Dr. Deep Anand

The upcoming flagship Habitable Worlds Observatory (HWO) promises to revolutionize nearly every field of astrophysics. The sensitivity and resolution of HWO will enable the detection of resolved stars out to at least 100 Mpc, thus allowing us to measure distances directly into the Hubble flow with stellar indicators such as Cepheid variables and the tip of the red giant branch. Here we discuss the requirements for a program to measure the Hubble constant to 1% without the use of Type Ia supernovae or other secondary distance indicators.

A fundamental prediction of the ΛCDM model is the existence of starless dark matter halos on sub-galactic mass scales. Here we present HST/ACS observations of a Cloud-9, a gas cloud around the nearby galaxy Messier 94 with characteristics that suggested it may be one of these relic objects. We confirm that there are no stars associated with Cloud-9, making it the first known system lying at the predicted boundary between failed and succesful galaxies, while also constraining the present-day threshold halo mass for galaxy formation.

The direct, empirical determination of the local value of the Hubble constant (H0) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance estimators. However, combining determinations from different estimators is non-trivial, due to their correlated calibrations and different analysis methodologies. Using covariance weighting and leveraging community expertise, we construct a rigorous and transparent Distance Network to find a consensus value and uncertainty for the locally-measured Hubble constant of H₀ = 73.50 ± 0.81 km/s/Mpc, in 7.1σ disagreement from results from the early Universe.

Cycle 1 JWST observations of Cepheids in Type Ia supernova (SN Ia) hosts resolved their red-giant-dominated near-infrared backgrounds, sharply reducing crowding and showing that photometric bias in lower-resolution HST data does not account for the Hubble tension. We present cycle 2 JWST observations of >100 Cepheids in NGC 3447, a unique system that pushes this test to the limit by transitioning from low to no background contamination. Using a tidal region in NGC 3447, we are able to measure Cepheids in a "crowding-free" environment, confirming prior measurements based on HST data. Our overall value of the Hubble constant from HST and JWST is H₀ = 73.18 ± 0.93 km/s/Mpc, or 6σ above the expectations from ΛCDM and the cosmic microwave background.

Here we use JWST TRGB distances calibrated using the megamaser galaxy NGC 4258 to determine a new Cepheid-independent SBF zero point for HST. This new calibration, along with improved optical color measurements from PanSTARRS and DECam, gives an updated value of the Hubble constant = 73.8 +/- 0.7 +/- 2.3 km/s/Mpc that is virtually identical to the previously published result based on SBF distances calibrated using Cepheids. JWST observations of the Coma cluster will soon establish a foundation for the SBF calibration that will extend to much larger distances than is possible with HST.

The Virgo Cluster is the nearest substantial cluster of galaxies to the Milky Way and a cornerstone of the extragalactic distance scale. Here, we present JWST/NIRCam observations that simultaneously cover the cores and halos of ten galaxies in and around the Virgo Cluster and are designed to perform simultaneous measurements of the tip of the red giant branch (TRGB) and surface brightness fluctuations (SBF). The work presented here is part of a larger program to develop a Population II distance scale through the TRGB and SBF that is completely independent of the prominent Cepheid + Type Ia supernova ladder.