Universe’s Growth Rate Mapped with Simulated Galaxies, Testing Cosmology Models

Researchers are increasingly utilising emission line galaxies (ELGs) to map the large-scale structure of the Universe with unprecedented accuracy in Stage-IV cosmological surveys. Ken Osato from Chiba University and Teppei Okumura from Academia Sinica Institute of Astronomy and Astrophysics, along with et al., present a study employing the IllustrisTNG simulations to create realistic mock ELG samples and rigorously test cosmological modelling. Their work focuses on anisotropic correlation functions, revealing a suppression in the quadrupole moment for ELGs potentially caused by infalling motion towards halo centres, which could lead to underestimation of the linear growth rate and indicates a velocity bias between ELGs and dark matter. This detailed analysis of ELG-halo connections and satellite phase-space distribution challenges assumptions about dark matter distributions and offers crucial insights for interpreting future cosmological observations?

Mock Emission Line Galaxy Samples for Validating Cosmological Clustering Models are crucial for precision cosmology

Scientists are employing advanced simulations to unravel the complexities of emission line galaxies (ELGs), the primary tracers of large-scale structures in next-generation cosmological surveys targeting redshifts of approximately 1.5 to 2. These Stage-IV surveys aim to measure the clustering of matter with unprecedented precision, and understanding ELGs is crucial to achieving this goal.

This study constructs realistic mock ELG samples using the IllustrisTNG hydrodynamical simulation and stellar population synthesis techniques to validate models of galaxy clustering. Researchers measured the anisotropic correlation functions of these mock ELGs, allowing them to infer the linear growth rate, a key cosmological parameter derived from galaxy clustering statistics.

As a comparative benchmark, mass-limited subhalo samples with the same density as the ELGs were also constructed. While the isotropic correlation functions in real space showed minimal difference between the two samples, a significant suppression was observed in the quadrupole moment of the anisotropic correlation function for ELGs.

This quadrupole moment is particularly sensitive to galactic velocities, suggesting that the observed suppression is linked to the infalling motion of ELGs towards the centres of their host halos. This infalling motion introduces a velocity bias between ELGs and dark matter, leading to an underestimation of the linear growth rate when analysing the data.

Importantly, this bias diminishes when the analysis focuses on larger scales exceeding 15 h−1 Mpc. Further investigation into the ELG-halo connection involved analysing the phase-space distribution of satellite ELGs within their host halos and examining galactic conformity in star formation activity. The phase-space distribution confirmed the infalling motion, challenging the conventional assumption that the radial distribution of satellite galaxies mirrors that of dark matter. This work highlights a crucial dynamic within ELGs that must be accounted for in future cosmological analyses, potentially refining our understanding of dark matter and the evolution of the Universe.

Mock Emission Line Galaxy Simulations and Anisotropic Correlation Function Measurements reveal key insights

Realistic mock emission line galaxy (ELG) samples were constructed utilising IllustrisTNG hydrodynamical simulations coupled with a stellar population synthesis framework to validate modelling of clustering statistics. The anisotropic correlation functions of these mock ELGs were measured to infer the linear growth rate, a key cosmological parameter derived from galaxy clustering analysis.

As a comparative baseline, mass-limited subhalo samples, matched to the ELG number density, were also created and analysed. While isotropic correlation functions in real space showed minimal difference between the two samples, the quadrupole moment of the anisotropic correlation function, sensitive to galaxy velocities, was notably suppressed for ELGs.

This suppression potentially arises from the infalling motion of ELGs towards the centres of their host halos, introducing a velocity bias between ELGs and dark matter, and leading to an underestimation of the linear growth rate. Analysis restricted to large scales mitigated this parameter bias, demonstrating the importance of scale selection in cosmological studies.

Further investigation focused on the ELG-halo connection by examining the phase-space distribution of satellite ELGs within halos and the galactic conformity of star formation activity. Confirmation of the infalling motion came from the phase-space distribution relative to the host halo, challenging the conventional assumption that the radial distribution of satellite galaxies mirrors that of dark matter.

