arXiv subjects

Dominik R. G. Schleicher

Publications and source records attributed to Dominik R. G. Schleicher.

97 records · Page 6Linked to original sources

arXiv2009

The formation of the first galaxies and the transition to low-mass star formation

The formation of the first galaxies at redshifts z ~ 10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In this context we emphasize the importance and influence of selecting appropriate initial conditions for the star formation process. We revisit the notion of a critical metallicity resulting in the transition from primordial to present-day initial mass functions and highlight its dependence on additional cooling mechanisms and the exact initial conditions. We also review recent work on the ability of dust cooling to provide the transition to present-day low-mass star formation. In particular, we highlight the extreme conditions under which this transition mechanism occurs, with violent fragmentation in dense gas resulting in tightly packed clusters.

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arXiv2009

The influence of magnetic fields on the thermodynamics of primordial star formation

We explore the effects of magnetic energy dissipation on the formation of the first stars. For this purpose, we follow the evolution of primordial chemistry in the presence of magnetic fields in the post-recombination universe until the formation of the first virialized halos. From the point of virialization, we follow the protostellar collapse up to densities of $\sim10^{12}$ cm$^{-3}$ in a one-zone model. In the intergalactic medium (IGM), comoving field strengths of $\gtrsim0.1$ nG lead to Jeans masses of $10^8 M_\odot$ or more and thus delay gravitational collapse in the first halos until they are sufficiently massive. During protostellar collapse, we find that the temperature minimum at densities of $\sim10^3$ cm$^{-3}$ does not change significantly, such that the characteristic mass scale for fragmentation is not affected. However, we find a significant temperature increase at higher densities for comoving field strengths of $\gtrsim0.1$ nG. This may delay gravitational collapse, in particular at densities of $\sim10^9$ cm$^{-3}$, where the proton abundance drops rapidly and the main contribution to the ambipolar diffusion resistivity is due to collisions with Li$^+$. After the formation of the protostar, the increased gas temperatures may enhance the protostellar accretion rate. Our model confirms that initial weak magnetic fields may be amplified considerably during gravitational collapse and become dynamically relevant. For instance, a comoving field strength above $10^{-5}$ nG will be amplified above the critical value for the onset of jets which can magnetize the IGM.

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arXiv2009

Dark stars: Implications and constraints from cosmic reionization and extragalactic background radiation

Dark stars powered by dark matter annihilation have been proposed as the first luminous sources in the universe. These stars are believed to form in the central dark matter cusp of low-mass minihalos. Recent calculations indicate stellar masses up to \sim1000 solar masses and/or have very long lifetimes. The UV photons from these objects could therefore contribute significantly to cosmic reionization. Here we show that such dark star models would require a somewhat artificial reionization history, based on a double-reionization phase and a late star-burst near redshift $z\sim6$, in order to fulfill the WMAP constraint on the optical depth as well as the Gunn-Peterson constraint at $z\sim6$. This suggests that, if dark stars were common in the early universe, then models are preferred which predict a number of UV photons similar to conventional Pop. III stars. This excludes dark stars with 100 solar masses that enter a main-sequence phase and other models that lead to a strong increase in the number of UV photons. We also derive constraints for massive as well as light dark matter candidates from the observed X-ray, gamma-ray and neutrino background, considering dark matter profiles which have been steepened during the formation of dark stars. This increases the clumping factor at high redshift and gives rise to a higher dark matter annihilation rate in the early universe. We furthermore estimate the potential contribution from the annihilation products in the remnants of dark stars, which may provide a promising path to constrain such models further, but which is currently still uncertain.

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arXiv2009

Cosmic constraints rule out s-wave annihilation of light dark matter

Light dark matter annihilating into electron-positron pairs emits a significant amount of internal bremsstrahlung that may contribute to the cosmic gamma-ray background. The amount of emitted gamma-rays depends on the dark matter clumping factor. Recent calculations indicate that this value should be of order $10^6-10^7$. That allows us to calculate the expected gamma-ray background contribution from dark matter annihilation. We find that the light dark matter model can be ruled out if a constant thermally-averaged cross section is assumed (s-wave annihilation). For more massive dark matter candidates like neutralinos, however, cosmic constraints are weaker.

