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ABSTRACT We present a new constraint on the Hubble constant ($$H_0$$) from the standard dark siren method using a sample of five well-covered gravitational wave (GW) alerts reported during the first part of the fourth observing run of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo and Kamioka Gravitational Wave Detector (KAGRA) collaborations (LVK) and with three updated standard dark sirens from third observation run in combination with the previous constraints from the first three runs. Our methodology relies on the galaxy catalogue method alone. We use a deep learning method to derive the full probability density estimation of photometric redshifts using the Legacy Survey catalogues. We add the constraints from well localized binary black hole mergers to the sample of standard dark sirens analysed in our previous work. We combine the $$H_0$$ posterior for 5 new standard sirens with other 10 previous events (using the most recent available data for the five novel events and updated three previous posteriors from O3), finding $$H_0 = 70.4^{+13.6}_{-11.7}~{\rm km~s^{-1}~Mpc^{-1}}$$ (68 per cent confidence interval) with the catalogue method only. This result represents an improvement of $$\sim 23~{{\ \rm per\ cent}}$$ comparing the new 15 dark siren constraints with the previous 10 dark siren constraints and a reduction in uncertainty of $$\sim 40~{{\ \rm per\ cent}}$$ from the combination of 15 dark and bright sirens compared with the GW170817 bright siren alone. The combination of dark and bright siren GW170817 with recent jet constraints yields $$H_0$$ of $$68.0^{+4.4}_{-3.8}~{\rm km~s^{-1}~Mpc^{-1}}$$, a $$\sim 6~{{\ \rm per\ cent}}$$ precision from standard sirens, reducing the previous constraint uncertainty by $$\sim 10~{{\ \rm per\ cent}}$$. 

Abstract We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration (or DELVE) survey. The low luminosity ( $M_V=-3.56_{-0.37}^{+0.47}$; $L_V=2300_{-700}^{+1200}{L}_{\odot }$), large size ( $R_{1/2}=90_{-30}^{+30}$pc), and large heliocentric distance ( $D=111_{-6}^{+9}$kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of $700_{-500}^{+1400}{M}_{\odot }/L_{\odot }$and a nonzero metallicity dispersion of ${\sigma }_{[\mathrm{Fe}/\mathrm{H}]}=0.19_{-0.11}^{+0.14}$, further confirming Leo VI’s identity as a UFD. While the system has a highly elliptical shape, $ϵ=0.54_{-0.29}^{+0.19}$, we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy–angular momentum space make it unlikely that Leo VI is part of the proposed infall group. 

Abstract We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity ( $M_V=-{2.5}_{-0.5}^{+0.3};$ $L_V={850}_{-260}^{+380}{L}_{\odot }$), extended ( $r_{1/2}={41}_{-8}^{+9}$pc) stellar system located in the outer halo (D_⊙= 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of $v_{\mathrm{sys}}=-{13.1}_{-0.9}^{+1.0}\mathrm{km}{\mathrm{s}}^{-1}$for the system and place an upper limit ofσ_v< 3.5 km s⁻¹(σ_v< 1.6 km s⁻¹) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= − 2.61 ± 0.21) with a statistically significant metallicity spread ( ${\sigma }_{\left[\mathrm{Fe}/\mathrm{H}\right]}={0.46}_{-0.14}^{+0.26}$dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (r_peri= 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly belowσ_v≲ 2 km s⁻¹, Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos.