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1. Inferring the Intermediate-mass Black Hole Number Density from Gravitational-wave Lensing Statistics

   https://doi.org/10.3847/2041-8213/ac7052  

   Gais, Joseph ; Ng, Ken K. ; Seo, Eungwang ; Wong, Kaze W. ; Li, Tjonnie G. ( June 2022 , The Astrophysical Journal Letters)

   Abstract The population properties of intermediate-mass black holes remain largely unknown, and understanding their distribution could provide a missing link in the formation of supermassive black holes and galaxies. Gravitational-wave observations can help fill in the gap from stellar mass black holes to supermassive black holes with masses between ∼100–10 4 M ⊙ . In our work, we propose a new method for examining lens populations through lensing statistics of gravitational waves, here focusing on inferring the number density of intermediate-mass black holes through hierarchical Bayesian inference. Simulating ∼200 lensed gravitational-wave signals, we find that existing gravitational-wave observatories at their design sensitivity could either constrain the number density of 10 6 Mpc −3 within a factor of 10, or place an upper bound of ≲10 4 Mpc −3 if the true number density is 10 3 Mpc −3 . More broadly, our method leaves room for incorporation of additional lens populations, providing a general framework for probing the population properties of lenses in the universe.
2. Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

   https://doi.org/10.3847/1538-4357/ac91d1  

   Nugent, Anya E. ; Fong, Wen-Fai ; Dong, Yuxin ; Leja, Joel ; Berger, Edo ; Zevin, Michael ; Chornock, Ryan ; Cobb, Bethany E. ; Kelley, Luke Zoltan ; Kilpatrick, Charles D. ; et al ( November 2022 , The Astrophysical Journal)

   We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of$z={0.64}_{-0.32}^{+0.83}$(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oft_m=${0.8}_{-0.53}^{+2.71}$Gyr, stellar mass of log(M_*/M_⊙) =${9.69}_{-0.65}^{+0.75}$, star formation rate of SFR =${1.44}_{-1.35}^{+9.37}$M_⊙yr⁻¹, stellar metallicity of log(Z_*/Z_⊙) =$-{0.38}_{-0.42}^{+0.44}$, and dust attenuation of$A_V={0.43}_{-0.36}^{+0.85}$mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution,more »with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.

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3. TDCOSMO: V. Strategies for precise and accurate measurements of the Hubble constant with strong lensing

   https://doi.org/10.1051/0004-6361/202039179  

   Birrer, Simon ; Treu, Tommaso ( May 2021 , Astronomy & Astrophysics)

   Strong lensing time delays can measure the Hubble constant H 0 independently of any other probe. Assuming commonly used forms for the radial mass density profile of the lenses, a 2% precision has been achieved with seven Time-Delay Cosmography (TDCOSMO) lenses, in tension with the H 0 from the cosmic microwave background. However, without assumptions on the radial mass density profile – and relying exclusively on stellar kinematics to break the mass-sheet degeneracy – the precision drops to 8% with the current data obtained using the seven TDCOSMO lenses, which is insufficient to resolve the H 0 tension. With the addition of external information from 33 Sloan Lens ACS (SLACS) lenses, the precision improves to 5% if the deflectors of TDCOSMO and SLACS lenses are drawn from the same population. We investigate the prospect of improving the precision of time-delay cosmography without relying on mass profile assumptions to break the mass-sheet degeneracy. Our forecasts are based on a previously published hierarchical framework. With existing samples and technology, 3.3% precision on H 0 can be reached by adding spatially resolved kinematics of the seven TDCOSMO lenses. The precision improves to 2.5% with the further addition of kinematics for 50 nontime-delay lensesmore »from SLACS and the Strong Lensing Legacy Survey. Expanding the samples to 40 time-delay and 200 nontime-delay lenses will improve the precision to 1.5% and 1.2%, respectively. Time-delay cosmography can reach sufficient precision to resolve the Hubble tension at 3–5 σ , without assumptions on the radial mass profile of lens galaxies. By obtaining this precision with and without external datasets, we will test the consistency of the samples and enable further improvements based on even larger future samples of time-delay and nontime-delay lenses (e.g., from the Rubin , Euclid , and Roman Observatories).« less
4. TDCOSMO III: Dark matter substructure meets dark energy -- the effects of (sub)halos on strong-lensing measurements of H0

   Gilman, Daniel ; Birrer, Simon ; Treu, Tommaso ( July 2020 , Astronomy astrophysics)

   Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H0. The lens models used in time delay cosmography omit dark matter subhalos and line-of-sight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing realistic mock lens systems that include full populations of dark matter subhalos and line-of-sight halos, applying the same modeling assumptions used in the literature to infer H0. We base the mock lenses on six quadruply-imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lens-by-lens basis. We show that omitting dark substructure does not bias inferences of H0. However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.6−2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and sets a precision limit on inferences from single lenses.
5. TDCOSMO: III. Dark matter substructure meets dark energy. The effects of (sub)halos on strong-lensing measurements of H ₀

   https://doi.org/10.1051/0004-6361/202038829  

   Gilman, D. ; Birrer, S. ; Treu, T. ( October 2020 , Astronomy & Astrophysics)

   Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H 0 . The lens models used in time delay cosmography omit dark matter subhalos and line-of-sight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing mock lens systems that include full populations of dark matter subhalos and line-of-sight halos, applying the same modeling assumptions used in the literature to infer H 0 . We base the mock lenses on six quadruply imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lens-by-lens basis. We show that omitting dark substructure does not bias inferences of H 0 . However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H 0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.7 − 2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and it sets amore »precision limit on inferences from single lenses.« less