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Register a new userETHOS -- An effective parametrization and classification for structure formation: the non-linear regime at $zgtrsim5$
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Authors
Sebastian Bohr
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We propose two effective parameters that fully characterise galactic-scale structure formation at high redshifts ($zgtrsim5$) for a variety of dark matter (DM) models that have a primordial cutoff in the matter power spectrum. Our description is within the recently proposed ETHOS framework and includes standard thermal Warm DM (WDM) and models with dark acoustic oscillations (DAOs). To define and explore this parameter space, we use high-redshift zoom-in simulations that cover a wide range of non-linear scales from those where DM should behave as CDM ($ksim10,h,{rm Mpc}^{-1}$), down to those characterised by the onset of galaxy formation ($ksim500,h,{rm Mpc}^{-1}$). We show that the two physically motivated parameters $h_{rm peak}$ and $k_{rm peak}$, the amplitude and scale of the first DAO peak, respectively, are sufficient to parametrize the linear matter power spectrum and classify the DM models as belonging to effective non-linear structure formation regions. These are defined by their relative departure from Cold DM ($k_{rm peak}rightarrowinfty$) and WDM ($h_{rm peak}=0$) according to the non-linear matter power spectrum and halo mass function. We identify a region where the DAOs still leave a distinct signature from WDM down to $z=5$, while a large part of the DAO parameter space is shown to be degenerate with WDM. Our framework can then be used to seamlessly connect a broad class of particle DM models to their structure formation properties at high redshift without the need of additional $N$-body simulations.
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We contrast predictions for the high-redshift galaxy population and reionization history between cold dark matter (CDM) and an alternative self-interacting dark matter model based on the recently developed ETHOS framework that alleviates the small-scale CDM challenges within the Local Group. We perform the highest resolution hydrodynamical cosmological simulations (a 36~Mpc$^3$ volume with gas cell mass of $sim10^5mathrm{M}_{odot}$ and minimum gas softening of $sim180$~pc) within ETHOS to date -- plus a CDM counterpart -- to quantify the abundance of galaxies at high redshift and their impact on reionization. We find that ETHOS predicts galaxies with higher ultraviolet (UV) luminosities than their CDM counterparts and a faster build-up of the faint end of the UV luminosity function. These effects, however, make the optical depth to reionization less sensitive to the power spectrum cut-off: the ETHOS model differs from the CDM $tau$ value by only 10 per cent and is consistent with Planck limits if the effective escape fraction of UV photons is 0.1-0.5. We conclude that current observations of high-redshift luminosity functions cannot differentiate between ETHOS and CDM models, but deep JWST surveys of strongly-lensed, inherently faint galaxies have the potential to test non-CDM models that offer attractive solutions to CDMs Local Group problems.
We present a simple physically motivated picture for the mildly non-linear regime of structure formation, which captures the effects of the bulk flows. We apply this picture to develop a method to significantly reduce the sample variance in cosmological N-body simulations at the scales relevant to the Baryon Acoustic Oscillations (BAO). The results presented in this paper will allow for a speed-up of an order of magnitude (or more) in the scanning of the cosmological parameter space using N-body simulations for studies which require a good handle of the mildly non-linear regime, such as those targeting the BAO. Using this physical picture we develop a simple formula, which allows for the rapid calculation of the mildly non-linear matter power spectrum to percent level accuracy, and for robust estimation of the BAO scale.
A cutoff in the linear matter power spectrum at dwarf galaxy scales has been shown to affect the abundance, formation mechanism and age of dwarf haloes and their galaxies at high and low redshift. We use hydrodynamical simulations of galaxy formation within the ETHOS framework in a benchmark model that has such a cutoff, and that has been shown to be an alternative to the cold dark matter (CDM) model that alleviates its dwarf-scale challenges. We show how galaxies in this model form differently to CDM on a halo-by-halo basis, at redshifts $zge6$. We show that ETHOS haloes at the half-mode mass scale form with 50~per~cent less mass than their CDM counterparts due to their later formation times, yet they retain more of their gas reservoir due to the different behaviour of gas and dark matter during the monolithic collapse of the first haloes in models with a galactic-scale cutoff. As a result, galaxies in ETHOS haloes near the cutoff scale grow rapidly between $z=10-6$ and by $z=6$ end up having very similar stellar masses, higher gas fractions and higher star formation rates relative to their CDM counterparts. We highlight these differences by making predictions for how the number of galaxies with old stellar populations is suppressed in ETHOS for both $z=6$ galaxies and for gas-poor Local Group fossil galaxies. Interestingly, we find an age gradient in ETHOS between galaxies that form in high and low density environments.
We introduce a novel approach, the Cosmological Trajectories Method (CTM), to model nonlinear structure formation in the Universe by expanding gravitationally-induced particle trajectories around the Zeldovich approximation. A new Beyond Zeldovich approximation is presented, which expands the CTM to leading second-order in the gravitational interaction and allows for post-Born gravitational scattering. In the Beyond Zeldovich approximation we derive the exact expression for the matter clustering power spectrum. This is calculated to leading order and is available in the CTM MODULE. We compare the Beyond Zeldovich approximation power spectrum and correlation function to other methods including 1-loop Standard Perturbation Theory (SPT), 1-loop Lagrangian Perturbation Theory (LPT) and Convolution Lagrangian Perturbation Theory (CLPT). We find that the Beyond Zeldovich approximation power spectrum performs well, matching simulations to within $pm{10}%$, on mildly non-linear scales, and at redshifts above $z=1$ it outperforms the Zeldovich approximation. We also find that the Beyond Zeldovich approximation models the BAO peak in the correlation function at $z=0$ more accurately, to within $pm{5}%$ of simulations, than the Zeldovich approximation, SPT 1-loop and CLPT.
We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of $2.8times 10^4,{rm M}_odot$ and a softening length of $72.4,{rm pc}$. We demonstrate that all alternative models have only a negligible impact on large scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small scale primordial damping in the power spectrum and late time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even over-solve the too-big-to-fail problem; thus ruling them out.
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