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arXiv2008

The prospects of finding the first quasars in the universe

We study the prospects of finding the first quasars in the universe with ALMA and JWST. For this purpose, we derive a model for the high-redshift black hole population based on observed relations between the black hole mass and the host galaxy. We re-address previous constraints from the X-ray background with particular focus on black hole luminosities below the Eddington limit as observed in many local AGN. For such luminosities, up to 20% of high-redshift black holes can be active quasars. We then discuss the observables of high-redshift black holes for ALMA and JWST by adopting NGC 1068 as a reference system. We calculate the expected flux of different fine-structure lines for a similar system at higher redshift, and provide further predictions for high-J CO lines. We discuss the expected fluxes from stellar light, the AGN continuum and the Lyman $α$ line for JWST. Line fluxes observed with ALMA can be used to derive detailed properties of high-redshift sources. We suggest two observational strategies to find potential AGN at high redshift and estimate the expected number of sources, which is between 1-10 for ALMA with a field of view of $\sim(1')^2$ searching for line emission and 100-1000 for JWST with a field of view of $(2.16')^2$ searching for continuum radiation. We find that both telescopes can probe high-redshift quasars down to redshift 10 and beyond, and therefore truely detect the first quasars in the universe.

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arXiv2008

Influence of primordial magnetic fields on 21 cm emission

Magnetic fields in the early universe can significantly alter the thermal evolution and the ionization history during the dark ages. This is reflected in the 21 cm line of atomic hydrogen, which is coupled to the gas temperature through collisions at high redshifts, and through the Wouthuysen-Field effect at low redshifts. We present a semi-analytic model for star formation and the build-up of a Lyman alpha background in the presence of magnetic fields, and calculate the evolution of the mean 21 cm brightness temperature and its frequency gradient as a function of redshift. We further discuss the evolution of linear fluctuations in temperature and ionization in the presence of magnetic fields and calculate the effect on the 21 cm power spectrum. At high redshifts, the signal is increased compared to the non-magnetic case due to the additional heat input into the IGM from ambipolar diffusion and the decay of MHD turbulence. At lower redshifts, the formation of luminous objects and the build-up of a Lyman alpha background can be delayed by a redshift interval of 10 due to the strong increase of the filtering mass scale in the presence of magnetic fields. This tends to decrease the 21 cm signal compared to the zero-field case. In summary, we find that 21 cm observations may become a promising tool to constrain primordial magnetic fields.

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arXiv2008

Reionization - A probe for the stellar population and the physics of the early universe

We calculate the reionization history for different models of the stellar population and explore the effects of primordial magnetic fields, dark matter decay and dark matter annihilation on reionization. We find that stellar populations based on a Scalo-type initial mass function for Population II stars can be ruled out as sole sources for reionization, unless star formation efficiencies of more than 10% or very high photon escape fractions from the parental halo are adopted. When considering primordial magnetic fields, we find that the additional heat injection from ambipolar diffusion and decaying MHD turbulence has significant impact on the thermal evolution and the ionization history of the post-recombination universe and on structure formation. The magnetic Jeans mass changes the typical mass scale of the star forming halos, and depending on the adopted stellar model we derive upper limits to the magnetic field strength between 0.7 and $5 $nG (comoving). For dark matter annihilation, we find an upper limit to the thermally averaged mass-weighted cross section of $10^{-33} \mathrm{cm}^3\mathrm{/s/eV}$. For dark matter decay, our calculations yield a lower limit to the lifetime of dark matter particles of $3\times10^{23}$ s. These limits are in agreement with constraints from recombination and the X-ray background and provide an independent confirmation at a much later epoch.

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