The study employed the two-point correlation function to quantify clustering, utilising both real-space and redshift-space measurements to capture the effects of peculiar velocities. Specifically, the quadrupole component of the correlation function was crucial in detecting the velocity bias, providing a sensitive probe of the dynamics within dark matter halos and their impact on cosmological parameter estimation.

Emission line galaxy clustering at redshift 1.5 from IllustrisTNG simulations

Researchers utilising IllustrisTNG hydrodynamical simulations have constructed mock emission line galaxy (ELG) samples to validate modelling of clustering and infer the linear growth rate, a key cosmological parameter. The study focused on samples at a redshift of 1.5, employing Hα and [O ii] line luminosities to define ELGs with luminosity thresholds of 2 × 1042 erg s−1 for Hα and 1 × 1042 erg s−1 for [O ii].

Resultant Hα and [O ii] ELG samples comprised 6,864 and 11,001 galaxies, corresponding to comoving number densities of 7.97×10−4 (h−1 Mpc)−3 and 1.28×10−3 (h−1 Mpc)−3, respectively. Control samples were also created, consisting of mass-limited subhalos matching the ELG number densities, with mass limits of 2.09 × 1012 h−1 M⊙ for Hα and 1.51 × 1012 h−1 M⊙ for [O ii].

Analysis of the isotropic correlation functions in real space revealed no significant differences between the ELG and control samples. However, the quadrupole moment of the anisotropic correlation function, sensitive to galaxy velocities, was suppressed for ELGs, potentially due to infalling motion towards the centre of hosting halos.

This suppression leads to an underestimation of the linear growth rate, indicating velocity bias between ELGs and dark matter. When limited to large scales, this parameter bias vanishes. Further investigation into the ELG-halo connection through phase-space distribution of satellite ELGs and galactic conformity of star formation activity confirmed the infalling motion, challenging the assumption that the radial distribution of satellites mirrors that of dark matter. The simulations were run with a box size of (205 h−1 Mpc)3, utilising 25003 dark matter particles and gas cells with mass resolutions of mDM = 5.9 × 107 h−1 M⊙ and mgas = 1.4×106 h−1 M⊙, respectively.

Emission line galaxy velocities influence cosmological parameter estimation significantly

Scientists utilising IllustrisTNG hydrodynamical simulations and stellar population synthesis have constructed realistic mock samples of emission line galaxies, crucial for interpreting data from Stage-IV cosmological spectroscopic surveys. These surveys aim to map large-scale structures and measure clustering statistics at high redshifts with greater precision.

Researchers measured anisotropic correlation functions from the mock ELGs and inferred the linear growth rate, a key cosmological parameter, comparing the results to a control sample of mass-limited subhalos. Analysis revealed that while isotropic correlation functions were similar between the two samples, the quadrupole moment, sensitive to galaxy velocities, was suppressed for ELGs.

This suppression potentially arises from the infalling motion of ELGs towards the centres of their host halos, leading to an underestimation of the linear growth rate and indicating a velocity bias between ELGs and dark matter. However, this bias diminishes when focusing on larger scales. Further investigation into the ELG-halo connection, through phase-space distribution and galactic conformity analysis, confirmed the infalling motion and challenged the assumption that satellite ELG radial distributions mirror those of dark matter.

The study establishes that the dynamics of ELGs significantly impact clustering statistics in redshift space, unlike the more accurate modelling previously achieved for luminous red galaxies. The inferred linear growth rate from ELG anisotropic correlation functions was found to be biased low due to the infalling motion of satellite ELGs, suggesting the no-velocity bias assumption is invalid for this population. Future research should focus on refining theoretical models to account for these ELG-specific dynamics, particularly the observed infalling motion and its effect on halo occupation distribution, to minimise systematic biases in cosmological parameter estimation from upcoming large-scale surveys such as DESI, PFS, Euclid, and the Roman Space Telescope.

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