Kevin Spencer McCarthy

arXiv2020

Constraining Delay Time Distribution of Binary Neutron Star Mergers from Host Galaxy Properties

Gravitational wave (GW) observatories are discovering binary neutron star mergers (BNSMs), and in at least one event we were able to track it down in multiple wavelengths of light, which allowed us to identify the host galaxy. Using a catalogue of local galaxies with inferred star formation histories and adopting a BNSM delay time distribution (DTD) model, we investigate the dependence of BNSM rate on an array of galaxy properties. Compared to the intrinsic property distribution of galaxies, that of BNSM host galaxies is skewed toward galaxies with redder colour, lower specific star formation rate, higher luminosity, and higher stellar mass, reflecting the tendency of higher BNSM rates in more massive galaxies. We introduce a formalism to efficiently make forecast on using host galaxy properties to constrain DTD models. We find comparable constraints from the dependence of BNSM occurrence distribution on galaxy colour, specific star formation rate, and stellar mass, all better than those from dependence on $r$-band luminosity. The tightest constraints come from using individual star formation histories of host galaxies, which reduces the uncertainties on DTD parameters by a factor of three or more. Substantially different DTD models can be differentiated with about 10 BNSM detections. To constrain DTD parameters at 10\% precision level requires about one hundred detections, achievable with GW observations on a decade time scale.

astro-ph.GA↗

arXiv2019

The Effects of Galaxy Assembly Bias on the Inference of Growth Rate from Redshift-Space Distortions

The large-scale redshift-space distortion (RSD) in galaxy clustering can probe $fσ_8$, a combination of the cosmic structure growth rate and the matter fluctuation amplitude, which can constrain dark energy models and test theories of gravity. While the RSD on small scales (e.g. a few to tens of $h^{-1}{\rm Mpc}$) can further tighten the $fσ_8$ constraints, galaxy assembly bias, if not correctly modelled, may introduce systematic uncertainties. Using a mock galaxy catalogue with built-in assembly bias, we perform a preliminary study on how assembly bias may affect the $fσ_8$ inference. We find good agreement on scales down to 8--9$h^{-1}{\rm Mpc}$ between a $fσ_8$ metric from the redshift-space two-point correlation function with the central-only mock catalogue and that with the shuffled catalogue free of assembly bias, implying that $fσ_8$ information can be extracted on such scales even with assembly bias. We then apply the halo occupation distribution (HOD) and three subhalo clustering and abundance matching (SCAM) models to model the redshift-space clustering with the mock. Only the SCAM model based on $V_{\rm peak}$ (used to create the mock) can reproduce the $fσ_8$ metric, and the other three could not. However, the $fσ_8$ metrics determined from central galaxies from all the models are able to match the expected one down to 8$h^{-1}{\rm Mpc}$. Our results suggest that halo models with no or incorrect assembly bias prescription could still be used to model the RSD down to scales of $\sim 8 h^{-1}{\rm Mpc}$ to tighten the $fσ_8$ constraint, with a sample of central galaxies or with a flexible satellite occupation prescription.

astro-ph.CO↗