rest frame equivalent width has a ${rm EW}_{{rm Ly}alpha,0} > 20 {rm AA}$ duty cycle of 62% with a non-negligible number of sightlines with $> 100 {rm AA}$, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Ly{alpha} and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites Ly{alpha} radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Ly{alpha} detections and spectroscopy of high-$z$ galaxies with the upcoming James Webb Space Telescope is feasible.
We investigate the process of metal-free star formation in the first galaxies with a high-resolution cosmological simulation. We consider the cosmologically motivated scenario in which a strong molecule-destroying Lyman-Werner (LW) background inhibits effective cooling in low-mass haloes, delaying star formation until the collapse or more massive haloes. Only when molecular hydrogen (H2) can self-shield from LW radiation, which requires a halo capable of cooling by atomic line emission, will star formation be possible. To follow the formation of multiple gravitationally bound objects, at high gas densities we introduce sink particles which accrete gas directly from the computational grid. We find that in a 1 Mpc^3 (comoving) box, runaway collapse first occurs in a 3x10^7 M_sun dark matter halo at z~12 assuming a background intensity of J21=100. Due to a runaway increase in the H2 abundance and cooling rate, a self-shielding, supersonically turbulent core develops abruptly with ~10^4 M_sun in cold gas available for star formation. We analyze the formation of this self-shielding core, the character of turbulence, and the prospects for star formation. Due to a lack of fragmentation on scales we resolve, we argue that LW-delayed metal-free star formation in atomic cooling haloes is very similar to star formation in primordial minihaloes, although in making this conclusion we ignore internal stellar feedback. Finally, we briefly discuss the detectability of metal-free stellar clusters with the James Webb Space Telescope.
The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures $T_{rm vir} < 10^{4}$~K. Cooling in these minihaloes is dominated by molecular hydrogen (H$_{2}$), and so Population III star formation is only possible in those minihaloes that form enough H$_{2}$ to cool on a short timescale. As H$_{2}$ cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale $M_{rm min}$ above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H$_{2}$ by a high redshift Lyman-Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on $M_{rm min}$ and on $M_{rm ave}$ (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on $M_{rm min}$ than the LW background, but also that both effects are additive. We also provide fitting functions quantifying the dependence of $M_{rm ave}$ and $M_{rm min}$ on the streaming velocity and the strength of the LW background.
It has been argued that low-luminosity dwarf galaxies are the dominant source of ionizing radiation during cosmological reionization. The fraction of ionizing radiation that escapes into the intergalactic medium from dwarf galaxies with masses less than ~10^9.5 solar masses plays a critical role during this epoch. Using an extensive suite of very high resolution (0.1 pc), adaptive mesh refinement, radiation hydrodynamical simulations of idealized and cosmological dwarf galaxies, we characterize the behavior of the escape fraction in galaxies between 3 x 10^6 and 3 x 10^9 solar masses with different spin parameters, amounts of turbulence, and baryon mass fractions. For a given halo mass, escape fractions can vary up to a factor of two, depending on the initial setup of the idealized halo. In a cosmological setting, we find that the time-averaged photon escape fraction always exceeds 25% and reaches up to 80% in halos with masses above 10^8 solar masses with a top-heavy IMF. The instantaneous escape fraction can vary up to an order of magnitude in a few million years and tend to be positively correlated with star formation rate. We find that the mean of the star formation efficiency times ionizing photon escape fraction, averaged over all atomic cooling (T_vir > 8000 K) galaxies, ranges from 0.02 for a normal IMF to 0.03 for a top-heavy IMF, whereas smaller, molecular cooling galaxies in minihalos do not make a significant contribution to reionizing the universe due to a much lower star formation efficiency. These results provide the physical basis for cosmological reionization by stellar sources, predominately atomic cooling dwarf galaxies.
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